Tendon driven motor actuators don't work #74

Infraviored · 2021-11-24T15:51:40Z

Infraviored
Nov 24, 2021

I am trying to simulate a tendon-driven humanoid robot in mujoco.

when using
<muscle name="motor0" tendon="motor0" />
the motor is controllable

using
<motor name="motor0" tendon="motor0" />
the motor stays fixed for all control inputs in [-1,1]

even when using
<cylinder name="motor0" tendon="motor17" forcerange="0 10000" ctrlrange="0 10000" lengthrange="0 10000" />
or replacing motor with position, velocity or cylinder there is no effect of contro input.

Help would be greatly appreciated

Answered by yuvaltassa

Nov 24, 2021

Your motor works just fine, it's just a bit weak so cannot pull your thing due to gravity.
Turn off gravity (in Disable Flags under the Physics tab) and you'll see that your motor works.
To increase the strength of your motor, increase the gear attribute to be bigger than 1.

<motor name="motor" tendon="red" gear="10"/>

View full answer

yuvaltassa · 2021-11-24T15:55:49Z

yuvaltassa
Nov 24, 2021
Maintainer

Hi,
If you make a minimal model (e.g. one joint and one tendon) and post here, we will help you debug.

1 reply

Infraviored Nov 24, 2021
Author

okay i did. Running this with
cd ~/.mujoco && ./mujoco200/bin/simulate /src/minimal.xml
if of relevance.

Infraviored · 2021-11-24T16:15:40Z

Infraviored
Nov 24, 2021
Author

Created a minimal model.
red tendon is attached to a motor, green one to a muscle. Green can be controlled, red can't

``

<default>
    <geom rgba=".9 .7 .1 1" size="0.01"/>
    <site type="sphere" rgba=".9 .9 .9 1" size="0.005"/>
    <joint type="hinge" axis="0 1 0" limited="true" range="-30 60" solimplimit="0.95 0.95 0.1"/>
</default>

<visual>
    <headlight diffuse=".7 .7 .7"/>
</visual>

<worldbody>
    <site name="s0" pos="-0.03 0.01 0.4"/>
    <site name="s1" pos="-0.03 -0.01 0.4"/>

  <body>
  <geom type="capsule" fromto="0 0 0.3  0.2 0 0.3"/>
        <geom name="g1" type="cylinder" fromto="0.0 0.015 0.3  0.0 -0.015 0.3"
                size="0.02" rgba=".3 .9 .3 .4"/>
    <joint pos="0 0 0.3"/>
        <site name="s2" pos="0.1 0 0.31"/>
  </body>
</worldbody>

<tendon>
    <spatial name="red" width="0.002" rgba=".95 .3 .3 1" limited="true" range="0 0.33">
        <site site="s0"/>
        <geom geom="g1"/>
        <site site="s2"/>
    </spatial>
    <spatial name="green" width="0.002" rgba=".3 .95 .3 1" limited="true" range="0 0.33">
        <site site="s1"/>
        <geom geom="g1"/>
        <site site="s2"/>
    </spatial>
</tendon>
<actuator>
    <motor name="motor" tendon="red"/>
    <muscle name="muscle" tendon="green"/>
</actuator>

``

0 replies

yuvaltassa · 2021-11-24T16:26:12Z

yuvaltassa
Nov 24, 2021
Maintainer

Your motor works just fine, it's just a bit weak so cannot pull your thing due to gravity.
Turn off gravity (in Disable Flags under the Physics tab) and you'll see that your motor works.
To increase the strength of your motor, increase the gear attribute to be bigger than 1.

<motor name="motor" tendon="red" gear="10"/>

1 reply

Infraviored Nov 24, 2021
Author

Got this working before my team meeting thanks to your help. Many thanks

i still do not understand two things
-why do the motors contract when applying negative control inputs (muscles contract for positive inputs)
-What is the physical property being controlled? I have now some unknown force*10(gear) projected onto [-1,1]

yuvaltassa · 2021-11-24T23:16:36Z

yuvaltassa
Nov 24, 2021
Maintainer

Muscles can only contract, because that's how muscles work. Positive input corresponds to efferent excitation. It could have been negative, it's just a convention.

Your motors are applying a force (implicitly in units of Newton) to the tendon, in the range [-10, 10].

2 replies

Infraviored Nov 26, 2021
Author

Thank you again.
I built another demo (block of mass 1 on vertical sliding joint) to show to my team that a gear of 10 is exactly what is needed to barely move the block (F=-9.81N)
I still do not understand forcerange. From the documentation i expect forcerange="-10 10" be equivalent to gear="10" but it is not. Forcerange has no effect yet.

yuvaltassa Nov 26, 2021
Maintainer

forcerange is just a clamping operator applied to the actuator output, just like ctrlrange is a clamping operator applied to the input. If e.g. you have gear="10" and forcerange = "-10 10", then for controls in [-1, 1] the force will change between -10 and 10 and outside [-1, 1] it will clamp to [-10, 10]. As explained here, the output of your actuator is in data.actuator_force.

GraceNiyo · 2023-05-08T22:59:51Z

GraceNiyo
May 8, 2023

Hello,
I have a follow-up on this question. I have a similar problem. I created a tendon attached to a box (ceiling) and a ball. The ball has one hinge joint. The actuator is a muscle and the muscle but the is muscle in uncontrollable. Here is my sample code. Any help is greatly appreciated.

<worldbody>
    <light diffuse=".5 .5 .5" pos="0 0 3" dir="0 0 -1"/>
    <geom type="plane" size="1 1 0.1" rgba=".9 0 0 1"/>

    <body name ="ceiling" pos = "0 0 0.4">
        <geom type= "box" size ="0.02 0.02 0.001" mass ="0.1"></geom>
        <site name="C_B" type = "sphere" pos = "0 0 0"/>
    </body>

    <body name= "ball" pos= "0 0 0.3">
      <geom type ="sphere" size = " 0.01" mass = "1" rgba ="0 1 1 1" ></geom>
      <site name="B_C" type = "sphere" pos = "0 0 0"/>
      <joint type = "hinge"  axis = "1 1 1" range = "-45 45"/>
     </body>
</worldbody>
<tendon>
    <spatial name = "tendon1" springlength="0.008" width="0.001" rgba ="1 1 1 1">
      <site  site= "B_C"> </site>
      <site site= "C_B"> </site>
    </spatial>
</tendon>
<actuator>
 <muscle name="muscle1" tendon="tendon1" lengthrange="0.001 1.6" force = "50" ></muscle>
</actuator>

1 reply

yuvaltassa May 12, 2023
Maintainer

I didn't load this, but I don't see how pulling on the tendon could achieve anything.

The hinge joint is at 0 0 0 and the site that you're pulling on is also at 0 0 0, how can it create motion with no moment arm? Did you mean to use <joint type="slide"/>?

GraceNiyo · 2023-05-12T23:03:15Z

GraceNiyo
May 12, 2023

Hi, Yes. The joint type is supposed to be "slide." Best, Niyo

…

On Fri, May 12, 2023 at 3:22 AM Yuval Tassa ***@***.***> wrote: I didn't load this, but I don't see how pulling on the tendon could achieve anything. The hinge joint is at 0 0 0 and the site that you're pulling on is also at 0 0 0, how can it create motion with no moment arm? Did you mean to use <joint type="slide"/>? — Reply to this email directly, view it on GitHub <https://urldefense.com/v3/__https://github.com/deepmind/mujoco/discussions/74*discussioncomment-5883449__;Iw!!LIr3w8kk_Xxm!pOmmjqMFjqsOGlU6jTLDujVyixU0_oC4BpiWDglE_1gposyYZuedHeMDCYu0hHUh5nh2g83V3HW1pE7Vn7j5$>, or unsubscribe <https://urldefense.com/v3/__https://github.com/notifications/unsubscribe-auth/AV63EDJBP4UPGY5YHSSUGVLXFYFOFANCNFSM5IWNEBKQ__;!!LIr3w8kk_Xxm!pOmmjqMFjqsOGlU6jTLDujVyixU0_oC4BpiWDglE_1gposyYZuedHeMDCYu0hHUh5nh2g83V3HW1pMQ5WJDj$> . You are receiving this because you commented.Message ID: ***@***.***>

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Tendon driven motor actuators don't work #74

Infraviored Nov 24, 2021

Replies: 6 comments · 5 replies

yuvaltassa Nov 24, 2021 Maintainer

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Infraviored
Nov 24, 2021

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Infraviored Nov 24, 2021
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Infraviored
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Infraviored Nov 24, 2021
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Infraviored Nov 26, 2021
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GraceNiyo
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