1. Wind speed range of 1 to 30 mph. 



2. Fast dynamic response which requires minimum 

 inertia and friction of all moving parts. 

 Frequency response to 1.5 cps . 



3. Design should be rugged, weatherproof and 

 capable of withstanding winds up to 60 mph 

 in the presence of rain and salt spray. 



The electronic circuitry in particular should 

 be stable over a wide range of environmental 

 temperature with an extreme range of -20 to 

 +60 & C. 



k. Electrical characteristics: 



(a) Minimum current drain since anemometers 

 may be battery operated. 



(b) Output should be compatible with standard 

 IRIG FM subcarrier oscillators, prefer- 

 ably to 5 volts DC. 



(c) Provision should be made for automatic 

 field calibration of all electronic 

 circuitry . 



interrupted and passed by the slots in the chopper 

 causing the photoelectric switch to alternately 

 open and close an electric circuit. 



The instrument case is fabricated from aluminum 

 tubing. The lower end plug and the case extension 

 which supports the shaft and bearing assembly, are 

 machined from aluminum and are fitted with "0" 

 ring seals, being secured to the tubular case with 

 small machine screws. The lamp housing, photo- 

 detector mount and the electronic circuit boards 

 are mounted on 3 small rods which are in turn 

 fastened to the lower end plug. A 1|— pin her- 

 metically sealed MS type electrical connector is 

 threaded into the lower end plug. The assembly 

 is finished with 2 coats of white epoxy paint 

 which not only provides corrosion protection but 

 also acts as a reflective coating to prevent the 

 electronic components from becoming overheated 

 when exposed to sunlight . 



ELECTRONIC CIRCUITRY 



Consideration of these specifications led to 

 the construction of a compact anemometer, com- 

 pletely self-contained except for a 2k volt 

 power source and field calibration equipment . 

 In order to minimize inertia and friction the 

 moving parts were reduced to the shaft which sup- 

 ports the cups and a slotted cylinder which acts 

 as a light beam interrupter. Several slots are 

 cut in the light "chopper" to give better resolu- 

 tion at low wind speeds and to enhance the dynamic 

 resolution. 



Mechanical Construction 



The mechanical layout of the anemometer is 

 shown in Fig. 1. The entire assembly is 13 inches 

 overall and the main instrument case is 2 inches 

 in diameter. The anemometer cups are press fit 

 into a hole in the upper cap. No retaining screw 

 is needed. The upper cap is locked onto a stain- 

 less steel shaft which is supported by two minia- 

 ture precision ball bearings. The upper cap, 

 which rotates with the shaft, forms the outer 

 portion of a double weather seal. There is an 

 inner cap, fixed to the case, inside of which is 

 a spinner which in turn rests on the inner race 

 of the upper bearing and which makes a loose con- 

 tact with the shaft . A second cap is locked 

 with set screws to the shaft at the lower bearing, 

 just inside, the instrument case. This is the 

 light chopper which projects downward and over 

 the lamp housing. It has 30 slots 0.01 inches 

 wide milled parallel to its axis. The lamp 

 housing consists of an aluminum tube surrounding 

 a low voltage incandescent bulb and having a 

 single slot 0.02 inches wide. A plastic ferrule 

 fastened to the lamp housing serves to mount a 

 solid state photoelectric switch. The photo- 

 switch is placed in line with the slot in the 

 lamp housing about 3/32 inch distant, while the 

 chopper rotates between the lamp housing and the 

 photodetector mount. Thus as the shaft rotates, 

 light passing the lamp housing slit is alternately 



The essential features of the electronic cir- 

 cuitry are shown in the functional diagram of 

 Fig. 2 and Fig. 3 illustrates one of many workable 

 circuits capable of performing the basic func- 

 tions. The photo-sensitive switch manufactured 

 by Solid State Products, Inc. of Salem, Mass., 

 and called a Photran, is of special interest 

 since it functions much like a switch with high 

 conductivity when exposed to light but with very 

 high resistance when dark. Once the device con- 

 ducts, however, it continues to do so until the 

 source of current is nearly spent. This accounts 

 for the high value of 1 megohm in the Photran 

 circuit . The pulses derived from the Photran 

 charging the 0.001 mfd capacitor appear on the 

 one-shot multivibrator as differentiated pulses 

 of a couple of microseconds duration. The one 

 shot shapes these into pulses of constant width 

 and amplitude, the amplitude being determined by 

 the zener diode. Since the pulses are of con- 

 stant amplitude and width and only change in fre- 

 quency, they may be integrated to produce a 

 voltage proportional to the incident wind velocity. 



Field calibration of the electronics is made 

 by extinguishing the lamp and introducing standard 

 frequency square waves which correspond to the 

 light chopper frequency for a given wind velocity. 

 The temperature environment limits of the instru- 

 ment are essentially those of normal transistor 

 tolerances when used in a one-shot multivibrator 

 and the Photran, being a silicon device, is 

 capable of equal or better performance over a 

 wide temperature range. Power consumption of the 

 instrument is approximately 170 milliamperes, 

 the rated current of the No. 313 lamp. The rest 

 of the circuitry adds very little to this drain 

 and depends somewhat on the frequency of the 

 pulses generated. 



1W 



