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Engineers at Johns Hopkins University are investigating the use of a robot (MrBot) that can operate within the closed bore of magnetic-resonance imaging (MRI) equipment to help physicians perform remote biopsies of tumors under MRI guidance.


However, such a system has yet to hit the market because electromagnetic motors typically used in robotics are incompatible with MRI. Ultrasonic (piezoelectric) motors are magnetism free, but they present conductive components and use electricity, which creates image distortions if operated within a certain distance of the object. Engineers at Johns Hopkins have developed a pneumatic step motor to overcome these challenges. PneuStep is the first pneumatic stepper and the first fully MRI-compatible motor because it does not interfere with the electromagnetic field. The motor is comprised of entirely nonmagnetic and dielectric materials, such as plastics, ceramics, and rubbers. Encoding was performed with fiber optics so that the motors are electricity free, exclusively using pressure and light.

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Great article and a great application for fluid power. But I do take exception to the statement that PneuStep is the first pneumatic stepper. I don’t know if I’m the first, but I designed a fluid power rotary stepper in the early ‘90s and received a patent in 1994. My patent covers a single-direction rotary stepper, but I have since designed a bi-directional stepper based on the same patent claims. My stepper concept is quite different from the PneuStep design, though. Like the PneuStep design, accuracy is achieved by using full strokes instead of mid-stroke, or proportional, positioning. But, my design is not rack-and-pinion, helical, lever-actuated, planetary, harmonic, nor differential piston. It uses no gears, cams, ratchets, pawls, springs or roller clutches. It is simply an arrangement of pistons and seals with only five (5) moving parts. A simple design ensures reliability and long life, and can withstand shock loads that other steppers cannot. Both the single- and the bi-directional designs are sequenced just like any double-acting cylinder while providing full torque during each entire step. It can also be constructed from MRI compatible materials. Because my torque density is probably much higher, the overall package could be made smaller. Another twist I would add (pun intended) is to use air-over-medical grade mineral oil for very smooth operation.

Greg Holmes
Holmes Automation Components, Inc.

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