Biomedical Engineering Reference
In-Depth Information
FIGURE 2-26
Magnetic
incremental encoder
for rotation.
for the complete displacement or angle, whereas incremental encoders include only a
single bit and rely on the associated electronics to maintain a count from some datum
point. Incremental encoders are simpler and cheaper, but if power is lost then position
information is lost and it is necessary to return to the datum point.
To increase accuracy and to enable incremental encoders to determine the direction
of travel requires that two magnetic sensors are used. The one is displaced from the other
along the axis of travel by 90
◦
so that the outputs are in quadrature. This allows a simple
D type flip-flop to determine the direction of travel and to control an up-down counter to
produce a quantized measure of the displacement, as shown in Figure 2-26.
By “unwinding” the ring magnet used in the previous application, a device that can
measure linear motion is produced. While a rigid rod magnet could be used to provide
the required alternating pole pattern, flexible magnetic materials have made this sensor
much easier to implement, especially for long linear runs. For low-resolution applications
where the sensor can be very close to the magnetic strip, flexible ferrite materials offer
good resolution. These materials can be purchased in rolls of varying thickness and width,
often with pressure-sensitive adhesive backing for easy installation. For more demanding
applications, either in terms of number of poles per mm or working distance from sensor
to magnet strip, rare-earth materials (neodymium-iron-boron or samarium-cobalt) mixed
with plastic binders can be used.
An example of a linear incremental encoder is the LM10 manufactured by Renishaw,
shown in Figure 2-27. This is a solid-state, noncontact encoder featuring a compact IP68
read head that rides between 0.1 and 1.5 mm above a self-adhesive magnetic strip. Devices
FIGURE 2-27
Photograph of the
LM10 linear
magnetic encoder.
(Courtesy of
Renishaw
http://www.
renishaw.com/.)
