Hoi Wong wrote:
"Dave Platt" wrote in message
...
The rotary encoder in my Wavetek 75 arbitrary waveform generator is
physically broken, and it's impossible to hook anything up to the broken
legs to probe the specs.
I'm totally new to encoders and I'd appreciate if anybody can offer any
advice on finding a suitable replacement. I couldn't find the service
manual for Wavetek 75, so all the information I have is the encoder
itself:
http://www.stanford.edu/~wonghoi/DC/...coderFront.png
http://www.stanford.edu/~wonghoi/DC/...ncoderBack.png
http://www.stanford.edu/~wonghoi/DC/...EncoderCap.png
Hmmm.
Based on the photos, it appears to be a mechanical rotary encoder,
with a two-bit quadrature output and a common terminal. By my count
there are 13 or 14 contact fingers per output per quarter-revolution
of the control... this would allow for 50-some pulses per revolution
for each of the two channels, or around 200 counts per revolution if
you're doing the usual sort of quadrature decoding.
I suspect that a direct replacement may be tricky to find. In looking
through the DigiKey and Mouser catalogs, it appears to me that
mechanical-contact encoders with this large a number of counts per
revolution are quite unusual. Mechanical encoders these days seem to
max out at around 32 counts per revolution, and usually have
detents. I'd guess that you'll probably want a detent-less encoder,
with a counts-per-revolution value not too different from that of the
broken one (which may very well be a custom part). That's probably
going to mean switching to an optical encoder, one way or the other.
I can think of at least two ways to do it, depending on how the
circuitry in the Wavetek 75 works:
- If the existing encoder's common wire (the one going to that
sliding contact) was grounded, and if the two quadrature leads were
of the "external pull-up resistor to +5 volts, and the encoder
pulls the quadrature output down to ground", then dropping in an
optical encoder could be fairly easy. Rotary optoencoders
generally need a +5v feed for their LED (many have build-in current
limiting resistors for the LED) and have phototransistor
pull-downs... so, substituting one of these might be as simple as
wiring it in with the common/ground and quadrature-outputs as
before, and adding a single lead for +5 for the LED drive.
- If the existing encoder was wired up any differently, you'd need an
interface board (transistors, relays, or some such) to take the
open-collector output of a rotary optoencoder and drive the Wavetek
circuit as it wants to be driven. Probably not hard to build on a
bit of perfboard once you know what you need to do.
In either case, having a schematic for the Wavetek 75 is likely to be
essential. There's a Wavetek test equipment special-interest mailing
list at YahooGroups - somebody there might have a schematic or even
the whole service manual.
As to specific encoder lines: Grayhill 61K or 61R series, Clarostat
600 series, or one of the Bourns encoders (the Bourns 14mm optos
carries in the new Digi-Key catalog are under $20) might all be
candidates.
--
Dave Platt AE6EO
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Hi Dave,
The part# on the service manual is no where to be found, neither does it
contain much information:
http://www.stanford.edu/~wonghoi/DC/4600-05-0211.png
I just probed the connections to the Wavetek 75 board, and it seems like one
wire is grounded while two are hooked up to +5V (My mechanical encoder was
too broken to tell which lead is which). Is there any opto-encoder that
you'll recommend for this case?
Thanks.
Cheers,
Hoi
http://search.digikey.com/scripts/DkSearch/dksus.dll?Cat=1114163;keywords=encoder
shows 549 encoders to chose from.
the mecanical encoders use pullup resistors, and the switch pulls the
+5 VDC lines low as the shaft is turned. The interface may need to be
modified to use the electronic encoders. The electronic encoders I've
used had a direction output, and a pulse output which will not work with
that Wavetek design.
The scan from the manual indicates that the part was customized for
Wavtek. We used a similar design in a product, 20 years ago. I have no
idea how many steps the detent has, per turn. The damaged encoder has
the Panasonic logo, but that body style is no longer made.
http://www.panasonic.com/industrial/components/electromechanical/elec_encd.htm
shows the current body styles, and something might fit, if you can
determine the shaft length, style and number of detents,
Personaly, I would see if any of the CTS encoders would fit. They
are a lot higher quality, and were used in some of our military grade
designs. The only bad ones I saw were damaged in shipping, when some
idiot returned equipment in a wood crate, and no packing material. The
equipment had over $2000 physical damage after being shipped half way
around the world so it was no wonder the shaft was pushed back through
the encoder's body.
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