WO2009103931A1

WO2009103931A1 - Sensor

Info

Publication number: WO2009103931A1
Application number: PCT/GB2008/000566
Authority: WO
Inventors: Anders Bengt Verner Nordin
Original assignee: Talaris Holdings Limited
Priority date: 2008-02-19
Filing date: 2008-02-19
Publication date: 2009-08-27

Abstract

A sensor assembly for detecting the presence of a sheet document in a transport path is disclosed. The sensor assembly comprises an optical transmitter (1), an optical receiver (2) and an optical fibre (3). The optical transmitter (1) and optical receiver (2) are each associated with and lie opposite a respective end (8a, 8b) of the optical fibre (3). At least one of the optical transmitter (1) and the optical receiver (2) is disposed across the transport path from the associated end (8a, 8b) of the optical fibre (3).

Description

SENSOR

This invention relates to a sensor, and in particular to an optical sensor for use in the processing of sheet documents such as banknotes. The sensor is used for detecting the passage of sheet documents as they pass along a transport path.

Prior optical sensor systems used in the processing of sheet documents generally fall into one of two categories. In the first category, light is emitted by an optical transmitter towards the transport path. A black plate is positioned across the transport path from the optical transmitter to absorb the light that it emits. However, when a sheet document travels along the transport path past the optical transmitter, the light emitted by the optical transmitter is reflected by the passing document towards an optical receiver allowing detection of the document. Clearly, the document must be sufficiently reflective for this technique to work.

In the second category, light is emitted by an optical transmitter across the transport path and detected by an optical receiver which is placed across the transport path from the optical transmitter. A passing document breaks the beam of light emitted by the optical transmitter, thereby allowing the passage of the document to be detected. With this technique, the document must be sufficiently opaque for it to be detected, although this is usually the case with most types of documents and especially documents such as banknotes.

An extension of this second technique is described in EP0604379. In this, an optical transmitter and an optical receiver are disposed on the same side of the transport path. A prism is placed across the transport path from the optical transmitter and optical receiver. Light emitted by the optical transmitter is reflected internally twice by the prism such that it impinges on the optical receiver. A passing document breaks the beam of light emitted by the optical transmitter, thereby allowing the passage of the document to be detected. Again, the document must be sufficiently opaque for it to be detected. There are various problems with these prior systems. Systems of the first category are only effective with documents that are sufficiently reflective. Whilst this problem is overcome by systems of the second category, these either require the optical transmitter and receiver to be placed on opposite sides of the transport path or the use of a prism as in EP0604379. Placing the optical transmitter and receiver on opposite sides of the transport path is undesirable because electrical connection must be made to both and this can be awkward and expensive to arrange. The use of a prism overcomes this issue, but it requires the additional expense of a prism and constrains the layout of the system significantly.

In accordance with one aspect of the invention, there is provided a sensor assembly for detecting the presence of a sheet document in a transport path, the sensor assembly comprising an optical transmitter, an optical receiver and an optical fibre, the optical transmitter and optical receiver each being associated with and lying opposite a respective end of the optical fibre, wherein at least one of the optical transmitter and the optical receiver is disposed across the transport path from the associated end of the optical fibre.

The sensor assembly of the invention overcomes all the stated problems with prior systems. Specifically, it is capable of detecting the passage of documents as they break the beam of light emitted by the optical transmitter (either between the optical transmitter and the optical fibre or between the optical fibre and optical receiver) so that the reflectivity of the documents is of no importance.

Furthermore, this is achieved with complete flexibility in the positioning of the optical transmitter and optical receiver. It is not necessary for them to be placed on opposite sides of the transport path. The use of an optical fibre rather than a prism is both cheaper and allows significantly more flexibility in the positioning of the optical transmitter and optical receiver because the optical fibre may follow an extremely convoluted path if required to convey light between the two. Indeed, only one of the optical transmitter and optical receiver need be placed adjacent the transport path. The other may be in a location entirely remote from the transport, for example it may be fitted directly to a controller printed circuit board.

In one embodiment, the optical transmitter is adapted to transmit visible light and the optical receiver is responsive to visible light.

Either as an alternative or in addition, the optical transmitter may be adapted to transmit infrared light and the optical receiver may be responsive to infrared light.

Typically, at least one of the ends of the optical fibre is received in a counterbored hole in a retaining member.

The optical transmitter and optical receiver may be angled such that the axes of optical transmission and reception converge. In this instance, the axis of optical transmission is considered to be the central axis of the light emitted by the optical transmitter, and the axis of optical reception is considered to be the central axis of the cone of acceptance of the optical receiver. This configuration is useful in some circumstances as it allows the bend radius imposed on the optical fibre to be less severe and more natural when short length of optical fibre are used.

Typically, the optical transmitter is a light emitting diode (LED).

The optical receiver is normally a photodiode or a phototransistor.

In a second aspect of the invention, a sensor system comprises a sensor assembly according to the first aspect of the invention and a controller adapted to energise the optical transmitter such that it emits radiation and to receive from the optical receiver a signal generated in response to radiation received by the optical receiver from the optical transmitter via the optical fibre, the controller being further adapted to indicate the presence of a sheet document in the transport path when the signal from the optical receiver crosses a predefined threshold value.

In a third aspect of the invention, there is provided a method for adjusting the operating point of the sensor assembly of the first aspect of the invention, the method comprising setting the intensity of light emitted by the optical transmitter to a first value, monitoring the signal generated by the optical receiver and increasing the intensity of light emitted by the optical transmitter until the signal generated by the optical receiver is saturated.

The intensity of light emitted by the optical transmitter is typically increased by increasing the current flowing through the optical transmitter.

Preferably, the current flowing through the optical transmitter is controlled by adjusting the duty cycle of a pulse width modulated (PWM) signal applied to the optical transmitter.

Normally, the first value is zero.

In a fourth aspect of the invention, there is provided a method for adjusting the operating point of the sensor assembly of the first aspect of the invention, the method comprising setting the intensity of light emitted by the optical transmitter to a first value, monitoring the signal generated by the optical receiver and adjusting the gain of an amplifier coupled to the output of the optical receiver until the output signal generated by the amplifier is saturated.

The intensity of light emitted by the optical transmitter is typically set by setting the current flowing through the optical transmitter.

Preferably, the current flowing through the optical transmitter is controlled by adjusting the duty cycle of a pulse width modulated (PWM) signal applied to the optical transmitter. Normally, the first value is zero.

An embodiment of the invention will now be described with reference to the accompanying drawings, in which:

Figure 1 shows a schematic overview of a sensor system according to the invention.

Figure 2 shows a view of the underside of the top part of a document bundler which forms part of a document transport and which houses a pair of optical transmitters and a pair of optical receivers.

Figure 3 shows a view of the top side of the top part of the document bundler shown in Figure 2.

Figure 4 shows a view of the underside of the bottom part, corresponding to the top part shown in Figures 2 and 3, which houses a pair of optical fibres.

Figure 5 shows a view of the top side of the bottom part of the document bundler shown in Figure 4.

Figure 6 shows a view of the top and bottom parts of the document bundler together.

Figure 7 shows a schematic view of a circuit for setting the operating point of the sensor system.

Figure 1 shows a very simple schematic view of a sensor system. The system comprises an optical transmitter 1 , an optical receiver 2 and an optical fibre 3.

The optical transmitter 1 is an LED (which may emit visible light, infrared light or both) and is mounted on a printed circuit board (PCB) 4. Electrical connection is made to the PCB 4 via an electrical connector 5. This allows the PCB 4 (and hence the optical transmitter 1) to be connected to a controller (not shown) so that power can be supplied to the optical transmitter 1 as required.

The optical receiver 2 is a phototransistor (which may be responsive to visible light, infrared light or both) and is mounted on a PCB 6. Electrical connection is made to the PCB 6 via an electrical connector 7. This allows the PCB 6 (and hence the optical receiver 2) to be connected to the controller so that the signal generated by the optical receiver 2 in response to light impinging on it may be processed by the controller.

In this embodiment, both the optical transmitter 1 and the optical receiver 2 are placed on the same side of the transport path. One end 8a of the optical fibre 3 is placed across the transport path from the optical transmitter 1 and the other end 8b is placed across the transport path from the optical receiver 2. Thus, a document passing along the transport path will pass between the optical transmitter 1 and the first end 8a of the optical fibre 3. It will also pass between the optical receiver 2 and the second end 8b of the optical fibre 3. The beam of light emitted by the optical transmitter 1 is thereby interrupted and the interruption or fluctuation in the signal generated by the optical receiver 2 is detected by the controller.

Figures 2 to 6 show the sensor system of Figure 1 deployed in a practical situation. Specifically, they show a document bundler which forms part of the document transport in our document handling apparatus for which a PCT application has been made on even date with this application, it is not necessary to refer to the other aspects of the document handling apparatus in order to describe the salient features of this invention.

The document bundler receives and stacks document such as banknotes. When the desired number of notes has been stacked, the notes are slightly compressed to form a tight bundle and then transported onwards to a final destination. The document bundler comprises a top plate 10 and a base plate 11. The documents to be bundled form a stack on the base plate 11. The base plate 11 is movable relative to the top plate 10 to accommodate different sizes of stack and is moved away from the top plate 10 as documents are added to stack. Once a stack has been formed, the base plate 11 is moved towards top plate 10 to compress the stack into a bundle, and the bundle is then conveyed on to a final destination. The mechanism for introducing the documents between the top plate 10 and base plate 11 and for forming the stack is not shown as it is not relevant to this invention.

The bundle of documents is conveyed out of the bundler by belts 12a and 12b housed in the top plate 10 and by belts 14a and 14b housed in the base plate 11. The belts 12a and 12b are coupled to a shaft 13 by a driving mechanism housed in the top plate 10, and the belts 14a and 14b are coupled to a shaft 15 by a driving mechanism housed in the base plate 11. The shafts 13 and 15 are coupled via pulleys and toothed belts (not shown) to the remainder of the document transport. Both these driving mechanisms are clearly visible and understood from Figures 3 and 4, but they will not be discussed further as they not relevant to this invention.

The top plate 10 houses the optical transmitter 1 and the optical receiver 2. These are exposed through the underside of top plate 10 through apertures 16 and 17 respectively. As can be seen from Figure 3, the PCBs 4 and 6, on which the optical transmitter 1 and optical receiver 2 are located, are mounted to the top plate by way of screws (not shown) which pass through holes 18 and 19 in PCBs 4 and 6 respectively thereby securing the optical transmitter and optical receiver 2 to the top plate 10.

A second optical transmitter 20 and second optical receiver 21 are mounted to the top plate 10 in a similar way such that they are exposed through the underside of top plate 10 through apertures 22 and 23. The optical fibre 3 is mounted on base plate 11 by pushing its ends 8a and 8b into pillars 26 and 27. The pillars 26 and 27 are integral with the base plate 11 and the ends 8a and 8b of the optical fibre 3 form an interference fit within the pillars 26 and 27 such that the optical fibre is held fast. The pillars 26 and 27 terminate in apertures 24 and 25 in the top side of base plate 11 through which the ends 8a and 8b of optical fibre 3 are exposed.

The two ends 31 and 32 of a second optical fibre 28 is similarly fitted in pillars 29 and 30 so that they are exposed to the top side of base plate 11 through apertures 33a and 33b respectively.

The optical transmitter 1, optical receiver 2 and optical fibre 3 in combination form a first sensor, and the optical transmitter 20, optical receiver 21 and optical fibre 28 form a second sensor. The first sensor is capable of detecting documents or bundles of documents entering or leaving the document bundler via the rear as shown in Figures 2 to 6. The second sensor is capable of detecting documents or bundles of documents entering or leaving the document bundler via the front as shown in Figures 2 to 6.

Each of the optical transmitters 1 and 20 and each of the optical receivers 2 and 21 are connected to a controller (shown in Figure 7 by reference numeral 100). The controller energises the optical transmitters 1 and 20 so that they emit light through apertures 16 and 22 respectively. A portion of this light is collected by ends 8a and 33a of optical fibres 3 and 28 and conveyed to the other ends 8b and 33b by the optical fibres 3 and 28. The light that is emitted by ends 8b and 33b is directed towards optical receivers 2 and 21 respectively. The optical receivers 2 and 21 generate respective signals in response to the light received by them from optical fibres 3 and 28 respectively.

Therefore, if a document passes into or out of the document bundler via the rear edge it will interrupt the beam of light between optical transmitter 1 and optical fibre 3 and between optical fibre 3 and optical receiver 2. This causes the signal generated by the optical receiver 2 to be interrupted or to fluctuate. The controller can detect this interruption or fluctuation (by comparison of the output of the optical receiver 2 with a threshold voltage or current for example) and thus detect the passage of a document over the rear edge of the document bundler.

Similarly, if a document passes into or out of the document bundler via the front edge it will interrupt the beam of light between optical transmitter 20 and optical fibre 28 and between optical fibre 28 and optical receiver 21. This causes the signal generated by the optical receiver 21 to be interrupted or to fluctuate. The controller can detect this interruption or fluctuation (by comparison of the output of the optical receiver 21 with a threshold voltage or current for example) and thus detect the passage of a document over the front edge of the document bundler.

Figure 7 shows schematically how the controller 100 is connected to the optical transmitter 1 and the optical receiver 2. The controller 100 can energise the optical transmitter 1 and monitor the signal generated by the optical receiver 2 in the manner set out above. In this way, the passage of documents past the sensor system can be detected.

The controller 100 can also be used to adjust the operating point of the sensor system. In order to do this, the controller 100 adjusts the dc current flowing through the optical transmitter 1 so that it emits exactly the right amount of light to just saturate the optical receiver 2.

In order to adjust the dc current flowing through the optical transmitter 1 to the desired value, the controller 100 outputs a pulse width modulated (PWM) digital signal. This is converted to an analogue signal by a digital to analogue converter 101 which is coupled to the optical transmitter 1 via a driver amplifier 102 and transistor 103. The current flowing through the transistor 103 is limited by a resistor 104 in the emitter circuit. This PWM control method is capable of controlling the dc current flowing through the optical transmitter within a range of 0 to 5OmA by adjusting the duty cycle of the PWM square wave signal generated by the controller 100. Initially, the current flowing through the optical transmitter 1 is set to a value of OmA. It is then increased in small steps, typically 5% of the full range of 50mA whilst the signal generated by the optical receiver 2 is monitored.

As can be seen from Figure 7, the light impinging on optical receiver 2 will modulate the current flowing through it which in turn generates a voltage across resistor 105. A buffer, filter and analogue to digital converter 106 convert the voltage to a digital signal which is monitored by the controller 100.

If the digital signal monitored by controller 100 changes in response to an increase in the current flowing through optical receiver 2 then this indicates firstly that the light received by optical receiver 2 is that emitted by optical transmitter 1 and conveyed by optical fibre 3 (which is shown schematically in Figure 7), and secondly that the optical receiver 2 has not yet saturated.

The current flowing through optical transmitter 1 is increased in this stepwise fashion until the optical receiver 2 does not respond to the step increase by changing the signal it generates. The optical receiver 2 is then known to be saturated and this value of current flowing through optical transmitter 1 is used as the operating point for normal sensing operation.

Adjusting the operating point in this way is required to ensure that optical transmitter 1 emits enough light to overcome any variations in the electronic components or mechanical geometry or contamination by dust. It is also desirable to set the operating point in this way rather than simply cause the optical transmitter 1 to emit its maximum current to avoid the possibility that the optical transmitter 1 emits so much light that it is not absorbed by passing documents, thereby causing the sensor system to fail in detecting passing documents.

The adjustment process for the operating point may be instigated by issuing a predetermined command to the controller 100. This may be required from time to time to take account of dust and debris that has settled on the optical transmitter 1 , optical receiver 2 or optical fibre 3.

It is also possible to adjust the operating point by causing a fixed current to flow through the optical transmitter 1 and adjusting the gain of an amplifier connected to the output of the optical receiver 2 such that the output of the amplifier is just saturated.

Claims

1. A sensor assembly for detecting the presence of a sheet document in a transport path, the sensor assembly comprising an optical transmitter, an optical receiver and an optical fibre, the optical transmitter and optical receiver each being associated with and lying opposite a respective end of the optical fibre, wherein at least one of the optical transmitter and the optical receiver is disposed across the transport path from the associated end of the optical fibre.

2. A sensor assembly according to claim 1 , wherein the optical transmitter is adapted to transmit visible light and the optical receiver is responsive to visible light.

3. A sensor assembly according to claim 1 or claim 2, wherein the optical transmitter is adapted to transmit infrared light and the optical receiver is responsive to infrared light.

4. A sensor assembly according to any of the preceding claims, wherein at least one of the ends of the optical fibre is received in a counterbored hole in a retaining member.

5. A sensor assembly according to any of the preceding claims, wherein the optical transmitter and optical receiver are angled such that the axes of optical transmission and reception converge.

6. A sensor assembly according to any of the preceding claims, wherein the optical transmitter is a light emitting diode (LED).

7. A sensor assembly according to any of the preceding claims, wherein the optical receiver is a photodiode or a phototransistor.

8. A sensor system comprising a sensor assembly according to any of the preceding claims and a controller adapted to energise the optical transmitter such that it emits radiation and to receive from the optical receiver a signal generated in response to radiation received by the optical receiver from the optical transmitter via the optical fibre, the controller being further adapted to indicate the presence of a sheet document in the transport path when the signal from the optical receiver crosses a predefined threshold value.

9. A method for adjusting the operating point of the sensor assembly of any of claims 1 to 7, the method comprising setting the intensity of light emitted by the optical transmitter to a first value, monitoring the signal generated by the optical receiver and increasing the intensity of light emitted by the optical transmitter until the signal generated by the optical receiver is saturated.

10. A method according to claim 9, wherein the intensity of light emitted by the optical transmitter is increased by increasing the current flowing through the optical transm itter.

11. A method according to claim 10, wherein the current flowing through the optical transmitter is controlled by adjusting the duty cycle of a pulse width modulated (PWM) signal applied to the optical transmitter.

12. A method according to any of claims 9 to 11 , wherein the first value is zero.

13. A method for adjusting the operating point of the sensor assembly of any of claims 1 to 7, the method comprising setting the intensity of light emitted by the optical transmitter to a first value, monitoring the signal generated by the optical receiver and adjusting the gain of an amplifier coupled to the output of the optical receiver until the output signal generated by the amplifier is saturated.

14. A method according to claim 13, wherein the intensity of light emitted by the optical transmitter is set by setting the current flowing through the optical transmitter.

15. A method according to claim 14, wherein the current flowing through the optical transmitter is controlled by adjusting the duty cycle of a pulse width modulated (PWM) signal applied to the optical transmitter.

16. A method according to any of claims 13 to 15, wherein the first value is zero.