Texas Instruments TIDA-00830 Manuale utente

Test Report

TIDA-00830 –November 2015

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

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. 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

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

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

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

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.

IMPORTANT NOTICE AND DISCLAIMER

TI PROVIDES TECHNICAL AND RELIABILITY DATA (INCLUDING DATA SHEETS), DESIGN RESOURCES (INCLUDING REFERENCE

DESIGNS), APPLICATION OR OTHER DESIGN ADVICE, WEB TOOLS, SAFETY INFORMATION, AND OTHER RESOURCES “AS IS”

AND WITH ALL FAULTS, AND DISCLAIMS ALL WARRANTIES, EXPRESS AND IMPLIED, INCLUDING WITHOUT LIMITATION ANY

IMPLIED WARRANTIES OF MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE OR NON-INFRINGEMENT OF THIRD

PARTY INTELLECTUAL PROPERTY RIGHTS.

These resources are intended for skilled developers designing with TI products. You are solely responsible for (1) selecting the appropriate

TI products for your application, (2) designing, validating and testing your application, and (3) ensuring your application meets applicable

standards, and any other safety, security, regulatory or other requirements.

These resources are subject to change without notice. TI grants you permission to use these resources only for development of an

application that uses the TI products described in the resource. Other reproduction and display of these resources is prohibited. No license

is granted to any other TI intellectual property right or to any third party intellectual property right. TI disclaims responsibility for, and you

will fully indemnify TI and its representatives against, any claims, damages, costs, losses, and liabilities arising out of your use of these

resources.

TI’s products are provided subject to TI’s Terms of Sale or other applicable terms available either on ti.com or provided in conjunction with

such TI products. TI’s provision of these resources does not expand or otherwise alter TI’s applicable warranties or warranty disclaimers for

TI products.

TI objects to and rejects any additional or different terms you may have proposed. IMPORTANT NOTICE

Mailing Address: Texas Instruments, Post Office Box 655303, Dallas, Texas 75265

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