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Mr. INTJ posted on August 17, 2008, 6:48 am
Hi,
I'm a software guy groping around in EE-land, and I could use a little
help.
A few old inkjet printers yielded up some nice motors with optical
rotary encoders built in. There are only six wires coming from the
connector on the back of the motor/encoder. Two of the wires go
directly to the DC motor. The other four are for the I-R emitter and
the two I-R detectors. Here are some lovely photos, in case they help:
http://www.minsmithphoto.com/mrintj/encoder-top.jpg
http://www.minsmithphoto.com/mrintj/encoder-back.jpg
http://www.minsmithphoto.com/mrintj/encoder-bottom.jpg
Using a low-voltage diode tester, and squinting at the traces on the
circuit board, I came up with the following circuit diagram...
http://www.minsmithphoto.com/mrintj/schematic.png
I've powered up the emitter, put an ohmmeter on one of the photodiodes
(in forward polarity), and watched the resistance go from high to low
as I slowly turn the motor shaft.
Even though it seems to work, I can't help but feel that something is
wrong. I would have expected the two photodiodes to share a common
anode or cathode, but that doesn't seem to be the case. Also, I'm
puzzled as to why they would be connected in the direction that they
are - so that I'd need a supply greater than +5V to read D1 and a
negative supply to read D3?
Surely, I've made a mistake somewhere, but when I go back through the
process of measuring and visual inspection, I end up with the same
goofy schematic.
What am I doing wrong? I'm assuming that the detectors (on the bottom
of the opto-interrupter assembly, closest to the circuit board) *are*
two individual photo diodes. An EE at work suggested that they may be
ICs with built-in amplifiers, etc...
Can anyone shed some light on this?
Thanks.
Mr. INTJ
San Diego, CA
BobH posted on
Re: Q: Motor w/optical rotary encoder from an inkjet
Mr. INTJ wrote:
What manufacturer printer did they come out of? Are the motors from the
paper path or the carriage?
Are there any part numbers on the sensor?
I am only familiar with the HP ones, but they did have an internal amp.
Those parts had 4 terminals: +5V, Ground and two phase output signals.
Some of them had logic level phase outputs and some of them had current
mode analog outputs. HP's Opto manufacturing group got renamed to
Agilent when the spinoff happened and then recently to Avago.
Re: Q: Motor w/optical rotary encoder from an inkjet
wrote:
These are from HP inkjet printers. You were right about the logic
levels - I just hooked both phase outputs up to individual traces on
my scope, and there they are - textbook logic signals, with a phase
difference according to direction. My mistake seems to have been
assuming that these were individual photo diodes. :-(
Thanks!
Re: Q: Motor w/optical rotary encoder from an inkjet
Mr. INTJ wrote:
Sure, glad to help.
Photo diodes would only be used in very high speed applications I think.
Normal low cost opto-interrupters use photo-transistors or
photo-darlingtons with open collector outputs. The fancier ones have
amplifiers and logic level outputs.
The HP sensors are pretty sophisticated, there are a number of detectors
at the proper spacing for the chopper wheel pitch, so that you still get
a clean output if there is junk on the chopper wheel or the sensor.
Bob
Curt Welch posted on
Re: Q: Motor w/optical rotary encoder from an inkjet
I don't know much of anything about these, but reading the wikipedia
article on photo diodes, it points out that a common mode of operation is
to reverse bias the diodes. As they are connected, they would be reverse
biased so it doesn't look backwards to me. You seem to be assuming they
must be forward biased to work.
I can only guess they are wried that way to drive some type of differential
sensor circuit.
Do they act like photo diodes when you test them without applying the power
to the +5 and GND? Maybe by shining a bright light or sunlight (to get IR
wavelengths) on them? If so, your schematic is probably correct. If it
needs the power to operate, it might be some type of receiver circuit
instead of the dual diodes you are assuming.
--
Curt Welch http://CurtWelch.Com/
curt@kcwc.com http://NewsReader.Com/
Re: Q: Motor w/optical rotary encoder from an inkjet
On Aug 17, 9:56 am, c...@kcwc.com (Curt Welch) wrote:
(snip)
Good point, I missed that.
(snip)
http://NewsReader.Com/
After some testing, I found that I'm getting the expected 5V
triggering signals out of both (phase differentiated), which makes
more sense to me than just bringing the photodiodes out to the
connector. I'm guessing that there's some magic inside the emitter-
detector assembly that I couldn't see. The two connections to GND and
+5V are correct, but internally, I think there's some kind of
amplifier or conditioner such that I'm getting logic-level pulses out
of the two remaining wires.
Thanks.
John Nagle posted on
Re: Q: Motor w/optical rotary encoder from an inkjet
Mr. INTJ wrote:
That's usually the case today. In fact, today you usually find
a single component with an emitter, two detectors, and buffers for
the detectors, so you get out a logic level. Once you figure
out which leads are power and ground, looking at the others with
a scope will tell you what you need to know. Power is usually 5VDC.
Some units have differential outputs, for noisy environments
near larger motors, and there you'll have power, ground, +X, -X,
+Y, -Y.
John Nagle
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