US3628102A

US3628102A - Exciter apparatus for impact member solenoid

Info

Publication number: US3628102A
Application number: US863824A
Authority: US
Inventors: Kenneth E Jauch; Charles W Koeller
Original assignee: NCR Corp
Current assignee: NCR Voyix Corp; National Cash Register Co
Priority date: 1969-10-06
Filing date: 1969-10-06
Publication date: 1971-12-14
Anticipated expiration: 1988-12-14

Abstract

A control for current interruption and performance regulation in an electrical solenoid is disclosed. In one embodiment of the invention, solenoid current magnitude is controlled independently of supply voltage and variation in electrical element characteristics, and current magnitude is determined in a manner enabling simultaneous adjustment of current level, and solenoid force, in a plurality of similar circuits. Means for control of solenoid excitation in response to any of a plurality of solenoid performance properties is also disclosed.

Description

United States Patent lnventors Kenneth E. Jauch 7 3,136,928, 6/1964 Avis 317 14s.5 sm 7 3,178,617 4/l965 Coker 317 33 Charl Koeller, Yellow p t Ohio 3,l83,830 5/1965 Fisheret al. 3l7/l48.5 x [2 pp N9 863,824 3,469,152 9/l969 Bosman 3l7/l48.5 x [2 1 Filed Oct-6, 1969 3,512,047 5 1970 Garde 317 33 Patent D 14, 1 1 2,997,632 8/l96l Shepard 3l7/l48.5 [73] Assrgnee The NationalCash Register Company 3,096,475 7/1963 Brooks 317/33 X Dayton, Ohio 3,274,446 9/l966 Nagata'. 317/33 3,437,905 4/1969 Healey et al. 323/9 X 3,445,751 5/l969 Easter 317/33 X [54] EXCITER APPARATUS FOR IMPACT MEMBER 3 473 lo I 9 bl SOLENOID r 6 l0/ 96 Gra 3l7/33 X 13 Claims, 3 Drawing Figs. Primary Examiner-Lewis H. Myers Assistant Examiner-Ulysses Weldon [52] U.S.Cl Almmeys Louis Kline and John L Callahan [51] lnt.Cl ..l-l0lh 47/32 of Search A onnfor current interruption and per- 157; 1 1/ RC. R formance regulation in an electrical solenoid is disclosed. in one embodiment of the invention, solenoid current magnitude [56] Rderences is controlled independently of supply voltage and variation in UNITED STATES PATENTS electrical element characteristics, and current magnitude is 2,978,630 4/1961 De La Tour 317/33 determined in a manner enabling simultaneous adjustment of 2,997,632 8/l961 Shepard 3l7/l48.5 current level, and solenoid force, in a plurality of similar cir- 3,048,748 8/1962 Carey.... 317/ I57 cuits. Means for control of solenoid excitation in response to 3,096,475 7/1963 Brooks 317/33 X any of a plurality of solenoid performance properties is also 3,125,7l5 3/1964 Brooks 3 l7/33 X disclosed.

Sl/ISO I35\ I49 I33 I36 I2] I47 122 I48 Patented Dec. 14, 1971 3,628,102

[ also INVENTORS I KENNETH E. JAUCH CHARLES W.KOEL ER BY WITNESS gem W MM/w THEIR ATTORNEYS EXCITER APPARATUS FOR IMPACT MEMBER SOLENOID BACKGROUND OF THE INVENTION l. Field of the Invention This invention is applicable to the art of solid state electronic controls usable with. the actuating solenoid of various mechanism, including a high-speed printing mechanism or other electromagnetically driven mechanisms.

The invention further pertains to a solid state feedback control having a plurality of inputs one of which is capable of limiting conduction in the solid state members in response to some performance property of the solenoid and also in response to some external command which is applicable to a plurality of similar circuits.

2. Description of the Prior Art It is well known in the prior art to control excitation of a solenoid coil by placing a switching circuit composed of one or more solid state elements, such as transistors, in the electrical excitation path of the solenoid coil. In that prior art solenoid excitation, the magnitude of current flowing in the excited coil becomes a function of the power source's voltage amplitude, the voltage across the switch element, and the impedance of the solenoid. Where precise mechanical action is needed from the solenoid's output member, as is true in a modern high speed printer, the dependence of solenoid excitation upon variables in the exciting circuit and upon power source voltage is undesirable; the high-speed printer environment often calls for solenoid excitation to be precisely controlled or, in some instances, to be responsive to a performance characteristic of the solenoid.

The prior art in solenoid excitation also discloses the use of a linear control element to control excitation of a solenoid coil, a common embodiment of this concept being a vacuum tube having a relay coil connected into its plate circuit or a transistor amplifier having a relay coil connected to its output. In prior art linear exciters such as these, the magnitude ofcurrent flowing in the coil is determined by the amplitude of a signal applied to the amplifier's input terminal or by the power supply's output voltage or by impedance properties of the coil or by a combination of these parameters.

In the present invention, prior art solenoid excitation is improved upon by providing means for the solenoid current to be controlled in a manner independent of variations in circuits parameters and also in response to solenoid performance and under the influence of an'adjustable reference level.

SUMMARY OF THE INVENTION In the 'present invention, current flow in solenoid coils is made to depend upon the magnitude of control signals which are separate and apart from that conveying the open and close DESCRIPTION OF THE DRAWING Fig. I shows an essential element block diagram of one embodiment of the present invention.

FIG. 2 shows one embodiment of a solenoid excitation circuit made according to the present invention.

FIG. 3 shows a high-speed printer mechanism which is typical of mechanisms that may have actuation solenoids excited by the circuit of the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT .In prior art high-speed printing mechanisms. it is common practice to control actuation of the electrical solenoid by means of a transistor switching circuit. According to this prior artmethod of control, the electrical solenoid, the transistor switching member, and the power source are all placed in series, and suitably timed pulses are applied to the transistor control electrode in order that timed excitation of the solenoid is accomplished.

In the precisely optimized environment of a third or fourth generation high-speed printer mechanism, this prior art method for controlling solenoid excitation is found to have disadvantages making it incompatible with the degree of precision maintainable in other parts of the mechanism.

The simple transistor switching circuit as used in prior art printing devices, while in the turned-on state, is possessed of a voltage drop having magnitude directly dependent on transistor saturation characteristics. In practice, it has been found that production variation in transistors and degradation of transistor characteristics during operating life make saturation characteristics vary to a degree which is unacceptable for a precisely designed mechanism. In one printer mechanism, similar to that shown in FIG. 3 ofthe drawing, for example, it is found desirable to maintain current flowing in the solenoid within 1 percent of the design value. It is found that transistor saturation characteristics when the transistor is placed in a prior art driving circuit are alone sufficient to make such l percent variation limits difficult to maintain over a long operating life.

Another difficulty often encountered when a conventional switching circuit drives a solenoid in a high-speed printer mechanism is that variations in the printer power supply voltage are reflected in performance of the printer. Since the high energy flows demanded of a power supply in a large data processing printer make the supply difficult to regulate within close voltage tolerance, it is desirable that some means to be found are making printing performance reasonably independent of power supply fluctuation. The conventional solenoid driving circuit, with its simple series arrangement of power supply, solenoid, and switching member, is incapable of compensating for'power supply fluctuation, so that both the force developed within the solenoid member and the time over which this force increases to a maximum may be expected to vary in response to power supply fluctuations.

Another disadvantage of the conventional prior art solenoid actuating control is that nomeans is available for adjusting the current in a solenoid on either a group or an individual basis. It is often found that tolerances introduced during fabrication of the solenoid and other portions of the printing mechanism make it desirable to adjust the solenoid current on an individual basis. It is also often found that testing of the printer or operationally varying its performance makes collective adjustment of the solenoid currents desirable. In the prior art conventional solenoid driver control, no provision is made for either individual or collective current adjustment.

Yet another disadvantage of the conventional prior art solenoid actuating control is that no provision is made for having the solenoid excitation determined by some property of the solenoid's performance, as opposed to being dependent on solenoid current or supply voltage.

In the present invention, there is disclosed a solenoid driver 1 control which overcomes the difficulties encountered with the comprises a linear circuit. in lieu of saturating the transistors which drive the solenoid coil, in Fig. 2 these transistors are maintained, after initial turn-on, in the linear,-or active, region, wherein the degree of conduction is controllable and wherein components external to the transistors are paramount in controlling the current level.

The essential parts of one embodiment of the present invention are shown in FIG. 1; these parts comprise an amplifiersensing network identified by the numeral 145, a source of time command signals 147 capable of commanding opening or closure of a solenoid 149 to be driven, a source of reference signal 146, and a path 170 for coupling a solenoid performance signal into the amplifier-sensing network 145.

The amplifier-sensing network 145 in FIG. 1 is composed of three

amplifier transistors

129, 130, and 131 (FIG. 2) connected into a Darlington amplifier; a comparator transistor 125; a solenoid current sensing resistance 132; a reverse current blocking diode 124; a reactive element 127; a voltagelimiting network comprising a resistor 133 and a diode 134; and terminals which are connectable to other portions of the mechanism. Among these terminals, the terminal 123 connects to the source of input or command signals 147; the terminal 126 connects to the source of reference signal 146; the terminals 136 connect to the driven solenoid coil 138; and the lead 135 connects to a source of electrical energy 160. The source 160 is capable of exciting the solenoid and is a source of positive potential direct current energy in the embodiment shown in FIG. 2.

Operation of the circuit shown in FIG. 2 is as follows. The

transistors

129, 130, and 131 are connected into a conventional Darlington'amplifier having common collectors and series-connected emitters and bases. Conduction in this Darlington circuit is initiated, or turned on, by the receipt of a positive level signal at the terminal 123.

The current flowing into the terminal 123 during turn-on of the Darlington amplifier is small with respect to the current flowing in the solenoid coil 138, the Darlington amplifier being capable of providing a current gain near 50,000. The current flowing into the terminal 123 is derived from the source ofcommand signal 147, which may comprise a conventional switching circuit having a current source resistor 121, a switching transistor 120, and an output terminal 122 in the present embodiment. The current flowing between the source ofcommand signal 147 and the terminal 123 of the solenoid actuating circuit may be in the order of one milliamp., a value which is small enough to be easily supplied by the source of command signal 147, which may be made from integrated circuits and can employ either the illustrated junction transistors or field effect transistors. 7

Current flowing in the output stage of the Darlington amplifier circuit and the solenoid coil 138 also flows in the series resistor 132. This series resistor 132 has a small value, in the order of one half-ohm or less, so that current levels near ten amperes flowing in the solenoid coil 138 will produce a volt age drop of five volts or less across the resistor 132. The voltage drop across the resistor 132 is applied to the base of the transistor 125. Also applied to the transistor 125 at its emitter terminal is the reference voltage derived from the source 146 and appearing on the terminal 126; this reference voltage has a value equal to the voltage drop expected across the resistor 132 when the desired current flows in the solenoid coil 138. The voltage applied to 'the terminal 126 is therefore determinative of the current level to be reached in the solenoid coil 138 when current limiting operation of the amplifier ensues. Once the voltage appearing across the resistor 132 is more positive by a few tenths of a volt than the voltage at the terminal 126, the transistor 125 begins conducting. Conduction in the transistor 125 removes driving current from the input transistor 129 of the Darlington amplifier, by altering the division of current at the node 148 This removal of driving current decreases the degree of conduction in the entire amplifier and the current level in the solenoid coil 138. The path through the transistor 125 may thus be considered a feedback path which is operative to limit the solenoid current by limiting conduction in the transistors of the Darlington amplifier once the desired current level has been attained in the solenoid coil 138.

An operating cycle of the circuit shown in F 16. 2 starts with current being supplied intothe terminal 123 to effect turn-on of the Darlington amplifier comprising the

transistors

129, 130, and 131. Since the current in the resistor 132 will initially be of low amplitude, the transistor will be nonconducting, and no diversion of the current flowing into the terminal 123 will occur through the transistor 125. With full driving current from the terminal 123 applied to the Darlington amplifier, saturation or full conduction will occur in its transistors for a brief time. Depending upon the parameters of the solenoid circuit, the voltage applied to. the solenoid, and theamplifier characteristics, this period of full conduction may last for a time measurable in milliseconds, during which essentially the full voltage from the power source 160 appears across the solenoid coil 138; the voltagedrop across the Darlington amplifier is small and on the order of one to two volts during this full conduction. Once current in the solenoid coil 138 has reached the magnitude determined by the resistor 132 and the voltage at the terminal 126, conduction in the transistor 125 commences, and current is diverted away from the Darlington amplifier at the node 148 via the path through the diode 124 and the transistor 125.

Once conduction in the transistor 125 commences, the four transistors of the circuit are operating in a closed feedback loop, and, as is typical of feedback loop circuits, some consideration must be given to the dynamic operating stability of the closed loop. Dynamic stability in the feedback system comprising the four transistors in FIG. 2 may be analyzed according to the techniques of Bode and Nyquist, as is common practice in the feedback art. in the present circuit, it is found that sufficient stability results if the frequency response of the Darlington amplifier is rolled off at the high frequency end by means of a capacitor 127 placed between the base and the collector of the transistor 129. This capacitor 127 decreases the gain of the feedback system at high frequencies to a level which is less than unity and thereby afiords dynamic stability. It is also to be observed that the capacitor 127 acts as a limiter upon the switching speed of the Darlington amplifier when current is first applied at the terminal 123. In practice, a small capacitor isadequate to provide dynamic amplifier stability; however, a larger value may be employed in order that a delayed and slowed solenoid current rise time may be achieved.

Several components are shown in FIG. 2 which are incidental to the operation of the solenoid driver circuit as explained above. Among these components are the

resistors

128, 139, and connected between base and emitter of the Darlington amplifier transistors. These three resistors provide a path for leakage current in each of the amplifier transistors, so that leakage current does not cause conduction during periods when the amplifier is to be held in the nonconducting state. With silicon transistors used in the amplifier, leakage currents to be conducted by these resistors are relatively small, and the value of these resistors may be large enough so as not to imburden the amplifying efficiency of the Darlington stages during their turned-on operation. Also shown in FIG. 2 is a protective and energy-absorbing network consisting of the resistor and diode network 133-134. which are connected across the coil 138 at its terminals; the resistor 133 and the diode 13 4 provide a current flow path for the inductive or flyback current flowing in the solenoid 138 upon removal of the power source by turnoff of the Darlington amplifier. it is well known, in the art of driving an inductive load, that the voltage across terminals of a solenoid member instantaneously reverses upon interruption of the applied potential, since energy stored in the inductance of the solenoid attempts to maintain current flow in the solenoid. Since a positive potential is applied to the circuit in the FIG. 2 embodiment, the diode 134 in the flyback circuit prevents current flow in the resistor 133 during the interval when the solenoid 138 is excited by conduction in the Darlington amplifier. Only upon opening of the inductive circuit does the diode 134 permit conduction to occur in the resistor 133. The resistor 133 also offers a means for withdrawing kinetic energy from the moving members of the driven mechanism, since magnetic flux in the solenoid member couples with these moving members and causes energy from such motion to be converted into electrical form, where it may be dissipated by the resistor 133.

The diode 124 in FIG. 2 is incorporated in the circuit in order that current from the reference voltage source 146 connected to the terminal 126 may not flow into the amplifier via the transistor 125 and current from the resistor 132 may not flow into the amplifier via the transistor 125. 1

As was mentioned in introducing the circuit of FIG. 2, it is desirable that current flow in the solenoid 138 be independent of the solenoid supply voltage. The circuit of FIG. 2 offers this independence from solenoid supply voltage, since the current level maintained by the amplifier-sensing network 145 is determined by comparing the voltage drop across the resistor 132 to the voltage applied at the terminal 126. In essence, the voltage applied at the terminal 126 acts as a reference in measuring the current flowing in the solenoid 138. Since the voltage applied at the terminal 126 does function as a reference, it is desirable that a well regulated and low impedance source be used to supply the terminal 126. In a practical embodiment wherein there is a plurality of mechanisms and a plurality of solenoid control circuits within a single cabinet, the low impedance well regulated source used to excite the terminal 126 in FIG. 2 may also be employed to excite the counterpart of the terminal 126 in other solenoid circuits. A feedback operational amplifier 142 in FIG. 2 has been found to offer a convenient source for an adjustable well regulated voltage at the terminal I26. With the proper selection of input and feedback resistances, the operational amplifier 142 can exhibit output resistances well below a value of one ohm, so the addition of loads similar to that of the terminal 126 produces negligible effect upon the operational amplifier's output signal at the terminal 144.

It is notable that the operational amplifier's output may be made virtually independent of variations in printer power supply levels by supplying its input as the terminal 143 from a source (not shown) which is stabilized by a device such as a Zener diode or some other stabilizing means.

Since the current level within a solenoid may be changed easily by varying the voltage applied to the terminal 126 of the amplifier-sensing network 145 in FIG. 2, adjustment of the current level in the solenoid can be used as a means for adjusting print density when the driven solenoid actuates a printing mechanism. By causing a low current level to flow within a printer solenoid, the printing impact may be caused to be slight and the print density may be caused to be low, while, if the current level is high, a heavy impact and a greater printing density will be produced. Low current in the printer solenoid may also be useful in printing a small number of media copies where it is desired to have low printing force in order that piercing of the media material not occur.

Current level in the printing solenoid may be varied in the embodiment shown in FIG. 2 of the drawings by changing the reference voltage level applied at the terminal 126. The potentiometer at 141 provides one way in which this may be accomplished; other ways of varying the voltage at the terminal 141 are numerous and include changing the value of resistances in the network surrounding the operational amplifier 142 and varying the voltage at the terminal 143 of the operational amplifier.

Regardless of how the reference voltage to the terminal 126 is varied, it may be controlled by a potentiometer, and the manipulation of this potentiometer can provide a convenient operatonaccessible adjustment for print density and for number of forms to beprinted with the printer having the present solenoid control circuit. It is found in practice that control of the print density by means of controlling solenoid current also affects the timing of the print operation; that is, the time between application of a signal at the terminal 123 and impact of the media material by the printer hammer varies with the adjustment of current level in the amplifier-sensing network 145. In printers which are designed'to accommodate this timing variation, control of print density and compensation for the number of media thicknesses by means of current control in the solenoid driver is a convenient and desirable feature.

The following parts have been found to be suitable for use in the circuit of FIG. 2: transistor 129, 2N4409; transistor 130, 2N3055; transistor 131, 2N3772; transistor 125, 2N3904; diode 124, 1N46l. For the resistances shown in FIG. 2, the following values have been found suitable: resistor 128, 33K; resistor 139, 510 ohms; resistor 140, Si ohms; resistor 132, 0.4 ohm; resistor 133, 3 ohms; capacitor 127, 680 picofarads.

The printing solenoid 138 in FIG. 2 is excited from a power source designated 160, which has a positive potential of 48 volts in the embodiment shown and is applied via the lead the current level in each solenoid driver is adjusted to be near I l amperes at the peak. The solenoid has a DC resistance of 1.2 ohms. Solenoid current rise time occurs over a period near 0.2 millisecond.

FIG. 3 of the drawings shows one embodiment of a printer mechanism which may employ a solenoid control circuit made according to the present invention. In FIG. 3, there is shown a printing mechanism having a movable typefont carrier 152, a printing ribbon 150, media to be printed upon 151, a ballistic print hammer 57, and a printing mechanism frame member 160, which is position-adjustable by way of adjusting members 161. The numerals 54 in FIG. 3 represent a plurality of printer-actuating solenoids as have their exciting coil represented by the coil 138 in FIG. 2. Also shown in FIG. 3 are a solenoid backstop assembly 24 and a hammer penetration stop assembly 55. Kinetic energy from the solenoids 54 is carried to the hammer members 57 by way of actuating arms such as the one identified by the numeral 17.

The mechanism of FIG. 3 is but one embodiment of a mechanism which may employ a solenoid control made in accordance with the present invention. It is intended that the scope of the present invention not be limited to any particular printing mechanism. It is also intended that the scope of this invention in many other arts; notable among these are that of high speed punching of media and that of document and package-handling equipment. In the high speed punching of media, for instance, it is possible that a regulating and adjustable solenoid control circuit could be used for varying the force with which a punching pin strikes the media to be punched in order that different media materials be acceptable to the punch and similarly powered. In the document and packagehandling art, it is possible for a regulating and adjustable solenoid control circuit to be used for varying the force with which a document or a package is deflected from one location to another, so that different trajectories are possible for the deflected article.

Numerous alterationsv are possible for the circuitry shown in FIG. 2 of the drawings while yet maintaining the essential features of the invention. A few of the more obvious of these alternations are cited here as examples of those which will be suggested to a person skilled in the art.

In FIG. 2, the use of NPN'junction transistors is shown; it is clear that PNP-junction transistors could be employed in the circuit with equally desirable results. It is also possible for field effect transistors to be employed at least in the low current portions of the circuit.

In FIG. 2, a voltage-limiting network composed of diode 134 and resistance 133 is shown; it is of course possible to replace this network with many other networks, which range in scope from a single diode or a single resistance to combinations of diodes, resistances, and capacitances. The particular network selected will depend upon the performance characteristics desired in the circuit.

in FIG. 2 and in the text of this disclosure, a Darlington connected amplifier is employed; for one embodiment of the invention, the Darlington amplifier proved to be a convenient amplifier configuration. However, it is intended that the scope of the invention not be limited to the Darlington amplifier, since other amplifying connections involving one or more transistors may be substituted for the Darlington amplifier with satisfactory results.

The current-sensing resistance in H6. 2 is shown to be embodied as a single resistive element 132; in a practical machine, it may be desirable to embody this resistance as a plurality of series or parallel connected elements in order that small adjustments to the current flow in an individual solenoid can be made by addition or elimination of an element. Such an embodiment, for instance, is a convenient way for compensating small performance difference between adjacent printing mechanisms within a printer.

In FIG. 2, comparison of the solenoid current level to a reference level is accomplished by means of comparing the voltage developed across the resistor 132 to a reference voltage introduced at the terminal 126 by an external source 146, the comparison being made within the transistor 125. A person skilled in the art will recognize that other element configurations can be employed to compare a current level to some reference and then adjust the conductivity of a current control element; for instance, the voltage from the resistor 132 could be coupled to some conduction-limiting point in the amplifier circuit through a Zener diode element. In this configuration, the Zener diode provides the desired reference against which signal from the resistor 132 is compared, and the operational amplifier and associated components are eliminated.

in FIG. 2, a capacitor 127 is connected between the base and collector terminals of the transistor 129 in order that frequency response of the closed loop circuit may be controllable. it has been found that frequency response of the closed loop circuit may also be controlled by connecting a capacitor between the base and collector terminals of the comparator transistor 125 and that the effectiveness of such a capacitor can be enhanced by placing a resistance in series with the reference signal from the resistor 132. A capacitor connected across the comparator transistor [25 is effective to control frequency response around the closed loop, just as was the location across the transistor 129; however, in the 125 location, this capacitance does not retard the Darlington amplifier's switching speed during initial excitation of the solenoid a trait which is desirable in some environments; It is clear that each of these capacitor locations represents just one of a plurality of ways in which frequency response ofa closed loop circuit may be controlled. Other possible ways for implementing such control include the use of inductance elements within the loop and the use of shunting capacitance elements between any point having a signal voltage swing and ground.

In FIG. 2, the

resistors

128 and 139 are shown connected across the base-to-emitter junctions of the

transistors

129 and 130. It has been found that satisfactory or improved operation results in some environments if these resistors are replaced with resistors connected between the base of each transistor, 129 and 130, and the emitter of the transistor 131.

In FIG. 2 and in heretofore describing the invention, means has been provided for sensing the current flowing in the sole noid coil [38 and for coupling a signal representative of that current flow into a comparator circuit and thence into the amplifiers input to regulate conduction in the amplifier.

In a broader sense of the invention, it is not necessary that the signal which controls amplifier conduction represent current flow in the solenoid coil; the signal which controls amplifier conduction may instead be derived from one of many per formance properties of the solenoid which vary during operation. One such property which may be sensed is force exerted by the solenoid upon some other member of the mechanism. With a signal representative of force coupled into the comparator, a solenoid which is controlled by the present invention could be made to operate in a constant force mode.

Other properties of the solenoid may also be sensed in developing the signal coupled into the comparator included in these possible other properties are:

voltage across the solenoid coil,

magnetomotive force within the solenoid coil,

magnetic flux within the solenoid, and

acceleration of the solenoid output member or some coupled member. For developing each of these alternate signals, a different form of transducer will be employed"; for developing a signal representative of solenoid acceleration, for example, an accelerometer or force-measuring transducer would be necessary; for developing a signal representative of magnetic flux within the solenoid, a Hall Efiect sensor could be employed.

The path 170 in FIG. 1 of the drawing is shown in general and in dotted form as an indication that some signal definitive of solenoid-operating properties is to be coupled into the amplifier-sensing network from the solenoid structure. The exact nature of this signal and the manner in which it is derived will depend upon the solenoid performance property which is being controlled. It is, of course, possible to combine two or more of the solenoid-operation-defining signals, such as solenoid current flow and a signal representing solenoid output force, so that the solenoids output is controlled in response to more than a single performance property.

Through use of the herein-described invention in one embodiment, it has been possible to achieve advancement in the state of the art for a high-speed printing mechanism; with this invention, it has been possible to eliminate the performance degradation often encountered upon heat-up of the coils in a printing solenoid and to eliminate the performance variations encountered from variation of semiconductor components in the printer solenoid control circuit and from power supply fluctuations in the printer mechanism.

What is claimed is: t 1. An excitation control circuit for a printer-actuating solenoid member ofa high-speed printer mechanism, comprising: a semiconductor amplifier including a plurality of semiconductor devices, with the collector electrodes of said transistor devices being connected to a common terminal and thence to agroundable terminal of said printer-actuating solenoid member. said solenoid member having another terminalconnected to a ground referenced source of direct current energy and having avoltage limiter circuit connected between said ground referenced source and said groundable terminal,

said semiconductor amplifier having the emitter electrode of a prior transistor device connected to the base electrode of a succeeding transistor device,

and having an input terminal connected to the base electrode of a first transistor device serving as an amplifier input terminal, with said common collector terminal serving as one amplifier output terminal and the emitter electrode of the final transistor device serving as another amplifier output terminal;

a current-sensing resistance of a low value connected between the emitter electrode of the final transistor device and ground so as to have flowing therethrough the current flowing in said solenoid member and said final transistor device;

a comparator transistor device having its base electrode connected to a junction point between said currentsensing resistance and the emitter electrode of said final transistor device, its emitter electrode connected to a source of reference potential, and its collector electrode connected through a diode to said amplifier input terminal;

a frequency response determining capacitance element connected between the collector electrode and the base electrode of one transistor device in said semiconductor amplifiet;

a plurality of leakage current compensating resistances each connected with the base electrode of one transistor device so as to conduct away collector to base leakage current occurring within said transistor device; and

a source of timed current signals for commanding desirable opening and closing of said actuating member, said source of signals being coupled to said amplifier input terminal.

2. Electrical apparatus for effecting intermittent mechanical actuation of a movable print hammer in a printing mechanism; the apparatus comprising;

a source of direct current energy;

electromagnetic transducer means having a movable member that is mechanically coupled with the movable print hammer, the transducer means being electrically connected with the source of direct current energy and including as parts thereof electrical windings and magnetic flux conducting structure, the transducer means providing means for converting energy from the source of direct current energy into energy that moves the print hammer;

cascaded semiconductor linear amplifier means having an output port that is connected with the source of direct current energy and the electromagnetic transducer means, for controlling the flow of energy from the source of direct current energy to the electromagnetic transducer means, the amplifier means also having an input port for receiving amplifier controlling input signals, the amplifier being linearly responsive to analog signals applied to said input port;

performance sensing means joined to the electromagnetic transducer means and the amplifier means for generating an analog electrical signal representative of the instantaneous magnetic performance of said electromagnetic transducer means;

means for mixing the analog performance sensing means signal with two or more electromagnetic transducer means control signals, at least one of which is an intermittent control signal, and one of which is an analog transducer means current controlling signal and for supplying the mixed signals to the amplifier input port;

whereby the current which excites the print hammer, and

the print hammer, are time responsive to the intermittent control signal and are analog responsive to the analog transducer means current controlling signal.

3. Electrical circuitry in combination with a print hammer electrical solenoid structure for producing precise intermittent excitation thereof,- said electrical circuitry being composed of:

electrical windings magnetically coupled with said print hammer electrical solenoid structure, said electrical windings including a first terminal and a second terminal at the start and finish thereof;

a source of direct current electrical energy capable of exciting said print hammer electrical solenoid electrical windings and having a first output terminal and a second output terminal;

a bipolar power transistor having a base terminal, an emitter terminal, and a collector terminal;

a current-sensing resistance element having electrical resistance less than one ohm and having a first electrical terminal and a second electrical terminal;

means for connecting said source of direct current electrical energy, said electrical windings, said bipolar power transistor, and said current-sensing resistance element into a closed electrical circuit wherein said electrical windings first terminal is connected to said source of directcurrent electrical energy first output terminal, said electrical winding second terminal is connected to said bipolar power transistor collector terminal, said bipolar power transistor emitter terminal is connected with said current-sensing resistance element first electrical terminal, and said current-sensing resistance element is connected with said source of direct current electrical energy second output terminal;

a transistor amplifier including a first junction transistor having collector, emitter, and base terminals and a second junction transistor having collector, emitter, and base terminals, said transistor amplifier having the collectors of the first and second junction transistors connected together and connected to the collector terminal of said bipolar power transistor and having the emitter terminal of the first junction transistor connected to the base terminal of the second junction transistor and the emitter terminal of the second junction transistor connected to the base terminal of said bipolar power transistor; whereby said transistor amplifier and said bipolar power transistor are connected into a Darlington circuit;

an adjustable reference signal source capable of generating reference signals comparable with the voltage developed across said current-sensing resistance element;

a junction transistor for comparing and mixing signals from said reference signal source and said current-sensing resistance element, said junction transistor for comparing and mixing signals having a base terminal that is connected to said current-sensing resistance element first electrical terminal, and an emitter terminal that is connected to said adjustable reference signal source, and a collector terminal that is connected with the base terminal of said transistor amplifier first junction transistor; and

timed binary control signal source means connected with said base terminal of said transistor amplifier first junction transistor for supplying to said base terminal a binary signal capable of placing said first and second junction transistors in said transistor amplifier and said bipolar power transistor in the substantially nonconducting state and in the controlled conductivity state sequentially, thereby opening and closing said print hammer electrical solenoid structure.

4. Electrical circuitry in combination with a print hammer electrical solenoid structure as in claim 3 wherein said circuitry also includes energy dissipating voltage limiting means including an electrical diode and anelectrical resistance element connected across said electrical windings first and second terminalsfor dissipating energy stored in the magnetic circuit of said electrical solenoid structure. I

5. Apparatus for effecting precisely controlled intermittent contact between a printing font and media to be printed, said apparatus comprising the combination of:

movable print hammer means for movingsaid printable media into'pressured contact with said printing font;

a source of direct current electrical energy;

electrical solenoid means including electrical windings coupled to said source of direct current electrical energy and also including mechanical means connected with said movable print hammer means for transducing electrical energy from said source ofdirect current electrical energy into mechanical energy vested in said movable print hammer means;

linear amplifier circuit means including an input port and cascaded variable conductivity elements the final one of which is connected electrically via an output port between said source of direct current electrical energy and said electrical windings of said electrical solenoid means for halting and for regulating the instantaneous magnitude of the flow of electrical energy between said source of direct current electrical energy and said electrical windings of said electrical solenoid means;

current sensing means connected electrically between said source of direct current electrical energy and said electrical windings of said electrical solenoid means for generating an electrical signal representative of the instantaneous magnitude of the flow of electrical energy between said source of direct current electrical energy and said electrical windings of said electrical solenoid means;

reference signal source means for producing an electrical signal comparable with the instantaneous magnitude of said current sensing means electrical signal at the instant when said flow of electrical energy between said source of direct current electrical energy and said electrical windings of said electrical solenoid means a attains a desired predetermined magnitude;

digital control signal source means including a source of binary electrical signals for determining the printing and the quiescent time intervals of said movable print hammer means; and

signal comparing and mixing means connected with said current sensing means, -said reference signal source means, and saiddigital control signal source means and said linear amplifier circuit means input port for comparing said current sensing means electrical signal with said reference signal source means electrical signal and generating therefrom a difference signal and for mixing said difference signal with said binary electrical signals from said digital control signal source means and for thereby generating and applying to said linear amplifier circuit means input port a binary gated negative feedback difference signal capable of dynamically maintaining said linear amplifier circuit means variable conductivity elements in a linear operating region during the printing time interval of said digital control signal source means binary electrical signals. 7

6. Apparatus for effecting precisely controlled intermittent contact as in claim wherein said current sensing means includes an electrical resistance element having resistance less than one ohm.

7. Apparatus for effecting precisely controlled intermittent contact as in claim 5 wherein said final one of said variable conductivity elements in said linear amplifier circuit means includes a bipolar junction power transistor which is connected to at least one other bipolarjunction transistor in a Darlington circuit configuration.

8. Apparatus for effecting precisely controlled intermittent contact as in claim 5 wherein said signal comparing and mixing means includes a bipolar transistor having a base electrode terminal which is connected with said current sensing means, an emitter electrode terminal which is connected with said reference signal source means, and a collector electrode terminal which is connected with said linear amplifier circuit means input port and to said digital control signal source means. I v

9. Apparatus for effecting rapid precisely controlled simultaneous pressure contact between a medium to be printed and plural locations on a continuously moving printing font. comprising the combination of: i Y

a plurality of movable print hammer means for moving said printable medium into pressure contact with said continuously moving printing font;

a source of direct current electrical energy;

plural electrical solenoid means each including a set of elec trical windings connected with said source of direct current electrical energy and mechanical means connected with one of said print hammer means for transducing electrical energy from said source of direct current electrical energy into mechanical energy vested in said print hammer means;

a plurality of linear amplifier circuit means each including an input port and cascaded variable conductivity elements at least one of which is connected electrically between said source of direct current electrical energy and said electrical windings of one of said electrical solenoid means for halting and for regulating the instantaneous magnitude of the flow of electrical energy between said source of direct current electrical energy and said electrical windings of said electrical solenoid means;

a plurality of current sensing means each connected electrically between said source of direct current electrical energy and said electrical windings of said electrical solenoid meansefor generating an electrical signal representative of the instantaneous magnitude of the flow of electrical energy between said source of direct current electrical energy and said electrical windings of said electrical solenoid means;

a single reference signal source means for producing an electrical signal comparable with the instantaneous magnitude of said current-sensing mean electrical signal at the instant when said flow of electrical energy between said source of direct current electrical energy and said electrical windings of said electrical solenoid means attains a desired predetermined magnitude;

a plurality of digital control signal source means each including a source of binary electrical signals for determining the printing and the quiescent time intervals of said movable print hammer means; and

a plurality of signal comparing and mixing means each connected with said single reference signal source means,

one of said current sensing means, one of said digital con-- trol signal source means, and one of said linear amplifier circuit means input ports for comparing said current sensing means electrical signal with said reference signal source means electrical signal and generating therefrom a difference signal and for mixing said difference signal with said binary electrical signals from said digital control signal source means and for thereby generating and applying to said linear amplifier circuit means input port a binary gated negative feedback difference signal capable of dynamically maintaining said linear amplifier circuit means variable conductivity elements in a linear operating region during the printing time interval of said digital control signal source means binary electrical signals. l0. Exciter apparatus for inducing a rapid energy controlled pressure contact between imprintablepaper and a continuously moving printing typefont, the exciter apparatus comprising the combination of: y

print hammer means including a movable ballistic print hammer for bringing said imprintable paper and said continuously moving printing typefont together in a pressured printing relationship; v 4 I electrical energy to mechanical energy transducer apparatus including an electrical solenoid having electrical windings and a mechanical connection with said print hammer means for converting bursts of electrical energy into bursts of kinetic energyyested in said movable bal-' listic print hammer of said print hammer means; I an electronic amplifier including at least two bipolar junction transistors having commonly connected collector terminals which are in turn connected to a first amplifier output terminal and having series-connected emitter and base terminals wherein an emitter terminal of a prior transistor is connected with a base terminal of a next succeeding transistor and having a base terminal of the first transistor connected to an amplifier input terminal and having an emitter terminal of the last transistor connected to a second amplifier output terminal; current-sensing means including an electrical resistance having a value less than one ohm and having one terminal thereof connected to said second amplifier output terminal for generating a voltage signal proportional in magnitude to the current flowing in said current sensing means and said electronic amplifier; direct current energy source means for exciting said electri cal solenoid; connecting means for connecting said electrical solenoid of said electrical energy to mechanical energy transducer apparatus. said first and second amplifier output terminals of said electronic amplifier, said current sensing means, and said direct current energy source meansinto a series circuit wherein excitation current for said electrical solenoid of said electrical energy to mechanical energy transducer apparatus flows;

frequency response determining means connected with the reference potential source means for generating a voltage signal comparable with said voltage signal generated by said electrical resistance of said current-sensing means when a predetermined current flows therein;

negative feedback means including a comparator transistor of the junction type having a base terminal that is connected to said second amplifier output terminal of said electronic amplifier and an emitter terminal connected to said reference potential source means and a collector terminal connected with said amplifier input terminal for comparing said current sensing means voltage signal with said reference potential source means voltage signal and for generating from said signals as a negative feedback signal capable of dynamically maintaining said electronic amplifier in a conducting but nonsaturated state and maintaining said current sensing means voltage signal comparable with said reference potential source means voltage signal;

. signal transmission path of said electronic amplifier for limiting the high-frequency response of said electronic amplifier to a frequency band wherein said amplifier is dynamically stable;

leakage current compensating means connected with at least one of said bipolar junction transistors in said electronic amplifier for precluding leakage current conduction in said electronic amplifier; and

digital signal source means including a source of digital signals connected with said amplifier input and capable of overcoming said negative feedback signal generated by said comparator transistor in said negative feedback means for terminating conductivity in said electronic amplifier between actuations of said print hammer means.

11. Apparatus for controlling the flow of electrical energy between a source of direct current energy and a print hammer magnetic actuator, said apparatus comprising:

ing the emitter-to-collector current flowpath therein for completing a series electrical circuit that also includes said source of direct current energy, said print hammer magnetic actuator and said current sensing resistance;

an adjustable source of reference signals capable of generating voltage signals comparable with voltage signals appearing across said current-sensing resistance when a predetermined magnitude of print hammer magnetic actuator current flows therein;

a linear mode current amplifier circuit having input and output ports and including at least one linear mode amplifier transistor, said current amplifier output port being connected to said bipolar junction power transistor base terminal;

a junction transistor having a base terminal connected to said current sensing resistance element, an emitter terminal connected to-said adjustable source of reference signals, and a collector terminal connected via a signal path with said input port of said linear mode current amplifier circuit; and

a switching transistor source of digital control signals connected with said linear mode current amplifier circuit input port for controlling the conducting and nonconducting time intervals of said bipolar junction power transistor,

whereby said junction transistor, said bipolar junction power transistor, and said linear mode current amplifier circuit are contained within a linear closed loop negative feedback circuit.

12. Apparatus for controlling the flow of electrical energy as in claim 11 wherein said signal path connecting said junction transistor collector terminal with said input port of said linear mode amplifier circuit includes a semiconductor diode element.

13. Apparatus for controlling the flow of electrical energy as in claim 11 wherein said linear mode current amplifier circuit includes frequency response controlling means for limiting the high-frequency response of said amplifier circuit and transistor leakage current-controlling means for nullifying the effect of transistor leakage currents in said amplifier.

Claims

1. An excitation control circuit for a printer-actuating solenoid member of a high-speed printer mechanism, comprising: a semiconductor amplifier including a plurality of semiconductor devices, with the collector electrodes of said transistor devices being connected to a common terminal and thence to a groundable terminal of said printer-actuating solenoid member, said solenoid member having another terminal connected to a ground referenced source of direct current energy and having a voltage limiter circuit connected between said ground referenced source and said groundable terminal, said semiconductor amplifier having the emitter electrode of a prior transistor device connected to the base electrode of a succeeding transistor device, and having an input terminal connected to the base electrode of a first transistor device serving as an amplifier input terminal, with said common collector terminal serving as one amplifier output terminal and the emitter electrode of the final transistor device serving as another amplifier output terminal; a current-sensing resistance of a low value connected between the emitter electrode of the final transistor device and ground so as to have flowing therethrough the current flowing in said solenoid member and said final transistor device; a comparator transistor device having its base electrode connected to a junction point between said current-sensing resistance and the emitter electrode of said final transistor device, its emitter electrode connected to a source of reference potential, and its collector electrode connected through a diode to said amplifier input terminal; a frequency response determining capacitance element connected between the collector electrode and the base electrode of one transistor device in said semiconductor amplifier; a plurality of leakage current compensating resistances each connected with the base electrode of one transistor device so as to conduct away collector to base leakage current occurring within said transistor device; and a source of timed current signals for commanding desirable opening and closing of said actuating member, said source of signals being coupled to said amplifier input terminal.

2. Electrical apparatus for effecting intermittent mechanical actuation of a movable print hammer in a printing mechanism; the apparatus comprising; a source of direct current energy; electromagnetic transducer means having a movable member that is mechanically coupled with the movable print hammer, the transducer means being electrically connected with the source of direct current energy and including as parts thereof electrIcal windings and magnetic flux conducting structure, the transducer means providing means for converting energy from the source of direct current energy into energy that moves the print hammer; cascaded semiconductor linear amplifier means having an output port that is connected with the source of direct current energy and the electromagnetic transducer means, for controlling the flow of energy from the source of direct current energy to the electromagnetic transducer means, the amplifier means also having an input port for receiving amplifier controlling input signals, the amplifier being linearly responsive to analog signals applied to said input port; performance sensing means joined to the electromagnetic transducer means and the amplifier means for generating an analog electrical signal representative of the instantaneous magnetic performance of said electromagnetic transducer means; means for mixing the analog performance sensing means signal with two or more electromagnetic transducer means control signals, at least one of which is an intermittent control signal, and one of which is an analog transducer means current controlling signal and for supplying the mixed signals to the amplifier input port; whereby the current which excites the print hammer, and the print hammer, are time responsive to the intermittent control signal and are analog responsive to the analog transducer means current controlling signal.

3. Electrical circuitry in combination with a print hammer electrical solenoid structure for producing precise intermittent excitation thereof, said electrical circuitry being composed of: electrical windings magnetically coupled with said print hammer electrical solenoid structure, said electrical windings including a first terminal and a second terminal at the start and finish thereof; a source of direct current electrical energy capable of exciting said print hammer electrical solenoid electrical windings and having a first output terminal and a second output terminal; a bipolar power transistor having a base terminal, an emitter terminal, and a collector terminal; a current-sensing resistance element having electrical resistance less than one ohm and having a first electrical terminal and a second electrical terminal; means for connecting said source of direct current electrical energy, said electrical windings, said bipolar power transistor, and said current-sensing resistance element into a closed electrical circuit wherein said electrical windings first terminal is connected to said source of direct current electrical energy first output terminal, said electrical winding second terminal is connected to said bipolar power transistor collector terminal, said bipolar power transistor emitter terminal is connected with said current-sensing resistance element first electrical terminal, and said current-sensing resistance element is connected with said source of direct current electrical energy second output terminal; a transistor amplifier including a first junction transistor having collector, emitter, and base terminals and a second junction transistor having collector, emitter, and base terminals, said transistor amplifier having the collectors of the first and second junction transistors connected together and connected to the collector terminal of said bipolar power transistor and having the emitter terminal of the first junction transistor connected to the base terminal of the second junction transistor and the emitter terminal of the second junction transistor connected to the base terminal of said bipolar power transistor; whereby said transistor amplifier and said bipolar power transistor are connected into a Darlington circuit; an adjustable reference signal source capable of generating reference signals comparable with the voltage developed across said current-sensing resistance element; a junction transistor for comparing and mixing signals from said reference signal source and said current-sensing resistance element, said junction transistor for comparing and mixing signals having a base terminal that is connected to said current-sensing resistance element first electrical terminal, and an emitter terminal that is connected to said adjustable reference signal source, and a collector terminal that is connected with the base terminal of said transistor amplifier first junction transistor; and timed binary control signal source means connected with said base terminal of said transistor amplifier first junction transistor for supplying to said base terminal a binary signal capable of placing said first and second junction transistors in said transistor amplifier and said bipolar power transistor in the substantially nonconducting state and in the controlled conductivity state sequentially, thereby opening and closing said print hammer electrical solenoid structure.

4. Electrical circuitry in combination with a print hammer electrical solenoid structure as in claim 3 wherein said circuitry also includes energy dissipating voltage limiting means including an electrical diode and an electrical resistance element connected across said electrical windings first and second terminals for dissipating energy stored in the magnetic circuit of said electrical solenoid structure.

5. Apparatus for effecting precisely controlled intermittent contact between a printing font and media to be printed, said apparatus comprising the combination of: movable print hammer means for moving said printable media into pressured contact with said printing font; a source of direct current electrical energy; electrical solenoid means including electrical windings coupled to said source of direct current electrical energy and also including mechanical means connected with said movable print hammer means for transducing electrical energy from said source of direct current electrical energy into mechanical energy vested in said movable print hammer means; linear amplifier circuit means including an input port and cascaded variable conductivity elements the final one of which is connected electrically via an output port between said source of direct current electrical energy and said electrical windings of said electrical solenoid means for halting and for regulating the instantaneous magnitude of the flow of electrical energy between said source of direct current electrical energy and said electrical windings of said electrical solenoid means; current sensing means connected electrically between said source of direct current electrical energy and said electrical windings of said electrical solenoid means for generating an electrical signal representative of the instantaneous magnitude of the flow of electrical energy between said source of direct current electrical energy and said electrical windings of said electrical solenoid means; reference signal source means for producing an electrical signal comparable with the instantaneous magnitude of said current sensing means electrical signal at the instant when said flow of electrical energy between said source of direct current electrical energy and said electrical windings of said electrical solenoid means attains a desired predetermined magnitude; digital control signal source means including a source of binary electrical signals for determining the printing and the quiescent time intervals of said movable print hammer means; and signal comparing and mixing means connected with said current sensing means, said reference signal source means, and said digital control signal source means and said linear amplifier circuit means input port for comparing said current sensing means electrical signal with said reference signal source means electrical signal and generating therefrom a difference signal and for mixing said difference signal with said binary electrical signals from said digital control signal source means and for thereby generating and applying to said linear amplifier circuit means Input port a binary gated negative feedback difference signal capable of dynamically maintaining said linear amplifier circuit means variable conductivity elements in a linear operating region during the printing time interval of said digital control signal source means binary electrical signals.

6. Apparatus for effecting precisely controlled intermittent contact as in claim 5 wherein said current sensing means includes an electrical resistance element having resistance less than one ohm.

7. Apparatus for effecting precisely controlled intermittent contact as in claim 5 wherein said final one of said variable conductivity elements in said linear amplifier circuit means includes a bipolar junction power transistor which is connected to at least one other bipolar junction transistor in a Darlington circuit configuration.

8. Apparatus for effecting precisely controlled intermittent contact as in claim 5 wherein said signal comparing and mixing means includes a bipolar transistor having a base electrode terminal which is connected with said current sensing means, an emitter electrode terminal which is connected with said reference signal source means, and a collector electrode terminal which is connected with said linear amplifier circuit means input port and to said digital control signal source means.

9. Apparatus for effecting rapid precisely controlled simultaneous pressure contact between a medium to be printed and plural locations on a continuously moving printing font, comprising the combination of: a plurality of movable print hammer means for moving said printable medium into pressure contact with said continuously moving printing font; a source of direct current electrical energy; plural electrical solenoid means each including a set of electrical windings connected with said source of direct current electrical energy and mechanical means connected with one of said print hammer means for transducing electrical energy from said source of direct current electrical energy into mechanical energy vested in said print hammer means; a plurality of linear amplifier circuit means each including an input port and cascaded variable conductivity elements at least one of which is connected electrically between said source of direct current electrical energy and said electrical windings of one of said electrical solenoid means for halting and for regulating the instantaneous magnitude of the flow of electrical energy between said source of direct current electrical energy and said electrical windings of said electrical solenoid means; a plurality of current sensing means each connected electrically between said source of direct current electrical energy and said electrical windings of said electrical solenoid means for generating an electrical signal representative of the instantaneous magnitude of the flow of electrical energy between said source of direct current electrical energy and said electrical windings of said electrical solenoid means; a single reference signal source means for producing an electrical signal comparable with the instantaneous magnitude of said current-sensing means electrical signal at the instant when said flow of electrical energy between said source of direct current electrical energy and said electrical windings of said electrical solenoid means attains a desired predetermined magnitude; a plurality of digital control signal source means each including a source of binary electrical signals for determining the printing and the quiescent time intervals of said movable print hammer means; and a plurality of signal comparing and mixing means each connected with said single reference signal source means, one of said current sensing means, one of said digital control signal source means, and one of said linear amplifier circuit means input ports for comparing said current sensing means electrical signal with said reference signal source means electrical signal and generating therefrom a difference signal And for mixing said difference signal with said binary electrical signals from said digital control signal source means and for thereby generating and applying to said linear amplifier circuit means input port a binary gated negative feedback difference signal capable of dynamically maintaining said linear amplifier circuit means variable conductivity elements in a linear operating region during the printing time interval of said digital control signal source means binary electrical signals.

10. Exciter apparatus for inducing a rapid energy controlled pressure contact between imprintable paper and a continuously moving printing typefont, the exciter apparatus comprising the combination of: print hammer means including a movable ballistic print hammer for bringing said imprintable paper and said continuously moving printing typefont together in a pressured printing relationship; electrical energy to mechanical energy transducer apparatus including an electrical solenoid having electrical windings and a mechanical connection with said print hammer means for converting bursts of electrical energy into bursts of kinetic energy vested in said movable ballistic print hammer of said print hammer means; an electronic amplifier including at least two bipolar junction transistors having commonly connected collector terminals which are in turn connected to a first amplifier output terminal and having series-connected emitter and base terminals wherein an emitter terminal of a prior transistor is connected with a base terminal of a next succeeding transistor and having a base terminal of the first transistor connected to an amplifier input terminal and having an emitter terminal of the last transistor connected to a second amplifier output terminal; current-sensing means including an electrical resistance having a value less than one ohm and having one terminal thereof connected to said second amplifier output terminal for generating a voltage signal proportional in magnitude to the current flowing in said current sensing means and said electronic amplifier; direct current energy source means for exciting said electrical solenoid; connecting means for connecting said electrical solenoid of said electrical energy to mechanical energy transducer apparatus, said first and second amplifier output terminals of said electronic amplifier, said current sensing means, and said direct current energy source means into a series circuit wherein excitation current for said electrical solenoid of said electrical energy to mechanical energy transducer apparatus flows; reference potential source means for generating a voltage signal comparable with said voltage signal generated by said electrical resistance of said current-sensing means when a predetermined current flows therein; negative feedback means including a comparator transistor of the junction type having a base terminal that is connected to said second amplifier output terminal of said electronic amplifier and an emitter terminal connected to said reference potential source means and a collector terminal connected with said amplifier input terminal for comparing said current sensing means voltage signal with said reference potential source means voltage signal and for generating from said signals as a negative feedback signal capable of dynamically maintaining said electronic amplifier in a conducting but non-saturated state and maintaining said current sensing means voltage signal comparable with said reference potential source means voltage signal; frequency response determining means connected with the signal transmission path of said electronic amplifier for limiting the high-frequency response of said electronic amplifier to a frequency band wherein said amplifier is dynamically stable; leakage current compensating means connected with at least one of said bipolar junction transistors in said electronic amplifier for precluding leakage current conduction in said electronic amplifier; anD digital signal source means including a source of digital signals connected with said amplifier input and capable of overcoming said negative feedback signal generated by said comparator transistor in said negative feedback means for terminating conductivity in said electronic amplifier between actuations of said print hammer means.

11. Apparatus for controlling the flow of electrical energy between a source of direct current energy and a print hammer magnetic actuator, said apparatus comprising: a current-sensing resistance element having electrical resistance less than one ohm; circuit means including a bipolar junction power transistor having emitter, base, and collector terminals and including the emitter-to-collector current flow path therein for completing a series electrical circuit that also includes said source of direct current energy, said print hammer magnetic actuator and said current sensing resistance; an adjustable source of reference signals capable of generating voltage signals comparable with voltage signals appearing across said current-sensing resistance when a predetermined magnitude of print hammer magnetic actuator current flows therein; a linear mode current amplifier circuit having input and output ports and including at least one linear mode amplifier transistor, said current amplifier output port being connected to said bipolar junction power transistor base terminal; a junction transistor having a base terminal connected to said current sensing resistance element, an emitter terminal connected to said adjustable source of reference signals, and a collector terminal connected via a signal path with said input port of said linear mode current amplifier circuit; and a switching transistor source of digital control signals connected with said linear mode current amplifier circuit input port for controlling the conducting and nonconducting time intervals of said bipolar junction power transistor, whereby said junction transistor, said bipolar junction power transistor, and said linear mode current amplifier circuit are contained within a linear closed loop negative feedback circuit.