WO2006123977A1

WO2006123977A1 - A method for detecting ice on the surface of a fixed structure, a device for performing the method and a fixed structure provided with a device for detecting ice

Info

Publication number: WO2006123977A1
Application number: PCT/SE2005/000748
Authority: WO
Inventors: Per SIKSTRÖM; Mattias Karlsson; Dennis Karlsson; Niclas Johansson
Original assignee: Fasitet Trollhättan Aktiebolag
Priority date: 2005-05-18
Filing date: 2005-05-18
Publication date: 2006-11-23

Abstract

The invention relates to a method for detecting ice on fixed structures (1). The method comprises the steps of placing at least one sensor (4) at a distance from a surface (5) of the fixed structure (1), determining the distance from the at least one sensor (4) to the surface (5) of the fixed structure (1) and subsequently using the at least one sensor (4) to determine whether the distance (A) to the surface (5) of the fixed structure (1) has changed. The method also relates to a device for carrying out the method and to a fixed structure having equipment for detecting ice.

Description

AMETHOD FORDETECTINGICEONTHE SURFACEOFAFIXED STRUCTURE₅ A DEVICEFORPERFORMINGTHEMETHOD ANDAFIXED STRUCTURE PROVIDED WITHADEVICEFORDETECTINGICE

BACKGROUND OF THE INVENTION

In cold weather, for example during wintertime, layers of ice may be formed on fixed structures such as buildings and bridges. Ice that has formed on such structures usually falls down sooner or later. This may cause injury to by-passers and to property. As an example, it can be mentioned that, in the year 2000, a suspension bridge in Uddevalla, Sweden, had to be closed for traffic during four days because of ice that was falling down from the cables of the suspension bridge. Experience has showed that the ice falling from the cables of a suspension bridge can actually be capable of causing serious damage to cars, for example by crushing the windshield. In order to prevent falling ice from causing damage, various methods for de-icing can be employed to remove ice before it has become thick enough to cause damage when falling down. For example, it has been suggested that cables of suspension bridges may be provided with equipment for heating a surface layer of the cable in order to cause ice formed on the cable to fall down. If the ice is caused to fall down when the ice layer is still relatively thin, experience suggests that the falling ice will not cause significant damage to property or significant injury to humans or animals. For example, US patent publication 2003/0155467 in the name of Victor Petrenko discloses "Systems and methods for modifying an ice-to object interface" that are described as being applicable to bridges and cables. A method for modifying ice adhesion strength is also disclosed in US patent No. 6027075 to Victor F. Petrenko. If such methods for de-icing are to be used, it is desirable to know when ice has formed and how thick the ice is. Such knowledge can be used to decide if de-icing is to be initiated. If de-icing methods are not used, it is still useful to know if ice has formed. Such knowledge may be used, for example, to close a bridge from traffic already before an accident has occurred.

Accordingly, it is an object of the present invention to provide a method for detecting ice on the surface of a fixed structure such as a bridge, for example a suspension bridge. The method should be useful for detecting both ice and snow. It should preferably be capable of being applied to any kind of surface, including for example metal surfaces, plastic surfaces or concrete surfaces. It is desirable that the method be capable of not only detecting the presence of ice but also to determine the thickness of the ice layer. Moreover, the method should preferably be a method that does not require large amounts of maintenance/service on the equipment that is used.

It is also an object of the invention to provide a device for detecting ice and snow on fixed structures and a fixed structure having equipment suitable for detecting ice. It is a further object of the invention to provide a method for providing ice detection on a fixed structure.

DESCRIPTION OF THE INVENTION The invention relates to a method for detecting ice on fixed structures. The method comprises the steps of placing at least one distance sensor at a distance from a surface of the fixed structure, determining the distance from the at least one sensor to the surface of the fixed structure and subsequently using the at least one sensor to determine whether the distance to the surface of the fixed structure has changed.

In preferred embodiments of the invention, the at least one distance sensor uses ultrasound to determine the distance to the surface of the fixed structure. The distance sensor may be placed at an initial distance of, for example, 200mm - 1300mm from the surface of the fixed structure. The choice of distance may be dependent on the choice of distance sensor. For some sensors, the distance could very well be less than 200 mm or more than 1300 mm. The distance sensor may take samples of the distance to the surface of the fixed structure during an extended period of time. The samples can be used to calculate an average value for the distance between the at least one distance sensor and the surface of the fixed structure.

In one embodiment of the inventive method, sample values outside a predetermined range are ignored when the average value is calculated.

In a preferred embodiment, a plurality of sensors may be placed around a circumference of an element of the fixed structure. Different distance sensors may then measure the distance to the surface of that element at different points of time. In the inventive method, a measurement of ambient temperature can be performed. The temperature can then be taken into account when the distance to the surface of the fixed structure is determined.

The invention also relates to a device for carrying out the inventive ice detection method. The inventive device comprises a fastening element for fastening the ice detection device on a fixed structure, at least one supporting arm connected to the fastening element and extending away from the fastening element and at least one distance sensor on the supporting arm. The distance sensor is placed at a distance from the fastening element and the distance sensor is preferably an ultrasonic sensor. In one embodiment, the fastening element is shaped to grip around the circumference of an element belonging to a fixed structure. The device may have only one single distance sensor. However, in preferred embodiments, the device comprises a plurality of distance sensors distributed around the circumference of the fastening element. Each sensor may then be placed on a supporting arm and at a distance from the fastening element.

In one embodiment, the distance sensors are connected to a control unit. The control unit may be arranged to activate the sensors according to a sequence such that adjacent sensors are not active simultaneously. Optionally, the device may be provided with electrical heating elements.

In preferred embodiments, at least a part of the device may be covered with spikes to discourage birds from landing on the device.

The invention also relates to a fixed structure. The fixed structure may be any kind of fixed structure. In particular, the fixed structure may be a suspension bridge. However, the fixed structure could conceivably be some other kind of bridge or a building such as a multistory building. The fixed structure is provided with at least one distance sensor arranged at a distance from a surface on the fixed structure and arranged to detect the distance from the sensor to the surface of the fixed structure. The distance sensor could be any kind of sensor. For example, it can be an optical sensor using a laser beam. However, in preferred embodiments, the at least one distance sensor is an ultrasonic sensor. The fixed structure may have only one distance sensor. However, in preferred embodiments, the fixed structure comprises a plurality of distance sensors. If a plurality of distance sensors is used, the sensors can be arranged around the circumference of an element of the fixed structure. In particular, they can be arranged around an element such as a cable on a suspension bridge. If the distance sensors are ultrasonic sensors, they can then be arranged to be activated in a sequence such that adjacent ultrasonic sensors are not active simultaneously. If adjacent ultrasonic sensors are not active at the same time, they are thereby prevented from interfering with each other.

According to an embodiment of the invention, the fixed structure comprises at least one temperature sensor and a control unit. The control unit may be connected to the at least one temperature sensor and the at least one distance sensor and arranged to determine the distance from the distance sensor to the surface of the fixed structure based on a signal from both the distance sensor and a signal from the temperature sensor. This is useful in particular if the distance sensor is an ultrasonic sensor since the speed of sound through air varies with the temperature. The fixed structure may in particular comprise a plurality of distance sensors placed at different vertical levels and at least two temperature sensors placed at different vertical levels. With knowledge of the vertical position of each ultrasonic distance sensor and each temperature sensor, suitable temperature compensation can be calculated for each ultrasonic distance sensor.

The fixed structure may also comprise a camera arranged to give a picture of a part of the fixed structure where one or several distance sensors are placed.

The invention also relates to a method of providing ice detection on a fixed structure. The method for providing ice detection comprises placing a distance sensor at a distance from a surface on the fixed structure such that the distance sensor can subsequently be used to determine whether the distance to the surface of the fixed structure has changed.

DESCRIPTION OF THE DRAWINGS

Fig. 1 shows a suspension bridge on which the invention can be applied.

Fig. 2 shows a device for detecting ice. In Fig. 2, the device is applied on a cable of a suspension bridge. Fig. 3 is a view similar to Fig. 2 but also showing a layer of ice formed on the cable. Fig. 4 is a partly cross-sectional and partly schematic illustration showing how the invention can be applied to an element of a fixed structure, for example a cable on a suspension bridge.

Fig. 5 is a perspective view corresponding to Fig. 4. Fig. 6 shows, in perspective, a part of a device that may be used for the inventive method.

Fig. 7 shows, in perspective, some further details of a device that may be used for detecting ice.

Fig. 8 shows in greater detail the suspension bridge of Fig. 1.

DETAILED DESCRIPTION OF THE INVENTION

With reference to Fig. 1 and to Fig. 8, the invention relates to a method for ice detection on fixed structures such as a suspension bridge. Fig. 1 shows a suspension bridge 1 that comprises pylons 2 and suspension cables 3. In cold periods, ice may form on the bridge 1 and in particular on the pylons 2 and the cables 3. Ice that has formed on for example the cables 3 can fall down and cause damage and injury. For example, falling ice can cause damage to cars crossing the bridge 1. Of course, ice can also be formed on the pylons 2 and fall down from the pylons 2.

To explain the inventive method, reference will now be made to Fig. 2 and to Fig. 3. In Fig. 2, a distance sensor 4 is shown arranged on a distance A from the surface 5 of an element 3 that is part of a fixed structure 1. In Fig. 2, the distance sensor 4 is shown as being arranged at the end of an arm 9 that is connected to a fastening element 8 that grips the element 3 that is a part of the fixed structure. In Fig. 2, the element 3 that forms a part of the fixed structure is a cable 3 on a suspension bridge 1. The exact distance A from the distance sensor 4 to the element 3 can be determined, for example by using the distance sensor 4. In Fig. 2, it is assumed that the cable 3 is free from ice. hi Fig. 2, the surface 5 of the cable 3 is thus the free surface (the ice-free surface) of the cable 3. In Fig. 3, a layer of ice E has formed on the cable 3. Since the cable 3 is now covered by ice, the surface 5 of the cable 3 will now be the surface 5 of the layer of ice E. When ice has formed on the cable 3, subsequent use of the distance sensor 4 can reveal that the distance A between the distance sensor 4 and the surface 5 of the cable 3 has changed and in particular if the distance has decreased. If the distance has decreased, this indicates that a layer of ice E has formed on the cable 3. Moreover, it is possible to determine exactly how thick the layer of ice is. This information can be used to determine whether a de-icing sequence should be initiated. In principle, the at least one sensor 4 may be any kind of sensor. For example, it could be a laser distance sensor. However, in preferred embodiments, the sensor 4 is an ultrasonic sensor 4 that uses ultrasound to determine the distance to the surface 5 of the fixed structure 1. Compared to optical sensors, ultrasonic sensors are less sensitive to disturbances caused by dirt and by salt. It must be expected that the distance sensors 4 may become covered by various substances, for example salt. This can disturb the operation of an optical sensor but an ultrasonic sensor can still give quite reliable readings even if it is covered by a thin layer of salt or dirt that could interfere with the operation of an optical sensor. Therefore, an ultrasonic distance sensor can operate during longer periods without maintenance.

An ultrasonic sensor suitable for this purpose can be obtained from, for example, EGE- Elektronik Spezial Sensoren GmbH, Ravensburg 34, D-24214 Gettorf, Germany. EGE- Elektronik Spezial Sensoren GmbH offers an ultrasonic sensor under the designation AGKU 1500 GI. The inventor of the present invention has found that this sensor is a suitable choice of sensor although it should be understood that also other sensors could be used.

For some ultrasonic sensors, it may be suitable to place the sensor at an initial distance of 200mm - 1300mm from the surface 5 of the fixed structure 1. For at least some ultrasonic distance sensors, the distance measurements can be made more accurate when distance between the sensor 4 and the surface 5 of the fixed structure lies in the range of 200 mm - 1300 mm. It should be understood that the choice of distance to the surface 5 of the fixed structure may depend on the exact choice of sensor.

The distance sensor 4 my conceivably be constantly active. However, it is preferred that the sensor 4 takes samples of the distance to the surface 5 of the fixed structure 1 during an extended period of time and where the samples are used to calculate an average value for the distance between the at least one sensor 4 and the surface 5 of the fixed structure 1. The term "average value" as used herein should be understood as referring to a calculated value for the thickness of the ice. The calculation can be performed by a control unit 13 connected to one or several distance sensors 4. As symbolically indicated in Fig. 2, the sensor 4 may be connected to the control unit 13 by a wire W. Of course, the connection could also be a wireless connection. The control unit 13 may be, for example, a computer 13. It should be understood that some measurement values may be incorrect for various reasons. For example, snowfall can interfere with the measurements and give a temporary reading that indicates that the distance A to the surface 5 of the fixed structure has decreased dramatically in a very short time. For a variety of reasons, individual samples can give incorrect indications that either exaggerate or underestimate the distance A to the surface 5 of the fixed structure. For this reason, a computer 13 that is used to calculate an average value (calculated ice thickness) may be programmed to ignore sample values outside a predetermined range. The same principle of ignoring values outside a predetermined range can be applied also if the average is calculated without the use of a computer. To further reduce the risk of false indications, it may also be possible to simply ignore a fixed percentage of the highest readings and the lowest readings. For example, if the control unit 13 receives 80 readings from a distance sensor 4, the control unit could be programmed to ignore the 30 highest values and the 20 lowest values (or the 20 highest and the 30 lowest, the 20 highest and the 20 lowest etc.). In this way, the risk of false alarms can be reduced.

As indicated in Fig. 4 and Fig. 5, a plurality of sensors 4 can be placed around the circumference of an element 3 of the fixed structure 1. For example, a plurality of ultrasonic distance sensors 4 can be placed around the circumference of a suspension bridge cable 3. The diameter of such a cable may typically be in the range of 160 mm - 250 mm although other dimensions are also possible. Preferably, different sensors 4 will then measure the distance A to the surface 5 of the element 3 at different points of time. One way of achieving this is to connect the distance sensors 4 to a control unit 13 that is arranged to activate the sensors 4 according to a sequence such that adjacent sensors 4 are not active simultaneously. As indicated above, the control unit 13 may be a computer 13. The computer 13 may be programmed to activate the sensors 4 according to a predetermined sequence. If the sensors 4 are controlled in such a way that adjacent sensors 4 are not activated simultaneously, the risk that adjacent sensors interfere with each other can be reduced or eliminated. When several sensors 4 are used, the measured average value can be made more accurate. It should be understood that an average value for the sample measurements can be based on readings from both one single sensor and a plurality of sensors. Conceivably, an average can be calculated by the computer 13 based on readings from several sensors during a certain period. It is also conceivable that an average value for each separate sensor 4 is calculated and the average values from several sensors 4 combined with each other.

Reference will now be made to Fig. 8. As symbolically indicated in Fig. 8, temperature sensors 6, 7 may be placed on the fixed structure 1. The temperature sensors 6, 7 are used to perform a measurement of the ambient temperature. The temperature can then be taken into account when the distance from a sensor 4 to the surface 5 of the fixed structure 1 is determined (e.g. a distance from the sensor 4 to the surface 5 of a cable 3). Since the ambient temperature affects the speed of sound, the temperature will also affect the measured value of the distance from an ultrasonic distance sensor 4 to the surface 5 of the fixed structure. The temperature sensors 6, 7 can be connected to the control unit 13 and the control unit 13 may be programmed to make a compensation for the temperature when ultrasonic sensors 4 are used. In principle, it is possible to use only one temperature sensor 6, 7. However, if there is more than just one temperature sensor 6, 7, the temperature sensors 6, 7 can be placed at different vertical levels. In Fig. 8, one temperature sensor 6 is placed in a relatively high position on one of the pylons 2 while another temperature sensor 7 is placed in a lower position. The reason is that the temperature may vary with the distance from the ground (or distance from the sea level). By using two temperature sensors 6, 7 on different vertical levels, a temperature for each intermediate vertical level can be extrapolated. Separate temperature compensation can then be made for ultrasonic sensors 4 at different levels. Temperatures can be extrapolated also for levels above the high temperature sensor 6 and the low temperature sensor 7.

Of course, wind speed can also affect the operation of an ultrasonic distance sensor. With the sensors tested so far, it has been found that very high wind speeds can adversely affect the reliability of the measurements. However, at such high wind speeds, other factors may be more decisive. For example, if the invention is used on a bridge, the bridge would normally be closed for traffic if the wind speed reached such levels that the reliability of an ultrasonic distance sensor was seriously affected. In practice, the wind usually becomes a problem at about 15 meter per second. Reliable measurements can be carried out even above 15 m/s (meters per second) but at wind speeds above 30 m/s, it is difficult to compensate for the wind. For a bridge, this may be less important since the bridge would most likely have been closed for traffic already before the wind had reached a speed of 30 m/s. The fixed structure may also comprise a camera 14 arranged to give a picture of a part of the fixed structure where one or several distance sensors are placed. The camera may be connected to the control unit 13. The camera 14 may be connected to a screen such that personnel responsible for supervising the fixed structure (e.g. a bridge) can use the camera 14 to inspect the fixed structure when the distance sensor(s) 4 or the control unit 13 give an indication that a layer of ice has formed on the fixed structure. The camera 14 may be, for example, a web camera 14 and accessible through the Internet.

Distance sensors 4 can also be placed on the pylons 2 of a suspension bridge to detect if ice is forming on the pylons 2. On each side of the pylon 2 (the front side or back side), four distance sensors 4 can be used.

The device 16 which is used for carrying out the inventive method will now be explained in greater detail with reference to Fig. 4, Fig. 6 and Fig. 7. The device 16 comprises a fastening element 8 for fastening the ice detection device on a fixed structure 1. The device 16 also comprises at least one supporting arm 9 connected to the fastening element and extending away from the fastening element 8. At least one distance sensor 4 is arranged on the supporting arm. The distance sensor 4, for example an ultrasonic sensor, is placed at a distance from the fastening element 8. If the device 16 comprises a plurality of distance sensors 4 distributed around the circumference of the fastening element 8, each sensor may be placed on a supporting arm 9 and at a distance from the fastening element 8.

In one embodiment of the invention, the fastening element 8 is shaped to grip around the circumference of an element 3 belonging to a fixed structure 1. As indicated in for example Fig. 4 and Fig 6, the fastening element 8 may comprise a piece of sheet metal that has been given a curved shape. A flange 17 on the curved sheet metal has been provided with through-holes 15. The through-holes 15 can be used in connection with, for example, screws, bolts or rivets to connect the curved sheet metal to a similar or identical element to grip around the circumference of a cable 3. Of course, if the ice-detecting device 16 is to be placed on other elements than cables, the fastening element 8 may have a different shape. This could be the case of the ice-detecting device 16 is to be placed on a pylon on a suspension bridge. As indicated symbolically in Fig. 4, the device 16 may be provided with electrical heating elements 12 in preferred embodiments of the invention. The electrical heating elements 12 can be used to prevent ice and snow from accumulating on the device 16 for detecting ice.

In Fig. 7, an embodiment is indicated where at least a part of the device is covered with spikes 10. The spikes 10 can be used to discourage birds from landing on the device 16.

As indicated in Fig. 7, the device 16 for detecting ice may be provided with a protective shield 11 that extends between adjacent supporting arms 9 of the ice-detecting device 16. The protective shield 11 may be formed by a sheet metal. If the distance sensors 4 communicate with a control unit 13, the communication may go through wires. The shield 11 can protect these wires from falling ice and snow.

It should be understood that the invention also relates to a fixed structure being provided with the inventive device and to a method for providing a fixed structure 1 with equipment for detecting ice.

The invention makes it possible not only to obtain an indication that ice has formed but also a precise indication of the thickness of the ice layer.

When the distance sensor(s) 4 used for ice detection is (are) placed at a distance from the surface 5 of the fixed structure, the surface 5 itself is not altered by the sensor or sensors. Consequently, the measured values coming from the sensor(s) are in fact representative also for parts of the surface 5 that are not directly monitored by the distance sensor(s) 4. The sensor (or sensors) does not interfere with the formation of ice. If the ice detection system is combined with equipment for de-icing, the ice detection system can be operated without interfering so much with the operation of the de-icing system since the sensor (or sensors) is not in contact with the surface from which ice is to be removed.

It should be understood that is possible to envisage embodiments where the distance sensor 4 or distance sensors 4 is/are placed in one or several holders that are not in contact with the fixed structure that is to be monitored. Hence, the fastening element 8 that has been described above could be attached to another object than the fixed structure 1. The invention can advantageously be combined with de-icing equipment. For example, the invention can be combined with de-icing equipment comprising a thin metal sheet that surrounds a cable on a suspension bridge. The thin metal sheet can be connected to a source of electricity. To perform de-icing, an electric current is sent though the thin sheet metal. This causes the metal to become warm and the ice immediately adjacent the sheet metal will melt. As a result, a surrounding layer of ice will start to glide and falls off from the cable.

While the invention has been described above with reference to a method for detecting ice and to equipment for detecting ice on a fixed structure, it should be understood that these categories only reflect different aspects of one and the same invention.

It should be understood that, while the invention has been described in terms of detecting the presence of ice, the invention is equally suitable for detecting snow. Hence, in the context of this application, the term "ice" shall be understood as meaning "ice and/or snow".

While the invention has been described in detail and by reference to preferred embodiments thereof, it will be apparent that modifications and variations are possible without departing from the scope of the invention defined in the appended claims. More specifically, although some aspects of the present invention are identified herein as preferred or particularly advantageous, it should be understood that the present invention is not necessarily limited to these preferred aspects of the invention.

Claims

1) A method for detecting ice on fixed structures (1), the method comprising the steps of: a) placing at least one sensor (4) at a distance from a surface (5) of the fixed structure (1), b) determining the distance from the at least one sensor (4) to the surface (5) of the fixed structure (1) and c) subsequently using the at least one sensor (4) to determine whether the distance (A) to the surface (5) of the fixed structure (1) has changed.

2) A method according to claim 1, wherein the at least one sensor (4) uses ultrasound to determine the distance to the surface (5) of the fixed structure (1).

3) A method according to claim 1 or 2, wherein the at least one sensor (4) is initially placed at a distance of 200mm - 1300mm from the surface (5) of the fixed structure (1).

4) A method according to claim 1, wherein the at least one sensor (4) takes samples of the distance to the surface (5) of the fixed structure (1) during an extended period of time and where the samples are used to calculate an average value for the distance between the at least one sensor (4) and the surface (5) of the fixed structure (1).

5) A method according to claim 4, wherein sample values outside a predetermined range are ignored.

6) A method according to claim 1 or 2, wherein a plurality of sensors (4) are placed around a circumference of an element (3) of the fixed structure (1) and where different sensors (4) measure the distance to the surface (5) of that element (3) at different points of time.

7) A method according to claim 2, wherein a measurement of ambient temperature is performed and the temperature is taken into account when the distance to the surface (5) of the fixed structure (1) is determined. 8) A device (16) for detecting the formation of ice on fixed structures (1), the device comprising a fastening element (8) for fastening the ice detection device on a fixed structure (1), at least one supporting arm (9) connected to the fastening element and extending away from the fastening element (8) and at least one distance sensor (4) on the supporting arm, the distance sensor (4) placed at a distance from the fastening element (8).

9) A device according to claim 8, wherein the sensor (4) is an ultrasonic sensor (4).

10) A device according to claim 8 or claim 9, wherein the fastening element (8) is shaped to grip around the circumference of an element (3) belonging to a fixed structure (1) and the device comprises a plurality of distance sensors (4) distributed around the circumference of the fastening element (8), each sensor being placed on a supporting arm (9) and at a distance from the fastening element (8).

H)A device according to claim 10, wherein the distance sensors (4) are connected to a control unit (13) that is arranged to activate the sensors (4) according to a sequence such that adjacent sensors (4) are not active simultaneously.

12) A device according to any of claims 8 - 11, wherein the device is provided with electrical heating elements (12).

13) A device according to any of claims 8 - 12, wherein at least a part of the device is covered with spikes (10).

14) A fixed structure, the fixed structure being provided with at least one distance sensor (4) arranged at a distance from a surface (5) on the fixed structure (1) and arranged to detect the distance from the sensor to the surface (5) of the fixed structure (1).

15) A fixed structure according to claim 14, wherein the fixed structure (1) is a bridge.

16) A fixed structure according to claim 15, wherein the fixed structure (1) is a suspension bridge (1). 17) A fixed structure according to any of claims 14 - 16, wherein the at least one distance sensor (4) is an ultrasonic sensor (4).

18) A fixed structure (1) according to any of claims 14 - 17, wherein the structure comprises a plurality of distance sensors (4) arranged around the circumference of an element (3) of the fixed structure (1).

19) A fixed structure according to claim 18, wherein the sensors (4) are arranged to be activated in a sequence such that adjacent sensors (4) are not active simultaneously.

2O)A fixed structure according to claim 17, wherein the fixed structure (1) comprises at least one temperature sensor (6, 7) and a control unit (13) connected to the at least one temperature sensor (6, 7) and the at least one distance sensor (4), the control unit (13) being arranged to determine the distance from the distance sensor (4) to the surface of the fixed structure (1) based on a signal from the distance sensor (4) and a signal from the at least one temperature sensor (6, 7).

2I)A fixed structure according to claim 20, wherein the fixed structure (1) comprises a plurality of distance sensors (4) placed at different vertical levels and at least two temperature sensors (6, 7) placed at different vertical levels.

22) A fixed structure according to any of claims 14 - 21, wherein the fixed structure (1) comprises a camera (14) arranged to give a picture of a part of the fixed structure (1) where one or several distance sensors (4) are placed.

23) A method of providing ice detection on a fixed structure, the method comprising placing a distance sensor (4) at a distance from a surface (5) on the fixed structure (1) such that the distance sensor (4) can subsequently be used to determine whether the distance to the surface (5) of the fixed structure (1) has changed.