Electricity: measuring and testing – Magnetic – With means to create magnetic field to test material
Patent
1991-02-04
1994-04-19
Snow, Walter E.
Electricity: measuring and testing
Magnetic
With means to create magnetic field to test material
324239, G01N 2772, G01R 3312
Patent
active
053049276
DESCRIPTION:
BRIEF SUMMARY
This invention relates to a method and apparatus for monitoring a series of products.
The invention is particularly applicable to the monitoring of mass-produced food products, such as loaves of bread, sausages, frozen chickens and so on, but is also applicable to the monitoring of other articles. The invention is particularly useful for the detection of metallic foreign bodies in such articles, but is not limited to such uses.
It is known to monitor food products by passing them through what is referred to as a metal detecting apparatus. The apparatus has a drive coil driven by alternating signal which produces an oscillating magnetic field, and a pair of detection coils connected in a precision balanced circuit, so that in the absence of a product in a detection zone in the vicinity of the coils a null detection signal is output from the detection coils. The drive signal and detection signal are fed to a phase detection circuit, which produces a pair of signals indicative of the detection signal component in phase with the drive signal and the detection signal component 90 degrees out of phase from the drive signal. From these two signals the phase difference between the detection signal and the drive signal can be determined. When a product is placed in the detection zone, a phase difference arises. For a series of identical products the phase difference for each is substantially the same. Furthermore, if the only difference between products in a series is their size, then the phase differences for the products are still substantially the same. However, if one of the products, say a loaf of bread, contains a foreign body, such a steel bolt, then a different phase difference will arise. Thus, in the known apparatus, the determined phase difference is compared with a pre-set value, and if there is a difference of more than a preset threshold value, an alarm signal is output.
In the known apparatus, the comparison value and threshold value are preset in accordance with the type of product. Although the phase differences produced for a series of apparently identical products are similar, slight variations do arise, and it is therefore necessary in the known apparatus to set the threshold value to a larger than ideal value in order to prevent spurious rejection of products, but thereby increasing the risk of accepting a contaminated product. The present invention evolves in part from a realisation by the inventors that the phase differences for a series of acceptable products do not, in most circumstances, change greatly from one product to the next in a series, even though there may be a more pronounced change over a long period of time. Various reasons for this phenomenon have been determined by the inventors; a major reason being change in temperature of the products, resulting in a change in phase difference, but other parameters may also have an effect. As one example, a sausage making machine and the sausages produced thereby may rise in temperature during a day's work as the machine "warms up". Also, the ambient temperature of the workplace may change during the day and have an effect on the phase difference. As a further example, a large batch of loaves taken from an oven at one time and then passed through the apparatus one after the other will be at progressively diminishing temperatures.
In accordance with one aspect of the present invention, the comparison value is updated automatically when an acceptable product is detected, in dependence upon the detected phase difference for the acceptable product. Accordingly, account is taken of progressive small changes in the phase difference for acceptable products, and therefore a smaller threshold value can be chosen, thus increasing sensitivity without increasing the risk of accepting unacceptable products.
The invention may also include the feature that, at the start of a series of products, a routine is provided whereby a few products can be detected and the initial comparison value is calculated from the results for those products.
The invention may
Averill Robert M.
Cox Ian W. D.
Kittel Clive F.
Marcus Christopher
Thomas Neil E.
Cintex Limited
Snow Walter E.
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