stepper motors made easy with autotune (rev. a) motors made easy with autotune sudhir nagaraj design...

7
Stepper motors made easy with AutoTune™ Sudhir Nagaraj Design Engineer, Motor Drive Business Unit Texas Instruments

Upload: phungngoc

Post on 01-Jul-2018

227 views

Category:

Documents


0 download

TRANSCRIPT

Page 1: Stepper motors made easy with AutoTune (Rev. A) motors made easy with AutoTune Sudhir Nagaraj Design Engineer, Motor Drive Business Unit Texas Instruments AutoTune 2 26 An intelligent

Stepper motors made easy with AutoTune™

Sudhir NagarajDesign Engineer, Motor Drive Business UnitTexas Instruments

Page 2: Stepper motors made easy with AutoTune (Rev. A) motors made easy with AutoTune Sudhir Nagaraj Design Engineer, Motor Drive Business Unit Texas Instruments AutoTune 2 26 An intelligent

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

Page 3: Stepper motors made easy with AutoTune (Rev. A) motors made easy with AutoTune Sudhir Nagaraj Design Engineer, Motor Drive Business Unit Texas Instruments AutoTune 2 26 An intelligent

3 Stepper motors made easy with AutoTune™ 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

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

Page 4: Stepper motors made easy with AutoTune (Rev. A) motors made easy with AutoTune Sudhir Nagaraj Design Engineer, Motor Drive Business Unit Texas Instruments AutoTune 2 26 An intelligent

4 Stepper motors made easy with AutoTune™ 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

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

Page 5: Stepper motors made easy with AutoTune (Rev. A) motors made easy with AutoTune Sudhir Nagaraj Design Engineer, Motor Drive Business Unit Texas Instruments AutoTune 2 26 An intelligent

5 Stepper motors made easy with AutoTune™ 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

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

Page 6: Stepper motors made easy with AutoTune (Rev. A) motors made easy with AutoTune Sudhir Nagaraj Design Engineer, Motor Drive Business Unit Texas Instruments AutoTune 2 26 An intelligent

6 Stepper motors made easy with AutoTune™ March 2016

SLYY066A

The platform bar is a trademark of Texas Instruments. All other trademarks are the property of their respective owners.

© 2016 Texas Instruments IncorporatedPrinted in the U.S.A.

Important Notice: The products and services of Texas Instruments Incorporated and its subsidiaries described herein are sold subject to TI’s standard terms and conditions of sale. Customers are advised to obtain the most current and complete information about TI products and services before placing orders. TI assumes no liability for applications assistance, customer’s applications or product designs, software performance, or infringement of patents. The publication of information regarding any other company’s products or services does not constitute TI’s approval, warranty or endorsement thereof.

References

1. DRV8846 product folder

2. DRV8880 product folder

3. Current Recirculation and Decay Modes Application Report

4. More information about motor drivers

Page 7: Stepper motors made easy with AutoTune (Rev. A) motors made easy with AutoTune Sudhir Nagaraj Design Engineer, Motor Drive Business Unit Texas Instruments AutoTune 2 26 An intelligent

IMPORTANT NOTICE

Texas Instruments Incorporated and its subsidiaries (TI) reserve the right to make corrections, enhancements, improvements and otherchanges to its semiconductor products and services per JESD46, latest issue, and to discontinue any product or service per JESD48, latestissue. Buyers should obtain the latest relevant information before placing orders and should verify that such information is current andcomplete. All semiconductor products (also referred to herein as “components”) are sold subject to TI’s terms and conditions of salesupplied at the time of order acknowledgment.TI warrants performance of its components to the specifications applicable at the time of sale, in accordance with the warranty in TI’s termsand conditions of sale of semiconductor products. Testing and other quality control techniques are used to the extent TI deems necessaryto support this warranty. Except where mandated by applicable law, testing of all parameters of each component is not necessarilyperformed.TI assumes no liability for applications assistance or the design of Buyers’ products. Buyers are responsible for their products andapplications using TI components. To minimize the risks associated with Buyers’ products and applications, Buyers should provideadequate design and operating safeguards.TI does not warrant or represent that any license, either express or implied, is granted under any patent right, copyright, mask work right, orother intellectual property right relating to any combination, machine, or process in which TI components or services are used. Informationpublished by TI regarding third-party products or services does not constitute a license to use such products or services or a warranty orendorsement thereof. Use of such information may require a license from a third party under the patents or other intellectual property of thethird party, or a license from TI under the patents or other intellectual property of TI.Reproduction of significant portions of TI information in TI data books or data sheets is permissible only if reproduction is without alterationand is accompanied by all associated warranties, conditions, limitations, and notices. TI is not responsible or liable for such altereddocumentation. Information of third parties may be subject to additional restrictions.Resale of TI components or services with statements different from or beyond the parameters stated by TI for that component or servicevoids all express and any implied warranties for the associated TI component or service and is an unfair and deceptive business practice.TI is not responsible or liable for any such statements.Buyer acknowledges and agrees that it is solely responsible for compliance with all legal, regulatory and safety-related requirementsconcerning its products, and any use of TI components in its applications, notwithstanding any applications-related information or supportthat may be provided by TI. Buyer represents and agrees that it has all the necessary expertise to create and implement safeguards whichanticipate dangerous consequences of failures, monitor failures and their consequences, lessen the likelihood of failures that might causeharm and take appropriate remedial actions. Buyer will fully indemnify TI and its representatives against any damages arising out of the useof any TI components in safety-critical applications.In some cases, TI components may be promoted specifically to facilitate safety-related applications. With such components, TI’s goal is tohelp enable customers to design and create their own end-product solutions that meet applicable functional safety standards andrequirements. Nonetheless, such components are subject to these terms.No TI components are authorized for use in FDA Class III (or similar life-critical medical equipment) unless authorized officers of the partieshave executed a special agreement specifically governing such use.Only those TI components which TI has specifically designated as military grade or “enhanced plastic” are designed and intended for use inmilitary/aerospace applications or environments. Buyer acknowledges and agrees that any military or aerospace use of TI componentswhich have not been so designated is solely at the Buyer's risk, and that Buyer is solely responsible for compliance with all legal andregulatory requirements in connection with such use.TI has specifically designated certain components as meeting ISO/TS16949 requirements, mainly for automotive use. In any case of use ofnon-designated products, TI will not be responsible for any failure to meet ISO/TS16949.

Products ApplicationsAudio www.ti.com/audio Automotive and Transportation www.ti.com/automotiveAmplifiers amplifier.ti.com Communications and Telecom www.ti.com/communicationsData Converters dataconverter.ti.com Computers and Peripherals www.ti.com/computersDLP® Products www.dlp.com Consumer Electronics www.ti.com/consumer-appsDSP dsp.ti.com Energy and Lighting www.ti.com/energyClocks and Timers www.ti.com/clocks Industrial www.ti.com/industrialInterface interface.ti.com Medical www.ti.com/medicalLogic logic.ti.com Security www.ti.com/securityPower Mgmt power.ti.com Space, Avionics and Defense www.ti.com/space-avionics-defenseMicrocontrollers microcontroller.ti.com Video and Imaging www.ti.com/videoRFID www.ti-rfid.comOMAP Applications Processors www.ti.com/omap TI E2E Community e2e.ti.comWireless Connectivity www.ti.com/wirelessconnectivity

Mailing Address: Texas Instruments, Post Office Box 655303, Dallas, Texas 75265Copyright © 2016, Texas Instruments Incorporated