The controller is critical to the function and performance of the active magnetic bearing (AMB) and therefore, the entire levitation system, as shown in Figure 4. The controller includes the hardware and software that interprets the measurement of position of the impeller and determines what force each actuator should exert. Within this section, we describe the fundamental elements of a controller, and one of the most common control strategies typically employed to achieve rotor stability are proportional-integral-derivative (PID). The remainder of the section includes a brief description of more advanced control scheme and of some of the hardware elements typical to a controller.