Steppers position precisely if loads are stable and speeds stay below 1,000 rpm.
PennEngineering Motion Technologies Harleysville, Pa.
Hybrid stepper motors combine the operating principles of variable reluctance with permanent-magnet stepper motors. The rotor shaft holds one or more pairs of stacked laminations containing many teeth along its outside diameter. A permanent magnet between each laminated stack pair creates a north and south pole oriented along the motor shaft axis.
Conventional electric motors rotate continuously, but stepper motors rotate or step in fixed angular increments. One revolution involves the motor taking a specific number of steps determined by the number of teeth, motor construction, and type of drive scheme used for control. This measurement, called the step angle or step resolution, can be stated as an angular measurement or a specific number of steps. The most commonly specified step resolution is 1.8°/step or 200 steps/rev.
Compared with servomotors, hybrid steppers have the advantage of rugged, simple construction; reliability with little maintenance; high torque at low speeds; and no need for position or velocity feedback devices. Comparative disadvantages include diminished torque at high speeds; resonance and noise; and high consumption of current. Where precise positioning is a must, hybrid steppers best suit applications characterized by stable loads and speeds under 1,000 rpm.
TORQUE AND SPEED
Developers must understand terms such as running torque, pull-out torque, pull-in torque, maximum speed, and cutoff speed as part of making design decisions. Motor torque is at a minimum when the rotor is halfway from one step position to the next. This minimum determines the running torque, the maximum torque the motor can drive as it slowly steps forward. Running torque is sometimes defined as the pull-out torque at higher stepping speeds. Motors powering loads exceeding their pull-out torque rating will be pulled out of step. The rotor skips a step instead of advancing forward and may even rotate backwards a short distance. Skipped steps destroy motor position accuracy.
Some motor data sheets identify a second torque value called the pull-in torque. This is the maximum frictional torque that the motor can overcome to accelerate a stopped load to synchronous speed. Trying to accelerate faster using more torque than the rated pull-in value will again make the rotor skip steps.
Maximum speed is defined as the speed at which the available torque falls to zero. Resonance problems make it difficult to measure maximum speed as torque prematurely drops to zero.
Cutoff speed is the speed above which the torque begins to fall. When the motor is operating below cutoff speed, the rise and fall times of the current through the motor windings occupy an insignificant fraction of each step. At cutoff speed the step duration will be comparable to the sum of the rise and fall times.
INFLUENCES ON RUNNING TORQUE
The way motor components are incorporated affects the operation, reliability, and service life of the motor. A hybrid stepper's rotor gets specific benefits from the type and quality of laminations, sharpness of teeth, and magnet materials.
Laminations. Rotor stacks typically carry up to 80 laminations of silicon steel (made up of two or four stacks), although this can vary. The longer the lamination the higher the torque.
Traditionally laminations have either been bonded using adhesives or pinned together. Innovative manufacturing processes now eliminate secondary operations involving glues or baking to promote manufacturing in large volumes. One process involves die-punching areas of each lamination which now interlock during assembly. Thin slices of insulation between each lamination reduce induced currents to minimize energy loss and boost performance.
Teeth: The teeth on the rotor must precisely match the teeth on the stator. Their corners must be sharp to get maximum torque output. For this reason teeth typically are precision ground.
Magnets: Magnet materials for hybrid steppers each exhibit their own special qualities, advantages, and disadvantages. Application requirements determine which magnet material is right for the job.
AlNiCo magnets offer stable strength during changes in temperature, but can become demagnetized if removed from the assembly. Alternatively, most hybrid steppers use neodymium-iron-boron magnets. These magnets are stronger than AlNiCo but their strength varies more over extended temperature ranges.
All in all, there are a lot of options that can be brought to bear on the demands of specific applications. These options let motor manufacturers offer OEMs specialized design and production expertise to devise motors for a variety of uses.
At specific step rates stepper motors often experience an undesired reaction called resonance. The indications are a sudden loss of torque with possible skipped steps and loss of synchronization.
Resonance is inherent in the design and operation of all stepping motors. Slow stepping rates combined with high rotor inertia and elevated torque produce ringing as the rotor overshoots its desired angular displacement and is pulled back into position. Resonance arises when the step rate coincides with rotor ringing, typically about 100 to 200 steps/sec. Unable to overcome the combined effects of both load inertia and ringing, the motor skips steps and loses torque and synchronization.
Changing any one of the three parameters inertia load, step rate, or torque will reduce or eliminate resonance. As a practical matter, only torque is the easiest to change using a technique called microstepping.
Microstepping applies power to the stator windings of the motor in incremental steps. Torque builds slowly reducing overshoot and canceling resonance.
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