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can i use piezoresistive fabric sensors to measure pressure exerted by backpack straps on shoulders?

i want to measure pressure exerted by backpack straps on shoulders. Will piezoresistive fabric sensors serve my purpose?

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I think strain gauges are the sensors you want. These are commonly used for measuring weight forces.
http://micah.navi.cx/2010/01/hacking-a-digital-bathroom-scale/
http://en.wikipedia.org/wiki/Strain_gauge

The article on load cells might be worth looking at too. It mentions piezo sensors.
http://en.wikipedia.org/wiki/Load_cell

Another trick that might work is using one or two air pressure sensor(s) connected to a compressible tube(s) that running through both(each) straps.  Here I'm assuming there are two straps, one on each shoulder.
http://en.wikipedia.org/wiki/Pressure_sensor

swetambri (author)  Jack A Lopez7 years ago
hii jack

can u plz suggest me how to measure pressure from piezosensor, circuit design and all.

Best regards

swetambri
What do you want exactly?  Some sort of display that tells you the number of newtons or pounds sensed by the loaded backpack straps?  Is this display going to be attached to the backpack?

Of course the easiest thing to do, just for testing purposes, is to just hook up your sensor to an ohmmeter.  This is shown in the first pic.

If you want to go beyond testing, I mean to actually build some some sort of display that can be worn on the backpack, probably the best way to do this would be to use some kind of microprocessor, a PIC(r), or a Arduino(r), or whatever your favorite is.  I myself don't have a lot experience with microprocessors, so I can really only speak about them in vauge terms.

Usually these microprocessors can measure analog voltages in the range 0-5 VDC.  So maybe the trick is to build a circuit that converts a resistance to a voltage, in this range, and then feeds that to one of the analog inputs.

Another thing microprocessors are good at is measuring time, or rather counting the number of clock cycles that have passed since some event, like a voltage on a pin changing. 

The second picture I have attached is a circuit that was pulled from one of Microchip(r)'s application notes.  It uses a very small number of components: just the  resistive sensor, plus a capacitor, plus a reference resistor, plus three GPIO (general purpose input output) pins on a PIC(r) microprocessor.  All the rest of the magic, alternately charging and discharging the capacitor with the reference-resistor and the sensor-resistor, and measuring the timing, this is done with software.  This is described in detail in the publication itself, "Microchip(r) Tips 'n Tricks for 8-pin FLASH PIC(r) Microcontrollers", tip #13,
http://ww1.microchip.com/downloads/en/devicedoc/40040b.pdf

The other pictures I have attached, are basically ideas for analog circuits for changing a resistance signal into a voltage signal. These could be used as a "analog front-end" for a microprocessor that wants analog voltages in the range 0-5 VDC. 

The relaxation oscillator circuit is my interpretation of the current balancing circuit described in the bathroom scale hack article that I mentioned earlier
http://micah.navi.cx/2010/01/hacking-a-digital-bathroom-scale/


resistance-sense-circuit-3.jpgpic-R-measuring_circuit.pngresistance-sense-circuit-1.jpgresistance-sense-circuit-2.jpg
swetambri (author)  Jack A Lopez7 years ago
Hii Jack.

Thnks for ur reply.

Ya i want to display pressure measured under the straps and this is possible through PIC.

Regards
Swetambri
swetambri (author)  Jack A Lopez7 years ago
for piezosensors i will hv to use a number of sensore to measure pressure from various sites on the back and shoulders.

load cell itself is so bulky , i think it wuld be uncomfortable to put under d straps.

That's a good point, load cells are bulky, and this backpack is intended to be worn by humans, rather than robots.

Have you seen this instructable?:
https://www.instructables.com/id/Flexible-Fabric-Pressure-Sensor/

I just noticed it on the right, "related" panel.  This sewn sensor actually exhibits a change in resistance in response to pressure.  You can see this effect from the included video (http://www.youtube.com/watch?v=uo6TJ-TsgHk)
This one might be worth looking into.

sensor-range.png
swetambri (author)  Jack A Lopez7 years ago
thanks!!  Ur advice was quite useful. Can u suggest me circuitry for implementing this task?

Strain gauges will measure force but not pressure, which is what you say you want to measure.

Jack has a good answer with the pipes idea, but in my experience the pressure change in tubes from the kinds of force and areas you are looking at isn't very much. When I've done it, I've nearly filled the pipes with water, and left a small air void at the ends in which I then measured pressure.

Steve
excaza7 years ago
There's no reason you couldn't.  As long as you properly calibrate your sensors, you shouldn't have a problem obtaining reasonable results.  I believe there's been a few projects about this on instructables already, perhaps they would be a good starting point?

www.instructables.com/id/Flexible-Fabric-Touch-Pad/
www.instructables.com/id/Flexible-Fabric-Pressure-Sensor/
They'll certainly tell you SOMETHING. It depends what you want to be told.
Do you need accuracy ?
Steve
swetambri (author)  steveastrouk7 years ago
will they be able to withstand 4 Kg of load, i can compromise on accuracy bcoz pressure variations would be very large. else suggest me other alternatives