Linear position transducer?

[mikem]

I need to measure the position of an object which moves slowly (< 1Hz)
along one axis. Max travel is about 6 inches. I want to read its
position to a resolution about 1 part out of 1024 (10 bits), but the
absolute accuracy only needs to be 1%. I can mechanically attach
something like a pushrod to the object.

I'm familiar with LVDTs and the old NE5521 chip, but dont want to
spend a $1000 for a 6" LVDT.

Anybody got any ideas?

MikeM

 

[Frank Bemelman]

"mikem" <mladejov@CharleyEchoDelta.utah.edu> schreef in bericht
news:kt2oqv0aqjkhej5iqu0g90du5d22bo4t5k@4ax.com...

I need to measure the position of an object which moves slowly (< 1Hz)
along one axis. Max travel is about 6 inches. I want to read its
position to a resolution about 1 part out of 1024 (10 bits), but the
absolute accuracy only needs to be 1%. I can mechanically attach
something like a pushrod to the object.

I'm familiar with LVDTs and the old NE5521 chip, but dont want to
spend a $1000 for a 6" LVDT.

Anybody got any ideas?

Vishay (and no doubt others) has linear displacement sensors, basically
just a potentiometer. $100 perhaps.

I wonder what laser displacement sensors cost these days (like the ones
from Keyence).

--
Thanks,
Frank Bemelman
(remove 'x' & .invalid when sending email)

 

[gagir]

Look at http://www.spaceagecontrol.com/reqrefer.htm
and you get a free position transducer... used to be any of their products
but now you get a sst ruler.

Eric Girard



"Frank Bemelman" <bemelmanx@euronet.nl.invalid> wrote in message
news:3fac11a0$0$56509$1b62eedf@news.wanadoo.nl...

"mikem" <mladejov@CharleyEchoDelta.utah.edu> schreef in bericht
news:kt2oqv0aqjkhej5iqu0g90du5d22bo4t5k@4ax.com...
I need to measure the position of an object which moves slowly (< 1Hz)
along one axis. Max travel is about 6 inches. I want to read its
position to a resolution about 1 part out of 1024 (10 bits), but the
absolute accuracy only needs to be 1%. I can mechanically attach
something like a pushrod to the object.

I'm familiar with LVDTs and the old NE5521 chip, but dont want to
spend a $1000 for a 6" LVDT.

Anybody got any ideas?

Vishay (and no doubt others) has linear displacement sensors, basically
just a potentiometer. $100 perhaps.

I wonder what laser displacement sensors cost these days (like the ones
from Keyence).

--
Thanks,
Frank Bemelman
(remove 'x' & .invalid when sending email)

 

[Spehro Pefhany]

On Fri, 07 Nov 2003 14:25:03 -0700, the renowned mikem
<mladejov@CharleyEchoDelta.utah.edu> wrote:

I need to measure the position of an object which moves slowly (< 1Hz)
along one axis. Max travel is about 6 inches. I want to read its
position to a resolution about 1 part out of 1024 (10 bits), but the
absolute accuracy only needs to be 1%. I can mechanically attach
something like a pushrod to the object.

I'm familiar with LVDTs and the old NE5521 chip, but dont want to
spend a $1000 for a 6" LVDT.

How much life do you need? A linear conductive plastic pot might do
the trick.

Best regards,
Spehro Pefhany
--
"it's the network..." "The Journey is the reward"
speff@interlog.com Info for manufacturers: http://www.trexon.com
Embedded software/hardware/analog Info for designers: http://www.speff.com

 

[Michael Gray]

On Fri, 07 Nov 2003 14:25:03 -0700, mikem
<mladejov@CharleyEchoDelta.utah.edu> wrote:

I need to measure the position of an object which moves slowly (< 1Hz)
along one axis. Max travel is about 6 inches. I want to read its
position to a resolution about 1 part out of 1024 (10 bits), but the
absolute accuracy only needs to be 1%. I can mechanically attach
something like a pushrod to the object.

I'm familiar with LVDTs and the old NE5521 chip, but dont want to
spend a $1000 for a 6" LVDT.

Anybody got any ideas?

MikeM

Just a thought:

How about using a scrapped HP inkjet printer?
They have an optical position sensor on the print carriage, that reads
a plastic tape with tiny binary markings photographically etched on
it.
They are very accurate, more than long enough, and don't wear out.
I'm not sure what code the sensor outputs the position in, but a
little experimentation would tell you that.

 

[Rob]

"mikem" <mladejov@CharleyEchoDelta.utah.edu> wrote in message
news:kt2oqv0aqjkhej5iqu0g90du5d22bo4t5k@4ax.com...

I need to measure the position of an object which moves slowly (< 1Hz)
along one axis. Max travel is about 6 inches. I want to read its
position to a resolution about 1 part out of 1024 (10 bits), but the
absolute accuracy only needs to be 1%. I can mechanically attach
something like a pushrod to the object.

I'm familiar with LVDTs and the old NE5521 chip, but dont want to
spend a $1000 for a 6" LVDT.

Anybody got any ideas?

MikeM

Servo pot with a machined pulley & a return "clock" sping.
Use light gauge nylon coated fine stranded stainless steel fishing trace
wire to go from the pulley to the moving structure.

Have fun
rob

 

[Frank Bemelman]

"Michael Gray" <fleetg@newsguy.spam.com> schreef in bericht
news:9cjoqv84jfn0p0ku0f85tolvhk5igdhbpa@4ax.com...

On Fri, 07 Nov 2003 14:25:03 -0700, mikem
mladejov@CharleyEchoDelta.utah.edu> wrote:

I need to measure the position of an object which moves slowly (< 1Hz)
along one axis. Max travel is about 6 inches. I want to read its
position to a resolution about 1 part out of 1024 (10 bits), but the
absolute accuracy only needs to be 1%. I can mechanically attach
something like a pushrod to the object.

I'm familiar with LVDTs and the old NE5521 chip, but dont want to
spend a $1000 for a 6" LVDT.

Anybody got any ideas?

MikeM

Just a thought:

How about using a scrapped HP inkjet printer?
They have an optical position sensor on the print carriage, that reads
a plastic tape with tiny binary markings photographically etched on
it.
They are very accurate, more than long enough, and don't wear out.
I'm not sure what code the sensor outputs the position in, but a
little experimentation would tell you that.

It's Gray code! ;-)

--
Thanks,
Frank Bemelman
(remove 'x' & .invalid when sending email)

 

[Stef Mientki]

optical mouse ?
Stef Mientki

mikem wrote:

I need to measure the position of an object which moves slowly (< 1Hz)
along one axis. Max travel is about 6 inches. I want to read its
position to a resolution about 1 part out of 1024 (10 bits), but the
absolute accuracy only needs to be 1%. I can mechanically attach
something like a pushrod to the object.

I'm familiar with LVDTs and the old NE5521 chip, but dont want to
spend a $1000 for a 6" LVDT.

Anybody got any ideas?

MikeM

 

[Roger Hamlett]

"Frank Bemelman" <bemelmanx@euronet.nl.invalid> wrote in message
news:3fac11a0$0$56509$1b62eedf@news.wanadoo.nl...

"mikem" <mladejov@CharleyEchoDelta.utah.edu> schreef in bericht
news:kt2oqv0aqjkhej5iqu0g90du5d22bo4t5k@4ax.com...
I need to measure the position of an object which moves slowly (< 1Hz)
along one axis. Max travel is about 6 inches. I want to read its
position to a resolution about 1 part out of 1024 (10 bits), but the
absolute accuracy only needs to be 1%. I can mechanically attach
something like a pushrod to the object.

I'm familiar with LVDTs and the old NE5521 chip, but dont want to
spend a $1000 for a 6" LVDT.

Anybody got any ideas?

Vishay (and no doubt others) has linear displacement sensors, basically
just a potentiometer. $100 perhaps.

I wonder what laser displacement sensors cost these days (like the ones
from Keyence).
A couple of other ideas.

If the motion is very slow/infrequent, consider a digital caliper. The
plastic versions of these are available in the UK, for under Ł20, and give
resolutions in the order of 1thou. Most designs don't draw power from (or
need) their internal batteries if connected to an external data cable. These
have four connections for power, and serial data. The sensor on these is a
strip of PCB, cut to form a 'tooth' pattern, which couples to a second
similar strip inside the sensor head, with the tooth pattern at a slightly
different spacing. The strips don't have to touch, so in fact the head can
be positioned over a different sensor strip, and arranged so there is no
contact at all. For the cost, they are unbeatable.
Another alternative, is to use a mouse. Provided you have an ability to
'resync' at intervals (a limit switch perhaps), you can either use a normal
mouse ball, or connect a drive directly to the optical sensor spindle. Again
the price (even with some added hardware), is the real 'feature'.
Third alternative, is to use a optical encoder strip. Honeywell make the
quadrature encoders used inside some mice, as a complete assembly. This has
the pair of decoders, and the quadrature decoder itself, to give a pair of
'step/direction' signals, from a slotted strip at the right interval passing
through the decoder. Look at 'hlc2705'. These are under Ł2, in 'one off'
quantities. The 'strip' to operate these sensors, can be made on plastic
film designed for printing slides for overhead projectors on laser printers.
I have printed such strips in the past, using a basic CAD package to make a
sheet of the lines, and printing an entire A4 sheet, then cutting strips
about 1/4" wide off this. I usually end up throwing away a couple of strips
with visible 'faults' (dust etc.). The entire system, then becomes the
optical detector, a suitable IR LED, with a gap between, and the strip
mounted onto the moving components (I usually 'hard mount' the strip at one
end, and pull the other with a rubber band or spring). Total cost is well
under Ł10. Resolution is in the order of 0.5mm with the system as described
(less than you need). However you can 'cheat', and improve the resolution on
this (provided the system that prints the sensor strip is accurate enough).
What you do, is instead of printing a 'linear' strip, you print the lines
angled. So, instead of a strip like:
||||||||||

you generate a strip with the lines like:
//////////

Then if the sensor sits parallel to the lines, you get an increased
positional signal change as the strip moves. At 30 degrees, you can get
resolutions of about 0.25mm. This approaches the sort of accuracy you need,
and for the price, might well be worth considering.

Best Wishes

 

[Michael Gray]

On Sat, 8 Nov 2003 10:46:12 +0100, "Frank Bemelman"
<bemelmanx@euronet.nl.invalid> wrote:

"Michael Gray" <fleetg@newsguy.spam.com> schreef in bericht
news:9cjoqv84jfn0p0ku0f85tolvhk5igdhbpa@4ax.com...
On Fri, 07 Nov 2003 14:25:03 -0700, mikem
mladejov@CharleyEchoDelta.utah.edu> wrote:

I need to measure the position of an object which moves slowly (< 1Hz)
along one axis. Max travel is about 6 inches. I want to read its
position to a resolution about 1 part out of 1024 (10 bits), but the
absolute accuracy only needs to be 1%. I can mechanically attach
something like a pushrod to the object.

I'm familiar with LVDTs and the old NE5521 chip, but dont want to
spend a $1000 for a 6" LVDT.

Anybody got any ideas?

MikeM

Just a thought:

How about using a scrapped HP inkjet printer?
They have an optical position sensor on the print carriage, that reads
a plastic tape with tiny binary markings photographically etched on
it.
They are very accurate, more than long enough, and don't wear out.
I'm not sure what code the sensor outputs the position in, but a
little experimentation would tell you that.

It's Gray code! ;-)

How Ironic...

 

[Fred Abse]

On Sat, 08 Nov 2003 10:46:12 +0100, Frank Bemelman wrote:

It's Gray code! ;-)

I've just looked at one on my HP printer, and it isn't Gray code, just
full width stripes. I assume there must be two photosomethings displaced
by 1/4 line pitch to read it, giving sin and cos, hence direction.

Plain ol' dumb incremental.

--
Then there's duct tape ...
(Garrison Keillor)
nofr@sbhevre.pbzchyvax.pb.hx

 

[Michael Gray]

On Sun, 09 Nov 2003 00:03:59 +0000, Fred Abse
<excretatauris@cerebrumconfus.it> wrote:

On Sat, 08 Nov 2003 10:46:12 +0100, Frank Bemelman wrote:

It's Gray code! ;-)

I've just looked at one on my HP printer, and it isn't Gray code, just
full width stripes. I assume there must be two photosomethings displaced
by 1/4 line pitch to read it, giving sin and cos, hence direction.

Plain ol' dumb incremental.

You are correct.

Showing my age there.
The older HP printers, such as the 500 series I distinctly remember as
having position encoded into the strip.
On inspection of my 990ci, with a magnifying lens, I see that it is as
you say.
Even so, the idea is sound, even if it requires a little more external
'intelligence'.

 

[Fred Abse]

On Sun, 09 Nov 2003 21:37:06 +1030, Michael Gray wrote:

On Sun, 09 Nov 2003 00:03:59 +0000, Fred Abse
excretatauris@cerebrumconfus.it> wrote:

On Sat, 08 Nov 2003 10:46:12 +0100, Frank Bemelman wrote:

It's Gray code! ;-)

I've just looked at one on my HP printer, and it isn't Gray code, just
full width stripes. I assume there must be two photosomethings displaced
by 1/4 line pitch to read it, giving sin and cos, hence direction.

Plain ol' dumb incremental.

You are correct.

Showing my age there.
The older HP printers, such as the 500 series I distinctly remember as
having position encoded into the strip.

The one I looked at *was* a 500 series ;-(

On inspection of my 990ci, with a

magnifying lens, I see that it is as you say.
Even so, the idea is sound, even if it requires a little more external
'intelligence'.

I suspect that "intelligence" comes cheaper than photo technique these
days.

--
Then there's duct tape ...
(Garrison Keillor)
nofr@sbhevre.pbzchyvax.pb.hx

 

[Graham W]

Michael Gray wrote:

On Sun, 09 Nov 2003 00:03:59 +0000, Fred Abse
excretatauris@cerebrumconfus.it> wrote:

On Sat, 08 Nov 2003 10:46:12 +0100, Frank Bemelman wrote:

It's Gray code! ;-)

I've just looked at one on my HP printer, and it isn't Gray code,
just full width stripes.
[...]

You are correct.

Showing my age there.
The older HP printers, such as the 500 series I distinctly remember as
having position encoded into the strip.

Specifically, the Deskjet 500 didn't have a strip! It had a rotary
'encoder'
on the bottom of the carriage motor at the right hand end.

But the 510 *does* have strip. I have just examined both models to
establish this info. 8Ź)

On inspection of my 990ci, with a magnifying lens, I see that it is as
you say.

My 880C also.


--
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WIMBORNE http://www.wessex-astro-society.freeserve.co.uk/ Wessex
Dorset UK Astro Society's Web pages, Info, Meeting Dates, Sites & Maps
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