Technical References
System Elements
Power Amplifiers
Power amplifiers receive a command signal, typically an analog
signal within the range of ±10 V, and amplify it to the required
level of current. Such amplifiers can be configured in the current
or velocity mode.The velocity mode is preferred when velocity
feedback is utilized; otherwise, a power amplifier typically is con-
figured 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 current and assures
that it is proportional to the command signal. Current amplifiers
are characterized by the current gain K
a
which indicates the
amplification per 1 V of command signal.
Amplifiers can also be configured in the velocity mode. Here
the amplifier includes a voltage amplification stage which com-
pares the applied voltage with the motor velocity and amplifies
the difference before it is applied to the current loop. Such an
amplifier is shown in the accompanying figure.
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 that gener-
ate a voltage switching 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 mini-
mize 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 posi-
tion sensors.The most common sensor is the incremental
encoder.
Incremental Encoders
Incremental encoders generate pulses that represent the shaft
position.The encoder output includes two signals, commonly
called Channel A and Channel B, which generate N pulses per revo-
lution.The two signals are shifted by a quarter of a cycle, as shown
in the accompanying figure.The shift between the two signals
enables the controller to determine the direction of rotation,
depending upon whether Channel A leads Channel B or vice-versa.
The shift of the two signals also increases the sensor resolution by
dividing each encoder cycle into four quarters, or quadrature
counts.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 with their complements.These
devices reduce system sensitivity to noise. Incremental encoders
may also produce a third signal known as the index or marker
pulse.This signal appears once per revolution and can be used for
initialization purposes.
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
2
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