A milling machine spindle relies on a shaft spinning in air to reach 45,000 rpm. Electrical current in the bearing magnets creates electromagnetic forces that suspend the shaft, preventing frictional contact with the bearings.

Bearings that support shafts with electromagnetic force have been around for a long time. But the aerospace industry was the only user of these friction-free devices until the early 1980s. Then magnetic bearings began to appear on large turbomachines that process chemicals and natural gas. Though these devices run faster and longer than conventional bearings, control costs have kept them out of most manufacturing applications.

In the 1990s, designers broke the cost barrier by switching from analog to digital controls. As a result, magnetic bearings became practical for smaller, specialized equipment such as vacuum pumps, used in manufacturing semiconductors, and machine tool spindles. As controls continue to get better, and costs fall with increasing production, magnetic bearings are likely to find a home in even more applications.

An uplifting approach

Magnetic bearings perform the same functions as conventional oillubricated bearings on rotating equipment. Unlike their counterparts, though, magnetic versions support rotating shafts without physical contact. This lack of contact virtually eliminates friction losses, letting machinery run at previously unattainable speeds. Without friction, the bearings don't need lubricants, which can contaminate the operating environment.

A milling machine spindle relies on a shaft spinning in air to reach 45,000 rpm. Electrical current in the bearing magnets creates electromagnetic forces that suspend the shaft, preventing frictional contact with the bearings.

The operating principle is simple: electromagnets in a bearing create a magnetic flux that levitates or suspends a shaft and lets it rotate freely in air. The system has three basic elements: a stationary bearing (called an actuator) that surrounds a rotating shaft, position sensors that measure radial movement of the shaft, and a controller.

Each bearing contains electromagnets with north and south poles oriented around the inner diameter. A small air gap separates the bearing and shaft radially. Applying electric current to the magnets creates a magnetic flux strong enough to suspend the rotating shaft within this air gap, so it doesn't contact the bearing.

Proximity sensors adjacent to the magnetic poles monitor radial shaft displacement in the X and Y axes (horizontal and vertical directions), usually by measuring inductance of the air gap between shaft and bearing.

Electromagnetic forces in the magnets keep the shaft centered within the bearing. When position sensors indicate that the shaft is off center, the controller signals power amplifiers to adjust the current, moving the shaft back to center.
Select figure to enlarge.

Using shaft position data from the sensors, a controller calculates the forces needed to support the shaft and keep it centered within the bearing. Then it sends signals to power switching amplifiers (usually two for each axis), which feed enough current to the magnets to generate these forces in the form of electromagnetic flux. The controller repeats this process at least 10,000 times each second, continually monitoring and adjusting shaft position.

Most magnetic bearing systems incorporate two radial bearings to support the shaft, plus one thrust bearing to accommodate forces in the axial direction. A thrust bearing consists of two fixed components (stators), oriented on either side of a disc-shaped rotor attached to the shaft. Electromagnets in the stators generate magnetic flux to keep the shaft positioned lengthwise. A proximity sensor measures axial displacement of the shaft and sends data to the controller in the same way as the radial bearing sensors.

Digital control is key

The control system for a magnetic bearing consists of an analog or digital controller and a power supply (amplifiers). Analog systems have been around for 30 years, but digital systems are rapidly replacing them because they cost less and take less space. These lower-cost digital controls are making it easier to move into smaller industrial equipment such as machine tool spindles.

An analog controller typically consists of five circuit boards, whereas a digital controller may use only one digital signal processor (DSP) and cost about 1/10th as much.

A controller contains information on bearing characteristics (stiffness and damping) used to stabilize the suspended shaft over its operating range. An analog system provides this data in hard-wired control circuits that are custom designed for each application. Digital systems, on the other hand, hold the information in software, which is easier to reprogram.

Continue to next page