7 nov 1983 anemometer

8/3/2019 7 Nov 1983 Anemometer

1/5

anemometer

anemometerwind speedon amoving-coilmeter The word 'anemometer' may sound a bitunusual to most people who are involved inelectronics. This is hardly surprising since itcomes from two old Greek words anemos(wind) and meter (to measure). When thetwo are combined, the result is an instrumentfamiliar to weather men the world over. Itcontains a number of rotating scoops thatcatch the wind and is used to measure thewind speed. We are not suggesting youshould set up your own weather station(that is one quick way to lose friends), but itis certainly nice to get your own idea of theweather, and the wind speed is one thing abarometer cannot tell you (no matter howhard you 'tap' it)!Before we get to the circuit of the anem-ometer; let us first see exactly what ananemometer is. As the photo of the proto-type at the heading of this article clearlyshows, it consists of a rotating (wind)millmounted in a holder. The actual mill consistsof three or four hemispheres, or somethingsimilar, which turn when caught by thewind. The speed of rotation depends, ofcourse, on the speed of the wind. Windspeed is generally stated according to theBeaufort scale. This is a system devised in1808 by Sir Francis Beaufort, an Englishadmiral, to relate the strength of the windwith the advisability of going to sea. Hedefined a twelve-way scalerangingfrom calmto hurricane force. Nowadays wind speed is

Not even meteorology is safe fromelectronics any more, it seems. Whileit is true that the rotating mechanicalelement is still an essential part of this'instrument', the bulk of the work isnow done by electronic components.The anemometer described here ismore than an instantaneous windvelocity meter, as it also stores themaximum and minimum values'measured over a certain period oftime.

often giveninunits of m/s or in knots and therelationship between the various scales isgiven in table l.The anemometer described here uses amagnet to open and close a reed switch onceper revolution of the mill. This informationcan be processed electronically so that thespeed of the wind causing this rotation canbe shown on a moving coil meter or adisplay. It is interesting to be able to see notonly what the instantaneous wind speed is,but also the maximum and minimum valuesmeasured over a certain period of time. Thisis a feature of the circuit that should appealespecially to amateur meteorologists.From wind speed to analogue voltageIn most 'cheap' (by which we mean'affordable for hobbyists') anemometers, therevolutions of the mill are converted into anumber of pulses. That can, for example, bedone with a reed switch and a magnet. Themagnet is fixed to the axle of the mill andthe reed switch is mounted firmly in the caseof the anemometer. Once every revolutionthe magnet comes close to the reed switchand this causes the contact to close. Thenumber of times the switch closes is there-fore equal to the number of revolutions ofthe mill per second. In other words, thenumber of pulses per second given by thereed switch is directly proportional to thewind speed.


2/5

It would be much easier to work with ananalogue voltaqe instead of a frequency forfurther processing of the signal. Thereforethe pulse frequency of the reed switch isfirst converted to a voltage with a small.converter circuit. This is the circuit shown infigure 1. The reed switch of the anemometeris connected between ground and the inpu tsof schmitt triggers NI ... N3. Resistor Rlensures that the inputs of these gates are '1'when the reed switch is open. Zener diodeD1rotects the inpu ts against noise thatcould be set up at the sensor or in long leads.Together with PI, R2 and CI, NI ... N3make up a monostable multivibrator. Atevery rising edge of schmitt triggersNI ... N3 a logic zero is present at theinputs of N4 ... N6. Because of the timeconstant CI/R2 + PI it takes a certain timebefore N4 ... N6 reach their upper triggeringthreshold. The pulse output from thesegates always lasts the same length of timeand this pulse is produced every time thereed switch opens. Three schmitt triggers inparallel are used here to ensure that enoughoutput current is produced.The pulse output from N4 ... N6 is sub-sequently converted to an analogue voltageby means of integrator R3/C2, and thisvoltage is buffered by lC2.The level of the analogue voltage can beadjusted using PI so that the circuit gives,for example, 1 V when the wind speed is30 m/s (this is dependent upon the type ofanemometer used). This voltage can inprinciple be applied directly to a moving coilmeter (1 V full scale deflection) or a digitalmeter.The memory sectionThe circuit shown in figure 2 is the diagramof the memory section of the anemometer.It may seem a bit complicated at first glancebut this is due to the fact that it is not easyto store an analogue value in memory for along period of time. In this case, the analoguevalue is first converted to its digital equival-ent which is stored in a counter. To find the.maximum and minimum values, the instan-taneous wind speed must constantly becompared with the previous maximum andminimum values stored in memory. For thiscomparison the digital value is first recon-verted to analogue form by means of aD/A converter.The 'memories' for the maximum and

Table 1. I ' anemometerBeaufort scale Description . Wind speed

m rs mph knots0 calm 0 ... 0.2 0 .. ,1 0 ... 11 light air 0.3 ... 1.5 1. .. 3 1 ... 32 light breeze 1.6 ... 3.3 4 ... 7 4 .. ,63 gentle breeze 3.4 ... 5.4 8".12 7 .. ' 104 moderate breeze 5.5., , 7.9 13, , . 18 11 .'. ' 165 fresh breeze 8.0 ... 10.7 19 ... 24 17 ... 216 strong breeze 10.8 ... 13.8 25, .. 31 22 ... 277 moderate gale 13.9,'.,17.1 32 ... 38 28., .338 fresh gale 17.2 ... 20.7 39 ... 46 34 ... 409 strong gale 20.8 ... 24.4 47 ... 54 41 ... 47.10 whole gale 24.5 ... 28.4 55 ... 63 48 ... 5511 storm 28.5 ' , , 32.6 64 .. ,75 56 ... 6512 hu rricane 32.6 + 75 + 65+

minimum values of wind speed are lC7 andlC8. These are dual four-bit binary counters,which can be reset by pressing push buttonS2. The clock input of each counter isprovided by a square-wave generator (Nl forlC7 and N2 for lC8) supplying a frequencyof about 200 Hz. Each generator can beswitched on or off via opamps A2 and A4.Diodes DI and D2 and resistors R9 and RIOprotect the inputs of NI and N2 fromnegative voltages (as the opamps have asymmetrical supply). The outputs of lC7 areconnected to three-state buffers, whereaslC8 uses the inverting type. The outputs ofall these buffers are connected to the inpu tsof D/A converter lC9. The oscillator aroundN3 and N4 (whose frequency is about100 Hz) defines which of the two countersis connected to the inputs of the D/Aconverter. If the output of N3 becomes logiczero the outputs of lC7 are connected tothe inputs ofIC9, and if the output of N4becomes logic zero the inverted outpu tsignals orIC8 are connected to the D/Aconverter. The buffers of the unused counterare switched to high impedance.The D/A converter gives an output ofbetween a and 1V, depending on the digitalinput signal it receives. This analogue voltage'is available at the output of opamp AS. Themaximum output voltage can be set withpotentiometer P2.The comparator section is built up aroundESI, ES2 and AI ... A4. The two electronicswitches are driven by ES3 and Tl. Theselatter two are needed to adapt the outputsignal from oscillator N3/N4 to the sym-

1

4V7400 mW N1 ... N6 = le1 = 40106

83103'

Figure 1: The measuring,section which converts thepulses generated by thewind mill into an analoguevoltage. The circuit consists,of a monostable multi-vibrator followed by anintegrator and buffer.

11-29


3/5

anemometer

5V

O'A 2 "''''t-----''CS''''---'=j''aZA" 4 N5 .. N121""Y--1~ -"lQlA s

le7 au 64520 OIS 11 11

7 RA Ol82 130383 15 ICSa...8 14 11 7 4 L S 244 /"' + -Ic + -t+ + -I-- 1 7.,,,SB:"l

N4

1 910

~ ~ I~QA 3 2 1802A 4 "N131 .. N20 16Q3A 5 6 8_

Ica


4/5


5/5

anemometer

Parts list. measuring boardResistors:R1 = 4k7R2=10k.R3 = 330 kP1 = 50 k preset.Capacitors:C1 = 100 nC2 =10 J.l/6 V tantalumSemiconductors:D1 = 4V7/400 mWzener diodeIC1 = 40106IC2 = CA 3140

Figure 5. The printedcircuit board layout forthe measuring/converterboard.

Figure 6. This is a scalewhich could be used forthe meter.6

5

and PI depend on the sensitivity of themoving coil meter used; for a 100 p.Ameter R17 is 6k8 and PI is5 k.The power supply is straightforward. Itsimply contains two voltage stabilizers anda few other components to give a sym-metrical supply of 5 V.The anemometerVarious manufacturers supply anemometers,but they are generally reluctant to supplythe mechanical part without the electronics.These are not cheap.in any case. With this in

10 15 205 \ 1 I. \ 20 25 30 35 40o 10 15 , I r I I if~ it:::~~~t~r~~notsm/s

I .

Figure 7. This is acalibration circuit givinga frequency of 50 Hz toadjust the measuringsection.11-32

25I 3050 55 II I 6010 ~

11

o~oKl 1 .0

. 1 1.

831036

7 50 Hz "'"17.8 m/s .., 0.593 V

mind we set ou t to see whether we couldbuild the mechanical part of the anemometerourselves. The design we came up withis shown in figure 3. This sort of 'd.i.y ,windmill' has the disadvantage that it is notcalibrated so that it is not possible to get anaccurate reading of the wind speed. However,it could be improved by comparing it witha 'real' anemometer, but that comes veryclose to being another 'catch 22' !The only comment about mounting thewindmill is that it should be located whereit will not be affected by 'false' winds.Calibrating the mounting stand should notbe any problem.

Constructing the electronicsThe measuring/converter and memory see-tions of the anemometer can be built on theprinted circuit boards shown in figures 4 and5. After assembling the boards, the wholecircuit, complete with switches, transformer,meter and so on, can be placed in a suitablecase. The scale for the meter is shown infigure 6.The converter section must now be adjusted.To do this, the auxiliary circuit shown infigure 6 isneeded. This produces a frequencyof 50 Hz and is connected to theinput ofthe converter section. If we want to measurewind speeds up to 30 m/s for a maximumoutput voltage of the section of 1 V, thewind speed corresponding to 50 Hz and thecorresponding theoretical output voltagefrom IC2 can be calculated for any wind-mill. Using a DVM at the output of IC2,this voltage can be set by adjusting poten-tiometer Pl.Next set the reference voltage of the D/Aconverter on the memory board. An accurate(digital) meter is needed here also. Connectthe meter to the MIN connection of S3(or the output of A3). Then press S2 andwhile it is pressed adjust P2 to give exactly1 V on the meter.Now, with S2 still pressed, adjust PI so thatthe meter of the anemometer gives exactlyfull scale deflection. The whole circuit isnow calibrated and ready for use.It can also be convenient to have twomeasuring ranges for the meter, for example,O... 10 m/s and O... 30 m/s. This canquite simply be done by using a changeoverswitch and an extra resistor and poten-tiometer, with resistances about, three timesas large as R17 and PI. The potentiometeris then set so that the meter gives full scaledeflection for an input voltage of 0.333 V(for a range of 0 ... 10 m/s),

Other applicationsThe memory circuit designed for thisanemometer is a fairly universal layout andcan easily be used for other applications,How about a thermometer with maximumand minimum memory, for example? For.this the whole memory board can be builtand only the measuring board has to bechanged for a circuit that converts ameasuredtemperature into an analogue voltage with amaximum value of 1 V. In that case, themeter has of course to be given a temperature~. "

7 nov 1983 anemometer

Documents