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How to simulate/create a spring in an already generated Block Diagram of a CAD Model in Simscape Multibody?

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Hello there,
i am trying to simulate a double wishbone suspension. For that, i created a CAD Model in Solidworks and exported it to Simulink.
The wishbone connected with a spring damper, is the part i am having trouble with. In fact how can I simulate the spring itself? Since the spring and damper component can not be visualized, how do I add the needed joints or blocks to the already generated block diagram of the imported CAD Model?
The requirements are that, the spring and damper component is not represented by a cylinder/piston (like shown in various videos) so only by a spring.
So again, how do i connect a spring with the wishbones so that the wishbone can "oscillate". Which joint(s) is/are required?

Accepted Answer

Yifeng Tang
Yifeng Tang on 1 May 2021
You'll need to first define how the two ends of the spring is connected to the wishbone part and the vehicle body, including the locations, orientations, and joint types. The locations and orientations is defined by Rigid Tranform blocks, and the joint types are probably two ball joints or one ball joint and one revolute joint.
Then the longitudinal motion of the spring can be constrained using prismatic or cylindrial joints. From there, you have two options to model the spring & damper force: (1) inside the joint itself, OR (2) couple with Simscape translational mechanical domain in 1D. For (1), you need to specify the "internal mechanics" in the joint, while for (2) you need to enable the force actuation and velocity sensing in the joint and a translational multibody interface block.
Yifeng Tang
Yifeng Tang on 6 May 2021
You were pretty close actually. So here are the list of minor fixes I put in:
  1. replace telescope joints with spherical. longitudinal DOF not necessary.
  2. replace revolute joint with spherical. direction and location were too hard to determine by hand for the revolute joint. If you have CAD data, it will be easier.
  3. add mass to both ends of the prismatic joint, to avoid zero mass degenerate system. Also made it ugly so it's very noticeable.
  4. Turn on simscape data logging, run the model, to find out the initial position of the prismatic joint.
  5. Use initial position of prismatic joint to set up internal mechanism with a spring. stiffness and damping randomly picked.
Now the model runs. The lower wishbone oscillates to a stop but the upper one is swinging. I suppose you need some kind of mechanical connection to bind the two wishbones together.

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