Re: Encoders from Wirz

From Ted Robbins rtr@...

A wordy tutorial on encoders

Manufacturers are not always clear on resolution specifications, but cpr
and ppr are the most common call outs.

CPR usually means square wave Cycles per revolution. This means, for
quadrature encoders, the kind we normally use, two square waves, each with
the number of square waves specified by the cpr number. since each square
wave has a positive going edge and a negative going edge, we can double the
number of counts by counting both edges of one square wave. We can get
four counts per square wave cycle by counting the edges on both square
waves.

The reason for these two square waves in quadrature, meaning 90 degrees out
of phase with each other, is to establish direction of rotation. Usually
the two square waves are called A and B outputs. Again usually, the A
output square wave has its positive going square wave occuring 90 degrees
before the B square wave when the shaft is rotated clockwise.

Sometimes, clockwise is determined by viewing the encoder as if it were a
clock, but sometimes clockwise is determined as if it were a motor and you
are straddling it and watching the shaft turn. This warning is inserted to
tell you to be really carefull and keep your hand on the Emergency Stop
button when starting any machine for the first time.

PPR means pulses per revolution and can mean one of three things. It can
mean
1. CPR as above.
2. Decoding has been done inside the encoder and there are two outputs,
one a string of pulses and one a gate telling you what direction the string
of pulses represents.
3. Decoding has been done inside the encoder but the two outputs each
offer a string of pulses, depending on which way the encoder shaft is
turning, ie; a string of clockwise pulses and a string of counterclockwise
pulses.

The clearest way to say what you mean is to refer to the number of slots in
an optical encoder. You can say slots because sometimes the encoder disk
is made of glass or plastic with opaque and clear pie wedges, and sometimes
the disk is made of a chemically milled thin mettal plate with pie wedges
etched out of the plate. When you say "slots" you are referring to the
physical construction of the encoder and forcing the use of the CPR
definition stated above. You know you can get that number of pulses from
the decoding logic, twice that number, or four times that number.

There are not always square waves coming out of the encoder, When there is
no squaring amplifier on the photosensors' outputs you can get anything
from a beautiful sine wave to a distorted square wave, depending on
internal construction. As a previous post explained, if a sine wave is
output, you can use differential amplifiers to get much more than four
divisions per cycle. The wave will usually shrink in high resolution
encoders as the shaft is turned faster. This is not usually a problem
because the extra resolution is usually important only at very slow speeds.
You can use saturation (Squaring) amplifiers till you get close to
position, then concern yourself with the higher resolution.

Mechanics

The pitch of a screw refers to the advance of the screw in a single
revolution. A Bridgeport typically has a five pitch, 200 thousandths per
turn lead screw on the table and saddle. Five times 200 equals 1000
thousandths per turn. This means you need 200 counts per revolution of
the encoder. With four count logic you only need 50 cycles per revolution.
Fifty slots times 4 counts gives you 200 counts per revolution. As a
previous post said, this works fine for steppers, but higher resolution
will give you smoother, an often more accurate positioning in servo driven
systems.

As machines get larger they need heavier lead or ball screws, so they
usually get coarser. A multi start screw which moves the axis faster is
also coarser than a single start screw. A four pitch lead screw gives a
lead of 250 thousandths per revolution. This means you must use a 125 slot
encoder and 2 count logic to get 250 counts per turn. You can't use 4
count logic because 1/4 of 250 counts is 62 and a half. You can only
divide by 2 or 4 as long as you get an integer result. You can't even
round off. Here, though, a higher resolution disk allows interpolation and
the use of four count logic. Remember, low resolution encoders are usually
cheaper than high resolution encoders. Plotters and pick and place
machines can often use ten and even 20 pitch screws.

Everything described here also applies to rack and pinion encoder drives
with the following cautions. The fewer teeth on the pinion, the worse the
cyclical error. Five teeth are pretty crude, ten teeth are better, 20
teeth are better yet.

Given the few integer per inch arrangements of gearing, you are usually
limited to one tenth or one twentieth inch pitch racks. This means you are
not going to get away with a 50 slot encoder. You will need more
resolution. The payback is that you will not have to calibrate your
machine and program an error correction table into the software, depending
on usage, of course. If you are foam cutting with a hot wire, a 1/10 pitch
rack, ten tooth pinion and 125 slot encoder will give you two thousandths
resolution, overkill, but what I might use for such a machine.

We have not discussed cut, rolled, and ground screws and racks, and how
they affect accuracy. We have also no discussed how to mount encoders on
racks and shield them, though in an earlier posting I explained how to
mount encoders directly on screws.
----------

> From: Roger Brower <rbrower@...>
> To: CAD_CAM_EDM_DRO@onelist.com
> Subject: [CAD_CAM_EDM_DRO] Encoders from Wirz
> Date: Sunday, June 27, 1999 11:51 AM
>
> From: Roger Brower <rbrower@...>
>
> I found: @ http://wirz.com/surplus/index.html
> QUBE Encoders from Danaher Controls
> We have a very limited supply of these high qualtity incremental
> industrial encoders from Danaher Controls,Model# 320120300000. The
> encoders use a magnetic technology with a 120 ppr TTL current sink
> quadatureoutput. The 1/4" x 3/4" shaft is on ABEC precison bearings with
> 30 lb axial load rating. Data sheet included.$25.00, similar units sold
> in industrial magazines for over $200.
>
> Are these going to be suitable for our servo systems? Those who know,
> Let me know, and Save me three!
>
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