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issue: January 2006 APPLIANCE Magazine

Motor Technology
Magnetic Kit Encoders


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A new magnetic kit encoder uses a sensor chip to provide a cost-effective motor feedback device with the same performance as an optical encoder.

New magnetic kit encoders are said to combine a non-contact, no-wear design and high-reliability silicon processes to yield exceptional reliability figures at elevated temperatures.

The new devices were designed by Renishaw Inc. to offer engineers a high-performance encoder in a small package and at a reasonable cost. Based on a AM256 sensor chip, the kit encoders are said to operate at speeds up to 60,000 rpm, temperatures up to 125°C, and with an accuracy of +/-0.3 degrees.

According to Hoffman Estates, Illinois, U.S.-based Renishaw, the encoders are ideal for end-of-shaft mounting and motor feedback. By adding a magnet to the end of the motor shaft and positioning a small printed circuit board module just clear of the shaft, high purity sine and cosine signals are produced at one cycle per turn. The company says that absolute shaft position can then be determined to high resolution for feedback and commutation control.

The key to the devices, however, is a sensor chip that uses a self-compensating array of Hall effect sensors and analog processing speeds. The large array of Hall sensors, coupled with the accurate conditioning of the signal outputs, reportedly ensure high purity signals, accuracy and resolution to 12 bit or 4,096 counts per revolution. The sensor is also said to be highly resistant to magnetic fields in the motor.

According to Howard Salt, business manager of Encoder Systems, the technology behind the encoders is relatively new and is possible because of improvements in semiconductor manufacture. "Up until a few years ago, the technology did not exist to produce encoders of this performance, for this cost, in high volumes," he says.

Even so, Salt admits that the concept behind the devices is quite simple. "Existing 'traditional' encoders are glass disks with 'captive' electronics requiring very precise disk to electronic alignment, using bearings and other parts," he explains. "Our encoders are just a chip and a magnet. Point the magnet at the chip [and] you have a rotary encoder with the same performance as an optical encoder. It's that easy."

The two-part design also makes the devices compact. They are available with an axial length of only 1/8 inch (3 mm) in diameters as small as 3/4 inch (19 mm). The encoders are also available with absolute, incremental and analog outputs in several industry-standard formats.

Possible applications for the magnetic encoders include motors used in medical scanners, vending machines and industrial power tools.

 

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