Clever sensors stand surrogate

April 10, 2016 // By Bill Schweber
Bill Schweber
Bill Schweber ponders the creativity of engineers in using the RLC basics of electronics to stand in for a parameter of interest.

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.