Position actuators' control range stuck in degrees (even with the compiler set to radians) #249

schoolrobot · 2022-04-23T13:26:20Z

schoolrobot
Apr 23, 2022

Hi!

Specifying a control range for position actuators in radians has minimal effect in moving the joints after loading a model. The compiler is set in radians, as are the joint range and ctrlrange.

As explained here, joint ranges are converted to radians after compilation, while ctrl and ctrlrange specify raw values and are not converted.
However specifying control range in degrees (much higher values than radians) allows a visible effect.

I tested this on the humanoid xml to verify it was not specific to my model.

On a side note, high gear values results in the simulation is unstable warning, and resets the model repeatedly. This only started after changing from motor to position actuators.
As a result, the gears were scaled down to 10% of the original (20 from 200).

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 you may not use this file except in compliance with the License.
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-->

<compiler inertiafromgeom="true" angle="radian"/>

<default>
    <joint limited="true" damping="1" armature="0"/>
    <geom condim="1" material="matgeom"/>
    <position ctrlrange="-4 4" ctrllimited="true"/>
</default>

<option timestep="0.005" iterations="500" tolerance="1e-10" solver="Newton" jacobian="dense" cone="pyramidal"/>

<size nconmax="500" njmax="200" nstack="10000"/>

<visual>
    <map force="0.1" zfar="30"/>
    <rgba haze="0.15 0.25 0.35 1"/>
    <quality shadowsize="2048"/>
    <global offwidth="800" offheight="800"/>
</visual>

<asset>
    <texture type="skybox" builtin="gradient" rgb1="0.3 0.5 0.7" rgb2="0 0 0" width="512" height="512"/>

    <texture name="texplane" type="2d" builtin="checker" rgb1=".2 .3 .4" rgb2=".1 0.15 0.2" width="512" height="512" mark="cross" markrgb=".8 .8 .8"/>

    <texture name="texgeom" type="cube" builtin="flat" mark="cross" width="127" height="1278"
        rgb1="0.8 0.6 0.4" rgb2="0.8 0.6 0.4" markrgb="1 1 1" random="0.01"/>

    <material name="matplane" reflectance="0.3" texture="texplane" texrepeat="1 1" texuniform="true"/>

    <material name="matgeom" texture="texgeom" texuniform="true" rgba="0.8 0.6 .4 1"/>
</asset>

<worldbody>
    <geom name="floor" pos="0 0 0" size="0 0 .25" type="plane" material="matplane" condim="3"/>

    <light directional="false" diffuse=".2 .2 .2" specular="0 0 0" pos="0 0 5" dir="0 0 -1" castshadow="false"/>
    <light mode="targetbodycom" target="torso" directional="false" diffuse=".8 .8 .8" specular="0.3 0.3 0.3" pos="0 0 4.0" dir="0 0 -1"/>

    <body name="torso" pos="0 0 1.4">
        <freejoint name="root"/>
        <geom name="torso1" type="capsule" fromto="0 -.07 0 0 .07 0"  size="0.07"/>
        <geom name="head" type="sphere" pos="0 0 .19" size=".09"/>
        <geom name="uwaist" type="capsule" fromto="-.01 -.06 -.12 -.01 .06 -.12" size="0.06"/>
        <body name="lwaist" pos="-.01 0 -0.260" quat="1.000 0 -0.002 0" >
            <geom name="lwaist" type="capsule" fromto="0 -.06 0 0 .06 0"  size="0.06" />
            <joint name="abdomen_z" type="hinge" pos="0 0 0.065" axis="0 0 1" range="-3 3" damping="5" stiffness="20" armature="0.02" />
            <joint name="abdomen_y" type="hinge" pos="0 0 0.065" axis="0 1 0" range="-3 1" damping="5" stiffness="10" armature="0.02" />
            <body name="pelvis" pos="0 0 -0.165" quat="1.000 0 -0.002 0" >
                <joint name="abdomen_x" type="hinge" pos="0 0 0.1" axis="1 0 0" range="-35 35" damping="5" stiffness="10" armature="0.02" />
                <geom name="butt" type="capsule" fromto="-.02 -.07 0 -.02 .07 0"  size="0.09" />
                <body name="right_thigh" pos="0 -0.1 -0.04" >
                    <joint name="right_hip_x" type="hinge" pos="0 0 0" axis="1 0 0" range="-25 5"   damping="5" stiffness="10" armature="0.01" />
                    <joint name="right_hip_z" type="hinge" pos="0 0 0" axis="0 0 1" range="-60 35"  damping="5" stiffness="10" armature="0.01" />
                    <joint name="right_hip_y" type="hinge" pos="0 0 0" axis="0 1 0" range="-120 20" damping="5" stiffness="20" armature="0.01" />
                    <geom name="right_thigh1" type="capsule" fromto="0 0 0 0 0.01 -.34"  size="0.06" />
                    <body name="right_shin" pos="0 0.01 -0.403" >
                        <joint name="right_knee" type="hinge" pos="0 0 .02" axis="0 -1 0" range="-160 -2" stiffness="1" armature="0.0060" />
                        <geom name="right_shin1" type="capsule" fromto="0 0 0 0 0 -.3"   size="0.049" />
                        <body name="right_foot" pos="0 0 -.39" >
                            <joint name="right_ankle_y" type="hinge" pos="0 0 0.08" axis="0 1 0"   range="-50 50" stiffness="4" armature="0.0008" />
                            <joint name="right_ankle_x" type="hinge" pos="0 0 0.04" axis="1 0 0.5" range="-50 50" stiffness="1"  armature="0.0006" />
                            <geom name="right_foot_cap1" type="capsule" fromto="-.07 -0.02 0 0.14 -0.04 0"  size="0.027" />
                            <geom name="right_foot_cap2" type="capsule" fromto="-.07 0 0 0.14  0.02 0"  size="0.027" />
                        </body>
                    </body>
                </body>
                <body name="left_thigh" pos="0 0.1 -0.04" >
                    <joint name="left_hip_x" type="hinge" pos="0 0 0" axis="-1 0 0" range="-25 5"  damping="5" stiffness="10" armature="0.01" />
                    <joint name="left_hip_z" type="hinge" pos="0 0 0" axis="0 0 -1" range="-60 35" damping="5" stiffness="10" armature="0.01" />
                    <joint name="left_hip_y" type="hinge" pos="0 0 0" axis="0 1 0" range="-120 20" damping="5" stiffness="20" armature="0.01" />
                    <geom name="left_thigh1" type="capsule" fromto="0 0 0 0 -0.01 -.34"  size="0.06" />
                    <body name="left_shin" pos="0 -0.01 -0.403" >
                        <joint name="left_knee" type="hinge" pos="0 0 .02" axis="0 -1 0" range="-160 -2" stiffness="1" armature="0.0060" />
                        <geom name="left_shin1" type="capsule" fromto="0 0 0 0 0 -.3"   size="0.049" />
                        <body name="left_foot" pos="0 0 -.39" >
                            <joint name="left_ankle_y" type="hinge" pos="0 0 0.08" axis="0 1 0"   range="-50 50"  stiffness="4" armature="0.0008" />
                            <joint name="left_ankle_x" type="hinge" pos="0 0 0.04" axis="1 0 0.5" range="-50 50"  stiffness="1"  armature="0.0006" />
                            <geom name="left_foot_cap1" type="capsule" fromto="-.07 0.02 0 0.14 0.04 0"  size="0.027" />
                            <geom name="left_foot_cap2" type="capsule" fromto="-.07 0 0 0.14  -0.02 0"  size="0.027" />
                        </body>
                    </body>
                </body>
            </body>
        </body>
        <body name="right_upper_arm" pos="0 -0.17 0.06" >
            <joint name="right_shoulder1" type="hinge" pos="0 0 0" axis="2 1 1"  range="-85 60" stiffness="1" armature="0.0068" />
            <joint name="right_shoulder2" type="hinge" pos="0 0 0" axis="0 -1 1" range="-85 60" stiffness="1"  armature="0.0051" />
            <geom name="right_uarm1" type="capsule" fromto="0 0 0 .16 -.16 -.16"  size="0.04 0.16" />
            <body name="right_lower_arm" pos=".18 -.18 -.18" >
                <joint name="right_elbow" type="hinge" pos="0 0 0" axis="0 -1 1" range="-90 50"  stiffness="0" armature="0.0028" />
                <geom name="right_larm" type="capsule" fromto="0.01 0.01 0.01 .17 .17 .17"  size="0.031" />
                <geom name="right_hand" type="sphere" pos=".18 .18 .18"  size="0.04"/>
            </body>
        </body>
        <body name="left_upper_arm" pos="0 0.17 0.06" >
            <joint name="left_shoulder1" type="hinge" pos="0 0 0" axis="2 -1 1" range="-60 85" stiffness="1" armature="0.0068" />
            <joint name="left_shoulder2" type="hinge" pos="0 0 0" axis="0 1 1" range="-60 85"  stiffness="1" armature="0.0051" />
            <geom name="left_uarm1" type="capsule" fromto="0 0 0 .16 .16 -.16"  size="0.04 0.16" />
            <body name="left_lower_arm" pos=".18 .18 -.18" >
                <joint name="left_elbow" type="hinge" pos="0 0 0" axis="0 -1 -1" range="-90 50" stiffness="0" armature="0.0028" />
                <geom name="left_larm" type="capsule" fromto="0.01 -0.01 0.01 .17 -.17 .17"  size="0.031" />
                <geom name="left_hand" type="sphere" pos=".18 -.18 .18"  size="0.04"/>
            </body>
        </body>
    </body>
</worldbody>

<actuator>
    <position name="abdomen_y"       gear="20" joint="abdomen_y" />
    <position name="abdomen_z"       gear="20" joint="abdomen_z" />
    <position name="abdomen_x"       gear="20.2" joint="abdomen_x" />
    <!-- <position name="right_hip_x"     gear="200" joint="right_hip_x" />
    <position name="right_hip_z"     gear="200" joint="right_hip_z" />
    <position name="right_hip_y"     gear="600" joint="right_hip_y" />
    <position name="right_knee"      gear="400" joint="right_knee" />
    <position name="right_ankle_x"   gear="100" joint="right_ankle_x" />
    <position name="right_ankle_y"   gear="100" joint="right_ankle_y" />
    <position name="left_hip_x"      gear="200" joint="left_hip_x" />
    <position name="left_hip_z"      gear="200" joint="left_hip_z" />
    <position name="left_hip_y"      gear="600" joint="left_hip_y" />
    <position name="left_knee"       gear="400" joint="left_knee" />
    <position name="left_ankle_x"    gear="100" joint="left_ankle_x" />
    <position name="left_ankle_y"    gear="100" joint="left_ankle_y" />
    <position name="right_shoulder1" gear="100" joint="right_shoulder1" />
    <position name="right_shoulder2" gear="100" joint="right_shoulder2" />
    <position name="right_elbow"     gear="200" joint="right_elbow" />
    <position name="left_shoulder1"  gear="100" joint="left_shoulder1" />
    <position name="left_shoulder2"  gear="100" joint="left_shoulder2" />
    <position name="left_elbow"      gear="200" joint="left_elbow" /> -->
</actuator>

`

Answered by yuvaltassa

Apr 25, 2022

Regarding radians and degrees, you are correct, this setting only applies to angles in the kinematic tree. Actuator values like ctrlrange are always in native units.

As explained here, gear scales the length of the actuator (imagine a mechanical gear mechanism between the joint and the actuator), which for position type actuators means that it changes the units and therefore the semantics of your position command. E.g., for your first actuator you are using units of radian/20. I am guessing that what you are looking for is actually the kp attribute (position feedback gain) of the position actuator.

View full answer

yuvaltassa · 2022-04-25T13:17:45Z

yuvaltassa
Apr 25, 2022
Maintainer

Regarding radians and degrees, you are correct, this setting only applies to angles in the kinematic tree. Actuator values like ctrlrange are always in native units.

As explained here, gear scales the length of the actuator (imagine a mechanical gear mechanism between the joint and the actuator), which for position type actuators means that it changes the units and therefore the semantics of your position command. E.g., for your first actuator you are using units of radian/20. I am guessing that what you are looking for is actually the kp attribute (position feedback gain) of the position actuator.

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Position actuators' control range stuck in degrees (even with the compiler set to radians) #249

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Position actuators' control range stuck in degrees (even with the compiler set to radians) #249

schoolrobot Apr 23, 2022

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yuvaltassa Apr 25, 2022 Maintainer

schoolrobot
Apr 23, 2022

yuvaltassa
Apr 25, 2022
Maintainer