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Motor Drive & Control

For various motor types; AC Induction (ACIM), Brushed DC, Brushless DC (BLDC), Permanent Magnet Synchronous and Stepper find the right devices, software and support to precisely control the position, velocity and torque.

Applications

Stepper Motors - Overview

Motor Control - Stepper Motors

Stepper motors are the "baby" of the motor family, having only become popular since the early 1960's. They were originally conceived of as a low-cost alternative to servo motors in expensive position control applications, and were immediately embraced by the emerging computer industry in peripheral applications. The main advantage of stepper motors is that they can provide open-loop position control for a fraction of the cost of a servo system requiring feedback. In the past, stepper motors have occasionally and inappropriately been referred to as "digital" motors, since they are commonly driven with quadrature square waves. However, such a narrow view of these motors can often lead to significant headaches later in the project development process. Stepper motors produce torque just like any other magnetic "analog" motor. The damping factor for most steppers is very low, resulting in underdamped operation and susceptibility to resonant problems at certain step frequencies. These problems often result in the stepper motor being more difficult to tame than other motor topologies.

Most stepper motors employ a doubly salient design with teeth on both the rotor and stator structures. Like a BLDC or PMSM motor, the permanent magnets are located on the rotor, and the electromagnets are contained in the stator. Most designs contain two stator phases which are driven independently by quadrature phased signals. There are many different ways to drive these phases, including full steps, half steps or micro steps, depending on the control techniques used. In each situation, a stator flux vector is established, and the magnets on the rotor try to line up with this vector. Since the rotor and stator have different numbers of teeth, the resulting movement, or step, may be very small. Shortly after this alignment takes place, the stator currents are changed in such a way as to advance the stator flux vector angle, causing the rotor to move to the next step. Since there is no position feedback in most applications, the rotor flux is allowed to align with the stator flux, which results in stator current flowing which is not contributing to motor movement. As a result, stepper motors are not as efficient as most other popular motors.

Because the step angle of most stepper motors is relatively small, they are not the optimum choice for high speed applications. In some applications, the stator current is required to completely change polarity at each step. The inductance associated with the stator coils tends to impede this change, and it takes a while for the currents to reach their new levels. At higher step frequencies, the current may not have completely reached its steady-state value before it is commanded to change again. As a result, the voltage driving the phases must be increased at higher speeds in order to drive the current change faster. But eventually a point of diminishing returns is reached where higher speed operation becomes impractical.

As mentioned earlier, stepper designs are often plagued with resonance problems due to their inherent low damping factors. This tends to increase audible noise, and can lead to errant stepping in severe conditions. To mitigate these problems (and also increase step resolution), the stepper windings are often driven with sinusoidal waveforms instead of square waves. When this occurs, the motor is said to be microstepping. One common way to drive a stepper in a microstepping application is to place each coil in a separate H-Bridge circuit, and utilize PWMs from a processor to modulate the sinusoidal waveforms. However, the designer must be mindful that increasing step resolution does not necessarily increase step accuracy, especially in open-loop applications. Two factors contribute to this:

  1. The actual vs. commanded rotor position will be affected by motor loading, as indicated by the stepper's static torque curve. Depending on the torque load, the actual shaft angle can vary from the commanded step angle by several microsteps.
  2. The step angle accuracy is also affected by the motor design and tolerances used to machine the rotor and stator structures. Steppers which are intended to be used for microstepping applications are machined to higher standards, and are consequently more expensive. However, microstepping is still often used with cheaper steppers simply for the improvement in resonance susceptibility.