19 – Controllers and power electronics

This chapter introduces a digital controller and a sample program for controlling a bearingless motor. Digital control using digital signal processors (DSP) is usually used in a bearingless drive to realize the system. This is due to the fact that digital control can protect the delicate derivative controller, which is essential for magnetic suspension. Digital control allows a higher degree of flexibility and universality compared with analogue control. It can also deal with complex control methods such as such as an observer, a nonlinear calculation (such as magnetic saturation) or any calculation using complex branching methods or extensive memory. In the motor speed controller the rotor angular position o is detected by the rotary encoder. The rotational speed ω is generated from the encoder counter and speed detector. In the rotor radial position controller, the rotor displacements x and y (on the x and y axes) are detected by the gap sensors through an A/D converter and these displacements are compared to the displacement commands x* and y*. The suspension force commands are generated by the proportional-integral-derivative (PID) controllers. It is necessary to transform the PID controllers from continuous-time operation to discrete-time operation in order to construct the rotor radial position controller using a digital controller.