GB2271962A

GB2271962A - Controlling a printing unit in response to the sensed speed of the substrate therethrough.

Info

Publication number: GB2271962A
Application number: GB9322585A
Authority: GB
Inventors: Simon Weekes
Original assignee: HUSSAIN MATLUB
Current assignee: HUSSAIN MATLUB
Priority date: 1992-11-02
Filing date: 1993-11-02
Publication date: 1994-05-04
Also published as: GB9322585D0; GB9222912D0

Abstract

The speed and registry of a printing unit are controlled in response to the output of a substrate-speed sensor, whereby to control the spacing of successive printed images and to match the speed of the print unit with the substrate when printing is being effected. In one form, the control determines the timing of the onset of printing after detection of a registration mark on the printstock, said timing being dependent on the sensed speed of the e printstock. The control may also be responsive to a sensor for detecting the temperature of the ink. <IMAGE>

Description

PRINTING APPARATUS AND METHOD The present invention relates to printing apparatus for use in high speed production lines for printing information on passing objects or material such as film wrapping used to package foods, for example.

Certain types of food items are typically wrapped in plastics film which is fed from a continuous roll to a printer which prints information such as brand name and sell-by date and then to the food wrapping line in which food items are wrapped. The film is then sealed and cut to separate the items. The sell-by date is typically printed using a rotary head printer operating at a fixed speed.

Thus, minor variations in the speed of travel of the film result in variation in the positioning or quality of print of the printed sell-by date for example on the packaging.

It is one object of the present invention to overcome this disadvantage.

In other known types of printing apparatus, printing is carried out in response to the detection of registration marks provided on the substrate itself. Whilst this solves alignment problems to some extent, it would be advantageous to provide a printing system which did not require the use of registration marks so that it could be used with all kinds of substrates.

One aspect of the present invention provides printing apparatus for use with a moving substrate comprising a print head, means for measuring the speed of travel of the substrate and means for controlling the operation of the print head according to the speed of travel of the substrate.

The invention also provides a method of printing on a substrate which is travelling past a print head comprising measuring the speed of travel of the substrate and controlling the operation of the print head according to the speed of travel of the substrate.

By synchronising printing with the speed of travel of the substrate, it is possible to ensure that printing is consistent, and in the case of printing information at intervals on a substrate it is possible to ensure that printing is regularly spaced on the substrate itself.

The substrate may be a plastics film as described above or any other sheet material.

The presently preferred embodiment of the invention is a print head designed for printing on sheet material and comprising a rotary print wheel. The operation of the print wheel is preferably controlled by means of a stepper motor.

The advantage of using a stepper motor is that the speed of operation is almost continuously variable over a given range rather than being variable in a wide step-wise fashion The speed of rotation of the print wheel is preferably electronically controlled by signals representing the speed of travel of the substrate. The signals may be provided by means of a rotary encoder arranged in contact with the substrate to rotate as the substrate travels, usually in a linear direction. The encoder provides pulses to a controller which controls the operation of the print head.

Thus, another aspect of this invention relates to a printing system comprising the print head, the controller and the encoder or some other device providing signals indicative of substrate speed.

In the preferred embodiment of the invention, it is the time delay between successive print operations which is controlled in response to the speed of travel of the substrate, to ensure that the linear spacing between successive prints is constant regardless of any variations in the speed of the substrate.

Thus, regularly spaced printing can be achieved without the need for detection of indicia on the substrate itself or externally applied synchronisation signals.

However, the apparatus of the invention is also operable in a mode in which the preselected time delay is not initiated until an additional signal is received. This additional signal can be supplied from another part of the apparatus which does not form part of the present invention, or it can be provided from an indicia detector which may be provided in the apparatus of the invention.

In other words the system of the invention is operable in a continuous mode in which printing continues at regularly spaced intervals or intermittent mode in which printing is interrupted until an additional signal is received.

Another aspect of this invention provides a printing system in which the temperature of the ink may be varied to vary the print quality and/or quantity of ink used, to be described in more detail below.

Another novel feature of this invention is the provision of print types made from silicon rubber or other rubber-like material containing metallic particles for more efficient heat conduction and ink transfer, to be described in more detail below.

The invention also provides printing apparatus having one or more sensors for detecting a physical condition and providing a signal indicative of this condition to control means whereby to facilitate fault diagnosis.

The preferred apparatus includes a "print head home" sensor. In the case of a rotary print wheel or print drum, the "print head home" sensor is positioned so as to be activated in response to rotation of the print wheel or drum. No signal from this sensor indicates that the print head is not properly located. The sensor may be a Halleffect sensor which detects the rotational position of the print wheel to ensure that it is correctly "parked" for the start of a print operation.

Sensors may also be provided for detecting the temperature of the ink roll and/or the print drum, which again can indicate faulty operation.

The absence of a signal from the means for measuring the substrate speed indicates faulty operation of this item.

The absence of a signal from the indicia detector, which preferably comprises a proximity switch, indicates faulty operation of this item.

The control means can be arranged to operate in a fault diagnosis mode in which it displays various fault codes indicative of different types of operating faults.

An embodiment of the invention will now be described by way of example only and with reference to the accompanying drawings in which: FIGURE 1 is schematic diagram of a printing system according to the present invention; FIGURES 2(a) and (b) are sketched graphs illustrating the operation of the print head in intermittent and continuous operation.

FIGURE 3 is a side sectional view of the ink roller carrier; FIGURE 4 is a side elevation of the encoder wheel and associated mounting components; and FIGURE 5 is a block diagram of the components of the control unit.

The illustrated printing system comprises three basic components, namely an encoder 1 for providing signals representing the speed of travel of the substrate, a control unit 2 and a print head 3.

The encoder 1 comprises a wheel 4 mounted on a shaft which is freely rotatable with respect to a housing 5.

By means of suitable electronic circuitry inside the housing 5, the encoder provides to the control unit 2 a set number of equally spaced electrical pulses for each revolution of the wheel 4. In other words, the spacing between pulses corresponds to a predetermined angular movement of the wheel 4. In this particular example, the set number of pulses is 400. In use, the wheel contacts the sheet material which forms the printing substrate so that as the material travels in the direction of the arrow A, the wheel is rotated in the direction of the arrow X. Thus, the linear movement of the substrate is translated to an angular velocity of the encoder wheel 4 which is representative of the speed of travel of the substrate.

One of the functions of the control unit 2 is to interpret the pulses from the encoder 1 to obtain a digital value of the speed of travel of the substrate. It will be appreciated that the rate at which pulses are received by the control unit from the encoder is proportional to the speed of the substrate, i.e. the closer the pulses are spaced in time, the faster the substrate is travelling. The control unit 2 contains circuitry for determining the number of pulses received per unit time and converting this into a digital value representing the speed of travel of the substrate. This value is used to control the operation of the print head, to be described in more detail below.

The print head 3 is designed to be clamped to a suitable part of the frame of the material handling machinery by means of screw clamps 6, in such a position that material passes the print head in the direction shown by the arrow A. Screw clamp 6 is operable to move a key 6a in the direction of arrow X to clamp a part of the host machinery. A further screw clamp 7 moves against a rotable cylinder 7a to adjust the angle of an adaptor 7b with respect to the remainder of the print head in the direction of arrow Y to vary the orientation of the print head in use.

The print head further comprises a print wheel 8 and an ink roller 9 which are rotatably mounted in a housing 10. Types 11 are mounted in channels in the surface of the print wheel 8 so as to protrude slightly from its surface and be wetted by ink from the ink roller 9 as the print wheel and ink roller rotate with respect to each other.

The print head may be operated either intermittently or continuously. Figures 2(a) and 2(b) are rough graphs which show the operation of the print head in intermittent and continuous mode respectively. In the illustrated example, the print wheel 8 bears types at diametrically opposite locations about its cylindrical surface so that two printing operations are performed for every complete revolution of the print wheel 8. The print wheel is driven by a stepper motor, not shown.

The print head 3 receives print commands from the control unit 2 and in response to each print command the print wheel is rotated through half of a revolution by the stepper motor to transfer ink via the types 11 onto the substrate. Figure 2a shows the angular velocity W of rotation of the print wheel 8 with respect to time. At the start of each print operation the angular velocity of the print wheel 8 gradually increases from rest to a predetermined velocity. Printing is arranged to take place when the predetermined velocity is reached, following which the angular velocity gradually decreases. The angular positions of the print wheel 8 are also shown in the Figures.

In continuous mode, assuming that a print operation is completed at time t1, there is a time delay T between time t1 and t2 before the next print operation commences. The amount of delay corresponds to a certain length of material passing the encoder wheel 4 and can be manually preset in the control unit 2. By continuously monitoring the speed of travel of the substrate, the control unit 2 functions to vary the time delay T to ensure that it always corresponds to the certain length of material preset by the operator.

In intermittent mode of operation the control unit 2 additionally receives signals from a proximity switch or optical sensor 12 which detects the presence of registration marks or indicia provided in the sheet material or other signals provided from the host machinery. When set in intermittent mode, each printing operation is triggered by the detection of a registration mark in the substrate by the switch or sensor 12. Thus, there is a delay D after an initial printing operation shown in Figure 2, at the end of which the control unit receives a signal from the sensor 12 and commands the print head to print after the additional preset delay T (if any).

The proximity sensor may be a magnetic sensor detecting the rotation of a metal cam on the host machine.

Alternatively it may comprise a photoelectric switch or simply a pair of relay contacts.

It will be appreciated that the print roller 8 may be designed to perform more or fewer print operations per revolution.

The control system described above ensures that the speed of travel of the substrate material is accurately synchronised with the rate of printing information, i.e. the faster the substrate travels, the higher the print rate.

This control is achieved automatically without the need for the operator to provide print rate information to the control unit 2.

A printing system as described above has been constructed by the present applicants to provide a selectable delay T between 0 and 999 via a digital display 13 on the controller 2 corresponding to a length of 1.2 metres.

The print head 3 is designed to print using thermofusible plastic ink. This ink is fluid when maintained at a certain temperature but sets on contact with relatively cold substrate. In order to keep the ink sufficiently warm to remain fluid, the print wheel 8 and the ink roller 9 are heated by means of heaters surrounding the wheel 8 and the roller 9 and arranged in the housing 10.

The housing 10 is preferably constructed from aluminium.

The temperature at which the wheel 8 and roller 9 are to be maintained is again selectable by the operator entering a desired temperature on the digital display 13. A sensor or transducer within the housing monitors the temperature of the wheel 8 and roller 9 and provides electrical signals representing the temperature to the control unit 2.

Circuitry within the control unit compares the actual temperature with the selected temperature to control the heaters accordingly. The ink may include an adhesive so that it adheres to a wide variety of substrate surfaces.

The print head 8 is mounted on a shaft which extends back in to the housing 10 and is caused to rotate by a stepper motor within the housing. A Hall-effect sensor detects the angular position of the shaft to ensure that an initial set up, the roller 8 takes up the correct angular position.

The ink roller carrier is shown in cross section by way of example in Figure 3. The carrier 15 is in the form of a solid block of rectangular cross section having a -part circular channel 16 for receiving the roller. The roller shaft is received in a circular hole 17 at the internal end of the holder (not visible in Figure 1).

Cylindrical holes 18 extending along the carrier house ink roller heaters.

The ability to vary the temperature of the ink allows the user to control the print quality and life of the ink roll 9, which is replaceable. The higher the temperature (up to a certain maximum), the better the print quality but the more ink is used, at the expense of the print roll life. Thus, for some applications, a lower ink temperature may be desirable.

The rotary encoder is shown in more detail in Figure 4. The wheel 4 is mounted on the housing 5 (Figl) by means of a grub screw, not shown. The housing 5 is behind the wheel 4 as shown in Figure 4. Wires extending from the housing 5 provide signals to the control unit 2 from the encoder 1.

The encoder is mounted on the machinery by means of two arms 30, 31 attached to a clamp 32. The clamp 32 is attached to the housing by means of screw 33. Arm 31 is fixed with respect to the clamp 32. Arm 30 is pivotably mounted to the end of arm 31. When clamped in place, a torsion spring operates to bias arm 30 in the direction shown by arrow Z. A stop (not shown) limits the amount of travel of arm 30 in this direction. The path of travel of the substrate is shown by a dotted line. The encoder wheel 4 is typically positioned opposite a feed roller 35 of the host machinery, such that the torsion spring biasses the wheel towards the substrate, ensuring good contact between the wheel and the substrate. The wheel 4 has a suitable frictional surface to ensure that there is no slipping of the substrate with respect to the roller.

The substrate may be driven by the host machinery, or it may simply be pulled at intervals by an operator, in which case its speed will vary very widely. The rotary encoder is used to ensure that printing takes place at desired intervals regardless of variations in the speed.

The types 11 may be made of metal or silicon. The invention may employ novel silicon types which incorporate metal particles for better heat retention and improved ink transfer.

Another advantage of the particularly described embodiment of the invention is that it provides a relatively large print "window" through the design of the print wheel 8.

A printing system constructed as described above has been found to be operable at speeds of up to 90 metres per minute providing up to 1100 prints per minute. The print wheel provides a printing area of up to 50mm x 50mm and with the fixed delay T set at zero, successive prints can be as little as 81mm apart (determined by the circumference of the print wheel 8).

The basic components of the control unit 2 are shown in Figure 4. The heart of the control system is the microprocessor 20 which is programmed to perform the control functions described above. The microprocessor receives signals from the sensors, indicated at block 21 and manually input signals to the control panel 22 via interface 23 to control motor drive 24 and heater drives 25. The other item in Figure 4 is an EEPROM 26 whose function will be described shortly.

The control unit also houses a power supply which provides power to all of the items -in Figure 4.

The microprocessor includes EPROM memory and RAM for data storage. A one time programmable part of the memory cannot be erased once programmed. The EEPROM 26 provides non-volatile storage of the delay and temperature settings when power is removed from the control unit. New data values are written to the EEPROM every time the temperature or delay settings are changed by the operator.

The current settings are read from the EEPROM as part of the system power-up initialisation routine. A special feature of the software detects the presence of a brand new device and loads it with default values for temperature and delay.

The processor is provided with a watchdog circuit, not shown, as a means of recovery in the event of a power hang-up caused, for example, by excessive power line noise or electromagnetic interference. If the processor crashes the watchdog circuit provides a reset pulse to the microprocessor so that it can continue operating.

The motor driver 24 contains various ICs and associated components to drive the stepper motor. Briefly summarised, pulses are supplied to the motor from the driver to advance the motor in increments or steps. The principles of stepper motors and their operation are well known and will not be described in detail herein.

The heater drives 25 supply power to the ink pad heater and print drum heater under control of the microprocessor.

The microprocessor operates in response to signals received from sensors indicated at 21. These include the ink temperature sensor, print drum position sensor, tachometer (rotary encoder) and proximity sensor (for intermittent mode).

All of the items of Figure 4 are housed in the control unit. The display interface may be on one circuit board and the rest on another located on the back panel with appropriate heat sinks.

Claims

CLAIMS:

1. Printing apparatus for use with a moving substrate comprising a print head, means for measuring the speed of travel of the substrate and means for controlling the operation of the print head according to the speed of travel of the substrate.

2. Printing apparatus as claimed in claim 1 wherein the means for measuring the speed of travel of the substrate comprise a rotatable member positioned in contact with the substrate in use so that movement of the substrate causes rotation of the rotatable member.

3. Printing apparatus as claimed in claim 2 in which the rotatable member is generally cylindrical and is positioned with its cylindrical surface in contact with the substrate whereby movement of the substrate in a direction tangential to the cylindrical surface will cause rotation of the rotatable member.

4. Printing apparatus according to claim 2 or 3 further comprising means associated with the rotatable member for producing an electrical signal every time the rotatable member advances by a predetermined angular distance, which signal is supplied to the control means.

5. Printing apparatus as claimed in any preceding claim further comprising means for setting a predetermined distance between successive print operations, the control means operating to vary the time delay between successive print operations whereby to maintain said predetermined distance.

6. Printing apparatus according to claim 5 in which said control means is operable to initiate said time delay only in response to an additional signal.

7. Printing apparatus as claimed in claim 6 further comprising means for detecting indicia on the substrate and for providing said additional signal to said control means in response to detection of indicia.

8. Printing apparatus as claimed in any preceding claim in which the operation of the print head is controlled using a stepper motor.

9. Printing apparatus as claimed in any preceding claim in which the print head includes a rotary print wheel.

10. Printing apparatus as claimed in any preceding claim in which the print wheel carries types incorporating metallic particles.

11. Printing apparatus as claimed in any preceding claim including an ink temperature sensor.

12. Printing apparatus as claimed in any preceding claim including a sensor for ensuring that the print head is correctly located.

13. A method of printing on a substrate which is travelling past a print head comprising measuring the speed of travel of the substrate and controlling the operation of the print head according to the speed of travel of the substrate.

14. A method as claimed in claim 13 comprising selecting a linear distance between successive print operations and varying the time delay between operations according to the speed of travel of the substrate.

15. A method as claimed in claim 14 wherein the time delay is initiated only in response to an additional signal.

16. A method as claimed in claim 14 wherein said additional signal is obtained from the detection of indicia provided on the substrate.

17. Printing apparatus substantially as hereinbefore described with reference to the accompanying drawings.

18. A method of printing substantially as hereinbefore described with reference to the accompanying drawings.