I am working on implementing a closed loop vibration control of a spring mass system of 3 DOFs. I have modelled the system using MASS42 and COMBIN14 elements as attached. m1=m2=m3=1 Kg and Spring constant is 300 N/m throughout.I have modelled the masses as areas (1mx1m) and the four nodes at the corners have been coupled to act as a rigid body along X direction with NSETs 1,2 and 3 for the three masses respectively. The modal and transient analysis are working fine. But when I try to perform the closed loop PID control algorithm it gives me the following message:
"Specified constraint not allowed on slave degree of freedom in coupled set. Coupled set number 1 First node= 1 Slave node = =4 degree of freddom = UX. Move constraint to the first node or remove coupling and constrain all".
I had initially given boundary condition as all nodes UY and UZ=0 and Node on the leftmost side as ALL DOF=0.
I tried doing the same using MASS21 element but I don't know how to give 1mx1m area to it so that I can make it's mass=1Kg.
Can someone tell me where I am doing wrong? Any help is very much appreciated. Thank you.
I think the reason is that constraints can only be applied to master nodes. However, in your model, a constraint is applied to some DOF of a slave node.
Probably this is not a complex problem. However, since problems appear during the PID control, can you show some details of the PID control analysis, such as how the force and constraints are applied to the model?
Sorry to reply a bit late. Actually I have overcome the problem by using point masses using MASS21 element. However I still don't understand the problem to my earlier set up. For control I am using a simple rate feedback control by applying a step response to the 1st mass by applying an initial displacement on the lower left corner of the first mass. Subsequently displacements from the other masses are obtained and with some requisite gain is fed back as the actuation force to the node 1. My script snippet is as follows
*get,x2,node,1,u,x
*get,x2,node,5,u,x ! 5 is the node for the lower left corner of 2nd mass area
*get,x3,node,9,u,x ! 9 is the node for the lower left corner of 3rd mass area
v1=g1*(x1-x1p)/dt
v2=g2*(x2-x2p)/dt
v3=g3*(x3-x3p)/dt
err=1-(v1 + v2 + v3)
actu = err
d,1,ux,actu
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