Background of the Invention

The present invention relates to an apparatus for detecting the position of label stock for printing. More particularly, this invention relates to a printer for label stock having a reflective sensor for detecting the position of an indicator stripe on the label stock in which an emitter, a concave mirror and a detector are all mounted on the back side of the label stock and a cassette for use in such a printer.

It is known to print label stock to produce individual labels to mark food products for example, with their description, weight, nutritional information, etc. Traditional ways of detecting the position of the labels include a "through beam" system wherein an emitter is placed on one side of the label stock and a detector is placed on the reverse side of the label stock. The back side of the label stock is imprinted with an indicator stripe. When the indicator stripe is present. The light from the emitter does not pass through the label stock and is not detected by the detector. When this occurs, the printer is signalled. However, this apparatus is not reliable. In particular, if graphics are printed on the labels, they tend to interfere with the detection of the indicator stripe, and generate a false indication that a new label is moving into position in the print station of the printer.

One example of a printer which employs a "through beam" detector is shown in U.S. Patent No. 5,336,003 to Tokyo Electric Co. This printer employs a prism on one side of the label stock to conduct the light

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emitted from an emitter to another prism located on the other side of the label stock. In this apparatus the light beam is used to locate the label after the label has been printed and peeled from its liner. The optical axis between the first and second prisms intersect at the surface of the label when the label is dispensed from the printer. This system suffers from the problems previously mentioned from positioning the emitter and detector on opposite sides of the label stock.

Therefore, it is an object of the present invention to provide a printing apparatus in which the emitter and the detector are on the same side of the label stock and in which a concave mirror is present to focus and reflect the light being detected to signal that a label is in position for printing. It is a further object of the present invention to provide a printing apparatus in which the label stock is supplied from a cassette which is free of electrical components.

Summary

In accordance with the present invention, a printer for labels comprises a printing station having a print head; label stock having labels on a first side and indicator stripes printed on a second side; a sensor having an emitter for producing light and a detector for receiving said light; a concave mirror for focussing and reflecting said light, said mirror being positioned with respect to said emitter, detector and label stock so as to reflect light originating at said emitter to said label stock and from said label stock to said detector; and a guide plate perpendicular to the base of the printer having a window therein; wherein said sensor and said mirror are positioned facing said second side of said label stock such that light emitted from said emitter is reflected from said mirror to said stock and

from said stock via said mirror to said detector. Preferably, said emitter and detector lie on the same plane which is substantially perpendicular to said label stock and said mirror is at an acute angle to said sensor and to said label stock.

As used herein, "light" defines any detectable radiation and is not limited to visible light. For example, ultraviolet an infrared radiation are also suitable for use in the present invention.

Also, in accordance with the present invention, a cassette for use in a label printer is provided comprising a base; a retainer on said base containing a roll of label stock having labels on a first side and indicator stripe imprinted on a second side; a guide plate perpendicular to said base having first label facing-side which is parallel to said label stock and a second mirror-facing side, said guide plate having a window therein; a concave mirror mounted on said cassette adjacent said window; and a sensor hole in said base of said cassette for an emitter and detector; said concave mirror being positioned with respect to said window and said sensor hole such that light emitted from an emitter is reflected from said mirror to said label stock and from said label stock to a detector; and wherein the guide plate window, mirror and sensor hole are aligned to each other such that said indicator stripe on the label stock can be detected.

Brief Description of the Drawings

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

Fig. 1 is a label cassette of the present invention.

Fig. 2 is a diagram of the path of light reflection.

Detailed Description of the Preferred Embodiment

In a typical embodiment of the present invention, a sensor pack having an emitter and a detector is mounted on the floor of a label printer. Label stock 40 is provided which includes labels 44 on the front side of the label stock and an indicator stripe 42 which is preferably a black bar on the back side of each label to signal when each new label is in a position for printing. A guide plate 22 having a window 24 is used in conjunction with a spring plate 26 to hold the labels in place during the detection. A concave mirror 14 is mounted and placed at an angle to the label stock. Light from the emitter is reflected off of the mirror 14 onto the label stock 40. Then the light from the label stock is reflected off of the mirror 14 down to the detector 18. When the indicator stripe 42 is present, no reflection from the label via the mirror is detected and the printer is signaled that a new label 44 is in position to print.

This system will now be described in detail below with respect to the figures. It is to be understood that the forgoing general description and the following detailed description are exemplary and explanatory but are not to be restrictive of the invention. The accompanying drawings which are incorporated in and constitute a part of this invention, illustrate the embodiments of the invention, and, together with the description, serve to explain the

principles of the invention in general terms. Like numerals refer to like parts throughout the disclosure.

Fig. 1 shows a removable label cassette 10 for use in the present invention which feeds the label stock (not shown in Fig. 1) to a print roller 32 which interfaces with a print station of a printer. The print station prints a desired image on each label when it comes in contact with a print head, preferably a thermal print head. A sensor pack 12 is shown in outline on the floor of the printer and emerges through a sensor hole in the base of the cassette. This sensor pack 12 is mounted on the printer and lies beneath a mirror 14. It includes a side by side emitter 16 and detector 18 as shown in Fig. 2. The emitter and detector used in this invention are commercially available. One emitter suitable for use in the present invention is QEC, 123, an infrared light emitting diode manufactured by Quality Technologies Corp. of Sunnyvale, California. A suitable detector is OP5704, an NPN silicon phototransistor manufactured by Optek Technology, Inc. of Carrollton, Texas. At least one cam 20 which activates a switch in the printer is also preferably placed next to the sensor pack which will signal that the cassette of labels is in place. The back of the label stock is imprinted with a black bar indicator. This black bar is sensed and used to notify the printer that a label is in position for printing.

The indicator stripe must be at a fixed position with respect to the label. It may be positioned at the leading edge of each label or at any point such that by detecting the indicator stripe, the exact position of the label within the printer can be determined. A typical black bar indicator stripe is .300 inches from the leading edge of the label.

The label cassette 10 uses a guide plate 22 to hold the labels in place. This guide plate has a window 24 which has a width smaller than the width of the black bars 42 on the label stock and through which the black bar is detected. In addition, the guide plate 22 should be painted black or be coated with a non-reflective surface in order to increase the contrast and reduce the reflection within the printing system. Otherwise, the detector 18 will become saturated with light reflected from the plate. The reflected beam is inherently diverging and if the guide plate is reflective, there may be enough light to trigger the detector erroneously. In addition, the cassette employs a spring plate 26 (e.g. a plate which is biased into engagement with the guide plate by a torsion spring) to hold the label stock against the guide plate. This is important from the standpoint of getting an accurate reading of the position of the label.

The mirror 14 is mounted on the base of the cassette. Typically, the mirror includes plastic tab feet 28 shown in Fig. 2, which snap into place in sockets provided on the base of the cassette. The mirror is preferably placed at an angle θ , preferably approximately a 45° angle to the emitter beam. In accordance with a preferred embodiment, this mirror is concave and, more particularly, is cylindrically concave. A pure concave mirror is more difficult to work with because it must be aligned with the light path in two dimensions, whereas the cylindrical concave mirror only requires alignment in one dimension. Stainless steel may be employed to make the mirror however, electroplated plastic is cheaper and more efficient.

The cassette 10 is preferably substantially light tight when in placed in the printer so that ambient light does not penetrate the cassette and interfere with

the detection of the indicator stripe. However, this is not that critical in many designs, such as, for example, if the cassette is loaded from the side of the printer. Only light coming from the bottom of the cassette is focused on the black bar by the mirror within the cassette. Light entering the chamber from the sides would not be a dangerous source of detection errors since the periphery of the mirror is in contact with both the base of the cassette and against the guide plate so as to provide a substantially light tight channel between the sensor pack and the window in the guide plate. This is a further advantage of using the cylindrical concave mirror. Because of its orientation, the mirror also does not accumulate dust on its interior reflective surface. This is a hazard since dust may diffuse the reflected light reducing the sensitivity and giving inaccurate or no readings.

The cassette further includes a pay out roller 30, an uptake roller for the liner, a drive roller for the label stock and a print roller 32 which is positioned directly opposite the thermal print head (not shown) . The print head is not a part of the cassette but is mounted in the printer.

Therefore, in accordance with the present invention, as shown in Fig. 2, the light from the emitter is sent from the emitter 16 to the mirror 14 where it reflects to the label stock 40, back to the mirror 14 and down to the detector 18.

The radius of curvature of the mirror is selected to collimate the light. The objective is to focus as much of the emitter beam onto the label stock as possible and to collect a sufficient amount of light reflected from the label stock to trigger the detector. The radius of curvature must be large enough to capture

the light beam and maximize its intensity. In one example, the mirror employed preferably has about a 0.4 inch radius.

The light path in the printer is typically about 1 to 2 inches. This is significantly longer than the light path used in the prior art printers. As a result of this longer light path, there is a need for a more efficient system to focus the light on the bar and to capture the reflected light. Therefore, the cylindrical concave mirror is preferred over a flat or simple concave mirrors. The inventors have discovered that this type of cylindrical, concave mirror focuses the light to the most intense brightness on the label stock. However, a flat or concave mirror may be used without departing from the scope of the invention.

By using the printing apparatus of the present invention, alignment of the label stock is not difficult. The label stock moves through the cassette on its side and is retained against the guide plate by the spring plate. The combination of the base of the cassette and the guide plate will assure proper alignment.

The printer system of the present invention employs a narrow path emitter and a wide detector. Specifically, the emitter preferably diverges less than about 10 or 20° from the center, whereas the detector preferably detects light deviating up to about 40° from the center line. A narrow beam, high output emitter was selected to get as much light as possible to the target. By using a wide detector, a tolerance is automatically built into the printer design for misalignments between the sensor and the cassette as it is inserted into the machine, and for manufacturing variation in the printer.

While a printing system incorporating a cassette has been described, those having skill in the art will appreciate that printers in accordance with the invention can be constructed without a removable cassette wherein a replaceable roll of label stock is set in the base of the printer.

The light employed to detect the black bar on the labels is preferably infrared radiation. However, other visible light or ultraviolet can be used without departing from the scope of this invention. In addition, to increase the intensity a pulsed emitter light may be used.

The printer preferably incorporates software wherein the system is continuously adjusted so as to be able to identify accurately the position of the label. Basically, the detector will detect the change in intensity of light from the reflective label background to the non-reflective black background and this change will occur over time as the black bar passes through the detector beam. For example, the black bar is not detected when it first appears in the window. Only when a substantial portion of the window is covered by the black bar is enough light blocked so that the detector will sense the black bar indicator and determine the label is in the printing position. It is critical to be able to determine the position of the label within about 0.5 mm so that the printed material is positioned fully on the label. The correction involves defining a threshold change in intensity from the maximum intensity white to the minimum intensity black which is indicative of a label being in position to be printed. The labels produced by some label manufacturers may not be as white as others and would have less intensive changes from black to white. Accordingly, the system includes compensating software to give it the reliable

intelligence to accurately determine the position of the label.

Having described the invention in detail and by reference to preferred embodiments thereof, it will be apparent that modifications and variations are possible without departing from the scope of the invention defined in the appended claims.

What is claimed is: