ENCODER.APR–pm
Optical encoders use leading-edge technology to conquer new markets
Fiber optics is one method being used to peel away the fragile label once
attached to optical encoders
BY PATRICK MANNION Associate Editor
Optical encoders–or rotary encoders, optical switches, or whatever
name you wish to give–are no longer the fragile, unreliable, niche-market
transducers they once were. Improvements in manufacturing processes have
eliminated many of the problems associated with these devices, such as
disk breakage, moisture ingress, short-life light source, poor temperature
performance, and susceptibility to electrical noise. Because of the
improvements made, these devices are now finding newer and more diverse
applications–from printers to forklift trucks. Many inherent qualities
endear the devices to designers and make them the only viable option for
many position control and switching applications. These advantages include
direct shaft-to-digital encoding, a minimum number of power supplies, low
power requirements, low cost (for incremental types), and high speed. With
these advantages, the encoder has found many applications in relatively
benign, interference-free environments. More extreme or rugged
environments require the use of the virtually indestructible synchro
transducers. The optical encoder can be divided into five main
categories; panel mount, light duty, heavy duty, industrial, and
precision. All these markets are served by both incremental and absolute
encoders (for a definition of incremental and absolute encoders, see
“Choosing the right optical encoder for your control system,” p. 23). The
type of encoder selected depends on several factors, including
environmental constraints, resolution required, accuracy, and cost
allowance. Whatever the application, an overall trend recognized by Paul
Chae, communications marketing manager with Allen-Bradley (Milwaukee, WI),
is toward faster and faster encoders. Already firmly entrenched in the
industrial, office automation, and military markets, Allen-Bradley is set
to launch into the demanding automotive scene as early as 1994. The
enabling factor for this launch is fiber-optic technology, which has given
the company the potential to manufacture reliable, high-speed, compact
encoders that are immune to electromagnetic interference (EMI). Along with
EMI, automotive applications also require a high tolerance of temperature
extremes and vibration. Fiber optics solves all these problems. For
regular encoders, the light output of the LED decreases with increasing
temperature. Thus, LEDs are limited to below 60 degreesC operation
(although products are available up to 125 degreesC using special optics).
With fiber-optic encoders, Allen-Bradley extends the encoder operating
range to 140 degreesC, with a minimum value of 0 degreesC. For high-speed
applications, incremental encoders have a maximum operating speed of about
210 kHz, with 100 kHz considered a typical value, as in the 845T, from
Allen-Bradley (see Fig. 1). Using fiber optics, this operating speed could
be extended to between 1/2 and 1 MHz. With the many advantages of fiber
optics, Chae is still surprised at the underuse of the technology on the
factory floor. He predicts, however, that a “cultural change” is
inevitable. Helping to effect this change is Computer Optical Components,
Inc. (COPI), of Chatsworth, CA. The company has released the FD221-R1
fiber-optic rotary incremental encoder that features a maximum operating
temperature of 300 degreesC. The resolution is as high as 2,000 and the
frequency response is 100 kHz. The outputs are A and B channel with A, B,
and index optional. For its incremental encoders, COPI uses the full
sinusoidal waveform generated by the disk and mask pattern. For a low
number of counts per revolution the waveform is triangular, while for high
counts, the waveform is sinusoidal. In most encoders this pattern is
squared off internally because of the greater familiarity of designers
with digital signals. The advantages of using the full waveform include
continuous position and speed information. Along with the fiber-optic
products, COPI is touting a 12-bit absolute encoder that weighs in at the
same price as an incremental device: around $150. According to Kees Van
Der Pool, president of COPI, the price reduction was achieved through the
use of proprietary techniques that reduce the optics to a one-diode,
one-LED pair. This brings about savings on construction and packaging.
Like many encoder manufacturers, COPI has found a ready market in
avionics, medical, and instrument applications. However, Van Der Pool makes
a point of staying away from the military and automotive segments. The
latter case is due to a reluctance to give the commitment an automotive
manufacturer tends to demand of its suppliers. However true this point of
view may be, it is not sufficient to ignore a potentially huge market.
Hewlett-Packard is aware of this and is set to launch into the market
using a steel alloy wheel to allow its encoder to operate in the
temperature range of -40 degreesC to 140 degreesC. While HP's overall aim
is to replace all analog transducers in the automobile, it is now devising
a means of controlling the exhaust emission to comply with California
guidelines. This will be done by inserting an optical encoder to control
the throttle valve for the mixture of air and fuel. HP is strong in the
office automation area, where its products find a ready market in laser
and ink-jet printers. Its core sales come from the high-volume, low-cost
optical-encoder market.
Panel-mount encoders
The end of the encoder market reserved for panel-mount devices is served
by a somewhat different beast than the regular shaft encoder discussed
above. Sometimes called glorified switches, the panel-mount optical
encoder comprises much the same detector mechanism as the shaft encoder,
except the wheel is turned by hand rather than a rotating shaft. The major
advantage of using the device as a switch or potentiometer is that there
is no wiper or contacts to wear out. For a typical 32-position switch, 25,
000 cycles of operation is the norm. For an optical encoder-based device,
up to 1 million cycles of operation are possible. The major cause of
encoder failure is no longer disk breakage, but the detent springs. The
panel-mount encoder serves similar markets as the shaft encoder. These
include avionics, medical, and industrial applications, in addition to
exercise equipment and gas pumps. Typical panel-mount devices include the
1000 Series from OakGrigsby Inc., of Sugar Grove, IL (see Fig. 2a), and
the Series 61 from Grayhill Inc., of LaGrange, IL (see Fig. 2b).
Measuring 1 in. in diameter, the 1000 Series absolute encoder has a 5-bit
output and up to 32 positions. It features continuous rotary action, a
sintered metal detent housing, and a stackable, triple-deck design–with
or without concentric shafts. The 1000 Series sells for $29.95 each in
lots of 1,000 and is available now.) Although OakGrigsby has been a
long-time supplier of panel-mount encoders, Brian Ward, marketing manager
for the company, sees a potential $20-million market in the light-duty
enclosed optical encoder end of the market. Consequently, the company has
plans to enter this area in the near future. With this in mind, Ward is
careful to stay away from the automotive market and its all-or-nothing
philosophy. In the military end, panel-mount encoders are still going
strong–despite cutbacks in government spending. Tom Menzenberger,
marketing services manager at Grayhill, explains that the panel-mount
encoder market has so far been untouched by cutbacks, as such devices are
usually put at the back end of major projects. Also, high reliability
makes the devices a must for critical applications. Grayhill's Series 61
sends out 2-bit quadrature code, has a contact bounce of 4 ms at make and
10 ms at break, and has an operating force of 250 to 330 g. It sells for
$10.47 and is available 4 to 6 weeks ARO.
CAPTIONS:
Fig. 1. The average incremental encoder has a maximum operating speed of
about 210 kHz, with 100 kHz considered a typical value, as in the 845T,
from Allen-Bradley. Fiber optics extend this to between 1/2 and 1 MHz
Fig. 2. Typical panel-mount devices include the 1000 Series from
OakGrigsby Inc. (a), and the Series 61 from Grayhill Inc. (b). These
devices have the advantage of longer life over potentiometers or switches.
For more information on the products mentioned in this article, call
the companies listed below.
Allen-Bradley Milwaukee, WI Jim Jerschefske 414-382-2000
Computer Optical Products, Inc. Chatsworth, CA Kees Van Der Pool
818-882-0424
Grayhill Inc. LaGrange, IL Tom Menzenberger 708-482-2132
Hewlett-Packard Co. Optical Components Div. San Jose, CA Sales Dept.
800-752-0900
OakGrigsby Inc. Sugar Grove, IL Brian Ward 708-556-4200
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