Low cost weight sensor ?

Anton Erasmus  wrote:
>... Have anyone used piezo electric material to
>measure weight ?

Yes, there are piezoelectric load cells. They may be difficult to use
because they require very high impedance analog circuitry to read the
output. ("Charge Amplifiers") See: 

http://www.kistler.com/do.productfinder.us.en-us?content=833_ProductFinder_Force&param=3XXX.9021A

>Would a piezo speaker be suitable ? 

May be... How stable will that be?

Roberto Waltman wrote:
>Anton Erasmus  wrote:
>>... Have anyone used piezo electric material to
>>measure weight ?
>
>Yes, there are piezoelectric load cells. They may be difficult to use
>because they require very high impedance analog circuitry to read the
>output. ("Charge Amplifiers") See: 
>
>http://www.kistler.com/do.productfinder.us.en-us?content=833_ProductFinder_Force&param=3XXX.9021A
>
>>Would a piezo speaker be suitable ? 
>
>May be... How stable will that be?

Anton, I just read again your original post: "I am trying to provide a
indication of the amount of paper available. The empty container is
approx. 1kg, and approx. 10kg full of paper."

(Stepping out from the box and trying to think.) You choose weight as
the parameter to be measured. What other options are there?
It would be very easy to measure the height of the stack of paper. 
(Measure the angle of a hinged rod lying on top of the stack,
ultrasound sensors, etc.)
Light transmitted through the stack will probably not work with 10kg
of paper.
Anybody has any suggestion other than weight?

On Thu, 25 May 2006 10:06:06 GMT, Peter Dickerson <first{dot}surname@tesco.net> wrote:
> "Anton Erasmus" <nobody@spam.prevent.net> wrote in message  news:42g97292vmtg52rbbu4negmk2nev5jaaui@4ax.com...
> [snip]
>> Thanks for all the ideas. It looks like all the weight measuring
>> sensors uses some sort of spring form material which deflects under
>> the weight. One then measures the amount of deflection, from which the
>> weight cab be calculated. The less deflection one can accommodate, the
>> more expensive the sensor. Have anyone used piezo electric material to
>> measure weight ? Would a piezo speaker be suitable ?
> 
> How about placing a piezo disk under the paper stack with the piezo wired as
> an electro-mechanical oscillator. The frequency of oscillation will depend
> on the load mass. Under excessive load the oscillator may not start though.

Anton,

As Peter points out, a piezo sensor is a "change" sensor:  it
produces an output when its state changes.  If you want to use this
aproach to measure something static (e.g.  paper stack size) you
have to find or create a changing characteristic (e.g.  movement in
response to vibration).

If this were a mechanically-created stack one could use a simple
photointerruptor to count the sheets coming in and those going out.
You'd still want some way to "zero out" the up/down counter for
those occasions when the count stopped reflecting reality, though.
<grin>

If you're _not_ going to count sheets, that leaves thickness and
weight (and three other characteristics I haven't thought of or
forgot <grin!>).  The first two vary across types of paper (20lb?
24lb?  cheap copier I-don-t-know-how-heavy?), which makes an exact
measurement tricky.

Thickness:  Someone previously mentioned capacitance varying
according to the stack height.  Several photocell approaches (or
even a simple switch) can detect "paper present" vs.  "no paper
present".

If you can handle the mechanics (and the situation allows), you
could add a "paddle" that lays down on top of the stack and use that
to mechanically translate height to light intensity, resistance, or
encoder ticks.  (Paddle needs to be out of the way when adding more
paper, of course.)

Thickness also translates to light intensity, but only up to a
limit, and it'll vary _strongly_ with paper color.  I'd expect
resistance and moisture content to vary even more widely. <gack!>

Weight: Not sure I can add much here to previous postings. Convert 
it to air or fluid pressure and measure that? (but remember that 
waterbedds leak from time to time. <grin>)

Again, good luck.



Frank McKenney, McKenney Associates
Richmond, Virginia / (804) 320-4887
Munged E-mail: frank uscore mckenney ayut minds pring dawt cahm (y'all)
--
   "Very few things happen at the right time, and the rest
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    correct these defects."  -- Herodotus
--

On Mon, 22 May 2006 21:26:37 +0200, in comp.arch.embedded Anton
Erasmus <nobody@spam.prevent.net> wrote:

>Hi,
>
>I am looking for a low cost weight sensor. I am trying to provide a
>indication of the amount of paper available. The empty container is
>approx. 1kg, and approx. 10kg full of paper. It should be able to
>handle industrial temperature range. A resolution of 1kg would be
>acceptable. If I can get 100g resolution, it would be great.  
>I would prefer SPI or I2C interface, but analog out would be
>acceptable - something that can directly interface with a MCU.
>
>Regards
>  Anton Erasmus
Get a normal  mechanical weighing machine, with the big dial, and
pointer and put a 10K put on the shaft of the needle. ADC, Done!


martin

On Thu, 25 May 2006 09:19:14 -0400, Roberto Waltman
<usenet@rwaltman.net> wrote:

>Roberto Waltman wrote:
>>Anton Erasmus  wrote:
>>>... Have anyone used piezo electric material to
>>>measure weight ?
>>
>>Yes, there are piezoelectric load cells. They may be difficult to use
>>because they require very high impedance analog circuitry to read the
>>output. ("Charge Amplifiers") See: 
>>
>>http://www.kistler.com/do.productfinder.us.en-us?content=833_ProductFinder_Force&param=3XXX.9021A
>>
>>>Would a piezo speaker be suitable ? 
>>
>>May be... How stable will that be?
>
>Anton, I just read again your original post: "I am trying to provide a
>indication of the amount of paper available. The empty container is
>approx. 1kg, and approx. 10kg full of paper."
>
>(Stepping out from the box and trying to think.) You choose weight as
>the parameter to be measured. What other options are there?
>It would be very easy to measure the height of the stack of paper. 
>(Measure the angle of a hinged rod lying on top of the stack,
>ultrasound sensors, etc.)
>Light transmitted through the stack will probably not work with 10kg
>of paper.
>Anybody has any suggestion other than weight?

We have used a switch that indicates when the paper is less than a
certain amount. Both mechanical and optical. The system is a ticket
dispensing machine - typically used in parking lots. The people
employed to operate it, is minimum wage employees (Which is about
US$0.70 per hour in South Africa). The systems are not well
maintained, the environment is extremely dirty and dusty, and the
enclosures in which the machines and paper are housed are never
cleaned. I am trying to think of a way to make a cheap robust sensor
which would work reliably for the 10 year life expected of the
equipment. Being able to measure more than just a specific quantity of
paper left would be a bonus.
Counting as suggested by someone else would not work since the paper
is periodically added, without any way to know exactely how much had
been added. The paper is also fairly low quality that generates lots
of paper dust, that loves to cling to things like photo diodes and
transistors.

Keep the ideas coming. Even if I cannot use it, I am sure someone
someday would get something they could use out of the archives.


Regards
  Anton Erasmus

On Thu, 25 May 2006 22:47:31 +0200, Anton Erasmus <nobody@spam.prevent.net> wrote:
--snip--
> We have used a switch that indicates when the paper is less than a
> certain amount. Both mechanical and optical. The system is a ticket
> dispensing machine - typically used in parking lots. The people
> employed to operate it, is minimum wage employees (Which is about
> US$0.70 per hour in South Africa). The systems are not well
> maintained, the environment is extremely dirty and dusty, and the
> enclosures in which the machines and paper are housed are never
> cleaned. I am trying to think of a way to make a cheap robust sensor
> which would work reliably for the 10 year life expected of the
> equipment. Being able to measure more than just a specific quantity of
> paper left would be a bonus.
> Counting as suggested by someone else would not work since the paper
> is periodically added, without any way to know exactely how much had
> been added. The paper is also fairly low quality that generates lots
> of paper dust, that loves to cling to things like photo diodes and
> transistors.


Anton,

Thanks for the detail; it helps a lot. A couple of questions more:

 1) Are these tickets something whose construction/printing are 
    under your control, and if so, to what extent?

    Barcodes, at least the traditional type, wouldn't help give you 
    quantity, but there may be some options based on the tickets
    themselves.

 2) How precise does the measurement have to be, and what will be 
    done with the information? That is, if all you care about is 
    being able to turn on a LED indicating that it's time to refill 
    the dispenser, what is it that makes you want more detailed
    information on the remaining ticket count?

 3) How is the level monitored now? (This overlaps a bit with (2)).

 4) The tickets _are_ stacked, and not dispensed from a reel or
    spool, yes? (Sorry, but I had to ask <grin>).

If, for example, the attendant periodically walks around the lot and
looks at each dispenser, you could use a simple mechanical "flag"
attached to a lever spring-pressed against the top of the stack to
give a "gauge" of how full the dispenser was.  The flag would
bevisible through a transparent window and would show "50% full",
"25% full", "near empty", and "empty" or even more detail if
desired.

Any better?


Frank McKenney, McKenney Associates
Richmond, Virginia / (804) 320-4887
Munged E-mail: frank uscore mckenney ayut minds pring dawt cahm (y'all)
--
   "We don't just borrow words; on occasion, English has pursued other
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    pockets for new vocabulary."   -- James D. Nicoll 
--

"Anton Erasmus" <nobody@spam.prevent.net> wrote in message
news:42g97292vmtg52rbbu4negmk2nev5jaaui@4ax.com...
> Thanks for all the ideas. It looks like all the weight measuring
> sensors uses some sort of spring form material which deflects under
> the weight. One then measures the amount of deflection, from which the
> weight cab be calculated. The less deflection one can accommodate, the
> more expensive the sensor. Have anyone used piezo electric material to
> measure weight ? Would a piezo speaker be suitable ?
>
> Regards
>   Anton Erasmus

maybe you can find idea in this application note:
http://www.acam.de/index.php?id=55

r.

Hello Anton,

> 
> We have used a switch that indicates when the paper is less than a
> certain amount. Both mechanical and optical. The system is a ticket
> dispensing machine - typically used in parking lots. The people
> employed to operate it, is minimum wage employees (Which is about
> US$0.70 per hour in South Africa). The systems are not well
> maintained, the environment is extremely dirty and dusty, and the
> enclosures in which the machines and paper are housed are never
> cleaned. I am trying to think of a way to make a cheap robust sensor
> which would work reliably for the 10 year life expected of the
> equipment. Being able to measure more than just a specific quantity of
> paper left would be a bonus.
> Counting as suggested by someone else would not work since the paper
> is periodically added, without any way to know exactely how much had
> been added. The paper is also fairly low quality that generates lots
> of paper dust, that loves to cling to things like photo diodes and
> transistors.
> 
> Keep the ideas coming. Even if I cannot use it, I am sure someone
> someday would get something they could use out of the archives.
> 

Looks like you may need a few pressure sensors. Example:

http://www.omega.com/prodinfo/pressuretransducers.html

-- 
Regards, Joerg

http://www.analogconsultants.com

On Thu, 25 May 2006 23:35:43 GMT, Frnak McKenney <frnak@far.from.the.madding.crowd.com> wrote:
> On Thu, 25 May 2006 22:47:31 +0200, Anton Erasmus <nobody@spam.prevent.net> wrote:
> --snip--
>> We have used a switch that indicates when the paper is less than a
>> certain amount. Both mechanical and optical. The system is a ticket
>> dispensing machine - typically used in parking lots.
--snip--snip--

(Hmph. Following up on my own post. I guess it's okay, as long as I
 don't start answering...  )


Anton,

It's amazing what just a few hours' sleep can do for one's thinking.
Scratch all that "sophisticated" stuff.

Why not just put a transparent (plastic, glass) window in the side of
the ticket dispenser so you can _see_ if any tickets are left? No
added mechanics, no electronics, and, best of all, it would make my
high school math teacher proud <grin!>:

   "Answer by inspection!"


Frank McKenney, McKenney Associates
Richmond, Virginia / (804) 320-4887
Munged E-mail: frank uscore mckenney ayut minds pring dawt cahm (y'all)
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    Before drawing conclusions from _any_ poll, it it absolutely vital
    to read the questions with greast care -- and that goes double for
    summaries released to the press, and triple for headlines based on
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    results were not what the sponsor hoped for,
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--

"Anton Erasmus" <nobody@spam.prevent.net> schreef in bericht
news:s65c729147mk6u97ha2tn7ie4b07j8svml@4ax.com...
>
> Keep the ideas coming. Even if I cannot use it, I am sure someone
> someday would get something they could use out of the archives.

Ultrasonic distance sensor?
http://www.parallax.com/detail.asp?product_id=28015

-- 
Thanks, Frank.
(remove 'q' and '.invalid' when replying by email)