Placing a Joint on a Moving Center of Mass in Simscape Multibody
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Baris Ucarsoy
on 1 Nov 2021
Commented: Steve Miller
on 1 Dec 2022
Hi, I wonder if there is any way to place a joint on the center of mass of a mechanism. But here is the catch, the center of mass is moving. I am trying to model a flying rocket and rocket mass varies by time. As a consequence COM is moving (upwards). Simcape lets me see where the center of mass relative to a selected frame with the inertia sensor but neither I can manipulate the mechanism's COM nor I can place a joint fixed with respect to the mechanism's COM.
Is there any solution for this?
Thanks.
4 Comments
Steve Miller
on 6 Nov 2021
Hi Baris,
You have described how a joint is connected to a frame on a body.
What do you plan to do with that joint? Why do you need the joint?
--Steve
Accepted Answer
Steve Miller
on 9 Nov 2021
You may need to superimpose this measurement onto another measurement.
To apply forces to a CG that is moving, you can use an External Torque and Force block.
- Measure the change in CG location using the Inertia sensor.
- Apply that motion to a joint (you may need a very small mass at the end of that joint)
- Apply the forces and torques to that frame.
--Steve
3 Comments
alberto corvi
on 15 Nov 2022
I cannot make this solution work. I have a similar problem where basically i need to sense the velocity of the Center of Gravity (CoG) of the system and possibly apply forces directly there. I tried to attach a 6-DoF Joint to the CoG but I haven't yet managed to do it. I cannot understand how to assign the motion of the CoG (sensed with INERTIA SENSOR) to the 6-DoF Joint block. I tried 'de-muxing' the signal but algebraic loops form and there seems to be no solution (Mux/Demux for physical signals - MATLAB Answers - MATLAB Central (mathworks.com)).
Steve Miller
on 1 Dec 2022
To avoid the algebraic loop, you either have to
- Break the using a transfer function loop OR
- Do not use the joint. Instead, apply the force at the same point always, but also apply an equivalent moment to compensate for the fact that the force is not being applied at the CG.
--Steve
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