GB2035218A - Improvements in printing devices - Google Patents

Abstract

A printing device comprises a transducer arrangement to scan the stationery 12 whose pair of sensors (e.g. photo-sensors) 21', 22' are spaced from one another along a line which is inclined relative to both the horizontal and vertical scanning directions. Provided scanning of the stationery does not take place in both directions simultaneously, the horizontal and vertical positions of the stationery can be detected e.g. for ensuring correct registration of print. The sensors are arranged in a potential divider configuration so as to be unaffected by changes in ambient light levels. <IMAGE>

Description

SPECIFICATION Improvements in printing devices This invention relates to printing devices.

In computer controlled typing systems, commonly known as word-processing systems, it is desirable in common with other typing devicesforthe documents printed to be as presentable as possible. To this end, a high quality electromechanical printer is usually employed, such as a so-called daisy wheel printer. The printer, once activated, prints from a memory in which the text is stored. In my copending British Patent Applications Nos. 48472/77 and 16244/781 describe and claim a printing device which is provided with an arrangement for ensuring the correct vertical and/or horizontal registration of print on stationery mounted within the printer.In one of my described printing devices, the typist is free to position in the horizontal sense paper any where across the plates of the printer, and it is sensed by a movable transducer arrangement which detects an edge of or marking on the paper to provide a signal to ensure the correct registration of print. A particular form of the transducer arrangement comprises two photoresistors connected in series and mounted on the carriage of the printing mechanism, side by side, in the direction of travel of the mechanism. A potential difference is applied to the resistors, and the potential at their interconnection varies when one of the resistors detects reflective stationery while the other still faces the platen of the printer.

(Variations in ambient light level affect both resistors equally as it is only when one receives light of different intensity to the other that a change in output occurs.) The change of potential is interpreted by control circuitry to inform the printer that stationery has been detected.

For some applications, it is desirable to sense both the horizontal and vertical position of stationery mounted within the printer.

According to the present invention, there is provided a printing device having: a printing mechanism for printing on stationery; transducer means arranged to scan stationery within the printing device in the longitudinal and lateral directions of the stationery to sense the position of the stationery, the transducer means comprising two sensors which are spaced from one another along a line which is inclined relative to both said directions; and control circuitry responsive to the output of the transducer means for providing a signal to ensure registration of the stationery and print applied thereto by the printing mechanism; the arrangement being such that, in use, scanning of the stationery does not occur in both said directions simultaneously.

The invention will now be described in more detail, by way of example, with reference to the accompanying drawings, in which: Figure lisa diagram of part of one embodiment of printing device, according to the present invention, for use in a word-processing system; Figure 2 shows to an enlarged scale, part of the transducer arrangement of the embodiment of Figure 1; and Figure 3 is a circuit diagram of the transducer arrangement of Figure 2.

The embodiment of printing device shown in part in Figure 1 is intended to be incorporated in a wordprocessor also incorporating an appropriately programmed central processing unit, mass storage unit for text to be printed, keyboard, display unit and the various items of interface circuitry required to interface the processing unit to the other components of the word-processor.

Referring to Figure 1, the platen of a printer is shown at 11. For clarity, the other chassis components of the printer are omitted. A slidable trolley 15 is mounted on two parallel bars 13 and 14 which extend lengthwise of the platen 11 and is free to travel along their length as indicated by the arrow.

Printing means, are mounted on the trolley 15 and comprise a "daisy-wheel" printing element 18 and its drive motor 17. A mounting bracket 19 affixed to the trolley 15 is used to support two photo-resistors 21 and 22, as well as a miniature tungsten filament bulb 23. Paper, embracing the platen, is shown at 12.

The two photo-resistors face the surface of the platen, but are spaced sufficiently therefrom to accommodate a print-ribbon shield and stationery. The spacing also permits light emitted by the bulb and reflected by the stationery to be detected by the photo-resistors.

Referring to Figure 2, the two photo-resistors 21 and 22 are depicted again. Horizontal and vertical edges of paper are shown schematically at 12a and 12b respectively. The figure is intended to illustrate the disposition of paper relative to the photoresistors which may occur within a printer as shown in Figure 1.

In Figure 2 the vertical (V) and horizontal (H) directions, in which relative movement of the daisy wheel and paper/platen, can occur correspond to rotation of the cylindrical platen and movement of the trolley 15 lengthwise and the platen, respectively. As shown, the transducers 21 and 22 are spaced apart along a line indicated as A which extends in a direction which is inclined relative to both the V and H directions. It will be appreciated that when either the trolley is shifted or the cylindrical platen rotated, so that a stationery edge or marking approaches the transducer arrangement, the output of one of the transducers (22 for upward movement of the paper or rightward movement of the trolley) will change its output before the other.Provided that vertical and horizontal shifts do not take place atthe same time, horizontal and vertical edges or markings can be discriminated.

The circuitry associated with the photo-resistors 21 and 22 is shown in Figure 3. As shown the photoresistors form a potential divider. The potentiometer 32 is set so that the reference potential applied to the comparator 31 is somewhat above the potential at the junction of the photo sensors 22 and 21. Then when the edge of an item of stationery comes into the field of view of the sensor 22 by way of the verti cal or horizontal leading edge of the item stationery approaching by relative movement the transducer arrangement, the output of the comparator 31 will change. Two pulse signals PH and PV can be derived from the printer control circuitry which are high for periods when the trolley 15 is moving lengthwise of the platen and the platen is rotating, respectively.By arranging the control circuitry to ensure that these two operations do not occur simultaneously (so that the high parts of these two pulses do not overlap when detection takes place), the output from two AND gates 40 and 41 to which these signals are respectively applied can be appropriately interpreted to indicate detection of the left hand and top edge of the item of stationery, respectively.

Thus, when for example, it is desired to sense the horizontal displacement of paper on the platen, (in order to ensure horizontal registration of print), the whole assembly is moved on the trolley 15 from left to right and at this time the pulse PH is high and PV is low. A change in value of the reflected light occurs as soon as the photo-resistors 21 and 22 approaches the stationery. (The platen of the printer is usually of a dark matt nature compared to the reflective nature of stationery.) This causes the output of comparator 31 to change state and the output of AND gate 41 goes high indicating detection of the edge 12b of the stationery. This change may cause the printing device control circuitry to enable printing to commence.

It will be appreciated that because of the relative disposition of the photo-resistors 21 and 22, a top or bottom edge or marking can be detected with the carriage anywhere across the width of the sheet.

However, if in addition it is desired to establish the vertical displacement of the paper before printing, to ensure vertical registration of print, the platen of the printer is rotated by its drive means and PV is set high with PH low. This brings the top or bottom edges of the paper past the photo-resistor combina- tion. In the case of detecting a top edge, the photoresistor 22 will drop in resistance once the paper is moved in the vertical sense towards it, or a corresponding change will occur for the photo-resistor 21 if the bottom edge of the paper were firstto be detected. Once again, the potential of the interconnection changes, which change can again be interpreted by control means for ensuring vertical registration.

Thus, the use of only two photo-resistors, arranged in this manner, enables both the vertical and horizontal edges of paper mounted within the printer to be detected provided, of course, thatvertical and horizontal movements do not occur simultaneously. Stationery bearing pre-printed captions can thus be inserted into the printer without careful positioning in eitherthe horizontal of vertical senses, and the printing mechanism, as directed by an output programme, will only print in the desired locations. The effects of ambient light and variations in temperature on the two photo-resistors alter the characteristics of each equally. Thus, in this arrangement, the quiescent value of the potential of the interconnection remains unaltered during such variations and is only changed when one resistor receives a different amount of light to the other.

By applying the outputs from the junction of the two photo-sensors to a fu rther comparator whose reference potential is set slightly below the quies cent potential at this junction, the right hand and bottom edge of the item stationery can also be detected. In another version of the apparatus, the common junction of the photo-sensors can be applied to inputs of four separate comparators each of which has an appropriate reference potential applied thereto so that the comparator outputs correspond individually to the left-hand, right-hand, top and bottom edges of the item.

As well as detecting the contrast between the relatively dark platen and the lighter surface of the item of stationery, the arrangement can also be used to establish registration with respect to areas of different contrast (e.g. dark marking) on stationery, be it continuous feed stationery or individual items.

Where two corresponding edges, that is the right and left or top and bottom edges, of an item of stationery are detected, by measuring the time between the two edges being detected or by counting the number of stepping pulses applied to the trolley drive or the platen drive as appropriate the width and length of an item of stationery can be measured.

Commercially available comparators have high sensitivity and this combined with appropriate adjustment of the reference potential (with which the sensor(s) output is compared) makes it possible to obtain a signal to ensure precise registration of print.

Numerous modifications of the present printing device will be apparent to those skilled in the art. For example, the control circuitry can be arranged to turn on the bulb 23 only when the position of the stationery is being detected; illumination of the bulb can then be controlled by a suitable signal originating in the processing unit.

Furthermore the control circuitry can be arranged to produce its output signal indicating detection of the stationery in such a mannerthatthere is delay of appropriate duration between the stationery being detected and the movement of the printing mechanism and transducer arrangement being stopped so that they stop a predetermined position beyond the edge or the marking being sought. In the case of detecting the edge of the stationery this can be used, for example to stop the printing mechanism one character width in from the edge so that printing can then start immediately. The delay may be generated by means of a monostable multi-vibrator.

It will be appreciated that the application of the present invention is not limited to daisy wheel printers of word-processors. The printing device may be of many other forms e.g. a line printer.

Claims (11)

1. A printing device having: a printing mechanism for printing on stationery; transducer means arranged to scan stationery within the printing device in the longitudinal and lateral directions of the stationery to sense the position of the stationery, the transducer means comprising two sensors which are spaced from one another along a line which is inclined relative to both said directions; and control circuitry responsive to the output of the transducer means for providing a signal to ensure registration of the stationery and print applied thereto by the printing mechanism, the arrangement being such that in use, scanning of the stationery does not occur in both said directions simultaneously.

2. A printing device according to Claim 1 wherein the transducer means and control circuitry are operative to detectthe difference in reflectivity of the platen and stationery or of different areas of the stationery.

3. A printing device according to Claim 1 or 2 wherein the transducer means and printing mechanism are mounted on a common support for move menttogether relative to stationery mounted within the device.

4. A printing device according to Claim 1,2 or 3 and including a cylindrical platen on which, in use, the stationery is mounted, at least the printing mechanism being movable relative to the platen longitudinally thereof to print across the width of the stationery and the platen being rotatable to effect line feed of the stationery.

5. A printing device according to any one of the preceding claims wherein the sensors are respon size to electromagnetic radiation and an electromagnetic radiation source is disposed to cause radiation to impinge on the stationery.

6. A printing device according to Claim 5 wherein the two sensors share a common radiation source.

7. A printing device according to any one of the preceding claims wherein the area of the stationery scanned by the transducer means is adjacent to the printing zone of the printing mechanism.

8. A word processor including a printing device according to any one of the preceding claims.

9. A word processor according to Claim 8 and having a central processing unit connected to interface with said control ci rcuitry to control the operation of the printing device, the central processing unit being programmed to cause the transducer means to scan an item of stationery to ensure registration of the stationery and the text to be printed thereon.

10. A printing device constructed and arranged to operate substantially as hereinbefore described with reference to and as illustrated in the accompanying drawings.

11. A word processor substantially as hereinbefore described.