piezos in motion · tals is in motive mode while two others cycle through locking; these push along...

Sponsored by

Presented by

In 1881, Pierre and Jacques Curie observed that quartz crys-tals generate an electric field when stressed — and motion when electrified. Now, technologies that use these substances to output signals — or motion — sport the suffix piezo, which comes from the Greek word piezein, which means to compress. Modern piezo elements are formed from a pow-der of plumbum, zirconate, and titanate (PZT) that is compacted, fired, and embedded with electri-cal connections.

Piezoelectric force sensors measure dynamic forces such as oscillation, impact, or high-speed compression or tension. Any force applied to the piezoelectric sens-

The piezoelectrical effect is the ability of materials called piezoceramics to generate an electrical charge in response to squeezing or pressing mechanical force — or motion when electrified. The effect is leveraged in piezomotors applied in an increasing number of applications.

Piezoelectric force sensor

Piezos in Motion:

Technology basics, motors, and more

1 July 2012www.micromo.com

high lateral force though limited travel, and can be set up for multi-axis operation. Fast-response stacked ver-sions output high force.

• Flexure-guided pi-ezomotors operate like the simple type but incorpo-rate motion amplifiers for longer, straight strokes — even to several mm.

• Standing-wave piezomotors — also called ultrasonic piezomotors — can be made in linear or rotary versions. Leveraging high-frequency oscillation of a stator, these exhibit unlimited motion, high speed, and fast response — to down to msec. Because motion is transmitted to a slide or rotor by friction, resolution is limited to 40 nm or so.

• In rotary ultrasonic piezomo-tors, pushers are attached to a piezoresonator through a vibra-tional shell. An ultrasonic radial standing wave is electrically ex-cited in the resonator, causing a ring to radially expand and con-tract, stimulating pusher move-ment on the radius. Because of their elasticity, the pushers vibrate with the same frequency (though phase shifted) in a direction or-thogonal to the ring’s radius. The superposition of the two orthogo-nal movements results in elliptical pusher movements. Because the pushers are spring-loaded against the rotor, their movement via fric-tion at the pusher contact area causes rotor rotation.

• In linear ultrasonic piezomo-tors, piezo elements orthogonally bonded to a car produce vibra-tion that rotates (and translates) an engaged screw for direct-drive linear actuation. Typically, two-

ing element produces a separation of charges within the atomic struc-ture of the material, generating an electro-static output voltage.

A piezomotor is an electric motor leveraging the shape change that piezoelectric materials exhibit when an electric field is applied. These often compact motors are suitable for myriad applications — offering better performance, effi-ciency, and miniaturiza-tion than conventional motors in everything from digital-camera focusing mechanisms,

industrial valves, toys, and military applications. Four types exist.

• Simple single-element piezo-motors expand and output mo-tion proportional to drive voltage. They’re typically run at crystal resonant frequency. Tube ver-sions are used in dispensing and scanning. Shear versions output

Piezoelectric lattices

Unstrainedlattice

Strainedlattice∆V

Lead zirconium titanate (PZT) doped with lanthanum is a widely used piezoelectric crystal. The lanthanum atoms do not fit neatly into the lattice of the lead zirconium titanate crystal.

When a stress is applied, the structure of the lattice shifts slightly. Positive ions tend to shift in one direction and negative ions in the other. The charge inequality generates a measurable voltage. Similarly, when a voltage is applied, the lattice shifts to equalize the charge and the crystal produces a measurable shape change. The strength of the ionic forces holding the crystal together means a high applied voltage is needed to produce a small shape change.

Piezoelectricity causes some ceramic crystals to produce a charge when stressed and to strain in response to an applied voltage. Piezoelectric crystals are inherently capable of delicate moves and their manufacture has been well honed — so modern piezoelectric motors integrating these crystals can make very fine steps, even to nanometer resolution.

Simple piezomotor — Stacked type

Piezomotors of the stacked subtype actually stacks piezo material to extend overall actuator stroke and force.


free group of crystals forward. Then, the first crystal group fol-lowed by the moving group is re-leased, retracting the third trailing group. Finally, both locking groups are returned to default.

• Linear piezo stepmotors also cycle their series of crystals (connected to either the motor’s casing or sta-tor) through locked and motive modes. Typically, one set of crys-tals is in motive mode while two others cycle through locking; these push along the parallel surfaces by which they’re sandwiched to out-put linear motion. Quick crystal distortion (and hence, response) allows the steps to be made at frequencies exceeding MHz, for speeds to 100 cm per sec.

Piezomotor benefitsIn addition to the high resolution and

accuracy already detailed, piezomotors also offer the following benefits.

• Piezomotors will never burn out, even if jammed while com-

channel sinusoidal or square wave is applied to the piezoelectric ele-ments at an ultrasonic frequency in the kHz range, matching the first bending resonant frequency of the screw — for orbital motion that drives the screw.

• Piezo stepmotors cycle a series of crystals through different states. The crystals are arranged in a “caterpillar legs” formation to allow for coordinated transla-tion. These stepmotors can be designed for almost any range or stroke, with picometer resolu-tion. Forces can exceed those of most other piezomotor types, be-yond 100 lb in some cases.

• Rotary piezo stepmotors cycle their series of crystals (connected to either the motor’s casing or sta-tor) through locked and motive modes. In short, one group of crys-tals is activated to lock an actuator rotor; a second crystal group is triggered and held to move a third

Digital control by Pulse Width Modulation (PWM)Ultrasonic piezomotor subcomponents

Elliptical movement of a pusher

Rotor

Vibration shell

Pushers connect to the piezoresonatorthrough a vibrational shell.

An ultrasonic radial standing wave is excitedin the resonator causing the ring to expand and

contract radially, stimulatingmovement of the pushers along the radius.

Because of their elasticity, the pushers vibrate with the same frequency

(though phase shifted) in a directionorthogonal to the ring’s radius.

Superposition of the two orthogonal movementsresults in elliptical movements of the pushers.Because the pushers are held pressed against

the rotor, Their movement via frictionat the pusher contact area,causes rotation of the rotor.

Left: In an ultrasonic piezomotor, ultrasonic standing waves are excited in a resonator — causing the ring to expand and contract, stimulating pusher movement and rotor rotation.Right: An ac electrical pulse train is supplied by a digitally controlled voltage source to piezoelements at its ultrasonic resonant frequency. Through the controller, motor speed is altered by varying 1) the pulse repetition rate, or 2) modulating the duration of individual pulses.

3www.micromo.com July 2012

• Piezos in medi-cal devices. Piezoelectric motors are used in ultrasonic emitters, fer-tilization, mi-cromonitoring, surgery robotics, pick-and-place, microdose dis-pensing, cell penetration and cell imag-ing in cytopa-thology, laser beam steering in dermatology and ophthal-mology, and 3D scanning.

For example, optical coherence tomography designs integrate piezo-motors to quickly move reference mirrors and imaging optics. For 3D image creation from optical inter-ference patterns, optical fibers are moved both axially and laterally dur-ing scans by piezomotors — moving precisely to deliver better image reso-lution than electromagnetic motors. Likewise, piezomotors are finding use in transdermal drug delivery, in the form of needle-free insulin injec-tors. Piezomotor-based noninvasive microsurgery robotic drills, tweezers, and scissors are also proliferating. Finally, in 3D cone beam imaging to treat orthodontics and sleep apnea patients, piezomotors are used dur-ing oral imaging makes exact mouth models to fit oral appliances.

• Hearing research. Inner ear hair cells are mechano-sensors that transduce nanometer-scale de-flections of sensory hair bundles into electrical signals. A 2007 research project in the depart-ment of neuroscience at the University of Virginia (U.Va.) Medical School, Charlottesville, used piezos to replicate ear hair functions, and advance through experiments understanding of both the normal physiology and pathophysiology of the inner ear. In the latest investigation

manded to move.• These motors exhibit high initial

torque and a wide range of torques.• The elimination of stick-slip (as-

sociated with some electromag-netic motors) allows full leverag-ing of a piezomotor’s quick start-stop capabilities.

• Piezomotors can be set up to be normally locked or normally free; in the former, drift is negligible — less than 1 arc-sec/hour for some rotary types.

• Electromagnetic motors must often be paired with gears and power-transmission components, and exhibit mechanical toler-ances, backlash, and hysteresis. Piezomotors are direct-drive units, so offer higher resolution and repeatability.

• Power requirements are often simple, and controllers can be low-voltage units (less than 12 Vdc) enabling the use of piezo-motors in mobile space and en-ergy-constrained designs.

• Piezomotors are generally im-mune to electromagnetic inter-ference, making them useful in MRI and similar applications.

Piezo applicationsMyriad products utilizing pi-

ezomotors have been designed or successfully commercialized for photonics, biomedical research, and nanotechnology — because piezomo-tors offer intrinsically dynamic per-formance. Applications abound:

Piezo stepmotor — Version that pushes rail along

Piez

o LE

GS

— Il

lust

ratio

n co

urte

sy M

ICRO

MO

Recommencing the motion, the motor continues pushing the drive rail leftward.

The drive pads lifted off the drive rail surface to allow the piezo elements to reposition.

When the first motion cycle is complete, the drive pads have moved as far to the left as possible.

The motor consists of two piezo elements, each with an attached drive pad. When activated, the piezo elements move, pushing the drive pads, which in turn cause the drive rail (dark blue) to move.


tion: If the three beams are joined to-gether at both ends, applying a voltage to one or more of the beams produces surprisingly large lateral movement.

• Better seatbelts. At the Ohio State University Smart Vehicle Concepts Center, professor Marcelo Dapino seeks to incor-porate piezoelectric devices into seat belts. Traditional seatbelts protect car occupants during a crash, but forces can reach 4,000 N in the shoulder belt and 2,500 N in the lap belt; traditional devices are effective, but designed around a narrow window of occupant size and weight. Dapino’s group seeks to streamline the entire seat-belt system while ensuring optimal restraint of any occupant. They plan to place solid-state piezoelectric actuators in seatbelt D-rings to control the force on the belt. Active nanofiber sensors in the belt webbing would measure forces as a crash unfolds.

• Piezomotors in unmanned drones. Recently, a 40-mm stacked piezomotor was in-tegrated into a uninhabited aerial vehicle’s morphing wing to supply hydraulic pressure to actuators. The design allows fluid to be confined to a closed

(read the summary by visiting jn.physiology.org and searching Andrea Lelli) mouse ear basal-hair bundles were mapped using piezomotors for replicated exci-tation. The hope is to eventually address problems with hearing and balance, the most common human sensory deficits.

• Tiny bug-sized robots. New robots the size of ants could soon be marching into new ap-plications with solid-state legs and mandibles. Developed by Perdue University researchers, West Lafayette, Ind., the design includes legs made of bundled piezoelectric beams — a different take on existing piezo technology concepts. Computer simulations suggest that the bugs could be mass-produced using manufac-turing technologies common to the semiconductor industry, and made to scavenge vibrational energy from the environment to recharge their power supply. A tripod gait — used by most in-sects — would enable the bugs to remain stable while traversing uneven terrain.

Some beams of piezoelectric mate-rial can exhibit expansion limits, but the new design overcomes this limita-

Piezo stepmotor — Version that cycles legs through excitation

In the last phase of the walking cycle, leg motion is slightly dif-ferent. The second pair moves in a lower right direction while the first pair head up and right. This completes the cycle and the unit is ready to repeat.

When all four legs are electrically activated, the piezo element is in this position: All legs are elongated and bending. The small red arrows show the direction of each leg tip.

Here, the first pair of legs is lifted, keeping contact with the rod, while the legs of the second pair are retracted. The small arrows show the direction of mo-tion: The first pair of legs moves in a lower right direction while the second pair instead head in a lower left direction, before elongating in the next step.

The second pair of legs is now reposi-tioned and placed on the rod, while the legs of the first pair are retracted, ready for the next step. This time, the direction of the first pair is lower left, and the second pair moves up and right.

Piez

o LE

GS

— Il

lust

ratio

n co

urte

sy M

ICRO

MO


technology, most are still limited in feed length and payload. In addition, these motors can take a long time to move parts into and out of a target “active measuring” area — particu-larly when multi-stage, wide-stepped, play-free reduction gearing is incor-porated to maintains the high resolu-tion of low-speed moves. In contrast, when using a piezomotor plus a dc motor mounted on a common spin-dle, the dc drive can move the mecha-nism to target quickly (with its quick feed rate) and then the piezomotor delivers high motion resolution once it reaches that target.

To deliver reach, two-drive po-sitioners typically use a ballscrew, with one motor coupled to each end. Theoretically, ballscrews have no feed-length limit, so the spindle can

loop surrounding each actuator, cutting fluid requirements, eliminat-ing hydraulic lines, and removing weight. High-frequency piezomotor motion speeds response, oscillating at 750 to 1,500 Hz with 600 to 1,200 VA

supplied. The resulting 0.13% strain pressurizes the hy-draulic fluid when coupled with an accumulator, an output piston, and a microcontroller to activate the system’s four valves.

• Fine-tuned positioning. Electronics and medical applica-tions that involve focusing, scan-ning, adjustment, and inspec-tion often use sub-micrometer positioning. Piezomotors are leveraged here because they move precisely, even down to a few nanometers. For example, in a measuring application, pi-ezomotors might power a linear actuator that slowly carries small objects past a sensor recording geometric data.

Piezomotors alone aren’t appropri-ate, because despite great strides in

Dual-axis stepper-motor magnetics

BOTTOM VIEW

Piezo encoder connectorDual-axis air-bearing stepper motor

Piezoelectric stages10 10-mm travel,20-nm resolution

Top coverDirection of travel fortop piezo stage

Motor housing

Stepper-motorconnector

Encoder connectors

Air supply

Stage — Traditional motors plus piezomotors for final positioning

Shown here is yet another design that integrates both electromagnetic and piezomotors — here, in a four-axis X-Y stage. Permanent-magnet linear stepper motors allow installation in any orientation; one or more piezo-positioning stages sit on the linear motor platform and output 10 mm of travel with 20-nm resolution.

In this application, piezomotors are paired with fast dc drives to output precise linear motion. Longer strokes, higher precision, and faster positioning than conventional models save valuable production time.


revolution.What are capabilities of the smaller

motor that drives the final position-ing? The piezo rotary motors are smaller — about one cubic inch and 70 grams or so. They work with control voltages from 0 to 3,000 Hz. Holding torque typically reaches 90 mNm; incremental stepwidth reaches 0.35 mrad. A small shaft mechani-cally links the motor to a permanent-magnet coupling that then connects to the ballscrew.

A high-resolution linear measur-ing system continuously records the movement and transmits the infor-mation to the motor controller. In this way, the drive moves the linear positioner in high-precision mode at a speed of 0.00002 to 0.15 mm/sec — 20 nm per second. The speed con-stancy at the bottom end of the range is only depends only on the resolu-tion of the linear scale. Repetition ac-curacy is better than 100 nm.

be as long as required. This allows larger objects to be measured at sev-eral key areas — without requiring that an operator repeatedly remove and reclamp the object to place key areas in the machine’s active sens-ing area. In contrast, conventional (stacked) piezo drives are restricted to just a few millimeters of position-ing width.

To deliver acceleration, dual-drive actuators switch powerlessly — with no heat — to the rotary piezo motor at a speed of 0.5 mm/sec by way of a permanent magnetic coupling. At rest, this drive then works as a pas-sive spindle brake, damping oscilla-tion and reducing settling time. The maximum-to-minimum speed ratio also exceeds 1,000,000:1 with the dual-actuator setup.

For the faster initial large-stroke positioning, a conventional brush motor with a rotary encoder con-nected to the shaft by a bellows cou-pling is adequate. The motor runs for a relatively short time in this application, so motor heat input is negligible. Depending on the spindle pitch used, speed from 0.5 to 100 mm/sec is possible — suitable for stan-dard positioning requirements. 7,000 rpm and 16 mNm are typical outputs. Gear output ratios vary; magnetic two-channel incre-mental encoders are typically used with up to 512 pulses per

Linear drive — Traditional motor for macro movements, piezomotor for fine, final positioning

A dc motor provides larger movesto get parts into position for measurement.

A piezo motor moves the parts during delicate scanning and live measurement actions.

A magnetic coupling transmits powersand doubles as a damper.

Parts to be measured are carriedon the ballscrew actuator.

Simply adopting a downscaled system is ineffective. Two motors attached to either end of a ballscrew offer more range of motion and precision. Ballscrews can also be fitted with extra components to enhance efficiency.

For more information, visit:http://machinedesign.com/article/precision-moves-with-magnetostriction-1118http://motionsystemdesign.com/motors-drives/piezomotors-actuators-streamlining-performance-20100401/index.htmlhttp://machinedesign.com/article/sensor-sense-piezoelectric-force-sensors-0207http://motionsystemdesign.com/linear-motion/motorized-drive-gives-linear-motion-0794/index.html


piezos in motion · tals is in motive mode while two others cycle through locking; these push along...

Documents