Texas Instruments TIDA-00830 User manual
Test Report
TIDA-00830 –November 2015
1
TIDA-00830 24V Stepper Motor Design with AutoTune™ TI Reference Design
Design Overview
TIDA-00830 is an application overview of TI’s
automatic stepper motor tuning feature known as
AutoTune™. This design showcases how quickly
AutoTune™ can adapt to changes in system inputs or
motor characteristics without the user having to apply
any type of parameter tuning. The hardware for this
design is sourced from the DRV8880 Evaluation
Module so the user has the ability to work with the
entire feature set of this device.
Design Resources
TIDA-00830 Design Folder
DRV8880 Product Folder
MSP430F2617 Product Folder
FT232RL Product Folder
STEP/DIR
Step size
Decay mode
UART
USB
Stepper
Motor
DRV8880
MSP430
FT232RL
3.3-V LDO
VM
Design Features
6.5- to 45-V operation with up to 2.0-A full-
scale (max drive current)
Configurable PWM Off-Time of 10-, 20-, or
30-µs
3.3-V, 10-mA LDO Regulator supply
AutoTune stepper motor tuning
Simple GUI for driver input control and motor
tuning
Onboard USB communication for easy
connectivity with external controller
100 mil header gives test probe access to all
driver input controls
Featured Applications
Automatic Teller and Money Handling
Machines
3D Printers
Factory Automation and Robotics
Video Security Cameras
1. Introduction
For stepper motors, a common design problem is tuning. Tuning involves selecting the best decay
mode to operate the stepper motor efficiently. A well-behaved current waveform makes for a well-
behaved stepper. These steppers will run smoothly (less vibration), quietly (less noise) and efficiently
(less power dissipation). Unfortunately, there are a lot of factors that can affect the waveform, such as
supply voltage, motor parameters, motor speed and load. Finding a fixed decay mode to match these
settings can sometimes be difficult, especially if these parameters shift during operation. Texas
Instruments AutoTune offered in the DRV8880 makes this process much simpler by determining the
proper decay mode and fast decay percentage based on individual motor characteristics and system
inputs.
TIDA-00830 24V Stepper Motor Design with AutoTune™ TI Reference Design
2
2. Current Limiting
In order to properly commutate a stepper motor, the controller device must continuously regulate the
current through the motor coils at predetermined current values. An overview of the current regulation
scheme found in the DRV8880 is shown in Figure 1 below.
Sine DAC
VREF TRQ[0:1]
Device
Logic 1/Av
Internal
H-Bridge Sense
Resistor
PWM Logic
+
-
1
2
3
4
TRQ[1:0] = 00 –100%
TRQ[1:0] = 01 –75%
TRQ[1:0] = 10 –50%
TRQ[1:0] = 11 –25%
TIME
VOLTAGE
Figure 1. Single Phase Current Regulation Control Scheme Figure 2. 1/4 Step Current Regulation Scaled by TRQ Percentage
The DRV8880 adjusts the current flowing through each phase of a stepper motor by internally
comparing a reference voltage generated by the internal sine DAC to the voltage measured across an
external sense resistor. This internal sine DAC is scaled based on the value of VREF and has a preset
table of voltage levels for each micro-stepping state. Current magnitude can be scaled using the
percent torque inputs TRQ[1:0] as shown in Figure 2 above. Adjusting the current magnitude using
just the TRQ[1:0] bits simplifies the control process by not requiring a change in VMor a change in
VREF.If the voltage across the sense resistor is larger than the internal comparison voltage, the device
enters a different decay mode depending on the state of the decay mode setting pins and ATE pin.
Depending on if the sine DAC table is increasing or decreasing steps will also affect the decay mode
the device uses. All of these features included in the DRV8880s current regulation system allow the
driver to have constant control over how much current is being supplied to the stepper motor.
3. Current Ripple
While the device is enforcing a specific current level during each step, there is a tradeoff between using
slow or fast decay modes for current regulation. If the device uses too much fast decay then the level of
current ripple in the power supply will increase and can result in unwanted noise and vibration in the
stepper motor. However, if the device does not use enough fast decay then the current in the motor
will gradually increase and result in the device not being able to effectively regulate current. This can
damage the motor and create an uneven driving pattern. Some stepper motor drivers utilize a decay
mode called mixed decay to operate while the device is lowering current through the motor windings.
Mixed decay works by including a percentage of fast decay during tOFF and then the remaining amount
of tOFF is held at slow decay. This allows the current to drop quickly during the fast decay interval and
then slowly dissipate during the slow decay period, thus keeping the current ripple to a minimum. The
downside to this form of current decay is that each tOFF interval is often enforced regardless of how
TIDA-00830 24V Stepper Motor Design with AutoTune™ TI Reference Design
3
much fast decay is needed. An example of a motor tuning that utilizes too much fast decay at some
steps is shown in Figure 3 below. This results in a large amount of power supply decay current ripple.
Figure 3. Supply Current Ripple Using Fast / Slow Decay Figure 4. Supply Current Ripple Using AutoTune
To avoid this issue, the DRV8880 has an adaptive decay mode called AutoTune™that automatically
adjusts the amount of fast decay inserted during tOFF to best suit the motor characteristics and load
conditions at any instance in time. An example of AutoTune working is shown in Figure 4 above.
4. AutoTune
By utilizing AutoTune™, the user can simply connect a motor and the driver will begin spinning it in the
most effective way possible without requiring any other parameter tuning. AutoTune operates by
actively calculating and adjusting the current decay. Each drive cycle has a mandatory tBLANK interval to
drive the motor. Once the tBLANK interval has elapsed, the device can continue with the remainder of
the tDRIVE interval if it has not hit the current limit ITRIP, or stop driving and begin decaying current if the
current limit has been reached based on whichever decay setting AutoTune™chooses.
tBLANK
tDRIVE
tSLOW
tOFF
tFAST
ITRIP
Increasing Phase Current (A)
Figure 5. Mixed Decay Timing Profile
If AutoTune™ selects a mixed decay mode the driver actively inserts different percentages of fast
decay during each tOFF interval, and effectively lowers the current ripple while driving the motor in the
most efficient way possible. An example of a mixed decay current profile is shown in Figure 5 above.
Since the driver is allowed to actively change the decay pattern it can also easily adapt to changing
motor operation factors without requiring the user to retune any of the driver parameters. This is an
especially relevant feature in applications where the motor will be run for long periods of time and aging
effects need to be considered.
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 VM (Motor Voltage)
changes is shown in Figure 9 below.
Figure 8. AutoTune Adaption to VMVariation Figure 9. Slow / Fast Decay Adaptation to VMVariation
TIDA-00830 24V Stepper Motor Design with AutoTune™ TI Reference Design
5
This additional chopping is an effect of the device trying to regulate the large amount of current that is
now present in the coils as compared to before. If the previous tune has a barely acceptable amount of
current ripple, then the change in VM will cause a larger amount of ripple. This can result in instability
and other issues on devices sharing the VM rail throughout the rest of the design. By utilizing
AutoTune™in the DRV8880, the device can quickly tune the stepper motor to run at this voltage and
regulate the current accordingly as shown in Figure 8 above.
5.3. Temperature Effects
A typical application may require the stepper motor to run at a continuous speed with a large load for a
prolonged period of time, which can cause the stepper motor to significantly increase in temperature.
This increase in temperature results in an increase in the motor resistance, causing the motor to start
drawing less current from the power supply. If the motor were tuned at room temperature and without
any load that same tuning may no longer be ideal once temperature increases. AutoTune™can
actively adapt to changes in motor resistance caused by the increase in temperature. Using a QSH
4218-41-10-035 stepper motor running at 1000 PPS under loaded conditions for 350 seconds at 1/8
micro-stepping the following temperature plot and current profile were captured in Figure 10 and 11
below.
Figure 10. Stepper Motor Thermal Characteristics Figure 11. Stepper Motor Current Profile
6. Board Layout
This design is intended to allow the user to quickly
test AutoTune’s capabilities against a variety of
different stepper motors without having to tune a lot
of different motor parameters. This design also offers
the full functionality of a Texas Instruments EVM,
allowing the entire feature set of the device to be
configurable via the supplied GUI and a 100 mil
header located at the center of the board for the
driver inputs. This board also includes an onboard
USB –UART IC for quick interfacing with an MSP430
that controls fault handling and motor commutation.
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