Patent Application
20140225970
This application claims the benefit of priority under 35 U.S.C. §119 of UK Patent Application No. 1302536.6, filed Feb. 13, 2013.
This invention relates to a tape drive, a method of operating such a tape drive and a printing apparatus including such a tape drive.
The invention is particularly useful in relation to a printing apparatus which utilises a printing tape or “ribbon” which includes a web carrying marking medium, e.g. ink, and a printhead which, in use, removes marking medium from selected areas of the web to transfer the marking medium to a substrate to form an image, such as a picture or text.
More particularly, but not exclusively, the invention relates to a so called thermal transfer overprinting apparatus in which the printhead includes a plurality of thermal heating elements which are selectively energisable by a controller during printing to warm and soften pixels of ink from the tape and to transfer such pixels to the substrate. The printhead presses the tape against the substrate such that the pixels of ink contact the substrate before the web of the tape is peeled away, thus transferring the pixels of ink from the tape to the substrate.
A thermal transfer overprinter is used to print on to a product's primary packaging and typically mounts within a packaging machine. The image to be printed is often a date code or other product information which needs to be applied to the product's packaging as close as possible to the time at which the product was packaged. The tape drive is used to move and position the thermal transfer tape.
In order to avoid wasting ink, whilst maintaining acceptable print quality, it is advantageous to be able accurately to control the movement of the tape, so as to position the next portion of tape to be used directly adjacent a portion of the tape from which the ink has previously been removed. It is desirable for a spacing between adjacent regions of tape from which pixels are removed to create an image, to be less than 1 mm.
It is also important to ensure that the regions of tape from which ink is removed during successive printing operations do not overlap, so that the printhead does not attempt to remove ink from a region of the tape from which the ink has already been removed. However, it is known to interlace images, such that a previously used region of tape is reused, but in the second and/or subsequent printing operations, different pixels of ink are removed from the tape to create an image.
It is known to provide thermal transfer printing apparatus in two different configurations. In the first, so called “intermittent” configuration, the substrate to be printed and the tape are held stationary during a printing operation, whilst the printhead is moved across the area of the substrate to be printed. Once the printing operation is complete, the printhead is lifted away from the tape, and the tape is advanced to present a fresh region of tape to the printhead for the next printing operation.
In the second, so called “continuous” configuration, the substrate to be printed moves substantially continuously and the tape is accelerated to match the speed of the tape before the printhead is brought into thermal contact with the tape and the printing operation is carried out. In this configuration, the printhead is maintained generally stationary during each printing operation.
In the case of a printing apparatus in continuous configuration, it is also necessary to accurately control the speed of the tape, to ensure that it matches the speed of the substrate. A typical thermal transfer printer operates with substrate that advances at linear speeds between approximately 0.01 metres per second and approximately 2 metres per second. Typical substrate accelerations are up to approximately 12 metres per second per second.
Printing apparatus of the kind described above includes drive apparatus for moving the tape relative to the printhead, to present fresh tape, from which pixels of ink are yet to be removed, to the printhead, such that successive printing operations can be carried out. It has long been known to provide tape drives which include two spool supports, one of which supports a supply spool on which unused tape is initially wound, and the other of which supports a take-up spool, onto which the tape is wound after it has been used. Tape extends between the spools in a tape path. Each of the spool supports, and hence each of the spools of tape, is drivable by a respective motor.
The tape used in thermal transfer printers is thin. Therefore it is important to ensure that the tension in the tape extending between the two spools is maintained at a suitable value or within a suitable range of tensions, in particular to enable the web to peel cleanly away from the heated ink. Too much tension in the tape is likely to lead to the tape being deformed or broken, whilst too little tension will inhibit the correct operation of the device. A slack tape is likely to affect print quality.
It is known to provide various types of tape drive which are compatible with thermal transfer overprinters. For example, it is known to provide a pair of stepper motors, each of which controls the movement of one of the spools so as to advance tape between the spools in a desired direction. The stepper motors are driven in a co-ordinated manner to transfer the tape from the supply spool to the take up spool and to accurately position the tape adjacent the printhead, whilst maintaining tension in the tape within an acceptable range. Various methods of determining and maintaining tension in the tape are known. Such methods require the tension in the tape to be measured, and for a correction to be applied in the event that the tension has strayed or is straying beyond a limit of the acceptable range. Therefore, such methods incur a delay of at least one printing operation between the tension in the tape falling outside the acceptable range and an appropriate correction being applied. It may even be the case that only a partial correction can be applied during the next printing operation, and thus the delay in correcting the tension in the tape such that it falls within the acceptable range is more than one printing operation.
It is also known to use a pair of DC motors to drive the tape spools (as described in FR 278459, for example). In such a system, both motors operate in a torque control mode to transfer the tape between the spools. A roller which is positioned near to the printhead is used to the determine movement of the tape along the tape path. Such a tape drive requires rollers on the inked side of the tape which can require regular maintenance. Furthermore, desired printing speeds and tape accelerations are increasing, leading to difficulties in successfully operating such a drive.
A motor control system of a tape drive including two brushless DC motors is described in the applicant's United Kingdom patent application number GB1113777.5.
In accordance with the present invention, there is provided a method of controlling tension in a tape, wherein the tape is transferable between a first spool and a second spool by a tape drive, the tape drive having a motor control system which includes two DC motors and a controller for controlling the operation of the motors, the tape drive also having two spool supports, each of which is suitable for supporting a spool of tape, and each of which is driven by a respective one of the motors, the method including only one tension setting step during a printing operation.
Maintaining the tension in the tape may not include separate steps of measuring tension and correcting tension.
Each of the motors may be operable in a first control mode and a second control mode, the method including, when the tape is substantially stationary, operating one motor in the first control mode whilst the other motor operates in the second control mode, to maintain tension in the tape.
The method may include switching the motor which was in the second control mode whilst the tape was stationary into the first control mode to transfer tape between spools.
Each printing operation may include a printing phase and a non-printing phase.
The tension may be set during a non-printing phase of the printing operation.
The tension may be set whilst the tape is stationary.
According to a second aspect of the invention, there is provided a tape drive for transferring tape between a first spool and a second spool, the tape drive having a motor control system which includes two DC motors, and a controller for controlling the operation of the motors, the tape drive also having two spool supports, each of which is suitable for supporting a spool of tape, and each of which is driven by a respective one of the motors, wherein the motor control system is operable in accordance with a method according to the first aspect of the invention.
Each of the motors may be operable in a first control mode and a second control mode.
The first control mode may be a position control mode.
The second control mode may be a torque control mode.
The controller may control operation of both of the motors such that each motor is switchable between the first control mode and the second control mode.
Each of the motors may have an associated sensor and each sensor may enable the controller to determine the position and velocity of a rotor of the respective motor.
The switch between the first control mode and the second control mode may be a smooth transition.
According to a third aspect of the invention, there is provided a printing apparatus including a tape drive according to the second aspect of the invention.
The printing apparatus may be a thermal transfer printer.
The invention will now be described, by way of example only, with reference to the accompanying drawings, of which:
Referring to
Each of the spool supports 12, 14 is independently drivable by a respective motor 16, 18. Each of the motors 16, 18 is a brushless DC motor. However, it will be understood that other functionally comparable motors could be used, for example Switched Reluctance motors (both with and without permanent magnets). The use of the terms “DC motor” and “brushless DC motor” herein is intended to include such functionally comparable motors.
Each of the spool supports 12, 14 is rotatable clockwise and anti-clockwise by means of its respective motor 16, 18. The movement of each motor 16, 18 is controlled and monitored by a controller 24 via a sensor 20, 22. The position of the controller 24 relative to the remainder of the printing apparatus 10 is irrelevant for the purposes of the present invention. The sensors 20, 22 typically are rotary encoders although it will be appreciated that other technologies are acceptable. The controller 24 is operable to control the mode of operation of each of the motors 16, 18 and the amount of drive provided by each of the motors 16, 18. Each sensor 20, 22 enables the controller 24 to determine the angular position and rotational speed of a rotor of the respective motor 16, 18.
The motors 16, 18, the sensors 20, 22 and the controller 24 all form part of a motor control system 25. The motor control system 25 allows the drive of each motor 16, 18 to be controlled such that each motor is switchable between a first control mode wherein position is a dominant control parameter and a second control mode where torque is the dominant control parameter. The first control mode will be referred to herein as “position control mode” and the second control mode will be referred to as “torque control mode”. In position control mode, the motor 16, 18 is driven to a demanded position and in torque control mode, the motor 16, 18 outputs a demanded torque. The control system 25 enables a user to adjust the proportion of torque control and the proportion of position control which is applied by each motor 16, 18. Each motor drive can be adjusted smoothly from fully position controlled to fully torque controlled and back again. Such a motor control system is described in the applicant's United Kingdom patent application number GB1113777.5, filed on 10 Aug. 2011 and in its U.S. patent application Ser. No. 13/237,802, filed on 20 Sep. 2011, the contents of which are incorporated herein by reference.
In more detail, the controller 24 receives inputs relating to a demanded position of each motor 16, 18 to advance the tape to a required position, the actual position of the motor 16, 18, the measured velocity of each motor 16, 18, the current drawn by the motor 16, 18, and a torque bias TB required by the motor 16, 18 at a given point in time. The purpose of the torque bias TB will be described in greater detail below.
In use, a supply spool 17, upon which unused tape is wound, is mounted on the spool support 14, and a take up spool 15, upon which used tape is wound, is mounted on the spool support 12. The tape generally advances in a tape path between the supply spool 17 towards the take up spool 15. The tape is guided in the tape path between the spools 15, 17 adjacent the printhead 19 by guide members 26.
The tape drive 11 requires calibration before printing operations can commence. Such calibration is generally required when the printing apparatus 10 is switched on, and when the spools of tape 15, 17 are replaced. The calibration process includes determining an initial estimate of the diameters of each of the spools of tape 15, 17 mounted on the spool supports 12, 14. An example of a suitable method of obtaining such an estimate is described in detail in the applicant's patent GB2310405. As tape passes from one spool to the other, for example from the supply spool 15 to the take up spool 17, it passes over a roller of known diameter. The roller is preferably one of the guide members 26. Tape is drawn from the supply spool 17, with the motor 16 which drives the take-up spool support 12 operating in position control mode. The motor 18 which drives the supply spool support 14 operates in torque control mode to deliver a predetermined torque.
During the calibration process, the current supplied to each of the motors 16, 18 is monitored and information relating to the current drawn by each motor 16, 18 is provided to the controller 24. The motor controller 24 monitors the current supplied to each motor 16, 18 via a respective current sensor 32, 34 connected between a driver of each motor 16, 18 and the motor 16, 18 itself.
Following the calibration process, the motor control system 25 maintains and updates values for the diameters of the spools 15, 17 by monitoring the amount of tape transferred from the supply spool to the take-up spool. The controller 25 takes into account the thickness of the tape to compute an expected change in the diameters of the spools 15, 17 over a period of time. This technique relies on the tension in the tape being kept substantially constant during printing operations and advancement of the tape between the spools 15, 17.
When the tape is at rest, the motor control system 25 maintains the desired tape tension by operating one motor, for example the supply spool motor 18, in position control mode. The other motor, for example the take up spool motor 16, is operated in torque control mode.
The motor 18 ensures that the absolute position of the tape relative to the printhead is accurately controlled, whilst the other motor 16 maintains the tension in the tape at the desired predetermined value.
In order to achieve this, a demanded position PD of the motor 18 is received by an S-curve generator 28, an output of which is used, along with an actual position PA of the motor 18 in an algorithm, preferably a PID algorithm, applied by an electronic filter 29 to determine the change in position required to be carried out by the motor 18. An actual velocity VA of the motor 18 is input to a second electronic filter 31, which performs an algorithm, again preferably a PID algorithm, and an output of the second electronic filter 31 is used in conjunction with an output of the first electronic filter 29, relating to the change in position of the motor 18, to determine a demanded torque TD to be provided by the motor 18. A demanded torque TD and the amount of current A drawn by the motor 18 are fed back to a torque controller 30 to provide a control output to the motor 18. Although the algorithms implemented by the filters 29, 31 are described as being PID algorithms, it will be appreciated that any Linear Time Invariant filter function may be used.
The motor 16 being operated in torque control mode does not use inputs relating to demanded position PD or actual position PA of the motor 16. The inputs relating to actual velocity VA may also be disregarded. The torque controller 30 receives a torque demand TD based only on the torque bias TB, and optionally upon the actual velocity VA of the motor 16. The current A of the motor 16 may also be fed back to the torque controller 30 to generate a control output for the motor 16. The intention of the torque bias TB is to apply a torque offset to the motor 18, which is in position control mode, to completely counteract the constant torque provided by the other motor 16, which is in torque control mode. This then means that the motor 18 in position control mode is only required to produce an instantaneous torque which will hold that motor in position, and does not need to compensate for the torque applied by the other motor 16. So if, for example, the motor 16 in torque control mode is applying 3N to the tape, the motor 18 in position control mode will have a torque bias TB applied to generate the equivalent of 3N to balance the tension in the tape.
When the tape is required to be advanced between the spools 15, 17, the controller 25 causes both of the motors 16, 18 to operate in position control mode. The transition of the motor 16, which was previously operated in torque control mode, into position control mode is smooth. This transition from torque control mode to position control mode is carried out by gradually reducing the torque bias TB to a nominal value, which may be zero.
During tape advance, the two motors 16, 18 advance the tape accurately along the tape path past the printhead 19, using the values of the diameters of the spools 15, 17 and a co-ordinated moving target position. The co-ordinated moving target position is arrived at by the control system 25 determining a desired position of the tape at a point in time, and the controller 24 controls the motors 16, 18 to achieve this desired position of the tape.
Once the advancement of the tape has been completed, one of the spool motors 16, 18, for example the take up spool motor 16, smoothly transitions from position control mode to torque control mode, whilst the other spool motor, for example the supply spool motor 18, remains in position control mode. Gradually increasing the torque bias TB from zero during deceleration of the tape causes a smooth transition of the motor 16 from position control mode to torque control mode, before the inputs relating to position PA, PD are disregarded. The other motor, in this case the supply spool motor 18, remains in position control mode, however the value of torque bias TB applied to this motor may be adjusted, so as to compensate for the increase in torque which is likely to be caused as a result of switching the take up spool motor 16 into torque control mode. In practice, it may be possible to retain a constant torque bias TB irrespective of whether the motors 16, 18 are stationary or in motion, however, the desired torque bias TB will be such that it causes the tension in the tape to remain substantially constant, by the two motors 16, 18 applying equal and opposite forces on the tape.
It is desirable, during tape advance, for the amount of tape fed into the tape path from the supply spool 17 to be equal to the amount of tape taken up by the take up spool 15, in order to maintain the tape tension substantially constant. However, this is difficult to achieve in known tape drives because disturbances of the tape which occur during printing operations and the fact that the spools 15, 17 are not perfectly cylindrical, mean that the control of the motors 16, 18 is based upon inaccurate estimates, and thus the tension is unlikely to be kept as near to constant as desired. In the present invention, the smooth transition of the take up motor 16 from position control mode to torque control mode prevents the accumulation of such errors increasing long term drift in the tape tension.
The motor control system 25 is capable of testing the accuracy of its control of the advancement of the tape in two ways.
The first method of testing is to determine the ratio of the torques applied to the two motors 16, 18 when the tape drive 11 is stationary. In such a situation, one motor 16, 18 is stationary, whilst the other motor 16, 18 supplies a torque so as to maintain its position, and to maintain the tension in the tape. The ratio of the torques should be the same as the ratio of the diameters of the spools 15, 17 at that time.
The second method of testing is carried out as the tape drive 11 is completing a movement of the tape. As the take up spool motor 16 transitions from position control mode to torque control mode, the controller 24 monitors the angular position change of take up spool motor 16 between its expected target position and its rest position at the correct ribbon tension, using the sensor 20. The angular position change that occurs together with the spool diameter gives a measure of the disturbances and errors in the position control of the motor 16.
The operation of the control system 25 is iterative, in that it takes into account the results of the testing method(s) carried out over a number of tape advancements (printing cycles) to correct the estimate of the diameters of the spools 15, 17 for future printing cycles.
The method of operation of the tape drive 11 described above retains the supply spool motor 18 in position control, as the supply spool 17 is more likely to be cylindrical than the take up spool, the tape on the supply spool 17 not having been unwound, and ink removed from it before being rewound on a different spool. Therefore this mode of operation is more likely to provide accurate positioning of the tape adjacent the printhead 19. However, it will be appreciated that either spool motor 16, 18 could be switched to torque control mode during tape advance.
During normal operation of the tape drive 11, the two motors 16, 18 effectively pull against one another to set and maintain tension in the tape which extends between the spools 15, 17. Acceptable tension in the tape of a thermal transfer overprinter using a typical 55 mm wide tape is generally between 2N and 8N and is preferably approximately 3N.
A printing operation typically includes a printing phase, during which ink is transferred from the tape to a substrate, and a non-printing phase, during which the tape is accurately positioned relative to the substrate and the printhead, such that the next printing phase is carried out using a desired portion of tape, so that use of the ink on the tape is optimised. In a printer which is operated in continuous mode, it is necessary for the tape to be accelerated up to the same speed as the substrate, during the non-printing phase, to ensure optimum print quality. The tape is then moved at a substantially constant speed during the printing phase, which is substantially the same speed as that at which the substrate is moving.
Following the printing phase, a second non-printing phase takes place, during which the tape is decelerated, and then reversed, to position the next portion of tape from which ink is to be removed adjacent the printhead. In actual fact, the tape drive allows the tape to ‘overshoot’ the desired position, in a controlled manner. The tape is then moved in the same direction as the substrate and is accelerated up to the speed of the substrate, during which time the tape moves a distance which is equivalent to the ‘overshoot’ distance, so that the tape is accurately positioned for the next printing phase to begin.
As shown in
As mentioned above, it is important for the tension in the tape to be maintained within an acceptable tolerance at all times, during both printing and non-printing phases.
An advantage of this system is that each printing operation includes only a single tension setting step. The tape tension is set only once during each printing operation, between printing phases, which eliminates the standard delay of one or more printing operations to correct the tension in the tape, once it has strayed beyond an acceptable limit. Thus, the system is more reliable, since the position of the tape is more accurately controlled. Furthermore, the problems associated with tape tension straying outside an acceptable range are reduced or eliminated, since the tension in the tape is more accurately controlled.
When power is removed from the motors 16, 18, the control system 25 manages the tension of the tape in the tape path. If the tape is in tension when power is removed from the motors 16, 18, one or both of the spools 15, 17 will be accelerated by the force exerted by the tension in the tape. Even when the tape is no longer in tension, the or each spool 15, 17 which has been accelerated will continue to rotate owing to the momentum of the spool(s) 15, 17, and tape may spill from the printing apparatus 10. Of course, this is undesirable, and unacceptable. To overcome this problem, the control system 25 operates at least one of the motors 16, 18, so as to enable a controlled release of tension from the tape, before power is removed from the motors 16, 18. Alternatively, a mechanical device may be used to inhibit or prevent the acceleration of the spools 15, 17 upon removal of power from the motors 16, 18.
Whilst the invention has been described in relation to thermal printing apparatus, it will be appreciated that the motor control system may be utilised in relation to other devices or apparatus.
When used in this specification and claims, the terms “comprises” and “comprising” and variations thereof mean that the specified features, steps or integers are included. The terms are not to be interpreted to exclude the presence of other features, steps or components.
The features disclosed in the foregoing description, or the following claims, or the accompanying drawings, expressed in their specific forms or in terms of a means for performing the disclosed function, or a method or process for attaining the disclosed result, as appropriate, may, separately, or in any combination of such features, be utilised for realising the invention in diverse forms thereof.
| Number | Date | Country | Kind |
|---|---|---|---|
| 1302536.6 | Feb 2013 | GB | national |
