Motion profile and Motion control scheme, how do they interact?

Question

I'm studying motion control and I start with the simplest case, I have to go from A to B in a straight line using a trapezoidal motion profile.

I understand that in a real case, due to the fact that the controller is discrete I need a position/velocity feedback to reach the desidered position with a good accuracy.

Reading some books I found that the most common control scheme is the one below, in which there are a controller for position, velocity and current (torque).

The questions I have is:

-> How can I use the control scheme with a given motion profile (for instance the trapezoidal) ?

I'm sure it's very trivial but I cannot solve.

At the time being, I understand that if I define the desired position for instance B (in the control scheme) I cannot use a custom motion profile but I'm restricted to the one the control system provide. So for instance if I'm in the deceleration stage (of the trapezoidal profile) the position/velocity/acceleration profiles will be provided by the control scheme and not but my needs.

I feel that what I understand is not correct but I don't know why: practically how to merge the two concepts.

Answer 1

These two concepts are complementary and you use them together, the motion profile providing the input to your control scheme. At each time-step the motion profile gives you the reference values for the control loop scheme (and also some feed forward values if needed).

This goes both for the acceleration and the deceleration phases. in both cases, the motion profile is generating the momentarily set-points at each timestep, which your closed loop control schemes tries to follow.

To understand the need for this you have to imagine a scenario where you want to get to point B, within a certain time (let's say 1 second). Your closed loop control loop is tuned for maximum performance, it would take you there in 0.1 seconds. This is when you need the motion profile, keep the closed loop controls at maximum performance but generate the set points accordingly. (In order to make sure the profile is executed smoothly a feed forward term is used at the acceleration and at the velocity level which is added as on offset at the summing blocks at your cascaded loop.)

The profile you have presented is a simple one (the simplest one which sees limited practical use), but this approach, trajectory planning, extended to mutiple axes and multiple waypoints and even higher degree polynomials is the basis for synchronizing high precision (e.g. CNC Machines) motion control. The profile is used to make sure the axes remain geometrically synchronized while moving with the desired on-path velocity.