Re: Low cost weight sensor ?

On Thu, 25 May 2006 10:06:06 GMT, Peter Dickerson <first{dot}surname@xxxxxxxxx> wrote:
"Anton Erasmus" <nobody@xxxxxxxxxxxxxxxx> wrote in message news:42g97292vmtg52rbbu4negmk2nev5jaaui@xxxxxxxxxx
[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)
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