Analog Angle Blog

Capacitance sensing reveals more than just the obvious

Engineers know that sensing of many common physical phenomena such as motion starts with fundamental electrical parameters of resistance, inductance, or capacitance (and time, of course). You might think that all the opportunities had already been explored, and that's why I am impressed when one of these basic parameters is used in a clever and innovative way to measure something that previously was difficult to assess.

A recent example was detailed in IEEE Spectrum , in the article “Bee Counters: Measuring a Nest’s Occupation By Its Capacitance.” The story explained how the Analog Devices AD7746 digital-to-capacitance converter IC, Figure 1 , with resolution down to 4 aF (here, equivalent to 21 ENOB and accuracy of 4 fF was used as the core of the instrument's design — but it took much more than a capacitance sensor going directly to an associated readout.

The designers (both application engineers, with one also an amateur bee enthusiast) realized that capacitance might be a marker for hive occupancy. Further, they felt that since bees and their living cells are mostly water, the bee collective should have a detectable and meaningful capacitance signature based on the dielectric constant.

Yet nothing is simple in the real world of sensing, especially when the “objects” to be measured are moving in random fashion despite the constrained environment. This is no simple mapping of capacitance to number of bees, even after basic calibration is performed; there's no simple function that says “x pf sensed = N bees”. The team had to add many compensation and calibration factors for temperature and humidity shifts, and even used video capture to correlate the capacitance data with the actual activity to create a “learning” data set. The situation gets even more complicated since the capacitance baseline shifts as the bees bring material into the nest. There are many other dynamics to providing a final reading, such as bee motion and location. Still, it's a very nice and interesting idea that's worth instrumentation and analysis.

Capacitance is not the only basic RLC parameter that sensor ICs are addressing. Inductance, which a mysterious factor to many engineers, is actually a very useful indicator once you can “tame” it. The recently introduced DRV421 magnetic-sensing IC from Texas Instruments combines a fluxgate sensor, signal conditioning, and compensation-coil driver, Figure 2 . It provides the functions needed for magnetic-based current sensing that is well suited to motor control, renewable energy, battery charging, and power-monitoring applications.

Of course, not all sensing situations can rely on basic RLC parameters, or even basic, easily grasped physics principles such as ultrasound or X-rays. A sharp contrast is the development of now-routine magnetic resonance imaging (MRI) which derives from a deep dive into quantum physics, and subtle properties of atomic particles when stressed by magnetic field, see ” MRI: Led by Physics, Followed by Lawsuits.”

Have you ever relied on basics such as capacitance or inductive sensing to implement a sophisticated measurement?

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