Stepping Motors
Microstepping
Microstepping systems are generally easy to install and use because they have no tuning or setup requirements like typical servosystems. And they are stable and free from drift when stopped.
Microstepping drives have been available for at least 10 years in varying degrees of sophistication. Fully packaged motor-drive systems, card-level indexers and drives, and even chip sets aimed at high-volume original equipment manufacturers (OEMs) are now available in the U.S., Europe, and Japan.
Drives now provide resolutions from 2,000 (the minimum for microstepping) to 50,000 steps/rev. The largest drives can microstep 6.5-in. motors and can produce torques to 5,000 oz-in. Though many drive systems operate from an external dc power supply, there is an increasing trend towards direct, off-line systems that run at 120 or 240 Vac and have no internal transformer.
The elimination of transformers combined with today's high efficiency PWM amplifiers have made possible 0.5-ft3 drives producing 2,000 W or more. Modern semiconductor power devices, including MOSFETs and IGBTs, help to increase package densities and often run cool enough to preclude fans.
Several indexers specifically for microstepping now have the ability to control multiple axes of motion. They can work from a variety of serial and parallel data buses including RS-232C, PC AT, IEEE-488, Multibus, STDbus, VMEbus, and NUBUS. They are also relatively easy to program because of features in some stand-alone machines such as touch screens and simple menus.
The most recent design trend is to integrate the indexer, drive, and power supply in one package. These systems are low in cost and sophisticated enough to control machines directly with no external computer or PLC. They store a variety of motion-control programs in nonvolatile memory. And some units accept position feedback from external optical encoders for critical positioning applications.
Rotary microstepping systems are by far the most widely used solution today, but direct linear microstepped motor-drive systems have also been developed. Most motion-control applications ultimately need linear motion and, therefore, require a leadscrew, belt, or band to convert shaft rotation into linear motion. The linear motor provides this motion directly and has virtually no backlash.
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