M O T I O N C O N T R O L S Y S T E M S
98
Motion Control Systems
·
Galil Motion Control, Inc.
·
800-377-6329
Power amplifiers receive a command signal--typically an ana-
log signal within the range of ±10 V--and amplify it to the
required level of current. Such amplifiers can be configured
in the current mode or in the velocity mode. The velocity
mode is preferred when velocity feedback is utilized; other-
wise, a power amplifier typically is configured in the current
mode.
In the current mode, the amplifier produces a current
that is directly proportional to the input voltage. This is
achieved by a current feedback loop which monitors the cur-
rent and assures that it is proportional to the command sig-
nal. Current amplifiers are characterized by the current gain,
K
a
, which indicates the amplifier for 1 V of command signal.
Amplifiers can also be configured in the velocity mode.
Here the amplifier includes a voltage amplification stage
which compares the applied voltage with the motor velocity
and amplifies the difference before it is applied to the current
loop. Such an amplifier is shown below.
Amplifiers use one of two methods to generate the
required voltage or current: linear amplifiers that produce
constant output voltage; or pulse-width-modulated (PWM)
amplifiers which generate a voltage that switches between the
high and low levels. Most amplifiers today, especially those
with power ratings above 100 watts, are switching amplifiers
using the PWM method to minimize power losses. Linear
amplifiers are more common when low power is required.
Now that we have discussed the operation of motors and
amplifiers, we will proceed to describe the operation of the
position sensors. The most common sensor is the incremen-
tal encoder.
Incremental Encoders
Incremental encoders generate pulses that represent the shaft
position. The encoder output includes two signals, common-
ly called Channel A and Channel B, which generate N pulses
per revolution. The two signals are shifted by a quarter of a
cycle, as shown below. The shift between the two signals
enables the controller to determine the direction of rotation,
according to whether Channel A leads Channel B or vice-
versa. The shift of the two signals also increases the sensor
resolution, as each encoder cycle is divided into four quar-
ters, each called a quadrature count. Thus, an encoder with
N cycles per revolution produces 4N quadrature counts per
revolution.
Most encoders produce square wave signals with TTL
levels. Other forms include sinusoidal signals or square
waves at higher voltages. Industrial systems often use
encoders with differential signals, i.e., channels A and B
and their complements. These devices reduce the system
sensitivity to noise. Incremental encoders may also pro-
duce a third signal known as the index or the marker. This
signal appears once per revolution and can be used for ini-
tialization purposes.
Power Amplifiers
OVERVIEW
Elements of an Amplifier in Velocity Mode
VELOCITY
COMMAND
CURRENT
COMMAND
MOTOR
TACHOMETER
VELOCITY
AMPLIFIER
CURRENT
FEEDBACK
VELOCITY
FEEDBACK
POWER
AMPLIFIER
M
T
+
+
-
-
Output Signals of an Incremental Encoder
CHANNEL A
POSITION
CHANNEL B
