TIDA-00830
24V Stepper Motor Design with AutoTune™
TI Reference Design
4
5. AutoTune Adaptation
5.1.
Torque DAC
In applications that require a driver to vary the amount of torque being supplied to the motor, the
DRV8880 allows the user to dynamically adjust a torque register (TRQ[0:1]) to change the magnitude
of current supplied to the motor. Once the torque register has changed, the device must change the
motor current regulation pattern and actively adapt the level of current supplied to the motor as shown
in
Figure 1
on page 2. The amount of time it takes for current in the motor windings to change is
dependent on the decay patterns ability to adapt to this new current limit while also considering the
inductance of the motor windings.
Figure 6
and
Figure 7
below show the interval where the torque
value is changed from 100% to 25% torque using AutoTune and Slow / Fast decay modes. In
Figure
6
,
AutoTune
™ is being used to quickly adapt to the change in required torque and keep the regulation
pattern constant. Minimal current ripple is observed during the lower current steps.
Figure 7
uses a
fixed decay setting where the current decreases more slowly through the winding until it falls below the
current limit.
Figure 6.
AutoTune 100% - 25% Torque Change
Figure 7.
Slow / Fast Decay 100% - 25% Torque Change
5.2.
Power Supply Variations
In a typical stepper motor power supply, there is the possibility for voltage variations caused by
capacitance instability, conducted / inducted noise, and other system design issues. With a typical
decay pattern and motor tuning, the driver may not be able to adapt to a sudden change in voltage
supplied to the motor rail. An example of how the current ripple can vary as V
M
(Motor Voltage)
changes is shown in
Figure 9
below.
Figure 8.
AutoTune Adaption to V
M
Variation
Figure 9.
Slow / Fast Decay Adaptation to V
M
Variation
