Feature Article

Piezo Motors and Actuators: Streamlining Medical Device Performance


Posted in Motors by Brian Buntz on March 8, 2010

 

Recently introduced piezoceramic motors and actuators offer a number of advantages over conventional electromagnetic (EM) motors for the execution of precise movements for medical device applications and are thus becoming increasingly popular with medical device manufacturers. Compared with EM motors, they are more compact, require lower voltage, deliver higher torque and generate less heat. In addition, they have shorter response time and have fewer mechanical component parts to wear out and service. They also are vacuum compatible and offer the ability to provide more accurate positioning than EM motors. 


Piezoelectric microscope objective nanofocusing device (Z-motor) provides 10x faster response and resolution than classical motor-driven units
 
Miniature piezo linear motor slide with on-board driver can reach velocities of 200 mm/sec
 
Miniature piezo ceramic rotary stages, linear stages and pushers
 
Variety of piezo-ceramic motion control devices for use in bio-medical applications: XY microscope stage, miniature stage, RodDrive pusher and sub-miniature slides
 
Custom ceramic encapsulated piezo stacks with aperture
 
Variety of piezo-ceramic disks and tube actuators for applications such as micropumps and nebulisers
 
This image depicts PZT disk in thinXXS Microtechnology AG micropump 2000. Custom piezoelectric disk actuators precisely dose liquids and gases in the thinXXS micropump. (Source: thinXXS Microtechnology AG)
 

This image sows the PZT disk in Pari Pharma GmbH eFlow Rapid Electronic Nebulizer. The atomiser head of the eFlow Rapid Electronic Nebulizer series employs an annular ultrasonic piezo transducer. (Source: Pari Pharma GmbH)

 
Basic design of an ultrasonic miniature linear motor (Click on image to open larger version)
 
Operating principle of a PiezoWalk type piezo stepping linear motor
 
Dynamic phases in a stator plate of a piezo ultrasonic motor
 
Basic principle of a piezo stack actuator
 
Design of a compact ultrasonic piezo motor linear translation stage

Improving the design of medical equipment to streamline functionality and performance is influenced by a number of critical factors. Among them are the research, design, modeling, testing, prototyping, and US FDA and EU approvals of new mechatronic devices and the integration of changes to existing designs. These factors usually represent a sizable capital investment well before the equipment goes into serial production. Factored into the medical device’s product development are considerations such as size of the equipment, 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.

The opportunity to capitalise on advances in technology for the manufacture of better operating, lower cost and more efficient equipment and devices is a key impetus for medical device companies focused on product development. A recent improvement in high-speed laser scanning, for example, has facilitated the development of a new optical imaging technique from Harvard Medical School known as optical frequency-domain imaging (OFDI). It is capable of providing unprecedented ultra-detailed 3D visualisation of a patient’s coronary arteries. OFDI operates at several magnitudes improvement over its predecessor, optical coherence tomography (OCT), which itself was enabled by earlier advances in laser scanning 15 years prior.

Just as refinements in laser scanning technology have had widespread applications, no less influential are recent advances in motor technology— specifically piezoelectric motors and actuators. Medical device manufacturers are increasingly choosing to use piezoelectric motors and actuators in preference to conventional electromagnetic motors because they exhibit inherent advantages in medical equipment design over conventional EM motor technology. Piezoelectric devices are used successfully in a widening range of medical applications including ultrasonic emitters, artificial fertilisation, medical nano-microliter pumps, micromonitoring, surgery devices, MRI-compatible robots, microdose dispensing, cell penetration and cell imaging in cytopathology, medical material handling such as pick-and-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 on 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.

The use of piezoelectric materials dates back to 1881 when Pierre and Jacques Curie observed that quartz crystals generated an electric 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 plumbum, zirconate, titanate (PZT) powders are processed, pressed to shape, fired, electroded and polarised. Polarisation is achieved using high electric fields to align material domains along a primary axis. Piezoelectric actuators in their basic form provide small displacement, but can generate huge forces. The minute size of the displacement is the basis for the high precision motion they can deliver.

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

Recent designs of piezo motors have a number of advantages over electromagnetic motors when being considered for use in medical equipment and devices. Two types of piezo motors in particular have considerable attributes making them well suited for medical applications. Ultrasonic piezo linear motors (also called resonant motors), and piezo stepper motors. Both versions can basically provide unlimited travel, yet they are very different in their 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 also required, ultrasonic linear drives are used. With resolutions as high as 50 nm, they become an attractive 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 its contact with the friction bar, it provides micro-impulses and drives the moving part of the mechanics (slider and turntable) forward or backwards. With each oscillatory cycle, the mechanics executes 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 PILine model developed by Physik Instrumente, which has played a pioneering role in advancing development of piezo devices for medical applications, are characterised by high speeds to 500 mm/s, in a compact and simple design. Such motors can produce accelerations to 10 g. They are also stiff, a prerequisite for their 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 PiezoWalk, also developed by Physik Instrumente, 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 high-precision positioning over long travel ranges, and when the position has been reached then performing 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 low temperatures.


 

Improving Medical Equipment Performance

Medical devices can be made smaller, more precise, lighter and easier to control by employing piezoelectric motors.

1) Higher Force Generation Supports Miniaturisation:
Piezoelectric motors are well suited for miniaturisation. They can easily be made smaller and more compact than electromagnetic motors, yet they provide greater force for their size. The efficiency of electromagnetic motors falls as their dimensions are reduced with more of the electrical power converted to heat, whereas with piezoelectric motors their efficiency stays virtually constant. Being the same volume and weight, the stored energy density of a piezo motor is ten times greater than that of an electromagnetic motor. The most advanced versions of piezo motors are configured into compact, high-speed micro-positioning stages that are smaller than a matchbox – the smallest piezo motor-driven stages are currently being used in autofocus devices for mobile phone cameras.

Because piezo motors provide a higher force per motor size, this has allowed equipment and instrumentation including medical devices to be reduced in size, while maintaining or increasing performance.

2) Improved Positioning Accuracy:
The direct-drive principle of the piezo motor eliminates the need for a supplementary transmission or gear train found in conventional electromagnetic motors; this avoids the usual backlash effects that limit accurate tracing which creates a critical reduction in positioning accuracy in electromagnetic servo-motors. The mechanical coupling elements otherwise required to convert the rotary motion of classical motors to linear motion are not necessary. The intrinsic steady-state auto-locking capability of piezoelectric motors does away with servo dither inherent in electromagnetic motors. Piezo motors can be designed to hold their positions to nanometer accuracy, even when powered down.

3) Faster Acceleration:
Piezo devices can react in a matter of microseconds. Acceleration rates of more than 10 000 g (response times of 0.01 milliseconds) can be obtained.

4) No Magnetic Fields:
Piezoelectric motors are suitable for medical and biotechnology applications as they do not create electromagnetic interference, nor are they influenced by it, eliminating the need for magnetic shielding. This feature is important for motors used within strong magnetic fields, such as with MRI equipment, where small piezo motors are used for MRI-monitored microsurgery and large piezo motors for rotating patients and equipment. Magnetic fields and metal components in conventional electronic motors make it impossible for motorised medical devices to function within MRI equipment.

5) No Maintenance or Lubrication, Aseptic Enabled:
Because the piezo motion depends on crystalline effects and involves no rotating parts like gears or bearings, they are maintenance free and do not require any lubrication. They can be sterilised at high temperatures, a significant advantage in medical applications.

6) Reduced Power Consumption:
Static operation, even holding heavy loads for long periods, consumes virtually no power. Also, since the efficiency of piezoelectric motors is not reduced by miniaturisation, they are effective in the power range lower than 30 W. This makes piezo motors attractive for use in battery-operated, portable and wearable medical devices because they can extend the life of a battery by as much as ten times.

7) No Heat Generation:
When at rest, piezo motors generate no heat. Piezoelectric motors also eliminate servo dither and the accompanying heat generation, which is an undesirable feature of electromagnetic motors.

8) Vacuum Compatible:
Piezo motors are in principle vacuum compatible, which is a requirement for many applications in the medical industry.

9) Operable at Cryogenic Temperatures:
Piezoelectric motors continue to operate even at temperatures close to zero kelvin, making them suitable for operation in extremely cold environments, such as in medical laboratory storage facilities and in cryogenic research.

10) Nonflammable:
Piezo motors are nonflammable and therefore are expected to be safe in the event of an overload or short circuit at the output terminal, a considerable advantage for portable and wearable medical devices.

11) Power Generation:
Piezo devices can be used to harvest energy. For example, using a person’s motion to power small medical or electrical devices such as pacemakers or health monitors.
 

Medical Equipment Manufacturers
Switching 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 have proven to provide more precise movements resulting in improved image resolution over conventional electromagnetic motors.

Point-of-care and medical test equipment engage piezo technology. Where fine-tuned positioning and measuring equipment is required, piezo motors fill the need, which can create motion with 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 starting to be employed using piezoelectric devices.

 

Basic Piezo Technology for Motion Control Applications

There are a number of different piezo actuators and motor types that are currently available. The most common ones are listed below:

A) “Simple” Piezo Actuator – expands proportionally to voltage. The motion is basically proportional to the drive voltage. Sub-groups include:

• Stacked actuator – most common type.  It offers high force, fast response and short travel.
• Shear actuator – lateral motion, fast, XY systems available. It provides high force and supports high frequency. Travel is typically limited to 20 µm.
• Tube actuator – mostly is for micro-dispensing applications and AFM scanners.
• Bender actuator – long travel (deflection) to several mm is possible, but low force and low frequency. 

B) Flexure-Guided, Piezo Actuator – frictionless flexures and motion amplifiers provide long travel and straight motion. The motion is basically proportional to the drive voltage.

• Integrated multi-axis systems available
• Motion range up to 2 mm and above.
• Frictionless, no wear and tear

 C) Ultrasonic friction motors

• Based on high frequency oscillation of a piezo plate (stator).
• Unlimited motion, high speed, fast response (10 to 10s of millisecs).
• Oscillation is transferred to a slide or rotor via friction.
• As a result of friction, resolution is limited to typically 50 nm. 

D) Piezo Stepping Motors

• Basically unlimited motion range.
• Based on accumulation of small controllable steps.
• Picometer resolution dither mode (direct piezo actuation).
• Compact and high force to 155 lbs. (for off the shelf units).
• Fast response (less than 1 millisec). High stiffness. 

E) UItrasonic Transducers

• Plate or disk-driven with a high frequency at resonance.
• Used as sensors or transmitters, and in nebulisers.

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 micro-surgery 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 uses piezoelectric motors. Precise motion of the optics is required to adjust the focal plane and for surface scanning. Piezoelectric positioning systems are integrated directly into the optics.

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 to miniaturisation, dynamics streamlining and interference immunity are pushing the physical limitations of electromagnetic drive systems. Piezoelectric motors are proving to be a viable alternative, finding their way into a growing number of medical device applications.

 

About Physik Instrumente L.P.

Physik Instrumente L.P. (PI) is a manufacturer of nanopositioning, linear actuators and precision motion-control equipment for photonics, nanotechnology, semiconductor and life science applications. PI has been developing and manufacturing standard and custom precision products with piezoelectric and electromagnetic drives for more than 35 years. The company has been certified to ISO 9001 since 1994. The manufacturer operates eight international subsidiaries.

For more information on this topic, please contact Stefan Vorndran, Director Corporate Product Marketing & Communications for Physik Instrumente L.P.; 16 Albert St., Auburn, MA 01501, USA; Phone: +1 508 832 3456, Fax: +1 508 832 0506; e-mail stefanv@pi-usa.us; www.pi-usa.us.

 


Jim McMahon writes on instrumentation technology. His feature stories have appeared in hundreds of industrial and high-tech publications throughout the world and are read by more than five million readers monthly. He can be reached at jim.mcmahon@zebracom.net.

 



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