Automatic plate bending system using high frequency induction heating

Abstract: An automatic plate bending system using high frequency induction heating has many universal poles (20,21) for bearing a steel plate (2), a member to be heated, by supporting it from below, the height positions of front end portions of the universal poles themselves being adjustable, and automatically moves a high frequency heating coil (24b) of a high frequency heating head (24) above the steel plate, which is placed on the universal poles, along predetermined heating lines (3) while retaining a constant clearance between the high frequency heating coil and the surface of the steel plate, whereby the steel plate is heated and automatically bent into a desired shape.

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention This invention relates to an automatic plate bending system using high frequency induction heating, and more specifically, to one useful for application to the bending of a steel plate having complicated curved surfaces, such as an outer panel of a ship hull. 2. Description of the Prior Art The outer panel of a ship hull is composed of a steel plate about 10 to 30 mm thick with a complicated undevelopable curved surface which reduces propulsion resistance for efficient navigation in the water. To form this curved outer panel, a processing method generally called line heating has been known for long. This method heats the surface of a steel plate locally by means of a gas burner or the like, to cause the extraplane angular deformation or intraplane shrinkage deformation of the steel plate due to plastic distortion, and skillfully combines these deformations to obtain the desired shape. This method is used at many shipyards. Fig. 1 is an explanation drawing conceptually showing an earlier technology concerned with a method for bending a steel plate to serve as an outer panel of a ship hull. Fig. 2 is a front view showing a wooden pattern for use in the bending in a state in which it is mounted on the steel plate. As shown in both drawings, according to the earlier technology, many (10 in the drawing) wooden patterns 1 following frame lines of the outer panel of the ship hull (lines extending along frame materials for the outer panel at positions where the frame materials are attached; the same will hold in the following description) as target shapes are mounted on a steel plate 2. Then, an operator compares the shapes of each wooden pattern 1 and the steel plate 2 by visual observation, and considers differences between their shapes, e.g., the clearance between the wooden pattern 1 and the steel plate 2. Based on this consideration, the operator studies what position to heat in order to bring the steel plate 2 close to the target shape. As a result, the operator determines each heating position (heating point). Concretely, the wooden pattern 1 is rolled along the frame line of the steel plate 2 in a vertical plane (the same plane as in Fig. 2). The points of contact of the wooden pattern 1 with the steel plate 2 during the rolling motion are watched to determine the heating points in consideration of the clearance between the wooden pattern 1 and the steel plate 2 in each state. Then, it is considered how to connect the respective heating points together in order to make the steel plate 2 similar to the target shape. Based on this consideration, a heating line is determined. As shown in Fig. 3, heating lines 3 that have been determined are marked on the surface of the steel plate 2 with chalk or the like, and the steel plate 2 is heated with a gas burner along the heating lines 3. With the earlier technology as described above, the steel plate 2 is heated with a gas burner by the operator along the heating lines 3 determined by the operator's sense based on many years of experience. As a result, a predetermined curved surface is obtained. Acquiring the ability to determine the heating lines 3 rationally is said to require more than about 5 years of experience. This has posed the problems of the aging and shortage of experienced technicians. The bending procedure also takes a large amount of time for incidental operations, such as the production, mounting and removal of the wooden pattern 1 for the steel plate 2, thus lengthening the entire operating time. Besides, the heating operation using a gas burner itself becomes heavily muscular activity in a hot, humid harsh environment involving the occurrence of steam associated with the evaporation of cooling water. Hence, a demand is growing for the advent of a device which realizes the automation of the plate bending operation. To solve the problem of the shortage of experienced technicians and reduce the operating time, it is necessary to improve, theorize and automate the bending operation while taking into consideration know-how that operators acquired through experience. Generally, bending of a plate material such as a steel plate is performed using a press or the like. To process the plate material into a complicated shape which is hard to form with a press, hot bending by a gas burner is used. The operation using a gas burner causes the problem of a deteriorated work environment due to noise, heat and combustion gases. Recently, therefore, high frequency induction heating has been studied. High frequency induction heating produces eddy currents in a member to be heated, e.g., a steel plate, by the action of electromagnetic induction, and applies heat by utilizing an eddy-current loss. Thus, a high frequency heating coil is required for high frequency induction heating. Fig. 4 shows an example of a high frequency induction heater for heating a flat plate-shaped member to be heated, such as a steel plate 1, from above. A high frequency heating coil 02 is provided opposite the steel plate 1 via a clearance Δt so as to be movable by a moving device 04 in the direction of an arrow A. The clearance Δt is about 5 mm. The high frequency heating coil 02 is secured to a lower end of a bar-shaped support arm 05 via a disk portion 03, and the support arm 05 is supported by a guide portion 04a of the moving device 04 so as to be movable vertically. Thus, the high frequency heating coil 02 moves linearly in a vertical direction integrally with the support arm 05. The moving device 04 has its moving speed controlled by a moving speed controller 06, and moves horizontally linearly along a guide rail 07. In the drawing, the reference numeral 08 denotes a matching transformer, and the numeral 09 designates a high frequency power source. To achieve desired uniform heating with such a high frequency induction heater, it is vital to keep the clearance Δt between the high frequency heating coil 02 and the steel plate 1 constant. This is because a heat input to the steel plate 1 is determined simply by the clearance Δt as a parameter along with an electric current supplied to the high frequency heating coil 02, its frequency, and the moving speed of the high frequency heating coil 02. High frequency induction heating thus requires that the clearance Δt between the high frequency heating coil 02 and the steel plate 1 be kept constant. To meet this requirement, the high frequency induction heater according to the earlier technology has a laser sensor provided near the high frequency heating coil 02, measures the distance between the high frequency heating coil 02 and the steel plate 1 by the laser sensor, and extends or contracts the support arm 05 to keep the clearance Δt between the high frequency heating coil 02 and the steel plate 1 constant. However, the laser sensor is vulnerable to high temperatures or steam. Thus, it is difficult to protect the laser sensor, for example, from radiant heat generated when the temperature of the steel plate 1 rises to 800°C, or from steam produced when the heated steel plate 1 is cooled with water. There is also the problem that laser light is disturbed by steam, and measurement errors will result. Hot bending of the steel plate involves various forms of heating, including line heating for heating in a linear form, spot heating for heating predetermined spots in a circular form, weaving heating for heating in a zigzag form, and pine needle heating for heating in a triangular form. To accommodate various forms of heating mentioned above, various coils adapted to the forms of heating are made ready for use, and a coil may be changed to agree with the form of heating. That is, an attachment type coil may be used. For such an attachment type, however, many coils in agreement with the forms of heating must be prepared, and coil replacement is required each time the form of heating is changed. This presents with the problems of boosted equipment cost and decreased operating efficiency. In the document WO-A-81/01258 there is already disclosed an automatic plate bending system using high frequency induction heating, comprising: a travel system free to travel in a horizontal plane, said travel system having a longitudinally travelling trolley stretching over two parallel rails and travelling along these rails, and a transversally travelling trolley travelling on the longitudinally travelling trolley in a direction perpendicular to the direction of the rails; a high frequency heating coil for induction heating the surface of a member to be heated, said high frequency heating coil being attached to the transversally travelling trolley so as to be vertically moveable, and being opposed, with a constant clearance to the surface of a member to be heated; universal poles disposed vertically at a multiplicity of specified positions between the rails, with a multiplicity of specified positions of front end portions of the universal poles themselves being adjustable, so as to bear the member to be heated, by supporting the member from below; and a control unit for controlling the travel of the travel system and the horizontal plane on the basis of predetermined heating line date, so that the high frequency heating coil heats the member to be heated, long predetermined heating lines via the travel system. In this document there is neither disclosed nor suggested, how the predetermined heating lines can be computed, SUMMARY OF THE INVENTION It is an object of the present invention to avoid this disadvantage. This object is attained by the characterizing features of claim 1. Preferred embodiments of the present invention are illustrated in the subclaims 2 to 9. BRIEF DESCRIPTION OF THE DRAWINGS Note that Fig. 15 - 21 are not covered by the claims. Fig. 1 is an explanation drawing conceptually showing an earlier technology concerned with a method for bending a steel plate which will serve as an outer panel of a ship hull; Fig. 2 is a front view showing a wooden pattern for use in the bending of a steel plate according to the earlier technology, the wooden pattern being mounted on the steel plate; Fig. 3 is a perspective view showing a state in which heating lines determined by the earlier technology are applied to a steel plate; Fig. 4 shows an explanation drawing conceptually showing a high frequency induction heater concerned with the earlier technology; Figs. 5(a) and 5(b) are schematic representations of the shape of a steel plate by contour lines for showing the results of experiments on the effects of the present invention; Fig. 6 is a perspective view showing the whole of an automatic plate bending system concerned with an embodiment of the present invention; Fig. 7 is an enlarged perspective view showing a high frequency heater I, an A portion in Fig. 6, in an extracted and enlarged manner; Fig. 8 is a perspective view showing a high frequency heating head concerned with the embodiment of the present invention as viewed from below; Fig. 9 is a plan view showing a coil portion of the high frequency heating head of Fig. 8 in an enlarged manner; Fig. 10 is a vertical sectional view of the high frequency heating head of Fig. 8 in an enlarged manner; Fig. 11 is a block diagram showing a control system of the automatic plate bending system concerned with the instant embodiment; Figs. 12 (a) to 12(e) are explanation drawings for illustrating an example of processing performed by a heating point determining unit 41 in Fig. 11; Figs. 13(a), 13(b) and 13(c) are explanation drawings showing displays of a display unit 43 associated with processing performed by the heating point determining unit 41 in Fig. 11; Fig. 14 is an explanation drawing conceptually showing the blank layout of a steel plate 2, an object to be processed, according to the instant embodiment; Fig. 15 is an explanation drawing for illustrating an example of processing performed by a heating line determining unit 44 in Fig. 11; Fig. 16 is a flow chart showing an example for determination of heating points; Fig. 17 is a flow chart 1 showing a first example for determination of heating lines; Fig. 18 is a flow chart 2 showing the first example for determination of heating lines; Fig. 19 is a flow chart 3 showing the first example for determination of heating lines; Fig. 20 is a flow chart showing part of a second example for determination of heating lines; Fig. 21 is a flow chart showing part of a third example for determination of heating lines; Fig. 22 is an explanation drawing for illustrating the principle of a curvature comparison method which is processing performed by the heating point determining unit 41 in Fig. 11 (a state in which the curve of a target shape is divided into fine zones that constitute arcs with radii of R 1 to R n ); Fig. 23 is an explanation drawing for illustrating the principle of the curvature comparison method which is processing performed by the heating point determining unit 41 in Fig. 11 (a state in which one of the arcs of Fig. 22 is approximated by a fold line defined by the bases of a plurality of isosceles triangles connected together while sharing their equal sides); Fig. 24 is an explanation drawing for illustrating the principle of the curvature comparison method which is processing performed by the heating point determining unit 41 in Fig. 11 (a comparison between the target shape and the measured shape when approximated by fold lines defined by the bases of a plurality of isosceles triangles); Fig. 25 is a flow chart 1 showing a further example for determination of heating points; Fig. 26 is a flow chart 2 showing the further example for determination of heating points; Fig. 27 is a flow chart 3 showing the further example for determination of heating points; Fig. 28 is a flow chart 4 showing the further example for determination of heating points; Figs. 29(a) to 29(d) are explanation drawings conceptually showing examples of the forms of heating using the coil portion 24b of the automatic plate bending system concerned with the instant embodiment; Fig. 30 is an explanation drawing conceptually showing a first modified example of a structure for retaining clearance with which the coil portion 24b is mounted; Fig. 31 is an explanation drawing conceptually showing a second modified example of a structure for retaining clearance with which the coil portion 24b is mounted; Fig. 32 is an explanation drawing conceptually showing a third modified example of a structure for retaining clearance with which the coil portion 24b is mounted; and Fig. 33 is an explanation drawing conceptually showing a fourth modified example of a structure for retaining clearance with which the coil portion 24b is mounted. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Embodiments of the present invention will now be described in detail with reference to the accompanying drawings. However, it is to be understood that these embodiments are given only for illustrative purposes and do not restrict the invention. Fig. 6 is a perspective view showing the whole of an automatic plate bending system concerned with an embodiment of the present invention. As shown in Fig. 6, two parallel travel rails 11, 12 are mounted on many frame legs 13 erected on a floor surface, and longitudinally traveling trolleys 14, 15 stretching over the travel rails 11, 12 run along these travel rails 11, 12 (in the X axis direction). Transversely traveling trolleys 16, 17 bear high frequency heaters I, II and run on transverse travel rails 14a, 15a provided on the longitudinally traveling trolleys 14, 15 in a direction perpendicular to the moving direction of the longitudinally traveling trolleys 14, 15 (i.e., in the Y axis direction). These longitudinally traveling trolleys 14, 15 and transversely traveling trolleys 16, 17 constitute a travel system which runs freely in a horizontal plane (XY plane). Power supply belts 18, 19 feed an electric power, high pressure air, and cooling water to the high frequency heaters I, II, and are composed of a flexible material so as to be able to move in response to the movement of the longitudinally traveling trolleys 14, 15. Universal poles 20, 21 are erected vertically on the floor surface at a multiplicity of specified positions between the travel rails 11 and 12, with the positions of front end portions of the universal poles themselves being adjustable, so as to bear steel plates 2, members to be heated in the instant embodiment, by supporting the steel plates 2 from below. That is, the position of each universal pole 20 or 21 (X coordinate and Y coordinate) in a horizontal plane (XY plane) is preset to be a predetermined position, and the height position of the front end portion of each universal pole 20 or 21 (i.e., Z coordinate) is adjustable by a built-in drive source, such as a drive motor. The system illustrated in Fig. 6 has two of the longitudinally traveling trolleys 14, 15 and two of the high frequency heaters I, II, and gives two working areas so that a bending operation can be performed simultaneously in each working area. However, the numbers of these trolleys, heaters and working areas can, needless to say, be set arbitrarily. Also, the constituent elements in the respective working areas, such as the longitudinally traveling trolleys 14, 15 and the high frequency heaters I, II, are constructed in exactly the same way. In the description to follow, therefore, the constitution concerned with the first working area, which comprises constituent elements, such as the longitudinally traveling trolley 14 and the high frequency heater I, will be explained. Fig. 7 is an enlarged perspective view showing the high frequency heater I, an A portion in Fig. 1, in an extracted and enlarged manner. As shown in Fig. 7, the transversely traveling trolley 16 running on the transverse travel rail 14a bears a shape measuring unit 22 as well as the high frequency heater I. The shape measuring unit 22 and the high frequency heater I move freely in a horizontal plane integrally with the transversely traveling trolley 16. The shape measuring unit 22 is movable vertically along a guide 23 secured to the transversely traveling trolley 16. The shape measuring unit 22 has a lower end portion in contact with the surface of the steel plate 2, traces the shape of this surface with the lower end portion, and detects displacements with a sensor such as a differential transformer, thereby supplying measurement data on the surface shape of the steel plate 2. The high frequency heater I has a high frequency heating head 24, high frequency flexible water cooled cables 25, a matching transformer 26, a power cable 27, an air cylinder 28, an air hose 29, and cooling water hoses 30. The high frequency heating head 24 is secured to a front end of a piston rod 28a of the air cylinder 28 so that a heating surface of its high frequency heating coil will be opposed to the surface of the steel plate 2. When driven by the air cylinder 28, the high frequency heating head 24 contacts or leaves the steel plate 2. The high frequency heating head 24 is also movable vertically, together with the air cylinder 28 and the matching transformer 26, along a travel rail 31 secured to the transversely traveling trolley 16. The high frequency heating coil of the high frequency heating head 24 is supplied with an electric power via the power cable 27, matching transformer 26 and high frequency flexible water cooled cables 25, and also supplied with cooling water through the cooling water hoses 30. The air cylinder 28 is fed with high pressure air through the air hose 29. The power cable 27, cooling water hoses 30 and air hose 29 are connected to the power supply belt 18 (see Fig. 6). Fig. 8 is a perspective view taken on line B-B of Fig. 7, showing the high frequency heating head 24 and its vicinity in an extracted manner. As shown in Fig. 8, the high frequency heating head 24 is secured to the piston rod 28a of the air cylinder 28 (see Fig. 7) via a disk portion 24a. The high frequency heating head 24 has a coil portion 24b secured to a central part of the disk portion 24a, and many steel ball portions 24c secured to the disk portion 24a along the outer periphery of the coil portion 24b. The steel ball portions 24c contact the surface of the steel plate 2 as a surface to be heated, thus smoothing the movement of the high frequency heating head 24 along the surface of the steel plate 2 in accordance with the movement of the high frequency heater I, and also function to retain a constant clearance between the coil portion 24b and the surface of the steel plate 2. The amount of heat input to the steel plate 2 during high frequency heating is determined solely by parameters consisting of an electric current supplied to the coil portion 24b, its frequency, the moving speed of the coil portion 24b, and the aforementioned clearance. To achieve the desired uniform heating, therefore, it is an essential requirement to keep this clearance constant. In Fig. 8, the numeral 32 denotes a nozzle, which supplies cooling water to a heating portion via the cooling water hoses 33 during heating with the coil portion 24b. Fig. 9 is a plan view showing the coil portion 24b of the high frequency heating head 24 of Fig. 8 in an enlarged manner. As shown in Fig. 9, the coil portion 24b is a portion which generates a magnetic flux for induction heating the steel plate 2. In this embodiment, the coil portion 24b is composed, in a generally circular form, of a conductive portion 24d comprising a spirally molded copper plate, and an insulating material 24e for filling up the gap of the conductive portion 24d. Around the coil portion 24b, a core portion 24f is provided which is formed of a polyiron core to serve as a magnetic path. The circular shape of the coil portion 24b is one whose diameter nearly equals the diameter of a flame of a gas burner used when heating the steel plate 2, the same member to be heated. Thus, the coil portion 24b can achieve heating comparable to heating with the gas burner. As a preferred example, the coil portion 24b is 52 mm in diameter, while the core portion 24f is 84 mm in diameter. Fig. 10 is a vertical sectional view showing the high frequency heating head 24 of Fig. 8 in an enlarged manner. As shown in Fig. 10, the core portion 24f is a disk-shaped member having a recess which the coil portion 24b faces. The core portion 24f serves as a magnetic path of a magnetic flux generated by the coil portion 24b. Pipes 24g, 24h vertically perforate through the core portion 24f, and cool the coil portion 24b with cooling water flowing through the pipes 24g, 24h. The disk portion 24a is a ring-shaped member, which has the core portion 24f fitted into its center for fixation. In the foregoing embodiment, the insulating material 24e is cooled simultaneously with cooling of the coil portion 24b with cooling water, and thus can be formed from a heat resistant resin. The frequency of an electric current for induction heating is preferably, say, 20 kHz to 30 kHz. Since the member to be heated is a steel plate in the instant embodiment, the frequency may be suitably determined by the depth of penetration of the magnetic flux, heating efficiency, and so on, but may vary by several kilohertzes depending on the heating conditions. The range of the heating frequency is generally from several kHz to 60 kHz for a steel plate, but may favorably be 50 kHz to 100 kHz for an aluminum alloy. Of course, the optimum frequency varies with the thickness of the member to be heated. For a steel plate about 10 to 30 mm in thickness, the optimum diameter of the coil portion 24b is about 52 mm, which is the same dimension as the diameter of a flame of a gas burner for steel plate bending by conventional gas burner heating. Fig. 11 is a block diagram showing a control system of the automatic plate bending system concerned with the instant embodiment. As shown in Fig. 11, a heating point determining unit 41 reads data on the target shape and data on measurements of the steel plate, and performs predetermined processings (to be described in detail later on) , thereby determining heating points on the steel plate 2. The target shape data are, for example, design data developed by CAD 42, and are given as three-dimensional coordinate data, while the steel plate measurement data are given as three-dimensional coordinate data on the steel plate 2 that have been obtained based on measurements by the shape measuring unit 22. Aheating line determining unit 44 performs predetermined processings (to be described in detail later on) on the basis of information on the heating points determined by the heating point determining unit 41, thereby determining heating lines 3 on the steel plate 2 (see Fig. 3; the same will hold below). The heating lines 3 determined by the heating line determining unit 44 are sent to a control unit 45 as data comprising a sequence of points expressed in three-dimensional coordinates. The control unit 45 controls the travel of the travel system III comprising the longitudinally traveling trolley 14 and the transversely traveling trolley 16 on the basis of the point sequence data on the heating lines 3, thereby to control the position of the coil portion 24b, the heating means for the steel plate 2. Thus, induction heating of the steel plate 2 is performed with the coil portion 24b being moved along the heating lines 3, thereby bending the steel plate 2. On this occasion, the control unit 45 performs the overall control of the system of the present invention, as well as the control of the travel system III. Concretely, its control includes, for example, control of an electric current for supply to the coil portion 24b, driving control for the air cylinder 28, control associated with the supply of cooling water, and positional control for the universal poles 20. During positional control of the universal poles 20, in particular, overbending of the steel plate 2 is also prevented. In detail, the control unit 45 performs control such that each universal pole 20 moves in response to changes in the shape of the steel plate 2 as the steel plate 2 is bent. Then, when any of the universal poles 20 after this responsive movement reaches a target front end position for each universal pole 20 that has been determined on the basis of the target shape data on the steel plate 2, a heating operation by the automatic plate bending system is stopped. To carry out the above control for preventing excessive bending, the target shape of the steel plate 2 when contacted with the universal poles 20 must be made known beforehand. Thus, the control unit 45 stores not only the position of each universal pole 20 in a horizontal plane, the position of its front end portion, but also design data given by the CAD 42, and steel plate measurement data given by the shape measuring unit 22, as three-dimensional coordinates data. Based on these data, the control unit 45 calculates coordinates data on the target shape of the steel plate 2 at the position of contact of each universal pole 20 with the steel plate 2, to determine the target front end position of each universal pole 20. The movement of the universal pole 20 in response to changes in the shape of the steel plate 2 can be easily achieved by controlling the front end position of the universal pole 20 so that the force of contact of the universal pole 20 with the steel plate 2 will become more than a predetermined value. In an initial state of bending by the automatic plate bending system, not all of the universal poles 20 contact the steel plate 2. For the universal poles 20 out of contact with the steel plate 2, the above-mentioned control for responsive movement of the universal poles 20 is performed after the steel plate 2 contacts these universal poles 20 as the bendingproceeds. In the initial state, the universal pole 20 has its front end position adjusted to agree with a curved surface corresponding to a bend of about 60% relative to the target shape of the steel plate 2. On the universal poles 20 in this state, the steel plate 2 subjected to primary cold bending by a bending roll or the like is placed by a rough positioning operation. Then, the first bending work by the automatic plate bending system is done, with a shape about 80% of the target shape being targeted. A display unit 43 visualizes information associated with various processings by the automatic plate bending system, and also functions as an external input unit for entry of information necessary for processing. Figs. 12 (a) to 12 (e) are explanation drawings for illustrating an example of processing performed by the heating point determining unit 41. In these drawings, the numeral 1' denotes a virtual wooden pattern for illustration, and the numeral 2' represents a similar virtual steel plate. The term "virtual" refers to the fact that the wooden pattern or steel plate at issue does not exist as a real one, but exists as electronic data or a graphic expressed in a visible form on the display unit 43. The processing in this example, as has been done by an operator, is to find the points of contact of the wooden pattern 1' with the steel plate 2' while rolling the wooden pattern 1', to determine a heating point. Thus, we call this method "a contact point finding method". As shown in Fig. 12(a), the steel plate 2', the object to be bent, is assumed to be one of a curved shape that has been subjected to primary bending. Such steel plate 2', when observed on a minuscule scale, is thought not to have a smoothly varying curved surface, but to be a collection of flat surfaces bent at certain linear sites. For example, as shown in Fig. 12(a), the steel plate 2' forms a flat surface in a certain range beginning on an M line, the centerline in the plate width direction, and is bent at a certain position to have an angle of 10°. On the other hand, a target shape that the wooden pattern 1' has is given as in Fig. 12 (a). Thus, the wooden pattern 1' is rolled along a frame line from the initial position shown in Fig. 12(a), whereby the wooden pattern 1' is brought into contact with the steel plate 2' as shown in Fig. 12(b). At this time, contact points on the steel plate 2' are designated as A, B, while contact points on the wooden pattern 1' are designated as C, D. Then, the wooden pattern 1' is rolled in the reverse direction to return it to the initial state (the state shown in Fig. 12(a)) as shown in Fig. 12(c). With the wooden pattern 1' being returned to the initial state, a straight line U connecting the contact points A, B and a straight line V connecting the contact points C, D are obtained to find an intersection point P of the straight lines U, V and an angle  at which the straight lines U, V intersect. Based on this intersection point P, a heating point is determined. The angle  (3° in Fig. 12) is deemed as a bending angle at the heating point. Actually, the intersection point P is extended vertically upward in Fig. 12 (d) until it reaches the steel plate 2', to determine a heating position. The steel plate 2' is heated at this heating position, whereby it is bent by the angle , beginning at the heating position. This is a case shown in Fig. 12 (e). As shown in this drawing, this heating results in the contact of the contact point B of the steel plate 2' with the contact point D of the wooden pattern 1', thus bringing the shape of the steel plate 2' close to the target shape (the shape of the wooden pattern 1'). Strictly speaking, there is a misalignment between the intersection point P and the heating position based thereon (there is a difference in the Z axis coordinate, the position in the vertical direction). In the bending at issue, however, the lengths of the straight lines U, V ranging from the intersection point P to the contact points B, D are sufficiently large relative to the angle . Hence, there is practically no harm in handling the intersection point P and the heating position based thereon as the same position. Then, the same procedure (the procedure shown in Figs. 12 (b) to 12 (d) ) is performed, provided that the state of contact of the contact point C of the wooden pattern 1' with the contact point A represents a reference position corresponding to the aforementioned initial position. By this measure, a heating point and a bending angle  at the heating point are determined. This procedure is repeated until the wooden pattern 1' is rolled to reach the end of the steel plate 2', whereby heating points and bending angles  at the heating points are determined sequentially. Figs. 13(a) to 13(c) are explanation drawings conceptually illustrating display screens of the display unit 43 when the heating point is determined by the heating point determining unit 41. Fig. 13(a) corresponds to the initial position, Fig. 13 (b) corresponds to a case in which the wooden pattern 1' is rolled once, and Fig. 13(c; corresponds to a case in which the wooden pattern 1' is rolled twice. Fig. 14 is an explanation drawing conceptually showing the blank layout of the steel plate 2, the object to be processed in the instant embodiment. As shown in Fig. 14, a virtual steel plate 2' which is a part of a cylindrical surface with a radius R taken out as in the drawing is assumed in the instant embodiment. To form this cylindrical surface approximately by bending, it is recommendable to bend the surface along the central axis of the cylinder so that its cross section is polygonal. That is, a roller reference line 16' is defined as indicating the direction of the central axis when the target shape is roughly deemed to be a cylindrical surface. Fig. 14 shows a case in which the M line, the centerline in the plate width direction, intersects the roller reference line 16'. The roller reference line 16' and the M line are not always in this relation. Since the steel plate 2' forms a part of the outer panel of a ship hull, for example, the roller reference line 16' and the M line may agree in a certain case. Figs. 15(a), (b), (c) and (d) are explanation drawings for illustrating an example of processing performed b the heating line determining unit 14 not covered by the claims. Determination by the heating line in this case is performed by connecting the heating points, which have been determined by the heating point determining unit 41, by a virtual straight line, examining the degree of parallelism between this straight line and a virtual roller line 16" drawn on a virtual steel plate 2', and grouping the heating points, whose straight lines show a predetermined degree of parallelism, into the same group. Grouping is performed while dividing the heating points into those above and those below the roller line 16". In Fig. 15, F 1 to F 7 represent virtual frame lines. The subscripts attached to the symbol F designate the frame line numbers. Many dots indicated narrowly at right angles to the respective frame lines F 1 to F 7 refer to the heating points. As shown in Fig. 15(a), a starting point 1 is set first of all. From this starting point 1, virtual straight lines (indicated as dashed lines in Fig. 15) are drawn toward the heating points on the respective frame lines F 1 to F 7 . The starting point is established on the frame line of a smaller frame line No. and at a site nearer to the roller line 16". Then, the degree of parallelism, relative to the roller line 16", of each of the virtual straight lines drawn toward the heating points on the respective frame lines F 1 to F 7 is examined as stated above. The heating points that give the parallel lines or whose straight lines intersect the roller line 16" at angles not larger than a predetermined angle are grouped together into the same group. Fig. 15(a) shows that the heating points of the same group satisfying the requirement for the degree of parallelism based on the starting point 1 are present on the frame lines F 3 , F 4 . Upon completion of grouping based on the starting point 1, grouping based on a starting point 2 is performed in accordance with the same procedure, as shown in Fig. 15(b). Fig. 15(b) shows that the heating points belonging to Group 1 based on the starting point 1 have been fixed, and the heating points based on the starting point 2 are being investigated. On this occasion, the heating points that have already been grouped are neither used as the starting points nor subjected to grouping. In this manner, the heating points lying below the roller line 16" are grouped. After grouping work is completed, a straight line (or a curve) is obtained from the sequence of heating points in each group, as shown in Fig. 15 (c), and this line is designated as a virtual heating line 3'. The heating line 3' is obtained by the method of least squares if it is a straight line, or by spline interpolation or the like if it is a curve. Fig. 16 is a flow chart showing a concrete procedure (example) using the heating point determining unit 41 when obtaining the heating points by the contact point finding method not covered by the claims. In the instant embodiment, the heating points are obtained on the frame lines, but needless to say, the way of obtaining them is not restricted to this manner. However, the frame materials are attached. Thus, data on their positions are stored as design data. The use of the frame lines in obtaining the heating points is advantageous in the applicability of such data. The above-mentioned procedure will be explained based on Fig. 16. 1) Design data such as CAD data are loaded to enter the target shape of the steel plate as three-dimensional data (step S 1 ). 2) The shape of the steel plate, the object to be processed, is measured to obtain three-dimensional coordinate data thereon (step S 2 ). This can be easily performed by an existing measuring method, such as laser measurement or image processing of an image shot with a camera. 3) The processings at step S 4 through step S 14 are performed for the respective frame lines (step S 3 ). The expression "Loop ..." indicated in the block for step S 3 refers to an operation in which the processings subsequent to the step at issue (in this case, step S 3 ) are deemed to be one loop, and the processings belonging to this loop are sequentially repeated for each frame line, as in the instant embodiment (the same will hold later on). At step S 3 , the frame line No. i is designated as "1", and the flow moves to the processing at a next step S 4 . "FLMAX" means the maximum frame line No. (the same will hold later on). 4) Since no heating point exists initially, j=0 is set as the initial value of the heating point No. (step S 4 ). 5) The position and posture of the target shape are recorded (step S 5 ). Concretely, records are made, for example, of the coordinates of the reference point of the target shape (the point of intersection between a curve of the frame line showing the target shape and a sight line, i.e., the point of the virtual wooden pattern showing the M line), and the inclination of the sight line (the inclination angle based on the horizontal line or the vertical line). The state on this occasion corresponds to the initial state in which during an operation using a conventional wooden pattern, an operator places the middle point of a portion of the wooden pattern extending along the target shape on the M line of the steel plate, and holds the sight line vertically. 6) The target shape is rolled along the steel plate (step S 6 ), and its rolling is repeated until the target shape reaches the end of the steel plate (step S 7 ). When the target shape and the steel plate are detected to have contacted at 2 points during the rolling (S 8 ), the processing described in the aforementioned "principle of the contact point finding method" is performed to determine the coordinates of the intersection point P and its angle  (steps S 9 , S 10 , S 11 and S 12 ). 7) "1" is added to the heating point No., and data on the respective heating points on specific frame lines are compiled (steps S 13 and S 14 ). These data on the heating points are given as three-dimensional coordinate and angle data with the respective frame line Nos. and the respective heating point Nos. specified. 8) When it is detected at the judging step (step S 7 ) that the end of the steel plate has been reached, it is judged whether the frame line No. at this time is larger than the maximum value of the number of the frame lines (FLMAX) for which the heating point determining processings are performed. If the frame line No. i<FLMAX, the processings at steps S 4 to S 14 are repeated for the frame line of the next No.. Whenever the flow returns to step S 4 , "1" is added to the frame line No. i. If the frame line No. i≥FLMAX, this means that the predetermined processings for obtaining the heating points have been completed for all the frame lines. Thus, the heating point determining processings are ended (steps S 15 and S 16 ). 9) When it is not detected by the processing at step S 8 that no contact at 2 points has been made, the flow returns to the processing at step S 5 , and the processing at steps S 5 to S 7 are repeated. That is, the target shape is rolled at a certain angle by a single processing, and the processings at steps S 5 to S 7 are repeated until contact at 2 points is detected. Thus, if the shape of the steel plate extending along the frame line for which the heating points are to be determined is a flat plane, it is detected by the processing at step S 7 that the end of the steel plate has been reached with no contact point being determined. Thus, a judgment is made that no heating point exists for this frame line, and the flow moves to the processing for the next frame line. If no contact at 2 points has been detected for all the frame lines, namely, if the entire steel plate is of a flat shape, no heating points can be determined by the "contact point finding method". Thus, the steel plate for which heating points should be determined by this method must have been subjected to primary bending with a bending roll or the like. According to the processing at step S 6 , the target shape is rolled along the steel plate, but the same effect is obtained if the steel plate is rolled along the target shape. In short, one of them may be rolled relative to the other so that the contact point of the two is obtained. The purpose of determining the heating points in the above manner is to obtain the heating positions and heating intensities (quantities of heat given to the steel plate) for causing the necessary change in shape. Between the heating intensity and the angle 8, there is a predetermined relationship, which can be found experimentally, Thus, at a time when the angle  is found, the heating intensity can be determined (needles to say, if the angle  is recorded as data, it can be converted to the heating intensity later, where necessary). Thus, at step S 14 , the heating intensity with respect to the angle  may be obtained along with data on the angle , although this is not directly related to the processing for finding the heating point. Figs. 17 to 20 are flow charts showing a concrete procedure (example) using the heating line determining unit 44 when obtaining the heating line determining unit 44 when obtaining the heating lines on the basis of the heating points determined not covered by the claims. This procedure will be explained based on these drawings. The following processings are performed as shown in Fig. 17: 1) Data on the heating points are entered (step S 21 ). Concretely, entry is made of the three-dimensional coordinate and angle data on the respective heating points on the respective frame lines that have been obtained at step S 14 of Fig. 16. 2) Since no predetermined group is formed initially, g=0 is set as the initial value of the group No. g (step S 22 ). 3) The processings at steps S 24 to S 54 are performed for the respective frame lines (step S 23 ). 4) It is judged whether the number of the upper heating points on the frame line of the frame line No. i is HPU (i) >0 (step S 24 ). "The number of the upper heating points, HPU" means the number of the heating points above the roller line 16" found when it is determined whether the heating point is above or below the roller line 16". For example, the heating point with a larger Y coordinate than that of the point of intersection of each frame line and the roller line 16" is regarded as the upper heating point. Thus, if the upper heating point exists, HPU (i) >0. In this case, the flow moves to the processing at step S 25 . 5) The processings at steps S 26 to S 38 are performed for the respective upper heating points on the frame line of the frame line No. i (step S 25 ). That is, the same processings are carried out for the respective heating points of the heating point Nos. j=1∼HPU (i) to perform their grouping. 6) It is judged whether grouping is finished or not (step S 26 ). Concretely, it is judged whether the group No. g is assigned to the heating points that are being judged. 7) When the judgment at step S 26 shows that the heating points, the objects being judged, have not been grouped, "1" is added to the group No. g (step S 27 ). Since the initial value of the group No. g is "0", the group No. g=1 is given at the processing for the first heating point concerned with the first frame line. 8) The heating point, the object being processed, is given the group No. g assigned at step S 27 (step S 28 ). 9) The number of the heating points belonging to the group is designated as "1" (step S 29 ). 10) A starting point is determined by the processings at steps S 27 to S 29 . 11) The processings at steps S 31 to S 37 are performed for the respective frame lines of the frame line Nos. i later than the frame line No. i (step S 30 ). These frame line Nos. are k=(i+1)∼FLMAX. 12) The processings at steps S 32 to S 36 are performed for the respective upper heating points on the frame line of the frame line No. k (step S 31 ). 13) It is judged whether grouping of the specific heating points on the frame line of the frame line No. k is finished or not (step S 32 ). Concretely, it is judged whether the group No. g is assigned to the heating point being judged. 14) When the judgment at step S 32 shows that the heating point being judged has not been grouped, it is judged whether this heating point is at a position parallel to the roller line 16" when viewed from the starting point (step S 33 ). For example, the heating point as the starting point and the heating point as the object being judged are connected together by a straight line, and the angle of this straight line to the roller line 16" is detected. If this angle is less than a predetermined value, a judgment is made that the heating point in question is at a parallel position. Alternatively, the same judgment can be made by measuring the distance between each end of the straight line and the roller line 16", and detecting whether the distances measured are each within a certain range. 15) When the judgment at step S 33 shows that the heating point being judged lies at a position parallel to the roller line 16", this heating point is assigned the same group No. g as that of the heating point as the starting point (step S 34 ). 16) "1" is added to the number of the heating points of the group No. g assigned at step S 34 (step S 35 ). 17) When the processing at step S 35 is completed, or when grouping of the heating points being judged by the processing at step S 32 is completed, or when the absence of a predetermined degree of parallelism is detected by the processing at step S 33 , the processings at steps S 32 to S 35 are repeated (step S 36 ) until the heating point No. 1 of the heating point being judged as belonging to the frame line of the frame line No. k becomes larger than the maximum value HPU (k). Whenever the flow returns from step S 36 to step S 32 , "1" is added to the heating point No.. In this manner, grouping of the heating points on the specific frame line is performed. 18) When it is detected by the processing at step S 36 that grouping of all the upper heating points on the frame line of the frame line No. k is completed, the processings at steps S 31 to S 36 are repeated until the frame line No. k becomes larger than the maximum value FLMAX (step S 37 ). Whenever the flow returns from step S 37 to step S 32 , "1" is added to the frame No. k. In this manner, grouping of the upper heating points for all the frame lines of the frame line Nos. later than i is performed. 19) When it is judged by the processing at step S 25 that grouping of the heating points, the objects being judged, on the frame line of the frame line No. i has been finished, or when it is detected by the processing at step S 37 that grouping of the upper heating points for all the frame lines of the frame line Nos. later than i has been finished, the processings at steps S 26 to S 38 are repeated (step S 38 ) until the heating point No. j of the heating point being judged as belonging to the frame line of the frame line No. i becomes larger than the maximum value HPU(i). Whenever the flow returns from step S 38 to step S 26 , "1" is added to the heating point No.. In this manner, grouping of the upper heating points on the frame line of the frame line No. i is performed. As shown in Fig. 18, the following processings are performed: 20) When it is detected by the processing at step S 24 that no upper heating points exist on the frame line of the frame line No. i, or when it is detected by the processing at step S 38 that grouping of all the upper heating points on the frame line where the starting point belongs is completed, grouping of the lower heating points on each frame line is performed by exactly the same procedure. That is, the processings at steps S 39 to S 53 corresponding to the processings at steps S 24 to S 38 are performed for the lower heating points. At step S 39 , "the number of the lower heating points, HPL" refers to the number of the heating points that is in contrast to the upper heating points when it is determined whether the heating point is above or below the roller line 16". In other words, HPL means the number of the heating points below the roller line 16". For example, the heating point with a smaller Y coordinate than that of the point of intersection of each frame line and the roller line 16" is regarded as the lower heating point. 21) When it is detected by the processing at step S 39 that no lower heating points exist on the frame line of the frame line No. i, or when it is detected by the processing at step S 53 that grouping of all the lower heating points on the frame line where the starting point belongs is completed, it is judged whether the frame line No. is larger than FLMAX. If it is smaller, the processings at steps S 24 to S 53 are repeated for each frame line. When these processings are completed for all the frame lines, i.e., when grouping of all the heating points belonging to all the frame lines is completed, the flow moves to the next processing (step S 54 ). As shown in Fig. 19, the following processings are performed: 22) For each heating point group established, the heating points of each group are sequentially connected together by a straight line, or a straight line or a curve is calculated by the method of least squares, spline interpolation or the like based on the coordinate values of the heating points, thereby to obtain a heating line (steps S 55 and S 56 ). At step S 55 , "G NO " refers to the maximum value of the number of the groups. 23) When it is detected that the group No. ≥ G NO , i.e., when it is detected that the heating lines 3 have been determined for all the groups, all the processings are completed (steps S 57 and S 58 ). Fig. 20 shows an example in which the heating intensity (determined by the bending angle ) at each heating point is taken into consideration during the processings shown in Fig. 19, and the information on the heating intensity is incorporated into the information on the heating line. As shown in Fig. 20, the distribution of the heating intensity is calculated for the determined heating line by the process subsequent to step S 56 in accordance with the instant embodiment (step S 59 ). The heating intensity has been directly obtained separately based on the bending angle  at the heating point, or is determined on the basis of information on the bending angle  at the heating point. According to the instant embodiment, the heating points on each heating line 3 can be heated with the most appropriate quantity of heat. In the case of bending by high frequency heating, for example, this can be easily achieved by controlling an electric current supplied to the high frequency heating coil to control the amount of heat input to the steel plate 2. Fig. 21 shows an example not covered by the claims in which the heating intensity (determined by the bending angle ) at each heating point is taken into consideration during the processings illustrated in Figs. 17 and 18, and this heating intensity is also incorporated into the conditions for grouping. As shown in Fig. 21, in accordance with the instant embodiment, it is judged by the processing subsequent to step S 33 or step S 48 whether the heating intensity is same as the heating intensity at the starting point (the heating intensity includes that within a predetermined tolerance range) (step S 60 ). If this judgment shows that the heating point in question does not have the same heating intensity, this heating point is excluded from the relevant group. In other words, the same group No. as that of the starting point is assigned to the heating point, provided that it has the same heating intensity. According to the instant embodiment, the heating points on each heating line 3 can be heated with the same quantity of heat. In the case of bending by high frequency heating, for example, the most appropriate amount of heat input to the steel plate can be given by keeping the electric current supplied to the high frequency heating coil constant for a single heating line 3. In the above-described embodiments, the term "virtual" has been defined as not existing as a real one, but existing as electronic data or a graphic expressed in a visible form on the display unit 43. However, such a restriction need not be applied to the technical idea of the present invention. A wooden pattern and a steel plate which an operator prepares by plotting are also included in the concept "virtual" as referred to herein, unless they are real ones. Figs. 22 to 24 are explanation drawings for illustrating another example of processing performed by the heating point determining unit 41. The processing shown in these drawings focuses on the fact that the curved shape of the steel plate 2 on a predetermined line, such as each frame line, can be regarded as a collection of arcs with a plurality of curvatures. The arc of the target shape is compared with the arc of an actually measured shape is compared with the arc of an actually measured shape corresponding to this arc portion on the basis of the curvatures of both arcs. Based on the results of comparison, the heating point is determined. This method is called "the curvature comparison method". Figs. 22 and 23 are views for illustrating the principle of the curvature comparison method. Fig. 22 shows the curve of the target shape (only its half to the right of M line, the reference line, is shown) divided into fine segments D 1 to D n which are arcs with radii of R 1 to R n . Whereas Fig. 23 shows a mode in which one of the divisional arcs indicated in Fig. 22 is approximated by a fold line defined by the bases of a plurality of (number m in Fig. 23) congruent isosceles triangles connected together while sharing their equal sides. As shown in Fig. 22, the target shape is divided into a plurality of fine segments D 1 to D n , these fine segments D 1 to D n are regarded as arcs, curvatures or radii are designated for the respective segments D 1 to D n , and the lengths 1 1 to 1 n of the arcs of the respective segments D 1 to D n are designated, whereby the target shape can be specified. Thus, if the target shape data in the respective segments D 1 to D n are compared with the steel plate measurement data, the amount of deformation of the steel plate 2 for making the target shape and the shape of the steel plate agree can be determined by the difference between the two types of data. Here, the deformation in heat bending is bending at the heating points. That is, the arcs in the respective fine segments are approximated by straight lines. As shown in fig. 23, when an arc with radius R is approximated by the fold line defined by the bases of the m number of the isosceles triangles connected together while sharing their equal sides, the length 1 of the arc is generally given by the equation (1): L = 2·R·m In the equation (1),  is the angle between the bases of the isosceles triangles. Fig. 24 is an explanation drawing which shows by a two-dot chain line a mode in which the arc of one segment of the target shape is approximated by a fold line N O defined by the bases of the m number of isosceles triangles connected together while sharing their equal sides, and which shows by a solid line a mode in which the arc of one segment of the measured shape corresponding to this segment is approximated by a fold line N c defined by the bases of the m number of isosceles triangles connected together while sharing their equal sides. As shown in Fig. 24, straight lines connecting the points (P O1 , P O2 ), (P O2 , P O3 ), (P O3 , P O4 ) ..... make the fold line N O , while straight lines connecting the points (P C1 , P C2 ), (P C2 , P C3 ), (P C3 , P C4 ) ..... make the fold line N C .  O is the angle that each subline of the fold line N O forms with the adjacent subline, while  C is the angle that each subline of the fold line N C forms with the adjacent subline. Referring to Fig. 24, one will see that when each subline of the fold line based on the measured shape indicated by the solid line is bent by Δ (= O -  C ), it coincides with each subline of the fold line based on the target shape. Let the length of the segment of the target shape and the measured shape of the steel plate 2 to be compared be l O , and the radius of the arc of the target shape in this segment be R O . When this arc is approximated by the fold line N O defined by the bases of the m number of isosceles triangles connected together while sharing their equal sides, the relation of the equation (2) is obtained from the equation (1): l O = 2 O ·R O ·m On the other hand, let the radius of the arc based on the measured shape of the portion corresponding to the segment to be compared be R C . When this arc is approximated by the fold line N C defined by the bases of the m number of isosceles triangles connected together while sharing their equal sides, the relation of the equation (3) is obtained from the equation (1): l C = 2 C ·R C ·m To heat-process the measured shape into the target shape, it is necessary to bend the m number of sublines of the fold line N C for the measured shape in the manner stated earlier. When the bending angle at this time is designated as Δ, the bending angle Δ is given as the difference between the angle formed by the adjacent sublines of the fold line N C and the angle formed by the adjacent sublines of the fold line N O . That is, the bending angle Δ is expressed by the equation (4): Δ =  O -  C = (l 0 /2R 0 ·m) - (l 0 /2R C ·m) = (l 0 (R C - R 0 )}/(2·R 0 ·R C ·m) Here, the lengths of the fold lines to be compared are equal, so that l 0 = l c . In heating of a single steel plate 2, its efficiency is high when the amount of heating (e.g., the amount of heat input based on parameters such as an electric current, and the clearance between a high frequency heating coil and the steel plate 2, during high frequency heating) is made constant overall. When the amount of heating is constant, the bending angle Δ is derived from the properties (material, thickness, etc.) of the steel plate 2. That is, a predetermined bending angle Δ is determined by determining the desired amount of heating, and the number m of the sublines of each of the fold lines N O and N C is given by the equation (5): m = {l 0 (R C - R 0 )}/(2·R 0 ·R C ·Δ) This means that if the bending angle Δ is given, it suffices to divide the length l C by the number m calculated from the equation (5). In other words, the heating points are obtained as respective positions found when the length l C is divided by the heating distance (l C /m). That is, if the radius R O of the arc of the target shape, the radius R C of the arc of the measured shape corresponding thereto, the length l 0 (length of the segment to be compared) of both arcs, and the bending angle Δ are given, then the three-dimensional positional coordinates of the corresponding heating points can be sought as solutions to geometrical problems by computations. In case the steel plate 2 is a flat plate, on the other hand, the radius R C in the equation (5) becomes infinity, so that m cannot be obtained. Thus, the equation (5) is converted into the equation (6): m = (l 0 (R C - R 0 ) }/(2·R 0 ·R C ·Δ) = {l 0 (1 - R 0 /R C )}/(2·R 0 ·Δ) Infinitizing R c in the equation (6) makes (R 0 /R c ) zero, thus giving the equation (7): m = l 0 /(2·R 0 ·Δ) The equation (7) is equal to calculating the number m of isosceles triangles for the length l 0 of the arc in the isosceles triangles which inscribe in the target shape with radius R 0 and whose adjacent bases form the angle Δ. In short, when a flat plate is bent, the heating distance can be found from the radius R 0 of the target shape and the bending angle Δ. To determine the heating points by the above-described curvature comparison method, the heating point determining unit 41 prepares the following data on the basis of the target shape data read in: 1 ○ position data on the reference line on each frame line, 2 ○ position data on the end of the steel plate 2 as the object to be processed, 3 ○ curvature data on the arc in each segment when the curved shape of the steel plate 2 on each frame line is regarded as a collection of arcs with a plurality of curvatures, and 4 ○ position data on the point of the boundary between each segment and the adjacent segment. The curvature data 3 ○ are values designated at the time of designing, or if these values are not designated, the data are calculated using the point sequence data of the target shape data. Similarly, data corresponding to 1 ○ to 4 ○ are compiled from the steel plate shape measurement data as well. At this time, the data 3 ○ correspond to the respective segments of the target shape. The heating point determining unit 41 processes the data 1 ○ to 4 ○ on the target shape and the measured shape, and calculates the heating points by the curvature comparison method described based on Figs. 22 to 24. An example of the relevant concrete procedure will be explained by reference to Figs. 25 to 28. Figs. 25 to 28 are flow charts showing this example. In this example, the heating points are obtained on the frame lines, but needless to say, the way of obtaining them is not restricted to this manner. However, the frame lines are lines corresponding to the positions at which frame materials are attached. Thus, data on their positions are stored as design data. The use of the frame lines in obtaining the heating points is advantageous in the applicability of such data. As shown in Fig. 25, the following processings are performed: 1) Design data such as CAD data are loaded to enter the target shape of the steel plate as three-dimensional data, and processings are also performed for the preparation of the data 1 ○ to 4 ○, such as curvature data on the arc in each segment constituting each frame line, and position data on the point of the boundary between each segment and the adjacent segment (step S 1 ). 2) The shape of the steel plate 2, the object to be processed, is measured to obtain three-dimensional coordinate data thereon, and processings are also performed for the preparation of the data 1 ○ to 4 ○ as for the target shape (step S 2 ). Measurement of the shape of the steel plate 2 can be easily performed by an existing measuring method, such as laser measurement or image processing of an image shot with a camera. 3) The bending angle Δ, a heat deforming angle, is set (step S 3 ). 4) The processings at step S 5 through step S 41 are performed for the respective frame lines (step S 4 ). The expression "Loop ..." indicated in the block for step S 4 refers to an operation in which the processings at steps subsequent to the step at issue (in this case, step S 4 ) are regarded as one loop, and the processings belonging to this loop are sequentially repeated for each frame line, as in the instant embodiment (the same will hold later on) . At step S 4 , the frame line No. i is designated as "1", and the flow moves to the processing at a next step S 5 . "FLMAX" means the maximum frame line No. (the same will hold later on). 5) Since no upper heating point exists initially, "0" is set as the initial value of the heating point No. (step S 5 ). "The upper heating point" means the heating point above a reference line, a straight line heading in the direction of a central axis of a cylinder whose part is deemed to approximate the target shape of the steel plate 2 (e.g., a point above the roller reference line 16' used in the explanation of a heating line determination method to be detailed later based on Fig. 14) when it is determined whether the heating point is above or below the reference line. For example, the heating point with a larger Y coordinate than that of a point on the reference line is regarded as the upper heating point. 6) The processings at step S 7 to step S 22 are performed for the respective segments, DM to DMAX, to be compared (step S 6 ). "DM" denotes the No. of the segment where the M line, the initial reference position, exists. "DMAX" designates the maximum value of the segment No.. 7) It is judged whether the segment is the segment where the M line, the initial reference position, exists (step S 7 ). 8) If the processing at step S 7 shows it to be the segment where the M line exists, a judgment is made that the reference point is at the position of the M line. Based on this judgment, this position is set (step S 8 ). 9) If the processing at step S 7 shows it to be the segment where no M line exists, a judgment is made that the reference point is at the end of the segment nearer to the M line. Based on this judgment, this position is set (step S 9 ). 10) The radius R C is found from the measurement data on the relevant segment (step S 10 ). 11) It is judged whether R C is larger than the radius R max (step S 11 ). The radius R max has been set at a value large enough for the steel plate to be regarded as a flat plate (radius = infinity). 12) If the processing at step S 11 shows R C > R max , the steel plate 2 as the object to be processed is deemed to be a flat plate. Thus, a calculation based on the equation (8) is done to determine the number m of the sublines of a fold line belonging to the relevant segment (step S 12 ). 13) If the processing at step S 11 shows R C ≤ R max , a calculation based on the equation (7) is made to determine the number m of the sublines of a fold line belonging to the relevant segment (step S 13 ). The value of m is treated such that the digits to the right of the decimal point are discarded to give an integer. 14) It is judged whether the number m of the sublines is larger than 1 (step S 14 ). As shown in Fig. 26, the following processings are performed: 15) If the processing at step S 14 shows m > 1, the length l of the heating distance (l = l 0 /m) is calculated (step S 15 ). If m ≤ 1, this means that two or more sublines are not present in the relevant segment, and there is no apex which should serve as the position of bending. Thus, the procedure moves to the processing for a next segment. 16) The processings at steps S 17 through S 21 are performed for the respective sublines of the fold line belonging to the relevant segment (step S 16 ). 17) It is judged whether a point apart from the reference point in the relevant segment by the length 1 of the heating distance exists in this segment (step S 17 ). 18) If the processing at step S 17 shows the existence of such a point in the segment, "1" is added to the upper heating point No. (step S 18 ). If that processing shows the absence of such a point, the flow moves to the processing for a next segment. 19) In addition to the upper heating point No. associated with the processing at step S 18 , the coordinate value of this heating point is recorded (step S 19 ). 20) The reference point is changed to the heating point determined at step S 19 (step S 20 ). 21) The processings at steps S 17 through S 20 are repeated until the No. of the subline belonging to the segment becomes k ≥ m (step S 21 ). Each time the flow returns from step S 21 to the processing at step S 17 , "1" is added to the subline No. k. 22) If the processing at step S 21 shows k ≥ m, if the processing at step S 17 shows the absence of a predetermined point in the segment, or if the processing at step S 14 shows m ≤ 1, the processings at steps S 7 through S 21 are repeated until the segment No. becomes j > DMAX (step S 22 ). Each time the flow returns from step S 22 to the processing at step S 7 , "1" is added to the segment No. j. As shown in Figs. 27 and 28, the following processings are performed: 23) The same processings as those at steps S 5 to S 40 are performed for the lower heating points (steps S 23 to S 40 ). 24) If the processing at step S 40 shows j > DM, this means that the upper and lower heating points have been determined for a certain frame line. Thus, the flow returns to the processing at step S 5 , and the processings at steps S 5 through S 40 are repeated until i > FLMAX (step S 41 ). Each time the flow returns from step S 41 to the processing at step S 5 , "1" is added to the frame line No. i. When i > FLMAX, all the processings are completed (step S 42 ). A concrete procedure using the heating line determining unit 44 for determining the heating lines based on the heating points that have been determined by the curvature comparison method is the same as that described in the flow charts for the aforementioned embodiment (Figs. 17 to 19). That is, the three-dimensional data on the heating points on the respective frame lines obtained at step S 19 of Fig. 26 and step S 37 of Fig. 28 are entered for "Enter sequence of heating points" at step S 21 of Fig. 17. The automatic plate bending system concerned with the instant embodiment has the coil portion 24b (see Fig. 8) whose portion generating a magnetic flux for induction heating the steel plate 2 is shaped like a circle with a diameter nearly equal to the diameter of a flame of a gas burner used when heating the steel plate 2. Thus, the automatic plate bending system can perform various forms of heating, including line heating along the heating line 3. Figs. 29(a) to 29(d) show forms of heating of the steel plate 2 using the coil portion 24b concerned with the above-described embodiment. In these drawings, the locus of movement of the coil portion 24b is indicated by a two-dot chain line. Fig. 29 (a) represents line heating. The line heating over an arbitrary length can be performed by linearly moving the coil portion 24b. Fig. 29(b) represents spot heating. In the case of spot heating, the coil portion 24b is moved spirally, whereby heating can be performed in a circular shape with an arbitrary radius. Fig. 28 (c) represents weaving heating. With the weaving heating, the coil portion 24b is moved in a zigzag form, whereby a wavy shape with an arbitrary width can be heated. Fig. 29 (d) represents pine needle heating. With the pine needle heating, an arbitrary triangular shape can be heated by moving the coil portion 24b while continuously varying its zigzag width. With induction heating using the coil portion 24b, it is vital, as stated earlier, that the clearance between the coil portion 24b and the steel plate 2, the member to be heated, be kept constant. To secure a constant clearance between the coil portion 24b and the steel plate 2, the high frequency heating head 24 is provided with the steel ball portions 24c in the aforementioned embodiment. Means of securing the clearance is not restricted to them. A constant clearance can be secured by utilizing a magnetic force or a reaction force by a high pressure gas. Fig. 30 is an explanation drawing conceptually showing a first modified example of a structure for retaining clearance with which the coil portion 24b is mounted. As shown in Fig. 30, the mounting clearance retaining structure according to this example has a magnet 51 disposed on an outer peripheral part of the coil portion 24b so as to surround the coil portion 24b. The magnet 51 is secured to the disk portion 24a. The steel plate 2, the member to be heated, is magnetized such that its surface opposed to the magnet 51 is of the same polarity as the polarity of a surface of the magnet 51 facing the steel plate 2. Thus, the coil portion 24b levitates under a magnetic repulsive force working between the magnet 51 and the magnetized surface of the steel plate 2, thereby keeping the clearance between the coil portion 24b and the steel plate 2 constant. Fig. 31 is an explanation drawing conceptually showing a second modified example of a structure for retaining clearance with which the coil portion 24b is mounted. As shown in Fig. 31, the mounting clearance retaining structure according to this example differs from the first modified example shown in Fig. 30 in that a magnetic force source 52 is disposed below the steel plate 42. This magnetic force source 52 magnetizes the steel plate 42 such that the surface of the steel plate 42 opposed to the magnet 51 is of the same polarity as the polarity of the opposed surface of the magnet 51. Thus, the coil portion 24b levitates under a magnetic repulsive force working between the magnet 51 and the magnetized surface of the steel plate 42, as in the first modified example, whereby the clearance between the coil portion 24b and the steel plate 42 is kept constant. In order for the portion of the steel plate 42 opposed to the magnet 51 to be magnetized always satisfactorily, the magnetic force source 52 is adapted to move synchronously with the movement of the coil portion 24b so as to be located below the magnet 51 as the coil portion 24b moves. Fig. 32 is an explanation drawing conceptually showing a third modified example of a structure for retaining clearance with which the coil portion 24b is mounted. As shown in Fig. 32, the mounting clearance retaining structure according to this example has a plurality of nozzles 53 disposed around the coil portion 24b, and jets high pressure air 56 vertically downwardly through the nozzles 53 toward the surface of the steel plate 2. By this measure, the coil portion 24b is levitated under a reaction force by jets of the high pressure air 56, whereby the clearance between the coil portion 24b and the steel plate 2 is kept constant. The nozzles 53 are secured to the disk portion 24a. Fig. 33 is an explanation drawing conceptually showing a fourth modified example of a structure for retaining clearance with which the coil portion 24b is mounted. As shown in Fig. 33, the mounting clearance retaining structure according to this example covers the coil portion 24b with a cover 54. The cover 54 has an opening which opens downwardly, and has its upper part secured to the disk portion 24a. The cover 54 has a pipe 55 which is attached thereto while piercing through a part of the upper surface of the cover 54, and high pressure air 56 is supplied into the cover 54 through the pipe 55. Also, the high pressure air 56 supplied into the cover 54 is jetted toward the surface of the steel plate 2 opposed to the aforementioned opening. Thus, the coil portion 24b is levitated under a reaction force generated by jets of the high pressure air 56, whereby the clearance between the coil portion 24b and the steel plate 2 is kept constant. In the first and second modified examples of the foregoing modified examples, the magnet 51 may be a permanent magnet or an electromagnet. In view of the controllability that the magnetic force can be varied arbitrarily by an electric current, the electromagnet is preferred. In the first to fourth modified examples, the position of the coil portion 24b is measured with a sensor, although this is not shown. Control is performed such that the position of the coil portion 24b relative to the steel plate 2 is detected on the basis of positional information obtained by the measurement, whereupon the clearance between the coil portion 24b and the steel plate 2 will become constant. This control can be achieved by feedback controlling the magnetic force of the magnet 51 or the steel plate 2 in the first modified example, or the magnetic force of the magnet 51 or magnetic force source 52 in the second modified example, on the basis of the positional information. In the third and fourth modified examples, on the other hand, the control can be achieved by feedback controlling the amount or pressure of jets of the high pressure air 56 on the basis of the positional information.

Claims: An automatic plate bending system using high frequency induction heating, comprising: a travel system free to travel in a horizontal plane, said travel system having a longitudinally travelling trolley (14, 15) stretching over two parallel rails (14a, 15a) and travelling along these rails, and a transversely travelling trolley (16, 17) travelling on the longitudinally travelling trolley (14, 15) in a direction perpendicular to the direction of the rails (14a, 15a); a high frequency heating coil (02) for induction heating the surface of a member to be heated, said high frequency heating coil (02) being attached to the transversely travelling trolley (16, 17) so as to be vertically moveable, and being opposed, with a constant clearance, to the surface of the member to be heated; universal poles (20, 21) disposed vertically at a multiplicity of specified positions between the rails (14a, 15a), with the height positions of front end portions of the universal poles (20, 21) themselves being adjustable, so as to bear the member to be heated by supporting the member from below; and a control unit (45) for controlling the travel of the travel system in the horizontal plane on the basis of predetermined heating line data so that the high frequency heating coil (02) heats the member to be heated, along predetermined heating lines via the travel system, characterized by a heating point determining unit (41) which reads in target shape data on a target shape of a steel plate (2) to be bend, and steel plate shape measurement data to be obtained by measuring a surface shape of the steel plate (2); divides a curve of target shape of the steel plate into a plurality of success segments (D 1 to D n ); similarly divides a curve of the measured shape of the steel plate into a plurality of successive segments (D 1 to D n ); in correspondence with the curve of the target shape; determines the number of a plurality (m) of congruent isosceles triangles, which are connected together while sharing their equal sides, for each segment (D 1 to D n ); on the basis of the radius (R 1 to R n ); of a division of the curve in each segment (D 1 to D n ); of the target shape of the steel plate, the radius (R 1 to R n ); of a division of the curve in each segment (D 1 to D n ); of the measured shape of the steel plate (2), and a separately said bending angle of the steel plate so that when the division of the curve in each segment (D 1 to D n ); of the target shape of the steel plate (2) is regarded as an arc, the arc in each segment (D 1 to D n ); of the target shape of the steel plate (2) can be approximated by a fold line(N 0 ) defined by the basis of the plural congruent isosceles triangles and that when the division of the curve in each segment (D 1 to D n ); of the measured shape of the steel plate (2) is regarded as an arc, the arc in each segment (D 1 to D n ); of the measured shape of the steel plate (2) can be approximated by a fold line (N 0 ) defined by the basis of a plurality (m) of other congruent isosceles triangles which are connected together while sharing their equal sides, the number of the latter isosceles triangles being the same as the number of the former isosceles triangles whose bases constitute the approximating fold line (N 0 ) for the target shape; divides the arc of the measured shape in each segment (D 1 to D n ); by the number (N 0 ) of the isosceles triangle to form respective points on the arc; and calculates the coordinates of the respective points as heating points. The automatic plate bending system using high frequency induction heating of claim 1, further comprising: a heating line determining unit (44) which reads in data on the heating points calculated by the heating point determining unit (41); draws straight lines from a certain heating point on a certain arc, as a starting point, to heating points on other arcs on the bases on the data on the respective heating points; examines the degree of parallelism between each of the straight lines and a reference line that is a straight line showing the direction of a central axis of a cylinder provided that the target shape is approximately deemed as a part of the cylinder; if this degree of parallelism is within a predetermined range, performs grouping of the relevant heating points as the heating points of the same group; and connects the respective heating points of the same group by a straight line or a curve to determine a heating line. An automatic plate bending system using high frequency induction heating according to claim 1, further comprising: a shape measuring unit (22) attached to the transversely travelling trolley (16, 17), for measuring the shape of the surface of the member to be heated. An automatic plate bending system using high frequency induction heating according to claim 1, further comprising: the clearance between the high frequency heating coil (02) and the surface of the member to be heated being secured by providing steel balls around the high frequency heating coil (02), and bringing the steel balls into contact with the surface of the member to be heated. An automatic plate bending system using high frequency induction heating according to claim 1, further comprising: the clearance between the high frequency heating coil (02) and the surface of the member to be heated being secured by providing a magnet around the high frequency heating coil (02), and causing a magnetic force to work between the magnet and the member to be heated. An automatic plate bending system using high frequency induction according to claim 1 heating, further comprising: the clearance between the high frequency heating coil (02) and the surface of the member to be heated being secured by providing a high pressure gas jetting unit near the high frequency heating coil (02), and directing a high pressure gas jetted by the high pressure gas jetting unit toward the surface of the member to be heated, thereby generating a reaction force. An automatic plate bending system using high frequency induction heating according to claim 1, further comprising: said high frequency heating coil (02) having a circular shape whose diameter nearly equals the diameter of a flame of a gas burner to be used when heating the same member to be heated. An automatic plate bending system using high frequency induction heating according to claim 1, further comprising: said control unit (45) further performing control such that as the member to be heated is bent, each of the universal poles (20, 21) moves in response to changes in the shape of the member to be heated, and such when any of the universal poles (20, 21) after responsive movement reaches a target front end position for each universal pole that has been determined on the basis of target shape data on the member to be heated, a heating operation is stopped.

Inventors: HAMAYA FUKUMI, IDE YASUKAZU, INOUE YOSHIAKI, KAWAKADO SHOUJI, KAWANO TAKAYUKI, KIKUTSUGI RYUUICHIROU, KOIWA HIDETSUGU, NAKAHAMA TAKESHI, OOTA KAZUAKI, SHIMAMOTO TAKIJIRO, HAMAYA, FUKUMI, IDE, YASUKAZU, INOUE, YOSHIAKI, KAWAKADO, SHOUJI, KAWANO, TAKAYUKI, KIKUTSUGI, RYUUICHIROU, KOIWA, HIDETSUGU, NAKAHAMA, TAKESHI, OOTA, KAZUAKI, SHIMAMOTO, TAKIJIRO, Hamaya, Fukumi, c/o Nagasaki Shipy.&amp;Machin. Works, Ide, Yasukazu, c/o Choryo Engineering Co., Ltd., Inoue, Yoshiaki, c/o Nagasaki R&amp;D Center, Kawano, Takayuki, c/o Nagasaki R&amp;D Center, Kikutsugi, Ryuuichirou, c/o Nagasaki Shipy.&amp;Machin, Koiwa, Hidetsugu, c/o Nagasaki Shipy.&amp;Mach. Works, Oota, Kazuaki, c/o Nagas. Shipyard&amp;Machinery Works, Shimamoto, Takijiro, c/o Choryo Engineering Co.Ltd
Applicants: MITSUBISHI HEAVY IND LTD, MITSUBISHI HEAVY INDUSTRIES, LTD.
Application year: 1998
Main classification: B21D11
IPC codes: B21D11