Low cost weight sensor ?

It looks like one of those problems that cost $300 for a few or $.30 for 100k, but not a lot in between.

Load cells would work, but will be quite expensive. Springs, levers and opto or hall sensors would be another possibility.

What's your volumes, and what's your willingness to build your own? As Jim Stewart mentioned there are possibilities, but the only commercially available weight (or force) sensors that I know of are all stainless steel devices meant to go into an industrial setting and last forever while being sprayed with salt water and whacked with hammers. That's good for the guy who needs one and doesn't want to spend a lot of engineering time qualifying the sensor, but not too good for a product.

Some roll-your-own possibilities:

Make a balance with an angle detector. The balance part is obvious, the angle detector could be a potentiometer or an LVDT or a fine encoder.

Make your own load cell with your own LVDT or strain gauge. There will be qualification issues, but I suspect that most really inexpensive scales out there (like bathroom and postal scales) use this scheme. The nice thing is that LVDTs are simple in their 1st-order approximation, and a load cell can be as easy as some spring steel that's been laser cut in a certain way. Strain gauges can be harder to implement, but there may be some possibilities there, too.

Use a big, compliant spring with a potentiometer or encoder. This is the same theory as a load cell, but it makes for an easier sensor design.

Hello Tim,

With the compliant spring version I'd look into capacitive sensing. Isolated plate pressed towards a grounded metal surface as spring is depressed, or the other way around. It's non-linear but a uC would likely be needed anyway in this setup.

Whatever is done, this field might be peppered with patents.

Regards, Joerg

It seemed that way to me from googling, but I hoped I just did not have the right "magic" search term.

I was hoping to not have to roll my own.

Regards Anton Erasmus

My volume is fairly low. Initially prototypes, and then 500 or so per year. Rolling my own seems to be the only option.

This looks like it can be quite simple witjout the need for complex or time consuming calibration.

We have used this sort of setup, but for the current product I would want something in the US$20 range.

A helix spring with a sliding potentiometer ? What sort of lengths / sizes of liding potentiometers are avialable ?

Regards Anton Erasmus

You mean that one will easily infringe some sort of patent ?

I was hoping one would find cheap MEMS based devices or something that implements a cheap robust weight sensor. Units with strange gauges already mounted seem to be quite expensive.

Regards Anton Erasmus

Strain gauges are even more expensive..

Best regards, Spehro Pefhany

Hello Anton,

Possibly. I have seen some rather bizarre patents in that field. Patents where I had no clue how on earth they could ever have been granted.

Sure, if money is not a concern. But if you want to produce oodles of these and it has to be cheap you'll have to be inventive and design it around nickel and dime parts. Of course then the chances are high someone else had had the same ideas and filed for a patent.

A resolution of 10% or even 1% is quite easy with a capacitive plate sensor and compliant springs. I don't know if that's patented though.

Regards, Joerg

Hello Anton,

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.

I can't look though and don't remember the brand. I returned it because it was less accurate than the old scales but it might have been good enough for you. Look at the stores (Taregt, Walmart, Mervyns etc.) I bet they still have something like that. This model was $34.99 with a big LCD display so the sensors must have been cheap.

Regards, Joerg

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I would suggest searching for Penny and Giles, their conductive plastics division makes all sorts of sliding pots, meant for machine position reading. So if the paper tray already ahs some form of spring for the paper of take up to move it could be attached to that or your mechanism.

:-) One writes strange things, when kids want attention.

Regards Anton Erasmus

Digikey have some precision linear pots by Panasonic IIRC, sort of 1% accuracy, quite cheap

martin

martin

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" ). If needed, this will add to the cost and complexity.

Good luck!

Frank McKenney, McKenney Associates Richmond, Virginia / (804) 320-4887 Munged E-mail: frank uscore mckenney ayut minds pring dawt cahm (y'all)

-- Anyone who is not genuinely addicted to the search for knowledge is unlikely to have the psychological energy to be a true scholar in any field. But in history this work clearly resembles more that of a detective than that of a scientist -- a search for and judgment of particular evidence rather than a repeatable experiment. The detective side of historical research needs skill, background, and intuition. -- Robert Conquest, "The Dragons of Expectation"

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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

Just a goofy thought...

There are cheap accelerometers these days (airbags, etc.); what is their precision and resolution? If one secured the target mass to a platform connected to a calibrated solenoid, applied a known force and measured the acceleration, the mass is easily computed...

Regards,

Michael

I had another idea you might find interesting. If the paper is neatly stacked, could you measure its dielectric constant with rf? I know that soil moisture and body fat can be inferred this way. You could avoid moving parts if you could make it work.

Hello Anton,

Something like that must have been in the bathroom scales I described. Maybe you could just hop into a store, buy one and take a look.

Regards, Joerg

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.

Peter