Stepper motors made easy
with AutoTune™
Sudhir Nagaraj
Design Engineer, Motor Drive Business Unit
Texas Instruments
An intelligent decay scheme continuously adapts
to provide the best possible decay solution
To handle re-circulation current in a current-chopping stepper motor drive, traditional
decay schemes such as fast decay, slow decay and fixed-mixed decay fall short of
optimal micro-stepping current regulation. End users often compromise certain
micro-stepping performance parameters in order to achieve others. What if there was
no compromise?
This white paper introduces an intelligent decay scheme that continuously adapts to
provide the best possible decay solution according to demands. This feature can now
be integrated into a motor driver integrated circuit (IC) eliminating the need for the end
user to tune the motor.
Stepper motor: A brief overview
Stepper motors are ubiquitous. They are used in a wide
range of applications from robots, printers, industrial position
control, projectors, cameras and so many more.
A stepper motor typically has two electrical windings.
An H-bridge is used to drive each winding. Motor position is
controlled by regulating the current in motor windings.
For a smooth motor motion profile and finer position control,
micro-stepping is desired. While micro-stepping, the current
Figure 1: Sine and cosine functions of micro-stepping
in these windings is regulated in a sine (red) and cosine (blue)
function (Figure 1). Each step corresponds to a preset current level. Having a non-optimal decay scheme
does not allow for good micro-stepping, which translates to poor motor position control.
xVM
What is decay?
Decay is defined as re-circulation current in the drive switches
and diodes once the drive is interrupted, which is common in
a pulse-width modulation (PWM) current regulation/chopping
technique. The drive current typically is interrupted once the
chopping current threshold is achieved. To handle this decay
current, the H-bridge can operate in two different states: fast
1
1
Drive Current
2
Fast Decay
3
Slow Decay
xOUT2
xOUT1
2
decay and slow decay. Mixed decay, a combination of fast and
slow decay, is also employed. These states are shown in
3
Figure 2 for a positive current flow.
Figure 2: Sine and cosine functions of micro-stepping
Stepper
motors made easy with AutoTune™
2
March 2016
A typical PWM cycle and sequence of events in time
is depicted in Figure 3. The various decay modes
from this figure are described below
In slow decay, current is re-circulated using both
low-side FETs. However, the slow rate of current
decrease during winding limits some current levels
being regulated.
In fast decay, the H-bridge reverses the voltage
across the winding. This decreases the current at
a much faster rate. The limitation with fast decay
is that the current charge and discharge rates are
Figure 3: Current waveforms in slow, fast and mixed
decay modes
similar; thus, the ripple current can be huge. This
leads to inefficiency and limits some current levels
compromises when choosing the best scheme,
that can be regulated.
such as:
Mixed decay is a combination of slow and fast
• Optimizing for quick-step rate (by setting a
decay. It begins with fast decay and after a fixed
higher mixed-decay percent) leads to excessive
time, switches to slow decay mode. Fixed-mixed
ripple in current regulation (while holding in
decay also has its limitations. A percentage of PWM
a step).
cycle or a combination of slow and fast decay
• Decay scheme for a fresh battery may not be
needs to be optimized for a given motor, stepping
the same for a battery declining in power.
rate, magnitude of load current and supply voltage.
• Optimal decay schemes differ greatly when
Lower load current levels typically need a different
handling current close to zero when compared
percentage mix compared to higher current levels.
to handling current at peak.
Problem statement
• An aggressive decay setting (higher percent of
There are several limitations of conventional fixed-
fast decay in the mixed-decay cycle) chosen to
decay schemes (slow, fast and mixed decay). One is
counter back electromotive force (BEMF) effects
the inability to precisely regulate current, which limits
causes excess ripple while regulating current in
micro-stepping resolutions. Conventional fixed-
most steps.
decay also needs to be tuned by the user to identify
• Initial tuned decay may not be good for a
the most favorable setting. Finally, fixed-decay does
resistive, end-of-life motor.
not adjust to varying parameters such as supply
When searching for improvement in handling decay
voltage, load current, back electromotive force
current, carefully analyzing the limitations can bring
(BEMF) and rate of micro-stepping.
about questions. Can we have different decay
The best scheme is often chosen by cycling through
schemes for different levels of micro-stepping
the available fixed-decay options while observing
current? Can we separate the decay approach for
current on the oscilloscope.
current regulation and step change? Can the decay
This is time-consuming and still leads to
scheme change as a response to changing loads,
varying supply voltage and changing BEMF? Let’s
find out.
Stepper
motors made easy with AutoTune™
3
March 2016
Problem solution
The answers to these questions are yes, yes
and yes. The proposed solution addresses two
main requirements.
The first is to identify the best possible decay
for a given level of current regulation for a
given step. In this approach, during current
regulation, the controller keeps track of where
Itrip (the signal when coil current reaches
target current) happens in a given PWM cycle. It
Figure 4: The DRV8846 has a noise advantage of 16.5% lower than the
nearest competition
recalls from memory the ‘Itrip event and timing’
The AutoTune decay scheme self-adjusts to changing:
from the previous cycle, and then dynamically
a. supply voltage
decides what decay action is needed for the
b. load inductance
current cycle.
c. load resistance
The second is to provide a quick transition from one
d. rate of stepping BEMF in a stepper motor
step to another. As a response to step command,
e. magnitude of current to be regulated (torque).
scaling up the percentage of fast decay allows us
This is all offered without sacrificing ripple and step
to aggressively reach a new level in a shorter time,
performance. As an example, Figure 5 shows
thereby providing a quick-step response.
a current waveform without employing adaptive
This solution is incorporated into a stepper motor
decay. The distortion due to BEMF is eliminated by
driver, such as the DRV8846, or AutoTune . It is an
AutoTune. Figure 6 shows the current waveform
all-inclusive digital scheme with no tuning required
when AutoTune is engaged.
TM
by the user. The solution gives the optimal decay
setting in any given situation. This decay setting is
modified in real time to changing parameters such
as current level, step change, supply, BEMF
and load.
Advantages
Figure 5: Shows loss of current regulation in the presence of BEMF
There are several advantages to this approach. No
tuning is required in an adaptive decay scheme.
Also, smaller ripple makes the average current more
accurate to the desired step current in peak current
detect regulation. This enables higher levels of
micro-stepping, leading to smoother motion for the
stepper motor. Smaller ripple also reduces noise in
the motor and drive electronics, shown in Figure 4.
Figure 6: This chart shows how a stepper motor with adaptive
decay tames BEMF
Stepper
motors made easy with AutoTune™
4
March 2016
This scheme saves device pins that set traditional
fixed decay, which reduces system cost. This
scheme also enables quicker step transition or
response time, (Figure 7, right plot) than most
conventional decay modes (left plot), without
Figure 9: Fixed slow decay mode versus AutoTune on TI’s
DRV8846
causing excessive ripple in current regulation
while holding a step between adjacent steps. This
Low-current regulation (near zero crossing of
example provides around a three-times faster step
micro-stepping sine) performance is improved with
response time.
this adaptive decay scheme. This is because the
adaptive decay scheme enables low ripple at lower
currents similar to slow decay. However, it does not
cause loss of regulation like slow decay does.
Slow decay causes loss of regulation because the
Figure 7: caption rewrite: A 600uS step transition in fixed decay
vs 200 uS transition in AutoTune adaptive decay mode
amount of current built up during the minimum ON
time is greater than the amount of current reduced
Using slow decay cycles wherever possible makes
by slow decay. Slow decay happens due to voltage
an adaptive decay scheme more power-efficient.
drop in the current path/loop. The lower the loop
This is because slow decay minimizes switching
current, the smaller the voltage drop. Hence, the
losses and is typically done using low-side FETs that
smaller amounts of current decayed.
are more power-efficient. In the plots in
Figure 8, blue is the current in the coil being
Conclusion
regulated. Pink and yellow are the H-bridge output
An adaptive decay scheme, like TI’s AutoTune,
voltage waveforms showing output switching. Pink
promises to be the future of decay in motor current
spikes indicate reverse FET voltage for fast/mixed
regulation. This plug-and-play solution enables
decay. The plot to the right employs AutoTune, TI’s
greater current regulation and micro-stepping
adaptive decay feature. It uses fast/mixed decay
performance. The intelligent solution keeps track
sparingly compared to the fixed-mixed decay case
and adjusts decay for varying supply voltages,
shown in the left plot. This makes using AutoTune
load currents, load inductance BEMF, and motor
power efficient.
variations over operating lifetime, ensuring the
optimal decay for any given situation. Power
efficiency improvement is another key benefit.
Higher performance that does not need tuning
enables quicker time-to-market. Users no longer
need to be concerned about having to tackle decay
on their way to spinning a motor successfully.
Figure 8: Fixed mixed decay versus AutoTune
Stepper
motors made easy with AutoTune™
5
March 2016
References
1. DRV8846 product folder
2. DRV8880 product folder
3. Current Recirculation and Decay Modes Application Report
4. More information about motor drivers
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Stepper
motors made easy with AutoTune™
SLYY066A
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March 2016
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