September
1997, Issue 86
PC/104
Quarter:
Motion Control with PC/104
SERVO-SYSTEM
BASICS
Servo
controllers come in several styles including DSP, processor,
and ASIC based. Typically, they include a feedback counting
mechanism, a gain structure that coordinates the actual
and desired system positions, and an output interface
signal to control the motor. These elements ensure that
the system load reaches the command position via the
requested move profile.
The
system load, inertia, profile, environment, and similar
factors govern the choice of any motor/amplifier package.
The electrical and physical differences of the motor
types listed in Table 1 offer enough performance variety
for you to judge which package satisfies your system
needs and budget.
| Motor
Type |
Moves |
Loads |
Power:Size
Ratio |
Armature
Inertias |
Accel/
Decel |
Notes |
| Brushless |
High-speed
short index short duty cycle |
Heavy
(high torque control) |
High |
Low-
Medium |
High |
Resolver
to quadrature converter option |
| PWM |
Long
high-
speed; Low-Medium-
speed short index; Light-Medium loaded index |
Medium |
Medium |
Low-
Medium |
Medium |
Brush
replacement required |
| DC |
Low-Medium
speed |
Light-
Heavy |
Low |
High |
Low-
Medium |
Velocity
mode or low accurate positioning; Not designed
for true servo application; Brute-force on/off
control |
| AC |
Low-High
speed |
Light-
Heavy |
Low |
Medium-
High |
Low-
Medium |
Brute-force
on/off control; Moderate servo positioning capability |
When
designing a system with a servo motor, consider the
system and encoder resolution, as well as the maximum
velocity and acceleration rate. Also, think about the
system type, system inertia and stability, and the motor
amplifier power and current requirements.
If
your application requires high-speed, short index moves
or short duty-cycle moves with high torque control,
use a DC brushless servo. It gives a high power-to-size
ratio and high acceleration/deceleration capabilities.
A
PWM DC brush servo motor works well for long, high-speed
moves, low-to-medium-speed short moves, or index moves
with light-to-medium loads. This motor needs brush replacement
and provides medium power-to-size ratios, armature inertias,
and acceleration/deceleration capabilities.
Stepper
motors provide low-to-medium acceleration/deceleration
along with high power-to-size ratios. For any application,
check how the stepper performs on its speed, torque,
and curve. A stepper's positional accuracy is generally
±3% of a full step but can be less for a higher cost.
General-purpose
DC motors aren't designed for true servo applications,
but they can move light-to-heavy loads requiring low-to-medium
acceleration/deceleration capabilities. They're well-suited
for velocity mode and low-accuracy positioning.
AC
motors--brushless servo, vector, or any other variety--provide
low-to-medium acceleration/deceleration capabilities,
moderate servo positioning using brute-force on/off
control, and medium-to-high armature inertias.
