perm filename ENCODR.REF[1,VDS] blob sn#083673
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C00002 00002 Encoder Specifications for Hand-Eye Work
C00005 00003 For reasons of simplicity and economy, and because of the rapidly
C00008 00004 Here are some typical encoder specs.
C00009 00005 LITTON ENCODER INFO.
Encoder Specifications for Hand-Eye Work
This file outlines some of the requirements we have for
encoders and their interfaces for some of the Hand-Eye project work.
Up till now we have been using Potentiometers as our general
purpose resolving devices. For increased resolution, improved
linearity, and better stability, it appears desirable to switch to
optical encoders on many of our "joints".
Two general types of encoders are available- "whole word" and
incremental". Whole word encoders have as
many tracks as bits of
output resolution and provide an n bit coded output of the position
of the encoder. Incremental encoders have two output tracks, (plus a
zero reference mark sometimes). They provide two output signals in
quadrature which are electronically decoded to drive an external
up-down counter. For unidirectional applications some of these
incremental encoders merely have one track and drive the counter
directly. Or else, a outside direction sensing switch changes the count direction
of the counter.
For reasons of simplicity and economy, and because of the rapidly
decreasing cost of i.c.electronics, most general purpose encoders are
of the incremental type. I am proposing to use these on most of the
new mechanical devices we design, and I am proposing to retrofit these
incremental encoders onto some of the already existing devices, starting
with the T.V. cameras.
The first device to be retrofitted will be the Sierra Camera because it
was designed to use encoders in the first place. Then the Cohu. Each camera will
use 2 encoders to start with. They will be quadrature output devices with a
zero reference track. 4X count multiplication circuitry
will be necessary to multiply the count output. Quadrature decoding will
also be required. The count should be stored in hardware counters to eleiminate
the need to reset the computer count everytime the system dies (done by
servoing the camera past the zero reference mark). I suggest that 16 bit counters
be designed to accomodate all possible encoders. These counters, and the related
logic must be stable and noise free to enable them to count error free for
periods of weeks or months. Possibly a battery pack, standby unit can be used to
provide power when the kludge bay power is off, otherwise the zeroing procedure will
have too be repeated.
Here are some typical encoder specs.
Number of lines- 1024 (gives 4096 counts per turn)
Output-integral amplifier model-
Max. expected count rate-50khz.
LITTON ENCODER INFO.
This page covers some information gleaned from a Mr. Rendler of
Litton Industries, Encoder Div.
Model 71 and Model 73. Model 71 is a larger version with
phototransistor output. Not differential. Prices for 512 lines are
$135, 145,155 for one two or three channels of amplified output. No
cover is supplied. Model 73 is newer and smaller. It fits into a 1.5
inch dia. package, has a cover (normally closed end, but available
with thru shaft opening), and differential transisitor output on all
channels. Price for 512 lines is $180,190,200 for 1,2,3 channels
output. All square waves. Both will take a
.5 inch dia shaft.
What follows is some information on special encoders and also
on encoder practice in general. They charge about $400 to make a
master disk and mask. Typical specifications are the following.
Concentricity of i.d. and track + or- .005 to .010 Conc. of o.d. to
track or i.d. + or- .005 to .010 Disk light to dark width is 50-50
Mask light to dark width is less than 50-50, dark is >50% (the
impression I get is that it is say like 40-60 or so. They will adjust
it to yeild a roughly uniform sine wave output. The standard glass
thickness is about .070-.080 for disks. For low inertia they make
the disks .020 thick. The masks are .010 thick glass. Litton grinds
their own glass, and will make it flat enough to suit the resolution.
They center the disk on the hub optically to about .0003 in.
concentricity with a microscope. For high resolutions they use an
electrical method which has two sensors 180 degs apart and alows
alignmeent to within 1/8 cycle. But not necessary even on 2048 line
disk. Typical track width is about .040 but it can be wider. The
mask is about .5 in wide and houses all the sources and sensors as
close together as possible. They generally use a differential
method, which uses two sources and sensors per channel. Only one
track is used for the disk. Quadrature is obtained from two pairs of
channels 90 degs. out of phase. These channels each have two mask
grids 180 degs apart. The zero ref. track is also differential.
They use a single outer track which is all clear, as a sort of
reference. An inner track has all light and a small line at zero
(masked). This gives a sort of differential output. Their typical
layout is an inner zero ref. track, then the count track, then the
zero calib. track(the one with no marks). The differential layout
adapts for greater temp. variations, also allows for ageing in
sources, sensors, voltage variations, etc. Gap. As a rule of thumb,
the gap whould be about equal to or less than the width of a line.
Their masks have each sensor placed about .10 apart and I think they
use the .062 dia. devices. This places them close together where
they have the least quadrature variation. Litton also has an aligning
tool good for up to 1000 lines. You slip it on the shaft and its two
pins engage the head holes. This centers everything without the need
to do any special electrical measurements. Oh yes, for glass
thickness, let them select things for us, the std. thickness may be a
variable, its around .060-.090.
They mount the disks to the hubs with a slightly flexible
epoxy to keep the bond from cracking with temperature changes, which
would occur if a rigid epoxy were used.
That about sums up my conversation of Jan. 20. We will order
one of their model 73 encoders with 512 cycles, and zero ref. incl.
all electronics. This seems to be a better deal. By the way, they
say that the phototransistor output on all channels is connected to
the circuit board by wires and we can unsolder these wires and
connect them to our outside electronics if desired. The collectors
are aparantly common. Light source is LEDs for each sensor.
Yes they still use the electron beam master maker, at a cost
of about $400 per disk and mask. They will also supply chrome on
glass disks as standard.