Specifications
121Motor Control
Controllers for Stepper Motors
Stepper motors are constant power
motors if driven with a constant supply
voltage. As speed increases, torque
decreases. This happens because of
the limitation on current ramp rates in
the windings due to their inductance.
Maximum torque is realized at zero
speed. So to increase torque at higher
speeds, high-voltage drivers with
current limiting are sometimes used
(Figure 6). These are called “chopper
drives,” and are designed to generate
a nearly constant current in each
winding rather than simply switching a
constant voltage. On each step, a very
high voltage is applied to the winding.
When the current limit is reached, the
voltage is turned o or “chopped.” At
this point the winding current starts
ramping down to a lower limit where
the voltage is again turned on, keeping
the winding current relatively constant
for a particular step position. The
additional electronics to sense winding
currents and to control the switching
adds some cost and complexity, but
it allows stepper motors to be driven
with high torque at high speed.
Microprocessors are commonly
incorporated in stepper motor drivers
to provide the controls needed.
Sophisticated control capability is
common for stepper motors since they
are often employed in machines that
require fast precision movements, such
as in robotics. Acceleration/deceleration
proles, holding torque, and other
parameters are often provided for.
Switched Reluctance Motors (SRMs)
Switched reluctance motors (SRMs)
are a form of stepper motor, but are
usually much larger and have fewer
poles than the traditional stepper
motor. The key to these motors is that
the rotor is made of only ferromagnetic
material and has no windings. It is a
very reliable, low-maintenance motor
with high power density at low cost,
all of which come at the expense of
more complex electronic controls.
Opposing stator poles are energized in
sequence and the rotor poles closest
to the energized stator poles become
magnetized and are attracted to them,
reducing magnetic reluctance when
brought into alignment. Before full
alignment is achieved, the next phase
is energized to keep the motor turning.
There is no need for any transfer of
electrical power to the rotor so there are
no brushes, commutators, or slip rings.
With electrical commutation there are
no sparks so these motors can be used
in explosive environments. They are also
good for holding a load in a stationary
position for long periods of time.
DISPLAY
DRIVER
SWITCH
DEBOUNCER
KEYBOARD
SCANNER
KEYBOARD
SWITCHES
DISPLAY
FRONT PANEL
µP
V
CC
V
CC
BUCK
V
DC
OP AMP
UART ISOLATION
TRANSCEIVER
FIELDBUS
ADC
GATE
DRIVER
GATE
DRIVER
STEPPER
MOTOR
SUPERVISOR
V
CC
V
DC
SHDN
BOOST
V
DC
TEMPERATURE
SENSOR
Figure 6. Controller for stepper motor. The boost regulator and the current sense per phase allow current to ramp quickly in each pole of the motor. Motor response is fast. When the maximum current per
phase is reached, the boost regulator is shut down until the minimum current per phase is reached again. The cycle is repeated until the next step is made.