STEPPER MOTORS BY: JOSEPH BURKE May 4, 2004

INTRODUCTION What do stepper motors do? How do stepper motors work? Advantages and disadvantages Inertia effects Stepping modes Types of stepper motors Slewing

WHAT DO STEPPERS DO? Capable of motion in precise increments. Move using control modules that energize the coils in the stator in a certain pattern that lines the magnetic poles in the rotor with the appropriate charge (N or S). Used in many pieces of equipment, from precision machine tools and controls to robots.

WHAT DO STEPPERS DO? Also used in XY plotters, printers, etc. Can go in forward or reverse, adding to their versatility and usefulness. Come in three types, Variable Reluctance, permanent magnet and hybrid.

HOW DO STEPPERS WORK? Steppers work in a very simple manner. 2 pole rotor with 3 poles Switch A is closed, so this creates a north pole to attract the south pole of the rotor. There are 2 options, either de-energize the A switch and energize the B switch, or de-energize the A switch and energize the C switch.

HOW DO STEPPERS WORK? This would cause the rotor to turn 60 degrees in either direction. This is a simple example, to illustrate the operation of a stepper. Most stepper motors have many poles on the rotor, and can step from 90 degrees all the way down to a fraction of a degree per increment.

ADVANTAGES AND DISADVANTAGES Stepper motors, like any piece of equipment have many benefits and drawbacks to their use.

ADVANTAGES Low cost Work in almost any environment Rugged Simplicity in construction High reliability No maintenance No feedback components are needed Inherently more failsafe than servo motors.

DISADVANTAGES Resonance effects and relatively long settling times Rough performance at low speed unless a microstep is used Liability to undetected position loss as a result as a result of operating open-loop Consume current regardless of load Losses at speed are relatively high and can cause excessive heating and they are frequently noisy (especially at high speeds)

DISADVANTAGES They can exhibit lag-lead oscillation, which is difficult to dampen. There is a limit to their available size, and positioning accuracy relies on the mechanics (e.g., ballscrew accuracy).

INERTIA EFFECTS When a coil energizes, pulling one of the poles of the rotor towards it, the rotor is accelerating. When the rotor reaches the coil, it overshoots the center position due to the rotating inertia. Oscillates there until it stabilizes. If the rotor has a load on it, this oscillation is lessened, due to the weight of the load. Another way to lessen the oscillation is viscous dampening (using air or oil to brake the rotor)

STEPPING When a stepper motor increments, each increment is referred to as a step. Depending on the number of poles in the stator, a stepper motor has the ability to either full step, or half step. Full stepping with a 4 pole stator is where 2 coils are energized at a time, so that the center of the two poles is the center point of the step.

STEPPING Half stepping is similar to the previous, except instead of 2 coils energizing, then the next 2 coils, it only energizes 1, then 2 coils, then the next 1. This cuts in half the number of steps that the motor would otherwise be capable of. This is possible with all 3 types of stepper motors.

THREE TYPES OF STEPPERS Stepper motors come in three major types. These types are VR or variable reluctance, PM or permanent magnet, and Hybrid. Each one is named for the type of rotor and the characteristics it has. The VR stepper has a soft iron core rotor and no permanent magnet. The PM stepper has a permanent magnet core, which is magnetic even when the motor is de-energized. The hybrid stepper is a combination of the two, with a permanent magnet sandwiched between two soft iron armatures. This means that one armature is completely N biased and the other is completely S biased.

VARIABLE RELUCTANCE VR stepper motors have no permanent magnet, so the rotor spins freely without “detent” torque. Torque output for a given frame size is restricted, although the torque-to-inertia ratio is good, and this type of motor is used frequently in small sizes for applications such as micro- positioning tables. VR motors are seldom used in industrial applications (having no permanent magnet). They are not sensitive to current polarity and require a different driving arrangement than the other motor types.

PERMANENT MAGNET PM motors have a permanent magnet in the rotor, giving it a detent torque, so it retains its position when de-energized. Also known as the tin can or “canstack” motor, it is the most widely used type in non-industrial applications. Low cost, low torque, and low speed, are suited to applications in fields such as computer peripherals. The motor construction results in relatively large step angles, but their simplicity lends itself to economic high-volume production at a low cost.

PERMANENT MAGNET The axial-air gap or disc motor is a variant of the permanent magnet design which achieves higher performance, largely because of its very low rotor inertia. This restricts the applications of the motor to those involving little inertia. (e.g., positioning the print wheel in a daisy-wheel printer). It is difficult to build small PM motors with many poles therefore the step sizes for PM motors are usually large. PM rotors are also bulkier and therefore these motors have higher inertia, and consequently, they accelerate slower. The maximum pulse rate is around 300 Hz.

HYBRID STEPPER MOTORS Combination of VR and PM type motors. Have a permanent magnet sandwiched between two soft iron armatures, so that all of one armature is N biased and all of the other armature is S biased. These two armatures are staggered in their construction, so that the incremental size the motor is capable is very small. One major benefit of a hybrid motor is the detent torque, like the PM steppers. Ability to retain its position. (for precision movement) Power can be saved by de-energizing the motor and re-energizing it when it needs to be used again.

SLEWING Capable of continuous operation, which is referred to as slewing. The motor is not starting and stopping at every step, and is therefore benefiting from the inertia effect. Also capable of carrying a greater load torque while sleqing as opposed to when it is stepping. When slewing, the motor is going at a steady rate of speed, and is not as erratic as when it is stepping.

CONCLUSION Stepper motors are very useful and versatile pieces of equipment that are used for precision movement in many aspects of industry. The three types of steppers, VR, PM, and Hybrid are each vital to industry as the strengths of each can be utilized to optimize your goal achievement.

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