Flexiforce Pressure sensors - does anyone have any experience with them?

Projects General Guidance
1

I am using one for my project, and I realized that it does not spit out linear values in the serial monitor.

For example, a weight of 1 pound may read as 256, 2 pounds at 459 and 70 pounds at 879 on the 1023 scale (this is for the 100 pound sensor)

This is my current code to map it down to 0-100 values

int pressure = analogRead(A0);
pressure = map(pressure,0,1023,0,100);
Serial.println("Pressure Reading: ");
Serial.println(pressure);

Is there a way to make it so that 1 pound = a readout of 1 with this sensor? How does calibration work?Thanks.

2

The best bet is to use Google to find information on them. I don't think they are very accurate.

3

Ive tried

really hard

Damnnnnn

4

Hi
what is the part number for the flexiforce sensors?
Where did you purchase them?

If its from sparkfun;

The graph of conductance, which is inverted resistance, is close to linear. So you could calculate using that as a basis for your conversion fucntion.

Tom...... :slight_smile:

5

Yeah that's the one.

It measures up to 100 pounds.

Anyway you can guide me on how to go about those calculations?

6

Hi,
Can you post a copy of your circuit please.
CAD or picture of hand drawn circuit in jpg or png would be fine.

Can you give me some analog values for some weights that you aim to apply, go as high as 80 pound if you can, the notes specify that close to limit response becomes saturated.

I'll get back to you soon.

Tom.... :slight_smile:

7

Imgur

You should be able to decipher that

Also, I dont have definitive weight measurements here at home. I'd need to wait to go back to school in a couple days.

What I can tell you is that if I press with little force I get a readout of about 650, and then it gets really hard to increase that number significantly with just my fingers

I can barely get it past 800 by squeezing very hard, as opposed to an automatic of 650 by barely pushing down on it and 210 from just touching it

I can respond with precise values in a few days if that is what you need

8

cross-post? Does anyone have any experience with FlexiForce Pressure sensors? - Sensors - Arduino Forum

9

Delete that thread, I think ill get more help here. Since you guys care so much about cross posting. I just need help. Sorry.

10

Imgur: The magic of the Internet
You should be able to decipher that

No not at all.
Why is there no resistor in line with the LED?
What value is the pull down resistor on the sensor?

Did you know that the sensor will not stand constant pressure and give a constant reading?

Since you guys care so much about cross posting

Yes we do, if you don't, now you have been told, then you are not welcome here.

11

Value of resistor connected to pressure sensor is 1 mega ohm, as shown by the tutorial
My LED doesnt need a resistor for what Im using it for

"Did you know that the sensor will not stand constant pressure and give a constant reading?"

I'm confused by what you mean with that.

This is a more updated picture, forgot a wire on the pressure sensor

Imgur

12

My LED doesnt need a resistor for what Im using it for

And what reason is that then? I don't believe you.

Value of resistor connected to pressure sensor is 1 mega ohm, as shown by the tutorial

Way too high no matter what the tutorial said.

"Did you know that the sensor will not stand constant pressure and give a constant reading?"
I'm confused by what you mean with that.

Well what i mean is that it does not work like you think it does. The output is not constant for a constant pressure.

Still cross posting then:-
http://forum.arduino.cc/index.php?reportsent;topic=314844.msg2181293#msg2181293

13

I cross posted before I was told not to. I'm sorry, ok?

My LED works without a resistor so I see no point in adding one. It only turns on when a certain pressure threshold has been reached.

What value resistor should I use instead, then?

And is there a way to fix no constant output? Is there a different sensor I can use to measure pressure/force/weight applied on an object?

Thanks

14

My LED works without a resistor so I see no point in adding one.

Sure they work, they will also damage your Arduino's output by drawing more than the maximum amount of current from them.

http://www.thebox.myzen.co.uk/Tutorial/LEDs.html

What value resistor should I use instead, then?

The A/D converter has a sample and hold capacitor on its input. If you have a high impedance source, which you will have with that sort of resistor then it will take a long time to charge from the value given by your pot to the value given by this high impedance potential divider. This is why the pot seems to affect the sensor's reading. There are things you can do to mitigate this like reading the same analogue input twice and only using the second reading. Put a delay between the two readings to allow it more time.
The normal value of the resistor should be the same as the half scale on your sensor so do some measurements.

And is there a way to fix no constant output?

No. This applies to this sensor and almost any low cost weighing solution. Have you noticed how your kitchen scales have a "tare" function for getting rid of that offset?

15

Wow ok, thanks.

16

Your hardware needs upgrading.

As said, a LED needs a current limiting resistor.

What is the value of the pot. If more than 10k, it needs a 100n cap from wiper to ground.

I would use the sensor from A-in to ground, and use a pull-up resistor.
Grounded things potentially have less problems. Adjust your code for inverted logic...
MEASURE the sensor with the average load you are going to use, and pick a pull-up resistor with that value.

If that's more than 10k, you have to do one of two things.
#1 multiple readings in your code
#2 10-100n capacitor from A-in to ground.

#2 could slow things down a bit more.
Leo..

17

Ok, I'll add the LED resistor. The pot is 10k, so I dont think I need to add that.

"I would use the sensor from A-in to ground, and use a pull-up resistor.
Grounded things potentially have less problems. Adjust your code for inverted logic..."

Mind explaining that a bit more? Sorry, I'm sort of a beginner at this. Thanks

18

The sensor and a resistor make up a voltage divider.

For the circuit it is the same if you use the sensor to +5volt and the resistor to ground.
Or the sensor to ground and the resistor to +5volt.

+5volt on one of the wires of the sensor is more risky.
You could accidently short it to ground and blow up things.

Also, keep the wiring short, or use shielded wiring.
A 1Megohm impedance picks up hash quickly.
Leo..

19

Thing is, it is not hash he is picking up. There is no noise pickup but a systematic increase where one reading affects the other. If you still use a 1M you still get the same problem.

20

Yes, I think I get that. The MUXing of the analogue inputs could move charge from one input to another.
That's why every input has to see a low impedance.
But long wiring with that high source impedance could add to that problem.
Leo..