GB2160736A

GB2160736A - Enlarging multicolour printing apparatus

Info

Publication number: GB2160736A
Application number: GB08410093A
Authority: GB
Inventors: Paul Fuller
Original assignee: XAFAX CORP
Current assignee: XAFAX CORP
Priority date: 1983-04-19
Filing date: 1984-04-18
Publication date: 1985-12-24
Also published as: GB8410093D0

Abstract

Apparatus for producing an enlarged colour copy of an original mounted on flat bed scanner (Fig. 2) includes a drum 1 on which the medium to be printed, eg. a carpet is mounted, and a spray-head assembly 6 on which four air brushes 6', three for the primary colours and one for black are mounted so as to be focussed on a single spot. The spray-head assembly is mounted for movement parallel to the axis of the drum along tie bars 7 by means of a locked bowden cable driven by motor 20. Signals produced by scanning the original are stored in a memory, and are caused to operate the air brushes in synchronism with rotation of the drum by shaft encoder 3. <IMAGE>

Description

SPECIFICATION Multi-colour printing apparatus The present invention concerns enlarged multi-colour printing apparatus for producing substantially enlarged copies of multi-coloured originals.

Apparatus of this kind is already well known. An early example of multi-colour printing apparatus can be found in the specification of US Patent No. 1,709,926 and a more recent example is British Patent No.

1,166,005. As can be seen from these prior patents the known apparatus basically comprises a drum on which the coloured original is mounted. The original is then scanned by a beam of light and the light reflected from the original broken down into the three primary colours to generate electrical signals which are used to control the feed of primary colour pigments onto a suitable print medium also mounted on a drum.

The present invention has for an object to provide apparatus of this basic type which is relatively simple to manufacture and yet robust and reliable in operation.

Accordingly the present invention consists in enlarged multi-colour printing apparatus comprising electro-optical scanning means for scanning an original to be printed in enlarged form, the scanning means including a flat bed on which the original is mounted, a light source for illuminating the original with light of known spectral composition, and an electro-optical scanning head for receiving light transmitted through the original to derive electrical signals which represent the intensities of the three primary colours in the light so received and memory means for storing the signals so generated, the apparatus further including a printing unit comprising a drum on which the medium to be printed on is mounted, means for rotating the drum relative to a printing head carrying a plurality of air brushes, means for scanning said printing head in a direction parallel to the axis of rotation of the drum, and means for control ling the operation of the air brushes in response to the stored signals and in response to a shaft encoder responsive to the angular position of the drum.

In order that the present invention may be more readily understood an embodiment thereof will now be described by way of example and with reference to the accompanying drawings, in which: Figure 1 is a diagrammatic plan view of enlarged multi-colour printing apparatus, Figure 2 is a perspective view of a scanning assembly forming part of the apparatus of Fig.

1, and Fig. 3 is a b;ock diagram of control circuitry associated with the operation of the apparatus of Fig. 1.

Referring now to the drawings the multicolour printing apparatus shown therein comprises a print drum 1 formed from three sections A, B and C coaxially mounted on a single shaft S. Each section is four feet in length and five feet in diameter. The drum 1 is intended to receive a print medium, for example a carpet, so that a coloured picture on an original can be transferred onto it.

Secured to the shaft S is a fail-safe electromechanical brake 2 and a shaft encoder 3.

The shaft S is rotated by an electric motor and reduction gearbox unit 10 which is coupled to shaft S by a belt drive 9. In normal operation the drum 1 is rotated at 25 RPM. Mounted to one side of the drum 1 is a sprayhead assembly 6 which carried four air brushes 6', one of each of the primary colours and one for black. The airbrushes are connected to a supply of compressed air 50 and resevoires of appropriately coloured inks are shown at 51.

It will be appreciated that in order to obtain the necessary gradations of colour the supply of compressed air to each air brush will have to be varied. Thus each supply line to each air brush includes a valve of variable through-put controlled by the process electronics of the apparatus. These valves are indicated at 52.

Furthermore and also in known fashion each air brush has a servo-contolled needle which determines its final flow aperture.

The sprayhead assembly 6 is mounted for sliding movement along two precision ground tie bars 7 which are exactly parallel to shaft S.

The sprayhead 6 is moved along the tie bars 7 by a locked bowdwen cable arrangement which includes a cable 4 helically wound around drums 5 and 5'. Drum 5 is driven by a motor 20 via a series of selectable electromagnetic clutches 21,22,23 and 24. Drum 5' is driven by an electric motor 25. This latter drum is used to drive the sprayhead assembly at high speed in its return direction after a printing operation. In this mode motor 20drives drum 5 via the clutch 21. During a printing operation motor 20 drives drum 5 through one of the clutches 22, 23 or 24 to determine the resolution of the final picture.

As can be seen in Fig. 1 the four air brushes 6' are focussed on a single spot on the printing medium so that there is no need to introduce an artificial delay into the control signals for the air brushes to compensate for them being staggered.

The control signals for the operation of the printing assembly are provided by a control console 11 on which is mounted a flat bed scanner 1 2. The console 11 is totally separate from the printing assembly and houses all the process electronics for the entire apparatus.

The console 11 incorporates a light box 30 over which the scanner 1 2 is, in operation of the apparatus, mounted. The scanner 12 is shown in greater detail in Fig. 2 and comprises a translucent base 31 on which any original to be scanned is laid. The scanner 1 2 also includes a light-sensing head 32 which receives light from the light box 30 transmitted through the original. The light so received is split by dichroic filters into the three primary colours R, G and B which are detected in appropriate photosensors to generate three colour control signlas. This arrangement is generally indicated at 33.

The scanning head 32 is mounted on a cross member 34 which can be moved along tie bars 35 by a bowden cable arrangement similar to, but smaller than the arrangement used to drive the spray-head assembly 6. This scans the sensing head 32 in the X direction over an original. The drive motor for the bowden cable drive is associated with a shaft encoder 36. For movement in the Y direction the sensing head 32 is mounted on a nut which forms part of a conventional lead screw assembly driven by an electric motor 38. A shaft encoder 39 monitors the operation of the lead screw and thus the position of the sensing head 32 in the Y direction.

The preceding description has been concerned with basic mechanical details of the apparatus. What now follows is directed more specifically to the way in which the electronic components of the apparatus operate.

Referring now to Fig. 3 of the drawings it can be seen that the three primary colour signals which are the output of sensing head 32 are taken to a circuit 60 which generates a black signal whenever the combined amplitudes of the three colour signals falls below a preset level. The four output signals from circuit 60 are strobed at 61 by output pulses from shaft encoder 36. Shaft encoder 36 is conventional and consists of a disc which rotates with the shaft on which it is mounted and which passes between a filament light source and a plurality of photosensors. The disc has a plurality of apertures which, as the disc rotates, allow light to fall on the photosensors. The number of light sources, the positions of the holes and the sensors are so arranged that the encoder produces accurately spaced pulses from which the angular position of the shaft can be measured.

In the present embodiment shaft encoder 36 gives 4,320 pulses per revolution which strobe the colour signals from sensing head 32.

Each strobed colour signal is converted by an analoque-to-digital converter into a four bit digital signal. Thus each pulse from encoder 36 generates a 16unit word so that one line scanned in the X direction requires memory 37 to store 4,320 16unit words. In fact the capacity of memory 37 is such that it can store two complete lines. In the present embodiment memory 37 comprises eight CMOS memory chips made by Plessey and sold under the type number MV5516. Each chip can store 2048 8-bit words.

Another feature of the embodiment being described is that scanning is always carried out in the same direction so that there is a flyback period when no data is being produced and during which the motor 38 can be incremented to move the crossbar 34 into position for the next line to be scanned. The shaft encoder 39 associated with motor 38 gives 385 pulses per revolution and the lead screw and nut arrangement has 26 turns per inch thread.

In operation the following sequence occurs.

The original to be reproduced is placed on the base 31 with sensing head 32 in position to start scanning line 1. Scanning of the original is started and lines one and two are stored in memory 37 in appropriate locations in the memory under the control of pulses from encoder 36. Odd lines are loaded into the left hand side of the memory and even lines into the right hand side. This is done by switch 63 which is also controlled by encoder 36. Drum 1 is not rotated by motor unit 10 until the first two lines have been loaded into memory 37. The fact that drum 1 is ready to receive the stored data and that printing can commence is indicated by the encoder 3 which is similar to encoder 36. Once encoder 3 has indicated printing can start then pulses from encoder 3 are used to read out in sequence the line one data stored in memory 37.The 16unit words are used to control the four air brushes 6' and are supplied to the control circuitry 70 of the sprayhead assembly. It will thus be appreciated that for the drum assembly to print a complete line the drum encoder 3 will have to generate 4,320 pulses.

Drum 1 is rotated continuously by motor unit 10 and the sprayhead assembly is also continally incremented along the tie bars 7 by motor 5 acting through one of clutches 22,23 or 24. The foacl point of the air brushes 6' thus describes a helix with respect to the print medium carried on the drum 1. There is thus no fly-back period for the sprayhead assembly 6 and it is for this reason that memory 37 stores two complete picture lines. Once line one has been read out and printed line two is immediately available for read-out. As the second, even line of data is read from memory 37 by the pulses from encoder 3 line three of the original is read into the memory locations previously occupied by line one. The arrangement is such that the printing rate of the drum assembly is slower by a factor of aproximately 25% than the rate at which the colour data is generated by the scanning assembly. Scanning is stopped when there are no locations available for the next line to be scanned but because of the speed diferential there is always data available for the drum assembly.

The horizontal magnification of the original is set by the amount of incrementation of the lead screw by motor 38 at the end of each line scanned.

in order to increase printing speed it is possible to decrease the optical resolution of sensing head 32 and increase the pitch of the helix described by the focal point of the air brushes. This is the reason for the clutches 22, 23 and 24 which can be appropriately selected to give pitches of 1 mm., 2mm. and 3mm. respectively.

Claims

1. Enlarged multi-colour printing apparatus comprising opto-electrical scanning means for scanning an original to be printed in enlarged form, the scanning means including a flat bed on which the original is mounted, a light source for illuminating the original with light of known spectral composition, and an opto-electrical scanning head for receiving light modified by the original to derive electrical signals which represent the intensities of the three primary colours in the light so received and also a signal representative of black in the original, and memory means for storing the signals so generated, the apparatus further including a printing unit comprising a drum on which the medium to be printed on is mounted, means for rotating the drum relative to a printing or spraying head carrying four air brushes, one for each of the three primary colours and one for black, means for scanning said spraying head in a direction parallel to the axis of rotation of said drum, siad scanning means comprising a locked bowden cable drive driven by an electric motor, and means for controlling the operation of the air brushes in response to the stored signals and in response to a shaft encoder responsive to the angular position of the drum.

2. Apparatus as claimed in claim 1, wherein the air brushes are focussed on a single spot of the printing medium in operation of the apparatus, and have associated therewith a source of compressed air, control valves being provided so that the supply of compressed air to each air brush can be varied in accordance with the intensity of the pigment to be printed.

3. Apparatus as claimed in either claim 1 or claim 2, wherein the sensing head of the scanning means is mounted on a cross member which can be scanned in one direction over an original by a bowden cable arrangement driven by an electric motor, the motor being associated with a shaft encoder thereby to digitise the output signals form the sensing head so that they can be stored in said memory means.

4. Apparatus as claimed in claim 3, wherein the sensing head is movable along said cross member by means of a lead screw and nut.

5. Apparatus as claimed in any one of the preceding claims wherein the drum is driven in operation by an electric motor driving a reduction gearbox the output of which is coupled to the shaft of the drum by a V-belt drive.

6. Apparatus as claimed in any one of claims 3 to 5, wherein said memory means are operative to store at least two lines of an original as scanned by said sensing head, and wherein said drum has a shaft encoder associated therewith whereby data stored in siad memory means can be read from said memory means under the control of pulses generated by the drum encoder.

7. Apparatus as claimed in claim 6, wherein said scanning means are arranged to operate at a faster rate than said printing unit whereby said memory means always contains data for read out to the printing unit.

8. Apparatus as claimed in claim 7 wherein said scanning unit always scans an original in one direction only and wherein said drum is continually rotated in operation.

9. Apparatus as claimed in claim 8, wherein in operation of the apparatus the spraying head is continually advanced relati e to the drum whereby the air bushes describe a helix with respect to the drum surface.