diff --git a/examples/README.md b/examples/README.md
index edada3d..8684c1a 100644
--- a/examples/README.md
+++ b/examples/README.md
@@ -2,8 +2,20 @@
 
 In this folder, you will find different examples on how to use the framework.
 
+Warning: this folder is currently being updated; few files might still have some bugs or not 
+implemented completely. Some other folders will be added in the upcoming days.
+
 You can check the following folders:
+- `worlds`: how to create a world in the simulator, load various objects inside and interact with 
+them, use the camera, and load or generate terrains.
+- `robots`: check how to load a specific robot (biped, quadruped, wheeled, etc) into the world.
+- `interfaces`: the various interfaces (game controllers, webcam, etc) and bridges that you can use.
+- `kinematics`: check how to use forward and inverse kinematics as well as position and velocity control.
+- `manipulability`: check how to use the velocity and dynamic manipulability ellipsoids.
+
+- `states`: how to query the states / observations.
+- `models`: the different learning models that you can use.
+
+- `imitation`: how to use imitation learning with the framework.
 - `gym/cartpole`: policies are trained with different algorithms on the gym Cartpole environment.
-- `robots`: check how to load a specific robot into the world.
-- `states`: how to query the states / observations.
 
diff --git a/examples/interfaces/README.md b/examples/interfaces/README.md
index 0a26b1a..5ca8360 100644
--- a/examples/interfaces/README.md
+++ b/examples/interfaces/README.md
@@ -1,4 +1,11 @@
-## Interfaces
+## Interfaces and Bridges
 
 In this folder, you will find examples on what interfaces you can use and on how you can collect the data from them.
-You will also be able to connect an interface with an element of the world (in this case, a robot) using bridges, and see that different bridges can lead to different behaviors while getting the data from the same interface. 
+You will also be able to connect an interface with an element of the world (in this case, a robot) using bridges, 
+and see that different bridges can lead to different behaviors while getting the data from the same interface. 
+
+Here are few examples that depict the various interfaces:
+1. `mouse_keyboard.py`: use the mouse keyboard interface
+2. `webcam.py`: use the webcam interface
+3. `playstation.py`: use the Playstation joystick controller interface
+4. `xbox.py`: use the Xbox joystick controller interface
diff --git a/examples/interfaces/mouse_keyboard.py b/examples/interfaces/mouse_keyboard.py
new file mode 100644
index 0000000..e5e3704
--- /dev/null
+++ b/examples/interfaces/mouse_keyboard.py
@@ -0,0 +1,27 @@
+#!/usr/bin/env python
+"""Load the mouse keyboard interface.
+"""
+
+from itertools import count
+
+import pyrobolearn as prl
+from pyrobolearn.tools.interfaces.mouse_keyboard import MouseKeyboardInterface
+
+
+# create simulator
+sim = prl.simulators.Bullet()
+
+# create mouse keyboard interface
+interface = MouseKeyboardInterface(sim)
+
+# run interface
+for _ in count():
+    # perform a step with the interface
+    interface.step()
+
+    # print pressed keys
+    if len(interface.key_down) > 0:
+        print("Keys that are pressed: {}".format(interface.key_down))
+
+    # perform a step with the simulator
+    sim.step(sleep_time=sim.dt)
diff --git a/examples/interfaces/playstation.py b/examples/interfaces/playstation.py
new file mode 100644
index 0000000..c136aa7
--- /dev/null
+++ b/examples/interfaces/playstation.py
@@ -0,0 +1,39 @@
+#!/usr/bin/env python
+"""Load the Playstation game controller interface
+"""
+
+import time
+from itertools import count
+import argparse
+
+from pyrobolearn.tools.interfaces.controllers.playstation import PS3ControllerInterface, PS4ControllerInterface
+
+# create parser to select the game controller
+parser = argparse.ArgumentParser()
+parser.add_argument('-c', '--controller', help='The Playstation game controller to use (ps3 or ps4)', type=str,
+                    choices=['ps3', 'ps4'], default='ps4')
+args = parser.parse_args()
+
+
+# load corresponding Playstation controller interface
+if args.controller == 'ps3':
+    controller = PS3ControllerInterface(verbose=True)
+elif args.controller == 'ps4':
+    controller = PS4ControllerInterface(verbose=False)
+else:
+    raise NotImplementedError("Unknown game controller")
+
+
+# run controller
+print('Running controller...')
+for _ in count():
+
+    # run one step with the interface
+    controller.run()  # same as `step()` if we are not using threads
+
+    # get the last update and print it
+    b = controller.X
+    print("X: {}".format(b))  # , controller[b]))
+
+    # sleep a bit
+    time.sleep(0.01)
diff --git a/examples/interfaces/xbox.py b/examples/interfaces/xbox.py
new file mode 100644
index 0000000..f32d3a5
--- /dev/null
+++ b/examples/interfaces/xbox.py
@@ -0,0 +1,39 @@
+#!/usr/bin/env python
+"""Load the Xbox game controller interface
+"""
+
+import time
+from itertools import count
+import argparse
+
+from pyrobolearn.tools.interfaces.controllers.xbox import Xbox360ControllerInterface, XboxOneControllerInterface
+
+# create parser to select the game controller
+parser = argparse.ArgumentParser()
+parser.add_argument('-c', '--controller', help='The Xbox game controller to use (xbox one or xbox 360)', type=str,
+                    choices=['360', 'one'], default='one')
+args = parser.parse_args()
+
+
+# load corresponding Xbox controller interface
+if args.controller == '360':
+    controller = Xbox360ControllerInterface(verbose=True)
+elif args.controller == 'one':
+    controller = XboxOneControllerInterface(verbose=True)
+else:
+    raise NotImplementedError("Unknown game controller")
+
+
+# run controller
+print('Running controller...')
+for _ in count():
+
+    # run one step with the interface
+    controller.run()  # same as `step()` if we are not using threads
+
+    # get the last update and print it
+    b = controller.last_updated_button
+    print("Last updated button: {} with value: {}".format(b, controller[b]))
+
+    # sleep a bit
+    time.sleep(0.01)
diff --git a/examples/manipulability/2d_manipulability.py b/examples/manipulability/2d_manipulability.py
new file mode 100644
index 0000000..85350d8
--- /dev/null
+++ b/examples/manipulability/2d_manipulability.py
@@ -0,0 +1,41 @@
+#!/usr/bin/env python
+"""Draw the 2D velocity and force manipulability ellipsoids on the end-effector of a 3-link planar manipulator.
+
+References:
+    [1] "Robotics: Modelling, Planning and Control" (section 3.9), Siciliano et al., 2010
+"""
+
+import time
+# from itertools import count
+import numpy as np
+
+import pyrobolearn as prl
+
+
+# create simulator
+sim = prl.simulators.Bullet()
+
+# create world
+world = prl.worlds.BasicWorld(sim)
+
+# create robot
+robot = world.load_robot('manipulator2d')
+robot.reset_joint_states(q=[0.64453457, -1.65045902, -0.31141744])
+
+# change camera view
+world.camera.reset(distance=2, yaw=-np.pi / 2, pitch=-np.pi/2.01)
+
+# draw 2d velocity manipulability ellipsoid
+# print(robot.end_effector_names)
+end_effector_id = robot.get_link_ids('gripper')
+jacobian = robot.get_linear_jacobian(link_id=end_effector_id)
+jjt = robot.get_JJT(jacobian)
+robot.draw_velocity_manipulability_ellipsoid(link_id=end_effector_id, JJT=jjt, color=(0, 1, 0, 0.7))  # green
+robot.draw_force_manipulability_ellipsoid(link_id=end_effector_id, JJT=jjt, color=(1, 0, 0, 0.7))     # red
+
+# TODO: fix bug
+
+time.sleep(10000)
+# run simulator
+# for t in count():
+#     world.step(sleep_dt=1./240)
diff --git a/examples/manipulability/README.md b/examples/manipulability/README.md
index 3c9e9a8..8f2b826 100644
--- a/examples/manipulability/README.md
+++ b/examples/manipulability/README.md
@@ -2,6 +2,16 @@
 
 We provide examples on how to use manipulability ellipsoids.
 
+Here are a short description of the various examples the user can try:
+1. `2d_manipulability.py`: draw the 2D velocity and force manipulability ellipsoids on the end-effector of a 
+3-link planar manipulator.
+2. `com_manipulability_tracking.py`: track the velocity manipulability ellipsoid of the center of mass of a robot 
+with a fixed base.
+3. `com_manipulability_tracking_with_balance.py`: track the velocity manipulability ellipsoid of the center of mass 
+of a floating-base robot while keeping its balance.
+4. `com_dynamic_manipulability_tracking_with_balance.py`: track the dynamic manipulability ellipsoid of the center 
+of mass of a floating-base robot while keeping its balance.
+
 References:
 - [1] "Robotics: Modelling, Planning and Control", Siciliano et al., 2010
 - [2] "Springer Handbook of Robotics", Siciliano et al., 2008
diff --git a/examples/robots/README.md b/examples/robots/README.md
index 3a46ca9..e945c60 100644
--- a/examples/robots/README.md
+++ b/examples/robots/README.md
@@ -1,5 +1,22 @@
 ## Robot examples
 
-You can try to load different robot by typing `python <robot>.py`.
+More than 60 robots (of various types) are available through `pyrobolearn`.
 
-To turn the camera in the simulator, keep pressing the `ctrl` key and the left button on the mouse, and move this last one.
+Here are the few examples that you can find in this folder:
+1. `load_robot.py <robot_name>`: load the given robot in the world.
+2. `visualize_robot.py <robot_name>`: test different visualization tools that can be used on the robot to show its 
+joint axis, bounding boxes, and others.
+3. `robot_with_sliders.py <robot_name>`: load the given robot in the world and allow you to manipulate the robot's 
+joints with sliders.
+4. `distribute_epucks.py`: distribute several e-pucks in the world and make them move forward.
+5. `quadcopter_controller.py`: move a quadcopter in the air using an Xbox or Playstation game controller.
+6. `robots/<robot>.py`: load the given robot in the simulator by directly instantiating it. Some of these files do 
+more than just loading the robot.
+
+Notes: to turn the camera in the simulator, keep pressing the `ctrl` key and the left button on the mouse, and 
+move this last one.
+
+
+#### What to check next?
+
+Check the `pyrobolearn/examples/interfaces` or `pyrobolearn/examples/kinematics` folder.
diff --git a/examples/robots/distribute_epucks.py b/examples/robots/distribute_epucks.py
new file mode 100644
index 0000000..3519785
--- /dev/null
+++ b/examples/robots/distribute_epucks.py
@@ -0,0 +1,60 @@
+#!/usr/bin/env python
+"""Distribute several e-pucks in the world and make them move forward.
+
+You can move in the world using the keyboard and mouse:
+- `ctrl + left click`: rotate the camera
+- `scroll wheel` or `ctrl + right click`: zoom in/out
+- `ctrl + middle click`: move the camera
+- `left click` on an object: if the object has a mass and a collision shape, you can interact with it with the mouse
+- `w`: wireframe (see collision shapes)
+- `g`: show/hide menu
+- `esc`: quit the simulator
+"""
+
+import numpy as np
+from itertools import count
+import argparse
+
+import pyrobolearn as prl
+
+
+# create function for the parser to check the number of robots
+def check(number):
+    """check that the number of robots is between 1 and 100."""
+    number = int(number)
+    if number < 1:
+        number = 1
+    if number > 100:
+        number = 100
+    return number
+
+
+# create parser to select the robot
+parser = argparse.ArgumentParser()
+parser.add_argument('-n', '--number', help='the number of epucks in the world', type=check, default=10)
+args = parser.parse_args()
+
+
+# create simulator
+sim = prl.simulators.Bullet()
+
+# create basic world (with a floor and gravity enabled by default)
+world = prl.worlds.BasicWorld(sim, scaling=1)
+
+# specify distribution ranges for position (x,y,z) and orientation (r,p,y)
+low_position, high_position = [-3, -3, 0], [3, 3, 0]  # x,y,z
+low_orientation, high_orientation = [0, 0, -np.pi], [0, 0, np.pi]  # r,p,y
+
+# distribute the epucks in the world
+robots = world.distribute(world.load_robot, size=args.number, position_range=(low_position, high_position),
+                          rpy_range=(low_orientation, high_orientation), return_body=True, robot='epuck')
+
+# run simulator
+for t in count():
+
+    # move each robot forward
+    for robot in robots:
+        robot.drive(speed=5)
+
+    # perform one step in the world
+    world.step(sleep_dt=1. / 240)
diff --git a/examples/robots/load_robot.py b/examples/robots/load_robot.py
index d13c838..125df9e 100644
--- a/examples/robots/load_robot.py
+++ b/examples/robots/load_robot.py
@@ -1,33 +1,44 @@
-# This file creates a basic world, load each robot that can be found in the PRL framework
+#!/usr/bin/env python
+"""Load a robot in a basic world.
 
-from pyrobolearn.simulators import BulletSim
-from pyrobolearn.worlds import BasicWorld
-from pyrobolearn.robots import implemented_robots
+You can move in the world using the keyboard and mouse:
+- `ctrl + left click`: rotate the camera
+- `scroll wheel` or `ctrl + right click`: zoom in/out
+- `ctrl + middle click`: move the camera
+- `left click` on an object: if the object has a mass and a collision shape, you can interact with it with the mouse
+- `w`: wireframe (see collision shapes)
+- `g`: show/hide menu
+- `esc`: quit the simulator
+"""
 
-robot_not_working = set(['icub'])
+from itertools import count
+import argparse
+
+import pyrobolearn as prl
+
+
+# get implemented robots
+robots = prl.robots.implemented_robots
+print("All the robots (total number of robots = {}): {}".format(len(robots), robots))
+
+# create parser to select the robot
+parser = argparse.ArgumentParser()
+parser.add_argument('-r', '--robot', help='the robot to load in the world', type=str,
+                    choices=robots, default='hyq2max')
+args = parser.parse_args()
 
-print("All the robots (total number of robots = {}): {}".format(len(implemented_robots), implemented_robots))
 
 # create simulator
-sim = BulletSim()
+sim = prl.simulators.Bullet()
 
 # create basic world with floor and gravity
-world = BasicWorld(sim)
+world = prl.worlds.BasicWorld(sim)
 
-# create one robot at a time
-for i, robot_name in enumerate(implemented_robots):
-    if robot_name not in robot_not_working:
-        # instantiate the given robot
-        robot = world.load_robot(robot_name)
+# load the robot in the world (note that you can create the robot outside the world (not recommended),
+# and then give it to the `world.load_robot` method to let know the world that a robot was loaded)
+robot = world.load_robot(robot=args.robot, position=[0., 0.])
 
-        # print info about the robot
-        print("Robot n{}: {}".format(i+1, robot))
-        # robot.print_info()
-
-        # run for few moments in the world
-        for t in range(250):
-            # run one step and sleep a bit
-            world.step(sleep_dt=1./240)
-
-        # remove the robot from the world
-        world.remove(robot)
+# run simulator
+for _ in count():
+    # perform one step in the world
+    world.step(sleep_dt=1. / 240)
diff --git a/examples/robots/quadcopter_controller.py b/examples/robots/quadcopter_controller.py
new file mode 100644
index 0000000..5cece42
--- /dev/null
+++ b/examples/robots/quadcopter_controller.py
@@ -0,0 +1,58 @@
+#!/usr/bin/env python
+"""Control a quadcopter in the air using an Xbox or Playstation game controller.
+
+how to run:
+```
+$ python quadcopter_controller.py --help                # for help
+$ python quadcopter_controller.py --controller xbox     # to use Xbox game controller
+$ python quadcopter_controller.py --controller ps3      # to use PS3 game controller
+```
+"""
+
+import numpy as np
+from itertools import count
+import argparse
+
+import pyrobolearn as prl
+
+
+# create parser to select the game controller
+parser = argparse.ArgumentParser()
+parser.add_argument('-c', '--controller', help='the game controller to use', type=str,
+                    choices=['xbox', 'ps3'], default='ps3')
+args = parser.parse_args()
+
+
+# load corresponding interface
+# if args.controller == 'xbox':
+#     from pyrobolearn.tools.interfaces.controllers.xbox import Xbox360ControllerInterface as Controller
+# elif args.controller == 'ps3':
+#     from pyrobolearn.tools.interfaces.controllers.playstation import PS3ControllerInterface as Controller
+# else:
+#     raise NotImplementedError("Unknown game controller")
+
+
+# create simulator
+sim = prl.simulators.Bullet()
+
+# create basic world (with a floor and gravity enabled by default)
+world = prl.worlds.BasicWorld(sim)
+
+# load quadcopter
+robot = prl.robots.Quadcopter(sim, position=[0., 0., 2.])
+world.load_robot(robot)
+
+# load interface
+# controller = Controller()
+
+# run simulator
+for t in count():
+    # robot.hover()
+    # robot.set_propeller_velocities(velocity)
+    robot.move([1., 1., 1.])
+
+    # follow quadcopter (seen from behind)
+    world.follow(robot, distance=2, yaw=-np.pi / 2)
+
+    # perform one step in the world
+    world.step(sleep_dt=1. / 240)
diff --git a/examples/robots/robot_with_sliders.py b/examples/robots/robot_with_sliders.py
new file mode 100644
index 0000000..9016de8
--- /dev/null
+++ b/examples/robots/robot_with_sliders.py
@@ -0,0 +1,47 @@
+#!/usr/bin/env python
+"""Manipulate the robot's joints with sliders.
+
+You can move in the world using the keyboard and mouse:
+- `ctrl + left click`: rotate the camera
+- `scroll wheel` or `ctrl + right click`: zoom in/out
+- `ctrl + middle click`: move the camera
+- `left click` on an object: if the object has a mass and a collision shape, you can interact with it with the mouse
+- `w`: wireframe (see collision shapes)
+- `g`: show/hide menu
+- `esc`: quit the simulator
+"""
+
+from itertools import count
+import argparse
+
+import pyrobolearn as prl
+
+
+# create parser to select the robot
+parser = argparse.ArgumentParser()
+parser.add_argument('-r', '--robot', help='the robot to load in the world', type=str,
+                    choices=prl.robots.implemented_robots, default='coman')
+parser.add_argument('-f', '--fixed_base', help='if we should fix the base when the robot has initially a floating '
+                                               'base', type=bool, default=True)
+args = parser.parse_args()
+
+
+# create simulator
+sim = prl.simulators.Bullet()
+
+# create basic world with floor and gravity
+world = prl.worlds.BasicWorld(sim)
+
+# load the robot in the world
+robot = world.load_robot(robot=args.robot, position=[0., 0.], fixed_base=args.fixed_base)
+
+# add a slider for each specified joint
+robot.add_joint_slider(joint_ids=robot.joints)
+
+# run simulator
+for _ in count():
+    # update the joint slider
+    robot.update_joint_slider()
+
+    # perform one step in the world
+    world.step(sleep_dt=1. / 240)
diff --git a/examples/robots/aibo.py b/examples/robots/robots/aibo.py
similarity index 89%
rename from examples/robots/aibo.py
rename to examples/robots/robots/aibo.py
index 33ebdff..6d98797 100644
--- a/examples/robots/aibo.py
+++ b/examples/robots/robots/aibo.py
@@ -3,12 +3,12 @@
 """
 
 from itertools import count
-from pyrobolearn.simulators import BulletSim
+from pyrobolearn.simulators import Bullet
 from pyrobolearn.worlds import BasicWorld
 from pyrobolearn.robots import Aibo
 
 # Create simulator
-sim = BulletSim()
+sim = Bullet()
 
 # create world
 world = BasicWorld(sim)
diff --git a/examples/robots/allegrohand.py b/examples/robots/robots/allegrohand.py
similarity index 91%
rename from examples/robots/allegrohand.py
rename to examples/robots/robots/allegrohand.py
index c91507e..c617da0 100644
--- a/examples/robots/allegrohand.py
+++ b/examples/robots/robots/allegrohand.py
@@ -3,12 +3,12 @@
 """
 
 from itertools import count
-from pyrobolearn.simulators import BulletSim
+from pyrobolearn.simulators import Bullet
 from pyrobolearn.worlds import BasicWorld
 from pyrobolearn.robots import AllegroHand
 
 # Create simulator
-sim = BulletSim()
+sim = Bullet()
 
 # create world
 world = BasicWorld(sim)
diff --git a/examples/robots/ant.py b/examples/robots/robots/ant.py
similarity index 89%
rename from examples/robots/ant.py
rename to examples/robots/robots/ant.py
index 8147102..39b1c84 100644
--- a/examples/robots/ant.py
+++ b/examples/robots/robots/ant.py
@@ -3,12 +3,12 @@
 """
 
 from itertools import count
-from pyrobolearn.simulators import BulletSim
+from pyrobolearn.simulators import Bullet
 from pyrobolearn.worlds import BasicWorld
 from pyrobolearn.robots import Ant
 
 # Create simulator
-sim = BulletSim()
+sim = Bullet()
 
 # create world
 world = BasicWorld(sim)
diff --git a/examples/robots/atlas.py b/examples/robots/robots/atlas.py
similarity index 88%
rename from examples/robots/atlas.py
rename to examples/robots/robots/atlas.py
index a82c71d..ec7496b 100644
--- a/examples/robots/atlas.py
+++ b/examples/robots/robots/atlas.py
@@ -3,12 +3,12 @@
 """
 
 from itertools import count
-from pyrobolearn.simulators import BulletSim
+from pyrobolearn.simulators import Bullet
 from pyrobolearn.worlds import BasicWorld
 from pyrobolearn.robots import Atlas
 
 # Create simulator
-sim = BulletSim()
+sim = Bullet()
 
 # create world
 world = BasicWorld(sim)
diff --git a/examples/robots/ballbot.py b/examples/robots/robots/ballbot.py
similarity index 87%
rename from examples/robots/ballbot.py
rename to examples/robots/robots/ballbot.py
index c9baeb3..7c0cdd9 100644
--- a/examples/robots/ballbot.py
+++ b/examples/robots/robots/ballbot.py
@@ -3,12 +3,12 @@
 """
 
 from itertools import count
-from pyrobolearn.simulators import BulletSim
+from pyrobolearn.simulators import Bullet
 from pyrobolearn.worlds import BasicWorld
 from pyrobolearn.robots import Ballbot
 
 # Create simulator
-sim = BulletSim()
+sim = Bullet()
 
 # create world
 world = BasicWorld(sim)
diff --git a/examples/robots/baxter.py b/examples/robots/robots/baxter.py
similarity index 88%
rename from examples/robots/baxter.py
rename to examples/robots/robots/baxter.py
index 1fedbed..8ed66d7 100644
--- a/examples/robots/baxter.py
+++ b/examples/robots/robots/baxter.py
@@ -3,12 +3,12 @@
 """
 
 from itertools import count
-from pyrobolearn.simulators import BulletSim
+from pyrobolearn.simulators import Bullet
 from pyrobolearn.worlds import BasicWorld
 from pyrobolearn.robots import Baxter
 
 # Create simulator
-sim = BulletSim()
+sim = Bullet()
 
 # create world
 world = BasicWorld(sim)
diff --git a/examples/robots/bb8.py b/examples/robots/robots/bb8.py
similarity index 87%
rename from examples/robots/bb8.py
rename to examples/robots/robots/bb8.py
index b1b5546..20c72c1 100644
--- a/examples/robots/bb8.py
+++ b/examples/robots/robots/bb8.py
@@ -3,12 +3,12 @@
 """
 
 from itertools import count
-from pyrobolearn.simulators import BulletSim
+from pyrobolearn.simulators import Bullet
 from pyrobolearn.worlds import BasicWorld
 from pyrobolearn.robots import BB8
 
 # Create simulator
-sim = BulletSim()
+sim = Bullet()
 
 # create world
 world = BasicWorld(sim)
diff --git a/examples/robots/cartpole.py b/examples/robots/robots/cartpole.py
similarity index 98%
rename from examples/robots/cartpole.py
rename to examples/robots/robots/cartpole.py
index 423bd20..138a055 100644
--- a/examples/robots/cartpole.py
+++ b/examples/robots/robots/cartpole.py
@@ -85,12 +85,12 @@ class LQR(object):
 if __name__ == "__main__":
     import numpy as np
     from itertools import count
-    from pyrobolearn.simulators import BulletSim
+    from pyrobolearn.simulators import Bullet
     from pyrobolearn.worlds import World
     from pyrobolearn.robots import CartPole
 
     # Create simulator
-    sim = BulletSim()
+    sim = Bullet()
 
     # create world
     world = World(sim)
diff --git a/examples/robots/cassie.py b/examples/robots/robots/cassie.py
similarity index 89%
rename from examples/robots/cassie.py
rename to examples/robots/robots/cassie.py
index 92c83af..30684fb 100644
--- a/examples/robots/cassie.py
+++ b/examples/robots/robots/cassie.py
@@ -3,12 +3,12 @@
 """
 
 from itertools import count
-from pyrobolearn.simulators import BulletSim
+from pyrobolearn.simulators import Bullet
 from pyrobolearn.worlds import BasicWorld
 from pyrobolearn.robots import Cassie
 
 # Create simulator
-sim = BulletSim()
+sim = Bullet()
 
 # create world
 world = BasicWorld(sim)
diff --git a/examples/robots/centauro.py b/examples/robots/robots/centauro.py
similarity index 90%
rename from examples/robots/centauro.py
rename to examples/robots/robots/centauro.py
index 2b9024a..ecd56b3 100644
--- a/examples/robots/centauro.py
+++ b/examples/robots/robots/centauro.py
@@ -3,12 +3,12 @@
 """
 
 from itertools import count
-from pyrobolearn.simulators import BulletSim
+from pyrobolearn.simulators import Bullet
 from pyrobolearn.worlds import BasicWorld
 from pyrobolearn.robots import Centauro
 
 # Create simulator
-sim = BulletSim()
+sim = Bullet()
 
 # create world
 world = BasicWorld(sim)
diff --git a/examples/robots/cogimon.py b/examples/robots/robots/cogimon.py
similarity index 88%
rename from examples/robots/cogimon.py
rename to examples/robots/robots/cogimon.py
index 8761d06..fb63156 100644
--- a/examples/robots/cogimon.py
+++ b/examples/robots/robots/cogimon.py
@@ -3,12 +3,12 @@
 """
 
 from itertools import count
-from pyrobolearn.simulators import BulletSim
+from pyrobolearn.simulators import Bullet
 from pyrobolearn.worlds import BasicWorld
 from pyrobolearn.robots import Cogimon
 
 # Create simulator
-sim = BulletSim()
+sim = Bullet()
 
 # create world
 world = BasicWorld(sim)
diff --git a/examples/robots/coman.py b/examples/robots/robots/coman.py
similarity index 92%
rename from examples/robots/coman.py
rename to examples/robots/robots/coman.py
index a853b93..9a7a68a 100644
--- a/examples/robots/coman.py
+++ b/examples/robots/robots/coman.py
@@ -3,12 +3,12 @@
 """
 
 from itertools import count
-from pyrobolearn.simulators import BulletSim
+from pyrobolearn.simulators import Bullet
 from pyrobolearn.worlds import BasicWorld
 from pyrobolearn.robots import Coman
 
 # Create simulator
-sim = BulletSim()
+sim = Bullet()
 
 # create world
 world = BasicWorld(sim)
diff --git a/examples/robots/crab.py b/examples/robots/robots/crab.py
similarity index 89%
rename from examples/robots/crab.py
rename to examples/robots/robots/crab.py
index 88a0add..5da82ac 100644
--- a/examples/robots/crab.py
+++ b/examples/robots/robots/crab.py
@@ -3,12 +3,12 @@
 """
 
 from itertools import count
-from pyrobolearn.simulators import BulletSim
+from pyrobolearn.simulators import Bullet
 from pyrobolearn.worlds import BasicWorld
 from pyrobolearn.robots import Crab
 
 # Create simulator
-sim = BulletSim()
+sim = Bullet()
 
 # create world
 world = BasicWorld(sim)
diff --git a/examples/robots/cubli.py b/examples/robots/robots/cubli.py
similarity index 94%
rename from examples/robots/cubli.py
rename to examples/robots/robots/cubli.py
index 405f4d3..f67b6a7 100644
--- a/examples/robots/cubli.py
+++ b/examples/robots/robots/cubli.py
@@ -5,12 +5,12 @@
 import numpy as np
 from itertools import count
 from pyrobolearn.utils.transformation import get_rpy_from_quaternion
-from pyrobolearn.simulators import BulletSim
+from pyrobolearn.simulators import Bullet
 from pyrobolearn.worlds import BasicWorld
 from pyrobolearn.robots import Cubli
 
 # Create simulator
-sim = BulletSim()
+sim = Bullet()
 
 # create world
 world = BasicWorld(sim)
diff --git a/examples/robots/darwin.py b/examples/robots/robots/darwin.py
similarity index 89%
rename from examples/robots/darwin.py
rename to examples/robots/robots/darwin.py
index af420af..2035b7b 100644
--- a/examples/robots/darwin.py
+++ b/examples/robots/robots/darwin.py
@@ -3,12 +3,12 @@
 """
 
 from itertools import count
-from pyrobolearn.simulators import BulletSim
+from pyrobolearn.simulators import Bullet
 from pyrobolearn.worlds import BasicWorld
 from pyrobolearn.robots import Darwin
 
 # Create simulator
-sim = BulletSim()
+sim = Bullet()
 
 # create world
 world = BasicWorld(sim)
diff --git a/examples/robots/edo.py b/examples/robots/robots/edo.py
similarity index 87%
rename from examples/robots/edo.py
rename to examples/robots/robots/edo.py
index 6b7b4e9..7f23baa 100644
--- a/examples/robots/edo.py
+++ b/examples/robots/robots/edo.py
@@ -3,12 +3,12 @@
 """
 
 from itertools import count
-from pyrobolearn.simulators import BulletSim
+from pyrobolearn.simulators import Bullet
 from pyrobolearn.worlds import BasicWorld
 from pyrobolearn.robots import Edo
 
 # Create simulator
-sim = BulletSim()
+sim = Bullet()
 
 # create world
 world = BasicWorld(sim)
diff --git a/examples/robots/epuck.py b/examples/robots/robots/epuck.py
similarity index 91%
rename from examples/robots/epuck.py
rename to examples/robots/robots/epuck.py
index 8ec101d..f93e0fd 100644
--- a/examples/robots/epuck.py
+++ b/examples/robots/robots/epuck.py
@@ -4,12 +4,12 @@
 
 import numpy as np
 from itertools import count
-from pyrobolearn.simulators import BulletSim
+from pyrobolearn.simulators import Bullet
 from pyrobolearn.worlds import BasicWorld
 from pyrobolearn.robots import Epuck
 
 # Create simulator
-sim = BulletSim()
+sim = Bullet()
 
 # create world
 world = BasicWorld(sim)
diff --git a/examples/robots/f10_racecar.py b/examples/robots/robots/f10_racecar.py
similarity index 89%
rename from examples/robots/f10_racecar.py
rename to examples/robots/robots/f10_racecar.py
index cf06cf0..bdb079c 100644
--- a/examples/robots/f10_racecar.py
+++ b/examples/robots/robots/f10_racecar.py
@@ -3,12 +3,12 @@
 """
 
 from itertools import count
-from pyrobolearn.simulators import BulletSim
+from pyrobolearn.simulators import Bullet
 from pyrobolearn.worlds import BasicWorld
 from pyrobolearn.robots import F10Racecar
 
 # Create simulator
-sim = BulletSim()
+sim = Bullet()
 
 # create world
 world = BasicWorld(sim)
diff --git a/examples/robots/fetch.py b/examples/robots/robots/fetch.py
similarity index 88%
rename from examples/robots/fetch.py
rename to examples/robots/robots/fetch.py
index ab1bbc7..cb07619 100644
--- a/examples/robots/fetch.py
+++ b/examples/robots/robots/fetch.py
@@ -3,12 +3,12 @@
 """
 
 from itertools import count
-from pyrobolearn.simulators import BulletSim
+from pyrobolearn.simulators import Bullet
 from pyrobolearn.worlds import BasicWorld
 from pyrobolearn.robots import Fetch
 
 # Create simulator
-sim = BulletSim()
+sim = Bullet()
 
 # create world
 world = BasicWorld(sim)
diff --git a/examples/robots/franka.py b/examples/robots/robots/franka.py
similarity index 90%
rename from examples/robots/franka.py
rename to examples/robots/robots/franka.py
index e04f5ff..0585912 100644
--- a/examples/robots/franka.py
+++ b/examples/robots/robots/franka.py
@@ -3,12 +3,12 @@
 """
 
 from itertools import count
-from pyrobolearn.simulators import BulletSim
+from pyrobolearn.simulators import Bullet
 from pyrobolearn.worlds import BasicWorld
 from pyrobolearn.robots import Franka
 
 # Create simulator
-sim = BulletSim()
+sim = Bullet()
 
 # create world
 world = BasicWorld(sim)
diff --git a/examples/robots/half_cheetah.py b/examples/robots/robots/half_cheetah.py
similarity index 87%
rename from examples/robots/half_cheetah.py
rename to examples/robots/robots/half_cheetah.py
index f99115d..fdcaa86 100644
--- a/examples/robots/half_cheetah.py
+++ b/examples/robots/robots/half_cheetah.py
@@ -3,12 +3,12 @@
 """
 
 from itertools import count
-from pyrobolearn.simulators import BulletSim
+from pyrobolearn.simulators import Bullet
 from pyrobolearn.worlds import BasicWorld
 from pyrobolearn.robots import HalfCheetah
 
 # Create simulator
-sim = BulletSim()
+sim = Bullet()
 
 # create world
 world = BasicWorld(sim)
diff --git a/examples/robots/hopper.py b/examples/robots/robots/hopper.py
similarity index 86%
rename from examples/robots/hopper.py
rename to examples/robots/robots/hopper.py
index dd17a83..8ddb43b 100644
--- a/examples/robots/hopper.py
+++ b/examples/robots/robots/hopper.py
@@ -3,12 +3,12 @@
 """
 
 from itertools import count
-from pyrobolearn.simulators import BulletSim
+from pyrobolearn.simulators import Bullet
 from pyrobolearn.worlds import BasicWorld
 from pyrobolearn.robots import Hopper
 
 # Create simulator
-sim = BulletSim()
+sim = Bullet()
 
 # create world
 world = BasicWorld(sim)
diff --git a/examples/robots/hubo.py b/examples/robots/robots/hubo.py
similarity index 88%
rename from examples/robots/hubo.py
rename to examples/robots/robots/hubo.py
index 1eb633a..51bc465 100644
--- a/examples/robots/hubo.py
+++ b/examples/robots/robots/hubo.py
@@ -3,12 +3,12 @@
 """
 
 from itertools import count
-from pyrobolearn.simulators import BulletSim
+from pyrobolearn.simulators import Bullet
 from pyrobolearn.worlds import BasicWorld
 from pyrobolearn.robots import Hubo
 
 # Create simulator
-sim = BulletSim()
+sim = Bullet()
 
 # create world
 world = BasicWorld(sim)
diff --git a/examples/robots/humanoid.py b/examples/robots/robots/humanoid.py
similarity index 89%
rename from examples/robots/humanoid.py
rename to examples/robots/robots/humanoid.py
index 889c1a9..aa71159 100644
--- a/examples/robots/humanoid.py
+++ b/examples/robots/robots/humanoid.py
@@ -3,12 +3,12 @@
 """
 
 from itertools import count
-from pyrobolearn.simulators import BulletSim
+from pyrobolearn.simulators import Bullet
 from pyrobolearn.worlds import BasicWorld
 from pyrobolearn.robots import Humanoid
 
 # Create simulator
-sim = BulletSim()
+sim = Bullet()
 
 # create world
 world = BasicWorld(sim)
diff --git a/examples/robots/husky.py b/examples/robots/robots/husky.py
similarity index 88%
rename from examples/robots/husky.py
rename to examples/robots/robots/husky.py
index 68a1ac8..9e8f4fc 100644
--- a/examples/robots/husky.py
+++ b/examples/robots/robots/husky.py
@@ -3,12 +3,12 @@
 """
 
 from itertools import count
-from pyrobolearn.simulators import BulletSim
+from pyrobolearn.simulators import Bullet
 from pyrobolearn.worlds import BasicWorld
 from pyrobolearn.robots import Husky
 
 # Create simulator
-sim = BulletSim()
+sim = Bullet()
 
 # create world
 world = BasicWorld(sim)
diff --git a/examples/robots/hyq.py b/examples/robots/robots/hyq.py
similarity index 90%
rename from examples/robots/hyq.py
rename to examples/robots/robots/hyq.py
index 633a9b1..1fcdab7 100644
--- a/examples/robots/hyq.py
+++ b/examples/robots/robots/hyq.py
@@ -3,12 +3,12 @@
 """
 
 from itertools import count
-from pyrobolearn.simulators import BulletSim
+from pyrobolearn.simulators import Bullet
 from pyrobolearn.worlds import BasicWorld
 from pyrobolearn.robots import HyQ
 
 # Create simulator
-sim = BulletSim()
+sim = Bullet()
 
 # create world
 world = BasicWorld(sim)
diff --git a/examples/robots/hyq2max.py b/examples/robots/robots/hyq2max.py
similarity index 90%
rename from examples/robots/hyq2max.py
rename to examples/robots/robots/hyq2max.py
index 7ed24fc..2f34c2f 100644
--- a/examples/robots/hyq2max.py
+++ b/examples/robots/robots/hyq2max.py
@@ -3,12 +3,12 @@
 """
 
 from itertools import count
-from pyrobolearn.simulators import BulletSim
+from pyrobolearn.simulators import Bullet
 from pyrobolearn.worlds import BasicWorld
 from pyrobolearn.robots import HyQ2Max
 
 # Create simulator
-sim = BulletSim()
+sim = Bullet()
 
 # create world
 world = BasicWorld(sim)
diff --git a/examples/robots/jaco.py b/examples/robots/robots/jaco.py
similarity index 86%
rename from examples/robots/jaco.py
rename to examples/robots/robots/jaco.py
index 6d86cea..2f2a463 100644
--- a/examples/robots/jaco.py
+++ b/examples/robots/robots/jaco.py
@@ -3,12 +3,12 @@
 """
 
 from itertools import count
-from pyrobolearn.simulators import BulletSim
+from pyrobolearn.simulators import Bullet
 from pyrobolearn.worlds import BasicWorld
 from pyrobolearn.robots import Jaco
 
 # Create simulator
-sim = BulletSim()
+sim = Bullet()
 
 # create world
 world = BasicWorld(sim)
diff --git a/examples/robots/kr5.py b/examples/robots/robots/kr5.py
similarity index 88%
rename from examples/robots/kr5.py
rename to examples/robots/robots/kr5.py
index 94d39f4..afbf260 100644
--- a/examples/robots/kr5.py
+++ b/examples/robots/robots/kr5.py
@@ -3,12 +3,12 @@
 """
 
 from itertools import count
-from pyrobolearn.simulators import BulletSim
+from pyrobolearn.simulators import Bullet
 from pyrobolearn.worlds import BasicWorld
 from pyrobolearn.robots import KR5
 
 # Create simulator
-sim = BulletSim()
+sim = Bullet()
 
 # create world
 world = BasicWorld(sim)
diff --git a/examples/robots/kuka_iiwa.py b/examples/robots/robots/kuka_iiwa.py
similarity index 95%
rename from examples/robots/kuka_iiwa.py
rename to examples/robots/robots/kuka_iiwa.py
index a2582a0..a2296dc 100644
--- a/examples/robots/kuka_iiwa.py
+++ b/examples/robots/robots/kuka_iiwa.py
@@ -4,12 +4,12 @@
 
 import numpy as np
 from itertools import count
-from pyrobolearn.simulators import BulletSim
+from pyrobolearn.simulators import Bullet
 from pyrobolearn.worlds import BasicWorld
 from pyrobolearn.robots import KukaIIWA
 
 # Create simulator
-sim = BulletSim()
+sim = Bullet()
 
 # create world
 world = BasicWorld(sim)
diff --git a/examples/robots/kuka_lwr.py b/examples/robots/robots/kuka_lwr.py
similarity index 88%
rename from examples/robots/kuka_lwr.py
rename to examples/robots/robots/kuka_lwr.py
index 696b77c..62f5e5d 100644
--- a/examples/robots/kuka_lwr.py
+++ b/examples/robots/robots/kuka_lwr.py
@@ -3,12 +3,12 @@
 """
 
 from itertools import count
-from pyrobolearn.simulators import BulletSim
+from pyrobolearn.simulators import Bullet
 from pyrobolearn.worlds import BasicWorld
 from pyrobolearn.robots import KukaLWR
 
 # Create simulator
-sim = BulletSim()
+sim = Bullet()
 
 # create world
 world = BasicWorld(sim)
diff --git a/examples/robots/laikago.py b/examples/robots/robots/laikago.py
similarity index 89%
rename from examples/robots/laikago.py
rename to examples/robots/robots/laikago.py
index 12fb442..9edfc10 100644
--- a/examples/robots/laikago.py
+++ b/examples/robots/robots/laikago.py
@@ -3,12 +3,12 @@
 """
 
 from itertools import count
-from pyrobolearn.simulators import BulletSim
+from pyrobolearn.simulators import Bullet
 from pyrobolearn.worlds import BasicWorld
 from pyrobolearn.robots import Laikago
 
 # Create simulator
-sim = BulletSim()
+sim = Bullet()
 
 # create world
 world = BasicWorld(sim)
diff --git a/examples/robots/littledog.py b/examples/robots/robots/littledog.py
similarity index 90%
rename from examples/robots/littledog.py
rename to examples/robots/robots/littledog.py
index 4767b59..39a87f6 100644
--- a/examples/robots/littledog.py
+++ b/examples/robots/robots/littledog.py
@@ -3,12 +3,12 @@
 """
 
 from itertools import count
-from pyrobolearn.simulators import BulletSim
+from pyrobolearn.simulators import Bullet
 from pyrobolearn.worlds import BasicWorld
 from pyrobolearn.robots import LittleDog
 
 # Create simulator
-sim = BulletSim()
+sim = Bullet()
 
 # create world
 world = BasicWorld(sim)
diff --git a/examples/robots/manipulator2d.py b/examples/robots/robots/manipulator2d.py
similarity index 89%
rename from examples/robots/manipulator2d.py
rename to examples/robots/robots/manipulator2d.py
index c881564..2636b67 100644
--- a/examples/robots/manipulator2d.py
+++ b/examples/robots/robots/manipulator2d.py
@@ -3,12 +3,12 @@
 """
 
 from itertools import count
-from pyrobolearn.simulators import BulletSim
+from pyrobolearn.simulators import Bullet
 from pyrobolearn.worlds import BasicWorld
 from pyrobolearn.robots import Manipulator2D
 
 # Create simulator
-sim = BulletSim()
+sim = Bullet()
 
 # create world
 world = BasicWorld(sim)
diff --git a/examples/robots/minitaur.py b/examples/robots/robots/minitaur.py
similarity index 90%
rename from examples/robots/minitaur.py
rename to examples/robots/robots/minitaur.py
index 11df4f1..1fdf45e 100644
--- a/examples/robots/minitaur.py
+++ b/examples/robots/robots/minitaur.py
@@ -3,12 +3,12 @@
 """
 
 from itertools import count
-from pyrobolearn.simulators import BulletSim
+from pyrobolearn.simulators import Bullet
 from pyrobolearn.worlds import BasicWorld
 from pyrobolearn.robots import Minitaur
 
 # Create simulator
-sim = BulletSim()
+sim = Bullet()
 
 # create world
 world = BasicWorld(sim)
diff --git a/examples/robots/mkz.py b/examples/robots/robots/mkz.py
similarity index 89%
rename from examples/robots/mkz.py
rename to examples/robots/robots/mkz.py
index 96e6119..6817fa0 100644
--- a/examples/robots/mkz.py
+++ b/examples/robots/robots/mkz.py
@@ -3,12 +3,12 @@
 """
 
 from itertools import count
-from pyrobolearn.simulators import BulletSim
+from pyrobolearn.simulators import Bullet
 from pyrobolearn.worlds import BasicWorld
 from pyrobolearn.robots import MKZ
 
 # Create simulator
-sim = BulletSim()
+sim = Bullet()
 
 # create world
 world = BasicWorld(sim)
diff --git a/examples/robots/morphex.py b/examples/robots/robots/morphex.py
similarity index 86%
rename from examples/robots/morphex.py
rename to examples/robots/robots/morphex.py
index 2c3a8cf..06e572a 100644
--- a/examples/robots/morphex.py
+++ b/examples/robots/robots/morphex.py
@@ -3,12 +3,12 @@
 """
 
 from itertools import count
-from pyrobolearn.simulators import BulletSim
+from pyrobolearn.simulators import Bullet
 from pyrobolearn.worlds import BasicWorld
 from pyrobolearn.robots import Morphex
 
 # Create simulator
-sim = BulletSim()
+sim = Bullet()
 
 # create world
 world = BasicWorld(sim)
diff --git a/examples/robots/nao.py b/examples/robots/robots/nao.py
similarity index 88%
rename from examples/robots/nao.py
rename to examples/robots/robots/nao.py
index 36da221..fd44718 100644
--- a/examples/robots/nao.py
+++ b/examples/robots/robots/nao.py
@@ -3,12 +3,12 @@
 """
 
 from itertools import count
-from pyrobolearn.simulators import BulletSim
+from pyrobolearn.simulators import Bullet
 from pyrobolearn.worlds import BasicWorld
 from pyrobolearn.robots import Nao
 
 # Create simulator
-sim = BulletSim()
+sim = Bullet()
 
 # create world
 world = BasicWorld(sim)
diff --git a/examples/robots/opendog.py b/examples/robots/robots/opendog.py
similarity index 88%
rename from examples/robots/opendog.py
rename to examples/robots/robots/opendog.py
index 991de4a..6fd94c7 100644
--- a/examples/robots/opendog.py
+++ b/examples/robots/robots/opendog.py
@@ -3,12 +3,12 @@
 """
 
 from itertools import count
-from pyrobolearn.simulators import BulletSim
+from pyrobolearn.simulators import Bullet
 from pyrobolearn.worlds import BasicWorld
 from pyrobolearn.robots import OpenDog
 
 # Create simulator
-sim = BulletSim()
+sim = Bullet()
 
 # create world
 world = BasicWorld(sim)
diff --git a/examples/robots/pepper.py b/examples/robots/robots/pepper.py
similarity index 89%
rename from examples/robots/pepper.py
rename to examples/robots/robots/pepper.py
index 2bc433d..b547a7e 100644
--- a/examples/robots/pepper.py
+++ b/examples/robots/robots/pepper.py
@@ -3,13 +3,13 @@
 """
 
 from itertools import count
-from pyrobolearn.simulators import BulletSim
+from pyrobolearn.simulators import Bullet
 from pyrobolearn.worlds import BasicWorld
 from pyrobolearn.robots import Pepper
 
 
 # Create simulator
-sim = BulletSim()
+sim = Bullet()
 
 # create world
 world = BasicWorld(sim)
diff --git a/examples/robots/phantomx.py b/examples/robots/robots/phantomx.py
similarity index 86%
rename from examples/robots/phantomx.py
rename to examples/robots/robots/phantomx.py
index 4e5ea5e..d12dfc4 100644
--- a/examples/robots/phantomx.py
+++ b/examples/robots/robots/phantomx.py
@@ -3,12 +3,12 @@
 """
 
 from itertools import count
-from pyrobolearn.simulators import BulletSim
+from pyrobolearn.simulators import Bullet
 from pyrobolearn.worlds import BasicWorld
 from pyrobolearn.robots import PhantomX
 
 # Create simulator
-sim = BulletSim()
+sim = Bullet()
 
 # create world
 world = BasicWorld(sim)
diff --git a/examples/robots/pleurobot.py b/examples/robots/robots/pleurobot.py
similarity index 90%
rename from examples/robots/pleurobot.py
rename to examples/robots/robots/pleurobot.py
index 16477c5..65acfe1 100644
--- a/examples/robots/pleurobot.py
+++ b/examples/robots/robots/pleurobot.py
@@ -3,12 +3,12 @@
 """
 
 from itertools import count
-from pyrobolearn.simulators import BulletSim
+from pyrobolearn.simulators import Bullet
 from pyrobolearn.worlds import BasicWorld
 from pyrobolearn.robots import Pleurobot
 
 # Create simulator
-sim = BulletSim()
+sim = Bullet()
 
 # create world
 world = BasicWorld(sim)
diff --git a/examples/robots/pr2.py b/examples/robots/robots/pr2.py
similarity index 88%
rename from examples/robots/pr2.py
rename to examples/robots/robots/pr2.py
index 0ae5c64..93152f8 100644
--- a/examples/robots/pr2.py
+++ b/examples/robots/robots/pr2.py
@@ -3,12 +3,12 @@
 """
 
 from itertools import count
-from pyrobolearn.simulators import BulletSim
+from pyrobolearn.simulators import Bullet
 from pyrobolearn.worlds import BasicWorld
 from pyrobolearn.robots import PR2
 
 # Create simulator
-sim = BulletSim()
+sim = Bullet()
 
 # create world
 world = BasicWorld(sim)
diff --git a/examples/robots/quadcopter.py b/examples/robots/robots/quadcopter.py
similarity index 86%
rename from examples/robots/quadcopter.py
rename to examples/robots/robots/quadcopter.py
index 1e03960..6b6db19 100644
--- a/examples/robots/quadcopter.py
+++ b/examples/robots/robots/quadcopter.py
@@ -4,14 +4,14 @@
 
 import numpy as np
 from itertools import count
-from pyrobolearn.simulators import BulletSim
+from pyrobolearn.simulators import Bullet
 from pyrobolearn.worlds import BasicWorld
 from pyrobolearn.robots import Quadcopter
 from pyrobolearn.utils.units import rpm_to_rad_per_second
 
 
 # Create simulator
-sim = BulletSim()
+sim = Bullet()
 
 # create world
 world = BasicWorld(sim)
@@ -24,7 +24,7 @@ robot.print_info()
 
 rpm = robot.get_stationary_rpm()
 print("Stationary RPM: {}".format(rpm))
-v = rpm_to_rad_per_second(rpm+20)
+v = rpm_to_rad_per_second(rpm + 20)
 v = [v, -v, v, -v]
 
 # run simulation
diff --git a/examples/robots/rhex.py b/examples/robots/robots/rhex.py
similarity index 89%
rename from examples/robots/rhex.py
rename to examples/robots/robots/rhex.py
index fb14874..2bb55cb 100644
--- a/examples/robots/rhex.py
+++ b/examples/robots/robots/rhex.py
@@ -3,12 +3,12 @@
 """
 
 from itertools import count
-from pyrobolearn.simulators import BulletSim
+from pyrobolearn.simulators import Bullet
 from pyrobolearn.worlds import BasicWorld
 from pyrobolearn.robots import Rhex
 
 # Create simulator
-sim = BulletSim()
+sim = Bullet()
 
 # create world
 world = BasicWorld(sim)
diff --git a/examples/robots/rrbot.py b/examples/robots/robots/rrbot.py
similarity index 94%
rename from examples/robots/rrbot.py
rename to examples/robots/robots/rrbot.py
index a0047d3..efeb5d4 100644
--- a/examples/robots/rrbot.py
+++ b/examples/robots/robots/rrbot.py
@@ -4,12 +4,12 @@
 
 import numpy as np
 from itertools import count
-from pyrobolearn.simulators import BulletSim
+from pyrobolearn.simulators import Bullet
 from pyrobolearn.worlds import BasicWorld
 from pyrobolearn.robots import RRBot
 
 # Create simulator
-sim = BulletSim()
+sim = Bullet()
 
 # create world
 world = BasicWorld(sim)
@@ -103,7 +103,7 @@ J = sim.calculate_jacobian(robot.id, 1, [0., 0., 0.])
 print(np.array(J[0]))
 
 a = robot.get_joint_positions()
-# print(robot.get_jacobian(1, np.array([0.,0.])))  # TODO: need to convert numpy array to list
+# print(robot.get_jacobian(1, np.array([0., 0.])))  # TODO: need to convert numpy array to list
 
 linkId = 2
 com_frame = robot.get_link_states(linkId)[2]
@@ -125,14 +125,14 @@ for i in range(10000):
     dx = np.array(robot.get_link_world_linear_velocities(linkId))
     tau = robot.calculate_inverse_dynamics(ddq, dq, q)  # Coriolis, centrifugal and gravity compensation
     Jlin = np.array(sim.calculate_jacobian(robot.id, linkId, com_frame)[0])
-    F = K.dot(xdes - x) - D.dot(dx)  # compute cartesian forces
+    F = K.dot(xdes - x) - D.dot(dx)  # evaluate cartesian forces
     # print("force: {}".format(F))
     tau += Jlin.T.dot(F)  # cartesian PD with gravity compensation
     # tau += Jlin.T.dot(- D.dot(dx))  # active compliance
 
     # tau = Jlin.T.dot(F)
 
-    # compute manipulability measure :math:`w = sqrt(det(JJ^T))`
+    # evaluate manipulability measure :math:`w = sqrt(det(JJ^T))`
     Jlin = Jlin[[0, 2], :]
     w = np.sqrt(np.linalg.det(Jlin.dot(Jlin.T)))
     # print("manipulability: {}".format(w))
diff --git a/examples/robots/sawyer.py b/examples/robots/robots/sawyer.py
similarity index 88%
rename from examples/robots/sawyer.py
rename to examples/robots/robots/sawyer.py
index b47e062..a60aa0e 100644
--- a/examples/robots/sawyer.py
+++ b/examples/robots/robots/sawyer.py
@@ -3,12 +3,12 @@
 """
 
 from itertools import count
-from pyrobolearn.simulators import BulletSim
+from pyrobolearn.simulators import Bullet
 from pyrobolearn.worlds import BasicWorld
 from pyrobolearn.robots import Sawyer
 
 # Create simulator
-sim = BulletSim()
+sim = Bullet()
 
 # create world
 world = BasicWorld(sim)
diff --git a/examples/robots/sea_hexapod.py b/examples/robots/robots/sea_hexapod.py
similarity index 86%
rename from examples/robots/sea_hexapod.py
rename to examples/robots/robots/sea_hexapod.py
index 37ee317..5d59a27 100644
--- a/examples/robots/sea_hexapod.py
+++ b/examples/robots/robots/sea_hexapod.py
@@ -3,12 +3,12 @@
 """
 
 from itertools import count
-from pyrobolearn.simulators import BulletSim
+from pyrobolearn.simulators import Bullet
 from pyrobolearn.worlds import BasicWorld
 from pyrobolearn.robots import SEAHexapod
 
 # Create simulator
-sim = BulletSim()
+sim = Bullet()
 
 # create world
 world = BasicWorld(sim)
diff --git a/examples/robots/sea_snake.py b/examples/robots/robots/sea_snake.py
similarity index 86%
rename from examples/robots/sea_snake.py
rename to examples/robots/robots/sea_snake.py
index 042a8df..0b1f5b8 100644
--- a/examples/robots/sea_snake.py
+++ b/examples/robots/robots/sea_snake.py
@@ -3,12 +3,12 @@
 """
 
 from itertools import count
-from pyrobolearn.simulators import BulletSim
+from pyrobolearn.simulators import Bullet
 from pyrobolearn.worlds import BasicWorld
 from pyrobolearn.robots import SEASnake
 
 # Create simulator
-sim = BulletSim()
+sim = Bullet()
 
 # create world
 world = BasicWorld(sim)
diff --git a/examples/robots/softhand.py b/examples/robots/robots/softhand.py
similarity index 93%
rename from examples/robots/softhand.py
rename to examples/robots/robots/softhand.py
index 7f8d171..64d5ab1 100644
--- a/examples/robots/softhand.py
+++ b/examples/robots/robots/softhand.py
@@ -3,12 +3,12 @@
 """
 
 from itertools import count
-from pyrobolearn.simulators import BulletSim
+from pyrobolearn.simulators import Bullet
 from pyrobolearn.worlds import BasicWorld
 from pyrobolearn.robots import SoftHand
 
 # Create simulator
-sim = BulletSim()
+sim = Bullet()
 
 # create world
 world = BasicWorld(sim)
diff --git a/examples/robots/swimmer.py b/examples/robots/robots/swimmer.py
similarity index 86%
rename from examples/robots/swimmer.py
rename to examples/robots/robots/swimmer.py
index 5668c4a..c85b93c 100644
--- a/examples/robots/swimmer.py
+++ b/examples/robots/robots/swimmer.py
@@ -3,12 +3,12 @@
 """
 
 from itertools import count
-from pyrobolearn.simulators import BulletSim
+from pyrobolearn.simulators import Bullet
 from pyrobolearn.worlds import BasicWorld
 from pyrobolearn.robots import Swimmer
 
 # Create simulator
-sim = BulletSim()
+sim = Bullet()
 
 # create world
 world = BasicWorld(sim)
diff --git a/examples/robots/walker2d.py b/examples/robots/robots/walker2d.py
similarity index 86%
rename from examples/robots/walker2d.py
rename to examples/robots/robots/walker2d.py
index 95b408b..e8b5afa 100644
--- a/examples/robots/walker2d.py
+++ b/examples/robots/robots/walker2d.py
@@ -3,12 +3,12 @@
 """
 
 from itertools import count
-from pyrobolearn.simulators import BulletSim
+from pyrobolearn.simulators import Bullet
 from pyrobolearn.worlds import BasicWorld
 from pyrobolearn.robots import Walker2D
 
 # Create simulator
-sim = BulletSim()
+sim = Bullet()
 
 # create world
 world = BasicWorld(sim)
diff --git a/examples/robots/walkman.py b/examples/robots/robots/walkman.py
similarity index 91%
rename from examples/robots/walkman.py
rename to examples/robots/robots/walkman.py
index c05d76c..f98e8d2 100644
--- a/examples/robots/walkman.py
+++ b/examples/robots/robots/walkman.py
@@ -3,12 +3,12 @@
 """
 
 from itertools import count
-from pyrobolearn.simulators import BulletSim
+from pyrobolearn.simulators import Bullet
 from pyrobolearn.worlds import BasicWorld
 from pyrobolearn.robots import Walkman
 
 # Create simulator
-sim = BulletSim()
+sim = Bullet()
 
 # Create world
 world = BasicWorld(sim)
diff --git a/examples/robots/wam.py b/examples/robots/robots/wam.py
similarity index 95%
rename from examples/robots/wam.py
rename to examples/robots/robots/wam.py
index d9787c4..c932802 100644
--- a/examples/robots/wam.py
+++ b/examples/robots/robots/wam.py
@@ -4,12 +4,12 @@
 
 import numpy as np
 from itertools import count
-from pyrobolearn.simulators import BulletSim
+from pyrobolearn.simulators import Bullet
 from pyrobolearn.worlds import BasicWorld
 from pyrobolearn.robots import WAM
 
 # Create simulator
-sim = BulletSim()
+sim = Bullet()
 
 # create world
 world = BasicWorld(sim)
diff --git a/examples/robots/youbot.py b/examples/robots/robots/youbot.py
similarity index 93%
rename from examples/robots/youbot.py
rename to examples/robots/robots/youbot.py
index e8824de..e0dede3 100644
--- a/examples/robots/youbot.py
+++ b/examples/robots/robots/youbot.py
@@ -5,12 +5,12 @@ These include: YoubotBase, KukaYoubotArm, Youbot, YoubotDualArm
 """
 
 from itertools import count
-from pyrobolearn.simulators import BulletSim
+from pyrobolearn.simulators import Bullet
 from pyrobolearn.worlds import BasicWorld
 from pyrobolearn.robots import YoubotBase, KukaYoubotArm, Youbot, YoubotDualArm
 
 # Create simulator
-sim = BulletSim()
+sim = Bullet()
 
 # create world
 world = BasicWorld(sim)
diff --git a/examples/robots/visualize_robot.py b/examples/robots/visualize_robot.py
new file mode 100644
index 0000000..b63138f
--- /dev/null
+++ b/examples/robots/visualize_robot.py
@@ -0,0 +1,76 @@
+#!/usr/bin/env python
+"""Test different visualization tools that can be used on the robot.
+
+
+Test different visualization tools on a robot. You can notably:
+- render the robot semi-transparent
+- draw the robot center of mass and the projected center of mass
+- draw the center of mass of each link
+- draw the link frames
+- draw the joint axis
+- draw bounding boxes around links
+- for legged robots:
+    - draw ground reference points such as the ZMP, COP, FRI, and CMP
+    - draw the support polygon
+    - draw friction cones
+- draw velocity and dynamic manipulability ellipsoids
+
+
+You can move in the world using the keyboard and mouse:
+- `ctrl + left click`: rotate the camera
+- `scroll wheel` or `ctrl + right click`: zoom in/out
+- `ctrl + middle click`: move the camera
+- `left click` on an object: if the object has a mass and a collision shape, you can interact with it with the mouse
+- `w`: wireframe (see collision shapes)
+- `g`: show/hide menu
+- `esc`: quit the simulator
+"""
+
+import time
+from itertools import count
+import argparse
+
+import pyrobolearn as prl
+
+
+# create parser to select the robot
+robots = ['coman', 'hyq2max']  # prl.robots.implemented_robots
+parser = argparse.ArgumentParser()
+parser.add_argument('-r', '--robot', help='the robot to load in the world', type=str,
+                    choices=robots, default='hyq2max')
+args = parser.parse_args()
+
+
+# create simulator
+sim = prl.simulators.Bullet()
+
+# create basic world with floor and gravity
+world = prl.worlds.BasicWorld(sim)
+
+# load the robot in the world
+robot = world.load_robot(robot=args.robot, position=[0., 0.])
+
+# move the simulation a bit forward (such that the robot touches the floor)
+for _ in range(100):
+    world.step()
+
+# change visualization
+robot.change_transparency()
+robot.draw_link_coms()
+robot.draw_link_frames()
+robot.draw_bounding_boxes(link_ids=-1)
+
+robot.draw_friction_cone(floor_id=world.floor_id)
+robot.draw_support_polygon(floor_id=world.floor_id, lifetime=0)
+
+robot.compute_and_draw_com_position()
+robot.compute_and_draw_projected_com_position()
+# robot.draw_cop(cop=world.floor_id)
+# robot.draw_zmp(zmp=world.floor_id)
+# robot.draw_cmp(cmp=world.floor_id)
+
+time.sleep(10000)
+# run simulator
+# for t in count():
+#     # perform one step in the world
+#     world.step(sleep_dt=1. / 240)
diff --git a/examples/worlds/README.md b/examples/worlds/README.md
index 5831848..4fd4b29 100644
--- a/examples/worlds/README.md
+++ b/examples/worlds/README.md
@@ -14,3 +14,8 @@ and with collisions) that are movable, fixed, or are moving.
 4. `load_robot.py`: load a robot in a basic world and distribute randomly few objects on the floor.
 5. `load_heightmap.py`: load a terrain from a heightmap (png) and load a robot on it.
 6. `generate_terrain.py`: generate a terrain and distribute randomly few objects on the terrain.
+
+
+#### What to check next?
+
+Check the `pyrobolearn/examples/robots` folder.
diff --git a/pyrobolearn/robots/__init__.py b/pyrobolearn/robots/__init__.py
index 77cf1c4..07b2c0f 100644
--- a/pyrobolearn/robots/__init__.py
+++ b/pyrobolearn/robots/__init__.py
@@ -129,6 +129,9 @@ for s in ['__init__', 'actuators', 'sensors', 'legged_robot', 'manipulator', 'wh
 
 implemented_robots = list(implemented_robots)
 
+# TODO: fix problem with icub
+implemented_robots.remove('icub')
+
 
 # create dictionary that maps robot names to robot classes
 robot_names_to_classes = {}
diff --git a/pyrobolearn/robots/quadcopter.py b/pyrobolearn/robots/quadcopter.py
index 29402b0..cdd0bc8 100644
--- a/pyrobolearn/robots/quadcopter.py
+++ b/pyrobolearn/robots/quadcopter.py
@@ -7,7 +7,7 @@ import numpy as np
 
 from pyrobolearn.robots.uav import RotaryWingUAV
 from pyrobolearn.utils.transformation import get_matrix_from_quaternion
-from pyrobolearn.utils.units import inches_to_meters, rpm_to_rad_per_second
+from pyrobolearn.utils.units import inches_to_meters, rpm_to_rad_per_second, rad_per_second_to_rpm
 
 __author__ = "Brian Delhaisse"
 __copyright__ = "Copyright 2018, PyRoboLearn"
@@ -62,6 +62,8 @@ class Quadcopter(RotaryWingUAV):
         [7] "Propeller Static & Dynamic Thrust Calculation":
             https://www.electricrcaircraftguy.com/2013/09/propeller-static-dynamic-thrust-equation.html
             https://www.electricrcaircraftguy.com/2014/04/propeller-static-dynamic-thrust-equation-background.html
+        [8] "Flying Principle of a Quadrotor" (from the course "Autonomous Navigation for Flying Robots" on EdX),
+            Jurgen, https://jsturm.de/publications/data/lecture_1_part_3.pdf
     """
 
     def __init__(self,
@@ -96,7 +98,7 @@ class Quadcopter(RotaryWingUAV):
 
         # joints 1 and 3 are CCW, and joints 2 and 4 are CW
         # CCW = +1, CW = -1
-        self.turning_directions = [+1, -1, +1, -1]
+        self.propeller_directions = np.array([+1, -1, +1, -1])
 
         # Propeller pitches are around 0.0762m (3 inches) and 0.127m (5 inches)
         # (from https://www.dronezon.com/learn-about-drones-quadcopters/how-a-quadcopter-works-with-propellers-and\
@@ -112,6 +114,12 @@ class Quadcopter(RotaryWingUAV):
         self.k1 = 1./3.29546
         self.k2 = 1.5
 
+        # joint velocity to fly on the spot (hovering)
+        v = self.get_stationary_joint_velocity()
+        self.stationary_velocities = np.array([v, -v, v, -v])
+
+        # initially: propellers[0] = +x, propellers[1] = -y, propellers[2] = -x, propellers[3] = +y
+
     def calculate_thrust_force(self, angular_speed, area, propeller_pitch, v0=0, air_density=1.225):
         r"""
         Calculate the thrust force generated by the propeller (based on [6]).
@@ -159,7 +167,7 @@ class Quadcopter(RotaryWingUAV):
         super(Quadcopter, self).set_joint_velocities(velocities, joint_ids, forces, max_velocity)
 
         # calculate thrust force of the given joints, and apply it on the link
-        for jnt, d, v in zip(joint_ids, self.turning_directions, velocities):
+        for jnt, d, v in zip(joint_ids, self.propeller_directions, velocities):
             if max_velocity and v > self.max_velocity:
                 v = self.max_velocity
 
@@ -188,18 +196,147 @@ class Quadcopter(RotaryWingUAV):
         p = self.propeller_pitch
         return (60 / p) * (fg / (self.air_density * self.area) * (p / (self.k1 * self.diameter))**self.k2)**0.5
 
-    # def gravityCompensate(self):
+    # def gravity_compensate(self):
     #     pass
 
+    def hover(self):
+        """Hover; let the quadcopter fly on the spot."""
+        self.set_propeller_velocities(self.stationary_velocities)
+
+    @staticmethod
+    def rpm_to_rad_per_second(rpm):
+        """
+        Convert the revolutions per minute to rad/sec.
+
+        Args:
+            rpm (float): revolutions per minute.
+
+        Returns:
+            float: rad/sec
+        """
+        return rpm_to_rad_per_second(rpm)
+
+    @staticmethod
+    def rad_per_second_to_rpm(omega):
+        """
+        Convert rad/sec to revolutions/minute.
+
+        Args:
+            omega (float): angular velocity (rad/sec)
+
+        Returns:
+            float: revolutions/minute
+        """
+        return rad_per_second_to_rpm(omega)
+
+    def move(self, velocity):
+        """Move the robot at the specified 3D velocity vector.
+
+        Args:
+            velocity (np.array[3]): 3D velocity vector defined in the xy plane. The magnitude represents the speed.
+        """
+        speed = np.linalg.norm(velocity[:2])
+        angle = np.arctan2(velocity[1], velocity[0])
+        angles = np.array([angle]*4) + np.array([0, np.pi/2, np.pi, 3*np.pi/2])
+        idx = angles > np.pi
+        angles[idx] = 2*np.pi - angles[idx]  # convert from range [0, 2pi[ to [-pi, pi[
+        velocities = np.abs(angles / np.pi)
+        velocities = self.stationary_velocities + speed * velocities * self.propeller_directions
+        if len(velocity) > 2:  # z direction
+            velocities += 10 * velocity[2] * self.propeller_directions  # 10 is a random constant
+        self.set_propeller_velocities(velocities)
+
+    def ascend(self, speed=0):
+        """Make the quadcopter ascend.
+
+        Args:
+            speed (float): speed to ascend.
+        """
+        self.set_propeller_velocities(self.stationary_velocities + speed * self.propeller_directions)
+
+    def descend(self, speed=0):
+        """Make the quadcopter descend.
+
+        Args:
+            speed (float): speed to descend.
+        """
+        self.set_propeller_velocities(self.stationary_velocities - speed * self.propeller_directions)
+
+    def turn(self, speed=0):
+        """Turn the quadcopter. If the speed is positive, turn to the left, otherwise turn to the right (using the
+        right-hand rule).
+
+        Args:
+            speed (float): speed to turn to the left (if speed is positive) or to the right (if speed is negative).
+        """
+        if speed > 0:
+            self.turn_left(np.abs(speed))
+        else:
+            self.turn_right(np.abs(speed))
+
+    def turn_left(self, speed=0):
+        """Turn the quadcopter to the left.
+
+        Args:
+            speed (float): speed to turn to the left.
+        """
+        velocities = self.stationary_velocities + speed * np.array([0, 1., 0., 1.]) * self.propeller_directions
+        self.set_propeller_velocities(velocities)
+
+    def turn_right(self, speed=0):
+        """Turn the quadcopter to the right.
+
+        Args:
+            speed (float): speed to turn to the right.
+        """
+        velocities = self.stationary_velocities + speed * np.array([1., 0., 1., 0.]) * self.propeller_directions
+        self.set_propeller_velocities(velocities)
+
+    def move_forward(self, speed=0):
+        """Move the quadcopter forward.
+
+        Args:
+            speed (float): speed to move forward.
+        """
+        velocities = self.stationary_velocities + speed * np.array([0., 0.5, 1., 0.5]) * self.propeller_directions
+        self.set_propeller_velocities(velocities)
+
+    def move_backward(self, speed=0):
+        """Move the quadcopter backward.
+
+        Args:
+            speed (float): speed to move backward.
+        """
+        velocities = self.stationary_velocities + speed * np.array([1., 0.5, 0., 0.5]) * self.propeller_directions
+        self.set_propeller_velocities(velocities)
+
+    def move_left(self, speed=0):
+        """Tilt the quadcopter to the left.
+
+        Args:
+            speed (float): speed to move to the left.
+        """
+        velocities = self.stationary_velocities + speed * np.array([0.5, 1., 0.5, 0.]) * self.propeller_directions
+        self.set_propeller_velocities(velocities)
+
+    def move_right(self, speed=0):
+        """Tilt the quadcopter to the right.
+
+        Args:
+            speed (float): speed to move to the right.
+        """
+        velocities = self.stationary_velocities + speed * np.array([0.5, 0., 0.5, 1.]) * self.propeller_directions
+        self.set_propeller_velocities(velocities)
+
 
 # Test
 if __name__ == "__main__":
     from itertools import count
-    from pyrobolearn.simulators import BulletSim
+    from pyrobolearn.simulators import Bullet
     from pyrobolearn.worlds import BasicWorld
 
     # Create simulator
-    sim = BulletSim()
+    sim = Bullet()
 
     # create world
     world = BasicWorld(sim)
diff --git a/pyrobolearn/robots/robot.py b/pyrobolearn/robots/robot.py
index b3cba32..9ed4e32 100644
--- a/pyrobolearn/robots/robot.py
+++ b/pyrobolearn/robots/robot.py
@@ -2182,7 +2182,7 @@ class Robot(ControllableBody):
         Me = logarithm_map([target_velocity_manipulability], velocity_manip[0:num_task_vars, 0:num_task_vars])[0]
         # print("Me: {}".format(Me))
         distance = distance_spd(target_velocity_manipulability, velocity_manip[0:num_task_vars, 0:num_task_vars])
-        # print("SPD dist: {}".format(distance))
+        # print("SPD distance: {}".format(distance))
 
         Jm_red = self.compute_velocity_manipulability_jacobian(jacobian, num_task_vars)
         # print("Jm: {}".format(Jm_red))
@@ -2899,7 +2899,7 @@ class Robot(ControllableBody):
         # print("Me: {}".format(Me))
         distance = distance_spd(target_dynamic_manipulability[0:num_task_vars, 0:num_task_vars],
                                 dynamic_manip[0:num_task_vars, 0:num_task_vars])
-        print("SPD dist: {}".format(distance))
+        print("SPD distance: {}".format(distance))
 
         Jm_red = self.compute_dynamic_manipulability_jacobian(jacobian, inertia, num_task_vars)
         # print("Jm: {}".format(Jm_red))
diff --git a/pyrobolearn/robots/uav.py b/pyrobolearn/robots/uav.py
index 41ca1da..02187fc 100644
--- a/pyrobolearn/robots/uav.py
+++ b/pyrobolearn/robots/uav.py
@@ -54,6 +54,39 @@ class RotaryWingUAV(UAVRobot):
     def __init__(self, simulator, urdf, position=None, orientation=None, fixed_base=False, scale=1.):
         super(RotaryWingUAV, self).__init__(simulator, urdf, position, orientation, fixed_base, scale)
 
+    def hover(self):
+        pass
+
+    def move(self, velocity):
+        pass
+
+    def ascend(self, speed=0):
+        pass
+
+    def descend(self, speed=0):
+        pass
+
+    def turn(self, speed=0):
+        pass
+
+    def turn_left(self, speed=0):
+        pass
+
+    def turn_right(self, speed=0):
+        pass
+
+    def move_forward(self, speed=0):
+        pass
+
+    def move_backward(self, speed=0):
+        pass
+
+    def move_left(self, speed=0):
+        pass
+
+    def move_right(self, speed=0):
+        pass
+
 
 class FlappingWingUAV(UAVRobot):
     r"""Flapping Wing Robot
diff --git a/pyrobolearn/simulators/bullet.py b/pyrobolearn/simulators/bullet.py
index 77d4780..312a733 100644
--- a/pyrobolearn/simulators/bullet.py
+++ b/pyrobolearn/simulators/bullet.py
@@ -125,6 +125,10 @@ class Bullet(Simulator):
         # given parameters
         self.kwargs = {'render': render, 'kwargs': kwargs}
 
+        # define default timestep
+        self.default_timestep = 1. / 240
+        self.dt = self.default_timestep
+
         # go through the global variables / attributes defined in pybullet and set them here
         # this includes for instance: JOINT_REVOLUTE, POSITION_CONTROL, etc.
         # for attribute in dir(pybullet):
@@ -160,6 +164,11 @@ class Bullet(Simulator):
         """Return the version of the simulator in a year-month-day format."""
         return self.sim.getAPIVersion()
 
+    @property
+    def timestep(self):
+        """Return the simulator time step."""
+        return self.dt
+
     #############
     # Operators #
     #############
@@ -386,6 +395,14 @@ class Bullet(Simulator):
         # set the render variable (useful when calling the method `is_rendering`)
         self._render = enable
 
+    def get_time_step(self):
+        """Get the time step in the simulator.
+
+        Returns:
+            float: time step in the simulator
+        """
+        return self.get_physics_properties()['fixed_time_step']
+
     def set_time_step(self, time_step):
         """Set the specified time step in the simulator.
 
@@ -401,6 +418,7 @@ class Bullet(Simulator):
             time_step (float): Each time you call 'step' the time step will proceed with 'time_step'.
         """
         # self.history.append(('set_time_step', {'time_step': time_step}))
+        self.dt = time_step
         self.sim.setTimeStep(timeStep=time_step)
 
     def set_real_time(self, enable=True):
diff --git a/pyrobolearn/simulators/simulator.py b/pyrobolearn/simulators/simulator.py
index 4a37bd6..805f4b0 100644
--- a/pyrobolearn/simulators/simulator.py
+++ b/pyrobolearn/simulators/simulator.py
@@ -184,6 +184,10 @@ class Simulator(object):
         # main camera in the simulator
         self._camera = None
 
+        # default timestep
+        self.default_timestep = 1. / 240
+        self.dt = self.default_timestep
+
         # TODO: this is really bad to have attributes like that... It doesn't generalize well to other simulators...
         # import pybullet
         # for attribute in dir(pybullet):
@@ -214,6 +218,11 @@ class Simulator(object):
         """Return the camera (yaw, pitch, distance, target_position) or None."""
         return self._camera
 
+    @property
+    def timestep(self):
+        """Return the simulator time step."""
+        return self.get_time_step()
+
     #############
     # Operators #
     #############
@@ -297,6 +306,14 @@ class Simulator(object):
         """Hide the GUI."""
         self.render(False)
 
+    def get_time_step(self):
+        """Get the time step in the simulator.
+
+        Returns:
+            float: time step in the simulator
+        """
+        pass
+
     def set_time_step(self, time_step):
         """Set the time step in the simulator.
 
diff --git a/pyrobolearn/tools/interfaces/controllers/playstation.py b/pyrobolearn/tools/interfaces/controllers/playstation.py
index d59ff5a..25cda1c 100755
--- a/pyrobolearn/tools/interfaces/controllers/playstation.py
+++ b/pyrobolearn/tools/interfaces/controllers/playstation.py
@@ -33,7 +33,7 @@ class PSControllerInterface(GameControllerInterface):
         [2] Hardware support: https://inputs.readthedocs.io/en/latest/user/hardwaresupport.html
     """
 
-    def __init__(self, use_thread=False, controller_name='Sony Interactive Entertainment Wireless Controller'):
+    def __init__(self, use_thread=False, sleep_dt=0, verbose=False, controller_name='Sony'):
         # Check if some gamepads are connected to the computer
         gamepads = devices.gamepads
         if len(gamepads) == 0:
@@ -46,6 +46,9 @@ class PSControllerInterface(GameControllerInterface):
                 self.gamepad = gamepad
                 break
 
+        if verbose:
+            print(self.gamepad.name + ' detected.')
+
         if self.gamepad is None:
             raise ValueError("The specified gamepad/controller was not detected.")
 
@@ -65,7 +68,7 @@ class PSControllerInterface(GameControllerInterface):
         # last updated button
         self.last_updated_button = None
 
-        super(PSControllerInterface, self).__init__(use_thread)
+        super(PSControllerInterface, self).__init__(use_thread=use_thread, sleep_dt=sleep_dt, verbose=verbose)
 
     ##############
     # Properties #
@@ -165,7 +168,7 @@ class PSControllerInterface(GameControllerInterface):
             self.__setitem(event_type, self.map.get(code), state)
 
             # display info
-            if self.verbose:
+            if self.verbose and self.last_updated_button is not None:
                 print("Pushed button {} - state = {}".format(self.last_updated_button,
                                                              self.buttons[self.last_updated_button]))
 
@@ -218,8 +221,9 @@ class PSControllerInterface(GameControllerInterface):
                 self.last_updated_button = 'Dpad'
             elif key == 'LT' or key == 'RT':  # max 1023
                 self.buttons[key] = value / 1023.
-                self.last_updated_button = key
+                # self.last_updated_button = key
         elif event_type == 'Key':
+            print(event_type, key, value)
             self.buttons[key] = value
             self.last_updated_button = key
 
@@ -237,8 +241,8 @@ class PS3ControllerInterface(PSControllerInterface):
         [2] Hardware support: https://inputs.readthedocs.io/en/latest/user/hardwaresupport.html
     """
 
-    def __init__(self, use_thread=False):
-        super(PS3ControllerInterface, self).__init__(use_thread=use_thread,
+    def __init__(self, use_thread=False, sleep_dt=0, verbose=False,):
+        super(PS3ControllerInterface, self).__init__(use_thread=use_thread, sleep_dt=sleep_dt, verbose=verbose,
                                                      controller_name='Sony PLAYSTATION(R)3 Controller')
 
 
@@ -255,15 +259,16 @@ class PS4ControllerInterface(PSControllerInterface):
         [2] Hardware support: https://inputs.readthedocs.io/en/latest/user/hardwaresupport.html
     """
 
-    def __init__(self, use_thread=False):
-        super(PS4ControllerInterface, self).__init__(use_thread=use_thread,
+    def __init__(self, use_thread=False, sleep_dt=0, verbose=False,):
+        super(PS4ControllerInterface, self).__init__(use_thread=use_thread, sleep_dt=sleep_dt, verbose=verbose,
                                                      controller_name='Sony Interactive Entertainment Wireless '
                                                                      'Controller')
 
 
 # Tests
 if __name__ == '__main__':
-    device = devices.gamepads[0]
+    device = devices.gamepads[1]
+    print(device.name)
     while True:
         events = device.read()  # blocking=False) # get_gamepad()
         for event in events:
diff --git a/pyrobolearn/tools/interfaces/controllers/xbox.py b/pyrobolearn/tools/interfaces/controllers/xbox.py
index 54a76c6..0edce40 100644
--- a/pyrobolearn/tools/interfaces/controllers/xbox.py
+++ b/pyrobolearn/tools/interfaces/controllers/xbox.py
@@ -1,7 +1,21 @@
 #!/usr/bin/env python
 """Provide the Xbox controller interfaces.
 
-This provides the interfaces for the PlayStation controllers (Xbox 360 and Xbox One) using the `inputs` library.
+This provides the interfaces for the Xbox controllers (Xbox 360 and Xbox One) using the `inputs` library.
+
+Troubleshooting:
+If the Xbox controller is not detected, please install the necessary driver. On Ubuntu 16.04, you can install the
+`xpad` driver by typing the following commands (see [1]):
+
+```bash
+sudo apt-get install git
+sudo apt-get install dkms
+sudo git clone https://github.com/paroj/xpad.git /usr/src/xpad-0.4
+sudo dkms install -m xpad -v 0.4
+```
+
+References:
+    [1] https://askubuntu.com/questions/783587/how-do-i-get-an-xbox-one-controller-to-work-with-16-04-not-steam
 """
 
 try:
@@ -73,6 +87,9 @@ class XboxControllerInterface(GameControllerInterface):
         if self.gamepad is None:
             raise ValueError("The specified gamepad/controller was not detected.")
 
+        if verbose:
+            print(self.gamepad.name + ' detected.')
+
         # translation
         buttons = ['BTN_SOUTH', 'BTN_EAST', 'BTN_WEST', 'BTN_NORTH', 'BTN_THUMBL', 'BTN_THUMBR', 'BTN_TL', 'BTN_TR',
                    'BTN_START', 'BTN_SELECT', 'ABS_Z', 'ABS_RZ', 'ABS_HAT0X', 'ABS_HAT0Y', 'ABS_X', 'ABS_Y', 'ABS_RX',
@@ -185,7 +202,7 @@ class XboxControllerInterface(GameControllerInterface):
             self.__setitem(event_type, self.map.get(code), state)
 
             # display info
-            if self.verbose:
+            if self.verbose and self.last_updated_button is not None:
                 print("Pushed button {} - state = {}".format(self.last_updated_button,
                                                              self.buttons[self.last_updated_button]))
 
@@ -249,8 +266,9 @@ class Xbox360ControllerInterface(XboxControllerInterface):
 
     """
 
-    def __init__(self, use_thread=False):
-        super(Xbox360ControllerInterface, self).__init__(use_thread=use_thread, controller_name='X-Box 360')
+    def __init__(self, use_thread=False, sleep_dt=0, verbose=False):
+        super(Xbox360ControllerInterface, self).__init__(use_thread=use_thread, sleep_dt=sleep_dt,
+                                                         verbose=verbose, controller_name='X-Box 360')
 
 
 class XboxOneControllerInterface(XboxControllerInterface):
@@ -258,8 +276,9 @@ class XboxOneControllerInterface(XboxControllerInterface):
 
     """
 
-    def __init__(self, use_thread=False):
-        super(XboxOneControllerInterface, self).__init__(use_thread=use_thread, controller_name='X-Box One')
+    def __init__(self, use_thread=False, sleep_dt=0, verbose=False):
+        super(XboxOneControllerInterface, self).__init__(use_thread=use_thread, sleep_dt=sleep_dt,
+                                                         verbose=verbose, controller_name='X-Box One')
 
 
 # Tests
diff --git a/pyrobolearn/tools/interfaces/vr/oculus.py b/pyrobolearn/tools/interfaces/vr/oculus.py
index 2d89e19..99c60e3 100644
--- a/pyrobolearn/tools/interfaces/vr/oculus.py
+++ b/pyrobolearn/tools/interfaces/vr/oculus.py
@@ -25,7 +25,7 @@ from pyrobolearn.tools.interfaces.vr import VRInterface
 __author__ = "Brian Delhaisse"
 __copyright__ = "Copyright 2018, PyRoboLearn"
 __credits__ = ["Brian Delhaisse"]
-__license__ = "GNU GPLv3"
+__license__ = "MIT"
 __version__ = "1.0.0"
 __maintainer__ = "Brian Delhaisse"
 __email__ = "briandelhaisse@gmail.com"
@@ -153,7 +153,7 @@ class OculusInterface(VRInterface):
                 # self.worldCamera.add_yaw_pitch(yaw, pitch, radian=False)
                 # print(pitch, yaw)
                 pos = self.world_camera.target_position
-                dist = self.world_camera.dist
+                dist = self.world_camera.distance
             elif name == 'BA': # button A: [touch, button]
                 pass
             elif name == 'BB': # button B: [touch, button]
diff --git a/pyrobolearn/worlds/world.py b/pyrobolearn/worlds/world.py
index 040166c..da642d4 100644
--- a/pyrobolearn/worlds/world.py
+++ b/pyrobolearn/worlds/world.py
@@ -19,6 +19,7 @@ from pyrobolearn.simulators import Simulator
 from pyrobolearn.worlds.world_camera import WorldCamera
 # from pyrobolearn.utils import has_method, has_variable
 from pyrobolearn.robots import Body, Robot, robot_names_to_classes
+from pyrobolearn.utils.transformation import get_quaternion_from_rpy
 # TODO: to install the `gdal` library, run the script `pyrobolearn/scripts/install_gdal.sh`, by default do not
 #  import it
 from pyrobolearn.worlds.utils.heightmaps.diamond_square import diamond_square_heightmap, diamond_square_heightmap_2
@@ -452,7 +453,8 @@ class World(object):
                             'an instance of Robot'.format(type(robot)))
 
         self.bodies[robot.id] = robot
-        self.ids[robot.id] = [robot]
+        # self.ids[robot.id] = [robot]
+        self.ids[robot.id] = [self.__get_method_and_parameters(frame=inspect.currentframe())]
         return robot
 
     def is_body_id(self, body_id):
@@ -952,7 +954,9 @@ class World(object):
             int: unique id of the floor in the world
         """
         # self.floor_id = self.sim.load_urdf('plane100.urdf', use_fixed_base=True, scale=scaling)
-        self.floor_id = self.sim.load_urdf('plane.urdf', use_fixed_base=True, scale=scaling)
+        self.floor_id = self.sim.load_urdf('plane.urdf', position=[0., 0., 0.], use_fixed_base=True, scale=scaling)
+        # distance = self.camera.distance
+        # self.camera.reset(distance=scaling * distance)
         return self.floor_id
 
     def load_terrain(self, heightmap, position=(0., 0., 0.), orientation=(.707, 0, 0, .707), scaling=1.,
@@ -1067,7 +1071,7 @@ class World(object):
         return heightmap
 
     # aliases
-    loadDEM = load_heightmap
+    load_dem = load_heightmap
 
     @staticmethod
     def generate_heightmap(algo=2, filename=None, width=256, height=256, n=8, min_height=0, max_height=255, noise=0,
@@ -1739,8 +1743,9 @@ class World(object):
                    return_body=False):
         pass
 
-    # TODO: add an orientation_range
-    def distribute(self, body, size=2, position_range=(-1, 1), return_body=False, *args, **kwargs):
+    # TODO: check collisions when distributing the various bodies (need to know the dimensions)
+    def distribute(self, body, size=2, position_range=(-1, 1), rpy_range=(0, 0), return_body=False, *args,
+                   **kwargs):
         r"""
         Spawn several bodies in the specified range.
 
@@ -1754,6 +1759,9 @@ class World(object):
             position_range (tuple of float, tuple of np.array): range of the uniform distribution interval for the
                 position of each body. The first element is the lower boundary, and the second one the higher boundary
                 of the interval.
+            rpy_range (tuple of float, tuple of np.array): range of the uniform distribution interval for the
+                orientation (expressed as roll-pitch-yaw angles) of each body. The first element is the lower boundary,
+                and the second one the higher boundary of the interval.
             return_body (bool): if True, it will return an instance of the `Body`, otherwise, it will return the
                 unique id.
             *args: list of arguments to be given to :attr:`body` if this last one is callable.
@@ -1776,6 +1784,10 @@ class World(object):
             else:
                 positions = np.random.uniform(low=low, high=high, size=(size, len(high)))
 
+        # create orientations (using uniform distribution)
+        low, high = rpy_range
+        rpys = np.random.uniform(low=low, high=high, size=(size, 3))
+
         # check given body argument
         bodies = []
         if self.is_body_id(body):  # unique id
@@ -1783,9 +1795,10 @@ class World(object):
         elif isinstance(body, Body):  # Body
             pass
         elif callable(body) and hasattr(self, body.__name__) and 'position' in inspect.getargspec(body).args:
-            body = body(position=positions[0], *args, **kwargs)
-            body = self.wrap(body, wrapper=Body)
-            positions = positions[1:]
+            body = body(position=positions[0], orientation=get_quaternion_from_rpy(rpys[0]), *args, **kwargs)
+            if not isinstance(body, Body):
+                body = self.wrap(body, wrapper=Body)
+            positions, rpys = positions[1:], rpys[1:]
         else:
             raise TypeError("Expecting the given `body` to be a unique id (int), an instance of Body, or a method of "
                             "`World`, instead got: {} (type={})".format(body, type(body)))
@@ -1801,11 +1814,16 @@ class World(object):
         method = getattr(self, method_name)
         kwargs = dict(kwargs)
 
+        if isinstance(body, Robot):  # if robot, get the class
+            if 'robot' in kwargs:
+                kwargs['robot'] = kwargs['robot'].__class__
+
         # distribute the various other bodies  # TODO: check for collisions
-        for position in positions:
+        for position, rpy in zip(positions, rpys):
 
             # update new position and create body
             kwargs['position'] = position
+            kwargs['orientation'] = get_quaternion_from_rpy(rpy)
             body = method(**kwargs)
 
             if return_body:
@@ -1944,10 +1962,10 @@ class DRCWorld(World):
 if __name__ == '__main__':
     import numpy as np
     from itertools import count
-    from pyrobolearn.simulators import BulletSim
+    from pyrobolearn.simulators import Bullet
 
     # create simulator
-    sim = BulletSim()
+    sim = Bullet()
 
     # create world
     world = BasicWorld(sim)
diff --git a/pyrobolearn/worlds/world_camera.py b/pyrobolearn/worlds/world_camera.py
index 6b30c51..8c3dd27 100644
--- a/pyrobolearn/worlds/world_camera.py
+++ b/pyrobolearn/worlds/world_camera.py
@@ -192,8 +192,8 @@ class WorldCamera(object):
         Set the yaw angle of the camera in radian. The yaw angle is positive when looking on the left and negative
         when looking on the right.
         """
-        pitch, dist, target_position = self.sim.get_debug_visualizer()[-3:]
-        self.reset(dist, yaw, pitch, target_position)
+        pitch, distance, target_position = self.sim.get_debug_visualizer()[-3:]
+        self.reset(distance, yaw, pitch, target_position)
 
     @property
     def pitch(self):
@@ -208,23 +208,23 @@ class WorldCamera(object):
         Set the pitch angle of the camera in radian. The pitch angle is negative when looking down and positive when
         looking up.
         """
-        yaw, _, dist, target_position = self.sim.get_debug_visualizer()[-4:]
-        self.reset(dist, yaw, pitch, target_position)
+        yaw, _, distance, target_position = self.sim.get_debug_visualizer()[-4:]
+        self.reset(distance, yaw, pitch, target_position)
 
     @property
-    def dist(self):
+    def distance(self):
         """
         Return the distance between the camera and the camera target.
         """
         return self.sim.get_debug_visualizer()[10]
 
-    @dist.setter
-    def dist(self, dist):
+    @distance.setter
+    def distance(self, distance):
         """
         Set the distance of the camera (in meter) with respect to the target position.
         """
         yaw, pitch, _, target_position = self.sim.get_debug_visualizer()[-4:]
-        self.reset(dist, yaw, pitch, target_position)
+        self.reset(distance, yaw, pitch, target_position)
 
     @property
     def target_position(self):
@@ -238,8 +238,8 @@ class WorldCamera(object):
         """
         Set the target position of the camera in the Cartesian world space coordinates.
         """
-        yaw, pitch, dist = self.sim.get_debug_visualizer()[-4:-1]
-        self.reset(dist, yaw, pitch, position)
+        yaw, pitch, distance = self.sim.get_debug_visualizer()[-4:-1]
+        self.reset(distance, yaw, pitch, position)
 
     @property
     def position(self):
@@ -257,12 +257,12 @@ class WorldCamera(object):
         """
         target = self.target_position
         vector = (target - position)
-        dist = np.sqrt(np.sum(vector**2))
-        vector = vector / dist
+        distance = np.sqrt(np.sum(vector**2))
+        vector = vector / distance
         pitch = np.arcsin(vector[2])  # [-pi/2, pi/2]
         # pitch = np.arctan2(vector[2], vector[1])
         yaw = np.arctan2(vector[1], vector[0])  # [-pi, pi]
-        self.reset(dist, yaw, pitch, target)
+        self.reset(distance, yaw, pitch, target)
 
     @property
     def orientation(self):
@@ -291,9 +291,9 @@ class WorldCamera(object):
                              "angles, instead got: {}".format(orientation))
 
         # reset the camera
-        dist, target_position = self.sim.get_debug_visualizer()[-2:]
+        distance, target_position = self.sim.get_debug_visualizer()[-2:]
         _, pitch, yaw = rpy
-        self.reset(dist, yaw, pitch, target_position)
+        self.reset(distance, yaw, pitch, target_position)
 
     ###########
     # Methods #
@@ -448,8 +448,8 @@ class WorldCamera(object):
         """
         if radian:
             yaw, pitch = np.rad2deg(yaw), np.rad2deg(pitch)
-        dist, target_pos = self.sim.get_debug_visualizer()[-2:]
-        self.sim.reset_debug_visualizer(dist, yaw, pitch, target_pos)
+        distance, target_pos = self.sim.get_debug_visualizer()[-2:]
+        self.sim.reset_debug_visualizer(distance, yaw, pitch, target_pos)
 
     def add_yaw_pitch(self, dyaw, dpitch, radian=True):
         """
@@ -460,12 +460,12 @@ class WorldCamera(object):
             dpitch (float): small amount to add to the camera's current pitch angle
             radian (bool): If the given pitch and yaw angles are in radian.
         """
-        yaw, pitch, dist, target_pos = self.sim.get_debug_visualizer()[-4:]
+        yaw, pitch, distance, target_pos = self.sim.get_debug_visualizer()[-4:]
         if radian:
             dyaw, dpitch = np.rad2deg(dyaw), np.rad2deg(dpitch)
         yaw += dyaw
         pitch += dpitch
-        self.sim.reset_debug_visualizer(dist, yaw, pitch, target_pos)
+        self.sim.reset_debug_visualizer(distance, yaw, pitch, target_pos)
 
     def get_rgb_image(self):
         """
@@ -641,10 +641,10 @@ class WorldCamera(object):
 if __name__ == '__main__':
     from itertools import count
     import time
-    from pyrobolearn.simulators import BulletSim
+    from pyrobolearn.simulators import Bullet
 
     # create simulator
-    sim = BulletSim()
+    sim = Bullet()
 
     # load floor
     floor_id = sim.load_urdf('plane.urdf', use_fixed_base=True)
@@ -662,13 +662,13 @@ if __name__ == '__main__':
         position = camera.position
         yaw = camera.yaw
         pitch = camera.pitch
-        dist = camera.dist
+        distance = camera.distance
         target_position = camera.target_position
         print("Position: {}".format(position))
         print("Target position: {}".format(target_position))
         print("Yaw: {}".format(np.rad2deg(yaw)))
         print("Pitch: {}".format(np.rad2deg(pitch)))
-        print("Distance: {}".format(dist))
+        print("Distance: {}".format(distance))
         print("##########\n")
 
         # move camera