Piezo motors, actuators streamline medical device performance

For the execution of precise movements with medical equipment, the latest piezoceramic motors and actuators are more compact, require lower voltage, deliver higher torque, have shorter response time, generate less heat, and are nonmagnetic and vacuum compatible, compared to conventional electromagnetic motors.

By Jim McMahon, for Physik Instrumente August 23, 2010

Improvements in the design of medical equipment for better streamlined functionality and performance are influenced by research, design, modeling, testing, prototyping, and U.S. FDA and EU approvals of new mechatronic devices, or the integration of changes to existing designs. These usually represent a sizable capital investment of resources well before the equipment goes into serial production. Design considerations include equipment size, speed of operation, heat generation, portability, handling of static or kinetic loads, power sources, measuring systems, vacuum and nonmagnetic requirements, sensors, machine controls, component part wear, and diagnostics.

Medical and bioresearch companies and others involved product development cycles can capitalize on advances in technology for the manufacture of better operating, lower cost, and more efficient equipment and devices. A recent improvement in high-speed laser scanning, for example, has facilitated the release of Harvard Medical School’s latest optical imaging technique, Optical Frequency-Domain Imaging (OFDI), said to be capable of providing unprecedented ultra-detailed 3D visualization of a patient’s coronary arteries. OFDI operates at several magnitudes improvement over its predecessor, Optical Coherence Tomography (OCT), which itself was enabled by advances in laser scanning 15 years prior.

Piezoelectric motors and actuators have had similar impact as refinements in laser scanning technology. Medical device manufacturers increasingly choose to use piezoelectric motors and actuators instead of conventional electromagnetic motors because of inherent advantages for medical equipment design. Medical applications using piezoelectric devices include ultrasonic emitters, artificial fertilization, medical nano-microliter pumps, micromonitoring, surgery devices, MRI (magnetic resonance imaging) compatible robots, microdose dispensing, cell penetration and cell imaging in cytopathology, medical material handling such as pick-n-place systems, drug delivery devices, 3D scanning, and for laser beam steering in ophthalmology, dermatology, and cosmetology.

Piezoelectric actuators

A piezoelectric actuator (piezo actuator) is a type of solid state actuator based upon the change in shape of a piezoelectric material when an electric field is applied. It uses a piezoelectric ceramic element to produce mechanical energy in response to electrical signals and, conversely, is capable of producing electrical signals in response to mechanical stimulus.

Use of piezoelectric materials dates back to 1881 when Pierre and Jacques Curie observed that quartz crystals generated an electrical field when stressed along a primary axis. The term piezoelectric derives from the Greek word piezein, meaning to squeeze or press, relating to the electricity that results from pressure applied to a quartz crystal.

Piezoelectric ceramics consist of ferroelectric materials and quartz. High-purity PZT (plumbum, zirconate, titanate) powders are processed, pressed to shape, fired, electroded, and polarized. Polarization is achieved using high electric fields to align material domains along a primary axis. Piezoelectric actuators in their basic form provide very small displacement, but can generate huge forces. The minute size of the displacement is the basis for high-precision motion.

For long travel ranges, a clever arrangement of multiple actuators, or the operation of a single piezoelement at its resonance frequency, have proven viable. These piezo motion devices are called piezo motors.

The latest piezo motor designs have advantages over electromagnetic motors for use in medical equipment and devices. Ultrasonic piezo linear motors (also called resonant motors) and piezo stepper motors can provide unlimited travel (movement) and differ in design, specifications, and performance.

In ultrasonic piezoelectric motors, the piezoelectric ceramic material produces high-frequency (inaudible to the human ear) acoustic vibrations on a nanometer scale to create a linear or rotary motion. For large travel ranges, especially when high speeds are required, ultrasonic linear drives are used. With resolutions as good as 50 nm they become a better alternative to electromagnetic motor-spindle combinations. The ultrasonic drives are substantially smaller than conventional EM motors, and the drive train elements needed to convert rotary to linear motion are not required.

Ultrasonic piezoelectric linear motors employ a rectangular monolithic piezoceramic plate (the stator), segmented on one side by two electrodes. Depending on the desired direction of motion, one of the electrodes of the piezoceramic plate is excited to produce high-frequency eigenmode oscillations (one of the normal vibrational modes of an oscillating system) of tens to hundreds of kilohertz. An alumina friction tip (pusher) attached to the plate moves along an inclined linear path at the eigenmode frequency. Through contact with the friction bar, it provides micro-impulses and drives the moving part of the mechanics (slider and turntable) forward or backward. Each oscillatory cycle produces a step of a few nanometers. The macroscopic result is smooth motion with a virtually unlimited travel range.

New ultrasonic resonant motors (such as the Physik Instrumente PILine model) are characterized by very high speeds to 500 mm/s (19.69 in/sec; 1.12 mph), in a simple, compact design. Such motors can produce accelerations to 10 g. They are also very stiff, a prerequisite for fast step-and-settle times (on the order of a few milliseconds) and provide resolution to 0.05 µm.

Piezo stepper linear motors usually consist of several individual piezo actuators and generate motion through a succession of coordinated clamp/unclamp and expand/contract cycles. Each extension cycle provides only a few microns of movement, but running at hundreds to thousands of Hertz, achieves continuous motion. Even though the steps are incremental, in the nanometer to micrometer range, they can move along at speeds in the 10 mm per second range, taking thousands of steps per second.

Piezo stepper motors (such as Physik Instrumente PiezoWalk) can achieve much higher forces of up to 700 N (155 lb) and picometer (one trillionth of a meter) range resolution compared to ultrasonic piezo motors. Resolution of 50 picometers has been demonstrated. The motor is capable of performing extremely high-precision positioning over long travel ranges. When the position has been reached, it performs highly dynamic motions for tracking, scanning, or active vibration suppression. Like the ultrasonic piezo motors, these motions can be conducted in the presence of strong magnetic fields or at very low temperatures.

Migration to piezoelectric devices

In optical coherence tomography, piezoelectric motors are used to impart rapid periodic motion to the unit’s reference mirror and imaging optics. To enable creation of two- and three-dimensional images from optical interference patterns, optical fibers must be moved both axially and laterally during the scan. Piezo motors can provide more precise movements resulting in improved image resolution over conventional electromagnetic motors.

Point-of-care and medical test equipment engage piezo technology. Where extremely fine-tuned positioning and measuring equipment is required, piezo motors fill the need, which can create motion with extreme precision, from inches to nanometers.

Piezoelectric actuators are beginning to be used for transdermal drug delivery, such as with a needle-free insulin injection system. Monitoring of endoscope-gastroscope devices is also beginning to be employed using piezoelectric devices.

Biomedical micro-tools, such as tweezers, scissors, and drills, have been adapted to a micro-robot base powered by piezo motors. Piezo motors are becoming more prevalent in microsurgery and non-invasive surgery tools.

3D cone beam imaging (used in orthodontics and for treating sleep apnea patients to obtain an exact model of the oral cavity for fitting oral appliances) employs the use of piezoelectric actuators.

Confocal microscopy (used in ophthalmology for quality assurance of implants) integrates piezoelectric motors into the optics. Very precise optical motion is required to adjust the focal plane and for surface scanning.

Electromagnetic devices dominate the drive mechanisms in medical equipment designs today. However, increasing accuracy requirements in the micron and nanometer ranges, along with an inclination toward miniaturization, dynamics streamlining, and interference immunity are pushing the physical limitations of electromagnetic drive systems. Piezoelectric motors are providing viable implementation alternatives for a growing number of medical device applications.

Physik Instrumente L.P. (PI) manufactures nanopositioning, linear actuators, and precision motion-control equipment for photonics, nanotechnology, semiconductor, and life science applications. PI has developed and manufactured standard and custom precision products with piezoelectric and electromagnetic drives for more than 35 years. The company received ISO 9001 certification in 1994 and has eight subsidiaries worldwide. Learn more at www.pi-usa.us.

Jim McMahon writes on instrumentation technology for Zebra Communications; jim.mcmahon@zebracom.net. Edited by Mark T. Hoske, Control Engineering, www.controleng.com

Also see:

A piezo stepping linear motion animation, a 17-second video, courtesy of Physik Instrumente;

Basic piezo technologies for motion control applications, with 5 diagrams;

11 ways piezoelectric motors improve equipment performance;

Machine Control Channel, Control Engineering; and

Machine Control Newsletter, Control Engineering.