How to implement differential drive (rotation in place)? #2392

husnoo · 2025-01-28T16:36:01Z

husnoo
Jan 28, 2025

Intro

Hi!

I'm a hobbyist programmer, and I'm trying to simulate a differential drive robot using Mujoco. I started from this: https://roboti.us/forum/index.php?resources/rover4we.28/ but the example ties the two back wheels together - I want the two wheels to be used for differential driving.

My setup

Python 3.8.18

mujoco.version == '3.1.2'

My question

I've modified the code so that the two wheels can be driven separately.

With simulate rover2.xml, you can set the two controls to 1.0 and the car speeds off into the right. If you set it to +1 and -1, instead of spinning with the same enthusiasm, it simply skids in place and doesn't move much.

The original code used a tendon, but it seems used to fix the two wheels together - I want them to rotate in different directions. It works well on physical robots, so I'm not sure why it's not working in simulation.

Minimal model and/or code that explain my question

<mujoco>

  <compiler inertiafromgeom="true" angle="degree"/>
  <option	timestep="0.02" gravity="0 0 -9.81"/>
  
  <asset>
    <material name="MatWheel" specular="1" rgba=".1 .1 .1 1"/>
    <material name="MatBody" specular="1" rgba=".6 .6 .1 1"/>  
    <texture name="floor_tex" type="2d" builtin="checker" rgb1=".2 .3 .4" rgb2=".1 .2 .3" width="100" height="100" />
    <material name="floor_material" texture="floor_tex" texuniform="true" />
  </asset>
	
  <default>
    <!--making every geom collidable. expect those that dont are default-->
    <geom contype="1" conaffinity="1"/>
  </default>
	
  <default class="acker_solver_param">
    <equality solimp="0.9950 0.9990 0.0001" solref="0.0100 0.7500"/>
  </default>
	
  <visual>
    <headlight active="1"/>
    <scale camera=".5"/>
    <map znear="0.001" zfar="1000"/>
  </visual>
	
  <worldbody>
    <geom name="ground" type="plane" size="5 5 1" pos="0 0 -.25" material="floor_material"/>

    
    <body name="rover" pos="0 0 0">		
      <light name="lamp" active="true" directional="true" cutoff="4" exponent="20" diffuse="1 1 1" specular="0 0 0" pos="0 0 2.5" dir="0 0 -1 "/>	
      <camera name="first-person" mode="fixed" pos=".22 0 .08" euler="180 -133 -90" fovy="90"/>
			
      <joint name="centroid" type="free" pos="0 0 0"/>

      <geom name="rover-body" type="box" pos="0 0 0" size="0.2 0.15 .095" mass="12" material="MatBody"/>
      <!-- <site type="box" size=".01 .07 .07" pos=".2 0 0"/> -->


      <!-- rear left -->
      <body name="r-l-wheel" pos="-0.2 0.15 -0.15"> 
        <joint name="r-l-drive-hinge" type="hinge" pos="0 0 0" axis="0 1 0" damping="0.01" />
        <geom type="ellipsoid" size=".05 .05 .05" fromto="0 0 0 0 0.05 0" mass=".125" material="MatWheel" condim="4"/>
	<site size=".01" pos="0 .05 .03"/>
	<site size=".01" pos="0 .05 -.03"/>
	<site size=".01" pos="0 0 .03"/>
	<site size=".01" pos="0 0 -.03"/>
      </body>

      
      <!-- rear right --> 
      <body name="r-r-wheel" pos="-0.2 -0.15 -0.15">
        <joint name="r-r-drive-hinge" type="hinge" pos="0 0 0" axis="0 1 0" damping="0.01"/>
        <geom type="ellipsoid" size=".05 .05 .05" fromto="0 0 0 0 -0.05 0" mass=".125" material="MatWheel" condim="4"/>
	<site size=".01" pos="0 -.05 .03"/>
	<site size=".01" pos="0 -.05 -.03"/>
	<site size=".01" pos="0 0 .03"/>
	<site size=".01" pos="0 0 -.03"/>
      </body>



          <body>
  	    <joint name="hinge" type="hinge" pos="0.2 0 -0.1" axis="0 1 0" damping="0"/>
	    <geom type="sphere" size=".10" pos="0.2 0 -0.1" friction="0 0 0"/>
          </body>

    </body>
  </worldbody>
  
  <!--<tendon>
    <fixed name="rear-differential">
      <joint joint="r-l-drive-hinge" coef="1"/>
      <joint joint="r-r-drive-hinge" coef="1"/>
    </fixed>
  </tendon>
  -->
  
  <actuator>
    <motor name="drive-l" joint="r-l-drive-hinge" /> <!--ctrllimited="true" ctrlrange="-1.0 1.0" gear="1.5"/> -->
    <motor name="drive-r" joint="r-r-drive-hinge" /> <!--ctrllimited="true" ctrlrange="-1.0 1.0" gear="1.5"/> -->
  </actuator>
  
</mujoco>

Confirmations

I searched the latest documentation thoroughly before posting.
I searched previous Issues and Discussions, I am certain this has not been raised before.

Answered by yuvaltassa

Jan 30, 2025

Sure, you can have as many actuators as you want. Some for individual wheels/tracks, some for linear combinations (fixed tendons). Multiple actuators can affect the same joint, forces/torques will be summed.

View full answer

shogan50 · 2025-01-28T17:07:15Z

shogan50
Jan 28, 2025

I've not played with tendons yet, but it looks to me like your tendon is acting like an axle tying the two rotations together. If you want differential rotation, I would think you just omit it.

1 reply

husnoo Jan 28, 2025
Author

Yes I've removed it, but the differential rotation doesn't work for rotation in place (+1,-1 or -1,+1) - it's very slow compared to the speed it zooms off the screen in forward/backward motion if the controls are +1,+1 or -1,-1.

yuvaltassa · 2025-01-28T17:15:47Z

yuvaltassa
Jan 28, 2025
Maintainer

Inspect this model, I hope it will answer all your questions:
https://github.com/google-deepmind/mujoco/blob/main/model/car/car.xml

3 replies

husnoo Jan 28, 2025
Author

That's perfect thank you!

husnoo Jan 28, 2025
Author

That makes sense, I can see how it's implemented - but could I ask a followup question please?

The reason I'm using a differential drive robot is to very roughly simulate a toy tank with tracks. The two tracks can be activated separately - but this use of tendons ties the two wheels together - do you think it's possible to have individual control of the two? Or would it be easier to just implement the forward/turn verbs on the real robot instead?

Thanks!

yuvaltassa Jan 30, 2025
Maintainer

Sure, you can have as many actuators as you want. Some for individual wheels/tracks, some for linear combinations (fixed tendons). Multiple actuators can affect the same joint, forces/torques will be summed.

Answer selected by husnoo

shogan50 · 2025-01-30T03:22:36Z

shogan50
Jan 30, 2025

I'm still mystified as to how to get xml to show up in full when I paste, so I can't paste your full modified xml in here. If you could tell me how you do it, I would appreciate it.

I deleted your default section
Your friction = 0 on the sphere doesn't look like it is zero, so I added another joint.
Setting the .ctrl to 1 and -1 makes for herkey jerkey motion, as the motors are pure torque with little damping. Setting them to +/- .05 makes it behave fairly nicely.

        <body>
            <joint name="fwd" type="hinge" pos="0.2 0 -0.1" axis="0 1 0" damping="0.1"/>
            <joint name="lateral" type="hinge" pos="0.2 0 -0.1" axis="1 0 0" damping="0.1"/>
            <geom type="sphere" size=".10" pos="0.2 0 -0.1" friction="0 0 0"/>
        </body>
    </body>
</worldbody>

<actuator>
    <motor name="drive-l" joint="r-l-drive-hinge"/>
    <motor name="drive-r" joint="r-r-drive-hinge" />
</actuator>

1 reply

husnoo Jan 30, 2025
Author

Thank you! The damping seems to be the key here!

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How to implement differential drive (rotation in place)? #2392

husnoo Jan 28, 2025

Intro

My setup

My question

Minimal model and/or code that explain my question

Confirmations

Replies: 3 comments · 5 replies

shogan50 Jan 28, 2025

husnoo Jan 28, 2025 Author

yuvaltassa Jan 28, 2025 Maintainer

husnoo Jan 28, 2025 Author

husnoo Jan 28, 2025 Author

yuvaltassa Jan 30, 2025 Maintainer

shogan50 Jan 30, 2025

husnoo Jan 30, 2025 Author

husnoo
Jan 28, 2025

Replies: 3 comments 5 replies

shogan50
Jan 28, 2025

husnoo Jan 28, 2025
Author

yuvaltassa
Jan 28, 2025
Maintainer

husnoo Jan 28, 2025
Author

husnoo Jan 28, 2025
Author

yuvaltassa Jan 30, 2025
Maintainer

shogan50
Jan 30, 2025

husnoo Jan 30, 2025
Author