I’ve been thinking about creating my own high pressure sensor, simply because the weight of commercial high pressure sensors is too large to be practical for the bike shock “chamber” I’m creating. I was going to build a capacitive sensor based on the idea that at high pressures the permittivity of air varies not insignificantly. My main concern there is current fluctuation over tiny time intervals. This might kill the idea, since I need capacitance readings +/- 20% over 20-50 millisecond intervals. This may be too short. So I’ve come up with something different I’d like to pass by you. It would be a circuit measuring differential current between two capacitors. It would essentially be measuring the magnitude of fluctuation of current. I think this could work well to at least measure pressure gradients accurately.
3 thoughts on “Ultra-fast dual capacitor high pressure sensor ”
Question: What will cause “noisy” capacitance fluctuations for the case of a single capacitor?
1. viscosity/turbulence
2. uneven heating
3. non-equilibrium conditions. (i.e. many body systems out of equilibrium rarely achieve equilibrium in efficient “least-path” ways. consider random walk paths toward lower energy states)
Now, consider the case of dual capacitors positioned asymmetrically as shown below …
By measuring differences in capacitance of C1 and C2, we are now taking advantage of the fluctuations rather than trying to mask them or work around them. Furthermore, there is another and likely even MORE significant cause of capacitance fluctuation between the two: that due to the different positions relative to the openings. So the list now looks like this …
1. pressure variations in time due to asymmetry of caps’s distance from chamber openings
2. viscosity/turbulence
3. uneven heating
4. non-equilibrium conditions.
For each of the above cases, the effect is amplified relative to average pressures. For example, for large nominal pressure occurring near maximum compression, each of the above 1 through 4 contribute a larger fluctuation relative to nominal pressure nearer to minimum compression.
More importantly, there are two significant advantages to the dual caps design …
1. much lower power is required since DC voltage operation is possible resulting in almost no current flow except for intervals of significant compression and significant pressure gradients.
2. the dual caps design can quickly determine when pressure inflection occurs. This is MOST significant, since a primary design goal is that actuators move as soon as pressure decreases from the maximum.
Back to the question: “why are relatively small current fluctuations an issue for the single capacitor design”
Answer: the current fluctuations are NOT small.
permittivity of air varies from about 1.005 to 1.04. In other words, over the largest pressure variance, permittivity varies less than 5%. Those are for the LARGEST swings. For nominal variance 2% is a peak. How to ensure that current fluctuations due to fluctuations in 1) turbulence/viscosity, 2) heat transfer and 3) other non-eq conditions … how to ensure that these fluctuations are well below 2% of nominal values of current flow? This is my concern. Too much uncertainty with the single cap design.
Excellent to find that servo technology is fast and light enough for my needs. This servo moves at 20 degrees in approx. 25 ms and weighs 20 gms.
Turnigy S306G-HV
Ridiculous