M O T I O N C O N T R O L S Y S T E M S
100
Motion Control Systems
·
Galil Motion Control, Inc.
·
800-377-6329
of 150 msec is divided equally between the acceleration,
slew, and deceleration. The slew velocity is 100,000 count/sec
and the total displacement is 10,000 counts. Typically, the
motion requirements are specified by the host in terms of
the total distance, slew speed, and acceleration rate. It
remains to the controller to generate the position profile
R(t) shown as a time-dependent function. Since the motor
position, C, follows R, the generation of the profile R con-
trols the motion path and rate. In addition to the basic
tasks described above, an advanced motion controller may
perform many high-level functions, such as processing com-
mands from a host computer, program sequencing, I/O
processing, and error handling. It is these high-level func-
tions that allow the controller to operate as a complete
stand-alone machine controller.
To describe the operation of the system, consider an
example with the following parameters:
Parameters Definition
Kt = 0.1 Nm/A Motor torque
constant
Jm = 10
-4
kg m
2
Moment of inertia
r = 2
Motor resistance
N = 500 lines/rev Encoder line
density
K
a
= 3 amps/V Amplifier current
gain
Now suppose that the system is required to be at position
zero (R=0), but the actual motor position is -20 (C = -20).
The controller determines the position error according to the
equation E = R - C, resulting in an error of 20 counts. The
error signal, E, is then processed by the filter. Assume, for
simplicity, that the combined gain of the filter and the DAC
is 0.1 V per count: this implies that an error signal of 20
counts produces a motion command signal, X, of 2 V.
The signal X is applied to the amplifier with the given
gain of 3 amps/V, resulting in a current of 6 amps. When the
current is applied to the motor, it generates a proportional
torque of 0.6 Nm. The positive torque drives the motor for-
ward in the direction that reduces the position error.
The applied current produces a torque Tg which equals
the product of the current and the torque constant Kt. The
generated torque accelerates the motor at the rate a where:
a = T
g
/ J
m
for the given system parameters, a = 6000 rad/sec
2
.
As the motor moves closer to the desired position, the
error, E, decreases and, along with it, the drive signal to the
motor. When the motor reaches the desired position exactly,
the position error drops to zero, as does the motion com-
mand signal. No current is applied to the motor while it
remains in the correct position; however, whenever the
motor moves away, a correction torque is applied to force it
back to the required position.
The motor may approach the required position in sev-
eral ways. If the motor approaches the required position
from one direction and stops, we say that the system
response is overdamped. If, on the other hand, the motor
position overshoots the target several times before settling,
the response is underdamped. Under some conditions the
motor response may oscillate and never stop; such a sys-
tem is called unstable. The system response is a function
of the system hardware and the settings of the PID filter
parameters. The ideal system response has minimum rise-
time, minimum overshoot, and quick settling into the
commanded position. Many tools, such as Galil's SDK
servo design kit software, are available for tuning the PID
filter for the optimum response.
OVERVIEW
