EP2729761A1

EP2729761A1 - Device and method for measuring deformation of a metal sheet

Info

Publication number: EP2729761A1
Application number: EP12807159.4A
Authority: EP
Inventors: Rickard ÅSTRÖM
Original assignee: Fotonic I Norden AB
Current assignee: Fotonic I Norden AB
Priority date: 2011-07-04
Filing date: 2012-01-18
Publication date: 2014-05-14
Also published as: EP2729761A4; WO2013006109A1

Abstract

The present invention relates to a sheet metal folding measurement method and device (100) for monitoring a folding process in real-time. The device (100) comprises a time- of-flight (TOF) sensor (120), a processing unit (140) and an output unit (160). The TOF sensor (120) is adapted and configured to capture a range image frame (170) representative of the distance between the TOF sensor (120) and a measured surface area (220) of the metal sheet (200). The processing unit (140) is configured to determine a two dimensional angle variable between the surface area (220) and a reference surface (R) and deliver the two dimensional angle variable as an output (165).

Description

DEVICE AND METHOD FOR MEASURING DEFORMATION OF A METAL SHEET

Technical Field

The present invention relates to metal folding. More specifically it relates to a device and a method for measuring deformation and deformation over time of a sheet of metal under pressure for folding purposes.

Background

EP0470 263 discloses a method in which a folding angle of a work piece is measured by a vision system. The vision system comprises a plane light source and an image pickup device for picking up an intersection line pattern of light induced onto the surface of the sheet by the plane light source. The vision system is placed in a reference position in accordance with a co-ordinate axis of a co-ordinate plane. The folding angle of the work piece is calculated based on the positional relationships between the pick-up direction, the direction of the emitted light, the intersection line and the co-ordinate axis of the co-ordinate plane. This method requires a precise attachment of the vision system with respect to the work piece and a calibration using a sample piece having a known angle. Furthermore, this vision system has to be attached at a well-defined distance away from the work piece, thereby enabling the emitted light to reach the focal point of the image pickup device.

US6727986 discloses a method and a device for measuring a folding angle of a sheet. Measurements are being made on the sides of an element, of a number of distances in a plane that crosses the sheet and the element. The distances comprise a number of distances between a measuring tool and discrete points on the sheet and a number of distances between a measuring tool and different points on the element. A distance profile of the measured distances is being determined. Then two straight lines are determined based on values of the distance profile. The angle between sheet and element from the angle between the two straight lines is also determined. The folding angle of the sheet will then be a function of the determined angle between the sheet and the element. The computational procedure of this method is complicated, the distances are not measured with great accuracy, and the results may not be reliable. Moreover, the measurement gives an estimated angle in discrete along the folding line. WO0203026 discloses an angle measurement system based on pattern projection. First the projected pattern must be localized, and the camera directed towards it. Then displacement and deformation of the projected pattern must be computed. To obtain a correct measurement, reference information is required. The position and form of a projection should be measured for several known calibration angles. The number of calibration angles depends on the complexity of the pattern. Each pattern has a certain number of degrees of freedom that are necessary to completely define it. This system is vulnerable in several aspects. It demands as in-data a multitude of measurements, assumptions and mathematical models, which means that even though each single piece of in-data may bring only a minor error, there may be a considerable summed error contribution. This summed error may propagate into the result.

Summary

It is the object to obviate at least some of the above disadvantages and provide improved methods, apparatuses and computer media products avoiding the above mentioned drawbacks.

According to a first aspect of the present invention a metal sheet folding measuring device is accomplished for measuring deformation and further folding of a metal sheet. The device comprises a time-of-flight (TOF) sensor, a processing unit and an output unit. The TOF sensor is adapted and configured to capture a range image frame representative of the distance between the TOF sensor and a measured surface area of the metal sheet. The processing unit is configured to determine a two dimensional angle variable between the surface area and a reference surface and deliver the two dimensional angle variable as output.

The device of the present invention may further be configured to perform measurements in a fixed position relative a folding press for folding and shaping the metal sheet. The device of the first aspect may further be adapted and configured to capture a range image frame set representative of the shaping of the metal sheet over time. The device of the first aspect may further be adapted and configured to measure movements of the metal sheet during or after shaping. The output of the device of the first aspect may be used to control the shaping operation of the folding press.

A second aspect of the present invention is a sheet metal folding measurement method for monitoring a folding and deformation process in real-time. The method of the second aspect of the invention comprises the steps of capturing a range image frame representative of the distance between the sensor and a measured area of a surface of a metal sheet; determining a two dimensional angle variable between the surface area and a reference surface; and delivering the two dimensional angle variable as output.

The angle variable may be output either in a format suitable for human reading, or in a format suitable for feeding back to and an automatic control engineering system that is controlling shaping of the metal sheet by controlling the operation of the folding press. The capturing step of the method of the second aspect of the invention may comprise the further step of real-time capturing a range image frame set representative of the shaping of the metal sheet over time.

Each image frame of the image frame set may be capturing an identical field-of-view. The determining step of the second aspect of the invention may comprise the further step of determining movements of the metal sheet during or after folding or shaping.

The delivering step of the second aspect of the invention may comprise the further step of using the output data to control the folding operation of the folding press by feeding it back to an automatic control engineering system that is controlling shaping of the metal sheet by controlling the operation of the folding press.

A third aspect of the invention is a computer program comprising code means for performing the steps of any embodiment of the method of the second aspect of the invention, when the program is run on a computer.

A fourth aspect of the invention is a computer program product comprising program code means stored on a computer readable medium for performing any embodiment of the method of the second aspect of the invention, when said product is run on a computer.

Brief Description of the Drawings

In order to explain the invention in more detail an embodiment of the present invention will be described in detail below, reference being made to the accompanying drawings, in which

Figure la is a cross-section of a metal sheet,

Figure lb illustrates the principle of the spring back angle when the folding die is released,

Figure lc illustrates the insufficiencies of measuring discrete points in order to determine if the folding angle is within tolerances,

Figure 2a is perspective view of how the device according to the present invention is placed in relation to a folding machine, Figure 2b is a schematic model of a device according to the present invention and

Figure 3 is a flow chart outlining a method according to the present invention. Detailed Description

A solution to the above mentioned problems is presented below. The solution is a deformation measurement method and a device that, based on real-time three- dimensional image frames, i.e. data from a time-of-flight sensor, can detect an angle distribution along the surface of a metal sheet, as well as the spring-back effect in real- time. The detection is instant, secure, accurate and reliable. It does not require complicated calibrations, precise attachments or inputting of reference parameters.

One underpinning faculty of the present invention is the ability to create instant distance data for individual pixels of an object image. A time-of-flight sensor, hereafter referred to as a TOF sensor, enables the generation of three-dimensional images in real-time. In such three-dimensional images and three-dimensional video sequences, objects may be easily and reliably localized, without complex signal processing, or space and time requiring triangulation or scanning equipment.

The general principles of a TOF sensor will first be described below. TOF distance data is created with the help of light signals, usually pulses that are switched on for a very short time, thus illuminating a detection scene. The signal is then scattered, i.e. reflected, by surfaces and objects in the scene. The light is collected onto a sensor plane. Light incident on the sensor plane from farther points of the object will be delayed compared to light reflected from nearer points. The delay has the magnitude of nanoseconds only. As each pixel of the sensor plane represents one unique point of the object, the spatial delay distribution represents a three-dimensional measurement of the object.

An optic unit of a TOF sensor comprises a lens and may comprise a band pass filter. The lens gathers the reflected light and images the scene onto the image sensor plane. The optical band pass filter only passes the light with the same wavelength as the illumination unit, and thus suppresses background light.

The image sensor comprises pixels. Each pixel may comprise two areas with alternating activation, a so called two-tap phase lock detection principle. The differences between intensities detected in the two areas correlates to the distance from the sensor pixel to one particular point of the detected scene. There is no mechanical scanning, with prisms or mirrors, which makes the construction robust. Further no electronics for mixing the received and reference signals are needed outside the image sensor.

In contrast to stereo vision or triangulation systems, the TOF sensor has an advantage in that it is very compact: the illumination may be placed just next to the lens, whereas other systems need a certain minimum base line. In contrast to laser scanning systems, no mechanical moving parts are needed.

Further, it is easy to extract the distance information out of the output signals of the TOF sensor. Therefore this task uses only a small amount of processing power, again in contrast to stereo vision cameras or the above mentioned radar application, where complex correlation algorithms have to be implemented. After the distance data has been extracted, object detection, for example, is also easy to carry out because the algorithms are relatively insensitive to the reflection from the object, whereas previously known detectors and detection methods have problems with uniform objects or repeatable patterns on the object.

Furthermore, TOF sensors are able to measure the distances within a complete scene with one shot. TOF sensors reach up to 100 frames per second, and are therefore suited to be used in real-time applications.

Some basic principles of metal folding will be described in relation to figure 1. Figure 1 a is a cross-section of a metal sheet 200, a folding punch 10, and a folding die 20 prior to folding. Both punch 10 and die 20 are longitudinally extended in the x- dimension. The metal sheet 200 rests on top of the folding die 20, and coincides with a horizontal plane z=0. During the folding operation the punch 10 exerts downward pressure along a desired folding line L parallel with the x-axis. The downward pressure causes the metal sheet 200 to incline increasingly relative the horizontal plane z=0 with an angle-under-pressure a (t) over time. Four problems complicate this process.

Firstly, as illustrated in figure lb, when the pressure from the punch 10 is released, the metal sheet 200 springs back with a spring back angle β so that the final folding angle γ in the cross-section equates to α-β. Therefore, in order to achieve a desired folding angle γο in a cross section, the metal sheet must be folded to an angle a= γο+ β. The spring back angle β is a function of, among other things, the metal sheets elastic modulus. However, so many other factors go in to the equation, that it is not practically possible to calculate exactly how the sheet metal is going to behave.

Therefore the final folding angle γ must be measured. Secondly, folding is a longitudinal deformation, and the angle-under-pressure a (t) as well as the final folding angle γ will vary along the folding line L. In other words, the final folding angle γ is a continuous variable γ (x) in two dimensions, and angle-under-pressure is a continuous variable a (x, t) in three dimensions. The continuous and variable properties of the angle-under-pressure a and the final folding angle γ means that folding control feedback or final measurements in discrete points along the folding line L give insufficient information. As illustrated in figure 1 c, measurements in any of the points marked with arrows will indicate that the final folding angle γ is within tolerances, when in fact it exceeds both upper and lower tolerance lines in other points.

Thirdly, the variation of a final folding angle γ is not symmetric along the folding line L, so a first continuous variable γι= γ_ζ>ο(χ) on a first side of the folding line L is not equal to a second continuous variable γ₂= γ_ζ<ο(χ) on a second side of the folding line L.

Fourthly, in order to prevent unpredicted and unwanted deformation to a point where the material must be scrapped, the folding operation, rather than the final folding result, must be monitored. The closer the tolerances, the more important is monitoring that is, for all practical applications, continuous in time.

Embodiments of the present invention offer solutions to all the above problems, and firstly measure accurately a folding angle between two surfaces, or for that matter any other geometric property, instantly. Secondly, embodiments of the present invention can measure a continuous variation, rather than an approximation based on point measures. Thirdly, as long as there is a line-of-sight, embodiments of the present invention are able to measure or monitor several continuous variations simultaneously, such as γι (x) and γ₂ (x). This also gives easy access to an angle∑ between the two surfaces 220 and 220', which, though in principle is equal to the sum of γι and γ₂, in fact is a completely different measurement. Fourthly, embodiments of the present invention enable true real-time measurements and monitoring of the folding as it develops.

Figure 2a is perspective view of how the device 100 according to the present invention is placed in relation to a folding press 300 machine.

An embodiment of the detection device 100 according to the present invention will now be described in relation to Figure 2b, which features a detection device 100 for measuring the shaping of a metal sheet 200, that is, measuring and monitoring the deformation and movement of the metal sheet 200 over time with the purpose of folding, to accomplish a final change in shape. The device 100 is adapted and configured to monitor momentary, incremental, temporary and/or permanent deformation of a piece of metal 200, notably a metal sheet. The device 100 comprises an illumination unit 110, a TOF sensor 120, a processing unit 140, a memory unit 150 and an output unit 160. The illumination unit 110 is adapted and configured to emit a detection signal comprising a certain wavelength band, with a certain modulation scheme, usually pulses. In order to obtain inconspicuous monitoring, IR or NIR light may be used in the illumination unit 110. That way, monitoring is unnoticeable, and will not interfere with or disturb other activities in the workshop. The detection signal may be composed by pulses generated from the illumination unit 110. Pulse frequencies up to 50fps may be obtained.

The TOF sensor 120 is adapted and configured to simultaneously convert variations in the reflected signal into an image frame 170, comprising range

information, i.e. a real-time three-dimensional image frame 170, using properties of the detection signal as a reference. The image frame 170 is a digital reproduction of the TOF sensor's 120 field-of-view. It may also capture a set 180 of range image frames 170 with a frame rate directly corresponding to the illumination unit's 110 pulse frequency.

The processing unit 140 is adapted and configured to collect range image frames comprising range information from the TOF sensor and perform various transformation procedures, comparing operations etc.

The memory unit 150 is configured and adapted for intermediate storing of range image frames and computing results. Generic information can be pre-stored in the memory 150. Information can also be generated during real-time situations and stored in the memory 150, so that the device 100 may be individually trained.

The device 100 is adapted and configured to use the range image frame 170 to calculate a continuous two-dimensional angle variable γ(χ) and a three-dimensional angle variable a (x, t) between the area surface 220 and a reference surface R. The reference surface R may be a virtual surface R, such as a vertical or horizontal plane through the folding line L. The reference surface R may be an actual surface. The actual surface may be the surface 220' on the other side of the folding line. The angle variable may be delivered as output 165, either in a form suitable for human reading, or in a form suitable for controlling a machine such as a folding press 300. This enables the device 100 to be used in an automatic control engineering system that controls shaping of the metal sheet 200 by controlling the operation of the folding press 300. Because the device 100 captures the range image frame instantly, no moving parts are necessary, and one device 100 is sufficient to measure a wide area where other systems would require an array of measuring devices, or a rail so that the measuring device could move back and forth over the measured area.

The device may further capture a range image frame set 180 representative of the real-time shaping of the metal sheet 200 over time, with the purpose of folding. This enables the use of a surface as measured at a previous point in time as a reference surface R. The reference surface R may be the surface 220 prior to shaping. Real-time measurements further enable measuring of movements of the metal sheet 200 after shaping, such as spring-back movement β(χ, t), or during shaping (x, t), such as abnormal movements indicating malfunction of the folding press 300 or structure flaws of the metal sheet 200.

The output 165 may be used to control the shaping and folding operation of the folding press 300, such that the shaping, or folding, may cease at the right time, or discontinued in the event of malfunction. This feature is a great advantage both for safety reasons, and for reasons of quality and economy.

The device 100 may be mounted permanently or temporarily on a folding machine. It may also be a stand alone device. It may be placed in the middle of a room, by a wall or in a corner, as long as there is line of sight to the area to be monitored. The measured surface 220 may be the upper surface or the lower surface of the metal sheet 200.

A folding measurement method according to one embodiment of the present invention will now be described in relation to figure 3. Firstly, the method may be used to monitor the folding process in real-time, from or before the instant when the punch 10 meets the sheet metal surface 220, during deformation-under-pressure, and during spring-back movement as the pressure is released. Measurements may be compared to a target folding angle or another target parameter, and the result may be fed back to the folding machine as control input, so that the machine ceases the folding operation at an appropriate time. The method may also be used to determine the spring back effect, i.e. the movement of the sheet metal as the punch releases its pressure against the sheet metal. The method 100 is performed in a measurement device 100 so placed that the measured area 210 of the surface 220 of the metal sheet 200 is within the TOF sensor's 120 field-of-view. As is shown in figure 3 the method comprises the steps of,

capturing 420 a range image frame 170 representative of the distance between the sensor 120 and a measured area 210 of a surface 220 of the metal sheet 200;

determining 440 a two dimensional angle variable between the surface area 220 and a reference surface R; and

delivering 460 the two dimensional angle variable as output 165; either in a format suitable for human reading, or in a format suitable for feeding back to and an automatic control engineering system that is controlling shaping of the metal sheet 200 by controlling the operation of the folding press 300.

The capturing step 420 may comprise the further step of real-time capturing

425 a range image frame set 180 representative of the shaping of the metal sheet 200 over time. Each image frame of the image frame set 180 may capture an identical field- of-view.

The determining step 440 may comprise the further step of measuring movements 465 of the metal sheet 200 during or after shaping.

The outputting step 460 may comprise the further step of using the output data 165 to control the folding operation of the folding press 300 by feeding it back to an automatic control engineering system that is controlling shaping of the metal sheet 200 by controlling the operation of the folding press 300.

Claims

1. A metal sheet folding measuring device (100) for measuring deformation of a metal sheet (200), comprising a time-of-flight (TOF) sensor (120), a processing unit (140), and an output unit (160), wherein the TOF sensor (120) is adapted and configured to capture a range image frame (170) representative of the distance between the TOF sensor (120) and a measured area (210) of a surface (220) of the metal sheet (200) and the processing unit (140) is adapted and configured to determine a two dimensional folding angle variable between the surface area (220) and a reference surface (R) and to deliver the two dimensional folding angle variable as an output (165).

2. The device (100) according to claim 1 , further configured to perform measurements in a fixed position relative a folding press (300) shaping the metal sheet (200).

3. The device according to claim 1 or 2 further adapted and configured to capture a range image frame set (180) representative of the shaping of the metal sheet (200) over time.

4. The device according to any of claims 1 to 3, further adapted and configured to measure movements of the metal sheet (200) during or after shaping.

5. The device according to any of the preceding claims, wherein the output (165) is used to control the shaping operation of the folding press (300).

6. A sheet metal folding measurement method for monitoring a folding process in real-time, comprising the steps of

capturing (420) a range image frame (170) representative of the

distance between a TOF sensor (120) and a measured area (210) of a surface (220) of a metal sheet (200);

determining (440) a two dimensional angle variable between the surface area

(220) and a reference surface (R ); and

delivering (460) the two dimensional angle variable as an output (165) either in a format suitable for human reading, or in a format suitable for feeding back to and an automatic control engineering system that is controlling the shaping of the metal sheet (200) by controlling the operation of the folding press (300).

7. The method according to claim 6, in which the capturing step (420) comprises the further step of

real-time capturing (425) a range image frame set (180) representative of the shaping of the metal sheet (200) over time.

8. The method according to claim 7, in which each image frame of the image frame set (180) is capturing an identical field-of-view.

9. The method according to any of claims 6 to 8, in which the determining step (440) comprises the further step of determining movements (465) of the metal sheet (200) during or after shaping.

10. The method according to any of claims 6 to 9, in which the delivering step (460) comprises the further step of using the output data (165) to control the folding operation of the folding press (300) by feeding it back to an automatic control engineering system that is controlling the shaping of the metal sheet (200) by controlling the operation of the folding press (300).

1 1. A computer program comprising code means for performing the steps of the method according to any of the claims 6 to 10, when the program is run on a computer.

12. A computer program product comprising program code means stored on a computer readable medium for performing the method of any of the claims 6 to 10, when said product is run on a computer.