Setup for a Pressure Sensor Smackdown!

As an aside to my testing of the amplified pressure sensor of the prior posts, I wanted to spend some time evaluating an alternative approach which would appear to not require amplification.

Initially, when I began researching flexible pressure sensors, I came across some exceedingly interesting projects made available at Instructables by a user named Plusea.

After looking over some of the sensor related project details and watching a few of Plusea's YouTube videos, I was convinced that my project was possible.

I immediately contacted Hannah ( ... Plusea) and ordered a fabric sensor kit from her Etsy shop with high hopes that operational range of this unique dynamic resistor would fit for my application.

Given that Hannah was going to be shipping my order out from Germany, and that I was pretty impatient and wanting to get up an running on this application, I did a little more Googling for other possible approaches and found a commercial manufacturer of flexible pressure sensors right here in the Boston area.

Both sensors appeared to work similarly in that they both were effectively acting as dynamic resistors, preventing current flow with no pressure (i.e. like a switch switched to off) and then slowly allowing more and more current (i.e. less resistance) with greater and greater application of pressure.

Of course, the package of sensors I ordered from the Boston supplier came almost overnight and thus I got up and running with them first.

Right from the start, there was more to using this sensor than the simple Velostat fabric swatch of Plusea's construction.

Most notably, it was suggested by the manufacturer that the sensor be paired with an either a single or dual supply OpAmp (... as we now well know from prior posts) which took me down the path of assuming some level of circuitry allocated just for proper function of the pressure sensor. 

Mind you, my intent was to minimize this circuitry (low voltage OpAmp, probably single supply, minimal supply voltage) but I was firmly on the path of detailing amplifier circuitry and vetting power scenarios just for the pressure sensor (... remember there are other sensors as well in my project!).

Well, last night I finally got around to putting together Plusea's fabric pressure sensor and sure enough it works great!

It produced what preliminarily appears to be vary consistent readings even at very heavy loads...  I even put my whole body-weight on it ( ... about 178 lbs)... and all with only the little current put out by my digital multimeter.

One thing though, the change in resistance appears very non-linear in that there is a large change in resistance with very light weight and then progressively less and less change as you pile on more weight, but the sensor appeared to be generating some very consistent readings in any event.

So, this appears to frame a bit of a smack down of sorts... which sensor is the best option for this application given all the constraints and requirements?

Obviously, I now need to create a comparison matrix.

Best to all...   

Testing OpAmp with Pressure Sensor (Part 2)

Building off my last test, I resolved to whip up a crude prototype ( ... very crude!) of my device with the dual-source OpAmp powered pressure sensor "installed" so I could make some preliminary measurements of its performance.

The goal of this test was to better answer questions 2 and 3 of the last post which concerned the stability, quality and repeat-ability of pressure measurements particularly with minimal amplification.

I decided to turn down the input voltage and gain to the minimal setting that allowed for a measurable response from a simple squeeze of the sensor with my fingers ( ... a ballpark estimate of the minimal functional pressure) and proceeded to sew up ( ... with my wife's sewing supplies) a prototype I call "p-alpha.1" for prototype-alpha 1.

P-alpha.1 provided, very nicely in fact, the basic foundation for putting the pressure sensor to the test under real conditions and overall, this exercise left me feeling very optimistic about the ability to make accurate measurements with my wearable device.

First, I tested the repeat-ability of the measurements by applying a basic range of weight ( ... 5 - 35lbs like the last post but not ultimately the full range required for the application) over and over again to get a "resting" measurement.

In the above graph, the green series titled "mid-measure" represents the trend of the "resting" measurement across the range of weight.

Again, as in the prior post, there was an obvious correlation between the weight applied and the output voltage but more importantly, the measurement was very repeatable and stable with each successive application of the same weight registering very nearly the same output voltage.

Then, I decided to put the device in motion and observe the range of fluctuation in the output voltage which I assumed would oscillate between some low and high value surrounding the initial "mid-measure" value.

My assumption was confirmed, there was an oscillation and it was, in my opinion, very workable.

Initially, I was concerned that this oscillation would be so sharp and severe as to make consistent analysis of pressure measurements very difficult but as it turns out the sensor, even under preliminary testing conditions, are very stable.

Looking at the chart above you can see the "high" measure in purple and the "low" measure in red.

Notice that while the spread between the "low" and "high" output voltage increased as more weight was applied to the device, there was never more than a 15% difference between low-mid or mid-high with may deltas in the range of 1-3% which ... in my book ... is very stable.

Preliminarily, I'm assuming that many values will be sampled from the sensor per second and that with lots of redundant measurements, I'll be able to perform some sort of noise reduction analysis, likely trimming errant high and low values and ultimately creating a very clean, predictable performance through the full range of motion of the device.

Next, I plan to improve p-alpha.1 ( ... to p-alpha.2) by firmly affixing the sensor to the device using a piece of Velcro and then running this test over as well as expanding the range of weight to 200 lbs.

Also, I'm putting in an order to Digi-Key for a low power OpAmp that I hope to power ( ... and maintain proper sensor function) using a single supply 3V coin battery.

Best to all...

Vetting OpAmp Options (Part 1)

Since one of the sensors in my project requires amplification, I've been spending a substantial amount of time getting down and dirty with various OpAmps in an effort to find one ( ... and a particular configuration) that meets all my criteria.

Most importantly, the sensor needs to be amplified enough to present a consistent and wide range of load measurement while not drawing/requiring too much power.

OpAmps generally allow for either "dual" or "single" supply configurations meaning that they require either one or two power supplies in order to operate.

By power supplies, I mean power sources which, in the case of my project, ultimately means batteries... and for this project I would like to keep both the overall power draw and number of replaceable or rechargeable batteries to an absolute minimum.

You would think this decision would be simple... just go with the "single" supply configuration and an OpAmp that can operate with a very low voltage.... but it's not quite that easy as there are many trade-offs and nuances.

First, generally speaking... ( ... remember this is all essentially new to me) "single" supply amps can deliver the similar dynamic range of amplification as the "dual" configuration but with the requirement of a higher single voltage.

For example, a "single" source amp might be able to take a single 5V supply and amplify the source signal within a range of 0 - 3V while a dual configuration might take two 2V supplies to create a range between -1.5V and +1.5V ... also a 3V total range.

Further... there is a the possibility of choosing an amp that supports "rail-to-rail" operation which allows either a "single" or "dual" configuration to generate an output voltage very near the input voltage which appears to me to be essential in a low power application.

Ultimately though, the configuration has to work for the application so the first order of business is test both the "single" supply and the "dual" supply configurations with the sensor across a range of voltages in order to establish the minimum voltage needed to make accurate ( ... and distinguishable) measurements across the desired range loads.

My wishful hope at this point is that I can make accurate measurements with an amplified single that ranges from 0V ( ... no load) to 1V ( ... many hundreds of pounds of load)... but I wont really know for sure until I capture and plot the output data.

Currently, I have the sensor in a "dual" configuration with two 1.5V supplies ( ... a tad under the recommended lower bound of voltage for this amp) and it appears to function pretty well but the proof has yet to be established so stay tuned!

Best to all...