Chapter 1 – Motors

Publisher Summary Motors work on the same principles of electromagnetism and function by applying power to an electromagnet in some form or another. They are key components of many embedded systems because they provide a means to control the real world. This chapter describes stepper motors that come in three flavors: permanent-magnet, variable-reluctance (VR), and hybrid. The variable-reluctance stepper has a soft iron rotor with teeth and a wound stator. As current is applied to two opposing stator coils, the rotor is pulled into alignment with these two coils. As the next pair of coils is energized, the rotor advances to the next position. The permanent magnet (PM) stepper has a rotor with alternating north and south poles. As the coils are energized, the rotor is pulled around. The PM stepper has more torque than an equivalent VR stepper. The hybrid stepper essentially adds teeth to a permanent magnet motor, resulting in better coupling of the magnetic field into the rotor and more precise movement. Stepper motors driven with constant current drive (chopped or analog) are more likely to have resonance problems at low step rates. Using half-stepping or microstepping can usually overcome these problems. Of course, going from a simple on-off H-bridge to a DAC-controlled microstepping scheme is a large step in system complexity.