Force Sensitive Resistor Calibration

Question

I have been trying to calibrate FSR06CE Force Sensitive Resistor.
Data sheet here.
For starters I started by placing deadweights on the top of the FSR's active area and using a multimeter and DSO I measured the resistance and frequency respectively I'm not sure if this was the right way to do it because the weight distribution is not equal as the weights are of different sizes.

The second methodology that I adopted to calibrate the FSR was to 3D print a frustum like structure with base being equal to the FSR's active area and the top being like a cavity allows me to place weights on top of it but it didn't give me accurate results and I was also not sure if the PLA material will actually distribute the weight equally or most of it will be observed on to itself.

The Third thing that I tried was to get a bush like structure machined ( metal ) such that the base sits on the FSR's active are and I took the readings for this as well.

All these three different methodology gave three different results altogether. I'm not sure which one I should move forward with. I have been referring to a IEEE Journal paper where they have calibrated the same FSR and I have been comparing my results to them but apparently none of this methods have given any closer result to the graph that they have obtained.

I would like to know where I am going wrong or what else I can improve ? Thank you.

Answer 1

The kind of information you need is typically supplied by the manufacturer in the form of application notes or manuals. I don't see any such information on the Ohmite site, assuming they are the actual manufacturer.

Such data from a manufacturer of similar products (Interlink) can be found here. You can also go to the manufacturer's website, however they demand registration to access the information. There are suggested circuits as well as application guidelines. You may be able to find similar information from other suppliers.

Referring to the application guidelines, a couple ideas as to where variations could come from include the way the load is applied (it should be applied evenly, with consistent distribution, perhaps with a layer with some compliance to smooth out the force) and whether the vent is obstructed.

Provide a consistent force distribution. FSR response is very sensitive to the distribution of the applied force. In general, this precludes the use of dead weights for characterization since exact duplication of the weight distribution is rarely repeatable cycle-to-cycle. A consistent weight (force) distribution is more difficult to achieve than merely obtaining a consistent total applied weight (force). As long as the distribution is the same cycle-to-cycle, then repeatability will be maintained. The use of a thin elastomer between the applied force and the FSR can help absorb error from inconsistent force distributions.

Do not block the vent. FSR devices typically have an air vent that runs from the open active area down the length of the tail and out to the atmosphere. This vent assures pressure equilibrium with the environment, as well as allowing even loading and unloading of the device. Blocking this vent could cause FSRs to respond to any actuation in a non-repeatable manner. Also note, that if the device is to be used in a pressure chamber, the vented end will need to be kept vented to the outside of the chamber. This allows for the measurement of the differential pressure.

It's also possible that your expectations are not in line with the way these devices actually perform.

Force accuracy ranges from approximately ± 5% to ± 25% depending on the consistency of the measurement and actuation system, the repeatability tolerance held in manufacturing, and the use of part calibration.

It would be be even better to have, in addition to the above type of information, some details on the apparatus used by the manufacturer to test and characterize their product.