Using MEMS technology to miniaturize the design, new inductive micro encoder
technology from Maxon Motor
offers a unique, miniaturized feedback solution. The
first implementation of the technology is mounted on the shaft of an EC6
brushless DC motor, which extends the length of the motor/encoder combination
by only 1 mm.
"We believe the MILE micro inductive encoder, which is only 6 mm in
diameter, is the world's smallest inductive encoder," says Paul McGrath, a
regional sales manager for Maxon Motor USA.
"The inductive encoder technology itself is not new, but it's the first
time this has been implemented on such a small scale."
The general principle used in inductive encoders is that the inductance
of one or more coils changes in relation to the material in their proximity. A
semi-circular iron core representing the material measure, for example, could
be directed to a coil which then changes its inductance. But there are
drawbacks to simple inductive encoders, such as the temperature dependence of
the soft iron/ferrite. External magnetic fields can also change the
permeability of the material used, well below saturation point.
This is why new highly accurate inductive encoders are ironless, and the
contrast is generated with eddy currents. McGrath says inductive encoders
have been featured in larger motors for a long time, but have not been produced
for micromotors until now.
Compared to optical, magnetic and magnetive resistive encoder solutions,
the new technology offers a series of benefits in specific applications. The new MILE encoder delivers 64 pulses at up
to 120,000 rpm and offers three channels including a line driver and integrated
commutation outputs. Typical application areas include medical technology,
robotics and harsh environment industrial applications.
McGrath says the new encoder can be compared, in some respects, to a
miniature resolver. The design utilizes eddy
current induced changes to an otherwise balanced, high frequency magnetic field
to excite the integrated differential coils.
And for that reason, it offers some of the traditional advantages of a
resolver including robust operation against electromagnetic interference, dust,
oil and magnetic fields. An integrated,
factory-set look-up table also makes it possible to correct repeatable errors
in real-time by comparing measured raw parameters to values in the look-up
table. The result is an almost perfect
linearity of the sensor.
The first implementation of the MILE encoder technology is supplied mounted
on the shaft of a motor, with further implementations to come in the near
future.
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