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Anemometer input processing

A project log for Climate & Environment Monitoring Station

Create a monitoring station to measure weather, soil, seismic, solar, magnetic, and gravity conditions - focus best accuracy/dollar w/COTS.

sparks.ronsparks.ron 10/01/2018 at 17:510 Comments

Just finished concept review for the anemometer input.  As I noted, some form of input conditioning will be required (and maybe isolation?).  When I did the spreadsheet to look at the time intervals per rotation across the range of the instrument I found that a low clock speed counter is all that is required.  I also did an complex math curve fit to the calibration data from the manufacturer to allow me to look at the edge cases and what timing would be required.  What I found is that for this particular anemometer sensor the curve "bottoms out" at a smidgen above 1 mph.  But from my experience in the past I think that anything below 1.5 MPH (0.67 m/s) is unlikely to be useful.  Wear, bearing drag, dust/dirt, etc. all create a start-up drag that is unpredictable.

The sensor is also unlikely to realistically survive at much above 100 MPH, so I looked at that and 120 MPH (45-54 m/s).  When using a period counter for timing, this upper speed limits the resolution at the top end and the starting speed creates the maximum count required without roll-over.

By balancing these two factors it led to a clock speed choice of between 30-60 Hz and a 16 bit count.  This can be achieved with about $5 of logic level integrated circuits (ICs).  I found a nice clock generator with an inbuilt divider that is programmable from 0-22 bit with an internal clock.  When I put that in my spreadsheet for the anemometer, it gave optimum results with a divider of 15.  That will output a clock of 31.9824 Hz leading to a count range of 59,921 to 625 across the desired range.

I plan to couple this with a Schmitt trigger and debounce input.  Then a parallel to I2C output finishes the process.  Also I selected a D-style latch that can be used to get the time count from the start of the rotation to the direction arrow pulse.  The time between these when compared to the time between speed pulses gives the wind direction (by multiplying that ratio times 360).

EDIT:

Oops, I just realized that the direction resolution will be dependent on the minimum number of clocks per revolution.  So I need to modify and re-validate my spreadsheet.

Once the wiring schematic is roughed out I will order the parts to breadboard it.

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