Piezoelectric motors and actuators usually get the nod in applications that require the most compact, precise motion components money can buy. Yet these ceramic-based motion systems have something else going for them: They are often non-magnetic, which makes them a good fit for devices that are in close proximity to medical imaging systems.
“Our ceramic servo motors increasingly operate in conjunction with medical systems that are sensitive to magnetics,” says Alan Feinstein, president of Johnson Medtech’s Nanomotion subsidiary. These applications include a variety of imaging systems such as scanning electron microscopes, where magnetics might produce artifacts in images. And they include robots, manipulators and other devices that work near or even within magnetic resonance imaging (MRI) machines.
Nanomotion’s motors run on a reverse piezoelectric effect in which a piezoceramic crystal transforms an applied electrical field into strain. By carefully designing the geometry of the crystal and the excitation electronics, Nanomotion harnesses that strain to drive linear or rotary stages.
As Feinstein explains, Nanomotion’s patented technology uses dual-mode excitation electronics to produce bending and longitudinal strains in the crystal simultaneously. The result is a small, quick (39,000 Hz) elliptical movement at the front edge of the crystal, which then sets the stage in motion. “It’s a little like a stick of Trident gum that wiggles 40,000 times per second and walks the stage along,” he says.
Nanomotion’s ceramic servo motors have recently been used to drive a new robot arm that will help doctors perform neurosurgery remotely with real-time guidance from an MRI machine. Researchers at the University of Calgary created the robotic system with help from MacDonald, Dettwiler and Associates, the firm responsible for the Canadarm remote manipulator system for the Space Shuttle.
Because the surgical robot mounts and runs within the confines of the MRI, the whole thing, including the actuators, had to be made from non-magnetic materials. And that’s where Nanomotion comes in. According to Feinstein, the robot uses six Nanomotion ceramic motors, one for each of the robot’s six rotational joints. The largest of these actuators, for the base of the robot, exerts about 12 lb of force and 24 inch-lb of torque. The smallest motors, those for the robot’s wrist joint, exert about 1 lb of force.
Feinstein says the individual actuators are accurate to “within a few microns” so the accuracy of the entire robotic system comes to 25 microns. In keeping with the non-magnetic requirement, Nanomotion supplied the actuators with PEEK and other non-metallic bearings.
Look for more of these MRI-compatible systems in the future. Feinstein says his company has already supplied ceramic motors for other MRI-compatible devices, including a small machine that simulates the beating of a human heart.