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Author Topic: Simmechanics Inverse and Forward Dynamics  (Read 2585 times)

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Offline oldskierTopic starter

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Simmechanics Inverse and Forward Dynamics
« on: March 01, 2011, 09:27:39 AM »
Hello everyone,

I beg your help, because I've been fighting this problem for more than a week without any particular result...

So, I have a Simulink block-diagram representing 5-links with 5 revolute joints robot-manipulator. First I solve Inverse Dynamics, motion is given to the model as a Nx3 vector with motion, velocity and acceleration, N - is number of steps. Normally I take 1 sec time period with 100 steps. In a simpliest case motion is rotation of 1st joint for pi/2 radians with acceleration within first 0.1 sec, constant motion within interval [0.1, 0.9], and decelearation within last 0.1 sec.
The result of Inverse Dynamics is a portion of computed torques on every joint. This part of simulation goes smoothly.
Next I want to perform Forward Dynamics with torques I recieved in previous step, just to check if model works well. And here I have a problem. Results of Forward Dynamics do not match initial motion I set in the beginning, although if I put a sensor on a joint drawing torques, the torques on output match torques on input. So I quess that everything should work... But it doesn't.

Please someone help me. Many thanks in advance.

PS: Here is the link for the model (for a reduced model actually - only 3 joints)
http://rapidshare.com/files/450423499/Simmech.zip
File Initial_motion.mat contains Nx3 vector of motion for the first joint. Load this file first.
File robomy_q123 contains simulink model for Inverse Dynamics. After running it, run file Q_fnc.m, which converts torques in function of time.
Than run simulink file robomy_q123_torq, which solves Forward dynamics.
Maybe it would help.

Offline kloutan

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Re: Simmechanics Inverse and Forward Dynamics
« Reply #1 on: April 29, 2011, 02:34:26 PM »
Hey,
i am having a similar problem. I actually broke down the model to a simple 2dof planar robot. I am having the same issue as you. Also, I derived the dynamic equations by hand and when simulated, got the same result for torques as the simmechanics model when theta, theta_dot and theta_double_dot is specified. When doing the reverse however, i.e when  constant torques are specified at the joints, graphs of  theta, theta_dot and theta_double_dot vs time are dissimilar to that from the simmechanics model. I am using MATLAB 6.5 and is baffled thus far.

 


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