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Nonzero joint velocities for static position

asked 2021-10-14 02:27:56 -0500

RamilK gravatar image

Hello everyone! I am using ROS Melodic with Gazebo 9 and working with Unitree A1 quadruped robot model. Recently I found that joint_states shows considerable non-zero joint velocities when robot stands on the ground in static position. When I plot joint velocities in Gazebo values are also non-zero. However when I hung up the robot and use the same servo velocities are zero. I use unitree_legged_control package as a servo controller.

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Seeing this:

When I plot joint velocities in Gazebo values are also non-zero.

makes me believe this is a problem in Gazebo, not the ROS integration.

Gazebo is not a part of ROS, it's a stand-alone piece of software for which plugins are available which make it usable with ROS.

If Gazebo itself reports non-zero velocities, the plugins will just copy those over into ROS JointState messages.

You should ask this question over at answers.gazebosim.org.

If/when you do, please post a comment here with a link to your new question there so we keep things connected.

gvdhoorn gravatar image gvdhoorn  ( 2021-10-14 02:47:37 -0500 )edit

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answered 2021-10-14 06:17:36 -0500

Mike Scheutzow gravatar image

Having non-zero velocity value reported for a joint is expected. You should see this on the real robot as well. If a joint is fighting gravity to support the weight of the robot above it, it's not really "static". That joint motor is moving slightly.

For your application, of course, you can decide that a velocity below some threshold value is "zero velocity".

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Well, it depends on how much non-zero it is. I have 0.1 rad/s, which is pretty high deviation.

RamilK gravatar image RamilK  ( 2021-10-14 09:34:25 -0500 )edit

Yes, 0.1 sounds high, but I know nothing about your hardware. I see 0.007 rad/sec on one of our robot arms.

Mike Scheutzow gravatar image Mike Scheutzow  ( 2021-10-14 11:28:09 -0500 )edit

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Asked: 2021-10-14 02:27:56 -0500

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Last updated: Oct 14 '21