Actuators in robot model

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Actuators in robot model

caron
Hi there,

I have a question regarding actuators in OpenRAVE: how do they change the mass properties in the dynamic model?

Can I remove them from my robot model, or are they used somewhere else in OpenRAVE?

By actuators, I mean the fields that look like this in a Collada file:

    <extra name="MY_JOINT" type="attach_actuator">
        <technique profile="OpenRAVE">
            <instance_actuator url="#actuator20"/>
            <bind_actuator joint="kmodel1/jointsid1"/>
        </technique>
    </extra>

While filing a bug today where ComputeInverseDynamics() returns wrong results due to actuators, I noticed that they actually change the mass properties of each link.

To see how they affect the model, I use norm(G) (the third term returned by ComputeInverseDynamics) and the norm of the COM jacobian. Here are the logs:

    # starting point
    norm(G) = 144.560927875
    norm(Jcom) = 1.76364237076

    # commenting one actuator
    norm(G) = 144.25879048
    norm(Jcom) = 1.76099226084

    # commenting a second actuator
    norm(G) = 144.611886893  # note that it went up
    norm(Jcom) = 1.76023420616

    # commenting a third actuator
    norm(G) = 143.360810669
    norm(Jcom) = 1.75756680734

So, actuators have a mass (because they change norm(Jcom)) and they are affecting the results of ComputeInverseDynamics()... How?
Stéphane Caron
https://scaron.info
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Re: Actuators in robot model

Rosen Diankov
Administrator
actually actuators don't have mass, they just store a bunch of properties.

i replied the reason to the github ticket, let me know if you have more problems.

best,


2016-05-18 0:26 GMT+09:00 caron <[hidden email]>:
Hi there,

I have a question regarding actuators in OpenRAVE: how do they change the
mass properties in the dynamic model?

Can I remove them from my robot model, or are they used somewhere else in
OpenRAVE?

By actuators, I mean the fields that look like this in a Collada file:

    <extra name="MY_JOINT" type="attach_actuator">
        <technique profile="OpenRAVE">
            <instance_actuator url="#actuator20"/>
            <bind_actuator joint="kmodel1/jointsid1"/>
        </technique>
    </extra>

While  filing a bug today <https://github.com/rdiankov/openrave/issues/402>
where ComputeInverseDynamics() returns wrong results due to actuators, I
noticed that they actually change the mass properties of each link.

To see how they affect the model, I use norm(G) (the third term returned by
ComputeInverseDynamics) and the norm of the COM jacobian. Here are the logs:

    # starting point
    norm(G) = 144.560927875
    norm(Jcom) = 1.76364237076

    # commenting one actuator
    norm(G) = <a href="tel:144.25879048" value="+14425879048">144.25879048
    norm(Jcom) = 1.76099226084

    # commenting a second actuator
    norm(G) = 144.611886893  # note that it went up
    norm(Jcom) = 1.76023420616

    # commenting a third actuator
    norm(G) = 143.360810669
    norm(Jcom) = 1.75756680734

So, actuators have a mass (because they change norm(Jcom)) and they are
affecting the results of ComputeInverseDynamics()... How?



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bring their own devices (BYOD) to work are irked by the imposition of MDM
restrictions. Mobile Device Manager Plus allows you to control only the
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Re: Actuators in robot model

caron
Hi Rosen,

Thanks for your straight-to-the-point answer :)

I didn't notice the change in DOF order, so yes, it one-shots this question.

However, it doesn't solve the discussion on github, where the actuator I commented out was for an inactive joint (=> DOF order unchanged).

Best,

Stéphane Caron
https://scaron.info
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