CA1128446A - Apparatus for synchronizing carrier speed and print character selection in on-the-fly printing - Google Patents

Abstract

APPARTUS FOR SYNCHRONIZING CARRIER SPEED AND
PRINT CHARACTER SELECTION IN ON-THE-FLY PRINTING

Abstract of the Disclosure A rotatable print disk is mounted on a carrier which traverses along a print line. The disk is rotated from each selected character position to the next selected character position by the shortest distance at a constant velocity; thus, the time for the disk to move from a given selected character position to the next will be variable dependent on this distance. The carrier is moved from one print position to the next at variable speeds selected such that the carrier reaches the next print position in synchronization with the disk reaching the next selected character position. Upon such synchronization, a print hammer is fired to print the character while the carrier continues on-the-fly towards the next print position. The apparatus includes means responsive to the time required for the print disk to rotate from one selected character position to the next for controlling the carrier escape-ment velocity to increase and/or decrease such that escapement occurs coincident with character selection without escapement velocity becoming zero.

Description

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APPARATUS FOR SYNCHRONIZING CARRIER SPEED AND
PRINT CHARACTER SELECTION IN ON-THE-FLY PRINTING

DESCRIPTION

Back~round of the Invention .
5 l. Field of the Invention .
This invention relates to a movable disk printer and in one of its aspects to such a printer in which printing is provided while at least a movable carriage on which the movable disk is mounted is on-the-fly. More specifically, 10 this invention relates to the synchronization of a movable disk which rotates for a variable distance with the car-riage moving at a variable velocity so that the moving carriage reaches a selected print position coincident with the rotatable disk's arrival at the print character , 15 selected for said print position.

2. Description of Prior Art Printers which utilize a rotating disk with characters on the periphery thereof are well known. Several of such printers are commercially available. Rotating disk .

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~z~46 - printers can be divided into categories by either focusing on how the disk rotates or by focusing on how the carrier traverses.

Focusing on how the disk rotates, such printers can ~e 5 divided into a first category where the disk constantly rotates and into a second category where the mo~ion of the disk is intermittent. In printers with a constantly rotating disk, printing takes plac~e when the hammer strikes the rotating disk. Rotation of the disk is not stopped 10 each time a character is printed. In printers with a disk that intermittently rotates, the disk is rotated to the desired print position and then stopped. There is no disk rotation while printing takes place.

An alternate division of disk printers can be made by lS focusing upon the motion of the carrier. In some printers, the traverse of the carrier is stopped each time printing takes place. In other printers, the carrier is moving at the instant when printing occurs. In both the type where the carrier is moving when printing occurs and in the type 20 where the carrier is stopped when printing occurs, the disk may or may not be rotating at the time of printing.
In some printers where the carrier is moving at a fixed speed when printing takes place, the carrier is slowed down and stopped between print positions in order to give 25 the rotating disk time to move to the desired character.

Th`e following are some of the issued and pending patents which show rotating disk printers:

The Willcox U. S. Pat. No. 3,461,235 issued Aug. 12, 1969, shows a disk printer with a constantly rotating disk. The 30 carrier stops at each print position.

~28446 The Ponzano U.S. Patent No. 3,707,214, issued December 26, 1972, discloses a disk printer which has separate controls for a print wheel and its carrier. The print wheel and the carrier move by the shortest distance to the next selected position. The print wheel and the carrier stop at each print position.

The Robinson U.S. Patent No. 3,356,199, issued December 5, 1967, describes a rotating disk printer wherein the disk is constantly rotating. The type elements on the disk are in a particular spiral configuration. The carrier also moves at a constant speed which is synchronized with the motion of the disk in such a manner that the desired character can be printed in each print position.

The Giani U.S. Patent No. 3,742,845, issued July 3, 1973, shows in Fig. 11 a drum printer which has a constantly rotating drum. It is suggested that this drum could be mounted on a carrier. The carrier would have to stop at each print position in order to give the rotating drum time to rotate to the desired character.

The Cahill U.S. Patent No. 3,794,150, issued February 26, 1974, discloses a drum printer which includes an incre- -menting carrier. The carrier stops at each print position until the drum rotates to the desired position.
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U.S. Patent No. 4,101,006, issued July 18, 1978, of Jensen et al, discloses a carrier control system for a start-stop disk printer in which the carrier normally traverses at a predetermined speed. Printing always occurs at the same predetermined speed; however, if there is not sufficient time to rotate the disk to the next desired character the carrier is slowed down between print positions and then returned to the predetermined speed.

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~28496 U.S. Patent No. 4,030,591, Martin et al, issued June 21, 1977, discloses a rotating disk printer where the carrier is moving at a variety of velocities when the printing by the firing of the print hammer takes place. Thus, the firing of the print hammer must be timed dependent on the velocity of the carrier or carriage at the particular instance.

In U.S. Patent No. 3,858,509, issued January 7, 1975, a rotating disk printing apparatus is disclosed in which the striking force applied to the hammer can be varied between "light" and "hard". However, in that patent the printing is not done on-the-fly and there is no need to coordinate the speed of the carriage and the travel time of the print hammer to insure that the position of the character to be printed is at the print impact point at the time it is caused to strike the printing medium.

U.S. Patent No. 4,035,781, L.H. Chang, issued July 12, 1977, mentions a procedure in a printer wherein upon a failure to print, at least one retry to print is made before the apparatus is stopped for an error correction routine. This patent does not involve on-the-fly printing wherein the carrier is never stopped. In the apparatus of the patent, the carrier appears to stop at each print position. Thus, it appears to be-unrelated to the problem of synchronization of time related parameters in on-the-fly printers.

Further developments with rotating disk printers covered in a Kane et al U.S. Patent No. 4,189,246, issued February 19, 1980, relate to rotating disk printers in which the carrier is moving at a variety of velocities, the rotatable char-acter disk is rotating over a variety of distances and the ;

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1~84~6 print hammer is driven at a variety of forces in order to achieve consistent and high print quality. Thus, the app-roach in U.S. Patent No. 4,189,246 adds a Eurther element, i.e., variable hammer force, which, unlike the apparatus of U.S. Patent 3,858,509, must be coordinated with a variable carriage velocity and variable disk rotation distance in order to achieve the desired synchronization of selected print character with the selected carrier with the selected carrier print position. Actually, as set forth in U.S.
Patent 4,189,246, the variations in impact force of the hammer are manifested by corresponding variations in hammer flight time. Thus, in order to achieve synchronization for printing a character, the apparatus depends on the synchro-nization of three time-related variables: the variable distance the character disk must rotate, the variable velocity of the carriage, and the variable flight time of the hammer. However, in U.S. Patent 4,189,246, the escape-ment velocity, i.e., carrier speed, is constant ketween each pair of characters but may vary from one character pair to the next character pair depending on the relative distance on the print disk between the characters in each pair. This necessitated that relatively large tolerances be set on the time for the carrier to escape to insure that the print disk would have sufficient time to complete its rotation before the carrier arrived at the next print position. The con-sequence of this reduced throughput due to slower carrier velocities.
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; U.S. Patent No. 4,178,108 issued December 11, 1979, by M.H.
Kane offered an improvement to U.S. Patent 4,189,246 by pro-viding means for correcting errors that occur when the print disk rotation and the carrier escapement lose synchroniza-tion. Loss of synchronization occurs when the carrier escapes to the next print position before the print disk ~Z84~6 rotates the next character into posi-tion to be printed while the carrier is still in motion, i.e., on-the-fly. U.S.
Patent 4,178,108 compensates for loss of synchronization by stopping the carrier, reversing its motion and then re-starting carrier motion in the direction of printing toregain synchronization. U.S. Patent 4,178,108 teaches that the tolerances of the escapement time can be reduced since loss of synchronization can be corrected. It further teaches that tolerances must be maintained such that loss of synchronization, and the resulting stopping of the carrier, will occur infrequently enough that throughput will still be improved over the tolerances required where occasional loss of synchronization cannot be tolerated.

SUMMARY OF THE INVENTION

The present invention is directed to on--the-fly printing apparatus wherein carrier means movable at variable velo-cities from a first print position on-the-fly past a second print position are coordinated with character printing means, for example, a rotatable disk, having a plurality of movable characters which are adapted to imprint upon the application of an impact force. The movable characters are controlled by character selection means which move the characters over variable distances during the movement of the carrier means from a first selected character coincident with the carrier at the first print position to a second selected character coin-cident with the carrier at a second print position. The apparatus further includes control means for controlling movement of the carrier means and the character selection means into coincidence at a print position such as said second print position. Such control means further includes ~ ~ . .. .

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means for synchronizing the carrier escapement time to the print disk rotation time by varying the carrier escapement velocity between print positions including acceleratlng and/or decelerating the carrier ve:Locity such that total 5 escapement time coincides with print disk rotation time, resulting in higher printing speeds and higher throughput than heretofore achievable.

BRIEF DESCRIPTION OF THE DRAWING
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Fig. 1 shows a printer apparatus adapted for use with the 10 present invention.

Fig. 2 is a schematic diagram, in block form, of the cir-cuitry for controlling the operation of the printer appa-ratus in Fig. 1.

Fig. 3 is a table of escapement velocities as related to 15 selection time magnitudes.

Fig. 4 is a table of delay times for escapement motor drive as related selection time magnitude and escapement velocity.

Fig. 5 contains graphs showing combinations of velocities 20 utilized to move the carrier certain distances as examples of carriage movement during printing.

Fig. 6 is a table showing the control bits for producing the velocity profile shown in Graph A of Fig. 5.

Fig. 7 is a flow diagram depicting the sequence of opera-25 tions executed by the escapement controller shown in Fig. 2.

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DESCRIPTION OF THE PREFERRED EMBODIMENT

Fig. 1 shows the main mechanical components of a printer of the well-known type which utilizes a rotating disk with characters on the periphery thereof. A laterally sliding 5 carrier 1 is mounted on a guide rod la and a lead screw 7 and carries a rotatable print wheel or disk 2 driven by a stepping motor 3. The carrier 1 is driven by lead screw 7 which is driven by a stepping motor 8. Alternatively, motor 8 could drive a belt which in turn could drive 10 carrier 1. The lead screw 7 is threaded such that each step of stepper motor 8 represents one increment of escape-ment for the carrier 1.

A type disk 2 comprises a disk having a number of movable type elements such as the flexible spokes or type fingers 15 9A, 9B, 9C, etc. Printing of any desired character is brought about by operating a print hammer 10, which is - acutated by a solenoid 11, both of which are mounted on carrier 1. When the appropriate type finger approaches the print position, solenoid 11 actuates hammer 10 into 20 contact with the selected type finger, driving it into - contact with a paper 12 or other printing medium. An emitter wheel 13 attached to and rotating with type disk 2 cooperates with a sensor FB2 to produce a stream of emitter index pulses for controlling the operation of the printer.
25 The emitter has a series of teeth each of which correspond to one finger 9A, 9B, 9C, etc., of the printing disk. A
homing pulse is generated for each revolution of the print wheel by a single tooth on another emitter (not shown).
The printer control can thus determine the angular position 30 of type disk 2 at any time by counting the pulses received since the last homing pulse. A toothed emitter lS is .
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1~2~4~6 mounted on the shaft of the motor 8 and in conjunction with a sensor FBl pxovides pulses which indicate the position of the carrier 1.

Stepper motors 3 and 8 are activated by conventional drive 5 circuits 21 and 22. Examples of the type of drive cir-cuitry that could be used are shown in U.S. Patent 3,636,429.
A hammer solenoid 11 is actuated by a hammer drive circuit 23 which is also conventional.

The actions of positioning the carrier 1 and positioning 10 the print wheel 2 are, in general, independent except that coordination is required at the instant printing occurs.
Both type disk 2 and carrier 1 must be in a selected posi-ton (but they need not be at rest), when hammer 10 strikes type disk 2.

15 Referring now to Fig. 2, a schematic diagram is illustrated of circuitry which may be utilized to provide the appropriate control signals to the escapement motor drive circuit 21, to print wheel drive circuit 22, and to the hammer drive circuit 23. The data which is to be printed comes from a 20 data source (not shown), which may be a conventional data buffer or a keyboard input device such as a typewriter.
~- Data from the data source is conducted to the input of a suitable computer or microprocessor, only the output of which is illustrated in Fig. 2, and the microprocessor can 25 be any suitable commercially available microprocessor or computer such as the IBM System 7. The processor receives the input data and will make certain calculations and then send a series of binary numbers out on either an address bus 40 or a data bus 41 as illustrated in Fig. 2. In 30 response to the data received from the microprocessor, the circuitry in Fig. 2 generates appropriate drive pulses to , ~lZ~4~;

circuits 21, 22, and 23 in order to cause stepper motors 8 and 3 to move the carrier 1 and the disk 2 to the correct positions, and to activate the print hammer 10 in orde~ to print the data supplies by the data source. The output 5 signals to each of the drive circuits 21 and 22 include information indicating the direction in which the stepper motor should move and the number of steps to be moved, it being understood that one pulse is provided by the appro-priate drive circuit for each step of the motors 3 and 8.

10 As illustrated in Fig. 2, the circuitry of this invention includes a plurality of buffer registers indicated gener-ally by the reference number 42 which receive appropriate information from the microprocessor through address bus 40 and data bus 41. Buffer registers 42 include an operating 15 state register 43, which controls the direction of movement of carrier, a hammer energy register 43 which stores data concerning initiation time and duration of the hammer energy pulse, and a selection register 46 which receives and stores data from the microprocessor concerning the 20 selection of the characters on the printing wheel 2. In order to load data into the buffer registers 42 from the microprocessor, address data from the microprocessor bus is inputted into a command decode circuit 47 and from there through a control bus 48 to the respective buffer 25 registers. Likewise, data from data bus 41 of the micro-processor is routed through data bus in gate 49 and data bus 50 to respective inputs of the buffer registers 42.
The microprocessor is also connected through the control bus 48, a data available line 51, and a data request line 30 52 to a sequence control circuit 53 which controls the sequence of operation of the circuitry of Fig. 2 and of the microprocessor, as hereinafter explained. Since printing is accomplished by the present invention while .

, ~28~9~6 carrier 1 is in motion, it is necessary to provide buf~er registers 42 in order that data from the processor may be stored therein prior to actual usage, to permit the micro-processor to accummulate subsequent data and to permit new 5 data to be stored in the buffer registers when the pre-viously stored data has been dumped. In this manner, the data is available to the operating registers described below when needed in order to permit the continuous opera-tion of the system.

10 In addition to the buffer registers described, the circuit of Fig. 2 also incluAes a plurality of operating registers 61, 62, and 64, and an escapement controller 63. A system reset signal on line 73 is input to sequence control 53 and distributed to the operating registers and escapement 15 controller for initializing the system at power on or after a system failure. In general, upon receipt of appropriate load command, the operating registers receive and store the information contained in the buffer regis-ters 42j thus permitting the buffer registers to then 20 intake new data while the data in the operating registers is being acted on. As illustrated in Fig. 2, an operating state output register 61 is provided to receive and store data from operating state register 43, a hammer delay and energy register 62 is provided to receive and store data 25 from hammer energy register 44, and a selection downcounter 64 is provided to receive and store data from selection register 46. The escapement controller is provided to receive the selection data from the selection register 46 and to generate escapement velocity profiles based on the 30 selection time magnitude as will hereinafter be described in detail. The outputs of the respective registers 61, 62, and 64 are connected as shown in Fig. 2 to escapement motor control logic 66 for controlling the direction of . . . ~
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l~Z8~46 motion of carrier 1, to hammer control logic 65 for con-trolling the actuation of print hammer 10, and to selection motor control logic 67 for controlling the motion of print wheel 2. The output of the escapement controller 63 is 5 connected to escapement motor control logic 66 for control-ling the escapement velocity of the carrier 1.

Referring to Fig. 3 there is shown in outline form a table of next velocity values stored in the escapement controller 63 for printing the next character. The tahle is arranged 10 as a matrix having a number of columns j equal to the number of selection time magnitudes S for rotating the print disk 2 and a number of rows i equal to the number of velocity values N at which the carrier 1 may be moving when printing occurs. The selection time magnitude S is 15 determined by the time required to rotate the print disk 2 from its present position to the position of the next character to be printed. In the preferred embodiment, motor 3 is capable of bidirectional rotation so that the maximum number of selection times j is one-half of the 20 number type fingers. The number of carrier velocities i at printing can occur is a designer's choice limited only by technology. In the preferred embodiment a range is set from zero to ten inches per second. The selection time magnitude S and the velocity N are combined to access the 25 next velocity table of Fig. 3 to produce a velocity value V at which the carrier will be moving when the selected ~ character is printed in order to synchronize selection and ;~ escapement. The escapement controller 63 receives from the selection register 46 the magnitude of the selection 30 time, S, for rotating the print disk to the next printing character. The selection magnitude S is combined with the velocity N of the carrier which is stored in a register in _ _ _ , _ . . . , . : .

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~2B44~6 escapement controller 63 to access the table and provide a next velocity V which is required to synchronize the carrier movement with the print disk rotation.

Referring to Fig. 4, a second table which is stored in the 5 escapement controller 63 is shown which contains the delay times used to control the escapement stepper motor 8 to provide carrier transition from the velocity N at which the last character was printed to the next velocity V at which the next character will be printed. The table of 10 Fig. 4 contains one matrix of values for each of the next velocities V from the table of Fig. 3. Each matrix of the table of Fig. 4 is arranged into columns corresponding to the selection time magnitude for the next character to be printed and rows corresponding to the number of carrier 15 increments between print positions. Recall the carrier increments are determined by stepper motor 8 and the threads on the lead screw 7. The preferred embodiment of the invention is described for illustrative purposes only using a 10 pitch printer, i.e. a printer which prints ten 20 characters per inch with each character occupying a fixed amount of space. It will be understood that some other pitch, such as 12 pitch could be selected or that propor-tional spacing can be employed within the scope of this invention. In the preferred embodiment the stepper motor 25 8 and lead screw 7 accomplish 240 steps per inch which provides 24 increments for each character of escapement.
This requires that stepper motor 8 make 24 steps between printing characters coincident with the selection time magnitude of the print disk 2. The matrix of the table of 30 Fig. 4 which corresponds to the next velocity V is accessed using the selection time magnitude S and the escapement increment density value D to provide a delay time t. The delay time t is the time between drive pulses to the escapement stepper motor 8 which controls the escapement velocity. The series of delay times t from the delay table combine to change the carrier velocity to the next velocity V in the selection time S.

5 Fig. S shows four graphs as examples of selection times required to rotate the print disk 2 from a first selected print character to a second selected print character and the corresponding escapement velocities used to synchronize the escapement with the selecton time. The horizontal leg 10 of each graph represents the incremental escapement den-sity, d, between characters. As previously stated, the preferred embodiment of this invention is described as a fixed pitch printer which prints ten characters per inch of escapement. The lead screw 7 contains 240 increments 15 per inch, therefore giving 24 increments of escapement per character position as shown in the graphs. The vertical leg of each graph in Fig. 5 represents the carrier velocity in inches per second. In operation, the escapement con-troller 63 provides a signal to the escapement motor 20 control logic 66 which determines the frequency at which - the stepper motor 8 is pulsed by the escapement motor drive circuit 21. The delay between pulses to the stepper motor 8 determines the velocity of the carrier. As is readily apparent, a small delay between pulses to the 25 stepper motor 8 causes the carrier to move at a higher velocity and conversely a longer delay between pulses to the stepper motor causes the carrier to move at a lower velocity.

Fig. 6 is a table of delay times for corresponding to the 30 escapement graph A of Fig. 5 and Fig. 7 is a flow diagram .

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~28~46 8 of the operation of the escapement controller 63. ~efer-ring now to Figs. 5, graph A, 6, and 7 in combination a typical escapement of the carrier from one print position to the next will be described. In Fig. S, graph A, it is 5 shown that the selection time equals 27.684 milliseconds.
This is the time required ~or the print wheel to rotate from the character just printed to the next printing character. This time is stored in the selection register 46. The graph A in Fig. 5 assumes that the carrier is 10 moving at six inches per second when the selection time is generated in the selection register 46. Referring to Fig.
7, the initial value of the velocity register is set to zero at block 101 when the system is turned on or in response to a system reset at block 102. When the start 15 line is energized by the sequence control 53 (Fig. 2) the escapement controller tests the start line at block 103.
When it is determined that a start signal has been set, the selection magnitude is read from the selection regis-ter by the escapement controller at block 104. Assuming 20 the carrier velocity is six inches per second, the velo-city register will contain 6. At block 107, the density counter is set equal to one representing the first incre-ment of escapement toward-the printing of the next charac-ter. The value of the density counter is tested at block 25 108 to determine if it is less than or equal to 24, the incremental position at which the next character is to be printed. If the result of the test is affirmative then the escapement controller 63 outputs a signal to the escapement motor control logic 66 to step the escapement 30 motor 8 at block 109. The velocity, selection, and density values are then combined to access the delay table tFig.
4) to determine the amount of delay to be executed prior to the next motor step. Referring to graph A of Fig. 5 .

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and Fig. 6 it can be seen that for the first increment of escapement density, D, the escapement velocity is six inches per second. The binary value representing the delay time between pulses to the stepper motor to maintain S escapement at six inches per second is counted down at blocks 112 and 113 in Fig. 7. When the delay time has been counted down an increment is added to the density counter at block 114 and a branch :is taken back to block 108 to test the magnitude of the density counter contents.
10 If it is still less than or equal to 24 a pulse is output once again at 109 to step the motor 8 and the delay is accessed from the delay table using the velocity and se-lection at the new density (Fig. 4). At the ourth incre-ment, 0 = 4, the delay time is increased and correspond-15 ingly the carrier velocty is decelerated to S.5 inches/second. At D = 5, D = 6, D = 7, D = 8, and D = 9 the carrier is further decelerated until the next print velo-city of three inches per second is attained. The delay value is then constant for the remaining increments of 20 this carrier escapement. When the density counter exceeds 24, the carrier has escaped to the next print position and a branch is taken to block 105. The value of the selection magnitude and the velocity register value are combined to access the next velocity table (Fig. 3) at block 105. As 25 was previously stated, the velocity value that is accessed in the next velocity table is the escapement velocity value for the carrier at the time the next character is to be printed. In graph A this value is three inches per second. At block 106 the velocity register is set equal 30 to the next velocity value. A finish signal is set at block 111. A branch is then taken back to block 102 and the process is repeated for the next character.

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~2~4~6 While the invention has been particularly shown and described wlth reference to a particular embodiment, it will be understood by those skilled in the art that vari-ous changes in form and detail may be made without depart-5 ing from the spirit and scope of the invention.

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Claims

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:
Claim 1 A printing apparatus for on-the-fly printing comprising in combination:

a carrier means movable from one print position past a next print position on a printing medium;

carrier drive means for moving said carrier;

printing means comprising a plurality of movable characters, each engagable with said printing medium;

character selection means for moving said charac-ters over a variable distance from one selected character to a next selected character;

means for urging said characters into contact with said printing medium; and printing control means for synchronizing the movement of said carrier means and said character selection means into coincidence at said next print position and for actuating said urging means when said coincidence occurs, said printing control means including carrier velocity determining means for generating a signal which is a function of the length of time required for the character selection means to move the next selected character into position to be printed, and carrier escapement control means responsive to said carrier velocity determining means for selec-tively actuating said carrier drive means to drive said carrier at a first on-the-fly selected velocity past said one print position coincident with said one selected character and selectively actuating said carrier drive means to drive said carrier at varying velocities between said one print position and said next print position such that said carrier arrives at said next print position coincident with said next printing character.

Claim 2 The apparatus of claim 1 wherein said carrier drive means includes means for moving said carrier in increments and wherein said carrier escapement control means includes means for controlling said carrier drive means to increase or decrease the velo-city of carrier escapement between increments of carrier movement.

Claim 3 The apparatus of claim 2 wherein said means for controlling said carrier drive means includes means for escuting variable amount of delay between incre-ments of carrier escapement such that the velocity of carrier escapement is increased or decreased.

Claim 4 A printing apparatus for printing on-the-fly at a plurality of print positions along a print line comprising in combination:

a carrier;

a carrier drive means for moving said carrier across said print line past said plurality of print positions;

a rotatable print element having a plurality of characters mounted on said carrier;

a print hammer mounted on said carrier operable to impact said print element when a selected charac-ter is positioned at a print position;

print element drive means for rotating said print element variable distances from one selected character coincident with said carrier at one print position to the next selected character coincident with said carrier at the next print position, the length of time required to move between any two selected characters being dependent on said variable distance;

carrier velocity determining means for generat-ing a signal which is a function of the length of time required to rotate said print element from one selected character to the next selected character-and carrier escapement control means for selectively actuating said carrier drive means responsive to said carrier velocity determining means to drive said carrier at a first on-the-fly selected velocity past said one print position coincident with said one selected character and to drive said carrier at vary-ing velocities between said one character position and said next character position such that the time to move said carrier from said one print position to the next print position is equal to the time required to rotate said print element from one selected charac-ter to the next selected character.