Abstract:
A motor control system includes a motor having an associated rotary position encoder, and a controller for controlling the operation of the motor, wherein the motor is switchable between a position control mode and a torque control mode.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application claims the benefit of priority (under 35 USC 119) of UK Patent Application No. 1113777.5, filed 10 Aug. 2011. 
     BACKGROUND 
     This invention relates to a motor control system, a method of operating a motor control system, a tape drive including a motor control system, a method of operating such a tape drive and a printing apparatus including such a tape drive. 
     Such printing apparatus 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. 
     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. 
     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. In the case of a printing apparatus in continuous configuration, it is also preferable 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 a substrate that advances at linear speeds between approximately 0.01 meter per second and approximately 2 meters per second. Typical substrate accelerations are up to approximately 12 meters per second per second. 
     Tape drives of various types have been proposed, for example a tape drive which includes a stepper motor for driving a take up spool so as to pull tape through along a tape path between a supply spool and the take up spool. Such a tape drive also includes a mechanical clutch for setting and maintaining the tension in the tape. Such tape drives are often mechanically complex and regular maintenance of the clutch is typically required. Furthermore, since the supply spool is operated at a fixed torque, the tension in the tape varies as the diameter of the supply spool varies over time. 
     Another example of a known tape drive is one in which a take up spool and a supply spool are rotated by respective stepper motors. 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 the tension in the tape. Various methods of determining and maintaining the tension of the tape have been proposed. Such methods typically require the measured tension in the tape to be compared with the desired tension, and for a correction to be applied. Therefore, such methods often incur a delay of at least one printing operation between the tension in the tape falling outside an acceptable range and the correction being applied. 
     A further example of a known tape drive includes a pressure roller in the tape path, which is driven by a motor. The roller directly controls the speed and position of the tape. The tape spools are driven through a mechanical clutch which maintains the tape tension between acceptable limits. Such tape drives are often mechanically complex. The tape drive is typically uni-directional and this tends to cause tape wastage. 
     A still further example of a known tape drive is one in which two DC motors are used to drive the spools of tape (as described in FR 2783459, for example). Both of the motors operate in torque control mode, and a roller which is positioned near to the printhead is used to determine the movement of the tape along the tape path. Such a tape drive includes 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 operating such a drive. 
     SUMMARY 
     This invention relates to a motor control system, a method of operating a motor control system, a tape drive including a motor control system, a method of operating such a tape drive and a printing apparatus including such a tape drive. 
     The invention can be 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 printing 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. 
     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. 
     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 better 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 the same region of the tape more than once. 
     In accordance with the present invention, there is provided a motor control system including a motor having an associated rotary position encoder, and a controller for controlling the operation of the motor, wherein the motor is switchable between a first control mode wherein position is a dominant control parameter to a second control mode where torque is the dominant control parameter. The motor may be a brushless DC motor or other functionally comparable motor. This invention has been developed using brushless DC motors. These motors are known by other names, for example, AC servo motors. The invention is also applicable to motors known as Switched Reluctance motors (both with and without permanent magnets). These motors are all controlled by the use of a software controlled system which generates a rotating magnetic field, and as such are functionally comparable with one another. 
     A measurement of the velocity of the motor may be fed back to the controller and used to determine an output of the controller which is received by the motor to control the movement of the motor. When the motor is in the first control mode, the controller may receive an input relating to a demanded position of the motor and an actual position of the motor, and may determine a change in position which is required to be carried out by the motor. In addition, the controller may use the change in position, the velocity of the motor and a torque bias value, to determine the output of the controller which controls the movement of the motor. 
     When the motor is in the second control mode, the controller may receive an input relating to a torque bias value which is used to determine an output of the controller which controls movement of the motor. The controller may receive an input relating to the velocity of the motor which is used in conjunction with the torque bias value to determine the output of the controller which controls movement of the motor. 
     The motor control system may include a pair of motors, each having an associated sensor and the controller controlling operation of both of the motors such that at least one is switchable between the first control mode and the second control mode. Each of the motors may be a brushless DC motor or other functionally comparable motor. Each sensor may enable the controller to determine the position and velocity of a rotor of the respective motor. Moreover, switching between the first control mode and the second control mode may be a smooth transition. 
     According to a second aspect of the invention, there is provided a method of operating a motor control system according to the first aspect of the invention, wherein the method may include providing an input to the controller relating to a torque bias, to determine the motor torque developed by the motor. The method may include using a user input to adjust the ratio of each control mode of the motor. 
     The method may include testing an accuracy of the control of the motor. The control system may be used to control a pair of motors, and the method may include determining a ratio of torques applied to the motors. The method may include determining a number of steps moved by the motor as it moves between a target position and a rest position. 
     According to a third aspect of the invention, there is provided a tape drive including a pair of tape spool supports, upon one of which a supply spool is mountable and upon a second one of which a take up spool is mountable, each tape spool support being drivable by a respective motor which has an associated sensor, the tape drive further including a controller to control each of the motors, wherein at least one of the motors is switchable between a first control mode wherein position is a dominant control parameter and a second control mode wherein torque is the dominant control parameter. Both of the motors may be switchable between the first control mode and the second control mode. Both motors may be drivable in the first control mode during movement of tape between the tape spool supports, and wherein at least one of the motors is switchable from the first control mode to the second control mode when the movement of the tape has been completed, and from the second control mode to the first control mode when tape movement is to be carried out. Moreover, a transition of the control mode of the motor between the first control mode and the second control mode may be smooth. 
     According to a fourth aspect of the invention, there is provided a method of operating a tape drive according to the third aspect of the invention, the method including maintaining tension in tape extending between the two spools, when the tape is substantially stationary, by operating one motor in the first control mode whilst operating the other motor in the second control mode. The method may include switching the motor which was in the second control mode whilst the tape was stationary into the first control mode in order to transfer tape between the spools. 
     The method may include determining estimated values of diameters of each of the spools and updating the estimated values of the diameters during use of the tape drive. The method may include testing an accuracy of the control of the motors by determining a ratio of torques applied to the motors and comparing the ratio of the torques with a ratio of estimated diameters of the two spools. The method may include testing an accuracy of the control of the motors by monitoring a number of steps taken by a motor between a target position and a rest position. The method may include driving the motors so as to release tension from tape extending between the spools before power is removed from the motors. 
     According to a fifth aspect of the invention, there is provided printing apparatus including a tape drive according to the third aspect of the invention. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Embodiments of the invention will now be described, by way of example only, with reference to the accompanying drawings, in which: 
         FIG. 1  is an illustrative view of part of a thermal printing apparatus including a motor control system according to the present invention; and 
         FIG. 2  is an illustrative view of a feedback circuit of the motor control system. 
     
    
    
     DETAILED DESCRIPTION 
     Referring to  FIG. 1 , there is shown a part of a printing apparatus  10 . The printing apparatus  10  includes a tape drive shown generally at  11 . The printing apparatus includes a housing  13 , in or on which is mounted a first spool support  12  and a second spool support  14 , which form part of the tape drive  11 . A spool of tape  15 ,  17 , for example inked printer ribbon, is mountable on each of the supports  12 ,  14 . The spool supports  12 ,  14  are spaced laterally from one another. The printing apparatus  10  also includes a printhead  19  for transferring ink from the tape to a substrate  21  which is entrained around a roller  23  adjacent the printhead  19 . Depending upon the configuration of the printer, the substrate  21  may be positioned adjacent the printhead  19  on a platen, rather than a roller. 
     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. Each of the spool supports  12 ,  14  is rotatable clockwise and anti-clockwise by means of its respective motor  16 ,  18 . Each motor  16 ,  18  is electrically connected to a controller  24  via a sensor  20 ,  22 . This sensor is typically a rotary encoder although it will be appreciated that other technologies are perfectly 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 each respective motor  16 ,  18 . Information relating to the current drawn by each motor  16 ,  18  is provided to the controller  24 . The motors  16 ,  18 , the sensors  20 ,  22  and the controller  24  all form part of a motor control system  25 . 
     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 T B  required by the motor at a given point in time. The purpose of the torque bias will be explained in more detail below. The position of the controller  24  relative to the remainder of the printing apparatus  10  is irrelevant for the purposes of the present invention. 
     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  should be calibrated before printing operations 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&#39;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. 
     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  24  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 a first control mode, in which position is a dominant control parameter. This first control mode will be referred to herein as “position control mode”. The other motor, for example the take up spool motor  16 , is operated in a second control mode, in which the dominant control parameter is torque. The second control mode will be referred to herein as “torque control mode”. 
     Therefore the tape drive  13  operates in a similar fashion to one in which one of the motors  16 ,  18  is a stepper motor and the other motor  16 ,  18  is a DC motor. One 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. 
     A demanded position P D  of the motor  18  is received by an S-curve generator  28 , an output of which is used, along with an actual position P A  of the motor  18  in an algorithm, preferably a PID (Proportional-Integral-Derivative) 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 V A  of the motor 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 T D  to be provided by the motor  18 . A demanded torque T D  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 P D  or actual position P A  of the motor  16 . The inputs relating to actual velocity V A  may also be disregarded. The torque controller  30  receives a torque demand T D  based only on the torque bias T B , and optionally upon the actual velocity V A  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 , such as the BLDC (Brushless Direct Current) motor shown in  FIG. 2 . 
     When the tape is required to be advanced between the spools  15 ,  17 , the controller  24  causes both of the motors  16 ,  18  to operate in position control mode. The transition of the motor  16 ,  18  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 T B  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 the 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. 
     During tape advance, it is desirable 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 from position control mode to torque control mode prevents the accumulation of such errors increasing long term drift in the ribbon tension. 
     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, by increasing the torque bias T B  relating to the motor  16 , whilst the other spool motor, for example the supply spool motor  18 , remains in position control mode. Gradually increasing the torque bias T B  from zero during deceleration of the tape causes a smooth transition of the motor from position control mode to torque control mode, before the inputs relating to position P A , P D  are disregarded. The other motor, in this case the supply spool motor  18 , remains in position control mode, however the value of torque bias T B  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 T B  irrespective of whether the motors  16 ,  18  are stationary or in motion, however, the desired torque bias T B  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. 
     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. 
     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, 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.