Placing a Joint on a Moving Center of Mass in Simscape Multibody

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.

[Steve Miller]

Hi Baris - why do you want to place a joint at the CG? Perhaps there is another way to get what you need without placing a joint.

--Steve

[Baris Ucarsoy]

Hi Steve, thank you for answering my question. Here is the thing, in simscape when I place a 6-DOF joint for a flying object, the center of rotation is always on the joint connection point. Since the center of rotation of a flying object is on its COM, I need the joint connection point always where the COM is. When the center of mass is stationary this is not a problem, I would put a 6-DOF joint on the COM and everything would be fine. But the rocket will lose mass and COM will move (but not the 6-DOF joint), what can I do for this situation?

I hope I have explained my problem clearly.

P.S.: For now I use Simulink only (with Aerospace Blockset) for the 6-DOF movement and it works fine. But I would reaaaaly love to do this with Simscape because it has amazing potential for this kind of job.

[Steve Miller]

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

[Baris Ucarsoy]

The 6-DOF joint is needed for connecting the rocket airframe to the world frame. I need unconstrained movement so I chose 6-DOF. I also need sensing ports from the joint so I can have feedback lines for changing drag and lift forces with respect to velocity. The problem is rocket airframe has a variable mass component (the motor), so during flight, my center of mass is moving upwards. I need my joint exactly connected to COM even when the COM is moving. The reason for that, to get an accurate stability response, rocket airframe needs to turn from COM.

From what I understand it is not yet possible to move joint location while in simulation. Thank you for your attention.

Answer 1

[Steve Miller]

To measure the location of the CG, you can use the Inertia Sensor block.

https://www.mathworks.com/help/physmod/sm/ref/inertiasensor.html

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.

1. Measure the change in CG location using the Inertia sensor.
2. Apply that motion to a joint (you may need a very small mass at the end of that joint)
3. Apply the forces and torques to that frame.

--Steve

[Baris Ucarsoy]

Thank you Steve, brilliant, I think I can make that work.

[alberto corvi]

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]

To avoid the algebraic loop, you either have to

1. Break the using a transfer function loop OR
2. 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