GB2275772A - Calibration of dimension measuring equipment - Google Patents

Abstract

Equipment for monitoring a dimension, such as, the thickness, of an article (1) employing a differential measuring technique wherein a pair of spaced distance measuring devices (2, 3) are disposed at opposed ends of the dimension of the article (1) and in spaced relationship therewith, requires calibration so that any parameter of the equipment which can change in dependence upon variation in operating conditions is taken in account in computation of the dimension of the article. The calibration comprises interposing between the distance measuring devices in spaced relationship therewith, an element (20) having a dimension (k) of known constant value, and calculating from the respective measured distances (x, y) and the known length (k) an error value corresponding to any change in parameters of the monitoring equipment resulting from variation in operating conditions. <IMAGE>

Description

~CALIBRATION DESCRIPTION This invention relates to a method of and apparatus for calibrating equipment used for monitoring the length of a dimension, such as, the thickness, of an article. The inventive method and apparatus are especially, but not exclusively, related to the calibration of equipment used in the monitoring of the thickness of a planar article, such as, a moving length of corrugated board, using a so-called differential measurement* technique, wherein a pair of distance measuring devices, such as, laser or ultrasonic distance measuring devices, on opposed sides of the moving planar article whose thickness is to be monitored on a continuous basis, are moved in unison across the width of the lengthwise-moving article whilst measuring their respective distances from the opposed surfaces of the article.Outputs from the distance measuring devices representative of their respective distances from the opposed surfaces of the article, in accordance with the technique mentioned above, are then processed and applied to the following equation to provide an indication of the thickness of the article across the width thereof: t = a - (x + y) where, X is the thickness of the article at any point thereon, A is the spacing between the pair of distance measuring devices, X is the measured distance of one of the devices from one surface of the article and y is the measured distance of the other of the devices from the other opposed surface of the article.

Although the spacing (A) between the two distance measuring devices is predetermined and should be constant, it has been found that, in practice, this spacing can vary, thereby causing errors in the computed thickness (X) of the article across its width. Such errors can be caused by a variation in an operating condition of the equipment resulting in a change in an operating parameter thereof, for example, a change, albeit slight, in the distance between respective guide means which extend across the width of the article whose thickness is to be monitored and upon which the distance measuring devices are mounted for movement, in unison, across the article's width.

Such changes have been found to be caused by thermal expansion and contraction of the components of the monitoring equipment. For instance, after start-up of the equipment, its components tend to expand as they warm up during operation, with the resulting expansion of the equipment components causing the guide means for the distance monitoring devices to deflect with respect to each other, thereby changing the spacing between those devices and resulting in an erroneous computation of the thickness (t) of the article being monitored.

Similarly, other operating parameters of the monitoring equipment, for example, those of the distance measuring devices, such as, laser or ultrasonic distance measuring devices, and any associated and electronic and other signal processing equipment and/or instrumentation, can change due to variations in their operating conditions brought about by, say, ageing effects and/or variations in ambient conditions, such as, temperature, resulting in a drift in the original calibration of the distance measuring devices, thus reducing the accuracy of data derived from the thickness monitoring equipment as a whole.

It is an object of the present invention to provide a calibration method and apparatus for use with monitoring equipment, such as, that described above, which overcomes, or at least substantially reduces, the operating disadvantages associated with it, as discussed above.

A first aspect of the invention resides in a method of calibrating equipment for monitoring the length of a dimension, such as, the thickness, of an article employing a differential measuring technique wherein a pair of spaced distance measuring devices are disposed at respective opposed ends of the dimension of the article whose length is being monitored and in spaced relationship therewith, wherein an operating parameter of the equipment can change in dependence upon a variation in an operating condition of the equipment and wherein accurate computation of the monitored length of the dimension of the article is dependent upon any such operating parameter change being taken into account in said computation, the calibration method comprising interposing between the distance measuring devices in spaced relationship therewith, an element having a dimension of known and substantially constant length, with the devices aligned with respective opposed ends of said dimension, providing representations of the respective measured distances of the devices from the corresponding ends of said dimension and computing said representations with the known length of said dimension, to provide an error value corresponding to any change in an operating parameter of the monitoring equipment resulting from a variation in an operating condition thereof.

A second aspect of the invention provides apparatus for calibrating equipment for monitoring the length of a dimension, such as, the thickness, of an article employing a differential measuring technique wherein a pair of spaced distance measuring devices are supported at respective opposed ends of the dimension of the article whose length is being monitored and in spaced relationship therewith, wherein an operating parameter of the equipment can change in dependence upon a variation in an operating condition of the equipment and wherein accurate computation of the monitored length of the dimension of the article is dependent upon any such operating parameter change being taken into account in said computation, the calibration apparatus comprising an element having a dimension of known and substantially constant length and interposable between the distance measuring devices in spaced relationship therewith, with the devices aligned with respective opposed ends of said dimension, means arranged to provide representations of the respective measured distances of the devices from the corresponding ends of said dimension when the element is interposed between the devices and means arranged to compute said representations with the known length of said dimension, to provide an error value corresponding to any change in an operating parameter of the monitoring equipment resulting from a variation in an operating condition thereof.

As indicated above, the inventive calibration method and apparatus are suited especially, but not exclusively, to the calibration of equipment used in the monitoring of the thickness of a planar article, such as, a moving length of corrugated board using a so-called "differential measurement" technique in which a pair of distance measuring devices, such as, laser or ultrasonic distance measuring devices, on opposed sides of the moving planar article whose thickness is to be monitored, are moved in unison across the width of the lengthwise-moving article whilst measuring their respective distances from the opposed surfaces of the article and in which errors in the monitored thickness of the article can be caused by changes in the spacing between guide or other support means on which the distance measuring devices are mounted for movement across the width of the article.

Accordingly, a third aspect of the invention resides in a method of calibrating equipment for monitoring the length of a dimension, such as, the thickness, of an article employing a differential measuring technique wherein a pair of spaced distance measuring devices disposed at respective opposed ends of the dimension (thickness) of the article whose length is being monitored and in spaced relationship therewith, are mounted upon respective support means whose spacing and, hence, the spacing between the devices, is substantially constant but can change slightly in dependence upon a variation in an operating condition of the equipment and wherein accurate computations of the so-monitored length of the dimension of the article is dependent upon the spacing between the devices being known, the calibration method comprising interposing between the distance measuring devices and in spaced relationship therewith, an element having a dimension (thickness) of known and substantially constant length, with the devices aligned with respective opposed ends of said dimension, providing representations of the respective measured distances of the devices from the corresponding ends of said dimension and computing said representations with the known length of the dimension, to provide an error value corresponding to any change in the spacing between said support means and, hence, between the distance measuring devices.

Further, a fourth aspect of the invention comprises apparatus for calibrating equipment for monitoring the length of a dimension, such as, the thickness, of an article employing a differential measuring technique wherein a pair of spaced distance measuring devices at respective opposed ends of the dimension (thickness) of the article whose length is being monitored are mounted, in spaced relationship therewith, upon support means whose spacing, and hence the spacing between the devices, is substantially constant but can change slightly in dependence upon a variation in an operating condition of the equipment and wherein accurate computation of the so-monitored length of the dimension (thickness) of the article is dependent upon the spacing between the devices being known, the apparatus comprising an element having a dimension (thickness) of known and substantially constant length which is interposable between the distance measuring devices in spaced relationship therewith, with the devices aligned with respective opposed ends of said dimension, means arranged to provide representations of the respective measured distances of the devices from the corresponding ends of said dimension when the element is interposed between the devices, and means arranged to compute said representations with the known length of said dimension, to provide an error value corresponding to any change in the spacing between said support means and, hence, between the distance measuring devices.

In one form of the monitoring equipment, the pair of spaced distance measuring devices are movable in unison along a path across the width of the article upon respective support means, in which case, the devices and element interposed therebetween may, in the inventive calibration method and apparatus, be movable in unison along a corresponding path across the width of the article, to provide a series of error values.

The method and/or apparatus in accordance with the various aspects of the invention defined above may be arranged to carry out a series of calibrations over a given operating period, for instance, from start-up to full operating conditions, during which the temperature of the components of the monitoring equipment, including that of the support means, can vary, thus tending to cause the spacing between the distance measuring devices to change over that period.

Means may be provided to programme the resultant series of error values into associated computing means, to correct subsequently computed values for the monitored dimension length of the article over the same operating period during which the equipment was calibrated previously.

Similarly, the error value or series of error values derived from a method and/or apparatus in accordance with the first and/or second aspects of the invention defined above, may be used in any subsequent computation of the monitored length of the dimension of the article, to eliminate any error(s) in that monitored dimension length which might otherwise be computed as a result of any change(s) occurring in the spacing between the distance measuring devices.

As also defined above, other operating parameters of the monitoring equipment, for example, those of the distance measuring devices and any associated electronic and other signal processing equipment and/or instrumentation, can change due to variations in their operating conditions brought about by, say, ageing effects and/or variations in ambient conditions, resulting in a drift in the original calibration of the distance measuring devices, thus reducing the accuracy of data derived from the monitoring equipment as a whole.

Thus, a fifth aspect of the invention provides a method of calibrating equipment for monitoring the length of a dimension, such as, the thickness, of an article employing a differential measuring technique wherein a pair of spaced distance measuring devices are arranged at respective opposed ends of the dimension (thickness) of the article whose length is being monitored in spaced relationship therewith, wherein an operating parameter of at least one of the distance measuring devices, any associated processing equipment and/or instrumentation, can change due to a variation in an operating condition thereof, resulting in a drift in the original calibration of the monitoring equipment, and wherein accurate computation of the monitored length of the dimension of the article is dependent upon any such drift in calibration being taken into account in said computation, the calibration method comprising interposing between the distance measuring devices in spaced relationship therewith an element having a dimension of known and substantially constant length, with the devices aligned with respective opposed ends of said dimension, providing representations of the respective measured distances of the devices from the corresponding ends of said dimension and computing said representations with the known length of said dimension, to provide an error value corresponding to any drift in the original calibration of the monitoring equipment as a result of a change in an operating parameter of at least one of the distance measuring devices, any associated processing equipment and/or instrumentation due to a variation in said operating condition.

A sixth aspect of the invention relates to apparatus for calibrating equipment for monitoring the length of a dimension, such as, the thickness, of an article employing a differential measuring technique wherein a pair of spaced distance measuring devices are arranged at respective opposed ends of a dimension (thickness) of the article whose length is being monitored in spaced relationship therewith, wherein, in use, an operating parameter of at least one of the distance measuring devices, any associated processing equipment and/or instrumentation, can change due to a variation in an operating condition thereof, resulting in a drift in the original calibration of the monitoring equipment, and wherein accurate computation of the monitored length of the dimension of the article is dependent upon any such drift in calibration being taken into account in said computation, the apparatus comprising an element having a dimension of known and substantially constant length which is interposable between the distance measuring devices in spaced relationship therewith, with the devices aligned with respective opposed ends of said dimension, means arranged to provide representations of the respective measured distances of the devices from the corresponding ends of said dimension when the element is interposed between the devices, and means arranged to compute said representations with the known length of said dimension, to provide an error value corresponding to any drift in the calibration of the monitoring equipment as a result of a change in an operating parameter of at least one of the distance measuring devices, any associated processing equipment and/or instrumentation due to a variation in an operating condition thereof.

The method and apparatus in accordance with the fifth and sixth aspects of the invention, as in the cases of the method and apparatii in accordance with the first, second, third and fourth aspects of the invention, may also be arranged to carry out a series of calibrations of the monitoring equipment over a given operating period at, say, spaced time intervals, during which an operating parameter of at least one of the distance measuring devices, any associated processing equipment and/or instrumentation can change due to a variation in an operating condition thereof, resulting in a drift in the calibration of the monitoring equipment.Means may also be provided to programme the resultant series of error values into associated computing means, to correct subsequently computed values of the monitored length of the dimension of the article over the same operating period during which the equipment was calibrated previously. Additionally or alternatively, such calibrations may be carried out at least once before, during and/or after operation of the monitoring equipment.

The error value(s) derived from a method and/or apparatus in accordance with the respective fifth and/or sixth aspects of the invention defined above, may be used in any subsequent computation of the monitored dimension length of an article, to eliminate any error(s) in the monitored dimension lengths which might otherwise be computed incorrectly as a result of any drift in calibration of the monitoring equipment.

The fifth and sixth aspects of inventive calibration method and apparatus are particularly suited for use in combination with the respective third and fourth aspects of inventive calibration method and apparatus.

Accordingly, a seventh aspect of the invention resides in a method of calibrating equipment for monitoring the length of a dimension, such as, the thickness, of an article employing a differential measuring technique wherein a pair of spaced distance measuring devices are disposed at respective opposed ends of the dimension (thickness) of the article whose length is being monitored in spaced relationship therewith, are mounted upon respective support means whose spacing and, hence, the spacing between the devices, is substantially constant but can change slightly in dependence upon a variation in an operating condition of the equipment, wherein accurate computation of the monitored length of the dimension of the article is dependent upon the spacing between the devices, wherein an operating parameter of at least one of the distance measuring devices, any associated processing equipment and/or instrumentation, can change due to a variation in an operating condition thereof, resulting in a drift in the original calibration of the monitoring equipment and wherein accurate computation of the monitored length of the dimension of the article is also dependent upon any such calibration drift being taken into account in said computation, the calibration method comprising (a) interposing between the distance measuring devices and in spaced relationship therewith an element having a dimension of known and substantially constant length, with the devices aligned with respective opposed ends of said dimension, providing representations of the respective measured distances of the devices from the corresponding ends of the dimension and computing said representations with the known length of the dimension, to provide an error value corresponding to any change in the spacing between said support means and, hence, between the distance measuring devices; and (b) at least once before, during and/or after operation of the monitoring equipment interposing between the distance measuring devices in spaced relationship therewith the or another element having the or a dimension of known and substantially constant length, with the devices aligned with respective opposed ends of said dimension, providing representations of the respective measured distances of the devices from the corresponding ends of said dimension and computing said representations with the known length of said dimension, to provide an error value corresponding to any drift in the calibration of the monitoring equipment due to a change in an operating parameter of at least one of the distance measuring devices, any associated processing equipment and/or instrumentation due to a variation in said operating condition.

An eighth aspect of the invention provides apparatus for calibrating equipment for monitoring the length of a dimension, such as, the thickness, of an article employing a differential measuring technique wherein a pair of spaced distance measuring devices are disposed at respective opposed ends of the dimension of the article whose length is being monitored and are mounted upon support means whose spacing, and hence the spacing between the devices, is substantially constant but can change slightly in dependence upon a variation in an operating condition of the equipment, wherein accurate computations of the so-monitored length of the dimension is dependent upon the spacing between the devices being known, wherein an operating parameter of at least one of the distance measuring devices, associated processing equipment and/or instrumentation, can change due to a variation in an operating condition thereof, resulting in a drift in the original calibration of the monitoring equipment, and wherein accurate computation of the monitored length of the dimension of the article is also dependent upon any resultant calibration drift being taken into account in said computation, the apparatus comprising:: (a) an element having a dimension of known and substantially constant length, which is interposable between the distance measuring devices in spaced relationship therewith, with the devices aligned with respective opposed ends of said dimension, means arranged to provide representations of the respective measured distances of the devices from the corresponding ends of said dimension when the element is interposed therebetween and means arranged to compute said representations with the known length of said dimension, to provide an error value corresponding to any change in the spacing between said support means and, hence, between the distance measuring devices; and (b) an element having a dimension of known and substantially constant length which is interposable between the distance measuring devices in spaced relationship therewith, with the devices aligned with respective opposed ends of said dimension, means arranged to provide representations of the respective measured distances of the devices from the corresponding ends of said dimension when the element is interposed between the devices, and means arranged to compute said representations with the known length of the dimension, to provide ian error value corresponding to any drift in the calibration of the monitoring equipment due to a change in an operating parament of at least one of the distance measuring devices, any associated processing equipment and/or instrumentation due to a variation in said operation condition.

The interposable elements may be constituted by a single element having a dimension of known and substantially constant length.

Again, the error value(s) derived from a method and/or apparatus in accordance with the respective seventh and eighth aspects of the invention defined above may also be used in any subsequent computation of the monitored dimension length of an article, to eliminate any error(s) in the so-determined dimension length which might otherwise be computed incorrectly as a result of any variation in the spacing between the distance measuring devices and/or any drift in the calibration of at least one of those devices and any associated processing equipment and/or instrumentation.

Preferably, laser distance measuring devices are employed in the equipment and the element(s) having a dimension of known and substantially constant thickness is, in the preferred embodiments of inventive calibration apparatus, a so-called "slip gauge". The preferred differential measurement technique employed in the monitoring equipment, operates in accordance with the above-identified equation, namely: t = a - (x + y) where t is the thickness of the article, A is the spacing between the pair of distance measuring devices, X is the measured distance of one of the devices from the corresponding surface of the article and X is the measured distance of the other of the devices from the other opposed surface of the article.

Thus, when employing the inventive calibration methods and apparatii, an error (K) in the spacing (a) between the pair of distance measuring devices results in any error ( & ) in the computed thickness (t) of the article. By using the inventive calibration methods and apparatii, therefore, errors (6a) in the spacing (g) can be computed, because the thickness (t) of the element of known thickness is constant. Such error values (a) can then be used to compute accurately the monitored thickness of the article.

In one preferred embodiment of inventive apparatii, and associated methods, to be described hereinbelow, the distance measuring devices are mounted upon guide or other support means in the form of respective leadscrews extending across, say, the width of a sheet-like article, such as, a web of corrugated board moving in its longitudinal direction, whereby the devices are movable in unison with each other either during calibration of the equipment, in which case, the element of known and substantially constant thickness interposed therebetween is also moved in unison therewith, or during the monitoring of the thickness of the article across the width thereof.

In another preferred embodiment, also to be described hereinbelow, the pair of distance measuring devices are mounted fixedly upon support means which, in a rest or calibration position, may be located on one side of a width of sheet-like material whose thickness is to be monitored, such as, a web of corrugated board moving in its longitudinal direction, and which is pivotable in an arc across the width of the sheet-like material during the monitoring of the thickness thereof. In the rest position of the support means, the calibration of the monitoring equipment is carried out in accordance with the various aspects of the invention defined above.

Alternatively, the distance measuring devices are mounted movably upon the support means, such that, when the support means can be moved from the rest or calibration position to a monitoring position through an arc of, say, 900 across the width of the sheet-like material, the devices can then traverse the width of the article during monitoring of the thickness thereof, as described above.

Yet further aspects of the invention provide: A. A monitoring and calibration process comprising providing equipment for monitoring the length of a dimension, such as, the thickness, of an article employing a differential measuring technique wherein a pair of spaced distance measuring devices are disposed at respective opposed ends of the dimension of the article whose length is being monitored and in spaced relationship therewith, wherein an operating parameter of the equipment can change in dependence upon a variation in an operating condition of the equipment and wherein accurate computation of the monitored length of the dimension of the article is dependent upon any such operating parameter change being taken into account in said computation, which process further comprises a method of calibrating the monitoring equipment in accordance with the first aspect of the invention defined above and/or any modifications thereof described herein.

B. A monitoring and calibration process comprising providing equipment for monitoring the length of a dimension, such as, the thickness, of an article employing a differential measuring technique wherein a pair of spaced distance measuring devices disposed at respective opposed ends of the dimension (thickness) of the article whose length is being monitored and in spaced relationship therewith, are mounted upon respective support means whose spacing and, hence, the spacing between the devices, is substantially constant but can change slightly in dependence upon a variation in an operating condition of the equipment and wherein accurate computations of the so-monitored length of the dimension of the article is dependent upon the spacing between the devices being known, which process further comprises a method of calibrating the monitoring equipment in accordance with the third aspect of the invention defined above and/or any modifications thereof described herein.

C. A monitoring and calibration process including providing equipment for monitoring the length of a dimension, such as, the thickness, of an article employing a differential measuring technique wherein a pair of spaced distance measuring devices are arranged at respective opposed ends of the dimension (thickness) of the article whose length is being monitored in spaced relationship therewith, wherein an operating parameter of at least one of the distance measuring devices, any associated processing equipment and/or instrumentation, can change due to a variation in an operating condition thereof, resulting in a drift in the original calibration of the monitoring equipment, and wherein accurate computation of the monitored length of the dimension of the article is dependent upon any such drift in calibration being taken into account in said computation, which process further comprises a method of calibrating the monitoring equipment in accordance with the fifth aspect of the invention defined above and/or any modifications thereof described herein.

D. A monitoring and calibration process comprising providing equipment for monitoring the length of a dimension, such as, the thickness, of an article employing a differential measuring technique wherein a pair of spaced distance measuring devices are disposed at respective opposed ends of the dimension (thickness) of the article whose length is being monitored in spaced relationship therewith, are mounted upon respective support means whose spacing and, hence, the spacing between the devices, is substantially constant but can change slightly in dependence upon a variation in an operating condition of the equipment, wherein accurate computation of the monitored length of the dimension of the article is dependent upon the spacing between the devices, wherein an operating parameter of at least one of the distance measuring devices, any associated processing equipment and/or instrumentation, can change due to a variation in an operating condition thereof, resulting in a drift in the original calibration of the monitoring equipment and wherein accurate computation of the monitored length of the dimension of the article is also dependent upon any such calibration drift being taken into account in said computation, which process further comprises a method of calibrating the monitoring equipment in accordance with the seventh aspect of the invention defined above and/or any modifications thereof described herein.

E. A monitoring and calibration assembly comprising apparatus in accordance with the second aspect of the invention defined above and/or any modifications thereof described herein, in combination with calibrating equipment for monitoring the length of a dimension, such as, the thickness, of an article employing a differential measuring technique wherein a pair of spaced distance measuring devices are supported at respective opposed ends of the dimension of the article whose length is being monitored and in spaced relationship therewith, wherein an operating parameter of the equipment can change in dependence upon a variation in an operating condition of the equipment and wherein accurate computation of the monitored length of the dimension of the article is dependent upon any such operating parameter change being taken into account in said computation.

F. A monitoring and calibration assembly comprising apparatus in accordance with the fourth aspect of the invention defined above and/or any modifications thereof described herein, in combination with equipment for monitoring the length of a dimension, such as, the thickness, of an article employing a differential measuring technique wherein a pair of spaced distance measuring devices at respective opposed ends of the dimension (thickness) of the article whose length is being monitored are mounted, in spaced relationship therewith, upon support means whose spacing, and hence the spacing between the devices, is substantially constant but can change slightly in dependence upon a variation in an operating condition of the equipment and wherein accurate computation of the so-monitored length of the dimension (thickness) of the article is dependent upon the spacing between the devices being known.

G. A monitoring and calibration assembly comprising apparatus in accordance with the sixth aspect of the invention defined above and/or any modifications thereof described herein, in combination with equipment for monitoring length of a dimension, such differential measuring technique wherein a pair of spaced distance measuring devices are arranged at respective opposed ends of a dimension (thickness) of the article whose length is being monitored in spaced relationship therewith, wherein, in use, an operating parameter of at least one of the distance measuring devices, any associated processing equipment and/or instrumentation, can change due to a variation in an operating condition thereof, resulting in a drift in the original calibration of the monitoring equipment, and wherein accurate computation of the monitored length of the dimension of the article is dependent upon any such drift in calibration being taken into account in said computation.

H. A monitoring and calibration assembly comprising apparatus in accordance with the eighth aspect of the invention defined above and/or any modifications thereof described herein, in combination with equipment for monitoring the length of a dimension, such as, the thickness, of an article employing a differential measuring technique wherein a pair of spaced distance measuring devices are disposed at respective opposed ends of the dimension of the article whose length is being monitored and are mounted upon support means whose spacing, and hence the spacing between the devices, is substantially constant but can change slightly in dependence upon a variation in an operating condition of the equipment, wherein accurate computations of the so-monitored length of the dimension is dependent upon the spacing between the devices being known, wherein an operating parameter of at least one of the distance measuring devices, associated processing equipment and/or instrumentation, can change due to a variation in an operating condition thereof, resulting in a drift in operating condition thereof, resulting in a drift in the original calibration of the monitoring equipment, and wherein accurate computation of the monitored length of the dimension of the article is also dependent upon any resultant calibration drift being taken into account in said computation.

In order that the invention may be more fully understood, preferred embodiments of calibration apparatii, and associated inventive methods, in accordance therewith, will now be described by way of example and with reference to the accompanying drawings wherein: Figure 1 is a diagrammatic elevation in partial cross-section of one form of monitoring equipment in use in which a method and apparatus in accordance with the invention can be employed for calibration purposes; Figure 2 is a diagrammatic representation of the monitoring equipment of Figure 1 when being calibrated in accordance with the invention; Figure 3 is a diagrammatic elevation of monitoring equipment in which a method and apparatus in accordance with the invention are employed for further calibration purposes;; Figure 4 is a side elevation of another form of monitoring equipment in which a method and apparatus in accordance with the invention are employed for calibration purposes; and Figure 5 is a plan view of the monitoring equipment of Figure 4.

Referring firstly to Figure 1, equipment for monitoring the thickness t of a continuous web 1 of corrugated board moving in its longitudinal direction, that is, in or out of the plane of paper, across the width thereof, comprises, inter alia, a pair of laser distance measuring devices 2, 3.

The upper and lower laser devices 2, 3 are arranged on respective opposed sides of the longitudinally moving web 1 and are arranged to measure their respective distances z and y from the corresponding confronting surfaces 4, 5 of that web across the width thereof.

The spacing g between the two laser distance measuring devices is generally known and is supposed to be constant, in which case, the following equation for computing the thickness X of the web 1, is applicable: X = v - (x + However, under continuous operating conditions of the monitoring equipment, the spacing A between the laser devices 2, 3 tends to vary, in which case, errors (eat) in the monitored web thickness occur when the thickness t of the web 1 is computed against the measured distances x and y.

The laser distance devices 2, 3 are mounted upon respective guide means in the form of a pair of parallel leadscrews (not shown) extending across the width of the longitudinally moving web 1, for reciprocal movement in unison thereacross. In this manner, the thickness t of the moving web can be monitored across its width in a continuous manner.

Variations in the spacing A between the laser distance measuring devices occur mainly due to temperature changes in, and resulting expansion and contraction of components of the equipment, particularly the leadscrews, as well as deflection of the leads crews which tend to be consistent over a period. Such temperature changes occur mainly during warm-up of the monitoring equipment after start-up.

With regard to Figure 2 of the drawings, here there is shown diagrammatically the thickness monitoring equipment which has been set up for calibration purposes in accordance with the invention.

Thus, the equipment comprises a pair of spaced, parallel leads crews 6 and 7 upon which the respective laser distance measuring devices 2 and 3 are threadedly mounted for reciprocable movement across the width of a longitudinally moving web 1 of corrugated board after calibration of the equipment has been effected, and which are rotated by a motor 8 via respective drives 9 and 10.

An encoder 11 linked to the motor 8 provides positional data for laser devices 2, 3 with respect to the width of the web 1.

Representations, in the form of electronic signals from the devices 2, 3, provide values for thickness t of the moving web 1 across the width thereof, which are computed in a processor 12 against the spacing A between the devices 2, 3 to provide a read-out at 13 of the thickness of the web 1 across its thickness. Such read-out 13 may be visual and/or in printed form.

Calibration of the thickness monitoring equipment is effected by replacing the web 1 with an element 20 of known and substantially constant thickness k in the form of a slip gauge, which is interposed between the laser distance measuring devices 2, 3 and supported therebetween in any suitable manner. The laser devices 2, 3 and the slip gauge 20 interposed therebetween are moved in unison along a path corresponding to the width of a longitudinally moving web 1 whose thickness t is to be monitored subsequently by the equipment.

Signals representative of the respective distances x, y between the devices 2, 3 and the corresponding surfaces 14, 15 of the element 20 are provided as the devices and element are moved along such path.

For, say, a path of 2 metres long, namely, the width of a web 1 whose thickness X is to be monitored, and a known thickness of 2.5 mm of the slip gauge 20, typical variations in the spacing A between the laser distance measuring devices 2, 3 along the path result in errors (8) as exemplified in the following Table.

3E

Position ition of laser distance measunn!nnd devices and slip gauge along path in millimetres

Thickness of element detected 2.47 2.50 2.52 2.54 2.50 from equation k=a-(x+y) in millimetres Error (ba) in spacing (8) between laser -0.03 0 +0.02 +0.04 0 distance measuring devices in millimetres Thus, for example, the error Sa in spacing a between the laser distance measuring devices 2, 3 at a position of 1500 mm along the path is +0.04 mm which means that the leadscrews 6, 7 have moved apart by that amount at that particular point along the path traversed by the devices 2, 3.

The errors fia obtained for parallelism between the leadscrews 6, 7, and hence the spacing between the laser devices 2, 3, at points along the path corresponding to the width of a longitudinally moving web 1 of corrugated board whose thickness t is to be monitored subsequent to calibration of the equipment, are inputted into the processor 12 and are used subsequently to convert the monitored and computed thickness of the web at corresponding points across its width. Any number of points along the path can be used, usually in dependence upon the length of the leadscrews 6, 7 and, thus, the width of the corrugated board web 1 whose thickness is to be monitored.

Referring now to Figure 3 of the drawings, equipment for monitoring the thickness of a continuous web of corrugated board moving in its longitudinal direction, such as that described above in relation to Figure 1 and 2, also comprises a fixed slip gauge element 20' of known and substantially constant thickness, which is mounted between the pair of spaced, parallel leads crews 6 and 7 at one end thereof where any deflection in the spacing between the two leadscrews 6, 7, and hence between the respective distance measuring devices 2, 3, is at a minimum.

At least once before, during and/or after operation of the distance monitoring equipment, signals representative of the respective distances between the devices 2, 3 and the corresponding surfaces 14', 15' of the slip gauge element 20' are provided. Such representations are then computed with the known thickness of the slip gauge element 20', to provide an error value corresponding to any drift in the calibration of at least one of the distance measuring devices and any associated processing equipment and/or instrumentation, as a result of a change in an operating parameter thereof, due to a variation(s) in said operating condition. Such a variation can occur due to ageing of the components of the distance measuring devices and any associated processing equipment and/or instrumentation and/or any variations in the operating temperatures thereof.

Those errors are inputted into a processor 12' and are used subsequently to correct the monitored and computed thickness of the moving web, as in the case of the equipment described above in relation to Figures 1 and 2.

In this manner, the read-out and/or print-out at 13 provides accurate data for the monitored thickness of the web 1 across its width, after computation thereof with the error values provided by the calibration method described above.

Turning now to Figures 4 and 5 of the drawings, another form of equipment for monitoring the thickness of a continuous web 31 of corrugated board moving longitudinally in the direction of the arrow A, comprises a pair of laser distances measuring devices 32, 33 which are mounted fixedly upon respective upper and lower support arms 36, 37 adjacent respective ends thereof. The other ends of the upper and lower support arms 36, 37 are connected to a support member 38 which is mounted for pivotal movement in a generally horizontal plane, along with the upper and lower support arms, about a vertical axis X located adjacent the right-hand side of the continuous web 1 of corrugated board.

In the rest or calibration position of the thickness monitoring equipment, the upper and lower support arms 36, 37 lie parallel to and spaced from the right-hand edge of the web 1, as shown in Figure 5. In this calibration position, the laser distance measuring devices 32, 33 are arranged in spaced relationship with respect to a fixed slip gauge element 40 of known and constant thickness.

Pivotal movement of the support member 38, and hence the upper and lower support arms 36, 37, about the vertical axis X moves the pair of laser distance measuring devices 32, 33 away from the slip gauge element 40 through an arc B of 900 across the width of the continuously moving web 1 into a final thickness monitoring position, as shown in Figure 5.

As described above in relation to the thickness monitoring equipment and associated calibration apparatus of Figures 1 to 3, changes in the spacing between the laser distance measuring devices 32, 33 can occur due to temperature variations during warm-up of the monitoring equipment. Also, drift in the calibration of at least one of the distance measuring devices 32, 33 and any associated processing equipment and/or instrumentation, can occur as a result of a change in an operating parameter thereof, such as, ageing of the components or variations in the operating temperatures of the distance measuring devices and any associated processing equipment and/or instrumentation.

As a consequence of such operating parameter changes, the computed thicknesses of the web 1 would be incorrect, thereby reducing the accuracy of the monitoring equipment.

Thus, the inventive calibration apparatus and method associated with the thickness monitoring equipment of Figures 4 and 5 eliminates any such errors in the computation of the thicknesses of the web 1 by carrying out the calibration procedure with the equipment in its rest or calibration position, namely, with the laser distance measuring devices 32, 33 disposed on opposed sides of the slip gauge element 40 of known and constant thickness adjacent the righthand edge of the web 1, as shown in Figure 5.

Such calibration, prior to the thickness monitoring procedure, provides signals representative of the respective distances between the devices 32, 33 and the corresponding upper and lower surfaces 34, 35 of the slip gauge element 40. Those representations are then computed with the known thickness of the slip gauge element 40, to provide an error value corresponding to any changes in the spacing between the devices 32, 33, as a result of any changes in the spacing between the upper and lower support arms 36, 37 due to temperature or other operating conditions of the support arms 36, 37.Also, such representations are computed further with the known thickness of the slip gauge 40, to provide an error value corresponding to any drift in the calibration of at least one of the distance measuring devices 32, 33 and any associated processing equipment and/or instrumentation, as a result of a change in an operating parameter thereof, as discussed above.

Once this calibration of the monitoring equipment has been completed, the equipment can then be used to monitor the thickness of the web 1 by moving the laser distance measuring devices 32, 33 along the arc B, with the moving web 1 interposed therebetween. In this particular arrangement, the devices 32, 33 are fixed adjacent respective ends of the upper and lower support arms 36, 37, such that the monitoring path is constituted by the arc B.

In a modification of this arrangement, the laser distance measuring devices 32, 33 may be mounted upon their respective supporting arms 36, 37 for reciprocal movement therealong, when the arms are in the position shown in dashed lines in Figure 5, namely, in the monitoring position of the equipment at 900 to the calibration position. This modified arrangement is similar to the previously-described arrangements of monitoring equipment described above with reference to Figures 1 to 3 of the drawings, in which case, the support arms 36, 37 may be in the form of parallel leads crews along which the distance measuring devices 32, 33 are movable reciprocably during monitoring of the thickness of the moving web 1.

Processing of the measured distances of the devices 32, 33 from respective upper and lower sides of the longitudinally-moving web 1 and the error values provided during calibration of the equipment, are carried out in any suitable manner, such as that described above in relation to Figures 2 and 3.

Claims (47)

1. A method of calibrating equipment for monitoring the length of a dimension, such as, the thickness, of an article employing a differential measuring technique wherein a pair of spaced distance measuring devices are disposed at respective opposed ends of the dimension of the article whose length is being monitored and in spaced relationship therewith, wherein an operating parameter of the equipment can change in dependence upon a variation in an operating condition of the equipment and wherein accurate computation of the monitored length of the dimension of the article is dependent upon any such operating parameter change being taken into account in said computation, the calibration method comprising interposing between the distance measuring devices in spaced relationship therewith, an element having a dimension of known and substantially constant length, with the devices aligned with respective opposed ends of said dimension, providing representations of the respective measured distances of the devices from the corresponding ends of said dimension and computing said representations with the known length of said dimension, to provide an error value corresponding to any change in an operating parameter of the monitoring equipment resulting from a variation in an operating condition thereof.

2. A method of calibrating equipment for monitoring the length of a dimension, such as, the thickness, of an article employing a differential measuring technique wherein a pair of spaced distance measuring devices disposed at respective opposed ends of the dimension (thickness) of the article whose length is being monitored and in spaced relationship therewith, are mounted upon respective support means whose spacing and, hence, the spacing between the devices, is substantially constant but can change slightly in dependence upon a variation in an operating condition of the equipment and wherein accurate computations of the so-monitored length of the dimension of the article is dependent upon the spacing between the devices being known, the calibration method comprising interposing between the distance measuring devices and in spaced relationship therewith, an element having a dimension (thickness) of known and substantially constant length, with the devices aligned with respective opposed ends of said dimension, providing representations of the respective measured distances of the devices from the corresponding ends of said dimension and computing said representations with the known length of the dimension, to provide an error value corresponding to any change in the spacing between said support means and, hence, between the distance measuring devices.

3. A method of calibrating equipment for monitoring the length of a dimension, such as, the thickness, of an article employing a differential measuring technique wherein a pair of spaced distance measuring devices are arranged at respective opposed ends of the dimension (thickness) of the article whose length is being monitored in spaced relationship therewith, wherein an operating parameter of at least one of the distance measuring devices, any associated processing equipment and/or instrumentation, can change due to a variation in an operating condition thereof, resulting in a drift in the original calibration of the monitoring equipment, and wherein accurate computation of the monitored length of the dimension of the article is dependent upon any such drift in calibration being taken into account in said computation, the calibration method comprising interposing between the distance measuring devices in spaced relationship therewith an element having a dimension of known and substantially constant length, with the devices aligned with respective opposed ends of said dimension, providing representations of the respective measured distances of the devices from the corresponding ends of said dimension and computing said representations with the known length of said dimension, to provide an error value corresponding to any drift in the original calibration of the monitoring equipment as a result of a change in an operating parameter of at least one of the distance measuring devices, any associated processing equipment and/or instrumentation due to a variation in said operating condition.

4. A method of calibrating equipment for monitoring the length of a dimension, such as, the thickness, of an article employing a differential measuring technique wherein a pair of spaced distance measuring devices are disposed at respective opposed ends of the dimension (thickness) of the article whose length is being monitored in spaced relationship therewith, are mounted upon respective support means whose spacing and, hence, the spacing between the devices, is substantially constant but can change slightly in dependence upon a variation in an operating condition of the equipment, wherein accurate computation of the monitored length of the dimension of the article is dependent upon the spacing between the devices, wherein an operating parameter of at least one of the distance measuring devices, any associated processing equipment and/or instrumentation, can change due to a variation in an operating condition thereof, resulting in a drift in the original calibration of the monitoring equipment and wherein accurate computation of the monitored length of the dimension of the article is also dependent upon any such calibration drift being taken into account in said computation, the calibration method comprising (a) interposing between the distance measuring devices and in spaced relationship therewith an element having a dimension of known and substantially constant length, with the devices aligned with respective opposed ends of said dimension, providing representations of the respective measured distances of the devices from the corresponding ends of the dimension and computing said representations with the known length of the dimension, to provide an error value corresponding to any change in the spacing between said support means and, hence, between the distance measuring devices; and (b) at least once before, during and/or after operation of the monitoring equipment interposing between the distance measuring devices in spaced relationship therewith the or another element having the or a dimension of known and substantially constant length, with the devices aligned with respective opposed ends of said dimension, providing representations of the respective measured distances of the devices from the corresponding ends of said dimension and computing said representations with the known length of said dimension, to provide an error value corresponding to any drift in the calibration of the monitoring equipment due to a change in an operating parameter of at least one of the distance measuring devices, any associated processing equipment and/or instrumentation due to a variation in said operating condition.

5. A method according to claim 4, wherein the interposable elements are constituted by a single element having a dimension of known and substantially constant length.

6. A method according to any of claims 1 to 3 and 5, wherein the element is a slip gauge.

7. A method according to claim 1, 2 or 3 or claim 6 when dependent upon claim 1, 2 or 3, wherein calibration is carried out at least once before, during and/or after operation of the monitoring equipment.

8. A method according to any of claims 1 to 7, wherein a series of calibrations are carried out over a given operating period of the monitoring equipment.

9. A method according to claim 8, wherein the series of calibrations are carried out from start-up to full operating conditions of the monitoring equipment.

10. A method according to claim 8 or 9, wherein means are provided to programme the resultant series of error values into associated computing means, to correct subsequently computed values for the monitored dimension length of the article over the same operating period during which the monitoring equipment was calibrated previously.

11. Apparatus for calibrating equipment for monitoring the length of a dimension, such as, the thickness, of an article employing a differential measuring technique wherein a pair of spaced distance measuring devices are supported at respective opposed ends of the dimension of the article whose length is being monitored and in spaced relationship therewith, wherein an operating parameter of the equipment can change in dependence upon a variation in an operating condition of the equipment and wherein accurate computation of the monitored length of the dimension of the article is dependent upon any such operating parameter change being taken into account in said computation, the calibration apparatus comprising an element having a dimension of known and substantially constant length and interposable between the distance measuring devices in spaced relationship therewith, with the devices aligned with respective opposed ends of said dimension, means arranged to provide representations of the respective measured distances of the devices from the corresponding ends of said dimension when the element is interposed between the devices and means arranged to compute said representations with the known length of said dimension, to provide an error value corresponding to any change in an operating parameter of the monitoring equipment resulting from a variation in an operating condition thereof.

12. Apparatus for calibrating equipment for monitoring the length of a dimension, such as, the thickness, of an article employing a differential measuring technique wherein a pair of spaced distance measuring devices at respective opposed ends of the dimension (thickness) of the article whose length is being monitored are mounted, in spaced relationship therewith, upon support means whose spacing, and hence the spacing between the devices, is substantially constant but can change slightly in dependence upon a variation in an operating condition of the equipment and wherein accurate computation of the so-monitored length of the dimension (thickness) of the article is dependent upon the spacing between the devices being known, the apparatus comprising an element having a dimension (thickness) of known and substantially constant length which is interposable between the distance measuring devices in spaced relationship therewith, with the devices aligned with respective opposed ends of said dimension, means arranged to provide representations of the respective measured distances of the devices from the corresponding ends of said dimension when the element is interposed between the devices, and means arranged to compute said representations with the known length of said dimension, to provide an error value corresponding to any change in the spacing between said support means and, hence, between the distance measuring devices.

13. Apparatus for calibrating equipment for monitoring the length of a dimension, such as, the thickness, of an article employing a differential measuring technique wherein a pair of spaced distance measuring devices are arranged at respective opposed ends of a dimension (thickness) of the article whose length is being monitored in spaced relationship therewith, wherein, in use, an operating parameter of at least one of the distance measuring devices, any associated processing equipment and/or instrumentation, can change due to a variation in an operating condition thereof, resulting in a drift in the original calibration of the monitoring equipment, and wherein accurate computation of the monitored length of the dimension of the article is dependent upon any such drift in calibration being taken into account in said computation, the apparatus comprising an element having a dimension of known and substantially constant length which is interposable between the distance measuring devices in spaced relationship therewith, with the devices aligned with respective opposed ends of said dimension, means arranged to provide representations of the respective measured distances of the devices from the corresponding ends of said dimension when the element is interposed between the devices, and means arranged to compute said representations with the known length of said dimension, to provide an error value corresponding to any drift in the calibration of the monitoring equipment as a result of a change in an operating parameter of at least one of the distance measuring devices, any associated processing equipment and/or instrumentation due to a variation in an operating condition thereof.

14. Apparatus for calibrating equipment for monitoring the length of'a dimension, such as, the thickness, of an article employing a differential measuring technique wherein a pair of spaced distance measuring devices are disposed at respective opposed ends of the dimension of the article whose length is being monitored and are mounted upon support means whose spacing, and hence the spacing between the devices, is substantially constant but can change slightly in dependence upon a variation in an operating condition of the equipment, wherein accurate computations of the so-monitored length of the dimension is dependent upon the spacing between the devices being known, wherein an operating parameter of at least one of the distance measuring devices, associated processing equipment and/or instrumentation, can change due to a variation in an operating condition thereof, resulting in a drift in the original calibration of the monitoring equipment, and wherein accurate computation of the monitored length of the dimension of the article is also dependent upon any resultant calibration drift being taken into account in said computation, the apparatus comprising:: (a) an element having a dimension of known and substantially constant length, which is interposable between the distance measuring devices in spaced relationship therewith, with the devices aligned with respective opposed ends of said dimension, means arranged to provide representations of the respective measured distances of the devices from the corresponding ends of said dimension when the element is interposed therebetween and means arranged to compute said representations with the known length of said dimension, to provide an error value corresponding to any change in the spacing between said support means and, hence, between the distance measuring devices; and (b) an element having a dimension of known and substantially constant length which is interposable between the distance measuring devices in spaced relationship therewith, with the devices aligned with respective opposed ends of said dimension, means arranged to provide representations of the respective measured distances of the devices from the corresponding ends of said dimension when the element is interposed between the devices, and means arranged to compute said representations with the known length of the dimension, to provide an error value corresponding to any drift in the calibration of the monitoring equipment due to a change in an operating parament of at least one of the distance measuring devices, any associated processing equipment and/or instrumentation due to a variation in said operation condition.

15. Apparatus according to claim 14, wherein the interposable elements are constituted by a single element having a dimension of known and substantially constant length.

16. Apparatus according to any of claims 11, 12, 13 and 15, wherein the element is a slip gauge.

17. Apparatus according to claim 11, 12 or 13 or claim 16 when dependent upon claim 11, 12 or 13, wherein the apparatus is arranged to carry out calibration of the monitoring equipment at least once before, during and/or after operation thereof.

18. Apparatus according to any of claims 11 to 17, wherein the apparatus is arranged to carry out a series of calibrations over a given operating period of the monitoring equipment.

19. Apparatus according to claim 18, wherein the apparatus is arranged to carry out the series of calibrations from start-up to full operating conditions of the monitoring equipment.

20. Apparatus according to claim 18 or 19, including means arranged to programme the resultant series of error values into associated computing means, to correct subsequently computed values for the monitored dimension length of the article over the same operating period during which the equipment was calibrated previously.

21. A monitoring and calibration process comprising providing equipment for monitoring the length of a dimension, such as, the thickness, of an article employing a differential measuring technique wherein a pair of spaced distance measuring devices are disposed at respective opposed ends of the dimension of the article whose length is being monitored and in spaced relationship therewith, wherein an operating parameter of the equipment can change in dependence upon a variation in an operating condition of the equipment and wherein accurate computation of the monitored length of the dimension of the article is dependent upon any such operating parameter change being taken into account in said computation, which process further comprises a method of calibrating the monitoring equipment according to claim 1 or any of claims 6 to 10 when dependent thereon.

22. A monitoring and calibration process comprising providing equipment for monitoring the length of a dimension, such as, the thickness, of an article employing a differential measuring technique wherein a pair of spaced distance measuring devices disposed at respective opposed ends of the dimension (thickness) of the article whose length is being monitored and in spaced relationship therewith, are mounted upon respective support means whose spacing and, hence, the spacing between the devices, is substantially constant but can change slightly in dependence upon a variation in an operating condition of the equipment and wherein accurate computations of the so-monitored length of the dimension of the article is dependent upon the spacing between the devices being known, which process further comprises a method of calibrating the monitoring equipment according to claim 2 or any of claims 6 to 10 when dependent thereon.

23. A monitoring and calibration process including providing equipment for monitoring the length of a dimension, such as, the thickness, of an article employing a differential measuring technique wherein a pair of spaced distance measuring devices are arranged at respective opposed ends of the dimension (thickness) of the article whose length is being monitored in spaced relationship therewith, wherein an operating parameter of at least one of the distance measuring devices, any associated processing equipment and/or instrumentation, can change due to a variation in an operating condition thereof, resulting in a drift in the original calibration of the monitoring equipment, and wherein accurate computation of the monitored length of the dimension of the article is dependent upon any such drift in calibration being taken into account in said computation, which process further comprises a method of calibrating the monitoring equipment according to claim 3 or any of claims 6 to 10 when dependent thereon.

24. A monitoring and calibration process including providing equipment for monitoring the length of a dimension, such as, the thickness, of an article employing a differential measuring technique wherein a pair of spaced distance measuring devices are disposed at respective opposed ends of the dimension (thickness) of the article whose length is being monitored in spaced relationship therewith, are mounted upon respective support means whose spacing and, hence, the spacing between the devices, is substantially constant but can change slightly in dependence upon a variation in an operating condition of the equipment, wherein accurate computation of the monitored length of the dimension of the article is dependent upon the spacing between the devices, wherein an operating parameter of at least one of the distance measuring devices, any associated processing equipment and/or instrumentation, can change due to a variation in an operating condition thereof, resulting in a drift in the original calibration of the monitoring equipment and wherein accurate computation of the monitored length of the dimension of the article is also dependent upon any such calibration drift being taken into account in said computation, which process further comprises a method of calibrating the monitoring equipment according to claim 4 or any of claims 5 to 10 when dependent thereon.

25. A process according to any of claims 21 to 24, wherein the monitoring equipment is used to monitor the thickness of a planar article.

26. A process according to claim 25, wherein the monitoring equipment is used to monitor the thickness of a moving length of corrugated board.

27. A process according to any of claims 21 to 26, wherein the distance measuring devices of the monitoring equipment are laser or ultrasonic distance measuring devices.

28. A process according to any of claims 21 to 27, wherein the distance measuring devices are moved in unison along a path across the width of the article upon respective support means.

29. A process according to claim 28, wherein the path is linear or arcuate.

30. A process according to claim 28 or 29, wherein the element having a dimension of known and substantially constant length, is moved in unison with the distance measuring devices along the path during calibration of the monitoring equipment.

31. A process according to any of claims 21 to 30,wherein the distance measuring devices are mounted fixedly upon support means.

32. A process according to claim 31, wherein said support means is, in a rest or calibration position thereof, located on one side of a sheet-like material whose thickness is to be monitored and is pivotal in an arc across the width of the material during monitoring of the thickness thereof.

33. A process according to claim 32, wherein calibration of the monitoring equipment is carried out with the said support means in its rest position.

34. A monitoring and calibration assembly comprising apparatus according to claim 11 or any of claims 16 to 20 when dependent thereon, in combination with calibrating equipment for monitoring the length of a dimension, such as, the thickness, of an article employing a differential measuring technique wherein a pair of spaced distance measuring devices are supported at respective opposed ends of the dimension of the article whose length is being monitored and in spaced relationship therewith, wherein an operating parameter of the equipment can change in dependence upon a variation in an operating condition of the equipment and wherein accurate computation of the monitored length of the dimension of the article is dependent upon any such operating parameter change being taken into account in said computation.

35. A monitoring and calibration assembly comprising apparatus according to claim 12 or any of claims 16 to 20 when dependent thereon, in combination with equipment for monitoring the length of a dimension, such as, the thickness, of an article employing a differential measuring technique wherein a pair of spaced distance measuring devices at respective opposed ends of the dimension (thickness) of the article whose length is being monitored are mounted, in spaced relationship therewith, upon support means whose spacing, and hence the spacing between the devices, is substantially constant but can change slightly in dependence upon a variation in an operating condition of the equipment and wherein accurate computation of the so-monitored length of-the dimension (thickness) of the article is dependent upon the spacing between the devices being known.

36. A monitoring and calibration assembly comprising apparatus according to claim 13 or any of claims 16 to 20 when dependent thereon, in combination with equipment for monitoring length of a dimension, such as, the thickness, of an article employing a differential measuring technique wherein a pair of spaced distance measuring devices are arranged at respective opposed ends of a dimension (thickness) of the article whose length is being monitored in spaced relationship therewith, wherein, in use, an operating parameter of at least one of the distance measuring devices, any associated processing equipment and/or instrumentation, can change due to a variation in an operating condition thereof, resulting in a drift in the original calibration of the monitoring equipment, and wherein accurate computation of the monitored length of the dimension of the article is dependent upon any such drift in calibration being taken into account in said computation.

37. A monitoring and calibration assembly comprising apparatus according to claim 14 or any of claims 16 to 20 when dependent thereon, in combination with equipment for monitoring the length of a dimension, such as, the thickness, of an article employing a differential measuring technique wherein a pair of spaced distance measuring devices are disposed at respective opposed ends of the dimension of the article whose length is being monitored and are mounted upon support means whose spacing, and hence the spacing between the devices, is substantially constant but can change slightly in dependence upon a variation in an operating condition of the equipment, wherein accurate computations of the so-monitored length of the dimension is dependent upon the spacing between the devices being known, wherein an operating parameter of at least one of the distance measuring devices, associated processing equipment and/or instrumentation, can change due to a variation in an operating condition thereof, resulting in a drift in the original calibration of the monitoring equipment, and wherein accurate computation of the monitored length of the dimension of the article is also dependent upon any resultant calibration drift being taken into account in said computation.

38. An assembly according to any of claims 34 to 37, wherein the distance measuring devices of the monitoring equipment are laser or ultrasonic measuring devices.

39. An assembly according to any of claims 34 to 38, wherein the distance measuring devices are movable in unison along a path across the width of the article.

40. An assembly according to claim 39, wherein the path is linear or arcuate.

41. An assembly according to claim 39 or 40, wherein the element having a dimension of known and substantially constant length, is movable in unison with the distance measuring devices along the path during calibration of the monitoring equipment.

42. An assembly according to any of claims 34 to 41, wherein the distance measuring devices are mounted fixedly upon support means.

43. An assembly according to claim 42, wherein said support means is locatable, in a rest or calibration position thereof, on one side of a sheet-like material whose thickness is to be monitored and is pivotable in an arc across the width of the material during monitoring of the thickness thereof.

44. A method of calibration substantially as hereinbefore described.

45. Calibration apparatus substantially as hereinbefore described with reference to the accompanying drawings.

46. A monitoring and calibration process substantially as hereinbefore described.

47. A monitoring and calibration assembly substantially as hereinbefore described with reference to the accompanying drawings.