When there is a need to move something around, the common solution is to use a motor of some form. Often this ends up being an electric motor with an external gear train or an actuator that is an electric motor with an internal gear train. The challenge when using either of these is knowing the precise location of the motor in terms of its angular position, or the position of the item being moved by the motor in space. Providing accurate location information in stepper motor systems is handled by an encoder that tracks the angular position of the motor. Also, by keeping track of time, both the velocity and acceleration can be determined. Two encoder types are commonly used: absolute encoders and relative encoders. Absolute encoders provide an absolute measure of the motor position, and relative encoders provide a measurement of how far the motor has moved from some reference point. Relative encoder systems are usually much less costly than absolute encoders and with the right choice of motor don’t require any additional parts.
Electric motors also come in two standard types: continuous-drive and stepper-drive motors. Both have excellent performance and work well in a wide range of applications. Continuous-drive motors that include servo motors can be driven by voltage pulses. Stepper motors are driven by pulsing the motor leads in the correct sequence. Our focus in this chapter will be on stepper motors, which have become very popular in many positioning applications. The bonus is that when a stepper motor is combined with a microcontroller, the need to use absolute encoders with their additional components and costs within the system is minimized.
Optical engineers can find themselves working with a wide array of motor, actuator, and encoding systems. In the past, the expense and awkwardness of electronics control of optical mounts and components limited their use. Today, it is not difficult to use a smart phone to communicate wirelessly with a microcontroller. In this chapter we will look at stepper motors and how to use them.
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