US 3769628 A
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United States Patent [1 Kenny  METHOD AND APPARATUS FOR ELECTROSTATICALLY RECORDING WITH A CLOSED LOOP WEB DRIVE  Inventor: Philip Charles Kenny, Los Altos,
 Assignee: Varian Associates, Palo Alto, Calif.
 Filed: Apr. 3, 1972  Appl. No.: 240,779
[ Oct. 30, 1973 Primary Examiner-Vincent P. Canney Assistant Examiner-Jay P. Lucas Attorney-Stanley Z. Cole et al.
[5 7] ABSTRACT A charge image is electrostatically recorded on a web through an array of writing electrodes. A stepping motor moves the web in response to a digital data source which also selects and activates the writing electrodes. The motor stepping and web motion is synchronized with the digital data input to the writing electrodes. The stepping motor is provided with a shaft encoder for detecting each step as it is completed and for feeding back this web change-inposition information to the digital data source. Upon receiving the step completion pulse, the digital data source releases the next raster of digital bits to the writing electrode array. Feedback between the shaft encoder and the digital data source preserves writing integrity on the web during start-up acceleration and subsequent deceleration of the web.
6 Claims, 1 Drawing Figure PATENTEDUCTZiO I975 552 25 mm v mm 356 E2 METHOD AND APPARATUS FOR ELECTROSTATICALLY RECORDING WITH A CLOSED LOOP WEB DRIVE BACKGROUND OF THE INVENTION l. Field of the Invention This invention relates to electrostatic recording and more particularly to web position feedback to the data source.
2. Description of the Prior Art l-leretofore, stepping motors in electrostatic recorders have been operated at a constant speed without position feedback to the data source. The rasters of digital bits were uniformly spaced with sufficient time therebetween to permit the stepping motor to complete the stepping action. An additional safety factor time was included to accommodate possible variances in motor response time dueto varying line voltage, motor wear, differing motor characteristics, etc. It was assumed that each step was completed within the time allowed and the raster information proceeded on time regardless of the web position. A less than optimum time efficiency and other difficulties accompanied this open loop mode of prior art operation. First, raster information was not processed until the stepping motor reached full speed of about 500 steps per second which requires about A second. One-half second of motor acceleration up to full speed operation is about 125 steps, or 12 lines of print. If ra'ster information was processed during start-up, the writing on the web would be unevenly spaced crowded at the slower initial speeds and approaching normal spacing as the stepping motor approached full speed. Second, subsequent accelerations and decelerations of the stepping motor produced undesirable discontinuities of the recording. Momentary increases in friction in the motor'or in the web, or momentary fluctuations or loss of power which exceeded the margin permitted by the safety factor produced line crowding in the recording. Third, stepping motors, unlike synchronous motors, stall after a critical slippage isreached. The stepping motor has to be started all over again with a series of progressively decreasing spaced step commands.'At increased stepping rates loss of synchronism may be introduced by a single stray noise pulse pickup by the stepping motor control circui t. These extraneouspulses advance the energization of the stepping motor winding inducing slipping. As the speed is increased, the sensitivity to extraneous pulse increases. At high speeds the stepping motor may be completely stalled by a flourescent lamp or nearby .induction apparatus.
SUMMARY'OF THE- INVENTION It is therefore an object of this invention to provide an electrostatic recording system in which web position feedback is established between the stepping motor and the data source; provide an electrostatic recording system in which the effects of stepping rnotor accelerations and decelerations is eliminated; provide an electrostatic recording system in which the raster input rate is dependent upon the web position; and to provide an electrostatic recording system in which stepping motor stalling is avoided.
BRIEF DESCRIPTION OF THE DRAWING Further objects and advantages of the electrostatic recorder and the operation of the closed loop web posi- & 3,769,628
tion control will become apparent from the following detailed description taken in conjunction with the drawing which is a schematic view of the electrostatic recording system operating in the closed loop web position showing the wave forms present at various points within the system.
Referring to the drawing, an electrostatic recorder 10 5110- Ait usaua 2 sqlz i fi n of 640 Writing electrodes 14 is positioned proximate a segmented backplate 16 with a dielectric coated paper or recording medium 18 passing therebetween. The styli and backplate segments are selected for writing by a stylus addresser 20 and a backplate addresser 22 in response to an output 24 of a digital data source 26. The writing electrodes are activated in groups of eight, about 60 microseconds per group, by stylus driver 28 over a raster period of from about 1200 to about 300 microsecout; b a ai Ttma iiaa; 6.115522; The segments of backplate 16 are activated in synchronism with styli 14 by packplate driver 30. The resulting current flowing between the selected and activated writing electrode and backplate segment pair transfers charge to the charge retentive surface of web 18. After each digital raster, data source 26 provides a step command at output 32 to a motor logic circuit 34 having three outputs, A, B and C which are amplified by power supply 35. A stepping motor 36 is provided having three sets of windings, A,'B and C, which are alternately activated by the three outputs of logic circuit 34. Stepping motor 36 turns one angular step in response to the step command initiated in output 32. Stepping motor 36 turns a shaft 38 which rotates a drive roller 40 to move web 18 o ne linez r step, about 1/80 t h of an inch, placing fresh paper underneath stylus array 12. A shaft eneo der 42 is provided on the other end of shaft 38 to detect the completion of each angular step of stepping motor 36. Shaft encoder 42 generates a separate step completion pulse for each of the three windings of motor 36. These pulses are returned to motor logic circuit 34 where they are combined or logically ORed to form a single pulse train which is communicated to data source 26 through feedback loop 44. Upon receipt of the step completed pulse, data source 26 releases the next digital raster to addressers 20 and 22. Synchronism between web motionand raster input is thus established and maintained. Any variation in the speed .of stepping motor 36 causes a corresponding variation in the raster feed rate in output 24 from data source 26. The structure and operation of shaft encoder 42 is described in more detail in IEEE Transactions On Automatic Control, Vol. AC- l3, No. 5, October 1968, entitled Applications Of The Closed-Loop Stepping Motor by T. Roland Fredriksen. The charge transfered from writing electrodes 14 onto moving web 18 forms a charge image 46 on web 18. The web passed through a toning station 48 developing the charge images 46 into visible images 50.
The time sequence of the operation of electrostatic recording system 10 is illustrated in the integrated waveform chart shown in the FIGURE. First, data source 26 provides a step command pulse in output 32 to motor logic circuit 34. In response to the step command pulse, motor logic circuit 34 provides an energizing pulse to winding A of stepping motor 36 causing the motor to step 10. Shaft encoder 42 determines when the step is completed and provides a step completed pulse in one of its three outputs. The three outputs of shaft encoder. 42 correspond to the three windings of motor 36. A step completion by each winding causes a pulse to appear in the shaft encoder output corresponding thereto. The three outputs are combined in motor logic 34 and a single train of step completed pulses appears in feedback loop 44 to data source 26. Data source 26 then releases raster pulses'in output 24 to addressers 20 and 22. After completion of the digital raster, data source 26 provides step command B in output 32 which'ultima tely energizes winding B in stepping motor 36' causing the stepping motor to rotate another At step B completion shaft encoder42 supplies the step completion pulse to data source 26 which releases the next raster of digital information. Each raster of digital information is released after the step completion pulse arrivesat data source 26. This ensures that web 18 is properly advanced to maintain writing integrity notwithstanding accelerations and decelerations of stepping motor '36.
The closed loop web position feedback permits writing during startup of stepping motor 36 because the raster digital information feed rate cannot be faster than the stepping rate of the motor Both the feed rate and the stepping rate increase together untilfull stepping rate is reached. The A second of wasted startup time of the prior art'is eliminated. If there is a slowdown in stepping speed due to a temporary increase in turning friction or-a temporary drag on web 18, or operater interference, the raster feed rate is.slowed'down a corresponding amount and. remains synchronized 1 with the web drive speed. Recorder 10 operating with web position feedback is so insensitive to irregularities in web "drive speed that the web may be manually pulled out of the recorder past stylus array. 12 with all of the imprecision that one might expect withoutdisturbing the writing integrity. The raster rate accommodates the non-uniform tugs and pulls of the operator because the raster pulses are'released by the step completed pulse which is necessarily in synchronism with the web movement. This manual feature is convenient in the event of stepping motorpowe'r supply failure.
The feedback control is effective in the opposite direction as well. Uneven rasterfeed'rates produce a complimentary change in the web drive. The stepping command pulses appearing at th e end of each raster become spaced thu's adjusting the steppingrate and synch'ronization'is maintained. I
Further, synchronization between the raster feed rate and the stepping speed prevents a type: of motor stalling which appears in stepping. motors. "Stepping motors proceed from one stable position of minimum reluctance to the adjacent-position of minimum reluctance in response to an energizing pulse on the adjacent windings. Slippage caused by. a stepping command rate faster than the angular stepping rate of the motor does not generate corresponding increase lin torque which increases the speed of the motor and [re-establish synchronization as is the case with conv 'ntional synchronous motors. lnstead, the turning t rque decreases after a critical slippage is reached an the motor stalls completely. Stray rf noise and arcing cause motor logic 34 to advance the winding energization of motor 36 causing loss of synchronization. Stepping motors are increasingly sensitive to this type stalling at higher speeds. Without web position feedback stepping motor speeds are limited to about 800 steps per second. The
feedback of the present invention permits a stepping rate of at least 2400 steps per second. In the present invention stalling caused by loss of synchronization is eliminated because motor logic circuit 34 receives feedback as to which winding has just been energized to cause the last step completion. With this information, logic circuit 34 can always provide the proper subsequent winding-energization. Stray pulses cannot effect this controlled loop to destroy synchronization.
Clearly, various changes may be made in the struc ture and embodiments shown herein without departing from the concept of the present invention. For example the web position may be determined in other ways besides the shaft encoder. The web position may be determined directly from the web motion.
What is claimed is:
1. An electrostatic recording apparatus for forming a charge image on the charge retentive surface of a recording medium in response to input signals, and for maintaining the properspacing of the charge image regardless of variations in the speed of the recording medium'or variations in the pulse rate of the input signals, the combination comprising:
a stylus array formed by a plurality of writing electrodes mounted proximate the charge retentive surface of the recording medium; I
backplate means mounted proximate the other side of the recording medium and spaced from the stylus array to permit passage of the recording medium therebetween;
data processing means responsive to the input signals for providing rasters of pulses foraddressing and activating the stylus array and the backplate means to cause charge to transfer from selected styli to the charge retentive surface, and also for providing a step command pulse for advancing the recording corresponding linear step increment to move the .recording medium past the stylus array; and vdetection means responsive to the step increment for generating a step completion pulse which is com- I municated to the data processing means to effect synchronization,between theraster rate and the stepping rate for preserving the proper spacing of the charge imageoln the recording medium. 2. The apparatus of claim 1, wherein the'detection means is a shaft encoder responsive to the angular step increments of the stepping motor, the shaft encoder having a plurality of outputs corresponding to the plurality of 'windingsof the stepping motor, and wherein .the shaft encoder provides step completion pulses for each winding which are communicated to the motor logic circuit permitting the motor logic circuit to energize the windings sequentially and maintain stepping synchronism unaffected by externally generated noise pulses. a
3. The apparatus of claim 2, wherein the stepping motor has three windings and the motor logic circuit and the shaft encoder have three outputs corresponding thereto.
4. The apparatus of claim. 1, wherein the data processing means consists of a data source, and addressing means for the stylus array and the backplate, and driving means for the styli and backplate.
5. The apparatus of claim 1, wherein the step command pulse is provided by the data processing means immediately after each raster of pulses is provided to the stylus array and the backplate means by the data processing means, and the step completion pulse is communicated to the data processing means permitting the data processing means to release the next raster of pulses.
6. A method of electrostatically recording a properly positioned charge image on a recording medium in response to input data, regardless of the speed of the recording medium or the rate of input data, the method comprising:
providing a raster of pulses to a stylus array and a backplate for selectively transferring charge to the recording medium positioned therebetween; providing a step command pulse for activating a stepping motor in increments which advances the recording medium in increments past the stylus ary; detecting the completion of each increment; and providing a step completion pulse which maintains synchronism between the input rate and the stepping motor stepping rate.