Errors in feedback control after setting the input of a prismatic joint to motion/provided by input

Hello, I am currently controlling the prismatic joint actuators of a Stewart platform using Simulink. I have set the input to motion/provided by input in order to change the position of the eight actuators over time. However, when I try to perform feedback control in the simulation, I encounter errors as shown in the image below. Feedback control works when the input is set to force, but why do these errors occur when the input is set to motion? If anyone knows the answer, I would be very grateful for your help.

Answer 1

[Divyanshu]

Hi @석준,

A possible reason for such errors is that an algebraic loop is formed in your model when input is set to motion.

For more information about identification and removal of algebraic loops have can refer the following links:

Identify Algebraic Loops in Your Model - MATLAB & Simulink (mathworks.com)

Remove Algebraic Loops - MATLAB & Simulink (mathworks.com)

Hope it helps!

[석준]

Thank you very much! By using the Unit Delay block, I was able to solve the algebraic loop problem in the feedback model.

[Yifeng Tang]

Hi @석준,

Unit Delay may work just fine, or create some numerical inefficiency, depending on the nature of the rest of your system. You may check the step sizes and solver resets using Solver Profiler and see if that's the case. One potential concern is mixing discrete block (Unit Delay) with continuous ones (Simscape). If indeed that gives you issues, consider replace the Unit Delay with a Transfer Fcn block, with a reasonable time constant.