A stepper motor (or step motor) is a brushless DC electric motor that divides a full rotation into a number of equal steps. The motor’s position can then be commanded to move and hold at one of these steps without any feedback sensor (an open-loop controller), as long as the motor is carefully sized to the application.
Switched reluctance motors are very large stepping motors with a reduced pole count, and generally are closed-loop commutated. Via Wikipedia, Figure 1 depicts an animation on how a stepper motor functions:
- Frame 1: The top electromagnet (1) is turned on, attracting the nearest teeth of the gear-shaped iron rotor. With the teeth aligned to electromagnet 1, they will be slightly offset from electromagnet .
- Frame 2: The top electromagnet (1) is turned off, and the right electromagnet (2) is energized, pulling the teeth into alignment with it. This results in 3.6° rotation in this example.
- Frame 3: The bottom electromagnet (3) is energized; another 3.6° rotation occurs.
- Frame 4: The left electromagnet (4) is energized, rotating again by 3.6°. When the top electromagnet (1) is again enabled, the rotor will have rotated by one tooth position; since there are 25 teeth, it will take 100 steps to make a full rotation in this example.
In an article from Control Design, Hank Hogen illustrates how stepper motors have great potential:
In the future, stepper motors promise to step it up by stepping down and stepping out. Smaller steps and greater integration, particularly via networking technology, push steppers into consideration for new automation and motion applications.
The emphasis is on more torque in the same motor size and a wider range of steps per revolution, according to Todd Walker, national marketing manager at Oriental Motor USA. The company’s products offer up to 1,000 steps per revolution.
Smaller steps cut vibration and noise. That can be important for applications such as a security camera that must move to scan an area or to image an object. “You can’t hear it pan and tilt,” Walker explains. “You can position it to wherever you want, and it provides improved accuracy.”
Accurate positioning also can be useful in an automated machining system. Having steppers rather than servos handle some of the motion can cut costs. Smaller steps means it takes more of them to cover a given angle, but properly designed drives and electronics compensate for this quite easily, Walker notes.
Some of the progress in reduced stepper noise arises from improved internal mechanical tolerances, as well as quieter and smoother bearings, says Scott Evans, director of global product planning for Kollmorgen. The result is less vibration than in the past. Those developments, combined with better drive electronics such as softer switching devices, have opened up new, yet old, applications. “Now because of being a bit quieter and with a lot less vibration out of that motor, you’re seeing them play very strongly in the medical and office applications,” Evans says. “Even areas where people got away from them 15, 20 years ago, they’re now coming back.”
In addition to mechanical improvements, stepper motors incorporate electronic and networking advances. For example, Advanced Micro Controls (AMCI) evolved its product line into stepper motors from the controller side, says Bob Alesio, director of sales and marketing. It makes sense that its products incorporate controllers and drives into the motor. “By integrating a controller and drive in a motor, we’ve simplified the specification process,” Alesio says. “They no longer have to look at three separate items and verify their compatibility, verify their performance together. We’ve taken the guesswork out.”
A step beyond that is to add networking capabilities that simplify things by making the integrated stepper motor show up like any other device in the programming scheme.
Integration of stepper motor and drive is an approach pioneered by Schneider Electric Motion USA, says Peter Crawford, applications engineer. One benefit is that it’s possible for simple motion control to be done locally, eliminating large main control panels.
However, there can be drawbacks to integration. Sometimes space is so tight that a bare stepper motor could be the only choice. Hot locations might require that electronics be cooler and, therefore, located remotely.
As for new technology, Schneider Electric has stall protection electronics, which essentially compensate for loading and keep the motor in the sweet spot of a speed torque curve. This opens up new applications because it eliminates the stall that can occur when someone, for instance, suddenly drops a package on a conveyor belt. “For when the torque goes up way over what a motor can typically handle, we’ve developed a hybrid motion technology to get past that,” Crawford says.
These types of advances haven’t changed the fundamentals of stepper motors. Their torque is highest when moving at lower speeds, says Mike Fisher, national sales representative for JVL Industri Elektronik, adding that those characteristics are actually a good fit for applications like the automated switchover of packaging machines from one size to another. The ability of stepper motors to replace the hand wheels traditionally used in such situations has been there for years. However, the relatively recent addition of networking and other capabilities has made the use of stepper motors in such settings simpler and more attractive. “It’s a lot easier to justify an integrated stepper motor with EtherNet/IP and absolute feedback in these applications,” Fisher says.