11 ways piezoelectric motors improve equipment performance
Piezoelectric Motors Help Equipment Performance: Ten reasons to consider piezoelectric motors for motion control application follow, along with one more point that many motion system or product designers might not have considered. This is an August 2010 Control Engineering Inside Machines feature article.
Jim McMahon, for Physik Instrumente
High-precision and other rugged devices, such as those for medical-related applications, can be made smaller, more precise, lighter, and easier to control by employing piezoelectric motors instead of many electromagnetic motors. Ten reasons to consider piezoelectric motors in an application follow, along with one more point that many motion system or product designers might not have considered.
1. Higher force generation supports miniaturization: Piezoelectric motors are better suited for miniaturization. They can easily be made smaller and more compact than electromagnetic motors, yet for their size they provide greater force.
Efficiency of electromagnetic motors falls as their dimensions are reduced with more of the electrical power converted to heat. With piezoelectric motors, 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 extremely 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 cell 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 often 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 within microseconds. Acceleration rates of more than 10,000 g (response times of 0.01 milliseconds) can be obtained.
4. No magnetic fields: Piezoelectric motors do not create electromagnetic interference, nor are they influenced by it, eliminating the need for magnetic shielding, beneficial for medical and biotechnology applications. This feature is particularly 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 motorized medical devices to function within MRI equipment.
5. No maintenance or lubrication, aseptic enabled: Piezo motion depends on crystalline effects and involves no rotating parts like gears or bearings, so piezo motors are maintenance free, without need for lubrication. They can be sterilized 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 miniaturization, 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 as much as 10 times longer.
7. No heat generation: When at rest, piezo motors generate no heat. Piezoelectric motors also eliminate servo dither and related heat generation, an undesirable feature of some electromagnetic motors.
8. Vacuum-compatible: Piezo motors are in principle vacuum-compatible, a requirement for many medical industry applications.
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 safer in the event of an overload or short circuit at the output terminal, a considerable advantage for portable and wearable medical devices.
One additional point that motion system designers may not have considered…
11. Power generation: Piezo devices can be used to harvest energy, such as using a person’s motion to power small medical or electrical devices, such as pacemakers or health monitors.
Physik Instrumente L.P. (PI) manufacturers nanopositioning, linear actuators, and precision motion-control equipment for photonics, nanotechnology, semiconductor, and life science applications; www.pi-usa.us. Jim McMahon writes about instrumentation technology for Zebra Communications; jim.mcmahon(at)zebracom.net. Edited by Mark T. Hoske, Control Engineering, www.controleng.com.