Equality connect constrain for a UPS- Parallel robot act unexpectedly #2294

AmirSamanMirjalili · 2024-12-16T16:01:45Z

AmirSamanMirjalili
Dec 16, 2024

Intro

Hi!

I am a undergrad student , I use MuJoCo for my research on Hyper Redundant parallel robots that has a stack of following 3UPS (universal-prismatic-spherical)

.

My setup

<default>
    <joint limited="true" />
    <motor ctrllimited="true" ctrlrange="-100 100" />
</default>

<asset>
    <material name="platform_base" rgba="0.2 0.3 0.4 1" />
    <material name="platform_top" rgba="0.4 0.5 0.6 1" />
    <material name="leg_outer" rgba="0.7 0.7 0.7 1" />
    <material name="leg_inner" rgba="0.6 0.6 0.6 1" />
    <material name="joint" rgba="0.5 0.1 0.1 1" />
    <material name="ground" rgba="0.5 0.5 0.5 1" specular="0.3" shininess="0.8"/>
</asset>

<visual>
    <global offwidth="1920" offheight="1080" />
    <quality shadowsize="4096" />
    <map shadowscale="0.5" />
</visual>

<worldbody>
    <!-- Lighting -->
    <light name="spotlight" pos="0 0 3" dir="0 0 -1" castshadow="true"/>
    <light name="frontlight" pos="2 2 2" dir="-1 -1 -1" castshadow="false"/>
    
    <!-- Ground plane -->
    <geom type="plane" material="ground" size="2 2 0.1" pos="0 0 -0.1"/>
    
    <!-- Fixed base platform -->
    <body name="base" pos="0 0 0">
        <geom type="cylinder" size="0.3 0.02" material="platform_base" />
        <geom type="cylinder" size="0.31 0.01" pos="0 0 0.015" material="platform_base" />
        
        <!-- First leg assembly -->
        <body name="p1_universal" pos="0.25 0 0">
            <joint name="p1_ux" type="hinge" axis="1 0 0" range="-45 45"/>
            <joint name="p1_uy" type="hinge" axis="0 1 0" range="-45 45"/>
            <geom type="sphere" size="0.02" material="joint" contype="0" conaffinity="0"/>
            
            <!-- Outer part of telescopic leg -->
            <body name="leg1_outer">
                <geom type="cylinder" fromto="0 0 0 0 0 0.3" size="0.015" material="leg_outer"/>
                
                <!-- Inner part of telescopic leg -->
                <body name="leg1_inner">
                    <joint name="p1_prismatic" type="slide" axis="0 0 1" range="0 0.3"/>
                    <geom type="cylinder" fromto="0 0 0 0 0 0.3" size="0.01" material="leg_inner"/>
                    <site name="leg1_top" pos="0 0 0.3" size="0.02" rgba="0.5 0.1 0.1 0.5"/>
                </body>
            </body>
        </body>

        <!-- Second leg assembly -->
        <body name="p2_universal" pos="-0.125 0.217 0">
            <joint name="p2_ux" type="hinge" axis="1 0 0" range="-45 45"/>
            <joint name="p2_uy" type="hinge" axis="0 1 0" range="-45 45"/>
            <geom type="sphere" size="0.02" material="joint" contype="0" conaffinity="0"/>
            
            <!-- Outer part of telescopic leg -->
            <body name="leg2_outer">
                <geom type="cylinder" fromto="0 0 0 0 0 0.3" size="0.015" material="leg_outer"/>
                
                <!-- Inner part of telescopic leg -->
                <body name="leg2_inner">
                    <joint name="p2_prismatic" type="slide" axis="0 0 1" range="0 0.3"/>
                    <geom type="cylinder" fromto="0 0 0 0 0 0.3" size="0.01" material="leg_inner"/>
                    <site name="leg2_top" pos="0 0 0.3" size="0.02" rgba="0.5 0.1 0.1 0.5"/>
                </body>
            </body>
        </body>

        <!-- Third leg assembly -->
        <body name="p3_universal" pos="-0.125 -0.217 0">
            <joint name="p3_ux" type="hinge" axis="1 0 0" range="-45 45"/>
            <joint name="p3_uy" type="hinge" axis="0 1 0" range="-45 45"/>
            <geom type="sphere" size="0.02" material="joint" contype="0" conaffinity="0"/>
            
            <!-- Outer part of telescopic leg -->
            <body name="leg3_outer">
                <geom type="cylinder" fromto="0 0 0 0 0 0.3" size="0.015" material="leg_outer"/>
                
                <!-- Inner part of telescopic leg -->
                <body name="leg3_inner">
                    <joint name="p3_prismatic" type="slide" axis="0 0 1" range="0 0.3"/>
                    <geom type="cylinder" fromto="0 0 0 0 0 0.3" size="0.01" material="leg_inner"/>
                    <site name="leg3_top" pos="0 0 0.3" size="0.02" rgba="0.5 0.1 0.1 0.5"/>
                </body>
            </body>
        </body>
    </body>

    <!-- Moving top platform -->
    <body name="top_platform" pos="0 0 0.5">
        <joint type="free"/>
        <geom type="cylinder" size="0.3 0.02" material="platform_top" />
        <geom type="cylinder" size="0.31 0.01" pos="0 0 -0.015" material="platform_top" />
        <site name="platform_attach1" pos="0.25 0 0" size="0.02" rgba="0.5 0.1 0.1 0.5"/>
        <site name="platform_attach2" pos="-0.125 0.217 0" size="0.02" rgba="0.5 0.1 0.1 0.5"/>
        <site name="platform_attach3" pos="-0.125 -0.217 0" size="0.02" rgba="0.5 0.1 0.1 0.5"/>
    </body>
</worldbody>

<equality>
    <!-- Connect legs to platform with ball joints -->
    <connect body1="leg1_inner" body2="top_platform" anchor="0.25 0 0.5"/>
    <connect body1="leg2_inner" body2="top_platform" anchor="-0.125 0.217 0.5"/>
    <connect body1="leg3_inner" body2="top_platform" anchor="-0.125 -0.217 0.5"/>
</equality>

<actuator>
    <!-- Motor actuators for the prismatic joints -->
    <motor joint="p1_prismatic" name="leg1_motor" gear="1"/>
    <motor joint="p2_prismatic" name="leg2_motor" gear="1"/>
    <motor joint="p3_prismatic" name="leg3_motor" gear="1"/>
</actuator>

My question

The problem that I encountered is shown in the following image. my spherical joints do not attach to my moving platform. do you have any suggestions on how I can solve this? in general, if I assume that the inertia of my legs can be neglected what do you suggest to use in order to mimic the prismatic effect accurately and I can command the length of the joint (this is the second priority for now)

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

Dec 16, 2024

Try using the site-based connect definition. example

View full answer

yuvaltassa · 2024-12-16T16:36:17Z

yuvaltassa
Dec 16, 2024
Maintainer

Try using the site-based connect definition. example

3 replies

AmirSamanMirjalili Dec 17, 2024
Author

Thank you @yuvaltassa. I managed to fix this using your suggestion, but I have another question: I want to know how accurate the calculated Jacobian (mj_jac) is for these types of robots (which have closed chains). Can I rely on them? or should I use a different approach

yuvaltassa Dec 17, 2024
Maintainer

You cannot, they are incorrect. They don't take the constraint into account.

AmirSamanMirjalili Dec 17, 2024
Author

@yuvaltassa Thank you very much for your help

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Equality connect constrain for a UPS- Parallel robot act unexpectedly #2294

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Equality connect constrain for a UPS- Parallel robot act unexpectedly #2294

AmirSamanMirjalili Dec 16, 2024

Intro

My setup

My question

Confirmations

Replies: 1 comment · 3 replies

yuvaltassa Dec 16, 2024 Maintainer

AmirSamanMirjalili Dec 17, 2024 Author

yuvaltassa Dec 17, 2024 Maintainer

AmirSamanMirjalili Dec 17, 2024 Author

AmirSamanMirjalili
Dec 16, 2024

Replies: 1 comment 3 replies

yuvaltassa
Dec 16, 2024
Maintainer

AmirSamanMirjalili Dec 17, 2024
Author

yuvaltassa Dec 17, 2024
Maintainer

AmirSamanMirjalili Dec 17, 2024
Author