Stepper Motors Overview
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Stepper motors offer many advantages. Although feedback is not usually required, stepper motors are compatible with feedback signals, either analog or digital. Error is noncumulative as long as pulse-to-step integrity is maintained by the stepper motor. A stream of pulses can be counted into stepper motors, and the stepper motor’s final position will be known within a small percentage of one step.
Since maximum dynamic torque occurs at low pulse rates, stepper motors can easily accelerate a load. When the desired position is reached and command pulses cease, the stepper motor shaft stops and there is no need for clutches or brakes. The stepper motor is generally left energized at a stop position. Once stopped, the stepper motor resists dynamic movement up to the value of the holding torque. An additional feature of the PM stepper motor is that when all power is removed, it is magnetically detented in the last position. A wide range of step angles are available — 1.8 to 80 , for example — without logic manipulation. Stepper motors have inherent low velocity without gear reduction. A typical stepper motor driven at 500 pps turns at 150 rpm. The stepper motor’s rotor inertia is usually low. Multiple stepper motors driven from the same source maintain perfect synchronization.
But the stepper motor’s efficiency is low; much of the input energy must be dissipated as heat. Load must be analyzed carefully for optimum stepper motor performance. And inputs must be matched to the stepper motor and load. Damping may be required when load inertia is exceptionally high to prevent oscillation.
Stepper Motor – Excitation Modes
Stepper motors can be excited in different modes, depending on stator winding and desired performance.
Stepper Motor – Two Phase
One entire phase (stator winding) of the stepper motor, end-tap to end-tap is energized at a given moment in time. Input current and wattage are halved (compared to four-phase excitation), and heat dissipation is decreased. Output can be improved by as much as 10%. In the stepper motor’s two-phase modified mode, both windings (end-tap to end-tap) are energized simultaneously. Energy input in this mode is the same as four phase, but output performance is increased by about 40%. The stepper motor control is complex and costly for this mode.
Stepper Motor – Three Phase
Many variable-reluctance stepper motors use three-phase windings. In modified mode, two adjoining phases are excited simultaneously and the rotor indexes to a minimum reluctance position corresponding to the resultant of the two magnetic fields. Since two stepper motor windings are excited, twice as much power is required as the standard mode (one phase at a time). The stepper motor’s output is not increased, but damping is improved.
Stepper Motor – Four Phase
Each of the stepper motor’s half winding is regarded as a separate phase, and phases are energized two at a time. Although this mode isn’t very efficient, the controller is simple. Compared to single-phase excitation, twice the input energy is required. Torque output is increased by about 40%, and maximum response rate is increased.
Stepper Motor – Five Phase
Five-phase stepper motors have 10 poles rather than the 8 poles typically used in other stepper motors. Rotor-to-stator offset becomes one-fourth to one-tenth the rotor tooth pitch. A 50-tooth rotor provides a full-step of 0.72 , and a 100-tooth version produces a 0.36 full-step (0.18 half-step). The stepper motors run at 500, 1,000, or 2,000 steps/rev with improved loaded-position accuracy and stiffer response. In addition to higher resolution, five-phase stepper motors produce less vibration than two to four-phase stepper motors with virtually no resonance effects.
Stepper Motor – Variable Reluctance
These stepper motors have soft iron multipole rotors and a wound stator. The number of teeth on the rotor and stator, as well as the number of winding phases, determines the step angle. Variable-reluctance stepper motors are generally medium step-angle devices (5 to 15 ) which operate at high step speeds. The stepper motor’s torque is generally low. Rotor inertia and, thus, inertial load capacity are extremely low. Motors of this type operate at maximum pulse rates from 300 to 1,000 steps/sec and have a maximum load inertia capacity of about two-thirds of rotor inertia. When excited in an overlap mode, these stepper motors can move at half step angles and double pulse rates. These stepper motors produce a net output velocity, which remains the same.
Stepper Motor – Permanent Magnet
PM stepper motors generally are thought of as low-torque, large step-angle devices. Torque developed by the stepper motors is far below that for equivalent-size hybrid stepper motors, and step angle generally is 90 or 45 . Position resolution, moreover, is on the order of +10% of step angle, a value that generally relegates the stepper motors to unsophisticated motion-control applications. Maximum pulse rates are for 100 steps/sec for large units to 350 steps/sec for small units. Stepper motors offer a rotor inertia, which is moderate between 5 and 75 gm-cm2.
Neodymium magnets make possible PM stepper motors having a large number of poles. With a suitable number of poles, PM stepper motors develop more torque than either hybrid stepper motors or dc servomotors. Speed range for the stepper motors is less than that for dc types but much higher than that for hybrids.
Position resolution of the PM stepper motors is less than that for hybrids. But unlike hybrids, some PM stepper motors perform well in closed-loop systems.
Both cemf and iron losses are proportional to the number of poles in the stepper motor. Thus, available torque from a PM stepper motor falls off more slowly with speed than in hybrids and more rapidly than in dc motors. The result is that PM stepper motors operate effectively at higher speeds — up to about 3,500 rpm — than hybrids but not as high as dc types. The speed range for PM stepper motors, however, suits a wide range of servo applications.
Stepper Motor – Hybrid
Hybrid stepper motors are frequently chosen for a wide variety of motion-control systems because they are easy to use. Stepper motors can maintain accuracy and reliability in open-loop mode, requiring less complex drive electronics than closed-loop servocontrollers. And absolute positioning accuracy for stepper motors is comparable to closed-loop servocontrollers for many applications.
Conventional hybrid stepper motors are rarely used in closed-loop systems because torque falls rapidly as current increases above the peak torque point — putting them outside typical control limits. The stepper motor’s torque also decreases as speed rises. If driven too fast, hybrids lose position accuracy by skipping steps.
A stepper motor’s peak torque is limited by the flux level that saturates the rotor and stator teeth. But an enhanced stepper motor is now available that reduces saturation effects and produces 50 to 100% more torque than conventional stepper motors for the same input power.
Both conventional and enhanced stepper motors develop maximum torque when the rotor teeth are offset by one-quarter tooth pitch from opposing poles in the energized phase. The stepper motor pole pairs develop appreciable torque even at zero current. Torque increases as current approaches the rated value.
At or near rated current in conventional stepper motors, a larger part of the air-gap flux traverses the gap from stator slot to rotor slot rather than from tooth to tooth, thus producing less torque.
The enhanced stepper motor uses a relatively new stator design to get around this problem. Here, samarium-cobalt or neodymium-iron-boron magnets are embedded in slots between the teeth. More concentrated flux lines result between the stepper motor’s rotor and stator teeth with fewer flux lines lost to the slotted air gap. These new slot magnets focus the air-gap flux, reduce leakage, and allow the stepper motor to produce more torque.
Torque is also produced by a second pair of poles of the same phase placed 180 away from each other, and 90 away from the first pair. The second pole pair of the conventional stepper motor produces a torque that opposes the positive-acting pair. This negative torque is large at low currents but diminishes near rated current.
Enhanced stepper motors also have large negative torques at low current. But positive-acting flux from the permanent magnets in the stator overcomes the small negative torque generated at rated current. The resulting torque then aids the pole pair producing the primary positive torque.
The slot magnets in enhanced stepper motors provide peak torques reaching twice that of conventional stepper motors. Moreover, these stepper motors can handle three times rated current compared to only two times for conventional stepper motors. Depending upon the inertial load, these new stepper motors reach speeds of 5,000 to 10,000 steps/sec. Corresponding torques are 200 oz-in. to 3,100 oz-in. in 2 to 4-in.-diameter packages. Hybrid stepper motors also generally have high inertia (30 to 40,000 gm-cm2), small step angles (0.5 to 15 ) and high accuracy ( 3%).