1. STEPPER MOTORS
BY: JOSEPH BURKE
May 4, 2004

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

3. 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.

4. 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.

5. 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.

6. 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.

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

8. 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.

9. 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)

10. 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).

11. 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)

12. 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.

13. 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.

14. 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.

15. 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.

16. 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.

17. 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.

18. 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.

19. 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.

20. 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.

21. THE END