Innovations improving smoothness of motion, miniaturizing designs, and networking are still moving microstepping technology ahead 25 years after it provided a revolutionary solution for position control.
Unlike servomotors, stepping motors advance and hold position intrinsically as a direct result of their magnetic structure. In "half-stepping" applications, stepper motors operate as purely digital devices, typically advancing 1.8 degrees and holding a new position for each planned "on-off" combination of their phase currents.
This simple open-loop operation accounts for the early adoption, low cost, and immense popularity of stepping motors in positioning applications. It also contributes to their primary weakness, a vulnerability to resonance that can cause a catastrophic loss of torque and desired position.
All that changed 25 years ago when the engineers who would soon start Compumotor developed a concept called microstepping. In microstepping, the phase currents are changed in small increments causing the motor to advance in much smaller "micro" steps. While many more steps per revolution is worthwhile, the more important advance is the resulting smoothness of operation and its quieting effect on motor resonance. Today, microstepping continues to be a dominant factor in the stepping motor market.
Ted Lin, founder of Lin Engineering, characterizes stepping motor applications today using three criteria:
The need to provide the required torque at the desired speed;
The desire for smooth motion; and
The ability to provide microstepping accuracy.
"Not all motors are created equal when it comes to microstepping." says Lin. "Actual positioning accuracies with 64 step microstepping can vary from ±15 arc minutes to as low as ±1.5 arc minutes." To put this accuracy in perspective, each step in a typical 64-step microstepping system translates to an average position change of only 0.6 arc minutes, providing a resolution of 12,800 counts per revolution. Another way to look at these typical accuracies is in terms of the corresponding digital increments, where they account for ±3 to ±25 counts of error. That's quite a difference, and it can become critical in some applications.
Microstepping has advanced tremendously over its history, and continues to move ahead. Mark Lamattina of Danaher Motion reports that their new P7000 product "has features that provide true servo-like performance at a fraction of the cost."
According to Lamattina, "their new stepper drives achieve smooth motion performance by 'probing the motor' for its electromagnetic characteristics and matching the drive to it.
Connectivity is one more area of active innovation in stepping motor controllers and drives. A wide variety of interfaces are available, from the tried-and-true step and direction to intelligent networks. RS-232 interfaces and 485 networks are common as are a variety of industrial networks such as DeviceNet and CANOpen. More advanced networks can save considerable applications engineering time, including both hardware and software development. Large systems, in particular, can be great beneficiaries of advanced networking which can also improve reliability and maintainability.
Another trend that has existed for a while yet continues to gain momentum is the integration of drive electronics into the stepping motor package. Muhammad Mubeen, a research analyst with Motion Tech Trends, reports that more and more suppliers are taking advantage of the ongoing miniaturization of electronics to provide enhanced integral stepping motor solutions. Mubeen says, "You can get a pretty intelligent drive with communications capabilities right on the back of the stepping motor."
Like many other automation products, prices for stepping motors, drives and controllers continue to decline. Mubeen notes that a major factor in stepper motor price decline is the availability of quality inexpensive motors from China.
Marc Feyh, a marketing product coordinator at Parker Compumotor, agrees with Mubeen's observations surrounding advances in packaging. Compumotor introduced an all-digital controller named Gemini about two years ago that, Feyh says, has received multiple patents and offers unique anti-resonance capabilities. "Recent efforts are not as performance-oriented, but are instead far more focused on packaging and footprint," Feyh reports.
One active project at Parker focuses on integrating drive electronics into the stepper motor package, and miniaturization is a primary challenge. Feyh says that performance of the integrated package is good, but will not be as good as traditional stepper motor/driver packages in the near future. He predicts a longer-term future where fully featured controllers and drives migrate downward in size and quite possibly all the way into the back of the motor.