NANOPOSITIONING: Piezo motors and actuators drive bio-optics precision

Precision and accuracy are at the heart of biomedical instrumentation, including imaging and treatment tools—and the motors these instruments incorporate have much to do with their performance. The physical limitations of traditional electromagnetic drive systems are being challenged by increasing accuracy requirements in the micron and nanometer ranges, along with an inclination to miniaturization, dynamics streamlining and interference immunity. Because medical devices can be made smaller, more precise, lighter and easier to control by employing piezoelectric motors, these alternatives are finding their way into a growing number of medical device applications; for instance, for cell imaging, 3D scanning, and laser beam steering in ophthalmology, dermatology and cosmetology.

In optical coherence tomography (OCT), for instance, 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 than conventional electromagnetic motors, and thus produce improved image resolution. Confocal microscopy, used in ophthalmology for quality assurance of implants, requires very precise movement of the optics for adjustment the focal plane and for surface scanning. To enable this, piezoelectric positioning systems are integrated directly into the optics.

The latest designs of piezo motors—including ultrasonic piezo linear (or resonant) and piezo stepper motors—have a number of advantages over electromagnetic motors, including unlimited travel (movement). According to Physik Instrumente L.P. (Auburn, MA; www.pi-usa.us), new ultrasonic resonant motors are characterized by speeds up to 500 mm/s, in a compact and simple design. They can produce accelerations to 10 g; are very 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 motors, on the other hand, can achieve much higher forces, up to 700 N (155 lb), and resolution of 50 pm. They also enable extremely high-precision positioning over long travel ranges, and can perform highly dynamic motions. Both operate in the presence of strong magnetic fields or at very low temperatures.