A4979 Datasheet, PDF (27/44 Pages) Allegro MicroSystems – The A4979 is a flexible microstepping motor driver with built-in translator for easy operation.

A4979
Microstepping Programmable Stepper Motor Driver
With Stall Detect and Short Circuit Protection
(see figure 11). The remaining bits in the Run register should be
set for the required configuration and sent with the step change
number each time.
The step rate is controlled by the timing of the serial interface.
It is the inverse of the step time, t_STEP, shown in figure 10. The
motor step only takes place when the STRn goes from low to
high when writing to the Run register. The motor step rate is
therefore determined by the timing of the rising edge of the STRn
input. The clock rate of the serial interface, defined by the fre-
quency of the SCK input, has no effect on the step rate.
Using the Phase Table Load Capability
Torque Ripple Reduction
The performance and audible noise of any motor drive system is
defined, to a large extent, by the torque ripple generated by both
the motor and the load. In most cases, when using a stepper motor
as the mechanical drive, the torque ripple of the load is not related
to the mechanical steps of the motor and must be reduced by
means unrelated to the motor and its drive system. However, for
stepper motors in particular, torque ripple produced by the motor
can be reduced by improvements in the mechanical design of the
motor and by improvements in the phase current control system.
Torque ripple will naturally be high when driving a stepper motor
in full step mode, due to the nature of stepping. However the
torque ripple can be reduced by using microstepping. Increas-
ing the number of microsteps per mechanical step will result in
reduced torque ripple. This is one of the major reasons for using
microstepping.
In the majority of cases the standard sinusoidal, microstep current
profile will be sufficient to achieve a good performance with
a good quality motor. In a few cases, further improvements in
torque ripple performance may be achieved by modifying the
microstep current profile to more closely match the motor charac-
teristics. This is usually only necessary for higher quality, higher
power stepper motors.
When using microstepping, the torque ripple is defined by the
variation in torque at each microstep. In a hybrid stepper motor
this is mostly determined by the mechanical construction of the
motor, particularly the shape of the teeth on the poles of the sta-
tor. The shape of these teeth determine the variation in the torque
constant, the ratio between current and torque, as the motor
rotates. The variation in the torque constant can be seen by mea-
suring the back EMF of the motor when being driven as a genera-
tor, that is when the shaft is driven by external means and the
phase voltage is monitored. The back EMF represents the motor
constant, which is essentially proportional to the torque constant.
If such torque ripple reduction measures are required, the A4979
provides the ability to modify the microstep current profile by
programming the internal phase current table through the serial
interface. The modified profile is then used, in place of the default
sinusoidal profile, to compensate for any variation in motor torque
constant. The current at each Step Angle Number can be set to suit
the microstep current profile requirements of a specific motor.
Note: This is an advanced feature of the A4979, which will not be
required for most applications. In general the default sinusoidal
profile will suffice and therefore the phase current table does not
have to be loaded.
Figure 11. Example current profile
Loading the Phase Current Table
The full phase current table in the A4979 contains one 6-bit value
for each phase, at each microstep position. With 16 microsteps
Allegro MicroSystems, Inc.
27
115 Northeast Cutoff
Worcester, Massachusetts 01615-0036 U.S.A.
1.508.853.5000; www.allegromicro.com