Re: Low cost weight sensor ?

On Wed, 24 May 2006 14:39:00 GMT, Frnak McKenney
<frnak@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxx> wrote:

On Tue, 23 May 2006 20:40:51 GMT, Joerg <notthisjoergsch@xxxxxxxxxxxxxxxxxxxxx> wrote:
??(someone else wrote):
Load cells would work, but will be quite expensive.
Springs, levers and opto or hall sensors would
be another possibility.
I was hoping to not have to roll my own.

Just an idea: A couple of years ago I bought fancy bathroom scales. Nice
glass design and four pillars. There must have been four pressure
sensors in the pillars since wires went there and there was neither any
mechanical cantilevering nor did this thing move down at all if I
stepped on it.

My apologies if someone else has already described this approach.

Some years back I pulled apar... er, "conducted an engineering
analysis" on a used ElCheapo bathroom scale. It was a remarkably
simple mechanism, consisting of an LED display, a photointerruptor,
four springs, a small PC board with a few components, and a slotted
encoder wheel with a gear driven by a moving flat bar. (The gear
mechanism is similar to the one used in those squeeze-to-spin
"sparking wheel" toys popular some time back)

The top plate is "suspended" from the bottom by the four springs,
and when a weight is placed on top it compresses the top and bottom
together forcing the bar to move, rotating the encoder. All the
circuit board has to do is count the photointerruptor pulses up and
down and reset the counter when no weight is present. F=-Kx.

Now, a bathroom scale doesn't have to meet commercial load-cell
precision or truck-weight loads, but there maybe something in the
concept that the OP can adapt.

Another problem, depending on the load range, may be the suspension
mechanism. A "sufficiently heavy" weight might require that it be
connected to the full load through some kind of weight-reduction
gearing or leverage in order to keep the sensor from being pushed
past its limits ("crushed" <grin>). If needed, this will add to the
cost and complexity.


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
.