Integration GuideHaptic Feedback & Vibration Alerting

for Handheld Products

Since its invention in the early90’s, simple vibration feedbackhas alerted billions of peopleand grabbed their attention.Now its successor, hapticfeedback can convey operatorinformation and radicallyimprove user interfaces.Both techniques are designedinto thousands of devicesevery year. This is our guidefor to how to design the bestattention­grabbing vibrationalerts, and build the mosttactile haptic user interfaces.

Contents3 Haptic Feedback vs Vibration Alerting4 ERM Technology Primer5 LRA Technology Primer & Comparison Chart6 Discrete Drivers & EMI Primer7 Driver ICs & Driving LRA Primer8 PWM Driver Signal & Advanced Vibration Alerting Primer9 Understanding Datasheets For Haptic Feedback

10 Placement of Vibration Motors in Typical User Interfaces12 Mounting Motors in Enclosures & PCB14 Long­Life Brushless & ATEX Motors15 Vibration Motor Lifetime16 PMD : Placing an Order

In the early 90’s, the market of miniature vibration motorswas built around pagers, but soon vibration alerting wasadded to mobile / cell phones, where it remains an essentialfeature. Today’s equipment consumers and designers areexpecting vibration alerting and haptic feedback to be presentas added value features.Precision Microdrives is the market leading manufacturer ofvibration alert products. Our mission is to support customersthrough design with best practice advice, and deliver ourproducts ‘On time & To Spec’ from 1+ prototypes to 100k+volume production.This booklet will guide you through the best ways to addhaptic feedback or vibration alerting to your product. See ourwebsite www.precisionmicrodrives.com for more in­depth

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HAPTIC FEEDBACK VS VIBRATION ALERTING

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ERM TECHNOLOGY PRIMER

Introduction to vibration motorsThe vibrations used in haptic feedback and vibrationalerting can be generated by two differentelectromechanical devices. These are known asEccentric Rotating Mass motors and Linear ResonantActuators. The term ‘vibration motor’ or ‘pagermotor’ usually refers to the ERM type. Piezo basedvibration motors are based on different technology,but are not manufactured commercially yet.

What is an ERM vibration motor?The term ‘Eccentric Rotating Mass’ refers to how thevibrations are created. These are normal DCmotors, with a small eccentric / offset weightattached to the shaft, and an unbalanced force iscreated when they rotate. This unbalanced forcedisplaces the motor, as well as whatever the motoris attached / secured to. It’s the high speeddisplacement that is known as ‘vibration’.

Characteristics of ERM motorsThe rotating eccentric mass generates a centrifugalforce on the motor body which causes vibration intwo axes. This force is governed by the formula left.A larger eccentric mass with a bigger offset fromthe shaft will produce more force and hence morevibration amplitude. Increasing the voltage suppliedto the motor will increase its speed, and thereforethe frequency of the vibration, with a linearrelationship. Furthermore, increasing the motorspeed will also increase the vibration amplitude witha quadratic relationship, which can be very useful ifbursts of high vs low amplitude vibration arerequired. With ERM’s it is not possible toindependently vary frequency and amplitude; theyremain interrelated. ERM motors can be drivensimply with plain DC or PWM drive signals (we coverthis later).

ERMs vibrate in two axes

A video game controller with 'dualshock'

Vibration Motor Relationships

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LRA TECHNOLOGY PRIMER

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What is an LRA motor?LRA stands for Linear ResonantActuator. It is comprised of amagnetic mass on a spring and avoice coil. When a current flowsthrough the voice coil, a force isgenerated due to the magneticfield. This force causes the mass todisplace. The repeateddisplacement of the mass producesa varying force which is felt asvibration.

Characteristics of an LRAmotorThe mass is restricted to move intwo opposite directions, creatingvibration in one axis. Changing thedirection of the current through thevoice coil causes the mass to bedriven in opposite directions,therefore LRAs are driven by ACsignals. The frequency of this signal

must match the ‘resonant frequency’ of the LRA, to produce any useful vibrationamplitude. Increasing the drive voltage, increases the amplitude linearly, whilstfrequency remains fixed to the frequency of the drive signal. The easiest drive methodis to use a dedicated driver chip. The LRA’s small size limits its maximum vibrationamplitude, but motor lifetime and haptic response time is better than ERMs.

Linear Resonant Actuators only vibrate in one axis

Precision Microdrives Y­axis LRA linear vibrator motor

Feature ERM LRAVibration Strength Low to High Low (Touchscreen / Handhelds)Vibration Frequency Varies with Voltage Fixed (Typically ~ 150 ­ 200 Hz)Vibration Direction Two Axes One AxisDevice Near Lifetime 150 ~ 1500 Hours BetterEfficiency Average BetterDrive Signal DC Voltage / PWM Sine Wave / PWMHaptic Performance Good BetterDevice Cost Low Higher

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DISCRETE DRIVERS & EMI PRIMER

Introduction to driving ERMs and LRAsERM vibration motors require a DC drive signal tooperate while LRAs require an AC drive signal. Drivesignals must respect both the minimum start voltageand a maximum operating voltage, shown in datasheets.Motors can be connected directly to an appropriatepower source (e.g. a battery), but cannot be drivendirectly from microcontrollers due to limited current.Most often, vibration motors are controlled from amicrocontroller via a driver circuit.

Discrete driver circuits for ERMsDiscrete drivers circuits for ERMs use transistors as aswitch to connect the motor to a DC power source. Thisallows the motor to be controlled by a low power signalfrom a microcontroller or digital logic / sensors. It alsosupports Pulse Width Modulation for varying thevibration amplitude and advanced signalling. MOSFETsare preferred to BJTs because of their higher efficiencyand better compatibility with lower battery voltages (1.5~ 4 V). Either N­type or P­type transistors can be used;typically a P­type is used with 1.5 V or lower suppliesdue to their lower typical gate­source voltage drop.Optionally a schottky diode can be added to protect the

transistor against high back­EMF voltages, but this is often only required for motors >12mm diameter. This type of driver will typically cost $0.1 USD ~ $0.2 USD per unit.

EMI considerations for vibration motorsElectromagnetic Interference (EMI) is the radiation ofelectromagnetic noise. This noise can affect controlsignals and other electronics which produces errors andreduces performance. The DC motors used in ERMs area common source of EMI due to commutator arcing.

A few simple components can reduce EMI and protect delicate circuitry such as RFcircuits. The level of EMI reduction is a design choice. The most common solutions area decoupling ceramic capacitor across the motor terminals though better performancecan be achieved with an X2Y ceramic filter instead.

N­type driver circuit

P­type driver circuit

Ceramic caps filter EMI

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DRIVER ICS & DRIVING LRA PRIMER

Motor driver integrated circuitsSingle integrated circuit chips for driving ERMmotors are freely available. They can reduce circuitcomplexity and simplify the process of addingadvanced features. For example they can becontrolled by digital signals or serial buses and havea wide range of input and output voltages. Somecan drive the motor in both directions, enablingadvanced vibration patterns, and most come withbuilt­in motor protection. The latest driver IC’s fromTI and ADI includes intergrated haptic feedbacksignal generators.

LRA driver integrated circuitsLRAs require an AC signal. To generate this from aDC source (such as a battery) requires a dedicatedLRA driver chip. These take digital PWM signalsfrom a microcontroller to control the frequency andamplitude sent to the LRA. The resonant frequencyof an LRA can shift depending on the target mass,operating environment, and age. The latest driverICs have an auto­resonance feature to solve thisproblem and offer active braking to improve hapticfeedback performance. Most LRA chips also supportERMs as well.

Texas Instrument DRV2603evaluation module

MAX1749 example circuit

DRV2603 example circuit

Supplier(Chip) ON Semiconductor(NCP5426) Maxim(MAX1749) Texas Instruments(DRV2603)2.5 ­ 5.2 V3.3 V (ERM) OR2 VRMS (LRA)Variable with PWM

400 mA (max)YesYes (10 ­ 250 kHz)Yes

$0.70 USD / 1kuYes

Analog Devices(ADUX1001i)2.3 ­ 4.8 V

YesYes (and I2C)Yes

Yes

Input Voltage 2.6 ­ 12 V 2.5 ­ 6.5 VOutput Voltage 1.2 ­ 2.0 (0.1 Vincrements) 1.25 ­ InputVoltageOutput Fixed /Variable Fixed in 0.1 Vincrements Varied by externalresistorsOutput Current 150 mA (max) 120 mA (max)Active Breaking No NoPWM No NoAuto­Resonance(LRA only) N/A N/A

Device Cost $0.50 USD / 1ku $1.41 USD / 1kuHaptics Processor No No

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PWM DRIVER SIGNAL & ADVANCED VIBRATION ALERTING PRIMER

Advanced Vibration AlertingAdvanced vibration alerting is aboutconveying more information to theuser than is present in a simplevibration alert. To do this, the vibrationmust be varied in some way, e.g.varying the amplitude of vibration viaPulse Width Modulation, or the pattern

of vibration via Pulse Coding.

A Pulse Width Modulated (PWM) signal is a string of high frequency (typically 20 kHz)pulses, where the period that the pulses are 'On' vs 'Off' (duty cycle) can be varied.When the pulses are averaged (e.g. filtered by the inductance of a motor winding), avoltage is produced that is proportional to the duty cycle which allows easy variation invibration amplitudes from motors.

Pulse Coding (PC) represents a string of low frequency pulses (typically 2~5 Hz) whichare not filtered. The 'On' vs 'Off' time and number of repetitions of the resulting pulsesof vibration are used to encode information.

Using PWM with vibration motorsAltering the duty cycle and hence motor voltage, allows us to vary vibration amplitudeand frequency. The PWM signal is fed from the microprocessor into either a MOSFET orIC driver which is connected to the motor. Driver ICs can also use PWM signals toproduce more advanced haptic effects and PWM is the most universal control input forLRA driver ICs.

Advanced Vibration AlertingThere are 3 types ofadvanced vibrationalerting:

• Amplitude Modulation ­ uses PWM to vary the amplitude, e.g. with Sine, Triangle,or Sawtooth envelopes. Used a lot in massagers to create vibration patterns.

• Pulse Duration Coding­ uses PC to vary pulse length to encode simple messgaes(e.g. SMS in Morse on earlier Nokia phones).

• Pulse Occurrence Coding ­ uses PC to vary pulse frequency to indicate proximity toan event, e.g. a reverse parking sensor vibrating a steering wheel.

Example of motor driven by a PWM signal

Vibration Pulse Occrance Modulation

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UNDERSTANDING DATASHEETS FOR HAPTIC FEEDBACK

General SpecificationsPrecision Microdrives datasheets are the mostcomprehensive in the industry, and are qualified onpurpose­built testing equipment. We test allvibration motors on a rated inertial load (typically100g, 250g or 1000g) suited to the motor size.Important specifications to note from an applicationperspective, are dimensions, max voltage andtypical current. Motors also have a rated startvoltage which should be met (drive voltage can berelaxed below this after the motor is spinning).

Vibration Alerting SpecificationsFor vibration alerting applications, most designersstart by comparing the available vibrationamplitudes against the motor form factors that theycan fit into their enclosure. Generally strongervibration will make for a better alert, but the motorspeed / vibration frequency can be influential. Forexample, lower frequencies will penetrate clothingsuch as gloves better. All vibration motor datasheets

have detailed vibration performance graphs. The relative strengths of vibration motorscan be compared by using the published normalised vibration amplitude figures, whichaccount for the differences in inertial loads during testing.

Haptic Feedback SpecificationsAll vibration motordatasheets have ahaptics performancesection. This is currentlybased on numeric hapticconstants, but will soon

be upgraded to include haptic response graphs. Numeric specifications are based on:

Example of the Key Features Table, inLRA vibration motor datasheet

Typical haptic characteristics, 304­109 (vibration motor)

Typical vibration motor performance,304­109 (vibration motor)

Lag­timeRise­timeStop­time

Active­brake time

The time from start to a vibration amplitude of 0.04G (limit of human feeling).The time from start to 50% of the rated vibration amplitude.The time from when the applied voltage is removed to when the vibration amplitude fallsbelow 0.04G.The time from when a reverse (brake) voltage is applied, to when the vibration amplitudefalls below 0.04G.

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PLACEMENT OF VIBRATION MOTORS IN TYPICAL USER INTERFACES

TouchscreenUser's fingertips press into thetouchscreen, so for vibrations to bereturned towards the user, acylindrical ERM or LRA should beused. Coin vibration motors are notsuitable as they vibrate indirections perpendicular to thepressed force of the finger. An LRAwill vibrate directly into the userfinger and can be mounted directlybehind the screen. ERMs are bestmounted at the side.

Control SticksFor gaming or as an industrialcontrol stick, joysticks are bestfitted with a cylindrical ERM tovibrate towards the users grasp atall times. These are easily fittedinside the main section of the stick.

Security ScannerDiscreet security scanners canhave audible alerts replaced withvibration alerting by inserting acylindrical ERM. Similar to ourjoystick example, the force isdirected outwards to the usersgrasp.

Jog DialsHigh end cars and consumerelectronics can add haptic feedbackto their dials. The low profile ofcoin motors coupled with vibrationsin two axes make them an idealchoice.

ERM in security scanner example

ERM in control joystick example

Linear Resonant Actuator in touchscreen example

ERM in touchscreen example

Coin vibration motor in jog dial / shuttle wheel example

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PLACEMENT OF VIBRATION MOTORS IN TYPICAL USER INTERFACES

Handheld InstrumentsLike mobile phones, handheldinstruments with haptics aredesigned to vibrate all over, soplacement of the vibration sourceis flexible, depending on otherdesign constraints.

Auto DashboardNew cars have dashboards withcapacitive touch buttons andhaptic feedback. LRAs producecrisp tactile feedback on smallpanels; larger panels require anERM. Touch surfaces are oftensuspended within the dashboardchassis so the haptic vibrations areisolated.

Body Worn VibrationAlertsFor applications in noisy ordistracting environments, vibrationalerting is a great alternative tolights and beeps. By mountingthem on the body, the users handscan be kept free to work. Theseare often used where protective orthick clothing is required, so themotor needs to be strong enoughto overcome the damping of softmaterials. This can be aided byensuring the vibrations aredirected towards the user. typicallymotors range from larger ERMs ingas detectors, to smaller lowprofile coin motors in wristwatches.

Vibration motor in secured foam housing

ERM in body worn vibration alerting pack

Linear Resonant Actuator in car dashboard panelsexample

Handheld gas monitor example

Vibration motor secured with adhesive

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MOUNTING MOTORS IN ENCLOSURES & PCBS

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LONG­LIFE BRUSHLESS & ATEX MOTORS

Motor brush lifetimeVibration motors have a finite life. For brushed vibration motors this is linked to wearof the brushes and commutator mechanism. Typically a brushed vibration motor canhave a Mean Time To Failure (MTTF) of 100 to 2000 hours. Beyond this lifetime,brushless designs are required.

Long Life & ATEXWhereas brushed motors have moving coils (armature) in a fixed magnets, brushlessmotors have fixed coils (stators) and a rotating magnet. Brushless motors have nocommutator mechanism, so the direction of the current in the stator coils is changedelectronically to keep the motor shaft rotating in the same direction. Since there are nobrushes to wear out, the brushless motors have much longer lifetimes (MTTFs of5000+ hours). Furthermore, since there is no DC arcing as the motor rotates (commonon brushed motor), brushless vibration motors are ideal for vibration alerting in ATEXenvironments. When a weight is attached to the shaft, the motion and hence vibrationcharacteristics remain identical to brushed motors.

Driving brushless vibration motorsTo drive a brushless motor, the current to the stator coils needs to be varied in aspecific way, which means brushless motors need a special driver IC. Some brushlessvibration motors like the coin type already have an internal driver IC, whilst othersrequire an external one. More details available in our Application Bulletin 18, online.

Implementing true haptic feedback with low latency start and stop times is notpractical with brushless vibration motors because the start algorithms take too long toget the motor running. However, brushless vibration motors can be used with theadvanced vibration alerting techniques, found on page 8.

Exploded brushless vibration motors

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VIBRATION MOTOR LIFETIME

Vibration Motor FailuresThere are two vibration motor failure modes;overheating, which is very rare, and much morecommonly, mechanical wear of the commutatormechanism. As the brushes (typically highlyconductive metal alloys) move over the coppercommutator, small metal particles from both partsare produced through mechanical attrition, andthese tend to fill gaps between commutatorsegments. Eventually, with enough build up, a

short circuit will occur. Alternatively, metal fatigue in the brushes might occur first, inwhich case a snapped brush will cause the failed motor to go open circuit. In eithercase, the motor will no longer run.

Measuring Motor Failure RatesPrecision Microdrives is leading the industry invibration motor lifetime testing. Product lifetimeand failure rates are determined through astatistical processes. For more popular vibrationmotors, we provide Mean Time To Failure (MTTF)and Failure In Time (FIT) analysis on thedatasheet. These measurements show the sameinformation in different ways, but the MTTF is most

useful as the average amount of hours until a motor fails. The distribution of failures isalso very important, and relevant parameters to are shown on the datasheets. Pleaseconsult our application notes online, for more information.

Our Longevity Testing MethodsNo vibration motor lifetime testing machines werecommercially available, so we built our own. Ourmotor longevity testing machine tests 48 motors ofthe same model simultaneously. Each motor isattached to a sled that has its own accelerometer tomonitor its performance and a server records allthe data. After 720 hours (28 days), the data isanalysed and MTTF and FIT rates are calculated

using Weibull Analysis. If there haven’t been enough failures in the period to obtain areasonable confidence level, we continue the test for another 720 hours.

A vibration motor end cap withdamaged and burnt metal brushes

Worn coin motor commutator whereworn metal fragments have caused ashort circuit

Recently tested 307­001 7mm PicoVibe motor produced a MTTF ofapproximately 1,800 hours