Forward dynamics problem in robotics systems toolbox

Hey,

I want to use model-based control scheme (such as sliding mode control, etc.) in kuka iiwa820. Main process is that continuously receive the target pose, calculate the error between current configuration and target pose, then calculate the joint torque to the dynamic model to update the current configuration.

To this purpose, i firstly did some tests and encountered some problems.

1. i expect to use the forward dynamics block to calculate the joint acceleration with iiwa820, and after integral i expect get the updated velocity and the updated joint position (configration) corresponding to the input joint torque. First the input torque is simply set by the "inverseDynamics" command. Indeed, it has achieved good results and the position is not change. However, if i slightly change the input （e.g. torque(4)=14.359387500001654 to 14.359487500001654), just change 0.0001，then the dynamic will change a lot. i don't know how to fix this problem. in my view, this change is so small and the joint configuration will not change a lot. If the controller is added, the torque will change more.
2. is there any forward kinematics method to calculate the end effector position? Or can it only be calculated through DH parameters
3. Is there any general idea to achieve my purpose？(use model-based control to control iiwa820)

Thank you very much for any suggestions！

Answer 1

[Karsh Tharyani]

1. It is hard to reason the sensitivity of dynamics without looking at the full robotic system and your simulated model.
2. You can use the "getTransform" function of a "rigidBodyTree" to compute the homogeneous transformation matrix of a given body with respect to anothe given body on the tree, given the rigid body tree's joint configuration vector. Documentation can be found here: https://www.mathworks.com/help/robotics/ref/rigidbodytree.gettransform.html
3. I would reach out to Technical Support https://www.mathworks.com/support/contact_us.html for further questions and perhaps provide some more details of your workflow so that we can convey them to the development team at The MathWorks.

Hope that helps,

Karsh