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# Is it possible to use Piezoelectric sensors to measure weight, like a hypersensitive set of scales? Answered

If Piezo is not appropriate in this case, which would be the best technology / method of weighing to use? I want to create a a hypersensitive set of scales which read up to 1 kg of total weight. But I want to create it as flat and flexible as possible. The footprint would be roughly 115 x 55 mm and be used to measure the weights of small foods for the prupose dieting, can anyone help me?

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## 11 Replies

bwrussell (author)2012-03-13

Aren't piezo elements stiff? Also I think they are used to measure vibration which is a change in pressure and as @rickharris points out once you reach a steady state your output drops off.

Do you want to measure mass or weight? If you actually want mass remember you will have to remove gravity from your output, which will be a force. If you want weight (force) you need to decide if you want Newtons or kilograms-force when you are calibrating.

oscarpf (author)2012-03-14

I would like to measure a total weight of something in Kilograms. I guess that would be kilograms-force in this case. I would like to create a small, flexible set of scales in the rough footprint of 115 x 55 mm, as flat as possible and prefer it to be slightly flexible. What would be the best technology / method of weight in this case?

rickharris (author)2012-03-13

Almost ll electronic scales use strain gauges. The resistance wire is wound or glued onto a stiffer springy metallic support so that when tension is released it springs back to the neutral position

Most self zero when turned on.

bwrussell (author)2012-03-13

I guess I interpreted "malleable" differently. I assumed it meant the bed, where objects to be weighed are placed, was flexible and could be draped over a non flat surface. That doesn't make much sense for a scale but neither does the other possibility that he wants the bed to be squishy, like a neoprene mat. I don't see strain gauges behaving predictably or repeatably without a stiff bed.

@oscarpf: What do you mean by malleable? What part exactly is malleable?

oscarpf (author)2012-03-14

Basically I would like to create a super thin/slim, but also flexible (perhaps malleable isn't the right term) set of scales which are portable, roughly within the footprint of about 115mm x 55mm. I would like them to be as accurate as possible as I want to measure weights of small foods for dieting. I just dont want to carry around something solid (hard), or would this drastically compromise the weight reading?

rickharris (author)2012-03-13

No - The Piezo element only gives an output as it is bend once it reaches a steady state the output goes away.

You need a strain gauge - These are/can be very flat.

steveastrouk (author)2012-03-13

There is a static voltage built up by the bend. Unfortunately, it leaks away as you measure it. The VERY best cells can show a "static" load with a very small error, for several minutes, before exponential decay beats you.

bwrussell (author)2012-03-13

Don't strain gauges work by changing the resistance of a wire during tension or compression? This means if the scale was flexible you would have to recalibrate every time the shape of the pad changed. It seems like a strain gauge would be hard to implement as a scale, especially a thin one since they can only detect strain in two axises.

lemonie (author)2012-03-13

What do you want to do better/different to scales that can be bought?
It's "hypersensitive" that I don't understand in that context.

L

Kiteman (author)2012-03-13

Forget piezoelectric, get hold of some quantum tunnelling composite pills.

They're fun little rubber pellets, with cleverly-shaped conductive particles embedded in the matrix.

The more you put them under pressure of any kind (squeeze, bend, twist), the lower the material's resistance becomes - from non-conductive, through to being able to carry a current of 10A.

The pills are 1.5mm thick.

Make your balance from two rigid metal plates, separated by five pills of QTC (one in each corner, and one in the centre).  Simply measuring the resistance between the two plates will give a reading that can be easily calibrated to load.

mpilchfamily (author)2012-03-13

Yes some load cells are piezoelectric. But how to go about creating a piezoelectric cell and accurately translating the electrical impulses to a weight is hard to say.