Stepper motors are a common choice for motion control applications that require high torque at low speeds, good holding torque, and relatively straightforward operation. And the basic premise of stepper motor construction gives them inherently high resolution and accurate positioning capabilities, making them suitable for open-loop operation.
In this session of Motion Control Classroom, we explain the differences between various stepper motor designs – including permanent magnet, variable reluctance, and hybrid types – and the performance characteristics of each. For example, you’ll learn which stepper motor design is best for high holding torque, which design eliminates detent torque, and when to consider a linear stepper motor.
You’ll also gain insights into best practices for selecting and operating stepper motors and how to choose the best drive scheme for your application. We also go in-depth on microstepping – a popular method for producing smoother motion and increasing resolution. And finally, you’ll learn when it makes sense to use a stepper motor in a closed-loop control system.
How does closed-loop stepper control work? (And why not just use a servo?)
Stepper motors are inherently open-loop devices. They don’t require feedback because each pulse of current delivered by the drive equals one step of the motor (or a fraction of a step, in the case of microstepping). And with small step sizes, or step angles, the motor’s position can be determined very precisely without the need for a feedback device and complicated control schemes.
So, if a stepper motor’s position can be determined in an open-loop system, why would you add the cost and complexity of closed-loop control to a stepper motor?
FAQ: What are stepper drives and how do they work?
A stepper drive is the driver circuit that controls how the stepper motor operates. Stepper drives work by sending current through various phases in pulses to the stepper motor. There are four types: wave drives (also called one-phase-on drives), two-phase on, one-two phase-on drives and microstepping drives.
Wave or one-phase-on drives work with only one phase turned on at a time. Consider the illustration. When the drive energizes pole A (a south pole) shown in green, it attracts the north pole of the rotor. Then when the drive energizes B and switches A off, the rotor rotates 90° and this continues as the drive energizes each pole one at a time.
Torque margin is the amount of extra torque capacity a stepper motor has to ensure successful operation without stepper motor stalls. A sufficient torque margin is critical when a motion design incorporates stepper motors for motion, as in many cases there’s no correction of motion if the motor is temporarily overloaded.
FAQ: How do I pick torque margin to prevent stepper motor stalls?
Schneider Electric Motion USA is a manufacturer of motion control components for automation equipment. The company is a proven leader in innovative motion control solutions for stepper motors and electronic controls, and the world leader in integrated motor drives with the MDrive® product line.