User manual

operating in the velocity mode. If no tachometer is used, typically you will be
operating in the current mode. In both these modes, the amplifier operates as a current
source, and the difference between them is in the setup of the servo preamplifier, the
section between the PWM power stage and the input differential amplifier. The
amplifier can also be operated as a voltage source. In this fashion, a given reference
voltage will produce an output voltage that will remain constant over a range of
output currents.
CURRENT MODE
Commonly used with microprocessor controls that use encoders for motion feedback
and don’t use tachometers. The amplifier acts as a wide bandwidth current source that
puts out its peak current rating in response to a +/-10V reference voltage input. In this
mode the servo preamplifier operates at low gain, and the PWM stage as a current
source, changing the output voltage in order to maintain a constant output current.
VELOCITY MODE
Used with a tachometer. In this mode the amplifier continues to act as a wide
bandwidth current source. The servo preamplifier operates at high gain, because it is
amplifying the difference of the tachometer and the reference signal. The PWM stage
again operates as a current source to accelerate / decelerate the motor to produce an
RPM which makes the tachometer signal equal to the reference signal.
VOLTAGE MODE
This mode is used less frequently because it produces lower performance from the
motor. As a voltage amplifier, an input voltage will produce an output voltage, which
will remain relatively constant over a range of output currents. The servo preamplifier
now amplifies the difference between the reference voltage, and the scaled down
sample of the output voltage.
VOLTAGE MODE WITH IR COMPENSATION
This is a variation of voltage mode operation in which the servo preamplifier sees not
only the reference and output voltages, but also the output current. Because the
armature voltage ( back emf, which produces the motor RPM ) is the difference
between the motor terminal voltage and the internal drop across the armature
resistance, the it will decrease when greater loads produce greater currents. This
current produces an “IR” drop ( current x resistance = voltage ) which subtracts from
the terminal voltage of the amplifier. IR compensation mode increases the amplifier
output when the load current increases. When adjusted correctly, the increase will just
compensate the IR drop and the motor speed will remain constant without a
tachometer. The actual regulation will not be as good as with a tach, but is simpler
and cheaper than using a tach, and is adequate for many situations.