Interlink FSR 406

jason0627 · 2014-01-23 13:46

Hello was wondering if any of you had experience with interlink FSR (force sensing resistors), or other FSR im sure they are pretty similar. I am having a problem using mine reliably.

Here is my set up. pretty sure i did set up A but i am still learning.

this is what i did . capacitor is .1uF

This is my code

' {$STAMP BS2}
' {$PBASIC 2.5}

'

[ Declarations ]

Force VAR Word ' Stores raw output
sensorPin CON 7
counter VAR Word
'

[ Main Routine ]

counter = 0

Measure:
HIGH sensorPin ' Discharge the capacitor
PAUSE 20
counter = counter + 1
RCTIME 7,1,Force ' Measure RC charge time
DEBUG DEC counter, " ,", DEC Force,CR
GOTO Measure

So when i go to run my program i get readings but i have found that the force decays over time. i inserted a chart into excel and this is what i get.

this does not make for a very accurate reading. I have not touched or breathed on the sensor. I have tried this with many different items and results are similar. has anyone else experienced this or am i doing something wrong? Thanks

Hal Albach · 2014-01-24 07:12

Have you used an ohm meter to verify that the output of the FSR remains constant while the force is applied? The data sheet states that the output is repeatable with a margin of +/- 6%. Also, try replacing the FSR in your circuit with a 10K resistor and see if the readings remain stable.

jason0627 · 2014-01-24 07:44

that is a good suggestion about replacing FSR to see if it is some other issue.

i did that and here are my readings.

as you can see pretty dead on every time so im thinking FSR. I am not sure how to test with the ohm meter to be honest. i will look up how to use that online to see what i should do . thanks for the feedback

Hal Albach · 2014-01-24 08:00

Connect the Ohm meter directly to the the FSR alone. Set the Ohm range high enough to observe the readings while the load (force) is applied. Per the datasheet 1000 grams should yield somewhere near 1K, the scale they show is logarithmic so don't look for a linear response.

jason0627 · 2014-01-24 08:07

i did that and the numbers were pretty inconsistent

I thought i would try the same thing to the resistor. so i tested the resistor and the numbers were consistent.

do you know what this mean? i take it as the FSR is not outputting consistent numbers.

Hal Albach · 2014-01-24 08:26

How much weight are placing on the FSR? Try it with a little over 2 pounds (1 Kg) and see if the readings become more stable. I'm thinking you might be near the edge of the sensitivity range, which is 0.1 Newton or a little over 10 grams-force.

jason0627 · 2014-01-24 08:46

I think you are on to something here.

i tried a brick ( i balanced it on my wedding ring) and the readings are stable. thanks for the help now i have some testing to see where my limits are.

thanks again .

Hal Albach · 2014-01-24 08:56

If my wife saw me using my wedding ring to hold up a brick I fear that brick would become part of my anatomy!

jason0627 · 2014-01-24 11:19

well i think the reason it remained constant is because the brick exceeded the range. the reason it goes down is because of Creep i think which is this

CREEPThe change in LOAD CELL SIGNAL occurring with time while under load and with all environmental conditions and other variables remaining constant. Normally expressed in units of % of applied load over a specified time interval. It is common for characterization to be measured with a constant load at or near CAPACITY.

and i found other users with same problem

https://forum.sparkfun.com/viewtopic.php?f=14&t=36728

Tracy Allen · 2014-01-24 12:09

I was going to suggest spreading out the force over a larger contact area, maybe with a soft pad of silicone or felt under a platform the same size as the sensor. I've more experience with the Tekscan, and I see that it came out better in the link that you provided.

Hal Albach · 2014-01-24 12:11

Well, it appears that what you are running into is a result of what the sensor is made of. How I see it is that when a continuous force or load is applied to the sensor it responds to the initial application with a somewhat proportional reduction in resistance. When the load remains on the sensor the active material will continue to compress slowly causing a further reduction in resistance as exhibited by the curve in your Excel graph. Quite a few web pages regarding FSR's indicate low accuracy. They suggest that the sensors are best used as bump sensors that can indicate that a force was applied and the approximate amount of force (strong, moderate, or light). Anything beyond that will probably require characterizing the sensor's output to include how long the weight has been on the sensor and what the settling error is with different weight over x amount of time.
Per your link in #9 it appears that the Flexforce sensors do not seem to have this problem.
If your intention was to make a low cost scale, I don't think the FSR is going to do it for you.

jason0627 · 2014-01-24 16:56

i got a response from Interlink and they confirmed that it is the creep/ drift

they stated polymeric force sensors will do this. the resistance will change downward about 5% per unit of log(time) with 1k load

which means 5% after the first 10s another 5% after 100s ect

i think i am going to look for a good load cell that is pretty cheap.

thanks for helping me figure it out

Interlink FSR 406

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