Method and apparatus for sensing sheets

Abstract: Apparatus for sensing the passage of sheets (1) through a nip formed by rollers (5 and 6), includes means for sensing the deflection of one roller relative to the other, the sensing means including an oscillator (18) providing a train of signals the frequency of which varies with the amount of the relative deflection of the rollers, and a counter (19) counting the signals from the oscillator in a fixed time interval. Means (23) are provided for storing counts representing the relative deflections of the rollers in a cycle of movement of the rollers when no sheet is present, and for comparing the difference between a subsequent count and a stored count for a corresponding position in the cycle of rotation of the rollers, with a predetermined threshold, to establish the presence or absence of a sheet between the guide members. The sensing means may comprise a core (14), moving with the shaft of one of the rollers, and a solenoid (15) connected to the oscillator circuit.

Description: This invention concerns a method and apparatus for sensing the passage of sheets through a nip between a pair of guide rollers by sensing the relative deflection of the guide rollers during the passage of a sheet through the nip. One example of such a method and apparatus is shown in WO-A-82 01698. In this example, a sheet passes under a roller which is connected to a linear variable differential transformer. A signal representing the change of signal from the sensor in response to the passage of an expected thickness of a sheet is preset and to this is added a signal representing the datum value obtained from the sensor when the roller rests on a guide surface. When a note passes through the nip, the output signal is fed to a comparator, which also receives the expected thickness signal and the datum value. the comparator determining whether or not a sheet is present. Between the passage of successive sheets through the nip, the datum value is rechecked and a suitable correction is made if necessary. Another example is shown in US-A-4420747 which describes a system for detecting missing or superimposed sheets. In this system a measuring device generates a signal which increases with the number of superimposed sheets. It uses an initial pulse which increments until the final pulse of the sheet then compares the resultant with a stored value. Such an arrangement works well if the rollers and the shafts on which they are mounted are made with high mechanical precision but to achieve such precision is expensive. The problem which arises when the roller and shaft system is not of high mechanical precision is that the "noise" level of the sensor output is comparable to the signal level corresponding to the passage of a single note, particularly for the thinner currencies. The object of the present invention is to achieve good results in sheet sensing with a roller and shaft system which is built to a lower precision and is consequently less costly. Apparatus according to the present invention, for sensing the passage of sheets through a nip, comprises: a pair of guide members whose surfaces define the nip, the guide members being movable in a cyclic manner to advance the sheet through the nip; sensing means 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 providing a train of signals the frequency of which varies with the amount of relative deflection of the guide members, and a counter arranged to count the signals from the signal generator in an interval of predetermined length; in that monitoring means 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 15 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. preferably, the detecting means includes means defining a second and higher threshold: a difference which lies between the two thresholds indicates the presence of a single sheet in the nip and a difference which exceeds the second and higher threshold indicates the presence of superimposed sheets in the nip. Such apparatus reduces the required mechanical precision, so that the components are less expensive and the apparatus is simpler to construct. The frequency-based system, in which each count effectively averages amplitude variations over a short, defined period, is less susceptible to spurious noise than a similar amplitude-based system. The sensing means may, for example. be a solenoid with a core which follows the relative deflection of the guide members. In such a construction, there is a very wide setting tolerance for the position of the solenoid relative to the core. Electrical and mechanical drift can be compensated in the microprocessor arrangements. In the preferred form, the frequency of the signal from the signal generator and the counter capacity are such that the counter in effect overflows. In other words, the high-order data bits are ignored and the changes in the low-order bits are used in the signal processing. The use of a high frequency signal and overflow enhances the linearity of the system, so that it is no longer essential to employ a "linear transformer" as a sensor. In the preferred method embodying the invention, calibration is effected automatically, so that it is not necessary to have prior knowledge of the standard thickness of the documents to be dispensed. In order that the invention may be better understood, an example of apparatus embodying the invention will now be described with reference to the accompanying drawings, in which:- Figure 1a is a view in front elevation of the sheet feeding apparatus and sheet sensor; Figure 1b is a view in end elevation of the apparatus shown in Figure 1a; Figure 2 is a typical waveform obtained using apparatus of the kind shown in Figure 1; Figure 3 is a block diagram of the electronic apparatus for processing the output of the sensor in Figure 1; and Figure 4 shows an oscillator circuit for use in the block diagram of Figure 3. The sheet feeding apparatus shown in Figures 1a and 1b comprises two parallel shafts 1 and 2 mounted between side plates 3 and 4. The shaft 1 is the sensor shaft and carries a roller 5 and the shaft 2 is the datum shaft and carries a roller 6. The rollers 5 and 6 are freely mounted for rotation on the shafts and are driven in rotation by belts 7 and 8. A sheet 10 fed through the system passes between the rims of rollers 5 and 6. The diameters of the rollers 5 and 6 is such as to space the belts 7 and 8 at the sensor station but elsewhere in the apparatus the belts contact the sheets and constitute the sheet transport means. The datum shaft 2 is fixed in the side plates 3 and 4 and the right-hand end of the shaft 1 is fixed in the side plate 3. However, at its other end, the sensor shaft 1 passes through a hole 11 in the side plate 4 and is mounted for rotation in an arm 12. The arm 12 is mounted at one end on a pivot 13 attached to the side plate 4. A core 14, cooperating with a solenoid 15, is mounted in the other end of the pivoted arm 12, i.e. remote from the pivot 13. The solenoid 15 is fixed in a housing 16 attached to the side plate 4. It will be clear that when a sheet passes between the cooperating rollers 5 and 6 on the shafts 1 and 2, the sensor shaft 1 will undergo a movement of separation from the datum shaft 2 and this will result in movement of the free left-hand end of the shaft 1, causing angular movement of the arm 12 about the pivot 13 and consequent movement of the core 14 into the solenoid 15. The passage of two superimposed sheets between the cooperating rollers 5 and 6 results in a greater separation of the shafts and a greater penetration of the core 14 into the solenoid 15. The movement of the core into the solenoid changes the inductance of the solenoid. Figure 2 is a typical waveform obtained from such a sensor. The small-amplitude variations indicate, in analogue equivalent form, the variable frequency pattern produced by the sensor due to the eccentricity of the pinch rollers. Thus, the periodic pattern corresponds to the composite profile of the pinch rollers in one revolution. It will be seen from Figure 2 that when a note passes between the pinch rollers the waveform pattern is offset by an amount corresponding to the thickness of the note but the offset pattern suffers little distortion. As shown in Figure 3, the solenoid 15 is connected to an oscillator circuit 18 which varies in frequency with the penetration of the core into the solenoid. An eight-bit counter 19 receives the oscillator output and counts the number of pulses present in a predetermined period or time "window". Thus, the count during such a period represents the penetration of the core 14 into the solenoid 15 and the relative deflection of the rollers. The duration of the predetermined period or time "window" during which the count is made is predetermined by a timer 20 supplied with pulses by a crystal-controlled reference oscillator 21 The count is fed to a data latch circuit 22 and thence over a data bus 24 to a microprocessor 23 responsible for monitoring the counts and detecting the presence of a sheet or sheets. As the data bus carries other data at other times, when the microprocessor requires data from the latch circuit it sends a suitable address signal over an address bus 25 to an address decode circuit 26 which signals the data latch circuit over a line 27. The microprocessor also supplies a data strobe signal over line 28 to the data latch circuit 22. The circuit shown in Figure 4 consists of an oscillator, the frequency of which depends upon the inductance of the solenoid 15, and a buffer, and is of conventional design. In the example described, the oscillator operates at 10 MHz and the duration of the sampling period or time "window" is such that the count obtained is greater than eight bits. However, the high-order data bits (those in excess of eight) are ignored and only the changes in the lower-order bits are used in the signal processing algorithm. This increases the resolution which can be obtained. The pinch rollers 5, 6 and a timing wheel (not shown) are driven by the main transport for the notes, producing thirty timing pulses per revolution of the pinch rollers. These timing pulses define thirty sampling points in each revolution of the rollers. At each timing pulse, the microprocessor supplies a "clear" signal over line 30 to clear the 8-bit counter and the timer 20. The timer 20 then counts the pulses from oscillator 21 and the counter 19 counts the pulses from the oscillator 18. When the timer 20 overflows it stops the counter 19 and sends a "data ready" signal over line 31 to the microprocessor. The microprocessor then addresses the data latch and acquires the data reading, as described above. To generate and store the eccentricity profile of the rollers prior to the arrival of a note the microprocessor samples the signal from the sensor at each of the thirty sampling instants and retains each sample value in a memory location. To smooth out the effects of random noise on the sampled values, a further reading is taken for each sample point and the value in the memory is replaced by the weighted average of the existing value in the memory and the new sample value for that point, the weighting of the average being towards the existing value in memory. However, before the average is calculated the existing value in memory is subtracted from the sample value and the result is then compared with threshold values which represent one-half the thickness value of a single note and one plus one-half of that thickness value. If either of these threshold values is exceeded, the average is not calculated. The note threshold values are determined by a calibration note-dispensing operation, during which notes are dispensed into a reject container. The microprocessor determines a thickness value for each dispensed note by averaging the thickness counts obtained for the entire note length (after subtraction of the values representing the roller profile). When the thickness values obtained for two notes are substantially the same, that value is accepted for the calculation of threshold values. If there is disagreement between the thickness values of the first two notes, even after allowance has been made for thickness tolerances and measurement inaccuracy, then extra notes are fed until there is agreement between the values obtained for the thinnest pair of notes. The number of calibration notes is limited, in this example, to nine notes; if there is still no agreement, a false condition is indicated. When the thickness value for a single note has been thus established, single and double-note thickness thresholds are calculated. The single note threshold may, for example, be one-half of the single-note thickness and the double-note threshold may be one plus one-half of the single note thickness. Thereafter, when a note is fed through the rollers 5 and 6, the count from the data latch at each sampling instant (i.e. at each timing pulse) is compared by the microprocessor with the stored count for the roller profile for the corresponding angular position of the rollers. The difference is noted for each sampling instant during the passage of the note and these difference are averaged to obtain the mean thickness of the note. The mean thickness is compared with the pre-established thresholds. In the preferred method, the gap between successive notes is such as to correspond to at least one revolution of the pinch rollers, enabling the entire pinch roller profile in the memory to be updated. This overcomes the problem which would otherwise arise due to the fact that the signal pattern will change with time, due to a gradual phase shifting between the pinch rollers. Although it is possible that the above calibration procedure would set a threshold based on the thicknesses of two double notes, the probability is extremely low. If this occurred, during a normal dispensing operation a single note might not be detected when it was fed to the apparatus; this would be interpreted as a feed failure. However, another sensor is provided to detect such notes and this sensor would initiate a new calibration dispensing operation. Once the initial calibration dispensing operation has been carried out, recalibration takes place only if there has been an interruption of the power supply, a resetting of the microprocessor, or a change of note denomination takes place, or an error condition occurs such that the calibration is suspected of being faulty. Further thresholds may be established in the microprocessor if desired. For example a threshold lower than the normal single note threshold may be used to define the lower end of a range (of which the upper end is the normal single note threshold) indicative of the presence of a sheet which is too thin. Similarly, a further lower range may indicate that no sheet is present but that the variations due to the roller profiles are too great.

Claims: 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. Apparatus in accordance with claim 1, 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. Apparatus in accordance with claim 1 or claim 2, in which the guide members (5,6) are rollers. Apparatus in accordance with claim 3, 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. 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 relative 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. A method of sensing the passage of sheets through a nip between a pair of guide members (5,6) whose surface define the nip, the guide members being movable in a cyclic manner to advance the sheets through the nip, comprising advancing sheets into the nip between the guide members, and sensing the deflection of one guide member relative to the other, the method being characterized by: generating a train of signals the frequency of which varies with the amount of the sensed relative deflection of the guide members; counting the signals from the signal generator in an interval of predetermined length; 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. A method in accordance with claim 6, wherein the difference between the subsequent and the stored count for the corresponding position of the guide members is compared with at least two further predetermined 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. A method in accordance with claim 6 or claim 7, 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. A method in accordance with claim 6, 7 or 8, 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.

Inventors: MUNN ERNEST ALFRED, SKINNER JOHN ALAN, MUNN, ERNEST ALFRED, SKINNER, JOHN ALAN
Applicants: DE LA RUE SYST, DE LA RUE SYSTEMS LIMITED
Application year: 1985
Main classification: G01D15
IPC codes: B41J29, B65H7, G01V9, G01D15