US2997632A

US2997632A - Hammer firing circuit for high speed printer

Info

Publication number: US2997632A
Application number: US760590A
Authority: US
Inventors: Jr Francis H Shepard
Original assignee: Individual
Current assignee: Individual
Priority date: 1958-09-12
Filing date: 1958-09-12
Publication date: 1961-08-22
Anticipated expiration: 1978-08-22
Also published as: GB912547A

Description

Aug. 22, 1961 F. H. SHEPARD, JR 2,997,632

HAMMER FIRING CIRCUIT FOR HIGH SPEED PRINTER Filed Sept. 12, 1958 INVENTOR fkmw; flap/1 90 JR.

AT ORN nite 2,997,fi32 Patented Aug. 22, 1961 2,997,632 HAMMER FIRING CIRCUIT FOR HIGH SPEED PRINTER Francis H. Shepard, Jr., Lee Lane, Berkley Heights, NJ. Filed Sept. 12, 1958, Ser. No. 760,590 5 Claims. (Cl. 317-1485) This invention relates to an improved electric circuit for firing a type-hammer in a high speed printer, such as the printer described in the inventors US. Patent No. 2,787,210.

An object of this invention is to provide a hammer firing circuit which can be made at lower cost than previously known circuits and which will operate with greater reliability and efiiciency.

A more specific object is to provide such a firing circuit which can be made more compactly and have greatly increased service life, together with increased ruggedness and reliability.

These and other objects will in part be understood from and in part pointed out in the description given hereinafter.

In a high speed printer of the kind described in the above-identified patent, a strip of paper is passed through the machine and printed upon by blows from an array of hammers positioned beneath the paper and fired up- Ward to drive the paper against corresponding ones of a like array of type wheels. These type wheels are mounted side-by-side in the form of a cylindrical drum and carry around their periphery alpha-numeric characters to be printed on the paper. The type wheel assembly is rotated at high speed, for example 1200 rpm, and the paper is moved step-by-step at high speed, for example twenty lines a second, so that the printing operation can be carried out at a very rapid rate. Each hammer is in the form of a long, thin rod which is fired end first upward against the paper to press it against a selected one of the characters around the rim of the corresponding type wheel. The time of printing is very short, of the order of fifty microseconds, so that even though the type wheel is continuously rotating its movement is effectively frozen during each hammer blow.

In this machine each hammer is propelled against the paper by an electromagnet which is energized at the proper instant by a firing circuit electronically controlled to print any desired character. Each hammer, and there may be for example 190 hammers in a machine, is accompanied by an electromagnet to fire it and by a corresponding firing circuit to energize the electromagnet. Thus in a single printer a vast number of electric circuits for firing the hammers is required. Accordingly, it is very desirable to have these circuits as inexpensive and as compact and reliable as possible. The present invention accomplishes these objectives.

In a high speed printer of this kind, the firing circuit for each type-hammer must be able to energize the coil or solenoid of the hammer-firing electromagnet with a carefully regulated current. This current must suddenly and accurately be applied to the coil in order to fire the hammer at the precise instant required for it to drive the paper against a particular type character of the continuously rotating type wheel. Moreover, the energy applied to the coil must not be too great nor too small, otherwise the hammer will print with too much or too little energy. Since a printer may be operated many hours a day for days-on-end, billions of hammer blows will be made during the life of the unit and this demands an extremely high degree of reliability in the electronic circuits which time and fire the hammer.

Previous circuits which have been used throughout the industry to fire hammers in a printer of this general kind employ a relatively large storage capacitor which is charged to a high voltage, for example 600 volts, and then suddenly discharged by means of a thyratron into the coil of its hammer-firing electromagnet. Not only does a circuit of this kind require relatively large stand by power and dangerous high voltages but it is relatively inefficient in its operation and poor in reliability. The service life of the thyratron tube in this heavy duty application is limited, and because of the overall inefficiency in the way this circuit operates it is expensive in terms of size of components and in power consumed. The present invention provides a much simpler and far more effective circuit for energizing the hammer-firing electromagnets.

l n accordance with the present invention a power transistor is operated as a switch at low voltage and very high current to energize an associated hammer-firing electromagnet. This transistor is operated and controlled by several other transistors to turn on at precisely the desired instant and for a carefully regulated time. is operated in such fashion that it is able to handle powers far in excess of its normal rating as an amplifier. By using an all-transistor switching arrangement of this kind, very high speed operation yet extremely precise firing control is obtained and there is no need of storage capacitors which might otherwise tend to cause oscillations in conjunction with the hammer electromagnet. By doing away with these storage capacitors, the delay in recharging them is eliminated and this increases the maximum speed of the printer. The standby power consumption of this transistor circuit is extremely low and this means that a multitude of these circuits can be enclosed in a small cabinet without harmful over-heating. With this circuit by controlling the on time of the power transistor, it is a simple matter to adjust the amount of energy applied to the hammer-firing electromagnet. The power transistor can be triggered on by its associated transistors in response to a very low energy pulse and accordingly the circuit is fully compatible with existing computing equipment normally associated with a high speed printer of this kind. By virtue of the unique way in which the circuit operates, the power or output transistor can be a very inexpensive one rather than one specially designed for high internal power dissipation and which at the present time would be very expensive.

A better understanding of the invention together with a fuller appreciation of its many advantages will best be gained from a study of the following description given in connection with the accompanying drawings wherein:

FIGURE 1 shows a type-hammer firing arrangement embodying features of the invention; and

FIGURE 2 shows typical current-voltage curves for a power transistor with certain load lines superimposed on these curves.

The arrangement shown in FIGURE 1 comprises an electric hammer-firing circuit, generally indicated at it which is connected by means of a pair of wires 12 to a type-hammer assembly generally indicated at 14. The latter, which is described in more detail in the aforesaid patent, includes an electromagnet 16 having two coils wound on a U-shaped iron core which in turn is part of a stationary frame 24). Positioned beneath these coils is an armature 22 which is pivoted to the frame at 24- and which when the coils are energized is drawn quickly upward against the core. The normal rest position of armature 22 is determined by a set screw 2-5, and the armature is urged into this position by a tension spring 28.

When the electromagnet is energized the right end of the armature is moved at high velocity upward and into contact with the rear end of a type hammer 39. This imparts an upward velocity to the hammer which then flies as a free projectile out of contact with the armature against a strip of paper 34 and an ink ribbon 36 to force them against a desired one of the type characters spaced around the rim of a continuously rotating type wheel 38. This hammer blow (which may be of the order of 50 microseconds in duration) prints a letter on the paper and then the hammer quickly returns to the position shown and the paper advances to the next line prior to printing another letter. The paper may be printed at speeds of the order of twenty lines per second. In a single machine there may, for example, be 190 hammers arranged side-by-side in a row perpendicular to the plane of FIGURE 1, each hammer being controlled by a separate circuit 10.

As will be appreciated, the instant of firing of hammer 38 must be precisely controlled in time in order for it to strike a desired one of the characters around wheel 38 as it rotates. At the precise instant, determined by a timing or trigger pulse applied to circuit 10, electromagnet 16 is suddenly connected via leads 12 by means of a power transistor 40 across a battery 42 (or other suitable low resistance source) whose positive side is grounded. The current build-up thereafter in coils 18 is very rapid (being determined by their inductance). The time delay between the triggering of circuit and the printing of a character is small, but whatever delay is present, however, is uniform from cycle to cycle so that once the triggering pulse has been adjusted for proper printing it can be left alone. The same triggering pulse can be used to actuate only one hammer or all of the hammers in the machine at the same time. Some control of the instant of printing of any given hammer without adjustment of the triggering pulse can thereafter be effected simply by controlling the length of time during which voltage from battery 42 is applied to coils 18. r

In order to turn transistor 40 on and fire the hammer, the base 44 of the transistor is suddenly made negative and a heavy current drawn from it. To this end base 44 is connected to the emitter 46 of a driver transistor 48, emitter 46 being connected in series with a resistor 50 to the positive side of a bias battery 52 whose negative side is connected to the emitter of transistor 40 and the positive terminal of battery 42. The collector 54 of transistor 48 is connected through a current-controlling resistor 56 to a common lead 59. This lead is connected through a decoupling resistor 60 to the negative side of battery .2 and is bypassed to ground by a large filter capacitor 61. Current drawn from base 44 of transistor 40 is returned to the emitter through transistor 48, resistors 56 and se, and battery 4-2. When transistor 48 is made conducting, it draws a heavy current from the base of transistor 48 so that the emitter-to-collector resistance of this transistor is extremely small. Thus, for load currents through the transistor far in excess of normal rated value, the voltage drop across and power dissipated in it are still well within permissible ratings. The maximum current which flows through this transistor when on is determined primarily by the voltage of battery 42 and the resistance of coils 18. To prevent transistor 40 from being damaged by inductive surge from coils 18 when the transistor is turned off, the collector of the transistor is bypassed to the negative side of battery 42 by a rectifier 58 which prevents the collector from going too negative.

The base 62 of transistor 48 is connected to the collector 63 of a transistor 64, this collector being also connected to lead 59 through a low-value load resistor 66. The emitter 68 of transistor 64 is connected to the positive side of battery 52, so that the transistor is normally biased on. The base 70 of transistor 64 is biased through a diode 72 and a diode 74 connected in series to the positive terminal of battery 42. The junction of

diodes

72 and 74 is connected through a high value bias resistor 75 to lead 59. Normally transistor 64 is turned on but when a positive pulse is applied to base 7 0 through capacitor 76 from a timing or trigger source (not shown) the transistor is turned off. This turns transistors 48 and 40 on to fire hammer 30.

When driver transistor 48 is turned on, a positive voltage is generated across load resistor 56 and this is fed back over a lead 78 through a capacitor 80 and diode 72 to base 7% of transistor 64. This voltage keeps transistor 64 turned 01f for a length of time determined by the charging time-constant of capacitor 80 and a resistor 82 connected between base '70 and lead 59. During this on time, a heavy current is drawn from the base of transistor 40 and this insures that it will conduct with an extremely low voltage drop between emitter and collector. Resistor 82 is made adjustable so that the on time can be varied. Diode 72 provides a high input impedance for the positive timing pulse to transistor 64 and diode 74 acts as a DC. restorer for capacitor 80. Alternatively, where a low impedance input can be tolerated, these diodes can be omitted and capacitor 80 connected to base 7% of transistor 64 through a coupling resistor 83, indicated by dotted lines.

To prevent the circuit from being turned on by a trigger pulse, if desired, transistor 64 is connected in parallel with a transistor 84 which is adapted to be turned on by a negative voltage applied to its base through a resistor 85 or, alternatively, off by a positive voltage. When transistor 84 is on, driver transistor 48 cannot be turned on; when transistor 84 is off, driver transistor 48 can be turned on to turn transistor 48 on.

FIGURE 2 shows curves of emitter-to-collector voltage (Eec) versus current (lee) for various emitter-to-base bias currents (Ieb) and illustrates the mode of operation of transistor 48. When off, the current through this transistor is substantially zero and the voltage across it is equal the voltage of battery 42 as indicated at point A. When the emitter-to-base current of the transistor is suddenly increased to saturation value, as indicated by point B, the internal resistance of the transistor drops from a value of many thousands of ohms to nearly Zero. At saturation, i.e. any value of base current along line S on which the current from emitter to collector is primarily determined by the load resistance and supply voltage, the voltage drop from emitter to collector for a germanium transistor is only a tenth of 21 volt or so. When the current flowing into coils 18 through the transistor begins to build up, because the coils are inductive the load line follows along the curve L in FIGURE 2 instead of the straight line R which illustrates a resistive load. Accord ingly, at no time in going from points A to B along curve L is the power dissipation of the transistor, indicated by curve P, exceeded. When the base of the transistor is again brought positive, the voltage across emitter and collector suddenly jumps to point C and thereafter rectifier 58 immediately reduces the current to its value at point A. The time required for the current and voltage to chance along line BOA is only a few microseconds, however, and hence though the normal power ratings of the transistor may be momentarily exceeded, it is not damaged.

In an actual circuit arrangement which has been built and successfully operated, the following elements and circuit values were used: battery 42, 27 volts;

battery

52, 2 volts; coils 18, 5 ohms total; transistor 40, RCA type 2N301 germanium and rated at 40 volts and 7 watts maximum;

transistors

48, 64, and 84, RCA type 2N217; resistor 56, 47 ohms; resistor 50, 150 ohms; resistor 66, 1000 ohms; resistor '75, 100,000 ohms; resistor 82, 33,000 ohms; resistor 83, 1000 ohms; capacitor 80, 0.1 microfarad;

resistor

60, 50 ohms; capacitor 61, microfarads. The time transistor 40 remained on in response to each trigger pulse was of the order of 1 to 3 milliseconds. The maximum current (Iec at point B) was about 6 amperes and reached this value near the end of each on cycle. The voltage drop across transistor 40 at point B was about 0.2 volt.

The above description of the invention is intended in illustration and not in limitation thereof. Various changes may occur to those skilled in the art and these may be made without departing from the spirit or scope of the invention as set forth.

I claim:

1. An improved hammer-firing arrangement for a high speed printer comprising: a hammer-firing solenoid, a low resistance source of voltage adapted to supply a relatively large pulse of current to said solenoid, a power transistor having its emitter and collector connected in series with said solenoid and said source, a rectifier connected across said solenoid and polled to prevent the voltage thereacross from reversing, a driver transistor having an input and an output, said output being connected to the base of said power transistor so that when said driver transistor is off said power transistor is off and when said driver transistor is on the base of said power transistor is biased to saturation and said power transistor conducts a large current with a small voltage drop across itself, a trigger transistor having an input and an output and which is normally turned on, said output being coupled to the input of said driver transistor, and time-constant positive feedback means for applying to the input of said trigger transistor a voltage pulse derived from the turning-on of said power transistor to keep said power transistor on for a controlled time.

2. The arrangement as in claim 1 wherein said trigger and driver transistors are interconnected so that when said trigger transistor is pulsed ofi by a momentary pulse,

said driver transistor is turned on and holds said power transistor on and said trigger transistor off for a precise time determined by said feedback means.

3. The arrangement as in claim 2 wherein said power transistor is a type 2N301 or its equivalent, said battery has a voltage of about 27 volts, and said solenoid has a resistance of about 5 ohms.

4. The arrangement as in claim 1 in further combination with inhibit means connected to said trigger transistor to prevent it from turning oil.

5. A high speed controllable switching circuit comprising a relatively low resistance load, a very low resistance direct voltage source, a power transistor having an input and an output said output being connected in series with said lead, and a plurality of transistors connected as a multivibrator arrangement and having an input and an output, said output being connected to the input of said power transistor and adapted to drive it to saturation so that its internal resistance is very low, said input of said multivibrator arrangement being adapted to be connected to an external signal source, said arrangement constituting a variable element whereby the power supplied to said load can be variably controlled.

References Cited in the file of this patent UNITED STATES PATENTS 2,864,978 Frank Dec. 16, 1 958 UNITED :STATES PATENTOFFICE D CERTIFICATE OF CORRECTION Patent N00 2 99 7 682 August 22 1961 Francis H. Shepard Jr.

corrected below.

In the drawings Figa l the upper resistor 75 which is connected to the positive side of battery 52 should be deleted.

' Signed and sealed this 27th day of March 1962.

(SEAL) Attest:

ERNEST W SWIDER DAVID L. LADD A t t e 5 ti ng Of f i C e r Commissioner of Patents UNITED STATES PATENT. OFFICE 7 CERTIFICATE OF CORRECTION Patent No. 2 997 632 August 22, 1961 Francis H. Shepard Jr.

It is hereby certified that error appears in the above numbered patent requiring correction and that the said Letters Patent should read as corrected below.

In the drawings Fig; l, the upper resistor 75 which is connected to the positive side of battery 52 should be deleted.

' Signed and sealed this 27th day of March 1962.

(SEAL) Attest:

ERNEST W. SWIDER DAVID L. LADD Attesting Officer Commissioner of Patents