Download piezos in motion

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-
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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
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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-
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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-
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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
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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
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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
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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
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July 2012