Abstract:
A method of controlling a DC motor is presented. In a tracking action, an input to the motor is tracked. In a comparing action, the input is compared against a baseline value. In an adjusting action, a usage of the motor is adjusted when the input passes a threshold related to a motor performance parameter. A method of estimating a parameter of an ink carriage moved by a DC motor is also presented. In a tracking step, an input to the motor is tracked. In a comparing action, the input is compared against a baseline value to predict a level of the parameter.

Description:
Direct current (DC) motors are able to produce torque because a current-carrying conductor located in a magnetic field experiences a force proportional to the magnitude of the flux, the current, the length of the conductor, and the sine of the angle between the conductor and the direction of the flux. When the conductor is a fixed distance (radius) from an axis about which it can rotate, a torque is produced that is proportional to the product of the force and the radius. In a DC motor, the resultant torque is the sum of the torques produced by each conductor. Each of the conductors are known as windings, and it is important for the windings to be insulated from each other so that the current flowing through them will not short circuit from one winding to another. 
     DC motors should be designed so that they will not overheat during usage. If the windings on a DC motor reach a temperature at which the protective coating, or insulation on the conductors melts, then the motor may short-circuit and fail. In addition to ambient conditions, things which affect the temperature of a DC motor can include the design and size of the DC motor, the magnitude of the load which the DC motor is coupled to, or even changes to the efficiency of the DC motor over time. If the temperature of a DC motor cannot be measured while the DC motor is in use, then the DC motor must be designed or selected robust enough to handle the worst-case loads it can possibly see over the expected lifetime of the DC motor. Often, this means a relatively large DC motor must be selected. Using such a large DC motor may add significant cost to a product containing the DC motor. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 schematically illustrates one embodiment of an imaging device with a DC motor. 
     FIG. 2 illustrates one embodiment of pulse-width-modulation (PWM) curves and corresponding load velocity curves over time. 
     FIGS. 3,  4 A, and  4 B illustrate embodiments of actions which may adjust usage of a DC motor based on an input to the DC motor. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIG. 1 schematically illustrates one embodiment of an imaging device  20  having at least one DC motor  22 . A variety of imaging devices are commercially available. For instance, some of the imaging devices that may embody the concepts described herein include desk top printers, portable printing units, wide-format printers, hybrid electrophotographic-inkjet printers, copiers, cameras, video printers, and facsimile machines, to name a few. The concepts introduced herein need not be limited to the environment of an imaging device, and can be applied to other devices having DC motors. However, for convenience the concepts introduced herein are described in the environment of an imaging device  20 . 
     The DC motor  22  is coupled to a load  24 . Since the embodiment of FIG. 1 is an imaging device  20 , the load  24  could be an imaging load, such as a photoreceptor belt, a toner sump, or a fuser roller, for example. The load  24  could be a carriage designed to hold ink cartridges for movement back and forth across a printzone while imaging. The load  24  could be a paper-path which transports an imaging media throughout the imaging device  20 . The load  24  could also be some type of imaging service station, such as a capping and wiping system in an inkjet device, or an electrostatic brush cleaner in an electrophotographic system. The load  24  can be anything driven, moved, or activated by the torque produced from the DC motor  22 . The coupling of the load  24  to the motor  22  may be accomplished directly, or by way of linking elements such as gears, pulleys, clutches, belts, friction rollers, or any combination thereof. Such linking elements are well known to those skilled in the art. 
     The motor  22  may be coupled to a controller  26 . The controller  26  may be a computer, a microprocessor, an application specific integrated circuit (ASIC), digital components, analog components, or any combination thereof. The controller  26  provides an input  28  to the DC motor  22 . A DC motor can have at least two possible inputs. In one instance, the magnetic flux may be varied, and in another instance, the current in the windings, or armature current may be varied. Thus, there are two common modes of operation for a DC motor: 1) armature control, where an adjustable voltage or current is applied to the armature or windings while the magnetic flux is held constant. In this case, the magnetic flux may be held constant by maintaining a constant magnetic field current or by using a fixed magnet. 2) Field control, where an adjustable voltage or current is applied to create a magnetic field which may be varied, while the armature current is held constant. 
     Regardless of what mode of DC motor operation is selected, a technique referred to as pulse width modulation (PWM) may be employed to vary the effective voltage or current seen by the armature or by the field. PWM controls the motor  22  with short pulses of voltage or current. These pulses are varied in duration to change the speed of the motor. The longer the pulses, the more torque the motor can produce and visa versa. For example, if the output of a DC voltage source is twelve volts, and the PWM signal is on 25%, 50%, or 75% of the time, the motor would receive the approximate average of three, six, and nine volts, respectively. 
     PWM is a way of digitally encoding analog signal levels. Through the use of high-resolution counters, the duty-cycle of a square wave is modulated to encode a specific analog signal level. The PWM signal is still digital, however, because at any given instant in time, the full DC supply is either fully on or fully off. Most motors require high PWM frequencies. The PWM period should be short relative to the motor&#39;s response time to a change in the switched signal state. Common PWM frequencies may range from 1 kHz to 200 kHz, for example. The duty cycle is the ratio of the on-time in a given period to the period. 
     For the sake of explanation, the following discussions will refer to the motor  22  in FIG. 1 as being controlled in armature control mode, with a fixed magnetic flux, using PWM from the controller  26  as the input  28  to the motor  22 &#39;s windings. It should be understood that PWM could be applied to the magnetic field current instead. 
     A position or time derivative sensor  30 , such as an encoder, may be coupled between the motor  22  and the controller  26 . The position or time derivative sensor  30  provides positional, velocity, or acceleration feedback  32  to the controller  26 . Based on the feedback  32 , the controller  26  may adjust the input  28  to the motor  22 , in this case adjust the PWM to the motor  22 , in order to result in a desired positional move, motor speed, or motor acceleration. 
     Various factors may affect the DC motor&#39;s  22  ability to respond to the motor input  28  in order to move the load  24 . An increase in temperature  34 , which is seen in the motor windings, can cause the resistance to increase, since copper has a positive temperature coefficient with respect to resistance. Also, magnets have a negative temperature coefficient with respect to flux, so the magnetic field will become weaker as the temperature increases. As a result, the change in temperature  34  can affect the motor&#39;s efficiency. Any change in the motor&#39;s efficiency  36  can change the DC motor&#39;s  22  response to the controller&#39;s input  28  with regard to the motor&#39;s ability to move or actuate the load  24 . The load  24  may also change  38 . For example, if the load  24  is a carriage which holds ink cartridges, as the ink cartridges are emptied through printing, the load  24  will have less mass, and therefore will reduce the load. On the other hand, the load  24  may increase, due to increased friction from aging or dirty parts. There are any of a number of reasons why there could be a change in load  38 , but a change in load  38  may also affect the ability of the DC motor  22  to respond to the motor input  28  in order to move the load. 
     FIG. 2 illustrates one embodiment of pulse-width-modulation (PWM) curves and corresponding load velocity curves over time. The embodiment of FIG. 2 is for a carriage being moved by a DC motor  22 . The carriage is for the transport of one or more ink cartridges. The carriage velocity  40  is shown on the left vertical axis. Time  42  is depicted along the horizontal axis. The carriage velocity  40  is measured by the controller  26  through the use of feedback  32  taken from a position or time derivative sensor  30 . Cold velocity curve  44  illustrates a velocity achieved versus time when the motor  22  is relatively cold. Based on the feedback  32  from the position or time derivative sensor  30 , the controller  26  adjusts the motor input  28 , in this case adjusts the PWM, to achieve the desired carriage velocity  40  at a given moment in time  42 . The PWM  46  is shown on the right vertical axis. Cold PWM curve  48  illustrates the PWM  46  values over time  42  used to result in the cold velocity curve  44  in this embodiment. At some point, the load could change  38 , for example, the load  24  may have more frictional resistance due to age or use. The motor efficiency could change  38 , for example, the bearings inside the motor could become aged or dirty. The temperature in the motor can also change  34 , simply through extended use of the motor. When the temperature of the windings increases, the resistance of the windings also increases. Thus, a change in load, efficiency, or temperature may require a larger or smaller input from the controller to maintain the same velocity. Warm PWM curve  50  illustrates the PWM  46  values over time  42  used to result in a warm velocity curve  52  in this embodiment. The start of the cold PWM curve  48  and the start of the warm PWM curve  50  are aligned in time  42  so that the two curves can be compared. Similarly, the start of the cold velocity curve  44  and the start of the warm velocity curve  52  are aligned in time  42  so that those two curves can be compared. As can be seen for this embodiment, in order to achieve a similar velocity under warmer conditions, the warmer PWM  50  must be significantly higher than the colder PWM  48 . In this embodiment, the difference between the warm PWM curve  50  and the cold PWM curve  48  is greatest during an acceleration phase  54  of the velocity curves  44 ,  52 . A substantially constant velocity phase  56  can also be seen on both the warm PWM curve  50  and the cold PWM curve  48 . In this embodiment, the difference between the warm PWM curve  50  and the cold PWM curve  48 , during the substantially constant velocity phase  56  is relatively small. This is indicative that the embodied system is an inertia dominated system. On the other hand, there are also systems which can be velocity or friction dominated. In these systems, the difference between the warm PWM curve  50  and the cold PWM curve  48  might be larger than illustrated for the embodiment of FIG. 2 during the substantially constant velocity phase  56 . 
     The position or time derivative sensor  30  allows the controller to adjust the DC motor input  28 , in this case, the PWM, so that a desired move, velocity, or acceleration is obtained by the load  24  coupled to the motor. Depending on the load  24  itself, or changes in temperature  34 , changes in motor efficiency  36 , and/or changes in load  38 , the controller  26  may be in a position where it has to increase the DC motor input  28  to the point where the increased input to compensate for changes in load, temperature, or efficiency can cause the temperature of the motor windings to increase to the point where the insulation on the motor windings melts, and one or more of the windings short circuit. As a result of the short circuit, the motor  22  can get weak, or stop, or stall. To avoid this situation, a large motor is often selected to take into account the worst case loads, temperatures, and changes in efficiency that the motor could be expected to see. Along with the margin against motor failure that a larger motor provides, it is often more expensive and the additional size is often undesirable when compared to a smaller motor. 
     FIG. 3 illustrates one embodiment of actions which may adjust the usage of a DC motor based on an input to the DC motor. The input to the motor is tracked  58 , and compared  60  against a baseline value. In comparing  60  against a baseline value, the controller determines  62  whether or not the tracked input passes a threshold related to a motor performance parameter. Examples of motor performance parameters include motor winding temperature, efficiency of the motor, age of the motor, and characteristics of the load coupled to the motor. Characteristics of the load may include mass of the load and friction or mechanical resistance of the load. In an imaging device, the characteristics of the load could be a quantity of ink cartridges being carried by a carriage moved by the DC motor. A characteristic of the load could also be an amount of ink in an ink cartridge. If there are fewer cartridges or less ink, then the motor usage may be adjusted by increasing the input to the motor as compared to the input levels when there are more cartridges or more ink because the motor would be less likely to overheat with a smaller load. If a threshold is not passed, the controller continues to track  58  input to the motor. If a threshold is passed  66 , then input to the motor is adjusted  68 . Thus, the controller  26  is able to control the DC motor  22  not only with regard to position, velocity, and/or acceleration, but also to indirectly monitor motor performance parameters such as temperature, age of the motor, motor efficiency, mass of the load (related to number of ink cartridges and amount of ink) and adjust the usage of the motor (increase or decrease) when necessary without needing a temperature sensor or other sensor. 
     FIGS. 4A and 4B illustrate one embodiment of actions which may adjust the usage of a DC motor based on a previous input to the DC motor in the environment of an imaging device  20 . The imaging device  20  has a carriage coupled to the DC motor for carrying ink cartridges. The actions illustrated in FIGS. 4A and 4B start with an arbitrary starting point  70 . The controller  26  decides  72  whether or not a motor move is desired. If no move is desired  74 , the controller determines  76  whether there have been any motor moves within the last forty-five minutes. If there have been no moves within the last forty-five minutes  78 , the motor state is set  80  to “ambient”. While forty-five minutes may be appropriate in this embodiment, other time periods may be determined to be appropriate for other embodiments. For example, if a motor is known to be at an ambient temperature after a shorter or a longer time, then that time period may be used instead. The controller  26  then decides  72  again whether or not a motor move is desired. If a motor move is desired the PWM is summed  84  during an acceleration phase of a carriage move. In a velocity or friction dominated system, it may be more desirable to sum the PWM after the acceleration phase. Other mathematical values based on the PWM could be used instead of the sum, for example, an average PWM value, or a peak PWM value could be used. For simplicity, however, only a sum of PWM values  84  during the acceleration phase of a carriage move is described. The controller determines  86  whether or not the motor state is ambient. The motor state in this case refers to temperature of the motor. If the motor state is ambient  88 , the PWM sum is stored  90  as a baseline value, and the controller  26  decides  72  again whether or not a motor move is desired. 
     After determining  86  whether the motor state is ambient, if the motor state is not ambient  92 , the PWM sum is compared  94  to a baseline value. The controller  26  determines  96  whether the PWM sum is greater than a “hot” percentage of the baseline value. For example, it may be desirable to set the hot percentage at 115% of the baseline value. Other hot percentages can be determined empirically based on the size of the motor being used and the load it is driving. If the PWM sum is greater  98  than the hot percentage of the baseline value the motor state is set  100  to “hot”, the carriage motor speed is limited  102 , and delays are inserted  104  between carriage moves. In other embodiments, it may be desirable to only limit the carriage speed  102 , or only insert delays between moves  104  , rather than do both. Adjusting motor speed and inserting or removing delays between motor moves are examples of ways to adjust the motor usage. If the PWM is being tracked for a motor performance parameter like mass of the load, then it could be desirable to adjust the motor usage by increasing the motor speed or removing delays. In the case of increased temperature, however, it may instead be desirable to limit the carriage speed  102  and/or insert delays between moves  104 . The carriage speed may be limited by decreasing the PWM values sent to the motor, or placing a maximum limit on the PWM values. By limiting the speed and/or placing delays between moves, the motor can be operated safely within a temperature range which will not allow the winding insulation to melt and the windings to short circuit. After limiting the carriage motor speed  102  and inserting delays between motor moves  104 , the controller can again determine  72  whether a motor move is desired. 
     If a motor move is not desired  74 , and if there have been motor moves within the last forty-five minutes  106 , the controller determines  108  whether there have been any motor moves within the last fifteen minutes. If there have not been any motor moves in the last fifteen minutes  110 , then the motor state is set  112  to “warm”. On the other hand, if there have been motor moves in the last fifteen minutes  114 , the motor state remains unchanged  116 . The controller can then again determine  72  whether a motor move is desired. In this embodiment, fifteen minutes is used as part of the decision point  108  to determine whether to set the motor state to warm  112 , or leave it unchanged  116 . Other time periods greater than or less than fifteen minutes may be used. It is recommended that the time period in the comparison of block  108  be less than the time period used in the preceding comparison of block  76 . This is because it should take a longer time to reach “ambient” than it takes to reach “warm” while the motor is cooling. 
     Referring back to the decision block  96 , if the PWM sum is not greater  118  than a predetermined hot percentage, then the controller examines  120  whether the motor state is “hot”. If the motor state is not hot  122 , then the controller can again determine  72  if a motor move is desired. If the motor state is hot  124 , the controller  26  determines  126  whether the PWM sum is greater than a “warm” percentage of the baseline value. For example, it may be desirable to set the warm percentage at 108% of the baseline value. Other warm percentages can be used, but the warm percentage should be less than the hot percentage. If the PWM sum is greater than  128  the warm percentage of the baseline value, nothing happens, the motor state remains “hot”, and the controller can again determine  72  if a motor move is desired. If the PWM sum is not greater than  130  the warm percentage of the baseline value, then the motor state is set to warm  132 , the carriage motor speed is no longer limited  134 , and the delays between carriage moves are removed  136 . 
     In discussing various embodiments of DC motor control methods, various benefits have been noted above. It is apparent that a variety of other functionally equivalent modifications and substitutions may be made to implement an embodiment of DC motor control according to the concepts covered herein, depending upon the particular implementation, while still falling within the scope of the claims below.