Driving Piezoelectric Actuators

7/30/2019 Driving Piezoelectric Actuators

1/5Power Electronics Technology April 2006 www.powerelectronics.com 40

Though the understanding of piezoelectrics ismore than a century old, there is, of late, a rap-idly expanding interest in piezoelectric actuators,because they are so extraordinarily fast and canproduce motions in precisely controlled, sub-

nanometer increments. Already well entrenched in indus-trial inkjet printers, their use is migrating into several otherdiverse applications, ranging from medical electronics toantivibration controllers for helicopter rotor blades.

Piezoelectr ic

actuators requirespecialized drivecircuits with high-speed and high-voltage capabilities.These circuits canbe implementedusing IC-style pow-er op amps config-ured according tospecific actuatorrequirements.

The three typi-cal piezoelectric ac-tuator drive circuitsdescribed in thisarticle address ac-tuators over a widerange of equivalentcapacitance withdifferent demandsfor voltage, currentand slew rate. Butbefore discussingdrive circuit op-erat on, one must

understand the principles behind piezoelectric actuatoroperat on.

Piezoelectric OperationThe word piezo is Greek for push. The effect known

as piezoelectricity was discovered by brothers Pierre andJacques Curie in 1880. They discovered that if a force wasapplied to a quartz crystal, an electric field was developed.Later they learned that the inverse was also true; that is, if avoltage was applied across the crystal, the electric field that

developed would cause it to deform.Crystals that exhibit these phenomena are said to be

piezoelectric, and barium titanate, lead zirconate and leadtitanate are a few of the ceramic materials employed today inindustrial applications that exhibit these characteristics.

Within the cylindrical piezoelectric actuator illustratedin Fig. 1a, when a voltage is applied longitudinally, a dis-placementL occurs along the axis of the device. Typically,a piezoelectric material can withstand a strain, or changein length, of 0.1%. This means an actuator that is 100 mmlong that is poled (energized) along its axis can be elongatedby 0.1 mm. The displacement, or change in length, of an

unloaded single-layer piezoelectric actuator can be closelyapproximated by:

L )O (Eq. 1)where L is the change in length (m), S is the strain per

unit length or relative length change (meters/meters, there-fore, dimensionless), E is the electric field strength (V/m)and L

Ois the length of the actuator (m). The term d

33is the

piezoelectric coefficient (m/V), where the first subscriptidentifies the axis of the field and the second subscript identi-fies the axis of the displacement.

The maximum electric field that most ceramic piezoelec-tric actuators can withstand is on the order of 1 kV/mm to2 kV/mm. To extend travel beyond the approximately 0.1%maximum of a single slice, and to avoid applying too large an

Fig. 1.Two linear piezoelectric actuators: A singlestack (a) and a multilayer (b) in which the dis-

placement is amplified by the number of slices.

+

(a)

VL

+

(b)

VL

L

Driving Piezoelectric

Actuatorsxtraordinarily fast devices that can produce

precise motions in subnanometer increments,piezoelectric actuators make unusual demandson the power op amps required to drive them.

By Sam Robinson, Applications Engineer,Apex Microtechnology, Tucson, Ariz.


2/5www.powerelectronics.com Power Electronics Technology April 2006 41

applied field, a multilayer actuator can be fabricated by gluingthin layers of the piezo material together to form a stack. Avoltage is then applied to each layer individually (Fig. 1b),so that each is powered independently. The voltage appliedto each slice is still the same, but the total displacement issimply the sum of the individual displacements:

L OT = (N)(L). (Eq. 2)When a piezoelectric actuator is driven by an ac volt-age, the equivalent circuit becomes quite complex. 1]

However, when a piezoelectric actuator is driven by aperiodic voltage source whose frequency is below theresonant frequency of the piezoelectric actuator, whichis often the case in inkjet driving circuits, then the devicecan be modeled by a single capacitor. Therefore, theimpedance presented to the driving source is, to a goodapproximation, simply:

Z fC1

(Eq. 3)2

where f is the frequency of the driving source and CPA isthe equivalent capacitance of the piezoelectric actuator.

When designing systems that employ piezoelectric ac-tuators, keep in mind some essential concerns. One suchconcern is an actuators limited strength in tension. Thetensile strength of a cylindrical actuator is approximately10% of its strength in compression. Specific values canbe obtained from data sheets from piezoelectric actuatormanufacturers.

Boundaries on acceleration also must be considered.When driven by a periodic waveform, the accelerationwill increase exponentially with frequency. Therefore, it is

important to identify the upper limit of the devices abilityto withstand high acceleration forces.

Another issue is power consumption. Though piezo-electric actuators consume virtually no power, the powerdissipating demands upon the operational power-amplifiercircuits employed to drive them are significant indeed. Followsound principles in designing the driving circuits. This meansmaking sure that the driving power op amps are operatingin their safe operating region and that current limiting isprovided to protect the circuitry from an inadvertent shortcircuit. Also, select a satisfactory heatsink, flyback diodes andcompensation capacitors.

Drive CircuitsWhat follows are circuits for driving actuators, as well as

for driving the deflection plates employed in a continuous-drop inkjet application. The principal distinctions amongthese circuits are the equivalent capacitance of the actuatorbeing driven, the voltage and currents required, and the slewrates necessary. The three circuits we will look at range fromone that drives a 500-nF load down to one that drives almostno capacitance (a deflection circuit with a few pico-faradsof distributed capacitance).

The design shown in Fig. 2 is intended for piezoelectricactuators such as those employed in large-format, billboard-type printing. In this application, the 500-nF capacitor repre-

sents the load impedance of a large number of piezoelectric-driven individual inkjets that are connected in parallel. Inthis particular circuit, a slew rate of approximately 20 V/sis required, which is well within the slew rate capabilitiesof Apex Microtechnologys PA69. This power op amp candeliver 200 V/s.

If the circuit was driven at this slew rate, it would be deliv-ering 100 A given that I = CdV/dT, so it is important to keep

the slew rate within bounds. The way this is done is with thecompensation networks formed by the 4.7-pF capacitors (C3

and C4) and the 3-k resistors (R3 and R4). These valuesare chosen from graphs in the data sheet. Since the PA69 isunable to deliver the required 10 A, it drives the MOSFETpair comprising X1 and X2, which deliver the current.

As depicted in F g. 2, this is a pulse-type application withthe driving source delivering a symmetric square wave thatswings between +5 V and -5 V, thereby providing the pulsetrain as the input to the noninverting (+) terminal of the

When a piezoelectric actuator is driven

y an ac vo tage, t e equiva ent circuit

becomes quite complex.

PIEZOELECTRICS



PA69. The source can be an FPGA, amicrocontroller or some other source.The compensation network of thePA69 controls the slew rate, which inturn controls the current through theexternal MOSFETs.

The 100- resistors (R5 and R6) arethe gate resistors for the MOSFETs thatprevent oscillations that might other-

wise occur, given that the MOSFETgates are capacitive and that the circuit-board layout contributes a distributedinductance due to the routing of thetraces on the epoxy-glass substrate.Without these resistors, ringing might

otherwise occur due to the various in-ductance-capacitance combinations.The gain of the PA69 is 10 as gov-

+VS,+48 V

V1- 5 V to +5 V

pulse25 kHz

C6

C2

CC-CR-

CC+CR+

ILIMOUT

PA69Power Operational

Amplifier

+

+ +

+

C5

C1

C4

C3

R3

R2

R1 3 k R5

100R7

X1

IRF530

X2

CPA

500 nF

IRF9540N

Piezoelectricactuator0.3R6

100R4

3 k

15 k

1.65 k

220

F

220F

1F

1F

4.7 pF

4.7 pF

VS, 48 V

Fig. 2. This piezoelectric driver circuit can deliver 10 A at slew rates of 200 V/s.

erned by resistors R1 and R2, so thatthe output of the PA69, which is beingdriven as hard as it can be, is swingingbetween -48 V and +48 V. Therefore,the PA69 is acting as the gate driverfor the MOSFETs (X1 and X2). The

output of these devices swings all theway to the supply rails, +48 V and -48V, as depicted in Fig. 2. The 0.3-

resistor (R7) is an isolation resistormodifying the output load so that it isnot completely reactive, which therebycontributes to stability.

The power-supply bypass capacitorsneed to be hefty. The 1-F capacitorsC2 and C5 are ceramic, whereas the220-pF C1 and C6 are electrolytics,which requires care that their polarity

is observed.

Bridge-Connected DriverFig. 3 shows a circuit for a piezo-

electric actuator that requires a high-voltage driver capable of deliveringhundreds of volts, peak-to-peak (p-p).Since a typical actuator looks like vir-tually a pure capacitance to the drivingamplifier, almost all the power dissipa-tion becomes the burden of the drivingamplifier. The source voltage V1 deliv-

ers 15 VP-P at 80 kHz. The circuit drivesthe actuator, which is represented bythe 1-nF capacitance, in series with a1- resistance (Fig. 3).

In this example, two power op-erational amplifiers are connected ina bridge circuit.[2] When configured inthis way, these ICs are able to deliver anoutput voltage swing that is twice thatof a single device. Also, this configu-ration doubles the single-device slewrate while making any nonlinearitiessymmetrical, thereby reducing second-harmonic distortion when comparedwith a single amplifier circuit.

To say the load is floating is to say itis not ground-connected at all. Whenthe left output (V

OUTA) swings from

10 V to 160 V and the right output(V

OUTB) descends from 160 V to 10 V,

a voltage swing of 300 V (-150 V to+150 V) develops across the load.

In Fig. 3, the outputs of the two

amplifiers are now out of phase. Theoverall gain of the two bridge-config-ured power op amps is +20, so that

PIEZOELECTRICS

CLOADRLOAD

1 nF 1

RB

V1

15 Vp-p

CB

CC

VOUTa VOUTb

CR

CR2 CR4

CR3

MUR160 MUR160

3 k

R4 20 k

R3 1 k

RBCB

3 k

80 kHz

RC

3 k

RF

20 k

RINb

10 k

+175 V

RINa

1 k

RLIMa

8.2

Piezoelectricactuator

RLIMb

6.8

6.8 pF 6.8 pF

RCCC

3 k

RF

10 k

R510 k

R610 k

+175 V

+175 V

-5 V

-5 V

6.8 pF6.8 pF

-5 V

MUR160 MUR160

CC CCCR- CR-

CC+ CC+

CR+ CR+

ILIM ILIMPA78-A PA78-B

IN+

IN-

IN+

IN

+VS +VS

VS VS

+

Fig. 3.A pair of bridge-connected Apex PA78 power op amps drive the piezoelectric actuator

and are powered by asymmetric power supplies at +175 V and -5 V.


4/5www.powerelectronics.com Power Electronics Technology April 2006 43

300 VP-P

is delivered to the piezoelectricactuator, as required. As shown in Fig.3, a dual-source, asymmetric powersupply delivers +175 V and -5 V to thetwo amplifier modules.[3]

The values of +VSand -V

Shave been

chosen so that there will be sufficienthead room during the positive andnegative excursions of both V

UTAand

VUTB .

This, as well as computing themaximum dissipated power per mod-

ule, is discussed in reference 4.In the Fig. 3 circuit, the principal

passive components are resistors R3and R4 and diodes CR1 through CR4.The feedback circuit comprising resis-tors R3 and R4 centers the output ofthe two power op amps around 85 V.The diodes protect the amplifiers asfollows.

In any piezoelectric actuator cir-cuit, it is essential to prevent signalsfrom inadvertently feeding back to theamplifier. A piezoelectric transducercan convert mechanical energy intoelectrical energy just as easily as it canconvert electrical energy into mechani-cal energy.

So if something were to bump thetransducer, it could create a lot ofenergy that would travel backwardinto the output of the amplifier, whichcould be destructive. However, byconnecting several ultrafast MUR160

diodes (CR1 CR4) from the outputof each amplifier to its correspondingpower-supply rails, as shown in Fig. 3,

each amplifier is protected. Ultrafastrectifiers typically have reverse recov-ery times of 100 ns or better, whichoffers good flyback protection.

A High-Speed Drive CircuitThe design goal of the circuit

shown in F g. 4 is to drive a pair ofdeflection plates. This circuit designis suitable for driving a piezoactuatorthat presents a load of approximately

10 pF. This circuit was actually devel-oped for continuous-drop printingapplications. In this circuit, the PA78power op amp connects directly to thedeflection plates.

lectrostatically charged ink drop-lets, typically 50 microns to 60 micronsin diameter, are emitted by the inksource at a high velocity and are thenpassed through the electrostaticallycharged region between a pair of de-flection plates. Each droplet is therebydeflected, as required to form the inkcharacters deposited on the printedsurface.

The programming information,arriving as a digital data stream atthe digital-to-analog converter (DAC),is converted into a sequence of squarewaves of differing voltages rangingfrom 0 V to 3 V. This wave-train se-quence is then fed to a fast op amp,such as Analog Devices AD817 (Fig. 5).

In turn, the output of this device drivesthe Apex PA78 power operationalamplifier.

PIEZOELECTRICS

ig. 4. Deflection plates amplifier circuit. The fast op amp, an AD817 in cascade with the PA78,

rovides a gain of approximately 100 enabling the output voltage to the defection plates to be

hanged instantaneously to any potential between 0 V and 300 V.

R1

3.3 k

R2

+VCC,15 V

VS,-VCC,-15 V

+VS,+330 V

AD817DACInput

0 V 3 V

+

PA78

+

C1

C210 k

R3

1 k

R4

25 kR5

3 k

R6

12

R7

3 k

Todeflectionplates

100 pF

1 pF

C3

1 pF

Hig Vo tage Magnetics

Single, Dual and Low Profile1553 Data Bus Magnetics

Military and Space LevelLAN Magnetics

LAN 10baseT,10 /100base T

MIL-PRF-27,MIL-PRF-21038,MIL-STD-981 Comp iance

ISO 9001/ 9002

> Free white paper and catalog

We offer a range of custom

& standard devices including:

Hi-RelMil/AeroMagnetics Solutions

isit us on the web for our

ree White Paper entitled

Measuring Transformer

Distributed Capacitance &

atalog or call 888-889-5391.

datatronicsromoland.com

For high performance and reliability,

Datatronics is the magnetics solutions

leader in mil-aero electronics.Our high

voltage,Data Bus and LAN components

offer exceptional reliability in harshenvironments for critical avionics,

navigation and control applications.



PIEZOELECTRICS

The AD817 has a slew rate that fitsvery well with the 300-V/s designgoal. A slower pre-amplifier will not

higher slew rates when the output isoverdriven, so this two-stage amplifierapprach provides better high-voltagestep response than a PA78 driven di-rectly from the DAC.

The output of the PA78 is applied

to the deflection plates, as a sequenceof square waves, differing in magni-tude and varying between 0 V and300 V at a repetitive rate of 100 kHz.At the beginning of each programmedvoltage value, the square wave mustreach its programmed voltage within1.5 s. This is essential because theprogrammed voltage must remainconstant throughout the final 8.5 s,for that is the time span for the nextdroplet to pass through the deflection-

plate field. ETech

AcknowledgementThe author would like to thank

Bob ONeil of MorganElectroCeram-ics for his assistance in preparing thisarticle.

References1. Piezoceramic Properties & Appli-cations Manual. Physical Basics,Chapter 2, www.morganelectro

ceramics.com.2. Application Note 20, Bridge ModeOperation of Power Operational Am-plifiers, Apex Microtechnology Corp.,www.apexmicrotech.com.3. Apex Microtechnology Corp., Ap-plication Note 21, Section 3.1 SingleSupply Operation of Power Opera-tional Amplifiers, www.apexmicrotech.com.4. PA78 Power Operational ReferenceDesigns, www.apexmicrotech.com.

+ VS

VS

IN +IN

Dynamic

Output

Q1 Q2 Q3 Q4

I2I1

Q7

Q9

Q10

Q8

C1

C1

Q5 Q6

D1

IQ5

IQ6

reach the design goal and a faster am-plifier may increase cost and powerconsumption. The PA78 achieves

Fig. 5. The output of the PA78 supports high voltages and slew rates, and can directly drive the

deflection plates of an inkjet printer.

Documents

Driving Piezoelectric Actuators