Control Engineering Basics
A motor is controlled with cascade control, with a current (torque) loop inside and a speed loop outside. Which of the following is the correct design approach?
- AMake the inner current loop respond much faster than the outer speed loop, so that from the outside the inner loop can be regarded as an almost ideal current source
- BMake the outer speed loop faster than the inner one, so that the speed error is removed before the current loop acts
- CGive the inner and outer loops the same bandwidth so that the phase lags of the two loops cancel each other out, which makes the whole system the most stable
Show answer and explanation
Answer: A. Make the inner current loop respond much faster than the outer speed loop, so that from the outside the inner loop can be regarded as an almost ideal current source
In cascade control, the output of the outer loop (the torque command calculated by the speed controller) becomes the setpoint of the inner loop. If the inner current loop is fast enough, the outer loop can regard the commanded current as flowing immediately, so the speed loop can be designed against the inertia of the mechanism alone. Disturbances such as supply-voltage changes are also suppressed by the inner loop before they affect the speed. Loops with similar bandwidths interact and tend to oscillate, so the inner loop is generally made several to about ten times faster than the outer one.
- A. Correct. Fast loops are built from the inside out.
- B. Making the outer loop faster lets the lag of the inner current loop delay the phase of the speed loop considerably, making it prone to instability.
- C. Loops with similar bandwidths interact and tend to oscillate.