Electro-optic sensor

Abstract

 This invention relates to a sheet sensor comprising a light source (62) coupled to an optical waveguide (60) arranged to direct light at the path of the sheet (54). At least a second waveguide is arranged to receive light after transmission through, or reflection from, the light. A transducer converts the received light into an electrical signal which indicates the presence or the absence of a sheet in the path. Both the light source and the «transmission» waveguide are secured to each other and to a common substrate (64).

Description

[0001] This invention relates to a sheet detection device to detect the presence of opaque or transparent sheets advanced along a path, and in particular to a differential fiber optic sensing device.

[0002] In feeding copy sheet material along a path, for example, through a copy processor, it is essential to be able to detect the presence or absence of a sheet. Without this detection capability, jams occur at the processing stations, resulting in machine malfunctions.

[0003] Many examples of sheet detectors are found in the prior art, for example US-A-3,278,254 disclosing a photosensitive double-sheet detector. In particular, a lamp as a source of light and a solar cell responsive to reflected light are used for generating a variable current proportional to the distance of the sheet from the detector. Fiber optic bundles communicating with the light source and the detector are separated by a specific angle to determine the distance of an object from the detector at which the maximum current is obtained. This type of system is extremely sensitive to the distance of the sheet from the detector and, therefore, the system must be positioned with a high degree of accuracy. It is also known in fiber optic sensing devices that error signals can be generated merely by the sheet being in different planes or locations along the transport path.

[0004] Other systems use mechanical sensors for detection of multiple sheets, as taught in US-A-3,396,965. A difficulty with mechanical detectors is that they are often limited to specific sheet thicknesses. If a different thickness is to be accommodated, it is necessary to make mechanical adjustments to the detector.

[0005] Other detectors, such as shown in US-A-3,778,051 use a transducer to produce signals proportional to the thickness of sheets of material. A binary signal representative of the thickness of an initially-fed sheet is compared with signals representative of the thickness of subsequently-fed sheets. When the thickness of a subsequently-fed sheet exceeds the thickness of an initially-fed sheet, the comparator produces an error signal, resulting in the ejection of the subsequently-fed sheet from the sheet-feeding mechanism.

[0006] It is known to use a pair of receiving elements in a sensor. For example, Japanese Publication 55-2528 discloses a light-emitting element with a polarizing filter and a pair of light-receiving photodetectors for sensing sheets in a sheet path. One of the photodetectors includes a polarizing filter to provide a high and low signal when sheets are not present in the path. In addition, US-A-3,900,738 shows a non-coherent light source projecting light onto two photodetector systems. The difference between the output from the photodetector systems represents the position of an object in the light path.

[0007] US-A-3,932,755 teaches another method for detecting multiple sheets along a path using a high-reflectivity plate and a low-reflectivity plate, and determining the difference between the quantity of reflected rays from the first plate and the quantity of reflected rays from the second plate to recognize superposed sheets. US-A-3,882,308 teaches a multiple sheet detecting system including a source of illumination and a photosensitive element. A comparison is made between the detected light rays from a sample sheet and from a sheet in the sheet path. The system can be calibrated for varying circuit parameters to accommodate different sheet weights and types. Another method of detecting multiple sheets is the use of inline suction ports on each side of a paper transport. The purpose of the suction port is to draw the sheet to the port and then sense when it blocks the port. If both ports are closed at the same time, it is assumed that a multiple feed has occurred.

[0008] In US-A-3,435,240, the surface characteristics of a material are determined by the quotient of output signals from two photomultipliers. The magnitude of these signals from the photomultipliers is determined by the simultaneous transmittance of energy from a single light source through small and large areas of the material being examined. In US-A-4,092,068, a single light source is directed onto a surface and the surface characteristics are examined by comparing the amount of light reflected at two different angles from the surface onto a pair of detectors.

[0009] One technique for detecting translucent paper comprises a light-emitter on one side of the paper path and a transmittance detector on the other side of the path. As paper enters the path between the emitter and detector, the light may be attenuated sufficiently by passing through the paper for the signal sensed by the detector to indicate paper in the path. The difficulty is that the sensor circuitry is generally tuned to detect only paper having a particular transmittance characteristic. For papers or documents having different transmittance characteristics, the sensor circuitry is often insensitive to the passage of those different sheets.

[0010] Another technique of paper sensing is to provide a reflectance detector on the same side of the paper as the light-emitter. With paper in the path, a predetermined amount of light reflected from the paper to the reflectance detector will indicate the presence of paper. Here again, the sensing circuitry is often sensitive only to a document or paper having certain reflectance characteristics, so that for paper with different characteristics, it is necessary to adjust the detector circuitry.

[0011] A difficulty with many of the known sheet detectors is that they are sensitive to the distance of the sheet from the detector. It is also often necessary to re-adjust the detector in order to be able to detect sheets of different thicknesses. Another difficulty with them is that they are often complex and costly and not easily adapted to material of different composition.

[0012] It is also known, as shown in US-A-4,432,599 to provide a movable optical fiber with its end face positioned opposite the end faces of a multiplicity of mutually adjacent fixed optical fibers. The axes of the movable optical fiber and the fixed optical fibers are located such that, with the movable optical fiber in its initial position, optical signals propagating therein couple optical signals of substantially equal intensity through the end faces to each of the fixed optical fibers. This optical energy balance is upset when a sensor mechanism, coupled to the movable optical fiber, causes a small displacement of the axis thereof. Small displacements of the movable optical fiber cause the optical energy distribution between the fixed optical fibers to vary substantially linearly with the positional shift of the movable optical fiber. The energy- altered signals and the fixed optical signals are converted into corresponding electrical signals by optical detectors which in turn may be coupled to sum and difference amplifiers to obtain appropriate signal sums and differences that may be utilized to establish the total displacement of the movable optical fiber axis.

[0013] It is also common practice throughout the electro-optic industry to couple radiant flux from a device such as a light-emitting diode (LED) into a fiber optic light guide by positioning the fiber normal to the top surface of the active element. This is necessary in most cases since it is desirable to couple the maximum amount of energy into the light guide so that losses are kept to a minimum. This is particularly important in communications applications of fiber optics. This procedure imposes restrictions on the packaging of this type of optical source which tends to keep the costs high because of the alignment required for optimum output power. Traditionally, coupling of optical flux into an optical waveguide (fiber optic element) has been of this type. Use of these conventional coupling methods is not advantegeous when low cost and small overall size is important. Components fabricated with these exacting processes are also more likely to be designed for a wide range of environmental conditions. This also makes designing simple low-cost sensors prohibitive.

[0014] It is an object of the present invention, therefore, to provide an improved sheet detection system and fabrication technique for the detection system.

[0015] Accordingly the present invention provides an electro-optic sensor which is as claimed in the appended claims.

[0016] Briefly, the present invention is a hybrid circuit fabrication technique to couple radiant energy from chip components to optical waveguides, and the differential fiber optic switch produced by the technique. The fabrication technique attaches an optical waveguide against the side surface of a light-emitting diode or photodiode in order that the alignment of waveguides requires precision in only two dimensions. A differential fiber optic switch of this invention includes a light source, a receiver having a pair of optical fibers side-by-side and an electro-optic device for converting light into electrical signals.

[0017] The present invention will now be described by way of example with reference to the accompanying drawings, in which:

Figure 1 is a representation of a reprographic apparatus incorporating the sensor of the present invention;

Figure 2 illustrates the geometry of sensing the presence of a reflective target using a pair of waveguides;

Figures 3(a) and 3(b) illustrate the response of a reflective sensor pair as a function of target distance;

Figures 4(a) and 4(b) are a side view and a top plan view of the edge coupling of an LED chip and optical waveguide in accordance with the present invention;

Figure 5(a) and 5(b) are end views of reflective sensing configurations in accordance with the present invention, and

Figure 6 is a differential fiber optic switch in accordance with the present invention.

[0018] Referring now to Figure 1, there is shown by way of example an electrophotographic printing machine having photoconductive surface 12 moving in the direction of arrow'16 to advance the photoconductive surface 12 sequentially through various processing stations. At a charging station, a corona-generating device 14 electrically connected to a high voltage power supply charges the photoconductor surface 12 to a relatively-high, substantially-uniform potential. Next, the charged portion of the photoconductive surface 12 is advanced through exposure station 18. At exposure station 18, an original document is positioned upon a transparent platen. Lamps illuminate the original document and the light rays reflected from the original document are transmitted onto photoconductive surface 12. A magnetic brush development system 20 advances a developer material into contact with the electrostatic latent image.

[0019] At the transfer station 22, a sheet of support material from stack 24 is moved into contact with the toner powder image. The sheet of support material is advanced to the transfer station by sheet-feeding apparatus 26 contacting the uppermost sheet of the stack 24. Sheet-feeding apparatus 26 rotates so as to advance sheets from the stack onto transport 28. The transport 28 directs the advancing sheet of support material into contact with the photoconductive surface 12 in timed sequence in order that the toner power image developed thereon contacts the advancing sheet of support material at the transfer station. Transfer station 22 includes a corona-generating device for spraying ions onto the underside of the sheet. This attracts the toner powder image from photoconductive surface 12 to the sheet.

[0020] After transfer, the sheet continues to move into prefuser conveyor 30 advancing the sheet to fusing station 32. Fusing station 32 generally includes a heated fuser rotter and a backup roller for permanently affixing the transferred image to sheet. After fusing, a chute drives the advancing sheet to catch tray 34 for removal by the operator. There is also included a cleaning mechanism 36 to remove residual toner that may have continued to adhere to the surface 12.

[0021] With reference to Figure 1, there are also illustrated a plurality of differential fiber optic sensors. In particular, there is illustrated a sensor 40 at the sheet feed apparatus 26. Other sensors 42, 44 and 46 are disposed before the transfer station 22, after the transfer station 22, and after the fuser station 32. Sensors 48 and 50 are positioned at the output tray 34, and along the photoreceptor surface 12 to detect any errant sheet that was not stripped from the photoreceptor drum. All sensors are electrically connected to suitable (not shown) control circuitry.

[0022] A butt-coupling method is used to join optical waveguides to side surfaces of an LED or photodiode. In this fabrication, the alignment of the waveguides with respect to the emitting and detecting devices requires precision only in two dimensions. This method of fabrication allows the coupling of radiant energy from inexpensive chip components, such as light-emitting diodes and photo detectors, into optical waveguide elements. When non-circular waveguides are used, conventional chip placement processes can be used to assemble the devices.

[0023] With respect to Figure 2, there is illustrated the geometry of a sensing device in accordance with the present invention. In particular, there is shown a pair of waveguides 56, 58, in a side-by-side, spaced-apart, relationship. Flux coupled from a light-emitting diode (not shown) is guided down the transmitting waveguide 56 to its face 57. It should be noted that the waveguide 56 is a transmitting waveguide and the waveguide 58 is a receiving waveguide, and a reflective mode sensing device is being described, although other sensor formats such as a transmission sensor, rather than a relective sensor, can be used.

[0024] Light rays projecting down the transmitting waveguide 56 radiate from the end 57 of the waveguide into the adjoining space and toward the target area. If a target 54 is in the target area, for example a copy sheet in a copy sheet path, light rays from the end 57 of waveguide 56 are reflected from the target. As illustrated, ray X projecting from the waveguide 56 is reflected from the target 54 as ray X', and ray Y is reflected from the target 54 as ray Y'. As illustrated, the angle of incidence and reflection of the ray from the target with respect to the vertical is defined by phi (₄₁). Optical power relationships can be defined for the light rays transmitted and received. For example, the maximum optical power is obtainable at:

where d is the distance from the target or copy sheet 54 to the edge 57 of the waveguide 56, and a is the centerline-to-centerline distance between the optical waveguides 56 and 58. It is also known that the relation of the power received to the power transmitted is given by equation:

where R equals the coefficient of reflection, equals the radius of the optical waveguide, and P_a and P_T are the power received and the power transmitted respectively.

[0025] Energy is guided along the transmitting waveguide 56, preferably 3 to 20 millimeters, to the radiating surface 57. Preferably, the end dimensions of the optical waveguides are compatible with the side face dimensions of the LED or light-emitting device in order that a maximum amount of flux is coupled into the waveguide.

[0026] As the distance d from the face or edge of a transmitting waveguide is varied, the power response varies as illustrated in Figure3 showing transmitting waveguide 56(a), receiving waveguide 58a, and target 54(a). In a preferred embodiment, the sensing range of sensor devices in accordance with the present invention is limited to approximately 5.0 millimeters. Since there is an overlapping of the fields of view of the transmitting and receiving waveguides 56(a) and 58(a), the occurrence of a target in the target area will cause a portion of the light rays or flux incident on the target to be reflected back toward the receiving waveguide 58a. The receiving waveguide in turn will guide a portion of the reflected flux to a (not shown) detecting device, to be converted to an electrical signal.

[0027] With reference to Figures 4(a) and 4(b), in accordance with the present invention, there is illustrated the coupling of the optical waveguides to a light-emitting diode or a photodetector. An optical waveguide 60 is butt-coupled to an LED chip or photodiode 62 on a hybrid substrate 64. That is, there is a side-to-side attachment of the waveguide 60 and the photodiode 62 rather than the waveguide being attached to the top of the diode 62. In general, the LED or photo-detecting element, such as a p-n or p-i-n diode, is chosen to be dimensionally comparable with the waveguide material. The chip devices or semiconductors are attached to the substrate 64 using conventional conductive die attach adhesives. The waveguides may be attached using similar epoxies. The final configuration of a transmitter and receiver pair must be evaluated in terms of the signal cross-talk or background level in the receiver because of the proximity to the transmitter. Adequate optical shielding can be done with the aid of an opaque overcoat material over the transmitting chip. Figures 5(a) and 5(b) illustrate sensing face configurations for both circular and polygonal waveguide cross-sections.

[0028] Combinations of this type of structure can be fabricated in order that pairs of optical waveguides operate side-by-side. If one waveguide is a transmitting source of flux, and the other waveguide is a receiver, a reflective type sensor is then provided. In the alternative, an interruptive or transmittive type sensor can be provided. In general, the size of the waveguides are in the order of the size of the chips used for sources and detectors. The devices can therefore be very compact and assembled along the edge of the substrate very readily. As illustrated, there are various ways to couple waveguides to the edge faces of the emitter and detector chips. Circular waveguides, such as plastics fiber optics, as well as non-circular waveguides, can be used, and various optically-transmissive materials may be used, such as molded acrylics, transparent ceramics and glasses. In addition, the use of hybrid substrates allows the incorporation of integrated electronics. Also, medium optical resolution can be achieved without the use of conventional optics, and high gain amplifiers can be used without noise pick-up problems.

[0029] Figure 6 is an illustration of a differential fiber optic switch in accordance with the present invention. With reference to Figure 6, there is shown a transmitting waveguide 70 receiving light from a light-emitting diode 76, and a receiving pair including a receiving waveguide 72 and a receiving waveguide 74. A paper path is illustrated by the arrow, showing the direction of movement of paper between the transmitting waveguide 70 and the receiving waveguides 72 and 74. Radiant flux is generated by the light-emitting diode 76 and is projected from the edge 76 of the transmitting waveguide 70, producing a cone of radiant flux incident upon the waveguides 72 and 74. Coupled to each of the receiving waveguides 72 and 74 are photodetectors 78 and 80, and suitable (not shown) electronic circuitry to determine the presence or lack of presence of a sheet in the paper path between the transmitting waveguide 70 and the particular receiving waveguide.

[0030] In the prior art, generally one receiving optic fiber or waveguide was used, and there was no precise determination of the presence of a sheet. That is, if the paper in the paper path shifted either toward the receiving waveguides or the transmitting waveguide, there was a margin of error, and the presence of paper would not always be accurately determined. In other words, the point of detection of the paper might shift out of the area or cone of reception of the receiving waveguide for receiving radiant flux projected by the transmitting waveguide.

[0031] With a pair of optical receiving waveguides side-by-side, there are immediate side-by-side responses to the presence of paper in the paper path, providing a high degree of resolution regardless of the plane of travel of the copy sheet. A differential fiber optics switch provides a high degree of resolution regardless of the copy sheet or target to sensor distance.

Claims

1. An electro-optic sensor comprising a light source (62) for generating a radiant flux;

a substrate (64) to which the light source is attached;

two optical waveguides (60) secured to the substrate, the radiant flux generated by the light source being transmitted through one waveguide and being intended to be incident upon the other waveguide after transmission through, or reflectance from, an object being sensed, and

electro-optic means communicating with the other waveguide for converting the radiant flux into electrical signals.

2. The sensor of any preceding claim, wherein the waveguides are in a side-by-side relationship.

3. A sensor device for detecting the presence of an object moving in a sensing station comprising:

a light source (62) having its output directed at the sensing station, the light source being attached to a substrate (64);

a first optical waveguide (60) projecting the output from the light source toward the sensing station;

a receiver disposed at the sensing station, the receiver including a second optical waveguide for receiving light rays from thefirst waveguide, and

electro optical means communicating with the second optical waveguide for converting the light transmitted along the second waveguide into electrical signals, whereby the sensor determines the presence of objects at the sensing station.

4. The sensor device of any preceding claim, wherein the first and second optical waveguides are secured to the substrate.

5. The sensor device of any preceding claim, wherein the first optical waveguide is butt coupled to a side face of the light source.

6. A differential optic sensor as claimed in any preceding claim, including a third optical waveguide positioned so that light from the first waveguide impinges on both the second and third waveguides after transmission through, or reflection from, the object being sensed, the second and third waveguides being side-by-side for at least a proportion of their lengths, the converter being in communication with both the second and third waveguides.

Drawing