The University of Calgary can boast that itâ€™s got the first surgical robot able to operate in the 1.5 to 3-Tesla fields of magnetic-resonance imaging (MRI) machines.
Dubbed the neuroArm, itâ€™s the brainchild of neurosurgeon Dr. Garnette Sutherland and his team of researchers.
The neuroArm is designed to operate on the head of a patient sitting in an MRI machine. A surgeon at a workstation controls the arm using real-time MR imaging as a guide. And the robotâ€™s hand is actually steadier than that of a human surgeon. A surgeonâ€™s hand is stable to about 100 microns, say developers, but the neuroArm is good to about 25 microns.
MacDonald, Dettwiler, and Assoc. (MDA), makers of the Space Shuttleâ€™s Canadarm, did the main engineering work. One of the tricky parts of the design was coming up with nonmagnetic motors and bearings. â€śMagnetic materials in the MRI field generate artifacts on the image the doctor uses to make decisions,â€ť explains Alan Feinstein, CEO of Nanomotion Ltd., part of Johnson Electricâ€™s Medtech group. Nanomotion supplied the motors that power neuroArmâ€™s six axes.
The Nanomotion motors use PZT, a piezoelectric ceramic, to move a small ceramic finger back and forth. The finger rotates a ceramic ring, thus creating motion through friction. The motors are adapted from versions originally used in vacuum environments for semiconductor manufacturing. This involved converting stainless-steel parts to ceramic and coming up with fully ceramic bearings.
â€śThe motor is putting pressure into the work surface. That means you need a certain amount of stiffness in the structure it works against,â€ť says Feinstein. â€śWe normally need a stiffness of about 40 N/micron. But the robotâ€™s stiffness was low because of the plastic and ceramic bearings. We had to collaborate closely with designers to make sure we didnâ€™t cause too much wear or noise despite compromises on stiffness.â€ť
The PWM signals driving the motors could also potentially interfere with MRI signals, so Nanomotion built in a tunable filter that shifts the motor drive pulse frequency out of harmâ€™s way.
MacDonald, Dettwiler, and Associates Ltd.,
Univ. of Calgary
neuroArm video and information,