Re: encoders

At 12:18 AM 7/14/99 EDT, you wrote:

>From: TADGUNINC@...
>
>I have a bunch of encoder disks with some kind of opti diode sensor, still
>trying to figure out the input output, with very poor resolution 40 slots

per

>rev. if I offset phase them equally and stack five of them 200 spr and run
>them, 5 to one, reduction rate of stepper to ball screw, and put the output
>in quadrature would that be sufficient resolution? if so how do i get
>quadrature output? I know what it is, but am unfamiliar with how to make the
>circuit.

You have a few different choices. The 40 slot encoders will give you 40,
80, or 160 edges (Pulses) per rotation. You can probably gear it most
easily to get it out of the way of the lead screw. The gear can be a
toothed belt or a real gear. A five to four ratio will give you 200 counts
if my arithmetic is ok.

If these encoders are really simple, only one led and sensor per channel,
two leds and sensors, at this resolution you will not get a sine wave, so
you probably can't successfully increase resolution by using differential
op-amps to get more switch points.

Decoder circuits are easy to make, but they are now available on single
chips, sometimes with up-down counters built on the same chip. You don't
really need the counters if you are dedicating a clone to the readout.

If you really want to design one, you can use "exclusive or" chips easiest.
Feed each channel to one "exclusive or". Run the channel output directly
to one input of the "exclusive or" and also run it through an "inverter" to
the other input. Add a resistor, about 1 to 3 K between the inverter
output and the "exclusive or". Then add a small capacitor, a few
picofarads will work, between that input to the "exclusive or" and ground.
The resistor and capacitor act to slow down the inverted input to the
"exclusive or", setting the length of the pulse. Your oscilliscope will
tell you when you get it right. For four count logic, 160 counts from a 40
slot encoder, use two "exclusive or" circuits, For two count logic, 80
counts from a 40 slot encoder, use just one "exclusive or". For one count
logic, 40 counts from a 40 count circuit, use only one circuit but
substitute an "and" gate for the "exclusive or". Use ordinary CMOS if you
don't want to order the special decoder chips.

Unless you have a really good reason, or are a masochist, use Dan Mouch's
new board. I've made many such circuits, and I bought his boards.

You still aren't done. You need the signals to feed the counter. Now you
have to use "and" gates and "or" gates to arrange the output to a counter
or your computer to be either a string of pulses and a (clockwise)/(not
counterclockwise) signal to interpret the string of pulses, or two seperate
strings of pulses, one for clockwise and one for counterclockwise.

Now you have to determine what kind of counter, computer, or display to
read the output of the decoder. Like I said before, use dan's stuff or
similar stuff for servo or stepper motors.

I can do all this stuff, but I want to cut chips sometime before I kick the
bucket. That's why I buy rather than build. Money is the barrier, so this
mail list makes buying rather than building practical.