Slide#
Description#
This environment was introduced in “MultiGoal Reinforcement Learning: Challenging Robotics Environments and Request for Research”.
The task in the environment is for a manipulator hit a puck in order to reach a target position on top of a long and slippery table. The table has a low friction coefficient in order to make it slippery for the puck to slide and be able to reach the target position which is outside of the robot’s workspace. The robot is a 7DoF Fetch Mobile Manipulator with a twofingered parallel gripper. The robot is controlled by small displacements of the gripper in Cartesian coordinates and the inverse kinematics are computed internally by the MuJoCo framework. The gripper is locked in a closed configuration since the puck doesn’t need to be graspped. The task is also continuing which means that the robot has to maintain the puck in the target position for an indefinite period of time.
The control frequency of the robot is of f = 25 Hz
. This is achieved by applying the same action in 20 subsequent simulator step (with a time step of dt = 0.002 s
) before returning the control to the robot.
Action Space#
The action space is a Box(1.0, 1.0, (4,), float32)
. An action represents the Cartesian displacement dx, dy, and dz of the end effector. In addition to a last action that controls closing and opening of the gripper.
Num 
Action 
Control Min 
Control Max 
Name (in corresponding XML file) 
Joint 
Unit 

0 
Displacement of the end effector in the x direction dx 
1 
1 
robot0:mocap 
hinge 
position (m) 
1 
Displacement of the end effector in the y direction dy 
1 
1 
robot0:mocap 
hinge 
position (m) 
2 
Displacement of the end effector in the z direction dz 
1 
1 
robot0:mocap 
hinge 
position (m) 
3 
 
1 
1 
 
 
 
Observation Space#
The observation is a goalaware observation space
. It consists of a dictionary with information about the robot’s end effector state and goal. The kinematics observations are derived from Mujoco bodies known as sites attached to
the body of interest such as the puck or the end effector. Only the observations from the gripper fingers are derived from joints. Also to take into account the temporal influence of the step time, velocity values are multiplied by the step time dt=number_of_sub_steps*sub_step_time. The dictionary consists of the following 3 keys:
observation
: its value is anndarray
of shape(25,)
. It consists of kinematic information of the puck object and gripper. The elements of the array correspond to the following:Num
Observation
Min
Max
Site Name (in corresponding XML file)
Joint Name (in corresponding XML file)
Joint Type
Unit
0
End effector x position in global coordinates
Inf
Inf
robot0:grip


position (m)
1
End effector y position in global coordinates
Inf
Inf
robot0:grip


position (m)
2
End effector z position in global coordinates
Inf
Inf
robot0:grip


position (m)
3
Puck x position in global coordinates
Inf
Inf
object0


position (m)
4
Puck y position in global coordinates
Inf
Inf
object0


position (m)
5
Puck z position in global coordinates
Inf
Inf
object0


position (m)
6
Relative puck x position with respect to gripper x position in global coordinates. Equals to x_{puck}  x_{gripper}
Inf
Inf
object0


position (m)
7
Relative puck y position with respect to gripper y position in global coordinates. Equals to y_{puck}  y_{gripper}
Inf
Inf
object0


position (m)
8
Relative puck z position with respect to gripper z position in global coordinates. Equals to z_{puck}  z_{gripper}
Inf
Inf
object0


position (m)
9
Joint displacement of the right gripper finger
Inf
Inf

robot0:r_gripper_finger_joint
hinge
position (m)
10
Joint displacement of the left gripper finger
Inf
Inf

robot0:l_gripper_finger_joint
hinge
position (m)
11
Global x rotation of the puck in a XYZ Euler frame rotation
Inf
Inf
object0


angle (rad)
12
Global y rotation of the puck in a XYZ Euler frame rotation
Inf
Inf
object0


angle (rad)
13
Global z rotation of the puck in a XYZ Euler frame rotation
Inf
Inf
object0


angle (rad)
14
Relative puck linear velocity in x direction with respect to the gripper
Inf
Inf
object0


velocity (m/s)
15
Relative puck linear velocity in y direction with respect to the gripper
Inf
Inf
object0


velocity (m/s)
16
Relative puck linear velocity in z direction
Inf
Inf
object0


velocity (m/s)
17
Puck angular velocity along the x axis
Inf
Inf
object0


angular velocity (rad/s)
18
Puck angular velocity along the y axis
Inf
Inf
object0


angular velocity (rad/s)
19
Puck angular velocity along the z axis
Inf
Inf
object0


angular velocity (rad/s)
20
End effector linear velocity x direction
Inf
Inf
robot0:grip


velocity (m/s)
21
End effector linear velocity y direction
Inf
Inf
robot0:grip


velocity (m/s)
22
End effector linear velocity z direction
Inf
Inf
robot0:grip


velocity (m/s)
23
Right gripper finger linear velocity
Inf
Inf

robot0:r_gripper_finger_joint
hinge
velocity (m/s)
24
Left gripper finger linear velocity
Inf
Inf

robot0:l_gripper_finger_joint
hinge
velocity (m/s)
desired_goal
: this key represents the final goal to be achieved. In this environment it is a 3dimensionalndarray
,(3,)
, that consists of the three cartesian coordinates of the desired final puck position[x,y,z]
. In order for the robot to perform a pick and place trajectory, the goal position can be elevated over the table or on top of the table. The elements of the array are the following:Num
Observation
Min
Max
Site Name (in corresponding XML file)
Unit
0
Final goal puck position in the x coordinate
Inf
Inf
target0
position (m)
1
Final goal puck position in the y coordinate
Inf
Inf
target0
position (m)
2
Final goal puck position in the z coordinate
Inf
Inf
target0
position (m)
achieved_goal
: this key represents the current state of the puck, as if it would have achieved a goal. This is useful for goal orientated learning algorithms such as those that use Hindsight Experience Replay (HER). The value is anndarray
with shape(3,)
. The elements of the array are the following:Num
Observation
Min
Max
Site Name (in corresponding XML file)
Unit
0
Current puck position in the x coordinate
Inf
Inf
object0
position (m)
1
Current puck position in the y coordinate
Inf
Inf
object0
position (m)
2
Current puck position in the z coordinate
Inf
Inf
object0
position (m)
Rewards#
The reward can be initialized as sparse
or dense
:
sparse: the returned reward can have two values:
1
if the puck hasn’t reached its final target position, and0
if the puck is in the final target position (the puck is considered to have reached the goal if the Euclidean distance between both is lower than 0.05 m).dense: the returned reward is the negative Euclidean distance between the achieved goal position and the desired goal.
To initialize this environment with one of the mentioned reward functions the type of reward must be specified in the id string when the environment is initialized. For sparse
reward the id is the default of the environment, FetchSlidev2
. However, for dense
reward the id must be modified to FetchSlideDensev2
and initialized as follows:
import gymnasium as gym
env = gym.make('FetchSlideDensev2')
Starting State#
When the environment is reset the gripper is placed in the following global cartesian coordinates (x,y,z) = [1 0.75 0.41] m
, and its orientation in quaternions is (w,x,y,z) = [1.0, 0.0, 1.0, 0.0]
. The joint positions are computed by inverse kinematics internally by MuJoCo. The base of the robot will always be fixed at (x,y,z) = [0.405, 0.48, 0]
in global coordinates.
The puck’s position has a fixed height of (z) = [0.42] m
(on top of the table). The initial (x,y)
position of the puck is the gripper’s x and y coordinates plus an offset sampled from a uniform distribution with a range of [0.1, 0.1] m
. Offset samples are generated until the 2dimensional Euclidean distance from the gripper to the puck is greater than 0.1 m
.
The initial orientation of the puck is the same as for the gripper, (w,x,y,z) = [1.0, 0.0, 1.0, 0.0]
.
Finally the target position where the robot has to move the puck is generated. The target can be in midair or over the table. The random target is also generated by adding an offset to the initial grippers position (x,y)
sampled from a uniform distribution with a range of [0.3, 0.3] m
. The height of the target is initialized at (z) = [0.42] m
.
Episode End#
The episode will be truncated
when the duration reaches a total of max_episode_steps
which by default is set to 50 timesteps.
The episode is never terminated
since the task is continuing with infinite horizon.
Arguments#
To increase/decrease the maximum number of timesteps before the episode is truncated
the max_episode_steps
argument can be set at initialization. The default value is 50. For example, to increase the total number of timesteps to 100 make the environment as follows:
import gymnasium as gym
env = gym.make('FetchSlidev2', max_episode_steps=100)
Version History#
v2: the environment depends on the newest mujoco python bindings maintained by the MuJoCo team in Deepmind.
v1: the environment depends on
mujoco_py
which is no longer maintained.