GB2064102A

GB2064102A - Improvements in electro- optical dimension measurement

Info

Publication number: GB2064102A
Application number: GB7940810A
Authority: GB
Original assignee: European Electronic Systems Ltd
Current assignee: European Electronic Systems Ltd
Priority date: 1979-11-26
Filing date: 1979-11-26
Publication date: 1981-06-10
Also published as: JPS5697812A; JPS61221610A; GB2064102B

Abstract

An optical measuring system in which an object (14) is imaged on an opto-electrical transducer 10 has a processing unit to modify the transducer output to compensate for inaccuracies of the optical system. The system may be used to measure the width and thickness of a strip 14 being rolled. The strip 14 is imaged onto an electro-optical transducer 10, 12 and to compensate for inaccuracies resulting from the imaging system 11, 13 a calibration of the imaging system is effected by means of a grid of known dimensions. The calibration results are recorded during a setting up process and used by a digital or analogue computer to correct any measurements taken by the imaging system during operation. <IMAGE>

Description

SPECIFICATION Improvements in electro-optical distance measurement The present invention relates to electro-optical distance measurement.

It has already been proposed to measure the length of an object by imaging the object onto, for example, a camera and performing measurements on the image. This method of measurement lends itself particularly to the measurement of the width of a strip being rolled, although it does have numerous other applications.

In one method of measuring the width of a strip being rolled, two cameras are arranged each to view one edge of the strip and the separation of the cameras is controlled by a servo-mechanism in such a manner as to centre the image of the two edges on the respective cameras. The separation of the cameras is then an indication of the width of the strip. Apart from the slowness of the reaction of the servo-control loop, this method of measurement is not sufficiently accurate since the camera supports will expand with temperature and impair the measurement accuracy.

Another and superior measurement system has been developed for the measurement of the width of a strip being rolled which involves the positioning of two cameras vertically one above the other and both simultaneously viewing the two edges of the strip being rolled. In this method, the spacing of the imaging systems is fixed and is not critical and the system can operate more rapidly and more accurately than the previously described prior art systems.

In either of the above described systems and, indeed, in any system relying on optical imaging, efficiency of the system depends upon the quality of the optics which serve to image the object onto the plane of the opto-electrical transducer, be it a television camera or a charge coupled device.

Measurements are normally carried out at the extremities of the field of view where the image quality is always at its poorest and the cost of high quality optical systems has hitherto limited the accuracy of such measuring systems.

According to one aspect of the present invention, there is provided an optical measuring system having one or more opto-electrical transducer(s) and an optical imaging system for imaging an object to be measured onto the transducer(s) and a processing unit connected to the electrical output(s) of the transducer(s) for modifying the output of the transducer in accordance with a predetermined correction function to compensate for any inaccuracies caused by the optical system.

The invention may be considered as providing electronic compensation for inaccuracies introduced by the optical system.

In order to determine the correction function, the system may be calibrated by the measurement of an object or preferably a grid having known dimensions. The measured values obtained with this calibration grid are entered into a microprocessor which may then compute the amount of correction to be applied at different points of the image field by analogue or digital interpolation.

It is preferable to use a charge coupled device as an opto-electrical transducer and in such a case the correction may conveniently be done by reference, in respect of each element of the charge coupled device, to a calibration table stored within a memory of the microprocessor during the calibration process.

The previously described measurement system employing two cameras lying vertically one above the other above the edges of the strip to be rolled suffers from the disadvantage that the higher of the two cameras must typically be some six feet above the strip in order to achieve the desired accuracy. Such a clearance has in practice been found to be unacceptable as the system could then interfere with or be damaged by other equipment in the rolling mill.

In accordance with a second aspect of the present invention, there is provided an optical measuring system for measuring the width of a strip being rolled, the system comprising two opto-electrical transducers arranged above the strip and lying on a line transverse to the longer dimension of the strip, each transducer being capable of viewing substantially the entire width of the strip, and a processing unit connected to the electrical outputs of the two transducers to determine the position in space of the upper edges of the two sides of the strip.

During the course of rolling a strip, it is quite possible for one edge of the strip to lift off the bed of the rolling mill or in some cases for the whole of the strip to lift off the bed. Such distortions of the strip would in all known apparatus result in inaccurate readings.

In accordance with a preferred feature of the invention, the processing unit compares the plane joining the two measured edges of the strip with a predetermined reference plane and if the planes are not coincident is operative to apply a correction to the positioning of the measured edges so as to evaluate the corresponding measured width of the strip had it been lying in said reference plane.

In calibrating a system as above described, it is convenient to position a reference grid in the reference plane and to generate from a comparison of the measured positions of the grid with the actual known positions of the lines on the grid a correction factor to be applied by the processing unit at the various viewing angles of the transducer optical system. Preferably, a second calibration is performed with the grid positioned at a predetermined height above the reference plane so as to enable a correction to be applied at any point on or above the bed of the rolling mill.

The invention will now be described further, by way of example, with reference to the accompanying drawings, in which: Figure 1 is a schematic representation of a first electro-optical distance measurement system, and Figure 2 is a similar representation of a second distance measurement system.

The system described in Figure 1 is intended to measure the width D and the thickness T of a strip 14 being rolled. The optical measurement system includes a first charge coupled device 10 mounted with its focussing system 11 a distance H1 above the plane on which the object 14 rests and a second charge-coupled device 12 mounted with its imaging system 13 a distance H2 above the plane on which the object 14 rests. The purpose of the equipment is to measure the width D of the strip 14 and its thickness T. This can be carried out in a known way from the measurements made by the charge-coupled devices 10 and 12.

If the strip 14 is a light emitter or a light reflector, its image can be sensed directly by the charge-coupled devices 10 and 12, but alternatively light source may be positioned beneath the strip 14 to measure the dimensions of the shadow. Knowing the distances H1 and H2, the sizes of the images on the charge-coupled devices and the magnifications of the imaging systems the dimensions D and T can be computed in a known way.

The accuracies of the measurements depend on the quality of the optical systems 11 and 13 which are shown only schematically in the drawings as consisting of a convex lens each. The cost of very high quality optics is extremely high and consequently the accuracy of the measurement is limited by the quality of the optical systems.

With a view to improving the accuracy of such a system, which is known per se, without significantly improving the cost, the invention proposes placing a grid in the position of the strip 14 in order to calibrate the system.

In a first embodiment of the invention, the dimensions of the image produced by lines having a predetermined distance apart are stored in a memory and the contents of the memory are read during operation in order to compensate for inaccuracies in the known system. Such a method of compensation can readily be performed by digital microprocessor but the invention is not limited to such construction and it is alternatively possible to feed the output of each chargecoupled device to a function generator each having a function predetermined during the calibration procedure to correct the inaccuracies resulting from the measurement system.The correction function may simply involve linear interpolation between two points measured during calibration but alternatively the function may be more complicated employing functions of higher orders to form a curve defining a best fit with the points measured during calibration. In the latter case, an analogue computation technique is involved which is believed evident to a person skilled in the art.

Although the lens calibration has been described with reference to a system comprising two vertically aligned electro-optical transducers, it will be clear that the invention may be used in any system in which an object to be measured is imaged onto an electro-optical transducer and furthermore the electro-optical transducer need not be a charge-coupled device but may, for example, be a television camera.

In the embodiment shown in Figure 1, the distance H1 may need in some cases to be as much as two metres and this has been found to be inconvenient in certain applications. Furthermore, the measuring system employed in the embodiment of Figure 1 assumes that the strip 14 being rolled is resting upon the bed of the rolling mill and this in practice may not be the case, it being cominon for one edge of the strip to lift off.

The system in Figure 2 shown very schematically disposes the two cameras 10' and 12' at a known height H above the bed of the rolling mill and at a known distance W apart. Each of the cameras is capable of seeing both edges of the strip 14 simultaneously. The method of computation in this embodiment, however, differs from that previously described. In place of assuming that the strip 14 is resting on the bed, the location in space of each of the edges 1 4a and 1 4b is calculated. This is a simple problem analogous to that solved in any navigation system wherein the location of a point is determined by taking bearings from two known points. Each of the cameras 10' and 12' provides a bearing and the point of intersection is a unique point in space which can readily be calculated.

If the points 1 4a and 1 4b are found to lie in a different horizontal plane from one another, the distance the points 1 4a and 1 4b is determined within conventional analytical geometry and it is then possible to complete the co-ordinates of the edges 1 4a and 1 4b if the strip is bent back to assume a horizontal position resting on the bed.

As in this embodiment, readings are taken not only when the strip 14 is resting on the bed, but when the edges are raised above it, the calibration of the lenses should be performed by placing the calibration grid in more than one horizontal position during the calibration process in order to correct for aberrations in all the different planes of measurement.

Claims

1. An optical measuring system having an optoelectrical transducer and an optical imaging system for imaging an object to be measured onto the transducer and a processing unit connected to the electrical output of the transducer to modify the output of the transducer in accordance with a predetermined correction function to compensate for any inaccuracies caused by the optical system.

2. An optical measuring system as claimed in Claim 1, having two optical transducers arranged one above the other to measure the width and thickness of a strip passing beneath the transducers.

3. An optical measuring system as claimed in Claim 1 or Claim 2 in which the processing unit includes a microprocessor and in which the said predetermined correction function is determined by calibration employing a grid of known dimensions.

4. An optical measuring system for measuring -the width of a strip being rolled, the system comprising two opto-electrical transducers arranged above the strip and lying on a line transverse to the longer dimension of the strip, each transducer being capable of viewing substantially the entire width of the strip and a processing unit connected to the electrical outputs of the two transducers to determine the position in space of the upper edges of the two sides of the strip.

5. An optical measuring system as claimed in Claim 4, wherein the processing unit is operative to compare the plane joining the two measured edges of the strip with a predetermined reference plane and to apply a correction to the positioning of the measured edges so as to evaluate the corresponding measured width of the strip had it been lying in the said reference plane.

6. An optical measuring system as claimed in Claim 1, substantially as hereinbefore described, with reference to and as illustrated in the accompanying drawings.

7. An optical measuring system as claimed in Claim 4 and substantially as hereinbefore described, with reference to and as illustrated in the accompanying drawings.