September
1997, Issue 86
PC/104
Quarter:
Motion Control with PC/104
STEPPER
MOTORS
When
designing a stepper system, think about the required
maximum resolution, accuracy, repeatability, and step
resolution. Also, consider the maximum needed velocity,
acceleration rates, system type, inertia, and the stability
of the product to be handled.
Once
you address these issues, you greatly improve your chances
for project success. Table 2 lists several different
stepper types and their characteristics.
| Stepper
Types |
Cost |
Linearity |
Detent
Torque |
Angles |
Notes |
| Variable
reluctance |
Moderate |
Very
nonlinear, nonsinusoidal |
No
(no magnets) |
Virtually
any |
Noisy;
Spins free; Good torque to inertia |
| PWM
(Canstack or claw) |
Lowest |
Fair |
Significant |
Coarse
step only |
Bearings
can affect motor stability; Low-Medium power
operation only; Best if it can do the job |
Two-/Five-
Phase Hybrid |
Moderate |
Nonlinear,
but repeats well |
Depends
on tooth design |
0.36-15º
available, 1.8º prevails |
Limited
duty, moderate power workhorse; Good torque/watt
accuracy |
| Axial
Air Gap Disc Rotor |
High |
Very
linear |
Depends
on design |
1.8º
and 3.6º only |
Easy
to model; Best torque to weight ratio; Efficient,
snappy, but limited power |
| Enhanced
Hybrid |
High |
Fairly
good |
Depends
on tooth design |
1.8º
only |
Models
like the hybrid type; Highest power out for
a given driver VA; Highest power per volume |
Be
cautious of servo controllers where voltage-to-frequency
converters run the stepper. The nonlinearity of these
converters generally compromises that stability.
Some
types of stepper motors include the moderately priced
variable reluctance motor (i.e., a nonlinear device
with nonsinusoidal characteristics) and the nonlinear
PM (e.g., canstack or claw type).
The
variable reluctance motor has no magnets, operates at
virtually any angle, and has a good torque-to-inertia
ratio, but it's noisy.
The
PM is the lowest priced stepper motor and is suited
only for low-to-medium power operation. Its bearings
can affect motion stability, and its step angles are
coarse. But if it can do the job, the PM is often the
most effective choice.
The
two- or five-phase hybrid is a moderately priced, nonlinear
device with good availability. Detent torque depends
on tooth design, and it has good torque/watt accuracy.
The hybrid is a moderate power workhorse for limited
duty.
The
axial air gap disc rotor and enhanced hybrid cost more
than other steppers. The axial air gap disc rotor is
linear, gives the best torque-to-weight ratio of the
group, is efficient and snappy, but has limited power.
The enhanced hybrid yields the highest power output
for a given driver power rating.
