High-Speed Stepper Motor Factors
Several factors become design and implementation challenges when you drive stepper motors at high speeds. Like many components, the real-world behavior of stepper motors is not ideal and far from theory. Stepper motors’ max speed varies by manufacturer, model, and the inductance of the motor, with speeds of 1000 RPM to 3000 RPM usually attainable.
Inertia
Any moving object has inertia, which resists changes to the acceleration of an object. In lower speed applications, it’s possible to drive a stepper motor at the desired speed without missing a step. However, attempting to drive a load on a stepper motor at high speed immediately is a great way to skip steps and lose the motor’s position. A stepper motor must ramp up from low speed to high speed to maintain position and precision except for lightweight loads with little inertial effects. Advanced stepper motor controls include acceleration limitations and strategies to compensate for inertia.
Torque Curves
The torque of a stepper motor is not the same for every operational speed. It drops as the stepping speed increases. The drive signal for stepper motors generates a magnetic field in the motor coils to create the force to take a step. The time it takes the magnetic field to come up to full strength depends on the inductance of the coil, drive voltage, and current limitation. As the driving speed increases, the time the coils stay at full strength shortens, and the torque the motor can generate drops off.
Drive Signal
The drive signal current must reach the maximum drive current to maximize the force in a stepper motor. In high-speed applications, the match must happen as quickly as possible. Driving a stepper motor with a higher voltage signal helps improve the torque at high speeds.
Dead Zone
The ideal concept of a motor allows it to be driven at any speed with, at worst, a reduction of torque as the speed increases. However, stepper motors often develop a dead zone where the motor cannot drive the load at a given speed. The dead zone arises from resonance in the system and varies for every product and design.
Resonance
Stepper motors drive mechanical systems, and all mechanical systems can suffer from resonance. Resonance occurs when the driving frequency matches the natural frequency of the system. Adding energy to the system tends to increase its vibration and loss of torque, rather than its velocity. In applications where excessive vibrations prove problematic, finding and skipping over the resonance stepper motor speeds is especially important. Applications that tolerate vibration should avoid resonance where possible. Resonance can make a system less efficient in the short term and shorten its life over time.
Step Size
Stepper motors employ a few driving strategies that help the motor adapt to different loads and speeds. One tactic is micro-stepping, which lets the motor make smaller than full steps. These micro steps offer decreased accuracy and make stepper motor operation quieter at lower speeds. Stepper motors can only drive so fast, and the motor sees no difference in a micro-step or a full step. For full-speed operation, you’ll usually want to drive a stepper motor with full steps. However, using micro-stepping through the stepper motor acceleration curve can significantly decrease noise and vibration in the system.
