March 2006 Mechanical Engineering Departments: Tech Focus, Pt. 2

This section was edited by
Associate Editor Alan S. Brown.

Power Transmission and Motion Control

Plant Growth in France
by Peter Easton

BorgWarner Transmission Systems of Auburn Hills, Mich., has broken ground for a new, 160,000-square-foot plant in France.

The company said the new plan is a response to increasing demand for BorgWarner DualTronic transmission technology in Europe and associated new business gained in transmission controls systems.

The new facility is in Eyrein, which is north of Tulle in the Department of Corrèze. The operation will include application engineering, testing, and manufacturing capabilities for transmission systems. BorgWarner currently manufactures solenoids and transmission control modules at a plant in Tulle, and plans to move that operation's employees and production to the new facility when it is completed in the summer of this year.

Serving Servoed Steppers
by Alan S. Brown

At first glance, the SilverDust IGB servo controller/driver from QuickSilver Controls Inc. looks like many similar devices. The Modbus-compatible controller provides 16 input/output and four analog input connections and plugs easily into PC and human-machine interface software.

What makes SilverDust different is that it isn't designed to control servo motors. Instead, it makes high-torque stepper motors act like servos.

There are several good reasons to do this, said Eric Dunn, QuickSilver's sales and marketing director. "Stepper motors cost less than servos," he explained. "A typical NEMA 23 servomotor and electrical encoder for feedback might cost $600 to $800, while our system costs more like $300 to $450."

The new SilverDust IGB controller makes high-torque stepper motors behave like servos.

Steppers deliver a lot of torque for their size. They typically contain 100 poles (50 north-south pairs), and move forward one pole pair for each sine wave of electricity applied to the motor. This generates high torque at low speeds, especially when compared with typical four- to 12-pole servo designs. The multiple poles also act as an electromagnet to lock the motor into each step when it stops.

QuickSilver, which is based in San Dimas, Calif., turns steppers into servos with variable frequency drives based on today's high-speed digital signal processors. For feedback, the drives use optical incremental encoders to measure the motor rotary position.

"We also did a lot of modeling of the motor to ensure we could produce a nice easy, controllable motion without cogging between poles," Dunn said. The company had to resolve several issues along the way, including inductance (stepper motors have a tendency to run hot) and electrical noise.

Steppers are not for everyone. Their profusion of poles generates electromagnetic forces. The faster they spin, the more the forces battle the applied voltage. As a result, torque starts to fall off between 1,000 and 2,000 rpm, while it remains flat on traditional servos until 4,000 to 5,000 rpm.

That still makes them an economical alternative in applications that require acceleration and deceleration rather than high running speeds. Five-axis mills, for example, use the motor to position workpieces quickly and apply torque needed to keep them from jiggling during machining. They are also used to position chips on printed circuit boards and to feed labels onto bottles and jars.