Wind Speed Monitor – How Discretely Bloweth the Wind?

INSPIRED by John Becker’s ingenious original design for measuring wind speed using ultrasonic transducers (EPE Jan ’03), but lacking his PIC programming skills, the author wondered if a similar method could be used but without a PIC. The design described here works quite well, although it does have some significant performance shortcomings compared to John’s elegant PIC-based version and it uses many more ICs.

Fundamentals

The equations associated with the discrete version are as follows, where:

d = the distance between two pairs of ultrasonic receivers and transmitters, A and B

vw = velocity of wind, blowing from B towards A

vs = velocity of sound in air

tr = response time of receivers (assumed to be the same for both)

The total apparent time for a pulse to travel from A to B against the wind is:

t2 = d/(vs + vw) + tr (2)

An up/down counter, driven by a clock frequency of f Hz, is enabled to count up during t1 and down during t2:

count up = f × tl
count down = f × t2

If C is the remaining count, then:

C = f × (t1 – t2) (C is positive as t1 >t2) Substituting for t1 and t2 from (1) and (2) gives:



But vs 2 is much greater than vw 2 so C is approx equal to f × 2d × vw/vs 2 (3)

As f, d and vs are constant, the number held in the counter will be directly proportional to the wind speed.

Calc-Free

To remove the need for any calculations,make:

f × 2d/vs 2 = 1 (4) then C = vw

Hence the number remaining in the counter will be the actual wind speed in whatever units have been chosen for d, f and vs. In practice, any convenient clock frequency can be chosen, say 500kHz, and d is then calculated.

Practical Example

For the display in MPH:
vs = 741 miles/hour

f = 500kHz = 500 × 103 count = f × 60 × 60 cycles per hour = 1.8 × 109

d = vs 2/2f from (4)

= 7412/(2 × 1.8 × 109) miles
= 1.53 × 10-4 miles
= 1.53 × 10-4 × 3 × 1760 feet
= 0.81 feet
= 9.68 inches (246mm)

The Circuit

The sequence of operation is controlled by the outputs of a decimal divider, IC7, driven by a 10Hz clock, IC14d (Fig.4). During the period of count 0 to 1, OR gate IC6b closes analogue switch IC2a and connects the first receiver, RX1 and TR1, through IC1a and IC1b (Fig.2), to the reset of one half of D-type flip-flop, IC3 (Fig.3). Resistor R4 and capacitor C3, on pin 5 of lC6b (Fig.2) ensure that its output remains high during the IC’s transition from 0 to l. At the same time, referring to Fig.1, transmitter TX1 is selected by switch IC2c and the synchronous up/down counter formed by IC10 and IC11 (Fig.5) is set to count up. At the start of count 1 from IC7 (Fig.4) in Fig.1, pin 3 of IC6a triggers a 1ms monostable, IC4, which enables the 40kHz oscillator IC5 which drives TX1, sending a pulse to receiver RX1 (Fig.2). Count 1 also clocks one half of flip-flop IC3 (Fig.3),TR1 in Fig.2 goes high and starts the up/down counter (Fig.5) via IC6c and IC14a (Fig.3).

While the pulse is travelling against the wind from TX1 to RX1, the counter is counting up. The counter is driven by a stable clock provided by the 10MHz oscillator module, IC8 (Fig.6). IC9 divides its output by 20 to give 500kHz. As IC8 and IC9 use a 5V stabilised supply (IC16), and the rest of the circuit employs 12V, an LF351 op amp, IC15, is used to restore the logic level to 12V. As the slew rate of the LF351 is inadequate for 500kHz, IC14b squares up the output for the counter. (This is a bit unconventional, but the author had a spares box of op amps!)

When the pulse arrives at RX1 (Fig.2), it is amplified by TR1 and demodulated by germanium diodes D1 and D2. The receiver has  a finite response time and as the demodulated pulse rises, it triggers IC1a and is inverted by IC1b, which resets flip-flop IC3 (Fig.3).(The response time is added to the true transit time of the pulse and would appear to cause an error. However, as shown previously, the response times of the two receivers cancel out, provided they are both the same.)

Article reproduced by permission of Wimborne Publishing. www.epemag.com