BACKGROUND OF THE INVENTION: In the severe environment prevalent in the steel industry, the hitherto most common way of measuring workpiece width has been to use linear array cameras mounted in an overhead position high above the workpiece where the cameras are able to view the entire width of the material to be measured. A further reason why the cameras are mounted high is so they are placed out of harms way of heat radiation and rising steam emanating from hot, watercooled workpieces. A number of apparent drawbacks are associated with this system. The high mounting position will likely interfere with transporting and lifting cranes needing access space. Since the workpiece also may move vertically to some extent, i. e. in a direction towards and away from the cameras, there will be a need to compensate the obtained measured value for this movement. This compensa- tion is usually made with software in a control system, and introduces an additional error in the measurement obtained.

The cameras will need some protection against heat radi- ation, if used in an environment where heat radiation is present, and the camera mounting devices will likewise need

to have some compensation means to correct heat expansion in the mounting devices. In some cases, a fully air- conditioned housing might be needed to permit accurate readouts of the measured width. As a result of the high mounting location, any maintenance will be made extra difficult because of the relative inaccessibility of the equipment.

Certain systems of this type will need background illumina- tion of the workpiece to allow accurate measurement. The illumination equipment thus will be placed under the workpiece and will be subjected to steam, mill scale and water spilling off the workpiece.

Examples of systems described above is one marketed and sold under the registered name of ACCUPLAN by George Kelk Corporation, and another system is marketed under the registered name of CCD2030 Stereoscopic Width Gauge by European Electronics System's Limited (EES).

Another system, consisting of diode line cameras on which the two side edges of an illuminated workpiece are pro- jected, is described in US-A-4 384 303. The number of diodes darkened by the workpiece edge are counted in the diode lines above the two edges. The resulting counts are added to the distance between facing diode lines to form a measured value indicating the width of the rolled product. Also this system appears to have the drawbacks described above, associated with having equipment underneath and close above the workpiece. The resolution, furthermore, can not be higher than the diode-to-diode distance within one diode line.

A known system for measuring a workpiece location relative to a known point, for example the location of the measuring system, is described in SE-B-461 116 which is hereby

incorporated by reference. A laser light source transmits a beam of light which is of a generally oblong shape. At least a part of the beam is reflected off an end of the workpiece and received in a camera. The camera consists of a first lens, a second lens and an array of light sensitive elements. The first lens focuses the beam onto the array whilst the second lens spreads the beam, in a direction perpendicular to the longitudinal direction of the array, to form a well-defined line of light. The relative position of the end of the workpiece is thus indicated by which single light sensitive elements of the array are being lit by the line of light.

SUMMARY OF THE INVENTION: A main object of the invention is thus to overcome the above-mentioned disadvantages and drawbacks and to provide an improved method and device for measuring the width of a workpiece under severe environmental conditions which is accurate under all operating conditions.

This object is accomplished by a method for measuring the width of a workpiece, the workpiece having a longitudinal direction, a first side having a first edge part and a second side having a second edge part, the edge parts being arranged substantially in the longitudinal direction, where the method includes the steps of: a) obtaining a first measurement of the position of the first edge part relative to a first laser position measuring system arranged on the first side of the workpiece; b) obtaining a second measurement of the position of the second edge part relative to a second laser position measuring system arranged on the second side of the workpiece; c) adding the first measurement to the second measurement to form a measurement sum, and

d) subtracting the measurement sum from a known distance between said laser position measuring systems, the result thus indicating the width of said workpiece.

This object is further accomplished by a device for measur- ing the width of a workpiece according to the invention comprising a first laser position measuring system arranged on the first side of the workpiece, to obtain a first measure- ment of the position of the first edge part relative to the first laser position measuring system; a second laser position measuring system arranged on the second side of the workpiece, to obtain a second measurement of the position of the second edge part relative to the second laser position measuring system, and a computing means for computing the width of the workpiece from the first and second measurements and a known distance between the first and second laser position measuring systems.

Preferred embodiments of the invention are detailed in the respective dependent claims.

BRIEF DESCRIPTION OF THE DRAWINGS : The invention will be described in the following in greater detail by way of example only and with reference to the embodiment shown in the annexed drawings, in which: Fig. 1 shows a schematic plan view of an embodiment of a device according to the invention, and Fig. 2 shows a schematic side view of a distance measure- ment unit used in a device according to the inven- tion.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS:

Fig. 1 shows one embodiment of the invention where a workpiece 100, for example a slab, plate or strip in a rolling mill, is transported on a roller table consisting of a plurality of rolls 400 mounted on a support structure 410. The workpiece 100 has a longitudinal direction A, a first side having a first edge part 110 and a second side having a second edge part 120. The edge parts 110,120 are arranged substantially in the longitudinal direction A of the workpiece 100 and the workpiece is transported substan- tially in the longitudinal direction A of the workpiece.

A first laser position measuring system 200 is arranged to measure the relative distance to the first edge part 110 of the workpiece and a second laser position measuring system 300 is arranged to measure the relative distance to the second edge part 120 of the workpiece. The laser position measuring systems 200,300 are advantageously of the type described below in connection with Fig. 2.

The exact location of the workpiece 100 on the roller table may vary within a large range, as long as the workpiece, during its transport movement, passes between the two laser position measuring systems 200,300 so that a wide laser beam 210 from the first laser position measuring system 200 and a corresponding wide laser beam (not shown) from the second laser position measuring system 300 can strike the respective first and second edge parts 110,120 of the workpiece. The wide laser beam 210 from the first laser position measuring system 200 and corresponding wide laser beam (not shown) from the second laser position measuring system 300 have an extended and flat shape substantially perpendicular to the longitudinal direction A of the workpiece 100.

An advantageous type of laser position measuring system is shown in Fig. 2. Two such systems are used, one as the

first laser position measuring system 200 and one as the second laser position measuring system 300, respectively.

The system comprises a laser light source 3 and a camera 4. The laser light source housing and the camera housing are omitted for clarity.

The system according to Fig. 2 will now be described in conjunction with the first laser position measuring system 200, but the second laser position measuring system 300 is arranged in an analogous manner. A laser beam 22 emanating from a laser source (not shown) is converted to a ribbon- shaped wide laser beam 210 when passing through a concave lens 23 and then a first convex lens 24. The workpiece 100 is shown in two locations in Fig. 2, a first location B closer to the laser light source 3 and a second location C further away from the laser light source 3. A part of the wide laser beam 210 light is reflected off the first edge part 110. A portion of this reflected light enters the camera 4 through a lens 12 and is spread out into a wide line of light by a cylindrical lens 27. The wide line of light shines upon an elongated array 26. The array 26 comprises a large number of narrow, light sensitive elements substantially arranged in the longitudinal direction of the array.

A method for measuring the width of the workpiece 100 includes the steps of: a) obtaining a first measurement of the position of the first edge part 110 relative to the first laser position measuring system 200 arranged on the first side of the workpiece 100; b) obtaining a second measurement of the position of the second edge part 120 relative to the second laser position measuring system 300 arranged on the second side of the workpiece 100; c) adding the first measurement to the second

measurement to form a measurement sum, and d) subtracting the measurement sum from a known distance between the laser position measuring systems 200, 300, the result thus indicating the width of said workpiece 100.

A computing means 500, for computing the width of the workpiece 100 from the first and second measurements and a known distance between the first and second laser position measuring systems 200,300, determines the relative distance between the laser light source 3 and the edge part 110 (and edge part 120) of the workpiece 100 by registering which light sensitive elements are hit by light. Suitably, a reference plane (not shown) is established with respect to the laser position measuring system 200, the location of the reference plane being known.

The equipment used for a method and a device according to the invention can be positioned to the side of the workpiece and is, thus, not subjected to any of the drawbacks described above relating to having equipment underneath or above the workpiece. The laser position measuring systems used are of high accuracy and low maintenance, making the system accurate and reliable. The equipment is readily accessible for maintenance, when and if this is necessary. No illumination is necessary from below the workpiece, which further improves the durability of the system and lowers the investment costs.

Whilst the invention has been described above with respect to certain preferred embodiments thereof, the invention is not limited to these but may be varied widely within the scope of the appended claims. For example, the workpiece has been shown as being substantially rectangular but it can be of any shape, as long as it has two sides which reflects light. An alternative to a wide laser beam as used

in the invention, a point-shaped laser beam can be used, i. e. a narrow beam. A fast mirror system then moves the laser beam across the height of the workpiece to be measured. A drawback associated with this method is that, since the workpiece is moving relative to the measuring system, there is a demand to synchronize the two movements: mirror and workpiece, which introduces errors in the measurements and increases the complexity and cost of the equipment used.