Make a FSR linear softpot

I'm trying to DIY an FSR linear softpot.

It is a long thin pressure sensitive resistor that can also detect where you press along the sensor.

Basically it works like a potentiometer and an FSR sensor in one. You take a measurement of the force and then a few microseconds after take a measurement of the potentiometer part to get both pressure and position.

The R-wiper in the picture is the FSR layer that changes resistance depending on how hard you press. The R-fixed is the fixed resistive track that changes resistance depending on where you press.

I have read on how to implement it in arduino but there is a detail that I don't quite understand.

This works like a regular potentiometer but the wiper has resistance that can change depending on how hard you press.

I'm wondering how this can work, won't the position detection be inaccurate if the resistance of the wiper changes?

I'm guessing the pressure sensing part may also be affected depending on where you press.

If I wanted to DIY one of these sensors in a custom shape for example what resistance would I use on the FSR and fixed resistance track to get the least amount of error? I know they use about 1000 Ohm per inch on the fixed resistance so I can just copy that but what I'm wondering is if it's better to use a less resistive FSR layer or more resistive?

So what I'm really wondering is how the resistance ratio of the wiper part vs the fixed resistor affect accuracy? Is it better to have the fixed resistance at say 10k and the FSR at 0-10k or the FSR at 0-300k? Or maybe the fixed resistor at 300k and the fsr at 0-10k?

I've tried to read up on resistance and voltage calculations in parallel and series circuits, voltage dividers etc. but I don't quite understand this.

Perhaps some reading on how "accuracy" is defined and calculated and reported. Put some numbers on your study.

Potentiometer accuracy is more a function of the width of the slider than the pressure. Some pots are wirewound and have very narrow sliders so only one or two wires are in contact at a time and are very "accurate". The plastic ones you get for cheap are just that. Temperature and the actual current flowing through the resistance also affects the slider contact.

Paul

Paul_KD7HB:
Perhaps some reading on how "accuracy" is defined and calculated and reported. Put some numbers on your study.

Potentiometer accuracy is more a function of the width of the slider than the pressure. Some pots are wirewound and have very narrow sliders so only one or two wires are in contact at a time and are very "accurate". The plastic ones you get for cheap are just that. Temperature and the actual current flowing through the resistance also affects the slider contact.

Paul

Thanks for the reply.

Ah you are absolutely correct. I didn't define what I mean. I'm not interested in super accuracy.

What I meant by accuracy is more like repeatability. I'm wondering if the potentiometer readings will affect the fsr reading depending on where you press and vice versa. Also if there is some way to minimise it.

For instance if the potentiometer is at a point where there is a resistance of say 5k it will show a certain voltage at the ADC when you measure position. Will that voltage be the same even if the wiper resistance varies? For instance if I press where the resistance is 5k on the potentiometer part twice. Once I press soft and the other time I press hard, will it register at the same position or will it drift off to some direction when the wiper resistance varies?

Also the same goes for the pressure reading. It is basically reading the wiper resistance by sending a voltage to the ADC. Will that reading vary depending on where you press?

Seems that testing will actually answer most of your questions!

Paul

Paul_KD7HB:
Seems that testing will actually answer most of your questions!

Paul

Yep but it could be useful to know how to calculate it beforehand so I don't spend hundreds of dollars making sensors that don't work.

I've looked at series and parrallel circuit tutorials on youtube but I don't know how to calculate this. There is no battery in this circuit lol. And what part is parrallel and what is series. Confusing.

I think I've figured it out. I think the position part is a non issue. The resistance of the FSR (wiper) doesn't matter, the ADC is like 10MOhm anyway and the FSR is in a parrallel circuit so voltage is the same.

However the pressure measuring will not be linear depending on where you press because the fixed resistor of the potentiometer part will form a parrallel circuit and the total resistance of the two parrallel resistors will not be the same depending on where you press.

As far as I understand the pressure sensing will be more sensitive the closer you are to the edges.

So basically having the potentiometer part be less resistive and the FSR be more resistive would be a better idea.

Hi,
What application are you looking at using this device?

Tom.... :slight_smile: