METHOD AND APPARATUS FOR DIMENSIONAL MEASUREMENT OF AN OBJECT

Abstract: The apparatus disclosed includes at least two optical measuring devices for forming independent images of the ob­ ject in their respective focal planes. From these images a specific dimension of the object, for example its diameter where a circular object is concerned, may be obtained. To cor­ rect for inaccuracies due to product movement, the dimension of the product obtained from one of the measuring devices is compensated by a positional measurement obtained from the other camera and vice versa. By this means accurate determi­ nation of the size of a product, for example a cable moving in an extrusion run, is possible without contact being necessary within the product.

Description: FIELD OF THE INVENTION The present invention relates to a method and apparatus for the measurement of the dimensions of an object. BACKGROUND ART In various continuous processes, it becomes desirable to measure the external dimensions of the objects being produced in a continuous format, and in this context various systems and apparatus have been devised to perform this duty For example reference may be made to U K patent specification 1395910 which discloses the use of two beams directed at the object under test particularly in the form of a machine part with stepped diameters, the radiation of those parts of the beam transmitted past the object being used to gauge the size of the object. The continuous processes with which the present invention is particularly concerned are extrusion processes for producing wire, cable and the like. However dimensional measurement may also be required in other processes such as those for glass tube or rod manufacture, or even in precision component manufacture. The basic need to measure object dimension in these processes is to ensure that the product being manufactured conforms to specification, in terms of the eventual use to which it is to be applied where critical tolerances are essential. A second reason is one of economy and this applies to the extrusion of plastic covered products where a minimum coating requirement has to be fulfilled to avoid excess coating going to waste. Existing methods are based on a shadow graph or scanning principle, which either looks at the shadow of an object or scans the object and hence obtains a measurement of the overall size. These existing systems suffer from inaccuracies resulting from the object moving with respect to the measuring apparatus itself. Object movement creates errors since this is interpreted falsely by the measuring instrument as a size change. This inherent difficulty has been overcome in some cases by guiding and restraining movement of the product through the measuring apparatus. This arrangement has had some but limited success. Thus where the product is either too hot or too soft, the mechanical guidance system employed is liable to deform the product or render it useless SUMMARY OF THE INVENTION It is an object of the invention to obviate the problems of the prior art. According to the invention there is provided apparatus for determining a dimension of an object comprising a source of radiation for directing at the object, a first optical head for forming an image of the object in relation to a first optical axis passing through the object and providing an output representative of the size and position of the image in a focal plane of the first optical head, a second optical head for forming an image of the object in relation to a second optical axis passing through the object and providing an output representative of the dimension and position of the image in a focal plane of the second optical head, and means for correlating said outputs of the first and second heads such that a dimension of the image obtained from the output of one of said first and second heads is compensated for object movement by the position of the image obtained from the output of the other of said first and second heads whereby to determine an accurate measure of a dimension of the product represented by the image. Other features and advantages of the invention will become apparent to the skilled reader as the invention is further described in relation to specific ways and means of putting same into effect. BRIEF DESCRIPTION OF THE DRAWINGS An embodiment of the invention will now be described by way of example with reference to the drawings wherein; Figure 1 is a diagrammatic illustration of measuring apparatus according to the invention; Figure 2 is a wave form output obtained from the apparatus of Figure 1; and Figure 3 is a diagrammatic representation of a measuring apparatus according to another embodiment of the invention. The measuring apparatus shown in Figure 1 comprising a pair of optical measuring heads 1 and 2. The optical heads 1 and 2 operate essentially on the camera principle and include lenses 3 and 4 which act to focus an image of the perceived object on light responsive devices within the heads 1 and 2 in the form of charge coupled devices (CCD's) 5 and 6. The object to be measured, in for the form of extruded cable 8, in this example, passes between the heads 3 and 4 and radiation source 7, to intercept the radiation beam 9. The heads 1 and 2 are arranged such that their respective optical axes 10 and 11 intercept or meet one another at the centre of the cable 8 as shown. Consequently a real image of the object 8 is formed on the radiation responsive devices 5 and 6 in the heads 1 and 2. In the example shown the optical axes 10 and 11 are at an acute angle α with respect to one another but in practice this angle may be of a value within say a range of 20° to 90°, but is not critical. The outputs 12 and 13 from the CCD 5 and 6 in each optical head 1 and 2 may be represented in electrical wave form on an oscilloscope as shown in Figure 2. It will be appreciated that the valley 14 represents the radiation which is obscurred by the real image of the object 8 falling on the CCD's 5 and 6. Hence the distance D between the sides 15 and 16 of the output pulses 17 and 18 of the waveform is a measure of the diameter of the product 8 passing along the extrusion line. By measuring this distance D by suitable electronic circuitry, it is possible accurately to determine the dimension (for example diameter) of the cable product 1. It is to be noted that this dimension measured simultaneously by optical heads 1 and 2, is that diameter of the object 8 perpendicular to the respective optical axes 10 and 11 of the heads 1 and 2. If the object 8 moves in space, for example towards the head 1, the real image formed on the radiation responsive device 5 will increase in size, since this image size is a function of the distance of the object 8 from the device 3. Similarly if the object 8 moves away from the head 1, the image formed on the CCD 5 will decrease in size. Consequently if reliance were placed entirely on the measurement of product size being obtained with a single optical head 1 or 2, there would be inherent inaccuracies brought about by product movement, as discussed earlier in this disclosure. When the object 8 moves towards head 1, the head 2 will detect a change in size of the real image of the object 8 which will be a function of the angle α between the optical axis 10 and 11 of the heads 1 and 2. The radiation responsive devices 5 and 6 will detect this change in image size, and signals representative thereof are fed to an indicator unit 17. By cross relating the output of one radiation responsive device 3 with the other 4 in the indicator unit 17, it is possible by suitable electronic circuitry in the indicator unit 17, including microprocessor means to compensate for the movements of the product 1 and obtain an accurate value of its diameter. This arrangement thereby clearly obviates the need to guide the product to prevent movement as has been necessary heretofore. The present invention therefore provides a truly non-contact measurement technique with minimum error. As mentioned earlier the angle α between the two heads 1 and 2 is not critical and could be anything from 20° up to 90° depending on the configuration, application and requirement of the measurements to be made. Considering a theoritical example to highlight the above, we may consider a practical situation where an image of the profile of the object 8 is projected onto the CCD's 5 and 6 of the measuring heads 1 and 2. In this example the CCD is a 12.70 mm 1024 pixel array. The profile is illuminated from behind as shown in Figure 1, so that the image is represented by no light reaching the array as shown with reference to Figure 3. Now: But 1 f = 1 u + 1 v where f = focal length of lenses 3 and 4. Therefore 1 v = uÿf uf ...... (4) and therefore from (3) Object height D = I(uÿf) f ........................ (5) Now consider the case where the angle α in Figure 1 is 90°. The nominal distance from the camera lenses 3 and 4 to the circular object 8 are U xnom and U ynom and the mean image positions detected by devices 1 and 2 are p x and p y , where the suffix "x" and "y" refer to devices 1 and 2 respectively. Therefore U x = U xnom + P y .......... (6) U y = U ynom + P x .......... (7) Substituting equations (6) and (7) in equation 5 in respect of both optical devices 1 and 2 we have where D x is the corrected image size in device 1 and D y is the corrected image size in device 2. U xnom and U ynom are entered according to the configuration of the system. They may be finely adjusted in order to calibrate the instrument against a fixed sample. It will be assumed that the camera devices 1 and 2 will be focused for U xnom and U ynom . If the devices 1 and 2 are mounted so that their lines of sight are at an angle α to each other, as shown in Figure 1, the object position may still be compensated as follows: In this case equations (6) and (7) become: U x = U xnom + P y cos α U y = U ynom + P x cos α The allowable movement is reduced in proportion to cos α. The invention, as shown in Figure 2, may be extended in some applications to having a multiplicity of heads H, say four or five, around the product 8 in order to effect more dimensional measurements and extract more information which may be required in the process of manufacturing elongated products. This method and apparatus lends itself suitable for not only measuring the dimension of the product but is also suitable for measuring small surface imperfections that may occur during the manufacture of various types of materials and as the product is viewed from a multiplicity of heads an analysis may be made of the surface quality of the product.

Claims: 1. Apparatus for determining a dimension of an object (8) comprising a source of radiation (7) for directing at the object, a first optical head (1) for forming an image of the object in relation to a first optical axis (10) passing through the object and providing an output (12) representative of the size and position of the image in a focal plane of the first optical head, a second optical head (2) for forming an image of the object in relation to a second optical axis (11) passing through the object and providing an output (13) representative of the dimension and position of the image in a focal plane of the second optical head, and means (17) for correlating sait outputs of the first and second heads such that a dimension of the image obtained from the output of one of said first and second heads is compensated for object movement by the position of the image obtained from the output of the other of said first and second heads whereby to determine an accurate measure of a dimension of the object represented by the image. 2. Apparatus as claimed in claim 1 wherein said dimension of the image is the diameter of said object. 3. Apparatus as claimed in claims 1 or 2 wherein said position is the mean position of the image in a respective focal plane of said first or second optical heads. 4. Apparatus as claimed in claim 1 wherein said outputs are provided by charge coupled devices (5, 6) respectively positioned at the focal plane of said first and second optical heads. 5. Apparatus as claimed in claim 4 wherein said correlating means includes processor means for processing the outputs of said charge coupled devices whereby to calculate a said accurate dimension of the product.

Inventors: TOLE, WALTER ROLAND
Applicants: TOLE, WALTER ROLAND
Application year: 1987
Main classification: G01B11
IPC codes: G01B11