flexiforce sensor - please help me

nnn123

Hi

I tried to calibrate the flexiforce sensor (by placing loads on the sensing area and measure the resistace across the terminals using multimeter) but i couldn't get a linear conductance graph although I followed all the steps provided in the user manual http://www.cytron.com.my/datasheet/sens ... Manual.pdf .
Furthermore, it does not gives stable reading when I conditioned/calibrated it. The resistance value kept on decreasing although if I waited for 3 minutes. Besides it also didn't give the same reading although the same amount of weight placed on it.

What should I do? Does this means the sensor has broken/damaged?

Thanks

ElectricAye

nnn123 wrote: »

....
I tried to calibrate the flexiforce sensor (by placing loads on the sensing area and measure the resistace across the terminals using multimeter)....

I took a look at Parallax's documentation, which I got from the following:
http://www.parallax.com/StoreSearchResults/tabid/768/txtSearch/flexiforce/List/0/SortField/4/ProductID/384/Default.aspx

What first grabs my attention is that this sensor has 5 megaohms of resistance when starting out at zero load. That's an extremely high resistance, so unless you have a special type of ohmmeter that can handle and discern mega-ohm levels of resistance, you are very likely to get weird results with a normal multimeter in the low force range.

Also, you have to be very careful how you apply the force to this type of sensor. Did you happen to see the following?

Flexiforce pdf. wrote:

Conditioning
Before using the FlexiForce Sensor for the first time or after long periods of inactivity, it is highly
recommended that it be conditioned in order to stabilize the output resistance. The output resistance
can fluctuate over the first few tests, and conditioning the sensor ensures repeatable results.
To condition the FlexiForce sensor, place 110% of the maximum test weight on the active sensing area
and let it sit for a few minutes. Repeat this process another four to five times to guarantee accurate
results.
Sensor Loading
The active sensing area of the FlexiForce Sensor should be treated as a single contact point, with the
load distributed evenly across the sensing area. For best results, the loaded area should cover 70-100%
of the sensing area and apply a minimum of 1 PSI.
If the load is smaller than the active sensing area, make sure the load is centered in the sensing area and
not near the edges.
If the load is larger than the active sensing area, you may need to make use of a “puck.” A “puck” is a
piece of rigid material smaller than the active sensing area that is placed between the object and the
sensing area. This will cause the load path go through the puck to increase the PSI, resulting in more
accurate measurements.

I'm just going to guess that the sensor is okay but that you have to be more careful how you handle your calibration. Set up your microprocessor (BS2 or whatever) as shown in the Parallax document, then try to calibrate it with that and pay special attention to how you apply your loads.

good luck!
Hope that helps,
Mark

Electronegativity

You might want to try putting the sensor between 2 pennies and squeezing it that way.

I used one of these by making it part of a voltage divider circuit and hooking it up to an analog to digital converter.