Issues with Tendon Position Control Accuracy, Actuator Coupling, and Force Sensing Implementation #2566

Kelly-234 · 2025-04-10T05:36:27Z

Kelly-234
Apr 10, 2025

Intro

Hi!
I'm a student and I'm using mujoco to build a model of three-section Cable Driven Robots.

My setup

MuJoCo 3.2.6 (x64 Windows 11)

My question

Model Overview:

3-DOF fully tendon-driven robotic arm model with cross-axis joints. The base segment is fixed at x=0, with tendon anchor points at x=-0.14. Using position control.
Each segment controlled by 3 evenly distributed tendons.
Tendon stiffness="100000"(to matching real-world stiffness).
Control signal range: [-0.14, 0.14] (matching slider range [-0.2, 0.2]).

Issues Encountered:

Unexpected Joint Discrepancy

Expected: Joint displacement = Control signal + Δtendon_length (with shortening as +)(perfect Joint displacement = Control signal)
Observed: Actual joint displacements deviate from this relationship
Question: What factors might cause this discrepancy between control signals and physical displacements?

2025-04-10.131502_20250410_13202592.mp4

-0.0153-(0.326838-0.325)=-0.017138≠-0.019
0.00391-(0.326955-0.325)=0.001955≠0

Unintended Actuator Coupling

Expected Behavior: Adjusting one actuator should only affect its corresponding joint
Current Behavior: All joints move simultaneously during single-actuator adjustment
Attempted Fix:

actuatorfrcrange="-1000 1000" actuatorfrclimited="true"

Result: failed.

2025-04-10.133100_20250410_13315723_20250410_13330311_20250410_13341680.mp4

Tendon Force Sensing Limitations

Challenge: No built-in sensors for distributed tendon forces
Current Workaround: Assume actuator_force ≈ tendon_force (valid only with negligible friction)
Need: Accurate force measurement with friction present
Request: Any methods to implement:
- Endpoint force measurement
- Distributed force monitoring along tendons

Minimal model and/or code that explain my question

<worldbody>
    <light pos="0 0 1" dir="0 0 -1" directional="true" diffuse="0.8 0.8 0.8" specular="0.5 0.5 0.5"/>
    <body pos="-.14 0 .05">
    <!-- <body pos="-.14 0 .05" mocap="true"> -->
        <geom type="sphere" size=".005" rgba=".55 .25 .25 .9"/>
        <joint name="s1_1" type="slide" actuatorfrcrange="-1000 1000" actuatorfrclimited="true" axis="1 0 0" limited="true" range="-.2 .2"/>
        <site name="s1_1"/>
    </body>
    <body pos="-.14 .043 -.025">
        <geom type="sphere" size=".005" rgba=".55 .25 .25 .9"/>
        <joint name="s1_2" type="slide" actuatorfrcrange="-1000 1000" actuatorfrclimited="true" axis="1 0 0" limited="true" range="-.2 .2"/>
        <site name="s1_2"/>
    </body>
    <body pos="-.14 -.043 -.025">
        <geom type="sphere" size=".005" rgba=".55 .25 .25 .9"/>
        <joint name="s1_3" type="slide" actuatorfrcrange="-1000 1000" actuatorfrclimited="true" axis="1 0 0" limited="true" range="-.2 .2"/>
        <site name="s1_3"/>
    </body>
    <body pos="-.14 .032 .038">
        <geom type="sphere" size=".005" rgba=".25 .45 .35 .9"/>
        <joint name="s2_1" type="slide" actuatorfrcrange="-1000 1000" actuatorfrclimited="true" axis="1 0 0" limited="true" range="-.2 .2"/>
        <site name="s2_1"/>
    </body>
    <body pos="-.14 .017 -.047">
        <geom type="sphere" size=".005" rgba=".25 .45 .35 .9"/>
        <joint name="s2_2" type="slide" actuatorfrcrange="-1000 1000" actuatorfrclimited="true" axis="1 0 0" limited="true" range="-.2 .2"/>
        <site name="s2_2"/>
    </body>
    <body pos="-.14 -.049 .009">
        <geom type="sphere" size=".005" rgba=".25 .45 .35 .9"/>
        <joint name="s2_3" type="slide" actuatorfrcrange="-1000 1000" actuatorfrclimited="true" axis="1 0 0" limited="true" range="-.2 .2"/>
        <site name="s2_3"/>
    </body>
    <body pos="-.14 .049 .009">
        <geom type="sphere" size=".005" rgba=".7 .6 .35 .9"/>
        <joint name="s3_1" type="slide" actuatorfrcrange="-1000 1000" actuatorfrclimited="true" axis="1 0 0" limited="true" range="-.2 .2"/>
        <site name="s3_1"/>
    </body>
    <body pos="-.14 -.017 -.047">
        <geom type="sphere" size=".005" rgba=".7 .6 .35 .9"/>
        <joint name="s3_2" type="slide" actuatorfrcrange="-1000 1000" actuatorfrclimited="true" axis="1 0 0" limited="true" range="-.2 .2"/>
        <site name="s3_2"/>
    </body>
    <body pos="-.14 -.032 .038">
        <geom type="sphere" size=".005" rgba=".7 .6 .35 .9"/>
        <joint name="s3_3" type="slide" actuatorfrcrange="-1000 1000" actuatorfrclimited="true" axis="1 0 0" limited="true" range="-.2 .2"/>
        <site name="s3_3"/>
    </body>
    <body name="link1" pos="0 0 0">
        <geom name="l1_1" class="link_left"/>
        <geom name="l1_2" class="link_middle"/>
        <geom name="l1_3" class="link_right"/>

        <site class="drive_rope" name="s1_l1_1_l" pos=".005 0 .05"/>
        <site class="drive_rope" name="s1_l1_1_r" pos=".115 0 .05"/>
        <site class="drive_rope" name="s1_l1_2_l" pos=".005 .043 -.025"/>
        <site class="drive_rope" name="s1_l1_2_r" pos=".115 .043 -.025"/>
        <site class="drive_rope" name="s1_l1_3_l" pos=".005 -.043 -.025"/>
        <site class="drive_rope" name="s1_l1_3_r" pos=".115 -.043 -.025"/>

        <site class="drive_rope" name="s2_l1_1_l" pos=".005 .032 .038"/>
        <site class="drive_rope" name="s2_l1_1_r" pos=".115 .032 .038"/>
        <site class="drive_rope" name="s2_l1_2_l" pos=".005 .017 -.047"/>
        <site class="drive_rope" name="s2_l1_2_r" pos=".115 .017 -.047"/>
        <site class="drive_rope" name="s2_l1_3_l" pos=".005 -.049 .008"/>
        <site class="drive_rope" name="s2_l1_3_r" pos=".115 -.049 .008"/>

        <site class="drive_rope" name="s3_l1_1_l" pos=".005 .049 .009"/>
        <site class="drive_rope" name="s3_l1_1_r" pos=".115 .049 .009"/>
        <site class="drive_rope" name="s3_l1_2_l" pos=".005 -.017 -.047"/>
        <site class="drive_rope" name="s3_l1_2_r" pos=".115 -.017 -.047"/>
        <site class="drive_rope" name="s3_l1_3_l" pos=".005 -.032 .038"/>
        <site class="drive_rope" name="s3_l1_3_r" pos=".115 -.032 .038"/>

        <body name="joint1" pos=".15 0 0">
            <joint name="joint1_pitch" class="joint_pitch"/>
            <joint name="joint1_yaw" class="joint_yaw"/>
            <geom name="j1_1" class="visual_joint_pitch"/>
            <geom name="j1_2" class="visual_joint_yaw"/>
            <geom name="j1_3" class="visual_joint"/>

            <body name="link2" pos=".03 0 0">
                <geom name="l2_1" class="link_left"/>
                <geom name="l2_2" class="link_middle"/>
                <geom name="l2_3" class="link_right"/>

                <site class="drive_rope" name="s1_l2_1_l" pos=".005 0 .05"/>
                <site class="drive_rope" name="s1_l2_2_l" pos=".005 .043 -.025"/>
                <site class="drive_rope" name="s1_l2_3_l" pos=".005 -.043 -.025"/>

                <site class="drive_rope" name="s2_l2_1_l" pos=".005 .032 .038"/>
                <site class="drive_rope" name="s2_l2_1_r" pos=".115 .032 .038"/>
                <site class="drive_rope" name="s2_l2_2_l" pos=".005 .017 -.047"/>
                <site class="drive_rope" name="s2_l2_2_r" pos=".115 .017 -.047"/>
                <site class="drive_rope" name="s2_l2_3_l" pos=".005 -.049 .008"/>
                <site class="drive_rope" name="s2_l2_3_r" pos=".115 -.049 .008"/>

                <site class="drive_rope" name="s3_l2_1_l" pos=".005 .049 .009"/>
                <site class="drive_rope" name="s3_l2_1_r" pos=".115 .049 .009"/>
                <site class="drive_rope" name="s3_l2_2_l" pos=".005 -.017 -.047"/>
                <site class="drive_rope" name="s3_l2_2_r" pos=".115 -.017 -.047"/>
                <site class="drive_rope" name="s3_l2_3_l" pos=".005 -.032 .038"/>
                <site class="drive_rope" name="s3_l2_3_r" pos=".115 -.032 .038"/>

                <body name="joint2" pos=".15 0 0">
                    <joint name="joint2_pitch" class="joint_pitch"/>
                    <joint name="joint2_yaw" class="joint_yaw"/>
                    <geom name="j2_1" class="visual_joint_pitch"/>
                    <geom name="j2_2" class="visual_joint_yaw"/>
                    <geom name="j2_3" class="visual_joint"/>

                    <body name="link3" pos=".03 0 0">
                        <geom name="l3_1" class="link_left"/>
                        <geom name="l3_2" class="link_middle"/>
                        <geom name="l3_3" class="link_right"/>

                        <site class="drive_rope" name="s2_l3_1_l" pos=".005 .032 .038"/>
                        <site class="drive_rope" name="s2_l3_2_l" pos=".005 .017 -.047"/>
                        <site class="drive_rope" name="s2_l3_3_l" pos=".005 -.049 .008"/>

                        <site class="drive_rope" name="s3_l3_1_l" pos=".005 .049 .009"/>
                        <site class="drive_rope" name="s3_l3_1_r" pos=".115 .049 .009"/>
                        <site class="drive_rope" name="s3_l3_2_l" pos=".005 -.017 -.047"/>
                        <site class="drive_rope" name="s3_l3_2_r" pos=".115 -.017 -.047"/>
                        <site class="drive_rope" name="s3_l3_3_l" pos=".005 -.032 .038"/>
                        <site class="drive_rope" name="s3_l3_3_r" pos=".115 -.032 .038"/>

                        <body name="joint3" pos=".15 0 0">
                            <joint name="joint3_pitch" class="joint_pitch"/>
                            <joint name="joint3_yaw" class="joint_yaw"/>
                            <geom name="j3_1" class="visual_joint_pitch"/>
                            <geom name="j3_2" class="visual_joint_yaw"/>
                            <geom name="j3_3" class="visual_joint"/>

                            <body name="link4" pos=".03 0 0">
                                <geom name="l4_1" class="link_left"/>
                                <geom name="l4_2" class="link_middle"/>
                                <geom name="l4_3" class="link_right"/>

                                <site class="drive_rope" name="s3_l4_1_l" pos=".005 .049 .009"/>
                                <site class="drive_rope" name="s3_l4_2_l" pos=".005 -.017 -.047"/>
                                <site class="drive_rope" name="s3_l4_3_l" pos=".005 -.032 .038"/>

                                <body name="end" pos=".115 0 0">
                                    <geom type="sphere" size=".01" rgba=".25 .35 .55 1"/>
                                    <site name="end_pos"/>
                                </body>
                            </body>
                        </body>
                    </body>
                </body>
            </body>
        </body>
    </body>
</worldbody>

<tendon>
    <spatial name="tendon1_1" range=".325 1" limited="true" width=".002" rgba="1 .4 .4 .4" frictionloss=".1" stiffness="100000">
        <site site="s1_1"/>
        <site site="s1_l1_1_l"/>
        <site site="s1_l1_1_r"/>
        <site site="s1_l2_1_l"/>
    </spatial>
</tendon>

<tendon>
    <spatial name="tendon1_2" range=".325 1" limited="true" width=".002" rgba="1 .4 .4 .4" frictionloss=".1" stiffness="100000">
        <site site="s1_2"/>
        <site site="s1_l1_2_l"/>
        <site site="s1_l1_2_r"/>
        <site site="s1_l2_2_l"/>
    </spatial>
</tendon>

<tendon>
    <spatial name="tendon1_3" range=".325 1" limited="true" width=".002" rgba="1 .4 .4 .4" frictionloss=".1" stiffness="100000">
        <site site="s1_3"/>
        <site site="s1_l1_3_l"/>
        <site site="s1_l1_3_r"/>
        <site site="s1_l2_3_l"/>
    </spatial>
</tendon>

<tendon>
    <spatial name="tendon2_1" range=".505 1" limited="true" width=".002" rgba=".4 1 .4 .4" frictionloss=".1" stiffness="100000">
        <site site="s2_1"/>
        <site site="s2_l1_1_l"/>
        <site site="s2_l1_1_r"/>
        <site site="s2_l2_1_l"/>
        <site site="s2_l2_1_r"/>
        <site site="s2_l3_1_l"/>
    </spatial>
</tendon>

<tendon>
    <spatial name="tendon2_2" range=".505 1" limited="true" width=".002" rgba=".4 1 .4 .4" frictionloss=".1" stiffness="100000">
        <site site="s2_2"/>
        <site site="s2_l1_2_l"/>
        <site site="s2_l1_2_r"/>
        <site site="s2_l2_2_l"/>
        <site site="s2_l2_2_r"/>
        <site site="s2_l3_2_l"/>
    </spatial>
</tendon>

<tendon>
    <spatial name="tendon2_3" range=".505 1" limited="true" width=".002" rgba=".4 1 .4 .4" frictionloss=".1" stiffness="100000">
        <site site="s2_3"/>
        <site site="s2_l1_3_l"/>
        <site site="s2_l1_3_r"/>
        <site site="s2_l2_3_l"/>
        <site site="s2_l2_3_r"/>
        <site site="s2_l3_3_l"/>
    </spatial>
</tendon>

<tendon>
    <spatial name="tendon3_1" range=".685 1" limited="true" width=".002" rgba="1 .84 .4 .4" frictionloss=".1" stiffness="100000">
        <site site="s3_1"/>
        <site site="s3_l1_1_l"/>
        <site site="s3_l1_1_r"/>
        <site site="s3_l2_1_l"/>
        <site site="s3_l2_1_r"/>
        <site site="s3_l3_1_l"/>
        <site site="s3_l3_1_r"/>
        <site site="s3_l4_1_l"/>
    </spatial>
</tendon>

<tendon>
    <spatial name="tendon3_2" range=".685 1" limited="true" width=".002" rgba="1 .84 .4 .4" frictionloss=".1" stiffness="100000">
        <site site="s3_2"/>
        <site site="s3_l1_2_l"/>
        <site site="s3_l1_2_r"/>
        <site site="s3_l2_2_l"/>
        <site site="s3_l2_2_r"/>
        <site site="s3_l3_2_l"/>
        <site site="s3_l3_2_r"/>
        <site site="s3_l4_2_l"/>
    </spatial>
</tendon>

<tendon>
    <spatial name="tendon3_3" range=".685 1" limited="true" width=".002" rgba="1 .84 .4 .4" frictionloss=".1" stiffness="100000">
        <site site="s3_3"/>
        <site site="s3_l1_3_l"/>
        <site site="s3_l1_3_r"/>
        <site site="s3_l2_3_l"/>
        <site site="s3_l2_3_r"/>
        <site site="s3_l3_3_l"/>
        <site site="s3_l3_3_r"/>
        <site site="s3_l4_3_l"/>
    </spatial>
</tendon>

<actuator>
    <position joint="s1_1" name="pos_ctrl1_1" kp="50000" kv="100" ctrllimited="true" ctrlrange="-.1 .1"/>
    <position joint="s1_2" name="pos_ctrl1_2" kp="50000" kv="100" ctrllimited="true" ctrlrange="-.1 .1"/>
    <position joint="s1_3" name="pos_ctrl1_3" kp="50000" kv="100" ctrllimited="true" ctrlrange="-.1 .1"/>
    <position joint="s2_1" name="pos_ctrl2_1" kp="50000" kv="100" ctrllimited="true" ctrlrange="-.1 .1"/>
    <position joint="s2_2" name="pos_ctrl2_2" kp="50000" kv="100" ctrllimited="true" ctrlrange="-.1 .1"/>
    <position joint="s2_3" name="pos_ctrl2_3" kp="50000" kv="100" ctrllimited="true" ctrlrange="-.1 .1"/>
    <position joint="s3_1" name="pos_ctrl3_1" kp="50000" kv="100" ctrllimited="true" ctrlrange="-.1 .1"/>
    <position joint="s3_2" name="pos_ctrl3_2" kp="50000" kv="100" ctrllimited="true" ctrlrange="-.1 .1"/>
    <position joint="s3_3" name="pos_ctrl3_3" kp="50000" kv="100" ctrllimited="true" ctrlrange="-.1 .1"/>
</actuator>

<sensor>
    <framepos name="end_pos" objtype="site" objname="end_pos"/>
</sensor>

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Issues with Tendon Position Control Accuracy, Actuator Coupling, and Force Sensing Implementation #2566

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Issues with Tendon Position Control Accuracy, Actuator Coupling, and Force Sensing Implementation #2566

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Kelly-234 Apr 10, 2025

Intro

My setup

My question

Minimal model and/or code that explain my question

Confirmations

Replies: 0 comments

Kelly-234
Apr 10, 2025