Stepper motors made easy with AutoTune (Rev. A) motors made easy with AutoTune Sudhir Nagaraj Design...
Transcript of Stepper motors made easy with AutoTune (Rev. A) motors made easy with AutoTune Sudhir Nagaraj Design...
Stepper motors made easy with AutoTune™
Sudhir NagarajDesign Engineer, Motor Drive Business UnitTexas Instruments
2 Stepper motors made easy with AutoTune™ March 2016
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
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.
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
decay and slow decay. Mixed decay, a combination of fast and
slow decay, is also employed. These states are shown in
Figure 2 for a positive current flow.
Figure 1: Sine and cosine functions of micro-stepping
1
2
3
xOUT1 xOUT2
xVM
Drive Current
Fast Decay
Slow Decay
1
2
3
Figure 2: Sine and cosine functions of micro-stepping
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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
similar; thus, the ripple current can be huge. This
leads to inefficiency and limits some current levels
that can be regulated.
Mixed decay is a combination of slow and fast
decay. It begins with fast decay and after a fixed
time, switches to slow decay mode. Fixed-mixed
decay also has its limitations. A percentage of PWM
cycle or a combination of slow and fast decay
needs to be optimized for a given motor, stepping
rate, magnitude of load current and supply voltage.
Lower load current levels typically need a different
percentage mix compared to higher current levels.
Problem statement
There are several limitations of conventional fixed-
decay schemes (slow, fast and mixed decay). One is
the inability to precisely regulate current, which limits
micro-stepping resolutions. Conventional fixed-
decay also needs to be tuned by the user to identify
the most favorable setting. Finally, fixed-decay does
not adjust to varying parameters such as supply
voltage, load current, back electromotive force
(BEMF) and rate of micro-stepping.
The best scheme is often chosen by cycling through
the available fixed-decay options while observing
current on the oscilloscope.
This is time-consuming and still leads to
compromises when choosing the best scheme,
such as:
• Optimizing for quick-step rate (by setting a
higher mixed-decay percent) leads to excessive
ripple in current regulation (while holding in
a step).
• Decay scheme for a fresh battery may not be
the same for a battery declining in power.
• Optimal decay schemes differ greatly when
handling current close to zero when compared
to handling current at peak.
• An aggressive decay setting (higher percent of
fast decay in the mixed-decay cycle) chosen to
counter back electromotive force (BEMF) effects
causes excess ripple while regulating current in
most steps.
• Initial tuned decay may not be good for a
resistive, end-of-life motor.
When searching for improvement in handling decay
current, carefully analyzing the limitations can bring
about questions. Can we have different decay
schemes for different levels of micro-stepping
current? Can we separate the decay approach for
current regulation and step change? Can the decay
scheme change as a response to changing loads,
varying supply voltage and changing BEMF? Let’s
find out.
Figure 3: Current waveforms in slow, fast and mixed decay modes
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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
recalls from memory the ‘Itrip event and timing’
from the previous cycle, and then dynamically
decides what decay action is needed for the
current cycle.
The second is to provide a quick transition from one
step to another. As a response to step command,
scaling up the percentage of fast decay allows us
to aggressively reach a new level in a shorter time,
thereby providing a quick-step response.
This solution is incorporated into a stepper motor
driver, such as the DRV8846, or AutoTuneTM. It is an
all-inclusive digital scheme with no tuning required
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
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.
The AutoTune decay scheme self-adjusts to changing:
a. supply voltage
b. load inductance
c. load resistance
d. rate of stepping BEMF in a stepper motor
e. magnitude of current to be regulated (torque).
This is all offered without sacrificing ripple and step
performance. As an example, Figure 5 shows
a current waveform without employing adaptive
decay. The distortion due to BEMF is eliminated by
AutoTune. Figure 6 shows the current waveform
when AutoTune is engaged.
Figure 4: The DRV8846 has a noise advantage of 16.5% lower than the nearest competition
Figure 5: Shows loss of current regulation in the presence of BEMF
Figure 6: This chart shows how a stepper motor with adaptive decay tames BEMF
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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
causing excessive ripple in current regulation
while holding a step between adjacent steps. This
example provides around a three-times faster step
response time.
Using slow decay cycles wherever possible makes
an adaptive decay scheme more power-efficient.
This is because slow decay minimizes switching
losses and is typically done using low-side FETs that
are more power-efficient. In the plots in
Figure 8, blue is the current in the coil being
regulated. Pink and yellow are the H-bridge output
voltage waveforms showing output switching. Pink
spikes indicate reverse FET voltage for fast/mixed
decay. The plot to the right employs AutoTune, TI’s
adaptive decay feature. It uses fast/mixed decay
sparingly compared to the fixed-mixed decay case
shown in the left plot. This makes using AutoTune
power efficient.
Low-current regulation (near zero crossing of
micro-stepping sine) performance is improved with
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
amount of current built up during the minimum ON
time is greater than the amount of current reduced
by slow decay. Slow decay happens due to voltage
drop in the current path/loop. The lower the loop
current, the smaller the voltage drop. Hence, the
smaller amounts of current decayed.
Conclusion
An adaptive decay scheme, like TI’s AutoTune,
promises to be the future of decay in motor current
regulation. This plug-and-play solution enables
greater current regulation and micro-stepping
performance. The intelligent solution keeps track
and adjusts decay for varying supply voltages,
load currents, load inductance BEMF, and motor
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 7: caption rewrite: A 600uS step transition in fixed decay vs 200 uS transition in AutoTune adaptive decay mode
Figure 8: Fixed mixed decay versus AutoTune
Figure 9: Fixed slow decay mode versus AutoTune on TI’s DRV8846
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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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