6.3.1. About the Demonstration
The demonstration shows how to improve speed control. Speed is never perfectly constant because motors include discrete parts such as magnets, electrical coils and steel cages to hold these components. As the motor rotates, the torque produced for a given current magnitude varies. These variations (cogging torques) produce accelerations leading to speed variations. The feedback controller adjusts the current to counteract the speed variations depending on the strength of the control gains. Stronger gains reduce speed variation, but may lead to control instability or amplify mechanical resonances.
The demonstration shows the level of speed variation with default speed control gains. It then shows the speed control step response. The step response should be fast and with little overshoot or oscillation, so the speed output should look similar to the square-wave input command. The speed control step response with standard gains is slow and may stop briefly at zero speed because of friction when the motor changes direction. Increasing the speed control loop proportional gain makes the response look much faster and without any visible stop at zero speed. However, the motor produces high-frequency noise that is audible and visible in the FFTs as a broad peak around 1kHz. In a real system with a mechanism connected to the motor, this noise can easily excite mechanical vibrations. To counteract this undesirable change, apply the suppression filter using a broad notch characteristic centred on 1kHz. The resulting waveform still has a fast, square shape, but the filter suppresses the noise. The speed variation with the combination of high gain and filter is much less than the original graph, showing the benefit of the faster control.