Method and apparatus for sensing sheets

Claim 2

Apparatus for sensing the passage of sheets through . a nip, comprising: a pair of guide members (5, 6) whose surfaces define the nip, the guide members being movable in a cyclic manner to advance the sheets through the nip; sensing means (14, 15, 18) for sensing the deflection of one guide member relative to the other; and detecting means responsive to the output of the sensing means to establish the presence or absence of a sheet between the guide members; characterized in that the sensing means includes a signal generator (18) providing a train of signals the frequency of which varies with the amount of relative deflection of the guide members, and a counter (19) arranged to count the signals from the signal generator in an interval of predetermined length; and the detecting means (23) is responsive to' a count from the sensing means to establish the presence or absence of a sheet between the guide members.

Claim 3

Apparatus for sensing the passage of sheets through a nip, comprising: a pair of guide members (5,6) whose surfaces define the nip, the guide members being movable in a cyclic manner to advance the sheets through the nip; sensing means (14, 15, 18) for sensing the deflection of one guide member relative to the other; and detecting means responsive to the output of the sensing means to establish the presence or absence of a sheet between the guide members; characterized in that the sensing means includes a signal generator (18) providing a train of signals the frequency of which varies with the amount of relative deflection of the guide members, and a counter (19) arranged to count the signals from the signal generator in an interval of predetermined length; in that monitoring means (23) are provided for monitoring the output of the sensing means and for storing counts representing relative deflections of the guide members in a cycle of movement of the guide members when no sheet is present; and in that the detecting means (23) is responsive to a subsequent count from the sensing means and to the stored counts for comparing, with a predetermined threshold, the difference between the subsequent count and a stored count for a corresponding position in the cycle of movement of the guide members, to establish the presence or absence of a sheet between the guide members.

Claim 4

Apparatus for sensing the passage of sheets through a nip, comprising: a pair of guide members (5,6) whose surfaces define the nip, the guide members being movable in a cyclic manner to advance the sheets through the nip; sensing means (14, 15, 18) for sensing the deflection of one guide member relative to the other; and detecting means responsive to the output of the sensing means to establish the presence or absence of a sheet between the guide members; characterized in that the sensing means includes a signal generator (18) providing a train of signals the frequency of which varies with the amount of relative deflection of the guide members, and a counter (19) arranged to count the signals from the signal generator in an interval of predetermined length; in that monitoring means (23) are provided for monitoring the output of the sensing means and for storing counts representing relative deflections of the guide members in a cycle of movement of the guide members when no sheet is present; and in that the detecting means (23) is responsive to a subsequent count from the sensing means and to the stored counts for comparing, with a predetermined threshold, the difference between the subsequent count and a stored count for a corresponding position in the cycle of movement of the guide members, to establish the presence or absence of a sheet between the guide members.

Claim 5

Apparatus in accordance with claim 2 in which the detecting means (23) includes means defining a second and higher threshold, a difference which lies between the two thresholds indicating the presence of one sheet and a difference which exceeds the second and higher threshold indicating the presence of two or more sheets between the guide members.

Claim 6

Apparatus in accordance with claim 2, in which the guide members (5,6) are rollers.

Claim 7

Apparatus in accordance with claim 4 in which one of the rollers (6) is mounted on a fixed shaft (2) and the other (5) is mounted to permit its displacement relative to the shaft of the first roller.

Claim 8

Apparatus in accordance with any one of the preceding claims, in which the sensing means includes a solenoid and a core so mounted that relative displacement of the guide members by the passage of a sheet causes realtive movement of the core and solenoid, and in which the solenoid is connected to the signal generator and controls the frequency of the signal generator in accordance with the degree of penetration of the core into the solenoid.

Claim 11

A method in accordance with claim 7, comprising: monitoring the output of the sensing means and storing counts representing relative deflection of the guide members in a cycle of movement of the guide members when no sheet is present; and comparing, with a predetermined threshold, the difference between a subsequent count from the sensing means and a stored count for a corresponding position of the guide members in their cycle of movement, to establish the presence or absence of a sheet between the guide members.

Claim 12

A method in accordance with claim 8, comprising comparing the difference between the subsequent count and the stored count for the corresponding position of the guide members with at least two thresholds, including one threshold which is the threshold for the passage of a single sheet and another threshold which is the threshold for the passage of superimposed sheets between the guide members.

Claim 13

A method in accordance with claim 8 or 9, in which the frequency of the signal generator, the interval in which a count is made, and the capacity of the counter, are such that the counter overflows, whereby only the lower-order bits of the count are considered.

Claim 14

A method in accordance with claim 8, 9 or 10, comprising setting the threshold value or values by a calibrating operation in which the sheets are passed in sequence through the nip and the resulting counts, after compensation by the stored count for the corresponding positions of the guide members, are checked for agreement, and in which when substantially the same value is obtained for two sheets that value is used in the calculation of a sheet thickness threshold.