Electronic – the main reason for using speed feedback in a motor positioning control system

In theory, one could use a BLDC motor as a 3-winding (six-phase) stepper motor, controlling rotation speed by controlling the frequency with which the windings are driven. Indeed, for certain situations where would be using a motor at only a fraction of its rated power (e.g. because the cheapest motor one could get was much bigger than necessary) this approach will work fine. A major problem with this approach, however, is that when a motor is driven at a fixed speed, a reduction in mechanical output torque will not cause a corresponding reduction in current draw. Instead, it will cause a massive increase in the amount of heat dissipated by the motor. Although stepper motors are generally able to safely dissipate 100% of rated power as heat, BLDC motors generally aren't.

One might regard the situation as being vaguely analogous to regulating the speed of an automobile purely by using the clutch and brake pedals while the engine was always running at wide open throttle. True, one could for a little while make the car go any desired speed using just two of the three pedals (and if one was going uphill, one could do it with the clutch alone), but fuel economy would be dreadful, and unless one added a truly massive amount of extra cooling to the clutch it would be destroyed in minutes.

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The block diagram is simplified industrial drive position control loop. For your app. you can ommit certain stages, you would neeed current controller PI and velocity controller PI. From your open loop trajectory planner you calculate the velocity according to your formula, then you give a velocity setpoint to the velocity controller.

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Now if you own a functional BLDC FOC kit, this should be no problem since the current loop and velocity is yet implemented. What you might add is the torque (current) feedforward. As you said \$M=J\alpha+M_{load}\$ we can anticipate the dynamic torque setpoint before the velocity lags the setpoint. So there is a torque feedforward path that forwards the torque setpoint "a priori". \$M_{ffwd}=J\alpha=\frac{d\omega}{dt}J;\; I_{ffwd}=M_{ffwd}*k_i;\; k_i[A/Nm]\$
\$I_{ffwd}=\frac{d\omega}{dt}J*k_i\$

Since the velocity loop "a posteriori" can't immediately follow the dynamic change in speed, there should be introduced a first order LP filter before the the velocity PI controller (it's ommited in the picture), to delay the action until there is a system response due to the feedforward loop action.

The other thing is a simple trajectory calculator, which calculates a ramp. This should be a recursive algorithm that increments the speed and checks limits each itteration loop. This part doesn't have any feedback from real speed or position, it just computes in open loop. It's all up to regulation loops to follow the speed setpoint.