FlexiForce Sensor Help!!!

ghost13

Hi Everyone,

So I'm trying to set up the FlexiForce sensor so that when I press on the sensor, the voltage changes linearly to the force being applied.
I followed the schematics on this download: http://www.parallax.com/Portals/0/Downloads/src/prod/flexiforce.zip but I get 0 volts when I try to get a reading (I'm using a multimeter for now). I'm using 5V for the input (I'm breadboarding this - I'm not using a parallax board).
I think the problems lies with the 0.01uF or 0.1 uF cap (I've tried about 10 different caps). But I get 5V going into one side of the cap, and 0 volts on the other side (I realize it is discharging and the multimeter is using up the voltage and staying at zero, but this still seems odd).

Any advice?

Or does someone have a better system for hooking it up?

ghost13

This is pretty urgent, by the way, so any help would really really be amazingly helpful [noparse]:)[/noparse]

Whit

Have you looked at the demo kit in the flexi-force download?

It says,

"Tekscan’s FlexiForce® sensor measures force between two surfaces. The sensor output exhibits a high degree
of linearity, low hysteresis and minimal drift compared to other thin-film force sensors. The Flexiforce single
element sensor acts as a resistor in an electrical circuit. When the sensor is unloaded, its resistance is very
high. Resistance decreases when force is applied to the sensor. The resistance can be read by connecting an
ohmmeter to the outer two pins of the sensor and applying a force to the round tab sensing area at the end of
the film.

The resistance of the Flexiforce sensor is proportional to weight."

So a reading from the flexiforce should be a resistance reading. If you want to vary a voltage based on pressure on the flexiforce, highest resistance occurs when unloaded and as it says above, decreases as force is applied.

Can you explain more about what you have tried and want to do?

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Whit+


"We keep moving forward, opening new doors, and doing new things, because we're curious and curiosity keeps leading us down new paths." - Walt Disney

ghost13

Thanks for your reply!

Here's a little background on what I am trying to do:
For a science project, I am attaching the FlexiForce sensor to the fin of a high powered rocket that will reach speeds of around Mach 1.5. As the rocket passes through transonic speeds (Mach 0.8 to 1.2), pressure is released from the nose down the side of the rocket. This pressure is great enough to rip the fins off some rockets if they are not well built (it can exert hundreds of pounds of force). I want to measure those forces. I don't plan to use a Basic Stamp chip - I only want to log the data. The logger I am using is this one (http://www.sparkfun.com/commerce/product_info.php?products_id=752), which has a built in 10 channel ADC.

I have downloaded the demo kit, and I am using the circuit in those instructions. Supposedly, according to that manual, the sensor can be hooked up to a pin on a Basic Stamp chip and RCTIME can be used to get a voltage reading from the sensor.
Would it work to run some power (5V) through the sensor connect it directly to the logger? Or do I need a better circuit?

So, overall, my end goal is to obtain a voltage reading from the sensor that is linear (or inversely linear) to the weight applied. Then that will hook up to the built in ADC.

Thanks!

whicker

that circuit is specific to the basic stamp and using the basic stamp's RCTIME instruction.
you're probably going to need something similar to the op-amp circuit that the actual datasheet shows. (resistance to voltage conversion).

i'd even search for circuits using a cds cell, because it appears to act very similar circuit-wise (both vary their resistance).

ghost13

But doesn't RCTIME act as a primitive ADC? So it measure voltage (basically)?

I know a bit about electronics, but I'm no expert. How does an op-amp work? Could you recommend a specific one?

And if you run voltage through a resistor, the voltage will slightly drop. So resistance does effect voltage? I know it really effects current, but doesn't Ohm's Law say it effects both?

What would be the absolute simplest way to get this up and running? The launch is in <2 weeks, so if I need to order specialty parts (All Electronics is 20 min away from my house so I'm talking about anything they don't carry, like ICs), which should I get?

Thanks!

skylight

Capacitors block DC so no surprise you are reading 0 volts on the other side of the cap

The sensor is resistive so either measure the volt drop across it or at the output side with reference to ground either way will give different readings
ie across the sensor you will read· the maximum volt drop when the sensor is unloaded and the voltage will lower as you apply pressure

on the output to ground reading the opposite will apply and voltage will increase as pressure is applied.

measuring with a meter is ok but beware of current restraints if hooking up to other circuits as being resistive it will get·hot if the current increases beyond specs.

Post Edited (skylight) : 6/6/2008 11:23:36 AM GMT

Bean

Ghost13,
· Measure the resistance of the flexforce with the expected load (weight).
· Then replace the capacitor with a resistor of about that value (make sure it is at LEAST a couple hundred ohms).
· You should then get a voltage output that varies with the weight on the sensor.
· You should get about 2.5 volts with the expected load (weight). The voltage will go down with more weight, and up with less weight. Just exchange the resistor and·flexforce to make the voltage higher with more weight.

· The RCTIME command is used to measure resistance, not voltage.

Bean.

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