How can I apply actuation to multiple joints in sequence?

I'm using Simscape Multibody (2nd gen) to model linkage that has two inputs (linkage is made of revolute joints and one elastic linear joint - aka a linear spring). First the linkage is oriented (rotated) and then a lever is turned which extends the linear spring linkage. While the linkage is positioned all of the other joints must be free to rotate. Then the lever is given a prescribed rotation to recover the torque needed to compress the spring.

Here's the initial orientation:

And here it is after it has been positioned:

The problem I'm having is that if I want to include an applied motion to the lever (dark component at the top) it is applied from the start of the simulation, but I need the lever to be free util the linkage gets into position.

Let me know if you need more information/clarification.

Dan Stuewe

[Dhruvesh Patel]

It would be helpful in understanding your question if you mark the joints/links according to the names used in the description text, show the motion using directional arrows and state which joints/links are fixed. Alternatively you can share a simplified labeled kinematic diagram for the linkage.

Answer 1

[Steve Miller]

It seems like you want to have prescribed motion for the second phase of a simulation and free motion for the first part. You cannot change an the motion status of joint mid simulation. The joint defining the degree of freedom must not have prescribed motion.

Your options:

1. During the first phase of simulation, apply no torque/force. For the second phase of simulation where you want prescribed motion, use a control system to apply the correct amount of torque/force. Tuned properly, the controller will have negligible error.
2. Have a series of 3 joints all along/about the same degree of freedom. One is the joint for the lever which moves freely, one has prescribed motion, and the third one is a variable damper. When prescribed motion should start, increase the damping of the variable damper to a very high level. This will ensure the other two degrees of freedom will move together (with some small error as the variable damper will slide a bit).

--Steve