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Inertial and unstability

asked 2015-10-02 11:28:38 -0500

debz gravatar image

updated 2015-10-08 13:55:08 -0500

Hi everyone,

I build a robotic arm (made of cynliders) without setting the inertials first. Everything was working well using a PID to control it. Then I set up the inertials using the list on wikipedia:


It appears that the values I get are very small and the model became unstable (all links are exploding in every directions). The different pieces of the arm are mostly like this.

  • radius = 0.025
  • length = 0.5
  • mass = 0.1

So their inertial are:

  • ixx = (mass/12)*(3*radius^2+length^2)
  • iyy = (mass/12)*(3*radius^2+length^2)
  • izz = mass*radius^2/2

I don't get why it is not stable. So here are my questions:

  • 1) Do you have any idea why it is not stable ?
  • 2) Do you know what happens if we set the inertials to zero ?
  • 3) Do you know what happens when we don't set the inertials ?


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answered 2015-10-02 16:14:36 -0500

scpeters gravatar image

If you are using ODE, can you try increasing the solver iterations?


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Yes thx I tried but nothing changed ! I multiplied it by 10 (50->500 iterations). By the way, do you know what are the default parameters of the inertials ?

debz gravatar imagedebz ( 2015-10-08 13:57:30 -0500 )edit

answered 2015-10-08 14:44:33 -0500

debz gravatar image

Given this topic, thevalues I get might be too small and interpreted as zeros... Anyone has an idea how to deal with that.

One says "Unfortunately, this usually gives a matrix that is rounded to all zeroes, so you might have to scale up your model, compute the inertia tensor, and divide the result by an appropriate factor.". But if you scale up your inertials parameters, then it will affect the behaviour of PIDs right ?


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Asked: 2015-10-02 11:28:38 -0500

Seen: 188 times

Last updated: Oct 08 '15