A linear motor for the masses 3-6-00

Zurich, Switzerland--It's a linear motor. It's an electric cylinder. It's the LinMot(R) P linear drive system, a technology developed by Ron Rohner and his colleagues at Sulzer Electronics Ltd. that is so unusual it almost defies any attempt to slap a conventional label on it.

The moving magnetic slider, stationary coil assembly with integral positioning sensors, sensor electronics, and bearings put the LinMot(R) P squarely in the linear motor camp. But its small-diameter cylindrical profile and low price compared to other linear motors make for a serious contender in the pneumatics applications arena.

"In fact, our primary design goal was to develop a small-format actuator at a reasonable price," says Rohner, director of engineering at the Zurich-based Sulzer Electronics Ltd. Meet it they did. Prices for the LinMot(R) P start at $600 for a single axis, compared to about $1,500 for linear motors of other designs.

It's not as if Rohner had many design options, since the permanent magnets alone typically account for 45 to 60% of the total cost of a linear motor--it's one reason prices remain so high--and the number of components is relatively few. But where there's a will, there's a way.

"To achieve our goal of small size and low cost, we did a couple of things differently with the design of the motor," says Rohner. "First, the magnet itself is part of the positioning system, which uses inexpensive Hall-effect positioning sensors. We also use a lower-power controller.

Small and low cost. Second, we integrated the bearing assembly into the motor itself, so that the body of the coils is the bearing system. There are no external components required to add cost and size."

The design is unique, since no other maker of tubular linear motors (and there aren't many) uses internal bearings or sensors. In the LinMot(R), the body of the coils functions as a slider bearing, which means there is a radial gap of approximately 0.004 inch. "But be- cause we are using the linear movement (axial direction), the gap in radial direction doesn't affect the accuracy directly," says Rohner. More problematic with an integrated design was dealing with an operating temperature range between -40 and 194F.

"The material as well as the behavior of the coils must adapt to this temperature range without affecting the mechanical dimensions," says Rohner. "For example, we had to be sure that we selected a material for the body of the coils that was both temperature stable and exhibited both low friction and a long life. We chose Delrin(R), which is an acetal resin that has metal-like properties."

Trade-offs involved. Life is full of trade-offs, however, and Rohner's team had to sacrifice a certain level of both accuracy and force in order to achieve a small size and low cost. For example, the linear resolution of the LinMot(R) is 0.004 inch, as compared to 0.5 to 10 microns for traditional linear motors.

"Because we use the magnet as a positioning sensor, the thermal expansion of the material limits us from achieving micron-level accuracy," explains Rohner.

Similarly, maximum peak force of the LinMot(R) P is 45 lbs, compared to 150 lbs for some slotless designs and 1,500 lbs for some iron core designs.

"Clearly, force is a function of size, and while nothing limits us going bigger from a technical standpoint, it just wouldn't make any sense from a manufacturing or marketing perspective," Rohner points out, adding that the largest diameter motor Sulzer plans to offer is 37 mm. The company also offers a 23-mm version of the motor.

Nonetheless, Rohner explains that the design team felt the trade-offs in accuracy and force are acceptable, particularly to engineers currently designing with pneumatics who are looking for a higher level of positioning accuracy but unable to justify servo prices. "The market we're targeting is completely different from the traditional linear motor market," says Rohner. "The ideal application for our motor is one that is cost sensitive, requires good positioning capabilities, and can get by with a reasonable level of precision."

Additional Details...Contact Ronald Rohner, Director of Engineering, Linmot, Sulzer Electronics, c/o Box 521, Rogers, MN 55374, Tel: (877) LINEAR-0; e-mail: [email protected].