Planar movement of a robotic joint in a rididBodyTree

Hello everyone

After a lot of struggling on my own I decided that it is time to ask the experts.

For my project I am trying to develop a space robot that would have:

A base, that would be able to move in x and y axis as well as rotate about z axis.

Two robotic arms with 2 revolute joints.

I have created the simscape model, however I am facing issues with the movement of the base.

According to the documentation:

[robot,importInfo] = importrobot(model) imports a Simscape™ Multibody™ model and returns an equivalent rigidBodyTree object and information about the import in importInfo. Only fixed, prismatic, and revolute joints are supported in the output rigidBodyTree object.

So my attempts to import a body that would have a planar joint or a rectangular and revolute joint failed.

When i read the description of prismatic joint, it is said to only be able to move in z axis.

Is it possible to change it to x or y?

Would a combination of a prismatic join in x, prismatic joint in y and revolute joint be possible, and would it be a proper substitute for a planar joint?

I will appreciate all the help, I've been stuck on this issue for ages trying to get a workaround.

Answer 1

[Umang Pandey]

Hi Szymon,

As per my understanding, it appears that you're trying to import your Simscape Multibody model into a “rigidBodyTree” object using the “importrobot” function.

1. It is indeed correct that the "rigidBodyTree" object exclusively supports fixed, prismatic, and revolute joints.

2. When working with the "rigidBodyTree" object, the "JointAxis" property enables you to modify the joint axis. Here is an illustrative example of how to establish a prismatic joint and set the joint axis to the X-axis:

 joint = robotics.Joint('joint1', 'prismatic'); 
 joint.JointAxis = [1 0 0]; 

3. About the possibility of substituting a planar joint with a combination of two prismatic joints and a revolute joint, yes, this is indeed possible. Infact, this is how Simscape models a planar joint. You can refer to the following documentation for more information :

https://www.mathworks.com/help/sm/ref/simscape.multibody.planarjoint-class.html

Best Regards,

Umang