Problem with the robot model collapsing [closed]

asked 2015-08-18 21:46:29 -0500

longforgotten
55 ●2 ●5 ●8

updated 2016-06-06 10:45:34 -0500

I'm a beginner in the robotics/ROS/Gazebo world. I started to work in a robotics research lab in my university, and I have to model an Hexapod. Apparently, the URDF Xacro is alright: the visuals, collisions, inertia tensors, COM, mass, joint tags, etc..

But when I spawn the model on GazeboSim, it doesn't get immovable. Instead, it get agitated. I thought is was caused by a bad inertia tensor, so I fixed the meshes from the original solid parts, calculated the mass properties and replaced the faulty inertial tags and meshes. But this didn't solve the problem. When I was recording the screen to post this question, I have noticed that the most I increased the effort limit, the most it became agitated. Here's the video that shows this behavior. And you can see, in the _rostopic_ terminal, that the effort value applied to the joint is the max value (positive or negative) defined in the robot URDF Xacro file, and I don't know why this is happening.

Here's the robot description.

What can be made to solve this problem? I'm getting crazy with this haha

Thanks :D

Closed for the following reason the question is answered, right answer was accepted by longforgotten
close date 2015-08-19 20:18:37.467125

answered 2015-08-19 01:29:24 -0500

Stefan Kohlbrecher

24361 ●173 ●299 ●444 https://www.energy-rob...

updated 2016-01-19 02:43:19 -0500

Real-time physics simulation is hard to make stable for every use-case a user throws at it. I also experienced quite a few cases of physically correct (or at least plausible) models having tremors or just exploding. A combination of these knobs makes things work for me most of the time:

Adjust controller gains. Note that per default, joint velocities in Gazebo often exhibit high jitter, so I'd recommend to use low D gains.
Adjust inertial parameters of the model. Too high differences between masses and inertia tensors of different links can lead to instabilities that make the physics engine fail (in my experience current standard Gazebo versions have to be restarted in that case). Of course, it's not really desirable to change the model to have inertial parameters the real thing does not have, but this can often cause things to work. What you can do is make a specialized model for Gazebo with (hopefully slightly) adjusted parameters, but keep using the correct model in other applications.
Adjust the physics engine parameters (time step and iterations) so the physics are more accurate, but slower.

See also useful answers here: Controller makes robot unstable,Gazebo model jittering.

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Thanks for your time! I'll do that and post the results here in the comments.

longforgotten ( 2015-08-19 09:50:43 -0500 )edit

I've noticed that the robot PID gains was not properly tuned, so I dropped down the PID values and the Hexapod has stabilized. My professor has commented about the Ziegler-Nichols method, so I'll use that to tune properly the PID controllers. Once again thanks, your answer helped me a lot!

longforgotten ( 2015-08-19 18:18:02 -0500 )edit

@emersonfs , Can you explain how you 'disabled' PID? It can be done by software or parameters can be set. (One of the most popular methods is to set I and D to 0 and controlling gain and offset with P. That will enhance the quality of answer by a lot.)

PrasadNR ( 2016-12-21 05:27:12 -0500 )edit

@PrasadNR I didn't disable PID, I set the PID values to a much much lower value than it was before.

longforgotten ( 2016-12-21 09:37:44 -0500 )edit

Thank you guys, my collapsed to the origin, then I modified the inertial matrix and Done... Inertia was my actual problem.

OsorioAlpha ( 2020-03-26 14:20:24 -0500 )edit

@OsorioAlpha can you please elaborate on what you meant when you said "I modified the inertial matrix" how did you change your values?

I actually built a small arm out of MDF and servos and weighed everything to the gram and I believe I have calculated my inertias correctly but my model is collapsed to gravity and jitters enough to slowly move itself across the map.

matthewmarkey ( 2020-05-12 06:33:34 -0500 )edit

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Problem with the robot model collapsing [closed]

Closed for the following reason the question is answered, right answer was accepted by longforgotten
close date 2015-08-19 20:18:37.467125

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Problem with the robot model collapsing [closed] edit

Closed for the following reason the question is answered, right answer was accepted by longforgotten close date 2015-08-19 20:18:37.467125

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Problem with the robot model collapsing [closed]

Closed for the following reason the question is answered, right answer was accepted by longforgotten
close date 2015-08-19 20:18:37.467125