I ran a quick, informal experiment to confirm the characteristic of the Interlink square FSR that we currently have. I fixed the FSR to a firm surface, placed a square acrylic block of variable surface area on top, and measured the resistance of the sensor with variable weight applied. I made a few significant findings: the FSR’s are sensitive to pressure rather than pure force; at some point in their response range, the two measurements converge (for example, a 500g weight applied over 9cm^2 gave roughly the same resistance as the same weight over 4cm^2, while 200g over 9cm^3 gave roughly 1.5 times the resistance as the same weight over 4cm^2); I confirmed the minimum threshold described in Interlink’s datasheet (the 50g weight over 16cm^2 left the sensor open, while the same weight over 9cm^2 gave resistances on the order of 20kOhm); there is a considerable amount of lag time and drift associated with the measurements. In fact, the drift was so significant that during some measurements the resistance seemed to almost continually decrease (for example, from 45 to 15kOhm over 20 seconds). In other cases, the resistance stabilized to (surprisingly consistently) -10% of the initial value after approximately 5 seconds. If I am reading the datasheet correctly, this is inconsistent with the specifications given of +/-2-5% part-to-part repeatability and a mechanical rise time (?) of 1-2 milliseconds.
These measurements may be attributed to experimental error, multimeter (Fluke) sampling, or hysteresis (something I don’t have the knowledge to comment on but is mentioned in reference to these devices), but I think they are significant enough to warrant further investigation.
The confirmation that the devices are pressure-sensitive will greatly affect their design intent: we will have to pay close attention to how we actuate them with respect to delivered pressure and saturation point (upper response threshold).
From page 8 of the integration guide:
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.
I’m not sure your quick test is completely valid.
Comment by gthomas — February 19, 2010 @ 12:27 am
I must not be understanding this passage. Does distribution refer to the actual physical location of the weight distribution over the area of the FSR? In that case, 5psi in the top right corner will not give the same resistance as 5psi in the bottom right corner, test to test. Or does it mean that a five separate 1psi areas will not give the same resistance as one 5 psi area? Or both of these things?
In my case, I would think that there would be little variation of the weight distribution between tests: I placed the acrylic block on the FSR, added a 500g weight, removed the weight leaving the block in the same location, and added the next weight, and so on. Undoubtedly the block shifted while I lifted/placed the weights, but I would think it would be a negligible amount. (Or does that passage say that even a negligible amount would have a disproportionate effect on the output?)
Comment by tdecker — February 19, 2010 @ 1:10 am
The way I interpreted the passage is that the position of the book on the acrylic block would be expected to change the reading, possibly because if the book was balanced on the block towards so that it’s center of mass was towards the top of the block, then the top row of cells would receive more pressure than the bottom row. Apparently this would give a different result than if the book was exactly centered or shifted slightly downward. At least that was my interpretation. Clearly it is saying that you can’t expect a deadweight test to give repeatable results.
Comment by gthomas — February 19, 2010 @ 11:51 am