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...
No comments:
Post a Comment