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Hello Sherm,

In the past, I had to deal with a special system requiring a MobilizedBody::Custom. In particular, the rotation of the body is a function of the generalized coordinates. Let's say, for example, that the mobilizer has 3 generalized coordinates (x,y,z) representing a 3D translation, and that the body rotation is a function of these, let's write Rotation=f(x,y,z).

I first tried finding scalar functions for each Euler angle (unwrapped to avoid jumps) and use a MobilizedBody::FunctionBased, but the rotation of the body can be any, so the interpolator was not good near singularities. Instead, I created a MobilizedBody::Custom, where f(x,y,z) actually defines a rotation matrix. That worked very well, but I am wondering if there is a "native" way of doing this with the supplied mobilized bodies, ideally without using constraints. At least, I have not found any!

Again, I am just asking out of curiosity. The ability of creating a custom mobilized body is immensely helpful.

Thank you very much for your insights!

Best regards,
Simone

Hi, Simone.

Congratulations on getting a custom mobilizer to work -- very nice!

I think you are correct that that's the only way to implement the singularity-free behavior you want. The fact that MobilizedBody::FunctionBased assumes 3 angles for rotation dooms it. (BTW that is itself a custom mobilizer anyway). I don't see how you could do this with any of the existing joint types. There are others that couple translation and rotation (e.g. a Screw) but none that would meet your need as you described it.

I presume you're familiar with our Mobilizer paper that describes the full theory, but in case someone else is interested it can be read here.

Regards,
Sherm

Hi Sherm,

Thank you for the immediate response!

Yes, I am familiar with your paper. I also relied on the advanced manual.

Best regards,
Simone