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

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

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

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

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

"You have to be careful not to get fingerprintson those code disks because they are usually pretty small," says Seiler. "Thegap requirements for optical encoders are much tighter than with capacitivetechnology, so you have to be careful that the spacing is precise. With ourencoder, there are also no LEDs to burn out, so the typical MTBF is an order ofmagnitude greater."

Seilersays the capacitive technology uses a known frequency from the transmitterboard which passes through a rotating disk with a pattern of metal on it thatis controlled and predictable to create a predictable perturbation to thesignal as the disk rotates. Knowing what kind of capacitive reactions willoccur as a result of the pattern on the rotor, the encoder can read those deflectionsor perturbations in the frequency and extrapolate pulses using an ASIC designedspecifically for that purpose.

"We have a static transmitted frequency andlook 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 isjust an incremental AB quadrature output code and the commutation code. Themain output of the encoder is really absolute, so we have to do interpolationto get the quadrature pulses out, as well as commutation output."

Because the output of the encoder is TTLlevel signals, the encoder can be dropped into any application that might usean optical encoder.

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

Low-Cost Capacitive Encoders_A