Self-aligning magetic bearing saves satellites

Cambridge, MA--Imagine a nearly perfectly balanced satellite momentum wheel with a bearing that never overheats, loses lubricant, resists spinning or needs service. NASA did, and the result is a 68 Nm-sec momentum wheel that rotates at 3,500 to 6,600 rpm around its center of mass under the control of a new self-aligning bearing system.

SatCon Technology Corp., the active- motion-control company that developed the hybrid propulsion system behind Chrysler's experimental Patriot racecar, designed and prototyped this electromagnetically controlled, no-contact journal bearing. Project spokesman Jim Downer thinks it is the first such practical magnetic spacecraft bearing. He attributes the bearing's development to contemporary compact electronics and high-strength magnetic materials.

For obvious reasons, components in orbiting spacecraft are usually not repairable. Thus a part as simple as a momentum wheel's bearing can be a satellite's Achilles' heel. Momentum wheels provide attitude control along a single axis, so losing one could cripple a satellite. Although they are precision-balanced, these wheels still exhibit some eccentricity or residual imbalance. Such imperfections cause vibrations that eventually wear out mechanical bearings.

To solve this bearing problem, SatCon suggested that NASA "go with the flow" and manage the momentum wheel's inevitable imbalance. "The idea is to allow spin around the point it wants to spin about (the center of mass)," says SatCon's Downer.

Here's how: the hub of the 33.2-cm momentum wheel is made from a ferrous material and supported radially by a magnetic journal bearing. Two stacked sets of proprietary electromagnetic coils direct motion in the x-y plane, exerting only 0.27N to keep the wheel spinning around its center of mass--a mechanical-bearing system would exert 4.7N.

In steady-state, the coils apply little or no force because the center of mass remains essentially fixed (and does not accelerate). In practice, transient forces are imparted to the hub as the wheel spins up (which takes 19 minutes), and when the motor driving the momentum wheel is used to impart attitude changes.

Eliminating transients requires an active-motion-control scheme that uses both measured and predicted wheel dynamics. Two differential pairs of eddy current sensors (from Kaman Instrumentation Corp., Colorado Springs, CO) provide position feedback to a closed-loop digital control system. A Texas Instruments TMS320C30 DSP chip processes this data and instructs amplifiers to update positioning-coil currents at about 3,000 Hz.

The DSP controller holds in memory an interrelated, dynamic mathematical model of the momentum wheel (called the plant model) and its disturbance dynamics. These models let the DSP predict how imbalances influence the wheel's motion.

"You want to feed back the center of mass position, but you only know the geometric center," Downer maintains. So the DSP must estimate center-of-mass position. The output of the disturbance dynamic model gives a measure of offset distance, and the output of the plant model gives an estimate of the center-of- mass position. These are summed to produce an estimate of the sensor output, which is then compared to the actual sensor output.

By controlling gains on both plant and disturbance models, "the loops converge so that the result nulls out. It drives the estimate of what you should be measuring and what you are measuring to be the same," says Downer. The result is a controller that lets the wheel rotate around its center of mass.

"This is an exciting innovation," exclaims Downer, and SatCon has applied for a patent on it. "There are a lot of applications for long-life, reduced-maintenance, high-speed operation and compactness in bearings," he adds. And because "magnetic bearings are in principle no different from electric machines of any kind," Downer envisions the day when factories churn them out and engineers design them into everyday down-to-earth products. In that situation, bearings of this type might become as commonplace and widely used as are miniature electric motors today.

Additional details...Bill O'Donnell, SatCon Technology Corp., 161 First St., Cambridge, MA 02142, (617) 349-0846.