The invention relates to snow presence detection.

According to the invention, there is provided snow presence detection apparatus, comprising a casing having a hollow interior and an open top which is closed by a cap presenting a convexly curved outer sensing surface, an electrically and thermally insulating seal mounted between the casing and the cap, an electrically energisable heater mounted in good thermal contact with the underside of the sensing surface but leaving clear a central portion thereof, a temperature detector mounted within the central portion of the sensing surface and in good thermal contact therewith, means making electrical connection to the heater and the temperature sensor, control means responsive to the temperature of the sensing surface as measured by the temperature sensor for carrying out a temperature-change cycle which comprises a heating time during which the heating means is electrically energised to raise the said temperature to a predetermined value and a cooling time during which the heating means is de-energised to allow the sensing surface to cool, and measuring means responsive to the heating and cooling times to compare them with predetermined values and thereby to assess whether or not snow is present on the

sensing surface and to produce a "snow present" or a "snow absence" signal accordingly.

Snow presence detection apparatus embodying the invention will now be described, by way of example only, with reference to the accompanying diagrammatic drawings in which:

Figure 1 is a diagrammatic side view of the apparatus;

Figure 1A shows a modification to the apparatus of Figure 1;

Figure 2 corresponds to Figure 1 but shows the internal wiring;

Figure 3 is a view of the underside of a cap of the apparatus;

Figure 4 is a graph for explaining the operation of the apparatus;

Figure 5 is a block circuit diagram; and

Figure 6 is a flow chart for explaining operation of the apparatus.

As shown in Figure 1, the apparatus comprises a hollow casing 5 made of any suitable material, such as metal. It is fitted with a dome-shaped cap 6 made of material such as metal with good heat-conducting properties and low specific heat such as aluminium. As shown in Figure 2 , the cap 6 is sealed to the casing 5 by means of a weather-tight seal 8 which has high thermal and electrically insulating properties. An electrically energisable heater 10 (Figure 3) is formed on the underside of the cap 6 (Figure 3 shows the heater 10 as extending over only part of the cap but in practice it extends over substantially the full extent). Electrical connections to the heater 10 are shown at 12. A temperature sensor 14 is mounted at the centre of the underside of the cap 6 and electrical connections to the temperature sensor are shown at 16. The temperature sensor 14 is bonded to the underside of the cap in a manner providing very good heat conduction and electrical insulation.

Figure 2 shows how the interior of the casing 5 incorporates a printed circuit board 18 which embodies the circuit of the apparatus, to be described in more detail below. The various connections to the printed circuit board, and the connections 12 and 16 to the heater 10 and the temperature sensor 14, are taken out of the casing

through a waterproof cable gland 20 and thence via a cable 22.

A mounting bracket 24 is shown for suitably mounting the apparatus.

In a manner to be described in more detail below, the apparatus of Figures 1 to 3 is mounted in a suitably exposed position in the area where the presence of snow is to be detected. The apparatus operates by electrically heating the cap 6 by means of the heater 10 (Figure 3) to a predetermined temperature and then de-energising the heater to allow the cap to cool. The heating and cooling times are measured - that is, the time taken to reach the predetermined temperature and the time taken to cool back towards ambient temperature. As will be explained in more detail below, these times enable an assessment to be made whether or not snow is present on the convexly domed outer surface of the cap 6.

The shape of the cap 6 is designed to facilitate the detection of snow but to prevent accumulation of water (from rain etc.) which might confuse the results. Thus, the cap has a smoothly radiussed edge 26 which is intended to prevent surface tension from retaining water on the cap.

The flared edge 28 facilitates assembly of the cap onto the casing 5. It is also intended to direct draining water away from the casing 5.

The finish of the outer surface of the cap 6 is advantageously roughened to retain a known mass of water. This assists in producing repeatable heating and cooling times.

Figure 4 illustrates the principle of operation. The vertical axis of the graph of Figure 4 represents the time required (in arbitrary units) to heat the cap 6 to the predetermined temperature which is preferably between 2°C and 3°C. The horizontal axis represents the time (in the same arbitrary units) for the heated cap to cool from the predetermined temperature to a lower predetermined temperature (e.g. 1.3 C) .

In the absence of snow on the cap 6, the heating time will be relatively short. The cooling time may also be short though not necessarily so: it will depend, in particular, on ambient temperature and wind and similar factors. For each heating and cooling cycle, therefore, the resultant plot of heating time against cooling time will lie below and to the left of the decision line 30 - that is, in the shaded area A. However, in the presence of snow, the

heating time will be significantly increased because of the need not only to heat the snow, as well as the cap, but also to change the state of the snow, that is, to melt it. The cooling time may also be increased but again this will depend on environmental conditions. Therefore, the plot of heating time against cooling time, in the presence of snow, will tend to lie above and to the right of the decision line 30, that is, in the un-shaded area B. The position of the decision line 30 is determined by calibrating and characterising the surface for no-snow conditions.

The apparatus incorporates a microprocessor and software which controls the heating and cooling cycles and assesses the results to determine whether or not snow is present on the cap.

The apparatus is substantially unaffected by wind, that is, it is substantially unaffected by the wind chill factor. In strong winds, the cap will take longer to heat to the predetermined temperature, but this is compensated for by the shorter cooling time caused by the wind.

In order to improve protection against the effect of wind in blowing snow off the cap 6, or preventing snow falling on the cap 6, a protecting ring 36 (shown partially broken

away in Figure 1A) may be mounted on the casing 5. The positioning of the ring 36 is adjusted to obtain optimum results according to the local conditions and the wind strength expected. The ring may create vortices which help to direct and retain the snow on the cap.

Figure 5 is a block circuit diagram of the apparatus.

As shown, the apparatus is controlled by a microprocessor 40. The microprocessor receives input signals from the temperature sensor 14 (Figures 2 and 3) via a signal conditioning unit 42 and an analogue to digital converter 44 which feeds signals representing the sensed temperature to a processing unit 46. Output signals for controlling the heater 10 of the apparatus (Fig. 3) are produced by the processor 46 and fed through an output block 48 to a solid state switch 50. The solid state switch 50 is connected to receive power on a line 52 from the input supply and thus energises the heater 10.

The power supply is also connected through a power conversion unit 54 and a reference voltage unit 56 to supply the microprocessor 40.

Output ports 58 of the microprocessor operate a control switch 60 which, when energised, produces a SNOW signal on

a line 62, that is, a signal indicating the presence of snow on the sensor cap 6.

A "watchdog" unit 64 carries out certain checks on the operation of the microprocessor 40 and can operate a control switch 66 via an AND gate 68 to produce a "fault" signal on a line 70. Switch 66 can also be operated directly from the microprocessor via a line 72.

All the units of the circuit diagram, including the microprocessor and the power supply circuitry, may be incorporated in the casing 5.

The operation of the apparatus will now be described in more detail with reference to the flow chart of Figure 6.

Referring to Figure 6, the first stage is the POWER ON state 100. The microprocessor now enters a WAIT state 102.

As indicated at 104, the microprocessor measures the time for which the system stays in the WAIT state. When this time (t) exceeds five minutes, the snow switch 60 is switched OFF (that is, to produce a NO SNOW or "snow absent" signal) .

In addition, while the system is in the WAIT state it

repeatedly checks whether a test "A" is or is not satisfied, as indicated at 108. Test A is a test to check whether the following logical statements is or is not satisfied:

t>D AND (T< -1.3°C OR (T< +1°C AND t>2mins))

where t is the elapsed time, D is a predetermined delay, and T is the temperature as measured by the sensor 14 (Figures 2 and 3) .

If Test A is satisfied, this indicates the existence of conditions in which snow may be present on the sensor cap 6. If the test is not satisfied, then the system remains in the WAIT state.

If Test A is satisfied, the system enters the HEAT state 110. This results in the heater 10 (Fig. 3) being switched ON, and the sensing surface 6 now begins to heat up.

In the HEAT state, the system continuously checks whether the temperature T of the sensing surface has reached +2.5°C, as indicated at 112 in Figure 6. So long as the temperature does not reach this level, the system remains in the HEAT state.

However, when the temperature has reached +2.5 C, the heater is switched OFF and the system enters the ACQUIRE state 114.

In the ACQUIRE state the system checks the time previously required (in the HEAT mode) to heat the sensing surface to +2.5 C and the time taken for the sensing surface to cool. Using the graph of Figure 4 in a manner to be explained in more detail below, the system therefore either produces a SNOW ENABLE output as indicated at 116 in Figure 6 or a SNOW INHIBIT output as indicated at 118. The SNOW ENABLE output causes the control switch 60 (Fig. 5) to be switched into the SNOW setting to indicate the presence of snow and also causes the system to enter a CLEAN state 120 (Figure 6) which causes the heater 10 to be switched ON again. During the CLEAN state, the heater remains on for 60 seconds and heats the heater surface 6 for the purpose of cleaning substantially all snow from it (ready for a further test). As shown in Figure 6 at 122, as soon as the heater has been turned on for 60 seconds in the CLEAN state, it is switched off and, after a short delay indicated at 124, the system returns to the WAIT mode.

If the SNOW INHIBIT output is produced in the ACQUIRE state instead of the SNOW ENABLE output, then, as shown in

Figure 6, the control switch 60 is turned OFF (to produce a "snow absent" signal), as well as the heater 10, and the system returns to the WAIT state after the short delay 124.

The operations carried out in the ACQUIRE state will now be considered in more detail with reference to Figure 4.

Firstly, the system determines whether the sensing surface has cooled to less than 1.3 C. If it has, the system then assesses the heating and cooling times and determines the position on the graph of Figure 4 of the resultant plot. If the heating and cooling times together produce a plot within the area B of Figure 4, the system produces a SNOW ENABLE output 116 (Figure 6). However, if the heating and cooling times produce a plot lying within the area A of Figure 4, the system produces a SNOW INHIBIT output 118.

If the ambient temperature around the apparatus is such that the sensing surface does not cool to 1.3 C, and the cooling time increases significantly, a further test is carried out to determine whether a SNOW ENABLE or a SNOW INHIBIT output is to be produced in the ACQUIRE state. Under such circumstances, it is found that simple use of the decision line 30 of Figure 4 will not necessarily produce correct results. Therefore, a further test is

needed. This further test involves a comparison of the heating and cooling times with two limit values. These are a cooling time limit value CTOUT indicated by the dotted line 130 in Figure 4 and a heating time limit HTOUT indicated by a dotted line 132 in Figure 4.

If the temperature of the sensing surface 6 is above 1.3°C and the cooling time has exceeded CTOUT, the system carries out a check on the corresponding value of the heating time. If the heating time is less than or equal to HTOUT (that is, below line 132 in Figure 4), a SNOW INHIBIT output 118 is produced. However, if the heating time is greater than HTOUT (that is, above line 132 in Figure 4), a SNOW ENABLE output 116 is produced.

The system may be programmed so that a delay is interposed before the snow indication is changed from SNOW to NO SNOW. If the system re-detects snow before the end of this delay, the intervening NO SNOW state of the indication is prevented, and this prevents fluctuation of the indication between SNOW and NO SNOW in conditions of intermittent snowfall.

The CLEAN state can also be directly entered from the HEAT state 110. If, during the HEAT state, the system determines that the heating time to reach the temperature

of +2.5°C is high (greater than 45 seconds in this example), as indicated at 124 in Figure 6, this may indicate such volume of snow present on the sensing surface that the system cannot operate normally. Therefore, the system directly enters the CLEAN state. At the same time, a counter 126 is incremented by one. At the end of the CLEAN state (when the heater has been on for 60 seconds), the heater is switched OFF and, as already explained and indicated in Figure 6, the system reverts to the WAIT state. When the system next operates normally in the HEAT state (that is, the heater is able to raise the temperature of the sensing surface 6 to +2.5 C within less than 45 seconds), the counter 126 is reset to zero, as indicated in Figure 6. If, during consecutive HEAT states, the heater is not able to raise the temperature of the surface to +2.5 C, the count in counter 126 will continue to increase. When it reaches a predetermined value, a FAIL indication is produced and control switch 66 (Figure 4) is actuated to produce a fault signal.

The microprocessor 40 may be arranged to transmit signals indicative of its current state (e.g. WAIT, HEAT, ACQUIRE or CLEAN) to a distant location for monitoring purposes (e.g. via an RS 232 link to a host computer). This is indicated in Figure 5 by the serial communications block

130

Many of the parameters affecting the operations carried out in the ACQUIRE state may be arranged to be programmable.

There can be more than one heater 10 and more than one temperature sensor 14 if required.