I have seen in the documentation of an old subcontractor's product that the controller of a robotic arm joint was basically feeding error position + error rate directly into one single PID.
It feels wrong to have two different variations mixed before acting on the mix, but I may be wrong given that, after all, speed is directly linked to position.
Is it correct? What are the drawbacks to that, for systems where both positions and speeds need to be tracked? Should state-space MISO controllers be used instead?