Force measure/detect

[mikrogut]

Hi!

I will measure/detect force applied to a plate. I.e. I will detect when someone apply 1 kg of force. The plate is made of plastics and is therefore a bit flexible. I can measure the force or just detect (boolean on/off) if the force is >=1 kg.

I have tried different force sensors, but non was accurate enough and now I need some new inputs.

Thanks for all inputs!

 

[Reloadron]

You don't mention what force sensors you have tried? You don't mention is these were sensors requiring amplification?

Given a choice I would likely employ the use of a strain gauge and configure it into a bridge configuration. Then amplify the voltage out from the bridge and apply it to a simple comparator if all you want is a 1 where greater than 1 kg and 0 when less than 1 kg. You may want to start with the basics and give this link a read. The strain gauge being the heart of a load cell.

Ron

 

[mikrogut]

I have tried the Flexi Force sensor, but it wasn't accurate enough. I need the accuracy to be +/- 200 g max. The Flexi Force is also a pretty expensive solution. I would like a cheaper solution.

 

[Reloadron]

I have tried the Flexi Force sensor, but it wasn't accurate enough. I need the accuracy to be +/- 200 g max. The Flexi Force is also a pretty expensive solution. I would like a cheaper solution.

+/- 200 g on a 1,000 g load is far from accurate. That is an allowable error of +/- 20% which is large. I don't know what your idea of expensive is but the application of a basic strain gauge to your platen as I suggested would likely be a viable solution. You could likely find a cheap digital bathroom scale and hack it. Strain gauges like these typically cost about $30 USD. Do you have any electronic skills, like working with strain gauges and instrumentation amplifiers?

Ron

 

[mikrogut]

I have electronic skills, so that is no problem. I may try some strain gauges. The real challenge is to set up the mechanics, not the elecronics.

 

[Reloadron]

If you work with them I suggest you do some reading and not on the electronics but those mechanics you mention. Just surface prep is something that requires work. However, this approach may work out. Also consider what I mentioned about a cheap digital scale to take apart.

Maybe someone else will wander into the thread with other suggestions.

Ron

 

[SABorn]

A very cheap and rather effective method is to use the conductive foam (the black stuff) that IC's come packed in.
If you measure the resistance of the foam you will find the resistance reduces as you compress the foam, so it can be used as a variable resistor. (or compression resistor)

The mechanics to control the foam compression would need a little working out, but it is a rather consistant result you get from the foam and im sure it would be way better than within 20%.

Doing an ADC reading on the foam with a micro would give a very reasonable result.

If this is for a commercial application then this method might not be the best but for a simple quick small use application i think it will work ok.

I know of a throttle position sensor made this way for a addon circuit that has worked for several years without fail.

Mount the foam between 2 bits of copper clad PCB board so wires can be soldered to the 2 boards.
A quick test i just done with the multimeter probes gave 30K to 10K full scale and i think a 1 inch square would take more than 1 kg to compress it.

Food for thought.

Pete.

 

[mikrogut]

Very interesting. That is a very cheap solution. I will do some testing on the linearity and repeatability.

Thanks!

 

[Reloadron]

Pretty cool there SABorn, well worth a try.

Ron

 

[ericgibbs]

Hi Ron,
The only problem is that as it ages [ drying out] the resistance changes a lot.

Those QTC pills from Maplin [ no joke] work OK.

**broken link removed**


EDITED.
To QTC pills,,,,,,,,,,,,,,,, Quantum Pills are some thing else !!!!!!!!!

 

[Reloadron]

Morning Eric

Pretty cool. Placing between two pieces of copper clad, spring loaded PCB would be something I would like to try.

Ron