Radiant energy – Photocells; circuits and apparatus – Optical or pre-photocell system
Patent
1982-07-06
1985-10-29
Nelms, David C.
Radiant energy
Photocells; circuits and apparatus
Optical or pre-photocell system
271263, G06M 700
Patent
active
045502528
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND OF THE INVENTION
This invention relates to apparatus for detecting the thickness of sheets as they are moved past the apparatus, and is especially applicable to the detection of banknotes where the thickness of two or more superimposed banknotes should be distinguished from that of a single banknote.
Machines for detecting the simultaneous passage of two banknotes where banknotes should normally pass singly are known, and one example of an optical detector is in commonly assigned British Pat. No. 1,344,986, the detector responding to the opacity of the note or notes; this is inadequate if the notes are greasy or soiled. Mechanical thickness detectors are also known using a roller whose axis is displaced by the passage of a banknote or banknotes to cause an electrical contact to be made or broken if the displacement is such as to indicate the passage of superimposed banknotes. Apparatus for gauging laminar materials, using nip rollers and a variable-reluctance transducer, is disclosed in commonly assigned British Pat. No. 1,497,181.
However, none of the machines has yet been able to compensate for long-term variations in the detecting heads and in the control circuitry which will tend to change the threshold value of the banknote thickness, i.e. the thickness at which the detector will signal the passage of a double banknote. In the case of the mechanical detector using a roller, this long-term drift could be caused by mechanical movement between the pivot of an arm attached to the axis of the roller, and the surface on which the note is placed as it is moved under the roller. This potentially degrades note thickness measurement if the output of the detector is compared against a fixed threshold level in the output stages of the control circuit. Mechanical stability of the order required for measuring banknote thicknesses, for example 0.005 cm to 0.015 cm, is difficult to achieve under reasonable manufacturing and wear rate conditions.
SUMMARY OF THE INVENTION
Apparatus according to the invention for the detection of single or multiple sheets comprises: a support on which the sheet or sheets is carried for movement in the plane of the sheets; a follower biased such that it normally abuts against the support and follows the profile of the surface of the support and the overlying sheet or sheets; a transducer responsive to the position of the follower to provide a thickness signal indicative of the linear displacement of the follower relative to the support due to the passage of the sheet or sheets; and a control circuit which compares the thickness with at least one stored threshold level signal to provide an output signal indicative of whether the detected thickness corresponds to that of a single sheet or multiple sheets; and characterized in that the control circuit includes means for repeatedly updating the or each stored threshold level signal from the thickness signal provided when there is no sheet between the support and the follower. There may be an arm for conveying the movement of the follower to the transducer. This can be a pivot arm, the follower being a cylindrical roller whose axis is attached to the end of the pivot arm. The position of the pivot point along the arm is then preferably such that small displacements of the follower are converted into larger displacements of the other end of the arm engaging with the transducer. The transducer is then ideally a linear variable differential transformer.
An alternative apparatus comprises a follower which is a cylindrical roller, the transducer being of variable reluctance and comprising a transformer whose magnetic circuit is completed by a portion of the periphery of the said follower, the thickness signal being the voltage across the transformer which depends on the degree of displacement of the follower roller towards a core of the transformer.
The control circuit preferably includes a manually adjustable level control by means of which the expected thickness of a single sheet is designated, the control circuit then deriving at
REFERENCES:
patent: 3593989 (1971-03-01), Crittenden
patent: 3660670 (1972-05-01), Howard
patent: 4151410 (1979-04-01), McMillan et al.
patent: 4237378 (1980-12-01), Jones
patent: 4378109 (1983-03-01), Takahashi et al.
patent: 4398711 (1983-08-01), Horst et al.
Brophy J. Jon
De La Rue Systems Limited
Nelms David C.
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