Pressure Sensor Smackdown!

After a few pretty labor intensive days of testing, I've firmly decided on the choice of pressure sensor for my project.

First, though I'd like to correct something I insinuated in the prior post, namely that the "Boston" pressure sensor required amplification in order to operate while the "Plusea" sensor did not.

As it turns out, this assertion was merely the result of my poor ( ... but steadily improving) skills at using a multimeter.

Both the "Plusea" sensor and the "Boston" sensor are perfectly capable of generating a full range of measurable resistance change correlating to the amount of pressure that the sensor is experiencing without requiring signal amplification.

The hitch is that the "Boston" sensor registers pressures in the rage of 15 mega-ohms (for the weight of a AA batter) to 33.7 kilo-ohms for about 180lbs while the "Plusea" sensor's range is more like 2 kilo-ohm to 144 ohms for the same weights.

So, initially when I hooked up the "Plusea" sensor to the multimeter, I quickly got readings in the default range that the multimeter was set to by default but for the "Boston" sensor, it took little more fiddling to get readings that were in the mega-ohm range.

But after some thorough testing I was able to establish the following comparison matrix:

Notice that for the "Maximum Measurable Resistance" item, the "Plusea" sensor was able to accurately and repeatedly measure the resistance caused by the weight of a minuscule SIM card at 30 k-ohm while the "Boston" sensor required the weight of a single AA battery to cause a measurable 15 mega-ohm response.

There's no doubt in my mind that the "Boston" sensor can accurately measure the resistance from the SIM card as well, it's just that I would need to use a more sensitive multimeter that could discern a much higher level of resistance in order to measure it.

Notice also that the "Standard", "Average" and "Delta" deviations as a percentage of the median sampled values for each weight are generally smaller for the "Plusea" sensor and, in general, for both sensors the deviations were smallest for the heaviest weight.

In my mind, this puts one major check mark in the "Plusea" column as I believe that the stability and repeat-ability of the pressure measurements is critical, though, on the other hand, both sensors performed pretty similarly.

Looking at the following box charts we can see that both sensors exhibited non-linear resistance across the range of weight and that the "Plusea" sensor had smaller and more even deviations while the "Boston" sensor had slightly larger deviations that were larger with lighter weights but eventually became very small with the highest weights.

In both charts, the top blue "whisker" indicates the "Maximum" value measures with the lower red whisker indicating the "Minimum" value.

The bottom of each both represents the 1st quartile with the bottom of the blue box portion indicating the median measured value and the top of each box indicating the 3rd quartile.

My interpretation of these charts is that the "Plusea" sensor showed a more linear response over the test range and generally showed a smaller degree of deviation for each weight while the "Boston" sensor exhibited a more consistent range with deviations that declined steadily with the application of more weight.

The final set of charts displays the "conductance" of the sensor for each weight in the test range.

Conductance, which is 1 / resistance, works to linear-ize the data and is ultimately the value I will be measuring with an un-amplified configuration for the pressure sensor.

Note that the conductance value for both series are being multiplied by a (different) factor for each sensor in order to bring the value into a integral range ( ... thus the "conductive index" terminology).

As you can see, the conductance is just a bit more linear for the "Plusea" sensor though it is important to note that there was a little range overlap with both the 2.5 and 5 lbs ranges as well as for the 25 and 35 lbs ranges while the "Boston" sensor exhibited no overlapping readings with these test weights.

In conclusion, I think that both sensors performed excellently and I'm pretty convinced that I can use either in an un-amplified arrangement ( ... thereby decreasing overall power and simplifying the circuitry) in my project.

The "Boston" sensor was a bit less linear with a more non-linear conductance curve as well, while the "Plusea" sensor was both more linear and a tad bit more stable.

The "Boston" sensors have the benefit of already being manufactured by a commercial supplier who is totally able to provide customized shapes and sizes while the "Plusea" sensor would require that I ultimately manufacturing the sensors.

Overall, there is just something that feels right about the "Plusea" sensor... its fabric, more or less trivial to construct, stable, produces a very linear conductance and is constructed of very inexpensive materials.

Further, I can experiment with all sorts of shapes and sizes and construction techniques without interacting with a third party.

So, my next steps are as follows:

1. Produce "p.alpha.2", the second incarnation of the "alpha" prototype with the "Plusea" sensor installed and firmly affixed with Velcro. 
2. Test the operational performance of "p.alpha.2" against the range of weights both statically and in motion. 

Best to all...