GB2241228A - Sheet thickness detection assembly - Google Patents

Abstract

A sheet thickness detection assembly e.g. for bank notes comprises juxtaposed and spaced reference and sensor rollers (2, 12) between which sheets pass in use. The passage of the sheet displaces the sensor roller (12) relative to the reference roller and this is sensed by linear voltage displacement transducers (17, 18). The ends of the shaft (9) on which the sensor roller is mounted are held in resilient supports (10, 11) to accommodate the displacement of the sensor roller (12) Coupled to the rollers are intermeshing gears (21, 22) which transfer the rotation of one roller to the other. The resilient supports (10, 11) allow the spacing of the centre of the gears to change as the sensor roller (12) is displaced but the gears remain intermeshed. The spacing of the gears while no sheets are present between the rollers is chosen to be non-standard. <IMAGE>

Description

SHEET THICKNESS DETECTION ASSEMBLY The invention relates to a sheet thickness detection assembly of the kind comprising juxtaposed and spaced reference and sensor rollers between which sheets pass in use to displace the sensor roller relative to the reference roller; and sensing means for sensing the relative displacement of the sensor roller. Such assemblies are hereinafter referred to as of the kind described.

An example of a sheet thickness detection assembly of the kind described is illustrated in EP-A-0337770. In this assembly the sensor roller is mounted rotatably on a shaft each end of which is supported between a lower rubber bung and an upper spring loaded plunger. During passage of the sheet between the rollers, the shaft is urged away from the reference roller against the spring action of the plunger.

The sensor roller is driven by a rubber tyred roller which engages the sensor roller at a position diametrically opposite from the reference roller, the rubber tyred roller being driven by a motor. Deflective movement of the sensor roller is opposed by the rubber tyred roller.

Three particular problems which arise with assemblies of the kind described are that a significant temperature rise takes place during operation even when no sheets are fed; that upon sheet arrival at the assembly the sensing means produces an overshoot initially; and that there is significant noise in the detection signal generated by the sensing means.

In accordance with the present invention, a sheet thickness detection assembly of the kind described is characterized in that the sensor roller is rotatably mounted to a shaft the ends of which are held in resilient supports which accommodate the displacement movement of the sensor roller; in that intermeshing gears are coupled to the rollers whereby rotation of one roller causes corresponding rotation of the other roller via the gears, the resilient supports permitting the distance between the centres of the gears to change in response to displacement of the sensor roller while remaining intermeshed; and in that the gears are mounted such that their centres are spaced apart by a non-standard distance when no sheets are present between the rollers.

It has not previously been thought that a direct gear driven system in which both rollers are driven through a gear coupling would be satisfactory due to the generation of significant noise in the sensing means output signal.

However, by using a larger than normal centre to centre distance and mounting the sensor shaft to resilient supports thus allowing the centre distance between the helical gears to vary it has been found that this problem is significantly reduced.

It has also been found that the problems of temperature mentioned above are significantly reduced.

Removing the drive tyres previously used and replacing them with a gear drive mechanism reduces the temperature of the rollers which was partly due to hysteresis of the tyres.

There are indications that the problems of overshoot are reduced due to the resilient mounting of the sensor roller shaft. These supports provide damping and restoring force to the impact of a sheet passing between the rollers.

The gap between the rollers has been found to be more stable under operational and evironmental conditions.

Preferably, the supports comprise blocks of resilient material such as rubber with resistance to compressive set/stress relaxation and good consistency of properties between the temperatures 100 and 500C has been found to achieve the greatest reduction in the overshoot problem.

A further advantage of using helical gears is that the drive torque required to drive the rollers is reduced by approximately 50% over the prior use of drive tyres.

The gears are preferably helical although other forms are possible.

An example of a sheet thickness detection assembly according to the invention will now be described with reference to the accompanying drawings, in which: Figure 1 is an end elevation, partly in section of the assembly; Figure 2 illustrates graphically the temperature response of the assembly shown in Figure 1 and of a conventional assembly; Figure 3 shows a pair of helical gears and their separation in detail; and, Figure 4 is a cross-section through Figure 3.

The assembly shown in Figure 1 comprises a bearing housing assembly 1 to which is rotatably mounted a steel reference roller 2 having integral axially extending shaft sections 3, 4 which extend through walls of the housing 1 and are rotatably mounted to the walls by ball-bearings 5, 6. A pulley 7 is fixed to the section 4 and is driven via a belt 8 by a motor (not shown). A sensor roller shaft 9 is mounted above the reference roller 2 and is fixed to the housing 1 by a pair of rubber blocks 10, 11. A sensor roller 12 is rotatably mounted to the shaft 9 by ball-bearings 13, 14 and defines a gap with the reference roller 2 with a thickness which can be adjusted. The larger this gap, the smaller the impact of a sheet entering the rollers. However, increasing the gap reduces the overall signal strength.The range of gap thickness chosen is to ensure that the notes are always detected and that overshoot is kept to a minimum.

The exact position of the shaft 9 and hence the thickness of the gap between the rollers 2, 12 can be adjusted.

The rubber blocks 10, 11 are surrounded by outer metal housing 23,24 which can be moved up and down to adjust the whole assembly including the position of the shaft 9. An alternative arrangement is to use screws directly onto the upper surface of the blocks 10, 11 although this can suffer from creepage and a change in the density of the rubber producing an uneven balance of forces.

The passage of a note between the rollers 2, 12 exerts an upward force on the roller 12 and hence on the shaft 9 which moves against the resilience of the rubber blocks 10, 11. This upward movement which provides a measure of the note thickness is detected by a pair of linear voltage displacement transducers (LVDT) 17, 18 each of which comprises an armature 19 urged under spring action onto a bearing pad 20 which runs on the surface of the roller 12.

As has been mentioned above, the roller 2 is driven via the pulley 7. This drive is communicated to the roller 12 via a pair of intermeshing, helical drive gears 21, 22.

The gear 21 is fixed to the shaft section 3 of the roller 2 while the gear 22 is fixed to the roller 12. The gears 21, 22 are mounted at slightly greater than their standard centre distance to minimise noise generation (on the detection signal). For example for a pair of helical gears having a standard centre distance of 28.39mm-28.49mm, the helical gears would be set at a centre distance 25 of 28.80mm-28.85mm (Figure 3). During the passage of a note between the rollers, the shaft 9 will be displaced as mentioned above causing relative displacement between the gears 21, 22 which also reduces noise. The gears are shown in cross-section in Figure 4.

To indicate the effect on temperature of the assembly shown in Figure 1, experiments have been performed to monitor the sensor roller temperature during operation of the assembly of Figure 1 and a conventional assembly using rubber tyred drive rollers. As can be seen in Figure 2 when the roller 2 is rotated with a tangential velocity of 1.7 metres per second the assembly of Figure 1 (line 30) causes a rise in sensor roller temperature to a level significantly lower than in the case of a conventional tyre driven system (line 31). The Section 32 indicates the likely maxima of the graphs.

The assembly shown in Figure 1 may be used in any suitable sheet feeding system but is particularly suitable for banknote handling apparatus.

Claims (6)

1. A sheet thickness detection assembly comprising juxtaposed and spaced reference and sensor rollers between which sheets pass in use to displace the sensor roller relative to the reference roller; and sensing means for sensing the relative displacement of the sensor roller characterized in that the sensor roller is rotatably mounted to a shaft the ends of which are held in resilient supports which accommodate the displacement movement of the sensor roller; in that intermeshing gears are coupled to the rollers whereby rotation of one roller causes corresponding rotation of the other roller via the gears, the resilient supports permitting the distance between the centres of the gears to change in response to displacement of the sensor roller while remaining intermeshed; and in that the gears are mounted such that their centres are spaced apart by a non-standard distance when no sheets are present between the rollers.

2. An assembly according to claim 1, wherein the gearsare helical gears.

3. An assembly according to claim 1 or claim 2, wherein the resilient supports comprise rubber blocks.

4. An assembly according to claim 3, wherein the rubber has a hardness in the range 40 to 90 IRMD.

5. An assembly according to claim 3 or claim 4, wherein the blocks are surrounded by an adjustable metal housing.

6. A sheet thickness detection assembly as hereinbefore described with reference to the accompanying drawings.