MATLAB Answers


SimMechanics - Joint Actuator/Motion problem

Asked by Alexander
on 14 Feb 2011

Hello everyone,

i am using SimMechanics and want to accelerate a body in a specified direction. My joint is a "prismatic" joint, with the axis of action in x direction. Now i connected a "JointActuator" with the joint and configured it to "Actuate with: Motion". With a tripple "Mux", i can input the position and that is working fine. But when i enter a value for the speed or acceleration (an the other Mux ports empty), my body isn't moving at all! What am i doing wrong?

Thanks in advance, Alex


1 Answer

Answer by Martin
on 14 Feb 2011
 Accepted answer

When actuating motion, you should specify position, velocity, and acceleration (not or). When nothing is attached to an input, it sees that as being 0. Effectively what has happened is that you have commanded it to keep the position at 0 for the entire simulation.

If you know the desired acceleration, you can integrate it once for velocity and again for position. However, it is not recommended to use a numerical derivative directly when going from position to acceleration. Numerical derivatives are notoriously messy, and numerical second derivatives are a recipe for trouble.

I would recommend you take a look at the documentation here regarding stabilizing numerical derivatives.

  1 Comment

on 15 Feb 2011

Ok, thats what i thought. I can only set the position, so i have to integrate.
About the numerical derivative problem... i think ist doesn't matter in my case.

Thank you very much!

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