Ned,
I went ahead and supplied 5 volts to the encoder and followed your
instructions for scope use. I could watch the transitions on the
screen. Thanks. If the carriage is moved back and forth .0002 the
transition will show. Still, the mysterious backlash seems to be
present. Small movements seem OK but not large ones. It's hard to
count transitions on the screen and watch the indicator too. I got
someone to count the transitions on the screen while I watched the
indicator. BTW, the indicator is OK, it doesn't have bachlash. Still,
I can't see if the shaft is moving or not. So the laser is probably
next. However, I think the problem is hysteresis in the rubber belt
driving the encoder from the carriage. We'll see.
Eric

Eric R Snow wrote:
Ned,
I went ahead and supplied 5 volts to the encoder and followed your
instructions for scope use. I could watch the transitions on the
screen. Thanks. If the carriage is moved back and forth .0002 the
transition will show. Still, the mysterious backlash seems to be
present. Small movements seem OK but not large ones. It's hard to
count transitions on the screen and watch the indicator too. I got
someone to count the transitions on the screen while I watched the
indicator. BTW, the indicator is OK, it doesn't have bachlash. Still,
I can't see if the shaft is moving or not. So the laser is probably
next. However, I think the problem is hysteresis in the rubber belt
driving the encoder from the carriage. We'll see.
Eric

Hi Eric,
I have designed the counters and state machines used to decode
quadrature encoders like you are using and there is a good possibility
that the problem you are seeing is in the readout head. By putting the
scope on the encoder outputs and confirming that they transition every
0.0002 inches including the first, you have confirmed that the encoder
is working correctly. The only other test would be to verify that the
transitions are on different 0.0002" boundaries for the two channels.
One should change on 1,3,5... and the other should change on 2,4,6...

Assuming that the encoder is wired correctly to the encoder, if you
don't see the backlash in the scope transitions, it is not in the
encoder/mechanical system. Look through the manual for the readout head
for configuration stuff about filtering the input data stream. Many of
the low cost quadrature encoders are pretty noisy and may generate
multiple transitions. It is common to build some kind of filtering into
the state machine that turns quadrature transitions into counts. If
there is an option to disable filtering on the head, try it.

Another possible source of the problem would be an "index" sensor input.
Since quadrature sensors are relative position sensors, a third sensor
that triggers once per revolution or at one end of a mechanism can give
you a calibrated "home" start point to give an absolute position. If
there is any mention of tying the index input to +5V or ground, make
sure you have done it. There may be exception states in the quadrature
decoding state machine to deal with the index sensor. Or if there is a
configuration option to disable it, make sure it is disabled.

Good Luck,
Bob

"Eric R Snow" wrote in message
...
Ned,
I went ahead and supplied 5 volts to the encoder and followed your
instructions for scope use. I could watch the transitions on the
screen. Thanks. If the carriage is moved back and forth .0002 the
transition will show. Still, the mysterious backlash seems to be
present. Small movements seem OK but not large ones. It's hard to
count transitions on the screen and watch the indicator too. I got
someone to count the transitions on the screen while I watched the
indicator. BTW, the indicator is OK, it doesn't have bachlash. Still,
I can't see if the shaft is moving or not. So the laser is probably
next. However, I think the problem is hysteresis in the rubber belt
driving the encoder from the carriage. We'll see.
Eric

You probably need a metal belt, a thin flat endless ribbon of stainless
steel or some such alloy. Dimensional stability is excellent at constant
temp. See
http://www.machinedesign.com/BDE/mec...emech1_20.html

Randy

"R. O'Brian" wrote in message
news:JBy0f.407$784.353@lakeread08...
next. However, I think the problem is hysteresis in the rubber belt
driving the encoder from the carriage. We'll see.
Eric

You probably need a metal belt, a thin flat endless ribbon of stainless
steel or some such alloy. Dimensional stability is excellent at constant
temp. See
http://www.machinedesign.com/BDE/mec...emech1_20.html

Or phosphor bronze... I can see that working for partial turns, but not
multiple turns as Eric is looking for. Two opposing straps wound not quite
the full circumference, fixed firmly at the ends, will track like a worm and
rack.

It would be instructive to disassemble a dial indicator and look inside.
Mebbe I would just find a way to mount the encoder to the de-faced
indicator, rather than mess with mirrors, lasers, belts, oscilloscopes,
quadrature waveforms, ... unless the idea is to mess, to reinvent, rather
than measure.

On Tue, 04 Oct 2005 19:15:41 GMT, "Mike Young"
wrote:

"R. O'Brian" wrote in message
news:JBy0f.407$784.353@lakeread08...
next. However, I think the problem is hysteresis in the rubber belt
driving the encoder from the carriage. We'll see.
Eric

You probably need a metal belt, a thin flat endless ribbon of stainless
steel or some such alloy. Dimensional stability is excellent at constant
temp. See
http://www.machinedesign.com/BDE/mec...emech1_20.html

Or phosphor bronze... I can see that working for partial turns, but not
multiple turns as Eric is looking for. Two opposing straps wound not quite
the full circumference, fixed firmly at the ends, will track like a worm and
rack.

It would be instructive to disassemble a dial indicator and look inside.
Mebbe I would just find a way to mount the encoder to the de-faced
indicator, rather than mess with mirrors, lasers, belts, oscilloscopes,
quadrature waveforms, ... unless the idea is to mess, to reinvent, rather
than measure.
I know how indicators work. And they involve anti-backlash gears. But
that misses the point. The encoder should have NO mechanical lash. The
disc with the lines is mounted directly to the shaft. A metal ribbon
if the friction was high enough. I ordered some fine cable today from
McMaster-Carr today. It will be wrapped a few turns around the encoder
shaft. I hope that this will provide enough friction while at the same
time have low enough tension to keep the friction low in the pulleys
the cable will travel over.
ERS

Eric R Snow wrote:

On Tue, 04 Oct 2005 19:15:41 GMT, "Mike Young"
wrote:

"R. O'Brian" wrote in message
news:JBy0f.407$784.353@lakeread08...
next. However, I think the problem is hysteresis in the rubber belt
driving the encoder from the carriage. We'll see.
Eric

You probably need a metal belt, a thin flat endless ribbon of stainless
steel or some such alloy. Dimensional stability is excellent at constant
temp. See
http://www.machinedesign.com/BDE/mec...emech1_20.html

Or phosphor bronze... I can see that working for partial turns, but not
multiple turns as Eric is looking for. Two opposing straps wound not quite
the full circumference, fixed firmly at the ends, will track like a worm and
rack.

It would be instructive to disassemble a dial indicator and look inside.
Mebbe I would just find a way to mount the encoder to the de-faced
indicator, rather than mess with mirrors, lasers, belts, oscilloscopes,
quadrature waveforms, ... unless the idea is to mess, to reinvent, rather
than measure.
I know how indicators work. And they involve anti-backlash gears. But
that misses the point. The encoder should have NO mechanical lash. The
disc with the lines is mounted directly to the shaft. A metal ribbon
if the friction was high enough. I ordered some fine cable today from
McMaster-Carr today. It will be wrapped a few turns around the encoder
shaft. I hope that this will provide enough friction while at the same
time have low enough tension to keep the friction low in the pulleys
the cable will travel over.
ERS


The turn-on and turn-off positions do not happen at exactly at the same
position when the shaft is reversed. This is where your "backlash" is
coming from. If you want better accuracy, use a higher count per rev.
encoder.

John

In article ,
says...
I ordered some fine cable today from
McMaster-Carr today. It will be wrapped a few turns around the encoder
shaft. I hope that this will provide enough friction while at the same
time have low enough tension to keep the friction low in the pulleys
the cable will travel over.
ERS

What concerns me about using a wrapped cable is that the turns of cable
want to walk across the drum as a result of the helical wrap. This will
cause the cable to be pulled out of line with it's attachment points,
which changes number of pulses per inch of carriage travel. If the
stroke is long relative to the drum's circumference the cable may either
slip back into line, cross over itself, or walk off the end of the drum.

I've done this, and even with a fairly short stroke, large drum, and low
accuracy requirement, it was a real pain to get working reliably. I
don't mean to be discouraging, but I haven't heard, or thought of, any
idea yet that gives me the warm fuzzies.

Ned Simmons

Ned Simmons writes:

In article ,
says...
I ordered some fine cable today from
McMaster-Carr today. It will be wrapped a few turns around the encoder
shaft. I hope that this will provide enough friction while at the same
time have low enough tension to keep the friction low in the pulleys
the cable will travel over.
ERS

What concerns me about using a wrapped cable is that the turns of cable
want to walk across the drum as a result of the helical wrap. This will
cause the cable to be pulled out of line with it's attachment points,
which changes number of pulses per inch of carriage travel. If the
stroke is long relative to the drum's circumference the cable may either
slip back into line, cross over itself, or walk off the end of the drum.

I've done this, and even with a fairly short stroke, large drum, and low
accuracy requirement, it was a real pain to get working reliably. I
don't mean to be discouraging, but I haven't heard, or thought of, any
idea yet that gives me the warm fuzzies.

Ned Simmons

Sorry didn't catch the previous post. One way to avoid the cable
walking is to set up a parallel shaft nearby and turn a couple
"windlasses"- bigger collars on each shaft so the cable can bear more on
each. Wouldn't hurt to put notches on the collars to help guide the
cable. So the coil goes around the first, then around the second, then
back around the first. Not wrapping all the way around each shaft, but
looping around the pair of shafts. The aux shaft should be free to
spin. Then as the cable drives the shaft (or vice versa), the cable
won't walk or slip. Maybe you only need a few loops around the shafts &
can keep things fairly compact.

Gregm

for what it's worth, I have a very nice Trav-a-dial that has a built in
encoder - look at what a trav-a-dial is on e-bay or on the web - this one
works perfectly (the mechanical part anyway) - I just overhauled it to get a
piece of swarf out of it, so now it's all clean inside and is as smooth as
new. It's the series 6 unit (one generation old) not the older silver ones.
this might be a much better solution to your problem than other mechanical
assemblies though I haven't yet gotten any information from the company as
to what the encoder specs are.

anyway, I don't really need one of these with an encoder inside, so
trade/sale/??? is possible


"john" wrote in message
...


Eric R Snow wrote:

On Tue, 04 Oct 2005 19:15:41 GMT, "Mike Young"
wrote:

"R. O'Brian" wrote in message
news:JBy0f.407$784.353@lakeread08...
next. However, I think the problem is hysteresis in the rubber belt
driving the encoder from the carriage. We'll see.
Eric

You probably need a metal belt, a thin flat endless ribbon of
stainless
steel or some such alloy. Dimensional stability is excellent at
constant
temp. See
http://www.machinedesign.com/BDE/mec...emech1_20.html

Or phosphor bronze... I can see that working for partial turns, but not
multiple turns as Eric is looking for. Two opposing straps wound not
quite
the full circumference, fixed firmly at the ends, will track like a worm
and
rack.

It would be instructive to disassemble a dial indicator and look inside.
Mebbe I would just find a way to mount the encoder to the de-faced
indicator, rather than mess with mirrors, lasers, belts, oscilloscopes,
quadrature waveforms, ... unless the idea is to mess, to reinvent,
rather
than measure.
I know how indicators work. And they involve anti-backlash gears. But
that misses the point. The encoder should have NO mechanical lash. The
disc with the lines is mounted directly to the shaft. A metal ribbon
if the friction was high enough. I ordered some fine cable today from
McMaster-Carr today. It will be wrapped a few turns around the encoder
shaft. I hope that this will provide enough friction while at the same
time have low enough tension to keep the friction low in the pulleys
the cable will travel over.
ERS


The turn-on and turn-off positions do not happen at exactly at the same
position when the shaft is reversed. This is where your "backlash" is
coming from. If you want better accuracy, use a higher count per rev.
encoder.

John

On Tue, 04 Oct 2005 22:55:13 -0400, john
wrote:



Eric R Snow wrote:

On Tue, 04 Oct 2005 19:15:41 GMT, "Mike Young"
wrote:

"R. O'Brian" wrote in message
news:JBy0f.407$784.353@lakeread08...
next. However, I think the problem is hysteresis in the rubber belt
driving the encoder from the carriage. We'll see.
Eric

You probably need a metal belt, a thin flat endless ribbon of stainless
steel or some such alloy. Dimensional stability is excellent at constant
temp. See
http://www.machinedesign.com/BDE/mec...emech1_20.html

Or phosphor bronze... I can see that working for partial turns, but not
multiple turns as Eric is looking for. Two opposing straps wound not quite
the full circumference, fixed firmly at the ends, will track like a worm and
rack.

It would be instructive to disassemble a dial indicator and look inside.
Mebbe I would just find a way to mount the encoder to the de-faced
indicator, rather than mess with mirrors, lasers, belts, oscilloscopes,
quadrature waveforms, ... unless the idea is to mess, to reinvent, rather
than measure.
I know how indicators work. And they involve anti-backlash gears. But
that misses the point. The encoder should have NO mechanical lash. The
disc with the lines is mounted directly to the shaft. A metal ribbon
if the friction was high enough. I ordered some fine cable today from
McMaster-Carr today. It will be wrapped a few turns around the encoder
shaft. I hope that this will provide enough friction while at the same
time have low enough tension to keep the friction low in the pulleys
the cable will travel over.
ERS


The turn-on and turn-off positions do not happen at exactly at the same
position when the shaft is reversed. This is where your "backlash" is
coming from. If you want better accuracy, use a higher count per rev.
encoder.

John
John,
This had better not be the case. At the most there should be only 1
count backlash, not 9.
Eric

On Tue, 4 Oct 2005 22:43:56 -0700, "william_b_noble"
wrote:

for what it's worth, I have a very nice Trav-a-dial that has a built in
encoder - look at what a trav-a-dial is on e-bay or on the web - this one
works perfectly (the mechanical part anyway) - I just overhauled it to get a
piece of swarf out of it, so now it's all clean inside and is as smooth as
new. It's the series 6 unit (one generation old) not the older silver ones.
this might be a much better solution to your problem than other mechanical
assemblies though I haven't yet gotten any information from the company as
to what the encoder specs are.

anyway, I don't really need one of these with an encoder inside, so
trade/sale/??? is possible
OK. Tell me more. Lets use e-mail.
Thanks,
Eric


"john" wrote in message
...


Eric R Snow wrote:

On Tue, 04 Oct 2005 19:15:41 GMT, "Mike Young"
wrote:

"R. O'Brian" wrote in message
news:JBy0f.407$784.353@lakeread08...
next. However, I think the problem is hysteresis in the rubber belt
driving the encoder from the carriage. We'll see.
Eric

You probably need a metal belt, a thin flat endless ribbon of
stainless
steel or some such alloy. Dimensional stability is excellent at
constant
temp. See
http://www.machinedesign.com/BDE/mec...emech1_20.html

Or phosphor bronze... I can see that working for partial turns, but not
multiple turns as Eric is looking for. Two opposing straps wound not
quite
the full circumference, fixed firmly at the ends, will track like a worm
and
rack.

It would be instructive to disassemble a dial indicator and look inside.
Mebbe I would just find a way to mount the encoder to the de-faced
indicator, rather than mess with mirrors, lasers, belts, oscilloscopes,
quadrature waveforms, ... unless the idea is to mess, to reinvent,
rather
than measure.
I know how indicators work. And they involve anti-backlash gears. But
that misses the point. The encoder should have NO mechanical lash. The
disc with the lines is mounted directly to the shaft. A metal ribbon
if the friction was high enough. I ordered some fine cable today from
McMaster-Carr today. It will be wrapped a few turns around the encoder
shaft. I hope that this will provide enough friction while at the same
time have low enough tension to keep the friction low in the pulleys
the cable will travel over.
ERS


The turn-on and turn-off positions do not happen at exactly at the same
position when the shaft is reversed. This is where your "backlash" is
coming from. If you want better accuracy, use a higher count per rev.
encoder.

John

On Tue, 4 Oct 2005 23:31:42 -0400, Ned Simmons
wrote:

In article ,
says...
I ordered some fine cable today from
McMaster-Carr today. It will be wrapped a few turns around the encoder
shaft. I hope that this will provide enough friction while at the same
time have low enough tension to keep the friction low in the pulleys
the cable will travel over.
ERS

What concerns me about using a wrapped cable is that the turns of cable
want to walk across the drum as a result of the helical wrap. This will
cause the cable to be pulled out of line with it's attachment points,
which changes number of pulses per inch of carriage travel. If the
stroke is long relative to the drum's circumference the cable may either
slip back into line, cross over itself, or walk off the end of the drum.

I've done this, and even with a fairly short stroke, large drum, and low
accuracy requirement, it was a real pain to get working reliably. I
don't mean to be discouraging, but I haven't heard, or thought of, any
idea yet that gives me the warm fuzzies.

Ned Simmons
Ned,
I've considered this. And I'm not quite sure how to keep the cable
where I want it. But I think I have figured it out. I'll let everybody
know when the cable arrives.
Eric

On 05 Oct 2005 00:19:27 -0400, Greg Menke
wrote:

Ned Simmons writes:

In article ,
says...
I ordered some fine cable today from
McMaster-Carr today. It will be wrapped a few turns around the encoder
shaft. I hope that this will provide enough friction while at the same
time have low enough tension to keep the friction low in the pulleys
the cable will travel over.
ERS

What concerns me about using a wrapped cable is that the turns of cable
want to walk across the drum as a result of the helical wrap. This will
cause the cable to be pulled out of line with it's attachment points,
which changes number of pulses per inch of carriage travel. If the
stroke is long relative to the drum's circumference the cable may either
slip back into line, cross over itself, or walk off the end of the drum.

I've done this, and even with a fairly short stroke, large drum, and low
accuracy requirement, it was a real pain to get working reliably. I
don't mean to be discouraging, but I haven't heard, or thought of, any
idea yet that gives me the warm fuzzies.

Ned Simmons

Sorry didn't catch the previous post. One way to avoid the cable
walking is to set up a parallel shaft nearby and turn a couple
"windlasses"- bigger collars on each shaft so the cable can bear more on
each. Wouldn't hurt to put notches on the collars to help guide the
cable. So the coil goes around the first, then around the second, then
back around the first. Not wrapping all the way around each shaft, but
looping around the pair of shafts. The aux shaft should be free to
spin. Then as the cable drives the shaft (or vice versa), the cable
won't walk or slip. Maybe you only need a few loops around the shafts &
can keep things fairly compact.

Gregm
Greg,
I can't quite see waht you are describing. Maybe I'm too tired. The
encoder shaft cannot get any bigger. This shaft is 1.000 in
circumference. This is so the encoder outputs 10,000 pulses per
revolution. Could you explain your idea more clearly so I can
understand it? I'm sure everyone else here has already visualized the
setup but my brain is fogged this morning.
ERS

"Eric R Snow" wrote in message
...
John,
This had better not be the case. At the most there should be only 1
count backlash, not 9.

With a tightly-lashed quadrature encoder, it should be, at most, one count,
and usually on the order of 1/2 count. As soon as the "reversing" leading
edge is sensed, the quad should pick up both count and direction.

LLoyd

Eric R Snow writes:

On 05 Oct 2005 00:19:27 -0400, Greg Menke

Sorry didn't catch the previous post. One way to avoid the cable
walking is to set up a parallel shaft nearby and turn a couple
"windlasses"- bigger collars on each shaft so the cable can bear more on
each. Wouldn't hurt to put notches on the collars to help guide the
cable. So the coil goes around the first, then around the second, then
back around the first. Not wrapping all the way around each shaft, but
looping around the pair of shafts. The aux shaft should be free to
spin. Then as the cable drives the shaft (or vice versa), the cable
won't walk or slip. Maybe you only need a few loops around the shafts &
can keep things fairly compact.

Gregm
Greg,
I can't quite see waht you are describing. Maybe I'm too tired. The
encoder shaft cannot get any bigger. This shaft is 1.000 in
circumference. This is so the encoder outputs 10,000 pulses per
revolution. Could you explain your idea more clearly so I can
understand it? I'm sure everyone else here has already visualized the
setup but my brain is fogged this morning.
ERS

Take your encoder shaft, put a 2nd parallel one "nearby", maybe 1
diameter away. On each shaft put in a series of notches, not deep, just
enough to help guide the cable- no more than 1/2 the diameter of the
cable. Align the shafts so the notches on one shaft are in line with
those on the other. The cable comes in, goes around the outside of one
shaft (in the notch), over and around the other shaft (again riding in
the notch), then back to the 2nd notch on the first shaft and so on
until it has traversed each notch on both shafts- looping around them
all the way. With tension on the cable, it will get nice and tight, but
it won't walk since its not taking a helical path along the shafts. You
should pack in the turns as closely as you can to reduce how far the
cable has to shift down the shaft to make the next notch.

I suggested the bigger diameter collars because this approach loses 1/2
of the engagement on each shaft- since its only bearing on the outside
surfaces of the shaft pair. Maybe you could cross the cables over as
they lead between the shafts & get more engagement that way.

Frictional loss is greater with the 2nd shaft, though maybe you could
make the 2nd shaft bigger to reduce the loss a bit.

Greg