Sonic Anemometer
Introduction: Sonic Anemometer
Most of the sensors in a weather station now use miniature electronic components to measure the environment. Light, pressure, temperature and humidity can all be quantified with sensors costing a few cents. The one last remaining element is wind speed and direction. Over the last few centuries, the rotating cup anemometer has reigned supreme. Three or four cups spin in the breeze to give an indication of air speed, granted there have been improvements in detecting the spin speed but the basic principal remains the same.
Wind speed may also be measured by detecting the change in the speed of sound caused by the air flow. The earliest attempt I found of applying this method comes from this article in Electronics, 1950. There are now appearing on the market, at a cost of hundreds or thousands of dollars, ultrasonic anemometers that measure wind using similar principles. The challenge of this Instructable was to make a sonic anemometer using hobbyist components costing only a few dollars
Supplies
The main components needed to make the ultrasonic anemometer are:
4 transducers removed from HC-SR04 ultrasonic range finders
3 CD4046B phase lock loop ICs
1 Piece of strip board
Various resistors and capacitors, see circuit diagram
1 2N2222 transistor or similar
1 Arduino Uno or similar
Arduino IDE for flashing and monitoring wind
Wood to make anemometer body
Sundries like wire, solder, screws, wood etc.<
Step 1: Wind Speeds
Understanding the expected range of wind speeds is a good place to start. The Beaufort scale in the picture tells us that average wind speeds can reach over 30 m/s although short term gusts can be higher. As the velocity of sound is 343 m/s, we can expect changes of about 10% in speed due to high winds.
Step 2: Proposed Design
The design for this anemometer is based on 3 ultrasonic receivers symmetrically arranged around a transmitter transducer. Sound waves from the 40 kHz transmitter radiate outwards to the 3 receivers 25 mm away. Under still air conditions, the sound waves will reach all the receivers at the same time.
The speed of sound increases at higher temperatures which makes the sound waves spread out faster and so there will be fewer waves between the transducers. As this design relies on a fixed number of waves between the transducers, the north receiver is phase locked to the sender. Any change in temperature will alter the sender frequency to keep the wavelength the same.
Step 3: How Does It Work
In calm conditions with no draughts, all three receivers will be experiencing the same part of the sound wave front from the central transmitter. 134
The pictures show what will happen with a north or west wind. A north wind will compress the sound waves on the north side of the sender and spread them out on the south side. However, the phase lock loop will keep the phase constant on the north receiver. Thus only the two southerly sensors will show a phase change related to the wind strength.
A similar situation happens for a westerly wind. The sound waves are compressed on the west side of the transmitter and expand out to the east. With a phase locked north sensor, we can estimate the wind speed from the difference between the east and west sensor phase.
By measuring the change in wavelength or phase at the E and W sensors, we can work out the north and west components of wind velocity. With a little arithmetic, we can then calculate the wind speed and heading.
Step 4: Construction
The body of the anemometer is made from two pieces of wood held apart with 3 mm bolts. A piece of 18 mm thick fibre board was used for the main body of the anemometer with 16 mm holes drilled for the sensors. The reflector plate was a piece of 4 mm plywood and fixed above the sensors with 3 mm nuts and bolts. Nothing rocket science, other materials could be used instead provided they are rigid and don’t bend in the wind. A hexagonal shape was used for the body because it is easier to cut than a circle! In fact, a circular design with rounded edges would present a much better aerodynamic profile to the wind... beyond my skill set.
A conical lid was added to protect the electronics from the weather and dissuade birds from using it as a perch.
Step 5: Transducers
As these sensors are run continuously, there is no need for any special dynamic performance. In fact, the sensors used in this project were unsoldered from the cheap and cheerful HC-SR04 range finder widely available. The 4 transducers are push fitted into the holes and connecting leads soldered on to the terminals. As the exact position may need a slight adjustment to tune the system, ream the holes with sand paper to give a firm but not tight fit. 226