US3678847A

US3678847A - Hammer firing system for a high speed printer

Info

Publication number: US3678847A
Application number: US49767A
Authority: US
Inventors: Charles B Pear Jr; Joseph A Ross
Original assignee: Potter Instrument Co Inc
Current assignee: Potter Instrument Co Inc
Priority date: 1970-06-25
Filing date: 1970-06-25
Publication date: 1972-07-25
Anticipated expiration: 1989-07-25
Also published as: GB1313273A; DE2131710A1; FR2097935A5; CA938559A

Abstract

In a high speed printer the hammers are fired by first applying a firing pulse to the hammer and then applying a damping pulse to the hammer as the hammer is rebounding. The damping pulse cancels out the kinetic energy of the hammer and brings it more quickly to rest each time the hammer is fired. The firing circuit comprises a transistor having the hammer coil connected in the collector circuit thereof and a resistance in the emitter circuit thereof. When the hammer is fired the transistor is initially driven into saturation. When the transistor comes out of saturation, negative feedback provided by the resistance in the emitter circuit maintains the firing pulse current constant to the end of the firing pulse.

Description

United States Patent Pear, Jr. et al.

[54] HAMMER FIRING SYSTEM FOR A HIGH SPEED PRINTER [72] Inventors: Charles B. Pear, Jr., Greenlawn; Joseph A.

Ross, Fort Salonga, both of NY.

[73] Assignee: Potter Instrument Company, Inc., Plainview, NY.

[22] Filed: June 25, I970 [21] Appl. No; 49,767

"25 PR'NT CLOCKA CONTROL 7-39 REFERENCE VOLTAGE [451 July 25, 1972 FOREIGN PATENTS OR APPLICATIONS 1,033,505 6/1966 Great Britain ..307/27O OTHER PUBLICATIONS Millman & Taub, Pulse, Digital and Switching Waveforms 1965 pp. 192-195 Primary Examiner-John Zazworsky Attorney-Lane, Aitken, Dunner & Ziems 5 7] ABSTRACT In a high speed printer the hammers are fired by first applying a firing pulse to the hammer and then applying a damping pulse to the hammer as the hammer is rebounding. The damping pulse cancels out the kinetic energy of the hammer and brings it more quickly to rest each time the hammer is fired. The firing circuit comprises a transistor having the hammer coil connected in the collector circuit thereof and a resistance in the emitter circuit thereof. When the hammer is fired the transistor is initially driven into saturation. When the transistor comes out of saturation, negative feedback provided by the resistance in the emitter circuit maintains the firing pulse current constant to the end of the firing pulse.

14 Claims, 7 Drawing Figures HAMMER HAMMER PATENTED L I912 3 3.678347 sum 1 or 3 n /25 LOCKA HAMMER HAMMER HAMMER PRINT CLOCK I CONTROL eENERATpR )CLOCK B Is I I 39 REFERENCE VOLTAG E I 20 I CLOCK 8. H FM H E PRINT I I I I CONTROL I I 28 I ENABLING SIGNAL I I I I I 30 I I I PULSESW TO HAMMER] I FIRING I CIRCUIT 2 FIRING 4 FlRlNG I PERIOD I I PERIOD I INVENTORS CHARLES B. P, EAR,Jr. a JOSEPH A, Ross FIG. 2

PATENTEDJUL25|912 3.678347 sum 2 or 3 NO RMAL TRANSI EN'T BEH AV IO R DISPLACEMENT 7| R T P T Es OSI ION L v TIME FIRE PULSE:%V73

TFI'E IMPROVED TRANSIENT BEHAVIOR DISPLACEMENT 7s F/ G. 4. to n TIME DAMPING PULSE FIRE PULSE Tl E HAMMER HAMMER HAMMER-VB I! r A HAMMER 9| FIRING PRINT 4; CIRCUIT CONTROL I MMv DELAY MMv INVENTOR-S F/ G 5.

CHARLES B. PEAR,Jr.8 JOSEPH A. Ross PATENTEDJULZS m2 3I678I847 I saw a or 3 TONE WHEEL PULSES I I I I I I i I I 1 Ema L I m m MMv 97 i l MMVIOS I: [1:

. I I PULSES TO I l HAMMER I l 09 I I FIRING l CIRCUIT I:

ll l2 HAMMER HAMMER HAMMEEL 1 m l I I AMME 29 9 I09 FIRING I I mvERfon PRINT I CONTROL 25 MMv 0ELAY MMv 97 (I03 los INVENTORS F l G. 7. CHARLES B. PEAR,Jr.8

JOSEPH A. ROSS RNEYS BACKGROUND OF THE INVENTION This invention relates to pulse actuated electromagnetic devices and more particularly to a hammer with an improved firing circuit for high speed printers.

In high-speed, impact printers, the repetition rate at which the hammers operate must be quite high to enable the printer to print at a high rate. This is particularly true in a printer of the type in which the hammers are controlled in a manner to print the dots in patterns to compose the selected alphanumeric characters. Accordingly, each hammer must be actuated many times to print each character. In order to obtain uniform printing and prevent what is known in the art as ghosting, it is preferable that the hammer be actuated from rest position or from near a rest position. Thus, it is desirable to bring the hammer to rest as quickly as possible following each actuatron.

The present invention provides a circuit for firing the hammer in a manner to very quickly bring it to rest following each actuation. In addition, the hammer firing circuit of this invention provides stable operation of the hammers despite impedance changes in the operating coil due to temperature changes and the like.

SUMMARY OF THE INVENTION In accordance with the present invention a pulse actuated electromagnetic hammer for a high speed printer is energized by a circuit which first apples an actuating pulse to the hammer operating coil and then applies a properly timed second pulse with a polarity to oppose the residual kinetic energy in the device and with an impulse content to approximately cancel the residual kinetic energy. The second pulse is applied as the hammer is rebounding after striking the print forming member and has the same polarity as the actuating or firing pulse. The second pulse, which is referred to herein as a damping pulse, cancels the kinetic energy of the hammer and quickly brings the hammer to rest. The hammer operating coil is connected in the collector circuit of a transistor which has an emitter feedback resistor to apply negative feedback to the transistor. In applying a pulse to the hammer coil, the transistor is initially driven to saturation and provides a steeply rising current pulse to the coil. When the transistor comes out of saturation, the negative feedback maintains a constant current through the coil until transistor is cut ofi, thus permitting the energy content of both the firing and damping pulse to be controlled by controlling the duration of the firing and damping pulses respectively.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 schematically illustrates a high speed printer with an improved hammer firing system in accordance with the present application.

FIG. 2 illustrates some wave forms which are utilized in the system of FIG. 1.

FIG. 3 illustrates the transient behavior of a hammer or other pulse actuated electromagnetic device without the present invention.

FIG. 4 illustrates the improved transient behavior with the present invention.

FIG. 5 illustrates an alternative embodiment of a hammer firing system in a high speed printer in accordance with the present invention.

FIG. 6 illustrates some wave forms used in the system of FIG. 5; and

FIG. 7 illustrates another alternative embodiment of a hammer firing system in a high speed printer in accordance with the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The printer schematically illustrated in FIG. I comprises a drum 1] which carries a raised helical bar 12 and hammers I3 for printing selected alphanumeric characters. For purposes of simplicity the circuit driving only one hammer I3 is shown and described. The remaining hammers l3 distributed along the length of the drum have similar driving circuits. The drum II is driven by a motor not shown, and drives a tone wheel I5 which produces a series of output pulses from a transducer I7. The characters are selected to be printed by timing the firing of the hammer 13 relative to the position of the drum 1] as it rotates. Usually a paper web and an ink or carbon ribbon are disposed between the drum 11 and a dot is printed each time a hammer fires and pushes the paper and ribbon against the helical bar. By firing the hammers 13 at the proper times, the dots will be printed in patterns to compose selected alphanumeric characters. Because a hammer must be fired several times to print a character, the present invention is particularly useful in this kind of printer.

Pulses produced by the transducer 17 are applied to a clock generator 19 which produces two synchronized output clock pulse trains, clock train A and clock train B illustrated in FIG. 2 as

wave forms

20 and 22. As shown in FIG. 2, each pulse of clock pulse train B is produced immediately before the next succeeding pulse of clock pulse train A. In the system of the present invention, the pulses of clock train A define hammer firing periods which start and end at the leading edges of adjacent pulses in clock train A. The hammer 13 can be fired once during each firing period. Thus, the hammer 13 can be fired at the same rate at which the pulses of the clock train A are produced.

The pulses of clock train A are applied to an AND gate 21 and the pulses of clock train B are applied to an AND gate 23. As will be described below, the hammer 13 is fired in response to pulses of clock train A and then is clamped in response to the pulses of clock train B. If the hammer is to be fired in a selected firing period, print control 25 will apply an enabling signal to both of the

gates

21 and 23 for a period long enough for both the pulse from the clock train A and the following pulse from clock train B to pass through the

gates

21 and 23. These pulses upon passing through the

gates

21 and 23 are applied through an OR gate 26 to a switch 27 in the hammer firing circuit 29. If the hammer 13 is not to be fired during a firing period then the print control 25 will not enable the

gates

21 and 23 during the firing period. If the hammer I3 is to be fired in two successive periods, the enabling signal from the print control 25 will be applied continuously over both firing periods. The print control signal is synchronized with the clock pulses by means of pulses applied thereto from clock train A. An exemplary control enabling signal is illustrated in FIG. 2 as wave form 28, showing the shape of the print control signal when the hammer is to be fired during a first, third, and fourth firing period of a sequence of four firing periods but not during the second firing period of the sequence. As shown in FIG. 2, the enabling signal is applied during the first period and continuously throughout the entire third and fourth periods but is not applied during the second period. As a result, during the first firing period a pulse from clock train A and a pulse from clock train B will pass through the OR gate 26, pulses from both clock trains will also pass through the OR gate 26 during the third and fourth firing periods but will not pass through the OR gate 26 during the second firing period. The resulting pulse train applied through the OR gate 26 to the hammer firing circuit 29 is illustrated in FIG. 2 as wave form 30. In response to each pulse applied thereto through the OR gate 26, the hammer firing circuit 29 will apply a pulse of is applied to the hammer. The time of occurrence of the firing pulses and damping pulses will be substantially simultaneous with the pulses of clock trains A and B respectively. In response to the firing pulse, the hammer 13 will strike the drum I1 and print. As the hammer is rebounding from the drum, the damping pulse is applied and cancels out the kinetic energy ofthe hammer. As illustrated in FIG. 2, the clock trains A and B are arranged so that the damping pulse occurs as near as possible to the time of the firing pulse in the next firing period without causing hammer jitter. The firing pulse is timed to terminate before the hammer strikes the drum. Preferably, the duration of the firing pulse is long so that a pulse of relatively low amplitude can be used to avoid excessive heating and achieve the desired hammer velocity.

FIG. 3 illustrates the normal transient behavior of a hammer in a prior art firing system. The curve 71 shows the displacement of the hammer with time in response to a firing pulse 73 applied at t At time t the hammer strikes the drum and rebounds as is represented by the change in direction of the curve 71. The hammer rebounds to its rest position and mechanically oscillates about the rest position over a significant time period as shown in FIG. 3. In the system of the present invention as shown in FIG. 4 following the firing pulse 73, a damping pulse 74 is applied to the coil as the hammer approaches its rest position, as represented by curve 76. This pulse opposes the kinetic energy of the hammer and effectively cancels most of it out, greatly reducing the amount of mechanical oscillation of the hammer about the rest position and thereby bringing the hammer much more quickly to rest. Preferably, the impulse content of the damping pulse 75 is related to the impulse content ofthe firing pulse in the same proportion as the rebound velocity ofthe hammer is to the impact velocity. The term impulse content" refers to the energy of an applied pulse and is proportional to the pulse width multiplied by the pulse current amplitude. For constant current pulses, the ratio of the firing and damping pulse widths will be equal to the ratio of the impact and rebound velocities. It will be apparent that the impulse content of the damping pulse, in other words the amount of damping required, is a function of the rebound velocity. Obviously the impulse content of the damping pulse necessarily is insufiicient to cause the hammer to restrike the drum.

When the electronic switch 27 receives a pulse from the OR gate 26, it closes and applies a positive voltage from a terminal 3] through a resistor 33 to the base of an NPN transistor 35. The base of the transistor 35 is connected through a clamping diode 37 to a positive reference voltage level applied at a terminal 39. The collector of the transistor 35 is connected through a resistor 41 to a positive source of 28 volts applied to a terminal 43. The emitter of the transistor 35 is connected through a kilohm resistor 45 to a minus source of 5 volts applied to a terminal 47. The emitter of the transistor 35 is also connected directly to the base of an NPN transistor 49, the emitter of which is connected to ground through a 3 ohm resistor 51 and the collector of which is connected to the coil of the hammer 13 in series to a source of positive voltage ap plied to a terminal 55. The coil of the hammer 13 is shunted by a diode 57 and a resistor 59 connected in series.

When a pulse from either the clock train A or the clock train B is applied to the switch 27, the switch 27 closes and applies the voltage applied to the terminal 31 to the base of the transistor 35. This causes the transistor 35 to conduct which in turn causes the transistor 49 to conduct. As a result, current is applied from the source 55 through the hammer coil and the transistor 49 connected in series. When the input pulse applied to switch 27 ceases, the transistor 35 will be cut off which in turn will cut off the transistor 49 and current flowing through the coil will be terminated. The diode 57 and the resistor 59 provide a current path to dissipate the energy inductively stored in the coil when the transistor 49 is cut off. The resistor 51 provided in the emitter circuit of transistor 49 providesa negative feedback signal to the transistor 49 which decreases the time it takes the hammer to get up to speed,

decreases the power dissipation in the hammer, reduces the power supply voltage regulation requirements as compared with the systems of the prior art, and minimizes the effects of changes in the impedance of coil 13 due to changes in temperature. When a pulse is first applied to the switch 27 causing the transistor 35 to turn on, the transistor 49 initially saturates and the current starts rising up rapidly towards a large value due to the low resistance in the circuit. In the initial stages of the current buildup the effect of this resistance, that is the internal coil resistance and the resistance of the resistor 51, is negligible and the slope of the curve is essentially the voltage divided by the inductance of the coil of the hammer 13. When the current reaches the desired value, the transistor 49 comes out of saturation and the current is maintained constant due to the feedback provided by the resistor 51. It should be noted that owing to the fact of negative feedback of the hammer driver circuit the damping pulse is a constant current pulse and its energy can be conveniently varied by varying the width of the pulse in clock train B.

FIG. 5 illustrates an alternative embodiment of the system of the present invention. In this system the print control 25 synchronized by tone wheel pulses from the transducer 17 applies a pulse to an AND gate 91 each time the hammer is to be fired. FIG. 6 shows an example of these pulses as wave form 93 and shows how they relate in time to the tone wheel pulses represented by wave form 95. The pulses 93 shown in FIG. 6 are intended to cause firing of the hammer 13 in the first, third and fourth firing periods represented in FIG. 6. The tone wheel pulses which define the boundaries of the firing periods are also applied to the gate 91. The print control pulses 93 are timed to occur simultaneously with the tone wheel pulses by means of the synchronization derived from the tone wheel pulses applied to the print control 25. The print control pulses 93 to insure simultaneity are wider than the tone wheel pulses, each starting before and ending after the corresponding tone wheel pulse. When the gate 91 is enabled by a pulse from the print control 25, the tone wheel pulse occuring at the time of this pulse will pass through the gate 91 and trigger a monostable multivibrator 97. Thus, in the example illustrated in FIG. 6, the tone wheel pulses will pass through the gate 91 at the start of the first, third and fourth firing periods. As a result, the monostable multivibrator 97 will produce a square wave output pulse at the start of the first, third and fourth firing periods as represented by the wave form 99 in FIG. 6. These pulses are designed to have a length equal to the length of the firing pulses to be applied to the hammer 13. The output pulses produced by the monostable multivibrator 97 are applied through an OR gate I01 to the hammer firing circuit 29 in exactly the same manner that the pulses passing through the OR gate 26 in FIG. I are applied to the hammer firing circuit 29 in the embodiment of FIG. 1. As a result, the hammer firing circuit 29 in response to each output pulse produced by the monostable multivibrator 97 will apply a firing pulse to the hammer l3 and cause it to strike the drum II. The output pulse produced by the monostable multivibrator 97 is also applied through a delay circuit 103 to a second monostable multivibrator to trigger the second monostable multivibrator 105. As a result the monostable multivibrator 105 will be triggered after a delay and produce an output pulse in response to each output pulse produced by the monostable multivibrator 97. These pulses are illustrated in FIG. 6 by the wave form I07. As shown in FIG. 6, pulses produced by the monostable multivibrator 105 are substantially smaller in width than the pulses produced by the monostable multivibrator 97 and are selected to have a width equal to the width of the damping pulses to be applied to the hammer 13 following each firing pulse applied to the hammer 13. The output pulses produced by the monostable multivibrator 105 are applied through the OR gate 101 to the hammer firing circuit 29 and cause the hammer firing circuit 29 to apply a pulse of the corresponding width to the hammer 13. Thus following each hammer firing pulse applied to the hammer 13, a damping pulse of the desired width is applied. The delay provided by the circuit 103 is selected so that the damping pulse is applied just before the next firing pulse would be applied if a firing pulse is to be applied in the next firing period. Wave form 109 illustrates the pulse wave form applied to the hammer firing circuit through the gate 101 and thus represents the timing of the firing pulses and damping pulses applied to the hammer 13.

In the embodiments described above, the damping pulses are applied to the hammer after the hammer has cleared the medium on which the printing is carried out. To further increase the speed of operation, the damping pulse may be started before the hammer is completely clear of the printing medium. This will tend to smear the printing when the hammer is fired in successive print periods. This problem can be avoided by eliminating the damping pulse in those instances when the hammer is to be fired in the next succeeding firing period.

FIG. 7 illustrates a system for carrying out this alternative in which the damping pulses are eliminated when a firing pulse is to be applied in the succeeding firing period. The system of FIG. 6 is just like that of FIG. 5 except that the output of the monostable multivibrator 105 is applied through an AND gate 109 to the OR gate 101. The AND gate 109 will be enabled by a signal derived from the output of the print control 25 through an inverter 113. The inverter 113 will enable the gate 109 only when the output of the print control 25 is not producing an output pulse. Because of the timing of the pulses produced by the print control 25, the gate 109 will be enabled when a damping pulse is produced only if no firing pulse is to be applied to the hammer 13 in the succeeding firing period. Thus, in the system of FIG. 7, the damping pulses can be applied before the hammer clears the medium on which the printing is being carried out. It should be noted that the width of the damping pulse may be varied, if desired, in accordance with whether or not the hammer was just previously fired. Such a system may be desirable owing to the fact that the velocity and certain second order effects such as hammer oscillation are a function of the just previous firing history of the hammer. in yet another embodiment of the invention, the actual velocity of the hammer may be sensed, and the width of the damping pulse determined by this actual width in a closed loop system. Conveniently, the actual hammer velocity can be sensed by means ofa coil attached to the hammer blade and in the field of the hammer magnet.

The above-described systems greatly reduce mechanical oscillation of the hammers after they are fired and thus enable the hammers to be fired at a much higher rate than the systems of the prior art. As pointed out above, the principles of the invention are applicable to other pulse actuated electromechanical devices in addition to hammers for high speed printers. The invention will enable such devices to be operated in response to pulses at a higher rate in the same manner that it enables the hammers of the high speed printer to be operated at a higher rate. The above-described systems are the preferred embodiments of the invention and many modifications may be made thereto without departing from the spirit and scope ofthe invention.

What is claimed is:

1. in a high speed printer having a print member and at least one pulse responsive electromagnetically operated hammer, said hammer being fired to strike said print member to thereby print in response to firing pulses applied thereto, the improve ment comprising means selectively operable to apply a firing pulse to said hammer to cause said hammer to strike against said print member and rebound therefrom and to apply a second pulse to said hammer following said firing pulse while said hammer is still moving, said second pulse being of a polarity to oppose the kinetic energy of said hammer and being of insufficient impulse content to cause said hammer to strike said print member.

2. In a high speed printer as recited in claim 1 wherein said second pulse is applied while said hammer is rebounding away 6 from said rint member.

3. In a igh speed printer as recited in claim 2 wherein the ratio of the impulse content of said second pulse to that of said firing pulse is approximately equal to the ratio of the velocity of said hammer upon rebounding from said print member to the velocity ofsaid hammer in striking said print member.

4. In a high speed printer as recited in claim 1 wherein the impulse content of said second pulse is such to approximately cancel out the kinetic energy of said hammer when said second pulse is applied to said hammer.

5. In a high speed printer as recited in claim 1 wherein said firing pulse is terminated before said hammer strikes said print member.

6. in a high speed printer as recited in claim 5 wherein said second pulse is not applied to said hammer until said hammer has rebounded from said print member and hascleared the medium on which said printing is carried out.

7. In a high speed printer as recited in claim 1 wherein said means to apply pulses to said hammer comprises an amplifier connected to apply pulses to said hammer in response to pulses applied to the input thereof and includes means to apply negative feedback signal to said amplifier.

8. In a high speed printer as recited in claim 7 wherein said amplifier comprises a transistor, wherein said hammer has an actuating coil connected in the collector circuit of said transistor, and wherein a resistance is connected in the emitter circuit of said transistor to apply said negative feedback signal to said amplifier.

9. In a high speed printer as recited in claim 3 wherein said means to apply pulses to said hammer comprises an amplifier connected to apply pulses to said hammer in response to pulses applied to the input thereof, said amplifier including negative feedback means for maintaining a constant current amplitude of said pulses applied to said hammer.

10. In a high speed printer comprising a print member and at least one pulse-responsive, electromagnetically-operated hammer, said hammer being fired to strike said print member and carry out printing in response to firing pulses applied thereto, the improvement comprising means selectively operable to apply a firing pulse to said hammer in each of a series of successive firing periods and to apply a damping pulse to said hammer following each firing pulse only if no firing pulse is to be applied to said hammer in the next succeeding firing period, said damping pulse having the same polarity of said firing pulse and being applied to said hammer while said hammer is rebounding from the medium on which the printing is being carried out as a result of the preceding firing pulse, said damping pulse having insufficient impulse content to cause said hammer to restrike said print member.

11. A combination comprising a pulse responsive electromagnetically operated device which operates to drive an output member against a stop in response to an applied pulse and means to apply a first pulse to said device to drive said output member against said stop and to apply a second pulse to said device following said first pulse applied thereto after said output member has been driven against said stop while said output member is still moving, said second pulse having a polarity to oppose the kinetic energy of said output member and having insufficient inpulse content to drive said output member against said stop.

12. A combination as recited in claim 13 wherein said second pulse is applied to said device while said output member is rebounding away from said stop.

13. A combination as recited in claim 14 wherein the ratio of the impulse content of said second pulse to that of said first pulse is approximately equal to the ratio of the velocity of said output member in rebounding from said stop to the velocity of said output member in striking said stop.

14. A combination as recited in claim 13 wherein the impulse content of said pulse is selected to approximately cancel out the kinetic energy of said output member.

Claims

1. In a high speed printer having a print member and at least one pulse responsive electromagnetically operated hammer, said hammer being fired to strike said print member to thereby print in response to firing pulses applied thereto, the improvement comprising means selectively operable to apply a firing pulse to said hammer to cause said hammer to strike against said print member and rebound therefrom and to apply a second pulse to said hammer following said firing pulse while said hammer is still moving, said second pulse being of a polarity to oppose the kinetic energy of said hammer and being of insufficient impulse content to cause said hammer to strike said print member.

2. In a high speed printer as recited in claim 1 wherein said second pulse is applied while said hammer is rebounding away from said print member.

3. In a high speed printer as recited in claim 2 wherein the ratio of the impulse content of said second pulse to that of said firing pulse is approximately equal to the ratio of the velocity of said hammer upon rebounding from said print member to the velocity of said hammer in striking said print member.

6. In a high speed printer as recited in claim 5 wherein said second pulse is not applied to said hammer until said hammer has rebounded from said print member and has cleared the medium on which said printing is carried out.