Dramatic reductions in voltage and power requireÂments are making
tiny piezo motors and drive systems an interesting option for portable, low-power
medical devices. By eliminating the need for the high voltage normally
associated with piezo systems, a new piezo motor design from New Scale
Technologies enables miniature motion systems that operate on a single 3-V
battery without using voltage boost circuits.
"Normally piezo technology requires input of 40
volts or more, which is a concern especially in medical applications," says
Ralph Weber, product manager for New Scale. Even though their previous systems
and ASIC could run on a 3.3-V input, Weber says that the boost circuits to
produce the higher input voltage required by all piezoelectric motors can scare
designers.
"This is a breakthrough because using
multi-layer ceramic technology, the system doesn't need 40 volts to operate,"
he says. "There is no aspect of the system which is high voltage anymore, and
our approach also enables the drive chip and control circuitry to be reduced to
very small sizes."
Piezoelectric micro motors are small enough for
use in porÂtable, handheld and implantable medical devices. The technology
allows engineers to add motion where traditional electromagnetic motors would
be impractical due to size and power limitations. Applications include
positioning devices that can be adjusted afÂter being implanted and robotic
surgical tools. Special non-magÂnetic versions can also be designed to operate
near MRI equipÂment without affecting image quality or motor performance.
Piezoelectric materials produce a stress or
strain when electriÂcally excited, yielding a few micrometers of motion with
very low force. Piezoelectric motor designs harness ultrasonic vibraÂtions of
these actuators with a mechanical coupling to produce many millimeters of
travel and high force, while retaining the benefit of sub-micrometer
resolution.
Weber says the smallest commercially available
piezo motor is New Scale's SQUIGGLE micro motor, a linear motor measurÂing less
than 2.8 x 2.8 x 6 mm. Using a patented design, four piezoelectric plates are
bonded to the sides of a rectangular tube which is threaded on the inside. A
two-phase drive voltage applied to the plates causes the nut to vibrate in an
orbital motion at its resonant (ultrasonic) frequency. Friction engages the
threaded screw and drives it forward. Reversing the phase reverses the screw
direction. The micro motor produces a smooth linear motion with no gears.
Weighing only 0.16 gm, it can produce up to 50 gm of force, a variable speed of
up to several millimeters per second and a position resolution of 0.5
micrometer.
Like most piezo motors, Weber says the first SQUIGGLE motors
employed "hard" PZT ceramic plates to minimize dielectric losses and associated
temperature rise. Unfortunately this material requires an applied voltage of
around 40V, requiring boost circuits in battery-powered applications. A
breakthrough in PZT ceramic production has reduced this voltage requirement to
2.8V, making these tiny piezo motors even more appealing to designers of
portable and handheld medical devices.
To achieve this breakthrough, New Scale collaborated with
TDK-EPC on an advanced multi-layer piezo ceramic plate. The patent-pending
plate is a co-fired assembly conÂsisting of many thin layers of hard piezo
ceramic, and each layer is less than 15 micrometers thick.
The new plate design has enabled the introduction of a new, reduced-voltage
SQUIGGLE RV micro motor, which operates directly from a battery at less than
2.8V without boost circuits. Weber says this milestone is significant because
it is the first piezoelectric motor to match the voltage requirements of tradiÂtional
electromagnetic voice coil, dc and stepper motors. It reÂmoves the perception
of risk associated with higher-voltage boost circuits and interconnections, and
also enables radically smaller drive electronics by eliminating the need for
boost circuits in the drive ASIC, as well as external inductors, capacitors and
diodes.
Additional power-saving technology designed into the new drive
ASIC for the SQUIGGLE RV produces the ultrasonic voltage signals that drive the
motor. The NSD-2101 drive IC was developed in conjunction with New Scale's
partner, austriamicrosystems. It incorporates patent-pending smart drive
technology that monitors motor performance and adjusts the ultrasonic drive
frequency to lock on to the mechanical resoÂnant frequency of the motor.
Because of the motor's self-locking design, no power is needed to
hold the motor position. A practical application is for periodic adjustment of
an implanted device because the motor can be powered on to make the adjustment
and then switched off entirely, drawing no battery power until the next
adjustment is needed.
The voltage reduction and the corresponding
shrinking of the drive electronics enable a new class of integrated micro
motion modules. The M3 (Micro-Mechatronic Module) design platform combines a
SQUIGGLE RV motor, a position sensor, the drive ASIC and a microprocessor in a
miniature, closed-loop motion assembly.
A TRACKER position sensor provides closed-loop
feedback with resolution of 0.5 micrometers and repeatability of 2 micrometers.
The onboard microprocessor provides PID control in the module. Input to the
module can be configured for I2C, SPI or USART, and simple, serial
communications from a PC or master microprocessor can command this miniaÂture
advanced motion control system.
The mechanical assembly and housing are
customized to OEM specifications. The reference design contains all components
and control electronics in a compact 12- x 30- x 8-mm package, with an actuator
arm that travels up to 6 mm to push an external load.
Another reference design incorporates these
advanced M3 platform controls into a miniature lens module, creating precise
autofocus capabilities in a package only slightly larger than the lens holder
for board-mounted camera systems.
Using this platform, modules can be developed
for nearly any configuration or application. They can also be sealed for use in
high-moisture applications or implantable devices.
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