GB2203246A - Wind measurement - Google Patents

Abstract

A device for measuring wind speed and direction comprises four identical tubes (12a-12d) equispaced about a solid column (10). Each tube (12) is open at the top, and is closed at the bottom by a film (14) of a polymeric piezoelectric material. Wind blowing across the open end induces Helmholz resonance of the air column within the tube (12) and hence movement of film (14) at the bottom. The average voltage output of the four films (14) is a function of wind speed, and the relationship of the output amplitudes from the several films (14) indicates wind direction. <IMAGE>

Description

"Wind measurement " This invention relates to the measurement of the speed, and preferably also the direction, of wind.

Wind speed is conventionally measured by a cup anemometer driving a tachogenerator or a digital encoder to produce a signal representative of wind speed. Wind direction is most commonly measured by a rotatable vane coupled to a digital encoder or a potentiometer. Such instruments rely on moving parts which must be accurately constructed and well maintained if accurate results are to be produced but the locations of use (e.g. in remote weather stations or at the masthead of sailing vessels) militate against ease of maintainance and often constitute an environment hostile to delicate moving parts.

An object of the present invention, which is defined in the appended claims, is accordingly to provide a means of measuring wind speed, and in preferred forms also wind direction, in a completely solid-state manner.

Embodiments of the invention will now be described, by way of example only, with reference to the drawings, in which: Fig. 1 is a perspective view of a wind speed and direction sensor embodying the invention; Fig. 2 is a plan view of the sensor of Fig. 1; Fig. 3 illustrates a modification of the sensor of Fig. 1; and Fig. 4 is a circuit diagram schematically illustrating suitableelectronics for us with the sensor.

Referring to Figs. 1 and 2, the sensor comprises a cylindrical post 10 around which, equally spaced, are secured four tubes 12a - 12d. The tubes 12 have their upper ends open and their lower ends closed by diaphragms to be described.

The post 10 extends above the open ends of the tubes 12 for a relatively large distance, at least several times the diameter D1 of the tubes 12. The post 10 has a diameter D2 1 equal to or greater than the tube diameter D1.

I The lower ends of the tubes 12 are closed by respective diaphragms 14 of a polymeric piezoelectric film such as polyvinylidene fluoride (PVDF); one suitable material is KYNAR (trade mark) polarized PVDF film by Pennwalt Corporation.

When wind passes across the open end of a tube 12, the air column within the tube is excited into an open Helmholz resonance such that the resonant amplitude is proportional to the square root of the wind velocity. The tube will resonate most readily if its length L is related to its diameter D1 such that L:D1 =8:1 (or 8n :1 where n is an integer). It has been found, however, that in the present instrument a ratio of 7:1 is preferred.

The resonance of the air column causes sinusoidal stretching of the PVDF membrane 14. The membranes 14 are each provided with electrodes on their opposed surfaces, as is well known per Se, and thus the piezo action of the membrane material causes the production of an a.c. signal whose amplitude is a function of the wind speed across the open mouth of the tube.

In the sensor of Fig. 1, the post 10 will influence the air speed across at least some of the tubes for any given wind direction. A tube directly downwind of the post 10 will receive a small amount of energy because of turbulence in the shadow of the post 10. For example, if the wind is in direction A, energy input to the tube 12b will be low, to 12d somewhat higher, and that to 12a and 12c higher still and equal. If the wind is in direction B, tubes 12b and 12c will have equal air speeds, while 12a and 12e will have somewhat lesser equal speeds. In general, the output signal from each tube is a function of the proportion of the open end of the tube exposed to uninterrupted airflow. The corresponding output signals can be processed to derive signals representing both wind speed and direction.

The embodiment of Figs. 1 and 2 uses a PVDF diaphragm which is driven directly by the resonant column of air. Fig. 3 illustrates a modification in which the tube 12 has a solid, integral end 12' and a PVDF film 14' is secured to the exterior of the tube 12. In this modification, the air resonance within the tube is transmitted to the PVDF film 14' via the tube wall, but otherwise the operation is as above.

Fig. 4 illustrates the processing of the signals from the membranes 14. All four outputs are summed at 20 to give a signal Us which is a function of the square root of the wind speed. The outputs of opposite films are differenced at 22 and 24 to give outputs Vx and Vy representing the X and Y components (see Fig. 2) of wind direction. It may also be desired to provide means for calibrating the device to compensate for environmental variations; for example, PVDF is highly temperature sensitive.

For this purpose, each film 14 is laminated with a similar film 26 connected to a signal generator 28. Each pair of films 14 and 26 may, as indicated, share a common intermediate electrode which is grounded. The signal generator 28 is arranged to periodically apply a standard signal, e.g. a tone burst, to the films 26 and circuitry (not shown) is provided to measure the resulting voltages produced by the films 14 to provide a calibration factor. Suitable circuitry for holding the periodic measurements and using these to proportionally modify the wind output signals will be apparent to those skilled in the art.

In one exemplary embodiment, the tubes 12'are of aluminium with an outer diameter of 12.7 mm, inside diameter of 11.0 mm, and length 88 mm. PVDF films 2 cm square on the sides of the tubes give a sensitivity of 1 mV/knot, and the PVDF diaphragm alternative gives a sensitivity of 5 mV/knot (both sensitivities approximate), with the relationship between voltage and the square root of wind speed being very close to linear.

It is desirable that the resonant "whistle" be an imperfect resonance. As discussed above, this may in most circumstances be achieved by choice of tube length. For very high wind speeds, additional means may be desirable to disturb the airflow across the end of the tube, for example a hole drilled diametrically through the tube adjacent the top, or a bar fixed across the open end of the tube. These measures obviate an abrupt change in frequency as the tube goes into resonance, which might otherwise occur.

Modifications may of course be made to the above embodiments within the scope of the invention. The invnetion includes the use of a single tube as a wind speed measuring instrument without measurement of wind direction. Direction could be sensed by the use of three tubes rather than four, or by a greater number of tubes, but at the expense of more complex electronics. It may be useful to provide, e.g. in the form of a plate, to direct the airflow at right angles across the top of the tubes, for instance when the instrument is to be mounted on the masthead of a sailing vessel.

Claims (10)

1. An instrument for measuring wind speed, comprising a tube having an open end and a closed end, the tube being so dimensioned that wind blowing across said open end induces Helmholz resonance of the air column within the -tube, and a piezoelectric element mounted on the tube to be coupled mechanically with said resonant air column.

2. The instrument of claim 1, in which the piezoelectric element comprises a polymeric piezoelectric film.

3. The instrument of claim 2, in which said film is secured to the sidewall of tne tube.

4. The instrument of claim 2, in which said film is in the form of a diaphragm closing said closed end of the tube.

5. The instrument of any of claims 2 to 4, including means for applying a predetermined mechanical force to said film, and means for measuring the resulting electrical output to provide a calibration signal.

6. The instrument of claim 5, in which said force-applying means comprises a further polymeric piezoelectric film secured to said first-mentioned film and connected to a signal generator.

7. The instrument of any of claims 2 to 5, in which said film is of polyvinylidene fluoride.

8. An instrument for measuring wind speed and direction, comprising at least three instruments in accordance with any preceding claim disposed around a central member which extends above the open ends of the tubes by a distance which is at least an order of magnitude greater than the tube diameter.

9. A method of measuring wind speed, comprising causing the wind to be measured to induce Helmholz resonance 2 an open ended tube, and measuring the amplitude of said resonance.

10. A method of measuring wind speed and direction, comprising causing the wind to be measured to flow past a solid object around which are disposed at least three open ended tubes to cause Helmholz resonance to be induced in at least one of said tubes, measuring the amplitudes of said resonances, and deriving wind speed from the sum of said amplitudes and wind direction from the differences in amplitude between the tubes.