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
"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
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.
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