MobilizedBody::Custom to off-the-shelf solution?

[Simone Hendriks]

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

[Michael Sherman]

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

[Simone Hendriks]

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