Translation after rotation of a body

[Laura Peeters]

Hi!

Let me first give a situation overview of my problem. I have a body (a tube) which needs to be able to rotate in 3 dimensions (x, y and z) and it needs to be able to translate along the z axis (this is the direction in line with the tube). But this translation should be around the current (after rotation) z axis of this body.

I'm using a free joint for this movement, but the translation of the body is always around the 'default' local axis of the body (see attachment for an illustration of the problem). I want this translation to be along the new z-axis after rotation of the body, so in line with the position of the tube. I've also tried to use a custom joint, but the problem is that I don't know the amount of translation that I need, so I can't define a function of the translation that comes with a certain rotation of the body.

Does anyone knows how I can solve this problem? Or how I have to define the joint?

Thanks in advance!

Regards Laura

[Michael Sherman]

Hi, Laura. There are several different ways to do this. By far the simplest would be to introduce a massless intermediate body. You would attach the massless body to the parent by a Ball joint, providing three rotational degrees of freedom. Then attach the tube by a Slider joint along the massless body's Z axis.

One thing to clarify is exactly how many degrees of freedom you want for the tube. I'm guessing 4, but that requires that the tube has reasonable inertia about its long axis. If it is essentially inertialess in that direction, you could model it with just 3 dofs (a U-joint plus a Slider). It is also possible that you want to give the tube 6 dofs but using a more convenient coordinate system than is provided by a Free joint; that wasn't clear from your post. In any case an intermediate massless (and inertialess) body is a great trick since, stripped of mass, a body is just a coordinate frame.

Would this approach work for your application?

Regards,
Sherm

[Laura Peeters]

Hi Sherm,

Thanks for your quick reply! It was very usefull. Creating a massless body solved my problem. (Or at least this part )

Regards,
Laura