GB2412735A

GB2412735A - Acoustic rain sensor

Info

Publication number: GB2412735A
Application number: GB0506767A
Authority: GB
Inventors: Stephen Lindsay Russell; John Bates
Original assignee: WRC PLC
Current assignee: WRC PLC
Priority date: 2004-04-01
Filing date: 2005-04-01
Publication date: 2005-10-05
Also published as: GB0506767D0; GB0407424D0

Abstract

A precipitation (rainfall) sensor comprises a microphone 1 housed in a weatherproof enclosure 2, which also contains a printed circuit board 3. The microphone senses the impact of raindrops as they fall on the surface of the housing. By integrating the size and number of detected audio impact signals the intensity of the rainfall is determined. The enclosure 2 may be 20 to 150 mm long, 20 to 150 mm wide and 15 to 50 mm deep. The detection surface must be damped to reduce oscillations when precipitation falls on the surface, and may be ABS polymer or polystyrene. The analysis circuitry is preferably housed in the enclosure 2, and may be powered by a rechargeable battery. The sensor may transmit a signal from the analysis circuit to a remote location. The sensor finds application in detecting possible overflows in sewer plants.

Description

24 1 2735 pRECIPITATION SENSOR This invention relates to a precipitation

sensor, and, more particularly, relates to a rain sensor.

Most measurement of rainfall is closely allied to meteorology and the instrumentation for this purpose is commonly the tipping bucket type of sensor, which can totalise the rain delivered over a given time. Tipping bucket gauges are not normally used for real time applications. An area where real time detection of rainfall has great commercial importance is in the control of irrigation equipment. This application is served by a range of sensor types such as those based on expanding discs and optical sensors which detect drops on the tip of a downward pointing cone. Another application of commercial interest is that of automatic operation of motor vehicle windscreen wipers.

Typically these devices use an optical sensor.

An application which has not yet been addressed is that associated with sewage treatment and sewerage. In the sewer network and at sewage treatment plants there are overflows designed to spill sewage to a watercourse during storms, when the network and treatment plant do not have sufficient capacity. If these overflows spill during dry or mildly wet weather, it indicates a malfunction of the network or treatment plant. In order to detect this condition automatically a low cost sensor is required which responds to heavy rain, but is able to ignore damp and misty weather. The sewage treatment works may also be able to improve performance of the plant by emptying storm tanks and increasing aeration levels in activated sludge processes in advance of heavy rainfall arriving at the plant. The numerous wet wells within the sewerage network could be pumped down to their lowest levels in advance of heavy rainfall to reduce the likelihood of a spill to the environment. More advanced control systems would be able to use rainfall data to minimise the environmental impact of a storm across a river catchment, by controlling both the sewerage network and the sewage treatment plant. Electronic measurement of prolonged dry weather is also valuable information, as it is an important factor in the demand for potable water in summer, and will help Water Utilities predict and meet demand. Prolonged dry weather also generates odours from some sewage and sewage treatment plants and the knowledge can be used to trigger preventative action such as flushing or deodorant sprays. Heavy rainfall in springtime increases the risk of cryptosporidium infecting water sources, and detailed knowledge of rainfall can be used to alert treatment plant operatives to the increased risk.

Rainfall information is currently available commercially from the Meteorological Office radar system, but it is not always available immediately, the spatial resolution, typically 1 - 2 km may not be good enough to capture local rainfall events, and in the UK it is too expensive for widespread use. 1g

W002086546 describes a sensor which measures impacts of raindrops using a strain gauge or piezoelectric sensor and integrates the measured impacts to obtain a measure of rainfall intensity. This principle is very suitable for the sewage plant and sewage network application, because it has the advantage of responding immediately to rainfall without being affected by damp or mist, and it could be applied so as to ignore light rain. Its main disadvantage for the application is the cost of purchasing and assembling a strain gauge or piezoelectric device, which makes the finished device too expensive for Water Utility companies to buy in large quantities.

We have now found a much lower cost technique for measuring the impacts of raindrops. In the present invention the sensing element is a very low cost audio microphone built into a small housing. Such microphones cost less than one UK pound moreover they do not need to be bonded to a surface to register rain drops, but can be directly soldered into an electronic circuit board like any other component thus avoiding assembly costs. The microphone is able to respond to the number and size of the impacts which are detected as the rain drops land on the surface of the housing, and by integrating the size and number of the audio impact signals, an analogue for rainfall intensity is readily generated. The signal processing can use the standard analogue electronic technique of integration of charge by a capacitor with a slow current drain to discharge the capacitor. The voltage across the capacitor is then a measure of the rainfall intensity. Alternatively the amplified signal from the audio output can be sampled and digitised directly and integrated by digital processing of the waveform.

Thus, according to one aspect of the invention, there is provided a precipitation sensor comprising a housing, a precipitation detection surface, and an audio microphone disposed within the housing, wherein the microphone is arranged to detect the sound of precipitation landing on the precipitation detection surface.

It is an advantage that the precipitation detector can be quite small. Typically, we envisage that the housing has a length in the range 20 to 150 mm, preferably 30 to 120 mm. The housing may have a width in the range 20 to 150 mm, preferably 20 to 80 mm, more preferably 30 to 60 mm. The housing may have a depth in the range 15 to 50 mm, more preferably 20 to 40 mm.

The length and width of the housing define the area on which precipitation will fall, and therefore encompass the precipitation detection surface. The precipitation detection surface typically comprises the entire length and width of the device. The length and width of the device may be identical. The shape of the precipitation detection surface may be, for example, rectangular, square or circular.

We have obtained good results with a detector 120 mm in length, 60 mm in depth and mm in depth, but we envisage that good results could equally be achieved with a detector 30 mm in length, 30 mm in depth and 20 mm in depth. The device would need to be calibrated depending on the area of the precipitation detection surface, as this would affect the number of counts per time.

It is important that the precipitation detection surface is heavily damped in order to reduce oscillations when precipitation falls on the surface. This is important in order to ensure that each raindrop, for example, can be individually detected. Damping can be measured in terms of quality factor "Q". We prefer that the quality factor "Q" of the precipitation detection surface is less than 10.

We have found that the best damping characteristics can be obtained with non-metallic materials. Preferred materials are acrylonitrile-butadienestyrene (ABS) polymer or polystyrene. The entire housing may be made of these materials.

Preferably, the thickness of the precipitation detection surface is in the range 1 to 3 mm, more preferably 1.5 to 2 mm.

Preferably, the housing is a waterproof housing, and the microphone is sealed within the housing. The housing may be any suitable material, such as a metal, plastic, or a combination thereof.

In a preferred embodiment, the precipitation detection surface is an integral part of the housing. Thus, the precipitation detection surface may be the upper surface of the 1 0 housing.

In an embodiment, the sensor further comprises precipitation analysis circuitry electrically connected to the microphone. The precipitation analysis circuitry is preferably disposed within the housing.

In an embodiment, the sensor according further comprises a power supply for the precipitation analysis circuitry. Preferably, the power supply is disposed within the housing. The power supply is a battery, most preferably a rechargeable battery.

It is preferred that the sensor further comprises a means to convert solar energy or wind energy into electrical energy, and wherein the battery is recharged by an electrical output from said converted electrical energy.

The sensor may also include a transmitter adapted to transmit an electrical output from the precipitation analysis circuitry to a remote location. The transmitter may be a radio or microwave frequency transmitter, for example, or it may be an acoustic transmitter.

Alternatively, the sensor may be electrically coupled to an apparatus at a remote location by means of a conductive wire.

According to another aspect of the invention there is provided a method of measuring precipitation comprising using a precipitation sensor as described above to measure the level of precipitation landing on the precipitation detection surface, then producing an electronic signal representing the level of the precipitation.

According to another aspect of the invention there is provided a method of measuring precipitation comprising using an audio microphone to detect the sound of the precipitation landing on a nearby surface, then producing an electrical signal representative of the level of precipitation landing on the surface.

The nearby surface may be a surface on a housing within which the microphone is disposed. The precise distance of the microphone from the nearby surface is not important, as long as the distance is close enough that the microphone can accurately detect precipitation landing on the surface. Typically the distance of the microphone from the nearby detection surface will be 50 millimetres.

Preferably the method further comprises feeding an electrical output of the microphone to a precipitation analysis circuit, then producing the electrical signal representative of the level of precipitation using said analysis circuit.

Preferably the method further comprises transmitting the electrical signal representative of the level of precipitation to a remote location.

According to another aspect of the invention there is provided method for detecting possible problems with overflows in sewer plants, comprising using an audio microphone to detect the sound of the precipitation landing on a nearby surface, then producing an electrical signal representative of the level of precipitation landing on the surface, then transmitting said electrical signal to a sewage control system, which is programmed to issue an alert, or to take remedial action, in the event that the level of precipitation falls outside a predetermined range.

A set of comparative data may be provided in a computer database, this comparative data indicating the levels of measured precipitation which are high enough to cause overflow problems. The measured level of precipitation can be compared with the comparative data, and the alert issued, or remedial action taken, if the measured level is above a predetermined level of the comparative data. The alert may be issued by way of an audible and or visible alarm.

The present invention can be used to measure any sort of precipitation in which a distinct sound is generated when the precipitation lands on a surface. Thus, the invention is particularly suitable for measuring rainfall, and can distinguish rainfall from damp or mist, which are not detected by the sensor.

The above apparatus and methods can be used to generate a measure of the amount of precipitation landing in a particular location over a specified time period.

Reference is now made to the accompanying drawings, in which: Figure 1 is a schematic cross-sectional view of a precipitation sensor according to the present invention; and Figure 2 is a schematic diagram showing electrical circuitry which may be used in the precipitation sensor according to the invention.

In figure 1 the microphone (1) is housed in a small weatherproof box (2) which also contains the electronics printed circuit board (3). In many sewage treatment applications, there is no mains power available to operate the sensor and power is provided by a battery. It may be advantageous to charge the battery using a solar panel or wind generator to avoid the need to replace the battery. For sewage applications it is not necessary to have a fully continuous measurement and it is therefore advantageous to operate the rain sensor intermittently to save power, for example at intervals of one minute. The microphone can detect precipitation landing on an upper surface (2 of the box (2).

The result of the rain sensor measurement must be available for comparison with data about storm overflow spills to determine whether there is a fault in the sewer network or treatment plant. It is therefore also advantageous for the sensor to include a communications device to transmit the rain sensor measurement to a central control facility, where the comparison with overflow spills can be made, and an alarm raised to remedy the fault.

Figure 2 shows a schematic diagram of a typical rain sensor device for use in a sewer network or plant. The microphone (1) detects the sound made by the impact of the raindrops. The signal from the microphone is amplified by amplifier (7) and thresholded to avoid integrating noise using a threshold circuit (8). The amplified and noise-free series of pulses is now integrated by the integrator (9) with its drain resistor. The output of the integrator can then be used directly as a measure of rainfall intensity, or it can be further processed by the use of one or more comparators (10) and (1 1). In this example the two comparators (10) and (11) are set at different levels so that comparator (10) switches when the rainfall intensity corresponds to medium rainfall of about 5 millimetres per hour and comparator (11) switches when the output voltage of the integrator corresponds to heavy rain of about 15 millimetres per hour. The operation is controlled by a microprocessor (12), which, for example, switches on the sensor once per minute waits for 5 seconds for the integrator to stabilise, reads the two comparators (10) and (1 1) and then transmits the state of the comparators to the base station (14) using the radio or other interface. It then goes into 'sleep' mode using minimal power until the next minute, when it repeats the process of switching on the sensor and transmitting the comparator states. The base station receives data from the sewer flow and level sensors and is able to generate alarms for parts of the sewer network and treatment plants in the area which have faults.

The device can be calibrated using drops of known mass dripping from a slow rate pump on to the device lid from a height of at least 2 metros. The volumetric flow can then be related to the rainfall rate in millimetres per hour.

It will be appreciated that the invention described above may be modified.

Claims

CLAIMS: 1. A precipitation sensor comprising a housing, a precipitation

detection surface, and an audio microphone disposed within the housing, wherein the microphone is arranged to detect the sound of precipitation landing on the precipitation detection surface.
2. A precipitation sensor according to claim 1, wherein the housing has a length in the range 20 to 150 mm, preferably 30 to 120 mm.
3. A precipitation sensor according to claim 1 or 2, wherein the housing has a width in the range 20 to 150 mm, preferably 20 to 80 mm, more preferably 30 to 60 mm.
4. A precipitation sensor according to claim 1, 2 or 3, wherein the housing has a depth in the range 15 to 50 mm, more preferably 20 to 40 mm.
5. A precipitation sensor according to any preceding claim, wherein the precipitation detection surface has damping characteristics such that the quality factor "Q" of the surface is less than 10.
6. A precipitation sensor according to any preceding claim, wherein the precipitation detection surface is non-metallic.
7. A precipitation sensor according to any preceding claim, wherein the precipitation detection surface is acrylonitrile-butadiene-styrene (ABS) polymer or polystyrene.
8. A precipitation sensor according to any preceding claim, wherein the thickness of the precipitation detection surface is in the range 1 to 3 mm, preferably 1.5 to 2 mm.
9. A precipitation sensor according to any preceding claim, wherein the housing is a waterproof housing, and the microphone is sealed within the housing.
10. A precipitation sensor according to any preceding claim, wherein the precipitation detection surface is an integral part of the housing.
11. A precipitation sensor according to any preceding claim, further comprising precipitation analysis circuitry electrically connected to an electrical output of the microphone.
12. A precipitation sensor according to claim 11, wherein the precipitation analysis circuitry is disposed within the housing.
13. A precipitation sensor according to claim 11 or 12, further comprising a power supply for the precipitation analysis circuitry.
14. A precipitation sensor according to claim 13, wherein the power supply is disposed within the housing.
15. A precipitation sensor according to claim 13 or 14, wherein the power supply is a battery.
16. A precipitation sensor according to claim 15, wherein the battery is rechargeable.
17. A precipitation sensor according to claim 16, further comprising a means to convert solar energy or wind energy into electrical energy, and wherein the battery is recharged by an electrical output from said converted electrical energy.
18. A precipitation sensor according to any one of claims 12 to 17, further comprising a transmitter adapted to transmit an electrical output from the precipitation analysis circuitry to a remote location.
19. A method of measuring precipitation comprising using a precipitation sensor according to any preceding claim to measure the level of precipitation landing on the precipitation detection surface, then producing an electronic signal representing the level of the precipitation.
20. A method of measuring precipitation comprising using an audio microphone to detect the sound of the precipitation landing on a nearby surface, then producing an electrical signal representative of the level of precipitation landing on the surface.
21. A method according to claim 20, further comprising feeding an electrical output of the microphone to a precipitation analysis circuit, then producing the electrical signal representative of the level of precipitation using said analysis circuit.
22. A method according to claim 20 or 21, further comprising transmitting the electrical signal representative of the level of precipitation to a remote location.
23. A method for detecting possible problems with overflows in sewer plants, comprising using an audio microphone to detect the sound of the precipitation landing on a nearby surface, then producing an electrical signal representative of the level of precipitation landing on the surface, then transmitting said electrical signal to a sewage control system, which is programmed to issue an alert, or to take remedial action, in the event that the level of precipitation falls outside a predetermined range.
24. A precipitation sensor substantially as herein described with reference to and as shown in the accompanying drawings.
25. A method of measuring precipitation substantially as herein described with reference to and as shown in the accompanying drawings.
26. A method for detecting possible problems with overflows in sewer plants substantially as herein described with reference to and as shown in the accompanying drawings.