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Why the robot gripper can not pick up the object in Simscape with Contact Forces Library ?

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snow John
snow John on 16 May 2020
Answered: Juan Sagarduy on 4 Nov 2020
I was learning to use the Simscape Multibody Contact Forces Library to simulate a demo for robot pick and place.I used the library and modeled it according to the below examples:https://ww2.mathworks.cn/matlabcentral/fileexchange/61370-robot-arm-with-conveyor-belts.
But neither method was successful .The TestPuma560.slx was built according to the Robot Arm with Conveyor Belts files.
And,The PumaGripper.slx was built according to the openManipulatorWaypointTracking.slx example.
During the simulation, I used Slider Gain to change the gripper to make it can grip of the ball.Both methods I have tried , and modify the damping parameters, but still report errors.I have uploaded my files and hope someone can help me solve this problem.
Best wishes!
-John

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Jose Avendano Arbelaez
Jose Avendano Arbelaez on 24 Aug 2020
John,
If the error occurs only after the robot contacts an object then it is likely that one of the stiffness set in either contact parameters or a hardstop, is too high for the solver to compute. Try using Simscape variable viewer and exploring forces on different joints. You might find signals that look like an impulse (Very quick, possibly infinite step). There are several things you could try adjusting to prevent the error:
  • Contact parameters (Here is another answer on this)
  • Solver settings (to have small enough steps that can capture the dynamics of your collision)
  • Actuation methods and signals (If your joint forces/torques are currently automatically computed then these could be a cause for the solver not to be able to calculate the necessary dynamics within the defualt time step)
Hope this helps.

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Accepted Answer

Juan Sagarduy
Juan Sagarduy on 4 Nov 2020
Hi John
I would recommend that you use the Spatial Force Contact block we added to Simscape Multibody in latest releases. It is likely that the simulation of contact is handled better and more robustly. It is a bit of work to replace the Contact Force Lib components, but I am sure it'll pay off in the long run.
Cheers
Juan

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