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Low-Cost Capacitive Encoders

Article-Low-Cost Capacitive Encoders

Low-Cost Capacitive Encoders

A new addition to the high-resolution, low-cost AMT encoder line from CUI Inc. generates standard U/V/W commutation signals for vectoring current to brushless dc motors. The AMT303 generates position information using a patented, capacitive code generation system coupled with a proprietary ASIC. The technology is immune to environmental particulates and magnetic interference, creating an economical and stable control and positioning solution to compete with optical commutation encoders.

"In the past it wasn't uncommon to package dc motors with Hall sensors already in place," says James Seiler, CUI's encoder product manager. "But many times, if you had an optical encoder, you could eliminate the necessity for the Hall devices. What we have now is a commutation encoder that not only has an excellent primary feedback of 1,024 cycles per revolution which can be quadrature decoded as 4,096 but it also has the commutation tracks that eliminate the need for the Hall sensors."

Seiler says that eliminating the Hall sensors, along with the encoder's capacitive technology, which is less expensive to manufacture than optical commutation encoders, are two reasons why the encoder is priced in the $20-35 range depending on OEM quantities.

"We believe the AMT303 series represents a breakthrough in commutation encoders," says Seiler. "When compared to optical encoders typically used in commutation applications, it will bring a level of ruggedness and ease of use."

Seiler says the capacitive encoder represents a proven, durable, reliable and accurate technology. It eliminates the glass disk that optical encoders use, and is convenient because most low-cost optical encoders are modular. The user mounts the base first, but then must also mount the disk and the cover.

"You have to be careful not to get fingerprints on those code disks because they are usually pretty small," says Seiler. "The gap requirements for optical encoders are much tighter than with capacitive technology, so you have to be careful that the spacing is precise. With our encoder, there are also no LEDs to burn out, so the typical MTBF is an order of magnitude greater."

Seiler says the capacitive technology uses a known frequency from the transmitter board which passes through a rotating disk with a pattern of metal on it that is controlled and predictable to create a predictable perturbation to the signal as the disk rotates. Knowing what kind of capacitive reactions will occur as a result of the pattern on the rotor, the encoder can read those deflections or perturbations in the frequency and extrapolate pulses using an ASIC designed specifically for that purpose.

"We have a static transmitted frequency and look at the perturbations in that frequency as the rotor changes," says Seiler. "Based on that, the unit extrapolates the output code, which in this case is just an incremental AB quadrature output code and the commutation code. The main output of the encoder is really absolute, so we have to do interpolation to get the quadrature pulses out, as well as commutation output."

Because the output of the encoder is TTL level signals, the encoder can be dropped into any application that might use an optical encoder.

An important feature for motor manufacturers is how easy it is to align the commutation position with the motor windings. Zero position may be set by an SPI (Serial Peripheral Interface) command or ground trigger, removing the need for time-consuming mechanical alignment in the mounting process. The unit's configuration also comes with nine sleeves that can accommodate nine different shaft sizes, which makes procurement and stocking requirements cleaner and more economical. Additionally, an onboard EEPROM can store up to 128 bytes of customer data. A demo board is available for stand-alone demonstration, PC access to SPI interface and example TCL code.

Low-Cost Capacitive Encoders
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