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Piezos in Motion: Technology basics, motors, and more 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 www.micromo.com In 1881, Pierre and Jacques Curie observed that quartz crystals 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 powder of plumbum, zirconate, and titanate (PZT) that is compacted, fired, and embedded with electrical connections. Piezoelectric force sensors measure dynamic forces such as oscillation, impact, or high-speed compression or tension. Any force applied to the piezoelectric sens- 1 Presented by Sponsored by July 2012 Piezoelectric lattices 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 ∆V 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. ing element produces a separation of charges within the atomic structure of the material, generating an electrostatic 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, efficiency, and miniaturization than conventional motors in everything from digital-camera focusing mechanisms, industrial valves, toys, and military applications. Four types exist. Simple piezomotor — Stacked type Piezomotors of the stacked subtype actually stacks piezo material to extend overall actuator stroke and force. www.micromo.com •Simple single-element piezomotors expand and output motion proportional to drive voltage. They’re typically run at crystal resonant frequency. Tube versions are used in dispensing and scanning. Shear versions output 2 high lateral force though limited travel, and can be set Unstrained up for multiaxis operation. lattice Fast-response stacked versions output high force. • Flexureguided piezomotors Strained operate like lattice the simple type but incorporate 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 piezomotors, pushers are attached to a piezoresonator through a vibrational shell. An ultrasonic radial standing wave is electrically excited in the resonator, causing a ring to radially expand and contract, stimulating pusher movement on the radius. Because of their elasticity, the pushers vibrate with the same frequency (though phase shifted) in a direction orthogonal to the ring’s radius. The superposition of the two orthogonal movements results in elliptical pusher movements. Because the pushers are spring-loaded against the rotor, their movement via friction at the pusher contact area causes rotor rotation. •In linear ultrasonic piezomotors, piezo elements orthogonally bonded to a car produce vibration that rotates (and translates) an engaged screw for direct-drive linear actuation. Typically, two- July 2012 Ultrasonic piezomotor subcomponents Elliptical movement of a pusher Digital control by Pulse Width Modulation (PWM) Rotor Pushers connect to the piezoresonator through a vibrational shell. Vibration shell An ultrasonic radial standing wave is excited in the resonator causing the ring to expand and contract radially, stimulating movement of the pushers along the radius. Because of their elasticity, the pushers vibrate with the same frequency (though phase shifted) in a direction orthogonal to the ring’s radius. channel sinusoidal or square wave is applied to the piezoelectric elements 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 translation. These stepmotors can be designed for almost any range or stroke, with picometer resolution. Forces can exceed those of most other piezomotor types, beyond 100 lb in some cases. •Rotary piezo stepmotors cycle their series of crystals (connected to either the motor’s casing or stator) through locked and motive modes. In short, one group of crystals is activated to lock an actuator rotor; a second crystal group is triggered and held to move a third www.micromo.com free group of crystals forward. Then, the first crystal group followed by the moving group is released, 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 stator) through locked and motive modes. Typically, one set of crystals is in motive mode while two others cycle through locking; these push along the parallel surfaces by which they’re sandwiched to output 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. Superposition of the two orthogonal movements results in elliptical movements of the pushers. Because the pushers are held pressed against the rotor, Their movement via friction at 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. Piezomotor benefits In 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- 3 July 2012 Piezo stepmotor — Version that pushes rail along www.micromo.com Piezo LEGS — Illustration courtesy MICROMO 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. • Piezos in medical devices. Piezoelectric motors are used in ultrasonic emitters, fertilization, micromonitoring, surgery robotics, pick-and-place, When the first Recommencing The drive pads microdose dismotion cycle is the motion, the motor lifted off the drive rail pensing, cell complete, the drive continues pushing the surface to allow the pads have moved drive rail leftward. piezo elements to penetration as far to the left as reposition. and cell imagpossible. ing in cytopathology, laser beam steering in dermatology manded to move. and ophthal• These motors exhibit high initial mology, and 3D torque and a wide range of torques. scanning. • The elimination of stick-slip (asFor example, optical coherence sociated with some electromagtomography designs integrate piezonetic motors) allows full leveragmotors to quickly move reference ing of a piezomotor’s quick startmirrors and imaging optics. For 3D stop capabilities. image creation from optical inter•Piezomotors can be set up to be ference patterns, optical fibers are normally locked or normally free; moved both axially and laterally durin the former, drift is negligible — ing scans by piezomotors — moving less than 1 arc-sec/hour for some precisely to deliver better image resorotary types. lution than electromagnetic motors. • Electromagnetic motors must Likewise, piezomotors are finding often be paired with gears and use in transdermal drug delivery, in power-transmission components, the form of needle-free insulin injecand exhibit mechanical tolertors. Piezomotor-based noninvasive ances, backlash, and hysteresis. microsurgery robotic drills, tweezers, Piezomotors are direct-drive and scissors are also proliferating. units, so offer higher resolution Finally, in 3D cone beam imaging to and repeatability. treat orthodontics and sleep apnea • Power requirements are often patients, piezomotors are used dursimple, and controllers can be ing oral imaging makes exact mouth low-voltage units (less than 12 models to fit oral appliances. Vdc) enabling the use of piezo• Hearing research. Inner ear hair motors in mobile space and encells are mechano-sensors that ergy-constrained designs. transduce nanometer-scale de• Piezomotors are generally imflections of sensory hair bundles mune to electromagnetic interinto electrical signals. A 2007 ference, making them useful in research project in the departMRI and similar applications. ment of neuroscience at the Piezo applications University of Virginia (U.Va.) Myriad products utilizing piMedical School, Charlottesville, ezomotors have been designed or used piezos to replicate ear hair successfully commercialized for functions, and advance through photonics, biomedical research, and experiments understanding of nanotechnology — because piezomoboth the normal physiology and tors offer intrinsically dynamic perpathophysiology of the inner formance. Applications abound: ear. In the latest investigation 4 July 2012 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. (read the summary by visiting jn.physiology.org and searching Andrea Lelli) mouse ear basalhair bundles were mapped using piezomotors for replicated excitation. 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 applications 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 manufacturing 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 insects — would enable the bugs to remain stable while traversing uneven terrain. Some beams of piezoelectric material can exhibit expansion limits, but the new design overcomes this limita- www.micromo.com The second pair of legs is now repositioned 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. 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 motion: 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. In the last phase of the walking cycle, leg motion is slightly different. 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. Piezo LEGS — Illustration courtesy MICROMO Piezo stepmotor — Version that cycles legs through excitation tion: If the three beams are joined together 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 incorporate 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 integrated into a uninhabited aerial vehicle’s morphing wing to supply hydraulic pressure to actuators. The design allows fluid to be confined to a closed 5 July 2012 Stage — Traditional motors plus piezomotors for final positioning Piezo encoder connector Top cover Dual-axis air-bearing stepper motor Direction of travel for top piezo stage Air supply Piezoelectric stages 10 10-mm travel, 20-nm resolution Encoder connectors Stepper-motor connector BOTTOM VIEW Motor housing Dual-axis stepper-motor magnetics 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. www.micromo.com loop surrounding each actuator, cutting fluid requirements, eliminating hydraulic lines, and removing weight. Highfrequency 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 hydraulic 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 applications that involve focusing, scanning, adjustment, and inspection 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, piezomotors might power a linear actuator that slowly carries small objects past a sensor recording geometric data. Piezomotors alone aren’t appropriate, because despite great strides in 6 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 piezopositioning stages sit on the linear motor platform and output 10 mm of travel with 20-nm resolution. 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 — particularly when multi-stage, wide-stepped, play-free reduction gearing is incorporated to maintains the high resolution of low-speed moves. In contrast, when using a piezomotor plus a dc motor mounted on a common spindle, the dc drive can move the mechanism 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 positioners typically use a ballscrew, with one motor coupled to each end. Theoretically, ballscrews have no feed-length limit, so the spindle can July 2012 Linear drive — Traditional motor for macro movements, piezomotor for fine, final positioning A magnetic coupling transmits powers and doubles as a damper. Parts to be measured are carried on the ballscrew actuator. A dc motor provides larger moves to get parts into position for measurement. A piezo motor moves the parts during delicate scanning and live measurement actions. Simply adopting a downscaled revolution. be as long as required. This allows What are capabilities of the smaller system is ineffective. Two larger objects to be measured at sevmotors attached to either motor that drives the final positioneral key areas — without requiring end of a ballscrew offer more ing? The piezo rotary motors are that an operator repeatedly remove range of motion and precision. smaller — about one cubic inch and reclamp the object to place key Ballscrews can also be fitted with and 70 grams or so. They work with areas in the machine’s active sensextra components to enhance control voltages from 0 to 3,000 Hz. ing area. In contrast, conventional efficiency. Holding torque typically reaches 90 (stacked) piezo drives are restricted mNm; incremental stepwidth reaches to just a few millimeters of position0.35 mrad. A small shaft mechaniing width. cally links the motor to a permanentTo deliver acceleration, dual-drive magnet coupling that then connects actuators switch powerlessly — with to the ballscrew. no heat — to the rotary piezo motor A high-resolution linear measurat a speed of 0.5 mm/sec by way of ing system continuously records the a permanent magnetic coupling. At movement and transmits the inforrest, this drive then works as a pasmation to the motor controller. In sive spindle brake, damping oscillathis way, the drive moves the linear tion and reducing settling time. The positioner in high-precision mode maximum-to-minimum speed ratio at a speed of 0.00002 to 0.15 mm/sec also exceeds 1,000,000:1 with the — 20 nm per second. The speed condual-actuator setup. stancy at the bottom end of the range For the faster initial large-stroke is only depends only on the resolupositioning, a conventional brush tion of the linear scale. Repetition acmotor with a rotary encoder concuracy is better than 100 nm. nected to the shaft by a bellows coupling 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- For more information, visit: http://machinedesign.com/article/precision-moves-withdard positioning requirements. 7,000 magnetostriction-1118 rpm and 16 mNm http://motionsystemdesign.com/motorsdrives/piezomotors-actuators-streamliningare typical outputs. performance-20100401/index.html Gear output ratios vary; magnetic http://machinedesign.com/article/sensor-sensepiezoelectric-force-sensors-0207 two-channel incremental encoders are http://motionsystemdesign.com/linear-motion/motorizedtypically used with drive-gives-linear-motion-0794/index.html up to 512 pulses per www.micromo.com 7 July 2012