Stepper motors require some external electrical
components in order to run. These components typically include a power supply,
logic sequencer, switching components and a clock pulse source to determine
the step rate. Many commercially available drives have integrated these
components into a complete package. Some basic drive units have only the
final power stage without the controller electronics to generate the proper
This is a very popular drive for a two phase
bipolar motor having four leads. In a complete driver/controller the electronics
alternately reverse the current in each phase. The stepping sequence is
"Stepper Motor Theory": Figure 5.
This drive requires a motor with a center-tap
at each phase (6 leads).
Instead of reversing the current in each phase, the drive only has to switch
current from one coil to the other in each phase ("Stepper Motor Theory": Figure
6). The windings are such that
this switching reverses the magnetic fields within the motor. This option
makes for a simpler drive but only half of the copper winding is used at
any one time. This results in approximately 30% less available torque in
a rotary motor or force in a linear actuator as compared to an equivalent
This type of drive is also referred to as a constant
voltage drive. Many of these drives can be configured to run bipolar or
unipolar stepper motors. L/R stands for the electrical relationship of inductance
(L) to resistance (R). Motor coil impedance vs. step rate is determined
by these parameters. The L/R drive should “match” the power supply output
voltage to the motor coil voltage rating for continuous duty operation.
Most published motor performance curves are based on full rated voltage
applied at the motor leads. Power supply output voltage level must be set
high enough to account for electrical drops within the drive circuitry for
optimum continuous operation.
Performance levels of most steppers can be improved
by increasing the applied voltage for shortened duty cycles. This is typically
referred to as “over-driving” the motor. When over-driving a motor, the
operating cycle must have sufficient periodic off time (no power applied)
to prevent the motor temperature rise from exceeding the published specification.
A chopper drive allows a stepper motor to maintain
greater torque or force at higher speeds than with an L/R drive. The chopper
drive is a constant current drive and is almost always the bipolar type.
The chopper gets its name from the technique of rapidly turning the output
power on and off (chopping) to control motor current. For this setup, low
impedance motor coils and the maximum voltage power supply that can be used
with the drive will deliver the best performance. As a general rule, to
achieve optimum performance, the recommended ratio between power supply
and rated motor voltage is eight to one. An eight to one ratio was used
for the performance curves in this catalog.
Many bipolar drives offer a feature called microstepping.
Microstepping electronically divides a full step into smaller steps. For
instance, if one step of a linear actuator is 0.001 inch, this can be driven
to have 10 microsteps per step. In this case, one microstep would normally
be 0.0001 inch. Microstepping effectively reduces the step increment of
a motor. However, the accuracy of each microstep has a larger percentage
of error as compared to the accuracy of a full step. As with full steps,
the incremental errors of microsteps are non-cumulative.