COMPENSATING CHANGES IN PRODUCT REFLECTANCE IN OPTICAL SCANNING DETECTION AND REJECTION SYSTEMS.

This invention ^' relates to a method and apparatus for providing continuous, automatic background compensation for a scanning optical detection system.

It is known that certain systems use a calibration circuit to adjust their base level to that of the background in view at the time of calibration. A common problem that exists in these systems is due to variations in illumination, and changes in product colour, this reduces the system efficiency and requires constant re-calibration.

In any optical detection system used to reject foreign material from a _. flow of product, it is essential that a visual difference exists between the normal product and the foreign material to be rejected. The system performance is directly related to the reject system's ability to act on this difference. In the system, fluctuations in illumination, product colour, dust, steam, or any other obscuring gas or material, will cause the normal product signal level to alter. When a fixed reject signal is compared with a camera signal, some fixed tolerance would be made above the level of these fluctuations, whereby only foreign material with a difference greater than these

fluctuations would be rejected.

An object of the present invention is to provide a method and apparatus for continuous automatic detection and compensation of changes in the level of a reflected background camera signal from a product stream, due to changes in illumination, product colour or other causes, in an optical scanning detection system.

It is a further object of this invention to provide such a method and apparatus that the background compensation signal will continually adjust the threshold level at which foreign material is rejected.

The above and other objects and advantages of the invention will be apparent from the following description _, of the invention, taken in conjunction with the accompanying drawing which shows a block diagram of the signal compensating apparatus.

The video signal from camera 1 is held by the valid data strobe in sample/hold device 2 the output of the sample/hold is compared with the output of an averaging circuit (e.g. running average over a period of 1.5 minutes) 4 to produce an error signal at the output of amplifier 3- This error signal is used to modify the current value of the averaging circuit.

A signal from a manual threshold level adjustment device 7 is summed together with the output of the averaging circuit 4, in a threshold summing circuit 5. The resultant output is compared with the video signal

in a reject pulse comparator 6. If the video signal is greater than the summed threshold signal a reject data pulse will be output from comparator 6. In this manner there is achieved:

1. A method of continuous background compensation of threshold value above which foreign material is rejected.

2. A method in which no calibration is required. The reject data pulse may be fed to a zone decoder logic circuit of an apparatus for providing programmable size and overlap in a detection system as described in our co-pending application no. 8625953 filed simultaneously with the present application.