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getForces #569

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costashatz opened this issue Dec 9, 2015 · 6 comments
Closed

getForces #569

costashatz opened this issue Dec 9, 2015 · 6 comments

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@costashatz
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Hello,

I am simulating a hexapod robot (18dof). I want to get my robot's force readings on every joint. I tried using the following:

  • using either velocity or force actuators
    • skeleton's getForces
    • joint's getForce

None of them seem to work. I am getting all zeros. Any ideas?
@mxgrey
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mxgrey commented Dec 9, 2015

I'll have to ask a few questions in order to help you debug your situation.

  • Are you using Gazebo while simulating, or are you using DART directly?
  • If you're using DART directly, are you calling these functions within a simulation loop? If you're running a dart::simulation::World using DART directly, could you toss the World into a dart::gui::SimWindow and provide a screenshot of what the simulation looks like?
  • Does your simulation world have any kind of object representing the ground or floor? If there is no floor, then your robot will simply fall forever in the simulation, and its joint torques may be zero if you're using velocity actuators with a command of 0.
  • If you are not simulating, and you don't want to run a simulation, then you'll need to call the function Skeleton::computeInverseDynamics() before the joint force information will be populated with anything.

@costashatz
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Are you using Gazebo while simulating, or are you using DART directly?

DART directly.

If you're using DART directly, are you calling these functions within a simulation loop? If you're running a dart::simulation::World using DART directly, could you toss the World into a dart::gui::SimWindow and provide a screenshot of what the simulation looks like?

I am using dart::simulation::World. The robot moves fine (simple gaits etc). No weird things.

Does your simulation world have any kind of object representing the ground or floor?

Yes, I have a floor.

I found 2 possible solutions:

  • Use dart::simulation::World::step(false)
  • Use Skeleton::computeInverseDynamics()

But I am getting different results. With the first solution I am getting 0 force for the first 6 DOF (6d pose), which makes sense, whereas with the second one I am not. What should I use?

@costashatz
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To make it more clear:

  • Use dart::simulation::World::step(false) and then use skeleton->getForces().
  • Use World::step(), skeleton->computeInverseDynamics() and finally skeleton->getForces().

@mxgrey
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mxgrey commented Dec 9, 2015

You'll almost certainly want to use the first solution, because that will accurately represent the joint forces that were present during the simulation. The default behavior for step is to reset all the joint commands (which may include joint forces) to zero so that your system is passive by default. To ensure that you get the joint forces after a step is complete, you'll want to pass in false. At that point, there should be no need to call computeInverseDynamics, although if you do call that function, then you'll probably want to pass in true for all its parameters so that it accounts for everything that is present in the simulation (like external forces).

@costashatz
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Thank you very much! Once again you've been very helpful.

@diegoferigo diegoferigo mentioned this issue Oct 9, 2020
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@omerkayan
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omerkayan commented Jan 18, 2023 •
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Hi @costashatz,

I have similar problem with you. Could you please help me? I am always getting zeros.

I have a skeleton and I want to read forces by skeleton for two nodes and one joint.
Node1: FemurL
Node2: TibiaL
Joint1: Revolute joint between FemurL and TibiaL

</Node>
<Node name="TibiaL" parent="FemurL" >
    <Body type="Box" mass="3.0" size="0.1198 0.4156 0.1141 " contact="Off" color="0.6 0.6 1.5 1.0" obj="L_Tibia.obj">
        <Transformation linear="0.9994 0.0348 -0.0030 -0.0349 0.9956 -0.0871 -0.0 0.0872 0.9962 " translation="0.0928 0.3018 -0.0341 "/>
    </Body>
    <Joint type="Revolute" axis ="1.0 0.0 0.0" bvh="Character1_LeftLeg" lower="0.0" upper="2.3">
        <Transformation linear="1.0 0.0 0.0 0.0 1.0 0.0 0.0 0.0 1.0" translation="0.0995 0.5387 -0.0103 "/>
    </Joint>
</Node>

This is my way to read forces in Joint1:
force_on_joint1= *(mCharacter->GetSkeleton()->getBodyNode("TibiaL")->getChildJoint(0)->getForces().data());

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3 participants
@costashatz @mxgrey @omerkayan