The ViaGard heating system available from Raychem HTS provides a polymeric self regulating heater that may be embedded in driveways, pathways or steps, for example, so that heating can be supplied to prevent the formation of ice thereon, or to melt any fallen snow. Control of the ViaGard heating system is effected by utilising the outputs from sensors that are embedded in the substrate, for example a concrete slab, that detect the temperature and any moisture present in the substrate. Thus, when the sensed temperature is detected as being below predetermined values and in the presence of moisture, the electrical supply to the heater is switched on. There are weather conditions, however, in which the response of such a system is not good enough, resulting either in the heater being switched or kept on unnecessarily, or in not being switched on soon enough.

It is one object of the present invention to provide improved heating of an outdoor substrate to prevent, or at least to alleviate, the formation of ice thereon.

In accordance with one aspect of the present invention, there is provided apparatus for preventing the formation of ice on a substrate exposed to an outdoor environment, the apparatus comprising a sensor for measuring the temperature of the ambient air around the substrate, a sensor for measuring the temperature of the substrate, and preferably a sensor for detecting the presence of moisture on the substrate, and a heater arranged to supply heat to the substrate in response to pre-arranged conditions dependent on the outputs of the said sensors.

In another aspect of the invention, the temperature and moisture sensing may be carried out on a test substrate and, under conditions conducive to the formation of ice thereon, heating may be applied to another, neighbouring substrate on which the formation of ice is to be prevented. Furthermore, detection of conditions conducive to the formation of ice on one substrate may be arranged to provide heating to two or more neighbouring substrates, which may be considered as susceptible to formation of ice thereon.

The substrate or substrates may be a pathway or driveway, especially if inclined, or steps, associated with a building, for example. The heating control of the present invention, however, may be applied to sports grounds, for example, where hot water pumped through pipes, which may be embedded in the ground, provides a preferred heater.

The output signals from the temperature sensors and from the moisture sensor may be supplied to an electronic control unit in which is stored an algorithm which has been predetermined so as to activate the heater of the apparatus under conditions regarded as conducive to the formation of ice.

In accordance with another aspect of the present invention, there is provided a method of preventing ice forming on a substrate exposed to an outdoor environment, wherein heat is applied to the substrate in an amount dependent on pre-arranged conditions derived from measurement of the temperature of the ambient air around the substrate, the temperature of the substrate, and preferably the presence of moisture on the substrate.

The processing of the sensed parameters in the control unit can thus be arranged to lead to activation of the heating of the substrate prior to the occurrence of rain or snow. This is in contrast with known ice protection systems in which under certain conditions the heating is activated only after the formation of ice ; for example, in the event of extremely cold weather, for example-15 ? C, for several days without there being any snow, the substrate can freeze down to that low temperature. Should the air temperature then rise rapidly, for example up to zero degrees within a few hours, and should rain occur, the heating of the substrate will be slow to dispel the ice because of the thermal inertia that has to be overcome.

The control unit for the heater of the present invention can be arranged to take into account not only the absolute values of temperature and moisture, but also the rate of change of temperature, for example.

The apparatus may also comprise a sensor for measuring the relative humidity of the ambient air around the substrate, wherein activation of the heater is responsive to the output of the humidity sensor. Furthermore, the apparatus may comprise a sensor for measuring the pressure of the ambient air around the substrate, and activation of the heater may then be responsive to the output of the pressure sensor.

It will be appreciated that the time and/or the power output of the heater may be controlled in response to the measured parameters and/or their rate of change.

The moisture sensor is preferably one that itself is heated, so as to avoid false output signals therefrom and advantageously the amount of heat applied thereto is arranged to be substantially equal to heat loss therefrom to its surroundings. Thus, in accordance with another aspect of the present invention, a moisture sensor is heated by means of an electrical resistance heater, for example a self regulating, or constant wattage, heater cable, in such a way as to compensate for the heat loss of the sensor to the surrounding substrate and air. The ambient temperature is sensed by a separate temperature sensor, and a proportional ambient sensing control algorithm is then used such that the heater of the moisture sensor is switched on full (100% heating) at the minimum design temperature for the associated heating apparatus, for example-20 ? C, and may be linearly reduced to zero percent (i. e. switched off) at a further predetermined temperature, for example +3 ? C. The switching of the heating element may be controlled by a triac, a solid state relay, or an electromechanical relay for example.

Weather conditions such as cold nights interspersed with warm days may give rise to unnecessary operation of the heater, that is to say, under conditions when in fact rain or snow leading to the formation of ice is not expected. Control of the heater may thus include an algorithm, whereby the requirement must also be met that the substrate has been sensed to be at less than a predetermined temperature, of say-8°C, for a minimum time, of say 24 hours, before the heater is activated.

Additionally or alternatively, control of the heater of the apparatus may be responsive to information received remotely, for example from a weather forecasting service.

Preferably, heating of the substrate is effected by a self regulating, preferably polymeric, electrical heater, which may advantageously be embedded in the substrate or other component to be heated. Electrical heating may alternatively be provided by a constant wattage heater. It is also envisaged, however, that the substrate may be heated by means of hot water, flowing through pipework.

Apparatus for, and method of, preventing the formation of ice on a substrate in an outdoor environment, in accordance with the present invention, will now be described, by way of example, with reference to the accompanying schematic electrical system diagram.

Referring to the drawing, a concrete ramped driveway 2 has a temperature sensor 4 and a moisture sensor 6 embedded therein. The outputs from the sensors 4 and 6 are supplied to an electronic control unit 8 containing a microprocessor. A temperature sensor 10 is located in the vicinity of the driveway 2 to determine the local air temperature, and the output from the sensor 10 is also supplied to the electronic control unit 8. The moisture sensor 6 has two electrodes, and a determination of the resistance therebetween is made by the control unit 8 as a measure of the presence of moisture. In order to ensure that ice, which is non-conductive and could thus give a false reading, is not formed from any moisture between the electrodes, the sensor 6 is heated by a proportional ambient sensing controlled heater 12. It will be appreciated that the heater 12 and the substrate temperature sensor 4 have to be thermally isolated from each other.

The microprocessor within the control unit 8 applies not only the absolute values of the parameters detected by the sensors 4, 6 and 10, but also the rate of change of the air temperature as detected by the sensor 10. An algorithm is stored in the microprocessor to which the measured and calculated values are applied. One suitable algorithm, which is effective independently of the presence of moisture in the substrate 2, is ; T,, ir > Tsubslrate + 2 Kelvin ? (K), AND Tsubstrate < -3 ?C.

A further suitable algorithm includes, in addition to the two criteria set out above, the additional feature that : d7'i,. dt > 2 Kelvin ? (K)/hour In the latter case, then in the event, and only in the event, that the three conditions of the algorithm are fulfilled, the control unit 8 switches on a heater 18 that is embedded in the driveway 2. The heater 18 is a polymeric self regulating heater, and is activated via an output relay 20 and a contactor 22.

A still further algorithm adds to any of the algorithms above the criterion that the sensed temperature of the pathway must have been at <-8 ? C for at least 24 hours.

An alternative algorithm may be arranged to ensure that the driveway heater 18 is always switched on EITHER when the above algorithm is fulfilled, OR whenever the moisture sensor 6 senses moisture AND when the substrate temperature sensor 4 senses a temperature of < +3°C.

The apparatus also includes an optional interface unit 14 which receives information relating to weather conditions in the geographical area in which the driveway 2 is situated, and feeds this information into the control unit 8. The control unit 8 can use the information received by the unit 14 from a weather bureau either to supplement or to override the input received from the local sensors 4, 6 and 10.

A further optional interface unit 16 can be linked into a management system of the building with which the driveway 2 is associated, for example to override operation of the heater 18 and/or 12 in certain circumstances.

The control unit 8 and the heater control signal outputted therefrom, in response to the sensors 4, 6 and 10, or the unit 14, or the unit 16, can also be used to activate heating of substrates other than the driveway 2. It is envisaged that such other substrates would also be subject to the same local weather conditions as the driveway 2, and may for example, comprise outdoor steps or pathways of an associated building. Thus, as shown in the drawing, a further heater 24 can be activated by the control unit 8 via the output relay 20 and a further contactor 26.

It will be appreciated that the formation of ice on a substrate, such as the driveway 2, will depend on the material in which the driveway is formed, and the algorithm that is set up in the microprocessor of the control unit can be arranged to take this into account.

Although mention has been made of only a single algorithm stored in the control unit 8, it is envisaged that a set of different algorithms may be stored within the unit 8, the particular one being selected by the operator in dependence on the geographical area and criticality that is required for use. Furthermore, the parameters of the or each algorithm that act as trigger values, for example the values 2 K,-3°C, 2 K/hour, 24 hours,-8 ? C, and +3°C of the algorithms exemplified above, may be adjusted to suit local operating conditions.

The moisture sensor exemplified by the sensor 6 consists of two electrodes which detect the presence of the conductive material, for example melted snow, therebetween. The electrodes have to be kept at a temperature slightly above the freezing point, since crystalline water is not electrically conductive. However, the temperature of the electrodes should not be kept so far above freezing point so as to vaporise the moisture and dry the surface of the sensor, which would lead to false results. Complex control circuits with several temperature sensors are presently used to maintain the temperature of the electrodes at a suitable level, for example as described in US-A-5418522 and DE-C- 4032734. However, in accordance with the moisture sensor of the present invention, the heating applied thereto is dependent on the heat loss to the surrounding, for example the substrate and the ambient air, taking into account the temperatures measured by the sensors 4 and 10.

The arrangements hereinbefore described could also be used for early detection of circumstances which might lead to harmful condensation in cabinets for electrical power distributions, for telecom uses, for traffic lights, for train ticket cabinets and all other electrical enclosures which are typically installed outside and for prevention of such condensation by switching on a heating device. In such uses, the sensor for measuring the temperature of the substrate might not be necessary.

It is known to remove condensation in such enclosures by means of a single point thermostat which turns a heater on at a certain set point. This has the disadvantage that condensation which is of harm to electric/electronic arrangements might have already occurred before the heater is turned on. Critical situations include A) low temperature for a long period of time, causing the electronic/electric arrangements to be at low temperature, followed by rapid increase of outdoor temperature. Due to the thermal mass of the electronic/electric arrangements their surface will be colder than the warming air and condensation might occur ; B) high temperature for long period of time causing the electronic/electric arrangements and the cabinet wall to be at high temperature, followed by rapid decrease of outdoor air temperature. Condensation might occur on inside of cabinet wall.

Using the air temperature sensor hereinbefore described according to the present invention and calculating the rate of change (dT/dt) and consequently switching the heating element on if Idt/dtI is greater than a given set point will prevent condensation before it occurs in both critical cases.

Measuring the relative humidity and its rate of change can be used to help in optimizing the decision process to switch the heater on at the right time. The temperature sensor which senses the temperature of the substrate could preferably be included and used also to assist such optimization.