1. Field of the Invention
The present invention relates to a system for detecting stray metal in articles to be inspected, and more particularly, to a system capable of securely detecting stray metal in articles despite changes of set conditions, such as changes of contents of the articles to be inspected, change of ambient condition, for example, change of temperature, etc.
2. Description of the Related Art
As is generally known, fine stray metal pieces or particles in articles or products, such as ham, sausage, miso (soybean paste), etc., are detected by the fluoroscopic method, induced voltage method, or variable induction method. In the fluoroscopic method, stray metal in articles is detected by means of soft X-rays or a radioisotope. According to the induced voltage method, an article is passed through a DC magnetic field in which a detecting coil is disposed. If a stray piece or pieces of magnetic substance, such as iron, exist in the article, an induced voltage is produced in the detecting coil. According to the variable induction method, an article is passed through an AC magnetic field in which a detecting coil is disposed. If a stray magnetic piece or pieces such as iron piece or pieces or a stray mon-magnetic piece or pieces such as stainless piece or pieces exist in the article, induced voltages generated in a detecting coil changes. Fluoroscopic metal detecting apparatuses are not popularly used because they are expensive and can be applied only limited kinds of objects. Metal detecting apparatuses based on the induced voltage method, on the other hand, cannot detect the non-magnetic piece or pieces at high sensitivity, so that their fields of application are limited. Conventionally, therefore, the variable induction method is popularly used for metal detecting apparatuses.
The metal detecting apparatus based on the variable induction method is based on the following principle of detection. In detecting section 1 of a metal detecting apparatus, as shown in FIG. 1A, an alternating signal is applied from oscillator S.sub.0 to magnetic field generating coil P, whereupon an alternating magnetic field is generated by the coil. First and second coils S.sub.1 and S.sub.2 are arranged facing coil P so that substantially equal lines of magnetic force delivered from coil P pass through coils S.sub.1 and S.sub.2, whereby substantially equal induced voltages E.sub.1 and E.sub.2 are generated by the alternating magnetic field. Article W is transported at a predetermined speed, between magnetic field generating coil P and first and second detecting coils S.sub.1 and S.sub.2, arranged in this manner, by means of a transporting apparatus (not shown). Thus, after it is passed through a magnetic field between coils P and S.sub.1, article W is passed through a magnetic field between coils P and S.sub.2. If no stray metal is in article W, the magnetic fields between coils P and S.sub.1 and between coils P and S.sub.2 make substantially no change despite the passage of the article, and substantially equal induced voltages E.sub.1 and E.sub.2 are generated from first and second detecting coils S.sub.1 and S.sub.2. If stray metal is in article W, on the other hand, lines of magnetic force of the magnetic fields between coils P and S.sub.1 and between coil P and S.sub.2 change each time the article passes through the magnetic fields, and different induced voltages E.sub.1 and E.sub.2 are generated from coils S.sub.1 and S.sub.2. Thus, if iron, a magnetic substance, exists in article W, the path of the lines of magnetic force passing through first detecting coil S.sub.1 is changed by iron, thereby increasing the lines of magnetic force, as the article passes through coil S.sub.1. As a result, induced voltage E.sub.1 detected by coil S.sub. 1 increases to a level higher than induced voltage E.sub.2 induced by coil S.sub.2.
If a nonferrous metal exists in the article, an eddy current is produced in the metal, as shown in FIG. 1C, and electromagnetic flux is consumed as energy to produce the eddy current, so that the lines of magnetic force passing through first detecting coil S.sub.1 are reduced. Thereupon, induced voltage E.sub.1 induced by coil S.sub.1 is reduced to a level lower than induced voltage E.sub.2 induced by second detecting coil S.sub.2. Thus, if stray metal is in article W, a difference is produced between induced voltages E.sub.1 and E.sub.2 from first and second detecting coils S.sub.1 and S.sub.2 while the article is passing through detecting section 1. A differential voltage equivalent to the difference between the two induced voltages is delivered as an unbalance signal, and the stray metal in the article can be detected by detecting the unbalance signal.
In the metal detecting apparatus described above, if the material or ingredients of the article, the ambient temperature, or the inductances of first and second detecting coils S.sub.1 and S.sub.2 change, the set conditions vary, so that the sensitivity of the apparatus is lowered. Thus, stray metal cannot be accurately detected.