Step motors have several qualities that make them particularly suited for control systems. They are stiff when stopped, produce high torque for their physical size, and are brushless — making them virtually maintenance free.
Step motors have a serious drawback, however, when used for precise positioning. Standard hybrid step motors have a relatively large step size, usually 1/200 of a revolution or 1.8 . Such large step size can also cause motor-shaft oscillations at resonance points that occur at low speeds, but there is a widely used technique that retains the advantages of step motors and overcomes low speed roughness and low resolution. The technique is called microstepping (micro stepping).
Microstepping increases the position resolution and smoothness of conventional hybrid step motors. This is done with electronic control in the drive circuits. The drive subdivides each full step electronically into a large number of smaller steps. For example, a microstepping drive that subdivides each full step of a 200-step/rev motor into 125 microsteps produces 25,000 steps/rev (200 X 125 = 25,000).
Motors and drives must provide high positional resolution in applications such as semiconductor fabrication. A 25,000 step/rev system attached to a 10-pitch leadscrew on an X-Y table can position a silicon wafer to one part in 250,000/in. This high positional resolution often eliminates gearboxes (and gear backlash) or other mechanical reducers otherwise needed to place wire bonds or test probes on exposed IC wafers, but many applications that do not need high resolution can also benefit from microstepping.
The biggest advantage of microstepping is smooth operation and the elimination of resonance over its entire speed range, typically 0 to 3,000 rpm. Smooth operation permits full torque utilization and freedom from rattling and mechanical wear.
The true accuracy of a microstepping system is usually less than its resolution. System accuracy is a complex function of motor accuracy, electronic tolerances, and errors in the mechanical transmission. But the combination of micropositioning and smooth operation has enabled microstepping systems to become standard in X-Y positioning systems requiring to 0.001 inch resolution, and precision grinding, turning, and surface-finishing machines. Other precision motion-control applications include optical scanning and inspection, disk memory media manufacturing, and fiber-optics manufacturing.
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.
An advantage of linear motors is that they can provide speeds in excess of 80 insec. Such speeds generally cannot be realized with lead screw transmissions. High-speed linear motors are ideal for applications like printed-circuit-board component placement, insertion equipment, and inspection machinery.
Though microstepping provides increased positional resolution and smoothness, simple setup and freedom from drift is not appropriate for all motion-control applications. Simple microstepping systems operate open loop. There is no position feedback device to guarantee that the shaft position is correct. Normally this is not a problem for applications where shaft loads are relatively constant, such as X-Y tables, scanners, and packaging machines.
To head off problems with shaft position uncertainties, an accepted design rule is to select a motor with twice as much torque as computed. Also, the stiffness of the shaft position is a function of the motor’s rated torque and the load. The high efficiency of step motors in torque-per-volume and torque-per-ampere usually minimizes any problem with using a larger motor. In fact, a step motor that provides twice the calculated torque requirements may even be physically smaller than a similarly specified servomotor.
Large intermittent shaft loads in excess of the motor’s available torque (which decreases with speed) can make the motor stall or lose position. Applications with widely varying torque demands, such as industrial robots, usually need a closed-loop system that can respond to these requirements efficiently. But closed-loop systems are more expensive. Feedback devices, such as encoders and resolvers, and a more sophisticated control system must be added. And these feedback elements tend to be more fragile than the motor itself.
The ability of repetition for a positioning system is often the most important design parameter. Open-loop microsteppers are very repeatable if the elements are selected as discussed. Systems with large and variable frictions that load a large percentage of the motor’s available torque can produce significant positional errors. As a guide, the shaft will deflect about 1 when a torque equal to half of the motor’s rated torque is applied. This is not a problem for systems with repeatable loads, such as lead screw-driven tables or scanners, but experts use a larger motor than torque calculations would normally indicate, simply to improve the overall system stiffness.
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