This invention relates to a hot metal detector, in particular a device for detecting the presence of hot metal, such as steel plates, on a transport path in a rolling mill.

In a rolling mill, it is desirable to be able to precisely and reliably detect arrival of a hot metal product. However, the environment is quite harsh and the presence of steam may reduce the accuracy of any detector.

There are a number of hot metal detection methods currently in use. One method uses a single detector 1 mounted above the hot metal 3 to view a large area 2 of material at once, as illustrated in Fig.1 a. This has the advantage that steam obscuring part of the sensor will have a relatively small effect. However, the point at which material is detected depends upon its temperature. As illustrated in Fig. lb, a head end of a hot metal 3 moving in a direction of travel as indicated by the arrow 5 will only be detected by the detector at position 4a if the metal is relatively hot. If the hot metal is somewhat cooler, then the detector may only be triggered when the head end reaches the second position 4b. Thus, there is inconsistency in the position at which the sensor detects the arrival of the hot metal.

An alternative solution has been to use a scanning device with a narrow angle of view across the width of a hot metal product, as shown in Fig.3. However, although detection is more reliable because of the narrow lateral field, a scanning system requires moving parts, or a special lens which add to the complication and expense.

In accordance with a first aspect of the present invention, a hot metal detector comprises a plurality of first infra-red sensors to detect radiation from a hot metal product moving along a path in a direction of travel; the detector further comprises one or more second infra-red sensors to detect radiation from the hot metal product before it reaches the first sensors; and a processor configured to receive a signal from the one or more second sensors and set a trigger threshold in the first sensors based on the signal from the one or more second sensors.

The second sensors are mounted closer to a head end of the hot metal product, so that the trigger level of the first sensors can be customised to the specific product.

Preferably, the first infra-red sensors comprise an array of sensors across the width of the path. In one embodiment, the second sensors comprise an array of sensors perpendicular to the array of first sensors, along the length of the path.

Alternatively, the second sensors comprise an array of sensors parallel to the array of first sensors, across the width of the path.

Preferably, an array of second sensors is positioned on both sides of the array of first sensors.

In a reversing mill, the sensors can be switched to operate only on the side closest to the head end of the hot metal product.

In accordance with a second aspect of the present invention, a method of detecting a hot metal product comprises sensing background infra-red radiation and setting a trigger level for a hot metal product in one or more primary sensors and in one or more secondary sensors; detecting infra-red radiation exceeding the background level at the one or more secondary sensors; in a processor, determining the amount by which the temperature of the hot metal product detected by the one or more secondary sensors differs from the trigger level set in the one or more primary sensors; and adapting the trigger level in the primary sensors accordingly.

Preferably, the method further comprises providing one or more second sensors on each side of the array of first sensors along a transport path; in the processor, controlling operation of the sensors on each side, such that only radiation detected in the second sensors before the first sensors are triggered causes the trigger level to be set.

Either the first or the second sensors may be used to detect the level of background radiation, but preferably the method comprises using the one or more second sensors to sense background radiation.

Preferably, the method further comprises providing one or more second sensors on each side of the array of first sensors along a transport path; in the processor, controlling operation of the sensors on each side, such that if no radiation is detected in the second sensors on an entry side before the first sensors are triggered, a trigger level is set for the sensors on the exit side and triggering of the sensors on the exit side is used to determine the position of a head end of the hot metal product.

A hot metal detector and a method of detecting a hot metal will now be described in more detail with reference to the accompanying drawings in which:

Figures la and lb illustrate a first known method of detecting hot metal; Figure 2 is a graphical illustration of the effect of temperature of the hot metal on detection position in the method of Fig.1;

Figure 3 illustrates a second known method of detecting hot metal;

Figure 4 shows a first example of a hot metal detector according to the present invention;

Figure 5 illustrates graphically, the effect of temperature of the hot metal on detection position for the detector of Fig.4;

Figure 6 illustrates a second example of a hot metal detector according to the present invention;

Figure 7a is a flow diagram illustrating an example of the method of the present invention using manual threshold setting for the secondary sensors; and,

Figure 7b is a flow diagram illustrating an example of the method of the present invention using automatic threshold setting for the secondary sensors. As referred to above known hot metal detectors include those which use a single detector 1 mounted above the hot metal 3 to view a large area 2 of material at once, as illustrated in Fig. la, or using a scanning device with a narrow angle of view across the width of a hot metal product, as shown in Fig.3. As can be seen from Fig.2, the detector relies on the sensors triggering when a trigger level 30 has been exceeded and sending a signal to indicate that a hot metal product, such as a metal plate, has been detected. The trigger level must be set above a background level 31 at which no hot metal product is present. Conventionally, the amount by which the trigger level 30 is set above the background level 31 is a preset value dependent upon the temperature of the metal plate based on the anticipated temperature at that point in the process.

However, depending upon plate temperature, the rate of rise of temperature may be different, so for two otherwise similar plates, one of which is relatively cold 34 and the other of which is relatively warm 35, the sensors may trigger at a different point. For example, a plate at 1150 °C may be considered to be hot, whereas a plate at 750 °C may be considered to be cold. Simply setting the trigger point to be a fixed amount above the background level means that the warmer plate 35 triggers first 32 and the cooler plate 34 triggers somewhat later and so detection of the front end occurs at a different position along the path. In order to overcome the disadvantages of the temperature effect on detection position without adding expense and complication, the present invention proposes an alternative solution shown in the examples of Figs. 4 to 6 which uses one or more infrared sensors in advance of the detector array to set the trigger level in the detector array.

In the first example, shown in Fig.4, the detector comprises a first, or primary, array 10 of sensors 11 aligned along an axis 18 across at least part of the width of a transport path 25 along which the hot metal product 3 moves in the direction of the arrow 12. Prior to a hot metal passing a detector unit 16, the detector unit determines a background level of infra-red radiation and sets a threshold in the sensor array 10 for the hot metal product. The threshold for the array 13 is either preset manually, or it is adjusted automatically based on the mean level of background radiation and set at a preset margin above this mean level. The automatic adjustment has the advantage that erroneous results from hot scale or reflections off the roller are avoided and the threshold is not set so high that cold plates fail to be detected. The background level may be determined using the sensors in the array or using other sensors.

The product 3 passes along the transport path 25 under the array 10, which has a relatively narrow viewing angle. Typically, the viewing angle is around 1 degree, although viewing angles of up to 3 degrees are possible. Even if some of the sensors in the array are obscured by steam, it is unlikely that they all will be at the same time, so detection still takes place and the sensors provide signals to the detector unit 16. The detector unit comprises a processor 21 which is able to process the received data, both for the background and in use measurements and to control the setting of a trigger threshold in the sensors of the array 10. In addition to the first array 10, one or more secondary arrays are provided. The secondary array 13 may comprise a plurality of infra-red sensors 17 aligned along an axis 19 parallel to the axis 18 of the array 10. As the hot metal product approaches the secondary array, the sensors 13 output a signal to the detector, the magnitude of the output from the sensors relating to the temperature of that particular hot metal product. The processor is then able to use this information to set a trigger threshold for the sensors of the first array, or to update the previously set trigger threshold in the first array.

Using sensors in the secondary array allows the trigger point to be set for the subsequent primary array positioned across the transport path, so that there is greater consistency of results from the primary array. As can be seen in Fig.5, the background level 31 is determined in the usual way, but as temperature data is obtained for each plate in the secondary array, then for the primary array, the trigger level 30b of the relatively warm plate is set at a higher value than the trigger level 30a of the relatively cool plate. As a result, detection of both plates is triggered at the same position 36 on the transport path regardless of the actual temperature of the metal plate.

An alternative embodiment is shown in Fig.6. In this example, instead of parallel secondary arrays of sensors either side of the primary array along the path of travel of the hot metal product, the secondary arrays 22, 23 are perpendicular to the primary array 10. Typically, an axis 24 of the secondary arrays is aligned with the centreline of the path along which the hot metal product is transported. However, the arrays could be offset to one side or the other of the centreline. The sensors 17 detect the presence of the hot metal product before it reaches the sensors of the primary array across the path 25. This ensures that the sensors 11 of the primary array are always triggered at the same point regardless of the temperature of the product.

In addition to setting the trigger levels for the primary array, the sensors 17 of the secondary array 22, 23 could even be used to detect the hot metal product if steam obscures all of the sensors of the primary array. To do this, the processor takes the outputs from one secondary array 23 and uses them to set a trigger point for the sensors of the other secondary array 22 (assuming the direction of travel shown in Fig.6).

An alternative mode of operation for a reversing mill is that the processor 21 , having received outputs from one secondary array 23 and an indication of head end detection from the primary array 10, then disables, or ignores inputs from the other secondary array 22 until the processor receives a signal indicating that the direction of travel along the transport path has been reversed and what was the tail end has become the head end and so needs to be detected by the secondary array 22 and primary array 10.

The examples described above show an array of secondary sensors both before and after the primary array along the transport path. If the backup for the primary array is not required and the mill operates in only one direction, then a detector which only has a secondary array on an entry side of the primary array is quite feasible. However, for a reversing mill, the secondary arrays need to be on both sides of the primary array along the transport path. A single secondary sensor could be used in the examples described, but with a single sensor, there is the risk that the sensor is obscured by steam as the metal product approaches, so multiple sensors are preferred.

Example methods of using a hot metal detector according to the present invention are illustrated in Figs.7a and 7b. The background infra-red radiation is determined 50 and the controller sets 51 the trigger level for the primary sensor array and for the secondary sensors. In the example of Fig.7a, the trigger level is set manually. A check 52 is made for a new hot metal product by determining whether level of radiation sensed 53 by any of the secondary sensors has exceeded the trigger level. When that occurs, the magnitude of the sensed radiation is output to the detector 16. The processor derives a trigger level and sends a signal to the primary array sensors to set 54, or update the trigger level. The primary array sensors detect received radiation 55 and check 56 whether this exceeds 58 the the trigger level. If not 57, they continue to check against the background level. If the sensed level exceeds 58 the trigger level, then an output to the processor is triggered and the sensor array checks for a return to below the level indicating the end of that product, then checks 52 for a new hot metal product.

In the example of Fig.7b, a similar set of steps are followed, but the adjustment of the trigger levels for the secondary sensors 60 is a preset margin above the background level, which is regularly measured and averaged, so that if the mean background radiation increases, the trigger level will also increase.

The detector and method of operation of the present invention provide reliable and precise detection of hot metal at different temperatures and in the presence of steam. The combination of a primary array 10 with a narrow viewing angle and secondary sensors or arrays of sensors 13, 14 which give the detector a large overall viewing area enables the hot metal to be detected even in the presence of steam and to trigger more accurately even if the plate temperatures vary from one plate to the next.