r^=l FUNCTIONAL COMPONENTS 



\P1 



PHOTO 

 SWITCH 



ROTARY 



LIGHT 



CHOPPER 



TYPICAL 

 PULSES 



PULSE TRAIN 



ONE SHOT 



INTEGRATOR 



4^ 



.002 MSEC 



i Jl_ IIIIMIIIII I I I 



I MSEC 



^N^^^^^^^^s^^^^ j 



-*■ TIME 



V. CO. 



TRANSMISSION LINE 



DEMODULATOR 



6 CPP >lTER 



Fig. 2. Functional diagram of anemometer circuitry. 



in the wind tunnel which required taking rather 

 long term averages to determine mean pulse rates . 

 Average pulse rate is 19-38 pulses/sec per meter/ 

 sec or . 6U6 rev/sec per meter/sec. Since the 

 distance from the center of rotation to the out- 

 side edge of the cups is 9-0 cm > "t ne ratio of 

 cup tip speed to wind speed is O.366. This means 

 that the outside edge of a cup will travel 

 O.366 meters while the air moving past the cup 

 travels 1 meter. 



The threshold velocity for four anemometers 

 varied from 0.5 to 0-9 meter/sec. The lowest 

 threshold velocity was observed on an anemometer 

 which had "been in outdoor service for over a 

 year. 



Assuming that the cup rotation rate is 

 linearly proportional to the wind speed, as 

 indicated above, the remainder of the electronic 

 circuitry may be evaluated by substituting a 

 square wave generator for the photoswitch output. 

 The integrating circuit (Fig. 2) is an essential 



part of the anemometer circuitry since the pulse 

 output of the one-shot multivibrator must be 

 smoothed somewhat before it can be used to modu- 

 late the subcarrier oscillator (VCO). Both the 

 integrating circuit and the VCO are nonlinear 

 circuit elements in this application. The VCO 

 output frequency is normally a linear function 

 of the input voltage. However, since there is 

 considerable ripple present on the integrator 

 output voltage and because the magnitude and fre- 

 quency of the ripple also vary with the pulse 

 rate, the output frequency of the VCO is not 

 strictly proportional to the average input 

 voltage. The RC-integrating circuit is, of 

 course, inherently nonlinear. Fortunately, the 

 two effects tend to oppose one another so that 

 the frequency output of the subcarrier oscillator 

 is a linear function of the pulse repetition 

 rate with a maximum deviation from the least 

 squares straight line fit of ±0.5$ of full scale. 

 The voltage output across the integrating circuit 

 as measured with a DC voltmeter has a maximum 

 deviation of ±2$ of full scale from the least 

 squares straight line fit. 



150 



