Optical scanning system utilizing linear drive motors
BACKGROUND 
     This invention relates generally to electrophotographic reproduction devices and, more particularly, to an optical system for such a device which scans a document at an object plane and transmits the image to a moving photoconductive surface. 
     Conventional scanning systems for scanning a document placed on a transparent platen are exemplified by the system disclosed in U.S. Pat. No. 4,332,460. Typically, a photoreceptor drum or belt is driven in a desired direction, and at a desired speed, by an ac or dc drive motor via an associated main drive shaft. The rotary motion of the main drive shaft is transmitted into a lateral motion via a system of cables, capstans, pulleys and clutches, the lateral motion being then applied to the various optical components such as scanning mirrors and lamps generally via a carriage upon which the components are mounted. Each optical component, such as a full-rate or half-rate scan mirror, can be driven at separate rates of speed by selection of appropriate pulley diameters and cable lengths. The scan system then goes through cycles of document scan and rescan motions. During the scan cycle, each carriage is driven at a fixed speed ratio relative to the photoreceptor. For rescan, the drive direction is reversed and the scan components are returned to start of scan position, generally at an accelerated speed. Changes in magnification require different combinations of carriage/photoreceptor scan ratios, rescan speeds, and start of scan positions. In order to accomplish these various actions, and as is generally known in the prior art, a complicated system of electro-mechanical clutches and variable transmission ratios are required as exemplified in the referenced patent. These systems are difficult to design and, once designed for a specific system, are also difficult to modify. The components and their assemblies are costly and are subject to deterioration due to inevitable mechanical wear. Further, the acceleration parameters are restrained because of the slow response time due to the inertia of the carriage components. 
     The present invention is directed towards a scanning system wherein the translational motions of the scanning components are provided by a plurality of independently driven linear motors. Each of the linear motors cooperates with an associated carriage support structure upon which is mounted the optical element to be moved. Each motor is under the control of a master control system which synchronizes the scanning component movement with that of the photoreceptor and which, further, provides variable position and speed relationships among the scan components, the platen and the photoreceptor through a range of magnifications. The control system comprises a digital controller and amplifier for each linear motor and incorporates a feedback circuit to monitor motion acceleration velocity or position of the scan components. 
     More particularly, the invention relates to an optical imaging system for scanning an original document in an object plane and projecting an image at a selected magnification, onto a moving photoreceptor, the system comprising: 
     optical means for scanning and illuminating an original document to be copied, said scanning means including at least two moving scan components, each with a separate drive means to provide an independent rate of scan motion, said drive means including a linear electric motor, 
     projection means for projecting a light image from said document onto said photoreceptor, and 
     digital control means adapted to control the operation of said drive means in selected modes of operation. 
     According to a further aspect of the invention, the digital control means includes supervising control means to feed back acceleration, velocity and position signals of said moving scan components. 
    
    
     FIG. 1 is a schematic side view of a document imaging system in an electrophotographic reproduction machine which requires three separate motions of the optical scanning components. 
     FIG. 2 is a block diagram of the control system of the invention for driving the scan components of FIG. 1. 
     FIG. 3 shows the mechanical mounting arrangement for one of the scan component carriages. 
     FIG. 4 is an enlarged view of the linear motor-carriage slider interface. 
     FIG. 5 is a block diagram of control elements for one of the scan assemblies. 
     FIGS. 6a and 6b are scan drive motion plots for the scanning carriages. 
    
    
     Referring now to FIG. 1, there is shown, in side view a document imaging system which requires three scanning component assemblies to move in a lateral direction beneath a fixed document platen. The operation and requirements of this system are described in general terms below followed by a more detailed description of the drive mechanism and control circuitry (FIGS. 2-5) which provide separate, independent motions to each of the three scan components. 
     Referring to FIG. 1, there is shown a six mirror imaging system 10 which provides variable magnification reproduction of a document 12 placed on a transparent platen 14. An illumination scan assembly 16, comprising an enlongated lamp 18 and associated mirror 19, is moved at a first scanning speed V 1  in a plane parallel to that of platen 14 and from a right to left direction, in the figure. The document scan line image is incrementally reflected from mirror 19 along optical path 20. Corner mirror assembly 21, comprising mirrors 22 and 23 is moving at a second rate of speed, V 2  to maintain a constant object-to-lens distance. Assembly 21 reflects the image into lens 24 which, in turn, projects the image onto a photoreceptor belt 26 moving at a third rate of speed V 3 . The image is reflected onto the belt via mirror assembly 27 comprising mirrors 28 and 29. Corner mirror assembly 27 is moved in a direction opposite to the travel of belt 26 and at a rate V 4  which accommodates travel along a chosen precession distance. Mirror 30 is moved at a rate V 5  in a direction opposite belt 26 travel to maintain a constant lens-to-image distance. 
     In operation, lens 24 is moved to the appropriate magnification position with the mirror components at the selected start-of-scan positions. As shown in FIG. 1, a portion AC of belt 26, representing a distance equal to document length P 1  -P 2  will be exposed during the scan cycle with point C defining the image point of document point P 1 . Illumination scan assembly 16 moves from right to left at a scan velocity V 1  greater than velocity V 3  of belt 26. Corner mirror assembly 21 moves at a second velocity V 2 , which, at 1X magnification, is equal to V 1/2 , to maintain a constant object-to-lens distance. A reflected image of the document, represented by a principal ray 20, is imaged through lens 24 and folded by corner mirror assembly 27 moving from left to right at velocity V 4 . The image is then reflected onto belt 26 by mirror 30, moving at V 5  so that the image is precessed during scan a distance equal to BC. Point A reaches point B at the end-of-scan position. Thus, incremental portions of the document are illuminated and incremental images are reflected along the optical path 20 and projected by lens 24 as a flowing image onto belt 26 forming a latent image of the document on the belt. This latent image is subsequentially developed and transferred to a final copy medium by process steps well known in the art and described in detail in, for example, U.S. Pat. No. 4,318,610, whose contents are hereby incorporated by reference. Following a scan cycle, the movable optical components are returned to their start of scan positions at some regulation rescan speed R. If the system magnification is to be changed, a different combination of mirror-photoreceptor scan and rescan motions must be used as well as new start-of-scan position and lens positions. 
     According to the invention, all of these varied motions V 1 , V 2 , V 4 , V 5  and R at different magnifications are provided by driving each scan component with its own independent, direct coupled, linear drive motor. Each drive motor is operated independently of the main photoreceptor drive motor and associated encoder. Each motion of the linear drive motor is separately controlled by means of an associated digital controller and amplifier which in turn, are under the control of a master supervisor controller. FIG. 2 represents a block diagram of the linear drive control system for these moving components of FIG. 1. As shown in FIG. 2, each moving component 16, 21, 27 and 30 has an associated control circuit 40, 50, 60, 70 respectively. Each control circuit is under the overall control of master supervisor control 80. Each control circuit generally comprises a digital controller 42, 52, 62, 72 amplifier 44, 54, 64, 74 linear motor 46, 56, 66, 76 and feedback path 48, 58, 68, 78. The operation of these circuits will be described in greater detail below. However, at this point, it may be useful to illustrate the manner in which the linear motor acts to create a lateral motion of the associated optical components. 
     Referring to FIGS. 3 and 4, there is shown the mechanical mounting arrangement of mirror 19 on linear motor 46. As shown, motor 46, which can be a type LP-20R40A motor as supplied by Japan Servo (2-phase, 4-winding non-polarized) is firmly mounted on a machine frame 90 in a desired location. The motor includes a fixed stator 92 with teeth 96, and a sliding member 100 with teeth 94. The sliding member 100 is adapted to move along guide rail 102 positioned above stator 92 with an air gap 98 between sliding member teeth 94 and stator teeth 96. Ilumination scan assembly 16 is mounted on carriage 104 (only mirror 19 is shown in FIG. 3). The carriage is connected at the inboard end to slider 100 and is adapted to ride along a guide rail 106 at the outboard end. 
     As shown in a partial, enlarged side view of FIG. 4, slider 100 comprises a plurality of magnetic motion elements or teeth 94 and windings 106 along its entire length. The slider teeth 94 cooperate with the stator teeth 96 so as to close transverse magnetic flux paths across the gap 98. The slider 100 can then be moved in a lateral direction by applying a predetermined electrical pulse to windings 106 from an external power supply 108. The slider increment of movement is thus transferred to the illumination scan assembly 16 providing the desired scan motion. 
     In a similar fashion components 21, 27 and 30 are mounted on carriages (not shown) and driven by linear motors 56, 66, 76 respectively. The carriage movement is governed by sliders associated with the linear motors in the same manner as described for motor 46 above. Further control details describing initiation of various movements, carriage velocities and lengths are provided below. 
     Referring now to FIG. 5, there is shown a block diagram of the controls for illumination scan assembly 16 control circuit (40) associated with providing the motion for the assembly. 
     Supervisor control 80 has input signals arriving from the operator control panel of the particular machine. These signals correspond to print commands, magnification selection, start and stop and carriage position signals. The output signals to digital controller 42 thus respond to magnification, homing and start commands. Controller 42 has the primary functions of executing one homing cycle or one scan and retrace cycle and of sending appropriate drive signals to the linear motor 46 via amplifier 44. Each controller generates a digital representation of the position/time cycle for its associated carriage in response to the input magnification signals and to inputs from feedback circuit 48 and clock signals from clock generator 82. This digital representation comprises a sequence of digital power command signals which controls motor 46 via associated amplifier 44. The sequence of motor commands is controlled by the homing or scan commands sent from supervisor control 80. 
     Amplifier 44 acts on each drive signal and translates the digital signal into currents in the motor windings. Feedback circuit 48 includes accelerator 49 which is used to damp out velocity variations of motor 46. Motor 46 responds to the set of driving currents by seeking the corresponding equilibrium position. If the command decisions are updated at a sufficiently high rate (4000 times/sec), a smooth motor response is achieved. 
     Supervisor control 80 controls drive systems 50, 60 and 70 in a similar fashion. 
     Scan Drive Trajectory 
     FIG. 6 shows the motion requirements for the scanning carriages in terms of position/time and velocity/time diagrams, with the nomenclature given in Table 1 with motion cycle terms provided in Table 2. Many of these terms are evaluated with respect to sampling time interval, discrete time, and discrete time index (clock) counts which, in turn, are used to design the digital controller. More information on these terms can be found in &#34;Discrete-Time and Computer Control Systems&#34;, by James A. Cadzow and Hinrich R. Martens, published by Prentice Hall (1970). 
     A FORTRAN computer program for the above values is provided in Appendix A. The program includes a set of position, speed and acceleration control values to be used in the unit controllers for the step-by-step calculation of carriage position commands. A different set is needed for each carriage and magnification combination. 
     Digital Control 
     Carriage motion, according to one aspect the invention, is controlled in an incremental manner by advancing carriage position in unequal increments at equal time intervals. To acomplish this, a control algorithm was derived expressed as follows (see Table 1 for nomenclature). 
     
         P(k)=P(k-1)+delP(k-1)                                      (1) 
    
     
         delP(k)=delP(k-1)+aTT                                      (2) 
    
     with the starting value 
     
         delP(k.0.)=V(k.0.)+aTT/2                                   (3) 
    
     The same algorithm is used for all segments of the motion cycle. Typical drive requirements for scan illumination assembly are: 
     Scan speed--714 mm/sec 
     Rescan speed--1150 mm/sec 
     Acceleration--1.5 g 
     Carriage motion may alternatively be controlled by advancing the carriage position in equal intervals at unequal time intervals upon generation of the appropriate algorithm. 
     
                       TABLE 1                                                     
______________________________________                                    
Scan Drive Nomenclature                                                   
          Term [SI units]                                                 
______________________________________                                    
Typed                                                                     
Symbol                                                                    
t           time [s].                                                     
k           sampling time index, =.0.,1,2, . . . .                        
P(k)        discrete time position value [m].                             
V(k)        discrete time velocity value [m/s].                           
m           control mode index, =1,2,3, . . . ,8.                         
km          value of k at end of mode m, =k1, . . . k8.                   
t67         retrace time interval [s].                                    
Assigned values:                                                          
Vscan       scan velocity, &gt;.0. [m/s].                                    
Vrtrc       retrace velocity, &lt;.0. [m/s].                                 
a           acceleration value, &gt;.0. [m/s/s].                             
k.0.        initial k-.0..                                                
T           sampling time interval [s].                                   
t.0.1       m=1 wait time interval [s].                                   
t23         m=3 scan time interval [s].                                   
t45         m=5 wait time interval [s].                                   
P(k.0.)     initial position [m].                                         
______________________________________                                    
 
    
     
                       TABLE 2                                                     
______________________________________                                    
Expressions for Motion Cycle Terms                                        
______________________________________                                    
Segment m = 1, from k = k.sub.0 to k = k.sub.1                            
k.sub.1 = k0 + t01/T                                                      
V(k) = 0                                                                  
P(k) = P(k0)                                                              
Segment m = 2, from k = k.sub.1 to k = k.sub.2                            
k2 = k1 + Vscan/aT                                                        
V(k) = V(k - 1) + aT                                                      
P(k) = P(k - 1) + V(k - 1)T + aTT/2                                       
Segment m = 3, from k = k2 to k.sub.3                                     
k.sub.3 = k2 + t23/T                                                      
V(k) = Vscan                                                              
P(k) = P(k - 1) + Vscan T                                                 
Segment m = 4, from k = k.sub.3 to k.sub.4                                
k.sub.4 = k3 + Vscan/aT                                                   
V(k) = V(k - 1) - aT                                                      
P(k) = P(k - 1) + V(k - 1)T - aTT/2                                       
Segment m = 5, from k = k.sub.4 to k.sub.5                                
k.sub.5 = k4 + t45/T                                                      
V(k) = 0                                                                  
P(k) = P(k4)                                                              
Segment m = 6, from k = k.sub.5 to k.sub.6                                
k.sub.6 = k5 + |Vrtrc|/aT                               
V(k) = V(k - 1) - aT                                                      
P(k) = P(k - 1) + V(k - 1)T - aTT/2                                       
Segment m = 7, from k = k.sub.6 to k.sub.7                                
t67 = {2(P(k2) - P(k1)) + (P(k3) - P(k2) -                                
2(P(k6) - P(k5))}/|Vrtrc|                               
k.sub.7 = k6 + t67/T                                                      
V(k) = Vrtrc                                                              
P(k) = P(k - 1) + VrtrcT                                                  
Segment m = 8, from k = k.sub.7 to k.sub.8                                
k.sub.8 = k7 +|Vrtrc|/aT                                
V(k) = V(k - 1) + aT                                                      
P(k) = P(k -  1) + V(k - 1)T + aTT/2                                      
______________________________________                                    
 
    
     The above discussion describes the operation of the invention in the context of moving scan carriages in fixed paths beneath a platen. The invention, however, is not to be limited solely to this configuration. The principle can be extended by those skilled in the art to provide linear motion to a scanning lens of the type as disclosed, for example, in U.S. Pat. No. 4,336,995 to a moving document/platen system or disclosed, for example, in U.S. Pat. No. 4,171,901 to combinations of moving optical elements and a moving platen/document as disclosed, for example, in U.S. Pat. No. 4,459,010. The following claims are intended to cover all such variations and modifications. ##SPC1##