Abstract:
A mail piece handler defines a paper path having a downstream direction. A first motor is mechanically coupled with the mail piece handler, and an encoder is operatively coupled with the mail piece handler. A mail piece sensor is positioned at the paper path, and a print rotor is positioned at the paper path downstream of the mail piece sensor, the print rotor having a print surface. A second motor is mechanically coupled with the print rotor, and an electronic controller is communicatively coupled with the encoder, with the mail piece sensor, and with the second motor, the controller disposed in the event of sensing of a mail piece by the mail piece sensor to control said second motor so as to cause the print surface of the print rotor to move at substantially a speed measured via the encoder.

Description:
This application claims priority from U.S. provisional application No. 60/098,584 filed Aug. 31, 1998, which is incorporated herein by reference. 
    
    
     The invention relates generally to paper and document transport, and relates particularly to bases for postage meters (franking machines). 
     BACKGROUND 
     A typical prior art postage meter (franking machine) system has a powerful AC motor which drives all of the moving parts of the meter when a letter or other mail piece enters the paper path. The postage meter is, by definition, prepared at all times to be able to rotate so as to print postage. If it is not ready to print postage, it blocks actuation of the AC motor. 
     Generally the system includes a base which contains the motor or motors and contains the lower portion of the paper path, for example a number of rollers or belts (or both) and provides a level and smooth surface defining part of the paper path. The rollers or belts protrude slightly above the level, smooth surface. 
     Above the base, and removably mounted to the base, is the postage meter proper. The postage meter typically has a secure housing within which are found accounting registers indicative of the postage value available for printing, and a print mechanism such as a print rotor. The meter typically provides additional rollers which, together with rollers in the base, define part of the paper path. 
     A mechanical linkage such as a gear engagement links the upper part of the paper path (within the postage meter) with the lower part of the paper path (within the base) and with the print mechanism such as the print rotor. 
     It should be appreciated that while some of the upper rollers or belts that help to define the paper path may be within the meter, others of the upper rollers or belts may be provided by part of the base. Thus, as a mail piece progresses along a level paper path, it may initially have the base both above it and below it, while later it may have the meter above it and the base below it. 
     In such prior-art systems, as mentioned above, it is commonplace to link the moving parts so that they are all powered by a single large AC motor. A single-revolution clutch may be provided which is actuated once for each time that a mail piece arrives at the postage meter. For example, there may be a trigger in the paper path just prior to the print rotor, and when the trigger is actuated then the single-revolution clutch causes the print rotor to rotate once. 
     Such a system has the advantages of simplicity and long-standing use in the field. But the AC motor is heavy and consumes a lot of power, and is actually rather crude in terms of speed control. Typically the entire system is forced to start at rest, accelerate to high speed, and then decelerate back to rest. The cycle is repeated hundreds of thousands or millions of times, and the extreme nature of the cycle requires the drive train to be quite strong and heavy. The cycle is also noisy. The AC motor is typically heavy and bulky. 
     Another prior art approach for a postage meter is that described, for example, in U.S. Pat. No. 4,631,681 or 4,774,446. In such a system, two motors are employed, the first an AC motor that actuates the paper path, and a second motor which is a DC motor having encoder feedback, coupled with the print rotor, driven with pulse-width modulation to match the velocity of the paper path driven by the AC motor, as measured with two document sensors in the paper path as driven by the AC motor. Such a system offers some advantages over the prior art, but at the expense of requiring more than one document sensor in the paper path. Such a system also has the possible drawback that it may in fact fail to measure the velocity accurately and may drive the print rotor at a speed that is too fast or too slow relative to the speed within the AC-driven paper path. 
     Still another prior art approach is that described in U.S. Pat. No. 4,933,616, in which a microcomputer provides control of the speed of first and second motors, a first motor connected with a sheet transport system and a second motor connected with a print drum assembly. Such a system has the drawback that both of the motors must be of a type that can have closely controlled speed, and each motor must have driving circuitry associated with the speed control capability. This adds to the cost of the system and to its weight and complexity. 
     It is desirable to have a postage meter system that overcomes the drawbacks of the prior art, in which the print rotor is driven in faithful correspondence with the paper path, and in which the operation is quieter, smoother, less expensive, and less bulky than in the prior art. 
     SUMMARY OF THE INVENTION 
     A mail piece handler defines a paper path having a downstream direction. A first motor is mechanically coupled with the mail piece handler, and an encoder is operatively coupled with the mail piece handler. A mail piece sensor is positioned at the paper path, and a print rotor is positioned at the paper path downstream of the mail piece sensor, the print rotor having a print surface. A second motor is mechanically coupled with the print rotor, and an electronic controller is communicatively coupled with the encoder, with the mail piece sensor, and with the second motor, the controller disposed in the event of sensing of a mail piece by the mail piece sensor to control said second motor so as to cause the print surface of the print rotor to move at substantially a speed measured via the encoder. 
    
    
     DESCRIPTION OF THE DRAWING 
     FIG. 1 shows a prior-art system; 
     FIG. 2 shows another prior-art system; and 
     FIG. 3 shows an embodiment of the invention. 
    
    
     DETAILED DESCRIPTION 
     FIG. 1 shows a prior-art system. In FIG. 1, an AC motor  38  is mechanically coupled to a gear  34  and to roller  35  and optionally to roller  36 , all contained within base  32 . Follower  37  together with roller  35  define an intake region for the paper path, generally horizontal and to the right in FIG.  1 . Postage meter or franking machine  31  is normally mounted on base  32  although it may be removed for example for inspection by postal authorities or for refilling (resetting). A gear  33  is mechanically engaged with the print rotor  30  and causes the print rotor  30  to rotate in response to rotation of gear  33 . Gears  33  and  34  are positioned within their respective base  32  and meter  31  so that they are engaged with each other when the meter  31  is positioned correctly on the base  32 . A sensor  39  senses arrival of a mail piece in the system and causes the AC motor  38  to start. 
     The system of FIG. 1 has been used for decades, and works well for its intended purpose. But it has to start and stop for each mail piece, which causes wear and tear. The AC motor is large and the mechanisms associated therewith are noisy. The speed of the transport along the paper path is not particularly well controlled and may be different from one mail piece to the next, which is undesirable in the case of a system that feeds mail pieces from a feeder. The system typically requires a single-revolution clutch in the drive train between the AC motor  38  and the print rotor  30 . Such single-revolution clutches represent a maintenance concern because they can wear out, and they add to the cost and complexity of the system. Some such single-revolution clutches are also noisy. 
     FIG. 2 shows another prior-art system, shown for example in U.S. Pat. Nos. 4,631,681 or 4,774,446. In such a system, two motors are employed, the first an AC motor  45  that actuates the paper path, and a second motor  40  which is a DC motor  40  having feedback from an encoder  41 , coupled with the print rotor  30 , driven with pulse-width modulation to match the velocity of the paper path driven by the AC motor  45 , as measured with two document sensors  43 , 44 . Controller  42  receives the edge triggers from the sensors  43 ,  44  from which a crude approximation of the mail piece velocity may be readily calculated, and then in a well-known way, the velocity at the print rotor  30  is measured via encoder  41 . The difference between the actual velocity (measured via the encoder  41 ) and the desired velocity (e.g. the velocity measured via the sensors  43 ,  44 ) is an input to a conventional control system which adjusts the pulse-width-modulated power fed to the DC motor. Such a system offers some advantages over the prior art, but at the expense of requiring more than one document sensor  43 ,  44  in the paper path. Such a system also has the possible drawback that it may in fact fail to measure the velocity accurately and may drive the print rotor  30  at a speed that is too fast or too slow. For example, the mail piece may speed up or slow down after it has passed the sensors  43 ,  44  and thus be traveling at a different speed than the edge of the rotor  30 . As another example, the speed measured by the sensors  43 ,  44  is at best an average of the speed during the time in which the leading edge of the mail piece passes therebetween. 
     FIG. 3 shows an embodiment of the invention. In this embodiment, a paper path is defined by rollers such as rollers  35 ,  36 , referred to generally as mail piece handlers. The mail piece handlers can be belts or rollers and the choice of particular types of mail piece handlers is not critical to the invention. These rollers move under the influence of a motor, preferably a brushless DC motor  53  which is mechanically coupled therewith through a drive train. An encoder  54 , mechanically coupled with the drive train of the DC motor  53 , provides reliable velocity information regarding the paper path. Importantly, the measured velocity at the paper path is correct on a more or less instantaneous basis, that is, what is measured is an instantaneous velocity rather than a velocity averaged over a longer interval. While the term “encoder” is used, it will be appreciated that other speed sensors could be used, such as resolvers, and the term “encoder” preferably encompasses such other sensors coupled with the paper path mail piece handler. While the exemplary motor is a brushless DC motor, it should appreciate that other types of motors could be used. The motor  53  is free-running at a predetermined speed; stated differently, in a preferred embodiment there is no feedback controlling the speed. Because (in a preferred embodiment) there is no control loop, the effective speed of movement of a mail piece in the paper path coupled with the motor  53  is or can be a function of the load from the mail piece. 
     When a mail piece enters the system it is sensed by a trigger  52 , which could optionally be the optical sensor set forth in the above-mentioned U.S. Pat. No. 5,495,103 entitled  Optical mail piece sensor for postage meter , assigned to the same assignee as the present application. Alternatively the mail pieces can be sensed by means of a mechanical detector such as a release lever. The sensor is preferably a phototransistor juxtaposed with a light-emitting diode. The base is controlled by an electronic signal which is integrated into a plug that provides an electrical connection between the base and the meter. This connection is used to tell the base whether the meter is ready or not. The meter will thus be largely unchanged from meters used in prior art systems, except for the need to generate this signal. 
     The trigger signal from the trigger sensor in the base prompts a controller  51  to actuate the stepper motor  50 , taking into account the velocity of the mail piece through the paper path as measured with the encoder  54 . Thus print rotor  30  is driven by the stepper motor  50 . The stepper motor  50  is at least a three-terminal device, and would generally have four terminals. Speed control is accomplished simply by controlling (via controller  51 ) the timing of the pulses to the stepper motor  50 , responsive to the encoder  54 . The stepper motor  50  is selected so as to have enough torque to overcome all loads, so that there is no slippage, that is, so that there is no need for any feedback from the stepper motor  50  or its drive train by which it is mechanically coupled with the print rotor  30 . There is desirably a sensor or two that can tell if the rotor  30  is in the “home” position, but otherwise no sensors are needed with respect to motion of the print rotor  30 . The stepper motor is driven so as to accomplish one rotation (one franking imprint) when the letter or other mail piece is detected by its sensor  52 . 
     It will be appreciated that there is no need to keep motor  53  turning indefinitely. In general the motor  53  is stopped some time after it is started, and preferably it is stopped after some timeout from the event of sensing a mail piece. It may also happen that a number of mail pieces arrive in succession, in which case the motor  53  may be kept moving until some timeout after the arrival of the last mail piece. 
     It will also be appreciated that while the event of the arrival of a mail piece will prompt starting the motor  53 , there are other events that would preferably also prompt starting the motor  53 . For example, when the system is switched on, it is desirable to start the motor  53 , since there may have been a mail piece in the system during the immediately preceding loss of power. Likewise, if the system has a sleep mode (for example, a low-power mode for power conservation), then it is desirable to start the motor  53  upon revival from the sleep mode. 
     In a system such as that just described, the letter speed through the meter can thus be set by a user within a particular range. For example, it may be arranged that the user can select any of several predetermined speeds for the motor  53 . The system including the encoder  54 , the controller  51 , and the motor  50  is able to accommodate any of several such speeds for the motor  53 , and in particular is able to accommodate differing speeds resulting from loads presented by different mail pieces. 
     Those skilled in the art will have no difficulty in devising obvious variations and improvements on the invention set forth herein, all of which are of course considered to be within the invention as defined by the claims which follow. For example, those skilled in the art will appreciate that the teachings of the invention could likewise give their benefits to systems that apply a postage indicium to a mail piece by some means other than a rotating print rotor.