US3919934A

US3919934A - Power minimization for electrostrictive actuated printers

Info

Publication number: US3919934A
Application number: US490780A
Authority: US
Inventors: Jack Beery
Original assignee: Burroughs Corp
Current assignee: Unisys Corp
Priority date: 1973-05-14
Filing date: 1974-07-22
Publication date: 1975-11-18
Anticipated expiration: 1992-11-18

Abstract

In a printer utilizing piezoelectric crystals, electrostrictive members, as the prime movers for the print hammers, significant power reduction may be achieved by applying a high voltage to contract the piezoelectric crystal and then discharging that voltage thereby causing the crystal to expand at a rapid rate thus driving the print hammer. The crystal is initially charged across its width, which is transverse to the direction of travel of the crystal, causing a reduction in its length. A transistorized switching circuit is utilized to discharge the high voltage applied to the crystal thus causing the crystal to expand at a rapid rate. By providing a charging time constant substantially longer than the time required to discharge the potential across the crystal, the current requirements, and thus the power requirements, for the system are substantially reduced.

Description

United States Patent [191 Related US. Application Data Continuation-in-part of Ser. No. 359,792, May 14, 1973, abandoned. which is a continuation-in-part of Ser. No. 110,802, Jan. 29, 1971, abandoned.

56 References Cited UNITED STATES PATENTS 11/1963 Harris 318/118 12/1963 Williams.. 101/1 X 9/1964 Stec 310/8 X 1/1966 Miller... 101/93 C 3/1966 Noll et 101/1 X 8/1969 Nyman 318/118 X Beery 5] Nov. 18, 1975 [5 1 POWER MINIMIZATION FOR 3,482,772 12/1969 Thayer mm x ELECTROSTRICTIVE ACTUATED 3,614,486 10/1971 Smiley lOl/DIG. 5

PRINTERS 3,735,153 5/1973 Shukla 307/246 [75] Inventor: Jack Beery, Farmington, Mich. Primaly E \.aminer Edgar Burr [73] Assignee: Burroughs Corporation, Detroit, Assis'am Emminef-Edward Cove" Mi h Attorney, Agent, or Firm-Michael B. McMurry;

Edwin W. Uren; Paul W. Fish [22] Filed: July 22, 1974 [21] Appl. No.: 490,780 57 ABSTRACT In a printer utilizing piezoelectric crystals, electrostric- -tive members, as the prime movers for the print hammers, significant power reduction may be achieved by applying a high voltage to contract the piezoelectric crystal and then discharging that voltage thereby causing the crystal to expand at a rapid rate thus driving the print hammer. The crystal is initially charged across its width, which is transverse to the direction of travel of the crystal, causing a reduction in its length. A transistorized switching circuit is utilized to discharge the high voltage applied to the crystal thus causing the crystal to expand at a rapid rate. By providing a charging time constant substantially longer than the time required to discharge the potential across the crystal, the current requirements, and thus the power requirements, for the system are substantially reduced.

10 Claims, 5 Drawing Figures US. Patent Nov. 18,1975 Sheet10f2 3,919,934

9 U5. Patent N0v.18,1975 Sheet2 0f2 3,919,934

POWER MINIMIZATION FOR ELECTROSTRICTIVE ACTUATED PRINTERS Cross-Reference to Related Parent Applications 1971 Ser. No. 121,753 by Narendra M. Shukla, now

U.S. Pat. No. 3,735,153 and assigned to the same assignee as the present invention, describes a high voltage pulse control circuit which is shown in FIG. as a part of the present system for charging and rapidly discharging the electrostrictive elements of the system.

BACKGROUND OF THE INVENTION 1. Field of the Invention This invention is related to an actuator system and in particular to a piezoelectric crystal actuator system as used in high speed printers.

2. Description of Prior Art Exemplary of the use of electrostrictive or piezoelectric elements as print hammer actuators in high speed printers are the US. Pats. to Thayer, Nos. 3,473,466 and Noll et al. 3,242,855 which teach the use of such crystals for controlling the movement of a hammer engaging a print drum. In both patents a plurality of crystals are individually actuated or charged in order to produce printing impressions, and large amounts of power of short duration are applied to the crystals when the desired characters are printed. Incorporated in such systems as disclosed in the cited patents are electrical power supplies for providing the amount of power applied to the crystals. If the lengths of the crystalsare of any significant size, the amount of power applied is exceedingly high in order to create the necessary electrical field to produce the sudden elongation of the crystals. Additionally, in a multicolumn printer having an electrostrictive element at each column position, the power surge on the power supply at each time of printing a line of characters becomes excessive considering that many of such crystal elements in the printer receive a substantially instantaneous surge of ,power when printing a line of characters.

SUMMARY OF THE INVENTION It is an important object of this invention to provide an electrostrictive actuator system wherein the power surgeson the power supply are minimal.

It is another important object of this invention to provide an electrostrictive actuator system for use in a high speed printer environment where the power requirements for a multicolumn printer are minimal.

In accordance with. the above enumerated objects andother objects which will hereinafter become apparent, there is described and defined anelectrostrictive or piezoelectric crystal actuator system. A member,

.such-as a print hammer, is supported to bemovable along a path of travel and preferably in substantially free flight condition. If the member is a print hammer,

then its path of travel is terminated with the impact of the hammer upon a print character or the like. The

electrostrictive provision, such as a piezoelectric crystal, is positioned at one end of the hammers path of travel and is normally in an abutting relationship therewith. In the present invention, the electrostrictive element is responsive to an electrical potential applied in a direction transverse to the hammers path of travel in order to.contract or reduce the length of the element. The memberwhich was in an abutting relationship with the electrostrictive element follows this contraction and remains in the abutting relationship therewith.

Control means is provided tocontrol the application of the electrical potential to the crystal for charging the crystal and thereby causingits contraction. Additionally, the control means will rapidly discharge the crystal thereby causing the crystal to suddenly restore to its original length. This restoration imparts motion to the hammer member to essentially launch. the member along its path of travel away from the electrostrictive means.

' BRIEF DESCRIPTION OF THE DRAWING FIG. 1 a graphic illustration of a crystal actuator system; I

FIG. 2 is a graphic illustration in perspective of the essential elements of a printing apparatus embodying the actuator system of this inven tion;

FIG. 3 is a fragmentary side view of one'of the hammer and actuator mechanisms employed in the printer of FIG. 2;

FIG.. 4 is a partial fragmentary sectional view taken alongline 4-4 of FIG. 3; and

FIG. 5 is a schematic view of a high voltage semiconductor pulse circuit for charging and discharging the electrostrictive elementsof the system.

DETAILED DESCRIPTION Referring to FIG. 1, there is illustrated in diagram matic form the essential elements of a crystal actuator system generally indicated at 10 according to the present invention. An elongated crystal, is mounted so as to support at one end thereof a hammer-like member 12 in an abutting relationship. Control means 14, such as described in more detail hereinafter, is electrically connected across the broadsides of the crystal 11 for controlling the application of power from an electrical power supply 16 for charging the crystal and then for sequentially controlling the discharge of the crystal. When the crystal 11 is charged, its length is contracted a distance Al. The hammer-like member 12, being biased into an abutting relationship with the crystal ll, follows this contraction. When the crystal 11 is discharged, the hammer-like member 12 is launched along a path of travel 18 into substantially free flight. In FIG. 1, the hammer-like member 12 is shown as guided along its path of travel 18 by a guide 20 and at the end of its flight, the member 12 is returned into abutting relationship with the crystal 11.

Referring to FIG. 2, there is illustrated the essential elements of a-printer and in particular a wide line drum printer generally indicated at 22 as commonly found in computer installations. The rotatable drum-24 of the printer 22 comprises a plurality of columns 26 of alpha or alpha-numeric printing characters spaced-circularly about the periphery thereof; Typically, such drums contain approximately to circular columns of print. However, for the purpose of disclosure, we will limit our description to one hammer actuator system .10 of the printer assembly, although the description herein is applicable to all of the hammer actuator systems along the drum. In use, the drum 24 is rotated at a constant speed by some power source such as an electric motor which is not shown, and information as to which of the characters to be printed is received by the printer 22 and synchronized with the rotational movement of the drum 24 to actuate the desired column hammer 28 at the proper time for printing.

Radially' aligned with the axis of the print drum 24 and closely spaced from its periphery are a plurality of biased or resiliently suspended hammers 28. As illustrated in FIG. 3, each hammer 28 is resiliently suspended by two pairs of parallelly spaced apart spring members 30-30 which are respectively attached to a pair of parallel spaced apart

support members

32 and 34. In the preferred embodiment, each hammer 28 is a single column hammer, although if it is desired, column-spanning hammers may be employed.

Positioned below and in an abutting relationship with each hammer 28 is a hammer actuator 36 which is an elongated bar-shaped, electrostrictive member which may be a piezoelectric crystal. In addition to gravitational force, each hammer 28 is biased by the resilient spring members30 into contact with the upper end of its associated piezoelectric crystal 36. A pair of

electrodes

38 and 40 arerespectively attached to each broadside of thepiezoelectric crystal with one electrode 40 connected through lead 41 to a source of ground potential and the other electrode 38 connected through lead 39 to the control means 14 for controlling the application of electrical potential to the crystal 36. In the preferred embodiment, there is illustrated in FIG. 4 side-by-side, two crystals which are electrically and physically connected together for each hammer, however, the selection of one or more such crystal layers for each hammer member depends upon the printing requirements of the printer 22 and the mass of the hammer system.

Interposed in the space between end faces 42 of the hammer members 28 and the periphery of the printer drum 24 is a continuous feed paper web 44 and an inked ribbon 46. The paper web is driven by at least two sprocket wheel mechanisms 48 having spokes 50 which engage the sprocket holes 52 along the edge of the paper. For each rotation of the print drum 24, the paper web 44 will be moved through a distance equal to one horizontal printing line. However, if the drum would have two complete character sets in a given column, then the web 44 would move one horizontal line for each half revolution of the drum.

Ass previously indicated, each actuator 36 may be an elongated piezoelectric crystal member having an

electrode

38 and 40 attached to each broadside. As illustrated in FIG. 4, there are two piezoelectric crystals hammer 28 into substantially free flight toward and into contact with web 44 and rotatable drum 24.

Referring to FIG. 3, the hammer 28 is shown suspended in the space between the two

parallel support members

32 and 34 by the two sets or pairs of parallel spaced-apart spring members 3030. These spring members 3030 function to bias the hammer 28 into physical contact with the upper surface of the actuator 36. In its normal operating position the actuator crystal is charged to the power supply voltage and its length which is the direction of the hammer travel, is contracted by a distance Al. The biasing force of the spring members 3030 insures that the hammer 28 follows this contraction to remain in physical contact with the upper surface of the crystal 36. When the crystal is discharged, it restores or returns to its normal uncontracted length thereby propelling or launching the hammer 28 forward against the printer drum 24 for printing on the paper web 44. After the hammer makes contact through the web to the printer drum, the spring members 30 30 function to return the hammer 28 to its normal position in contact with the actuator 36. The crystal actuator assemblies are supported along their length by an insulated support bracket, which for reason of clarity is not shown in FIGS. 2-4, but is illustratively represented in FIG. 1, allowing the crystals 36 to contract and expand according to the state of charge of the crystal without any hinderance or restriction.

The electrostrictive members such as the piezoelectric crystals employed in the illustrated embodiment of the invention are basically high energy, high voltage devices. The voltage magnitude for operation of a piezoelectric crystal is on the order of kilovolts, and in the preferred embodiment the voltage used of the power supply 16 is 3,000 volts. Each crystal is charged to 3,000 volts during the time between successive printing impressions on the drum 24. Then, at the proper time for printing, the crystal 36 is discharged and the hammer 28 is launched into contact with the printing drum 24. By charging the crystal between actuations, the charging rate of the crystal can be much lower or slower and the requirements on the power supply are much less stringent.

As previously indicated, the piezoelectrical crystal 36 is selected according to the mass of the hammer 28. When voltage is applied to the piezoelectric crystal in the preferred embodiment, potential energy is stored in the crystal by means of its contraction. When the discharge circuit in the control means 14 for the crystal is closed, the potential energy stored within the crystal 36 is converted into impulse energy which is imparted to the hammer 28 causing the hammer to be launched into contact with the paper and the printer drum. One of the parameters of the crystal 36 which is important for its selection in this type of application is its K factor. This factor is defined as the transverse or lateral coupling factor which defines direction of electric field transverse to the longitudinal axis and the direction of the mechanical strain is in the direction of the longitudinal axis of the crystal. Also, the crystal is selected to have a high modulus of elasticity which reflects the quickness of response of the crystal returning to its normal size. The length of the crystal is determined by the velocity desired at the hammer for effective. printing. The lower the mass of the hammer, the higher the velocity required for high energy impact printing. In such a situation, the crystal must expand at a large rate indicating a very quick response time, and dictating a high voltage be applied to a long length crystal. In the preferred embodiment, the length of the crystal is 4.500

inches long and when fully charged is contracted a distance of A1 of 0.003 inches. The cross section of the crystal need not be rectangular but may be slightly curved or bowed for rigidity.

In the normal state when the electrical power is first applied to the circuit, the crystal charges up to the power supply voltage which is 3,000 volts. During this time the character to be printed is being synchronized with the rotation of the drum and is being moved by the drum to print position. This time period allows a much longer charging time for the crystal than would be encountered if the charging of the crystal was employed to impel the hammer member, thereby reducing the amount of current in the charging circuit and the power output from the power supply. When the character to be printed is proximate the end of the flight path of the print hammer, the control means 14 causes the crystal to rapidly discharge, the crystal returning to its normal linear length to thereby launch the hammer member toward the print drum. After printing, the spring members 30 return the hammer to its normal position atop the crystal and the crystal begins to recharge for the next printing operation thereby again contracting in its length for this purpose.

A desirable charging and discharging circuit for use in the present electrostrictive actuator system is that disclosed and claimed in the'hereinabove referenced patent application of Narendra M. Shukla, Ser. No. 121,753, now US. Pat. No. 3,735,153 and herein schematically shown in FIG. 5. Referring to this Figure by the characters of reference, there is illustrated in schematic representation a high voltage semiconductor control circuit for initially charging each electrostrictive element, such as the piezoelectric crystal 36 described in connection with FIGS. 3 and 4, having a pair of

electrodes

38 and 40 electrically and'physically connected to the opposite sides thereof. The first electrode 38 from one side of the crystal is electrically connected in a charge circuit through a current limiting charging resistor 58 to the high voltage power supply 16 and in a discharge circuit through another current limiting discharge resistor 62 to a plurality of cascaded semiconductor switches 64, 65 and 66, which in the preferred embodiment are transistors. In the normal state with electrical power on, the piezoelectric crystal 36 is electrically charged to the supply voltage 16 through the charging resistor 58. When utilization of the system is desired, the charge on the piezoelectric crystal is discharged through the discharge resistor 62 and the cas- ,caded transistors to ground potential 68.

The piezoelectric crystal as might be used in the present embodiment is basically a high energy, high voltage device. This crystal material may be that produced by 'Gulton industries and identified as G-l5l2. The voltage magnitude for operating such a device is on the order of kilovolts and in the preferred embodiment the supply voltage 20 as previously mentioned herein is three thousand volts. When the crystal is discharged, the cascaded

transistors

64, 65 and 66 provide a circuit capableof discharging the current due to the 3,000 volts. In both charging and discharging, the high voltage pulse circuit is able to control the high voltage without destruction to the circuit. In semiconductor technology at the present time, single switching units having kilovolt ratings are generally not available in quantity and whenthey are, they are very expensive. Therefore, it is necessary to cascade a plurality of single transistors 64-66, each having a collector to emitter breakdown voltage rating substantially less than the supply voltage. By means of cascading these individual units according to the referenced'Shukla patent, a high voltage semiconductor pulse circuit is provided wherein the summation of voltage ratings of each of the semiconductor switches 64-66 equals or exceeds the supply voltage and the voltage applied to each individual transistor does not exceed its breakdown rating.

In order to protect the

transistors

64, 65 and 66 during the charging time of the crystal 36 andwhen the crystal is charged, there are electrically connected in parallel to the collector-emitter circuits of each of the switching transistors,

voltage balancing resistors

70, 71 and 72 in a voltage divider means generally indicated at 74. These resistors are serially connected together and connected between the high voltage supply voltage 16 and the ground potential 68 forming a voltage divider means. Since each individual resistor -72 has an extremely high resistance value, one megohm in the preferred embodiment, the standby current required from the power supply 16 through the voltage divider 74 is small. The voltage divider functions to insure that the voltage division across each transistor 64-66 is within the breakdown voltage rating of each transistor.

Electrically connected in the

base circuit

76, 77 and 78 of each of the transistors is a resistor-capacitor tim ing network generally indicated at 80, 81, and 82. It is a function of each of these networks 80-82 to supply sufficient base drive current to their respective transistors 64-66 for a predetermined period of time. It is necessary that the transistors remain in their on state until the charge on the piezeoelectric crystal 36 is dissi pated and then'be turned off thereby eliminating ex cessive and unnecessary power consumption in the several circuit components. As will hereinafter be shown, the sequential order for turning the transistors 64-66 on is of prime importance. If the order of turn on is other than the predetermined order, there will be transistor failure or breakdown due to the voltage rating of the transistor being exceeded.

For ease of discussion of the operation, the following component values are used for several of the circuit components of the circuit.

Component Capacitance Resistance Crystal 36 .003 mfd Resistor 62 200 Capacitor 83 .005 mfd Resistor 84 4K Capacitor 85 .Ol mfd Resistor 86 2K Capacitor 87 .47 mfd Resistor 88 50 By applying the familiar time constant equation: T RC, the discharge time constant of the crystal 36 is seen to equal 0.66 microseconds. In a similar manner, the time constant in the base timing circuit 80 and 81 of the two

transistors

64 and 65 electrically nearest the crystal As previously mentioned, the sequence order of turn on or conduction of the transistors 64-66 is important in order to prevent excessive voltage across the collector-emitter circuits of each of the transistors. In a preferred embodiment, the first or lowermost transistor 66, the one electrically furtherest from the crystal is initially turned on which in turn causes the middle or next succeeding tran'sitor 65 to turn on. As the second transistor' is being turned on, the third transistor 64 begins to conduct. In this manner, the voltage across the collector-emitter circuits of each transistor does not exceed its rating.

In the normal state when the electrical power is first applied to the circuit, the crystal 36 charges up through the charging resistor 58 to the high voltage supply 16 which is 3,000 volts. Likewise each of the

capacitors

83, 85 and 87 in the base circuits 80,81 and 82 of each transistor are charged through their

respective diodes

90, 91 and 92 to the voltage at the electrical connection between its transistor and the next adjacent transistor in the direction of decreasing voltage magnitude. In FIG. 5, this is the voltage at the

emitter

94, 95 and 96 of each transistor as determined by the voltage divider 74. For example, in the base circuit 80 of the transistor 1 64 closest to the crystal, the capacitor 83 charges to a voltage which in the preferred embodiment is 2,000 volts. Likewise, the capacitor 85 in the middle transistor 65 chargesto a voltage of 1,000 volts and the capacitor 87 in the lowermost transistor 66 is charged to ground potential 68. The control circuit generally indicated at 98 in the lower portion of FIG. is actuated, serving to connect the side of the capacitor 87 electrically opposite the base 78 of the transistor 66 to ground. In such a condition, the charge across the capacitor 87 of that transistor is zero volts.

The switching control circuit 98 comprises three switching

stages

100, 101 and 102 wherein the first two

stages

100 and 101 comprise oppositely poled transistors and the third stage 102 comprises a power switching transistor. In the preferred embodiment, the first stage transistor 100 is controlled by a pair of cross coupled

NAND gates

104 and 105 forming a flip flop circuit. The circuit may be initiated by a mechanical switch 106 which supplies a low or ground potential 68 to either one of two

input gates

108 or 110 of the flip flop.

The first stage transistor 100 in the preferred embodiment is a high speed medium power transistor used as a saturated switch. The second stage transistor 101, which is oppositely poled to the first stage and in the preferred embodiment is a PNP transistor, is a high speed transistor used as switching driver. As will hereinafter be shown, this transistor 101 supplies the proper amount of base current for the third or final stage 102 which is a switching power transistor.

In the initial condition all three

stages

100, 101 and 102 of the switch control network 98 are off or nonconducting. The input switch 106 is positioned, as illustrated in the drawing, for supplying ground potential to one input 108 of the lower NAND gate 105. Both

NAND gates

104 and 105 are responsive to logic signals wherein the false level is ground potential and the true level is plus 5 volts. With the switch 106 in a position as shown in the drawing, one input 110 to the upper NAND gate 104 is floating which allows that NAND gate to be controlled by the other input 112. The other input 112 to the upper NAND gate 104 is the output 114 of the lower NAND gate 105 and is at the low level. The output 116 of the upper NAND gate 104 is low which is cross coupled to the second input 118 of the lower NAND gate. The output 116 of the upper NAND gate 104 is also electrically connected through a resistor 120 to a plus 25 volt supply 122. Additionally, the output 116 of the upper NAND gate 104 is connected through a pair of diodes 124 and 125 to the base 126 of the first stage transistor 100. With the output of the upper NAND gate 104 at ground potential, the voltage on the base 126 of the first stage transistor 100 is essentially at ground potential.

To initiate operation of the high voltage pulse circuit, the switch 106 is transferred thereby grounding the floating input 110 of the upper NAND gate 104 switching its output 116 to a true level. As previously indicated, a true level is a signal of plus 5 volts, therefore the input to the double diode 124 and 125 connection is also at plus 5 volts. With plus 5 volts on the input to the double diodes, the base-emitter junction of the first stage transistor 100 is forwarded biased causing that transistor to go into conduction. The function of the double diodes 124 and 125 in the base circuit is to insure that the transistor 100 remains in the 011 or nonconducting state when the output 116 of the upper NAND gate 104 is at a false level. These two diodes accomplish this by preventing any significant value of current from being applied to the base lead 126 of the transistor at this time. In the preferred embodiment, each of the two diodes have a forward voltage drop of 1 volt.

With the first stage transistor 100 in conduction current flows through the collector lead 130 from the two series resistors 132 and 134 in the collector circuit. At the junction of the two series resistors 132 and 134, the base 136 circuit of the second stage transistor 101 is electrically connected.

The second stage transistor 101, as previously indicated, is a PNP transistor wherein the emitter 138 circuit is electrically connected to the plus 25 voltage supply 122 and the base 136 circuit is electrically connected through one of the series resistors 132 to the collector 130 of the first stage transistor 100. The collector 140 circuit of the second stage transistor 101 is electrically connected to the base 142 circuit of the third stage transistor 102. When the first stage transistor begins to conduct, the base 126 circuit of the second stage transistor 101 is placed at a potential which forward biases the emitter-base junction of the second stage transistor 101. This brings the second stage transistor into conduction for supplying the large amount of base current necessary to bring the final stage 102 into conduction.

When the collector of the second stage transistor 101 is raised from essentially ground to some positive voltage level, the third stage 102 transistor begins to conduct. An emitter voltage signal which swings from ground to some positive voltage, which in the preferred embodiment is approximately plus 10 volts, is generated and is a-c coupled through the timing capacitor 87 to the base 78 of the first transistor 66 in the high voltage pulse circuit causing that transistor 66 to begin to conduct. As that transistor 66 begins to conduct, the current flows from its collector 144 through the emitter 96 pulling the collector voltage down towards ground potential. As the voltage on the collector 144 is pulled toward ground, the base-emitter junction of the second transistor 65 is forward biased causing that transistor 65 to begin to conduct. Just prior to the instant of con- .duction', the voltage on the base 77 of the second transistor, as previously indicated, is approximately onethird of the supply voltage 16 of 1,000 volts. Likewise the voltage on the emitter 95 of the second transistor 65 is at the same voltage and as the first transistor 66 begins to conduct, the voltage on the emitter 95 drops causing the base-emitter junction of the second transistor 66 to be forward biased. As soon as the second transistor 65 begins to conduct, the voltage on the emitter 94 of the third transistor 64 drops causing the baseemitter junction of that transistor 64 to be forward biased and that transistor 64 begins to conduct. With all three

transistors

64, 65 and 66 in the state of conduction, the collector 146 electrically nearest the crystal 36 is approximately at ground potential and the voltage drop across the three

transistors

64, 65 and 66 is negligible.

From the previously described component chart, the

capacitors

83, 85 and 87 in the base circuits 80, 81 and 82 begin to discharge with a time constant of approximately 20 microseconds in each base circuit. As soon as the discharge current drops below a base current value necessary to maintain conduction, the

transistors

64, 65 and 66 begin to turn off. As each transistor turns off, the capacitor in its base circuit begins recharging to stant of approximately 470 microseconds.

As previously indicated; the high voltage discharge circuit is ready for refiring as soon as the electrostrictive member 36 is recharged. In a preferred embodiment wherein the member 36 is essentially a piezoelectric crystal, the time between sequential firing of the crystal or discharge of its capacitors is such that the charging resistor 58 has a value of two megohm. This is a charging time constant of approximately 6.6 milliseconds and the maximum charging current per crystal is approximately 1% milliamps. Therefore,.in accordance with one of the objects of this invention, the charging time of the crystal 36 is substantially longer than the discharging time thereby minimizing the drain on the power supply. It should be understood that the values illustrated in this description are basically a matter of selection and are used herein for illustration purposes the voltage of the voltage divider through its

respective I diode

90, 91 and 92 to the voltage at the voltage divider '74 and through a common resistor 148 to ground. When all of the

transistors

64, 65 and 66 are fully turned off, the voltage at the junction of each

resistor

70, 71 and 72 in the voltage divider 74 is returned back to its normal value.

When all three

transistors

64, 65 and 66 in the high voltage pulse circuit are all turned off, the swith control circuit 98 may still be in the same state as that which initiated conduction of the pulse circuit. However, even though the third state 102 of the control circuit is on, this has no effect on the base circuit 82 of the first transistor 66 in the pulse circuit.

When it is desired, the switch 106 is transferred back to its normal position as shown in FIG. 5. This causes the one input 108 of the lower NAND gate 105 to become low causing the output 114 of that NAND gate to go low which is cross coupled to the other input 1 12 of the upper NAND gate 104. The one input 110 of the NAND gate 104 resumed its floating potential because the switch 106 has been transferred and the output 116 of the upper NAND gate 104 is then switched from the high to a low signal. Voltage-wise the output of the upper NAND gate 104 is returned to ground potential. .This reduces the voltage input to the double diode 124 and 125 in the base circuit of the first stage 100 which jCflllSCS that transistor to turn off and in succession each succeeding

state

101 and 102 is turned off. The power transistor 102 is likewise turned off causing the emitter circuit of that transistor to return to ground potential. This circuit will return to ground as the charge on the capacitor 87 of the base circuit 82 of the first transistor '66 in the pulse circuit discharges through the emitter resistor 150 of the power transistor. The time constant in this circuit is extremely fast causing that capacitor 87 to discharge relatively quickly. Effectively, when the capacitor 87 is completely discharged, the switch control circuit 98 is again ready for effective firing. As in the example of the preferred embodiment, the emitter resistor 150 of the power transistor is a 1,000 ohm resistor and the capacitor 87 in the base circuit of the first only.

As is commonly employed in high speed printers, especially of the type having a cyclically movable character bearing member such as the rotatable drum 24, a control signal is generated when a selected character is fast approaching print position. Such a control signal is that the hammer responds to the crystals elongation and is launched into substantially free flight for contact with the selected character on the printing drum. The time span of the free flight of the hammer and the peripheral speed of the printing drum are constants which can be utilized to determine the time the hammer should be placed in flight for proper impact with the selected character on the drum.

When voltage is applied to the crystal, potential energy is stored in the electrostrictive element or crystal 36 by means of its contraction. When the crystal is discharged, the potential energy stored therein is converted into kinetic energy which is imparted to the hammer causing the same to strike the paper 44 and impress it against a selected character 26 on the print drum. In the normal state, when electrical power is first applied to the circuit, the crystal charges up through the charging resistor 58 to the voltage supply which in the illustrated embodiment of the invention is 3,000 volts. While the crystal is in this charged state a character on the rotating print drum is selected for printing and thereafter is advancing with the movement of the drums periphery toward the print position. Likewise, each of the

capacitors

83, and 87 in the base circuits of the cascaded transistors are charged through their respective diodes to the voltage at the emitter of each such transistor. In the preferred embodiment of the invention, the charging of the crystal 36 takes place during the time that the position of the selected character on the print drum is being determined. This allows for a much longer charging time than utilizing the charging action to expand the long dimension of the crystal and impel the print hammer as heretofore in this art. This reduces the amount of current in the charging circuit and the power output from the power supply.

When a selected character 26 borne by the drum is to be printed and it approaches the path of the print hammer 28, the input represented by the switch 106 is ef- 1 1 fectively transferred from ground to a plus voltage, such as 5 volts, which by means of the switch control circuit 98 applies a plus voltage, such as 25 volts, to the timing network 82 of the first transistor 66 causing that transistor to begin to conduct. The remaining two transistors 65 -and 64 are immediately thereafter brought into conduction in sequential order as previously described. This provides a conductive path for the discharge of thecrystal 36 and the launch of the hammer toward the print drum. It is evident that the

transistors

64, 65 and 66 remain in their on state until the charge on the crystal is fully dissipated and then be turned off." This will eliminate excessive and unnecessary power consumption in the several circuit components. As soon as the discharge current drops below a base current value necessary to maintain conduction, the cascaded transistors begin to turn off, and as each turns off, the capacitor in its base circuit begins recharging through its respective diode to the voltage of the voltage divider 74 and through a common resistor 148 to ground. When all of the cascaded transistors are fully turned off, the voltage divider 74 functions properly and the voltage at each resistor in the voltage divider is returned back to its normal value.

In the foregoing description, the discharge circuit of FIG. 5 has been initiated by the transfer of the switch 106 from ground to a plus voltage. In the environment of high speed printing drums, such as illustrated in FIG. 2, it would be the usual practice to replace the simple mechanical switch 106 by a type of high speed electronic switch which would perform essentially the same function. Such a switching instrumentality would be designed to provide a controlled time voltage pulse to the left side of the capacitor 87 associated with the transistor 66 furthest from the crystal 36 to allow that transistor to conduct. Moreover, in most high speed applications the electronic gate 105 could be omitted and gate 104 would then be controlled by a logic pulse on input lead 110 for triggering the system into discharge operation. Such a pulse would beequivalent in function if not also in value to the change in voltage level from ground to plus 5 volts on lead 110. Typically, such triggering pulse would be equal to or longer than the time constants of the timing network 82 for transistor 66.

While one particular embodiment of the invention has been shown and described, it will be understood, of course, that it is not desired that the invention be limited thereto since modifications may be made, and it is therefore contemplated by the appending claims to cover any such modifications as fall within the true spirit and scope of the invention.

What is claimed is:

1. An electrostrictive actuator system for use in a printer including:

an electrostrictive member;

power supply means for applying a potential difference across the opposed surfaces of said electrostrictive member wherein said potential difference is effective to contract said electrostrictive memher;

a plurality of switching means connected in series to said power supply means, for discharging said potential difference across said electrostrictive member;

divider means connected to said power supply means and said switching means, for reducing the potential across each of said plurality of switching means;

12 timing means connected to said switching means for causing each of said switching means to selectively open thereby being effective to discharge said potential across said electrostrictive member and thereby causing said electrostrictive member to expand; .and print means responsive to the expansion of said electrostrictive member for performing print operations. i

2. The electrostrictive actuator system of claim 1 wherein said electrostrictive member is an elongated piezoelectric crystal having electrode positions on opposite sides thereof for receiving said potential difference.

3. The electrostrictive actuator system of claim 2 wherein said piezoelectric crystal has a predetermined time modulus of elasticity.

4. The electrostrictive actuator system of claim 1 wherein said power supply means includes a high voltage power supply that provides, in combination with said electrostrictive member, a charging time constant substantially longer than the discharging time thereby minimizing the maximum current to be provided by the said high voltage power supply.

5. The electrostrictive actuator system of claim 1 wherein said switching means includes transistors and additional circuit means electrically connected to said transistors for sequentially turning on said transistors commencing with the transistor electrically farthest from said power supply means thereby providing for a sequenced opening of said transistors so that none of said transistors are subject to excess voltage.

6. The electrostrictive actuator system of claim 5 wherein said divider means comprises a plurality of high resistant elements electrically connected to said power supply for supplying each of said transistors with a voltage proportional to the voltage value to that provided by said power supply means.

7. The electrostrictive actuator system of claim 1 wherein said print means includes an element guided for movement in a direction along a prescribed path of travel and capable of relative displacement with respect to one end of said electrostrictive member when said member is restored to its normal length by the discharge of said potential.

8. In a line printer:

a plurality of print characters;

a cyclically movable member, having said print characters disposed thereto in such a manner as to have each character traverse the print line of the printer as said cyclical member performs its cyclical motion;

a row of electrostrictive elements extending parallel to the print line of the printer with one such element individually located at each print position thereof;

a row of print hammers extending parallel to the print line of the printer in interposed relation between said row of electrostrictive elements and said cyclically movable member, said print hammers being individually associated with each of said electrostrictive elements and capable of being impelled by said electrostrictive elements toward said print characters;

power supply means for applying a potential difference across the opposed surfaces of said electrostrictive elements wherein said potential difference is effective to contract said electrostrictive elements;

switch means, one associated with each of said electrostrictive elements, connected to said power supply means, for discharging said potential difference across said associated electrostrictive element; divider means, one for each of said switch means, connected to said power supply means and its said associated switch means, for reducing the potential across selected elements of said associated switch means; and timing means, one for each of said switch means,

connected to said associated switch means for causing each of said associated switch means to open thereby being effective to discharge said potential across said associated electrostrictive elethereof for receiving said potential difference.

Claims

1. An electrostrictive actuator system for use in a printer including: an electrostrictive member; power supply means for applying a potential difference across the opposed surfaces of said electrostrictive member wherein said potential difference is effective to contract said electrostrictive member; a plurality of switching means connected in series to said power supply means, for discharging said potential difference across said electrostrictive member; divider means connected to said power supply means and said switching means, for reducing the potential across each of said plurality of switching means; timing means connected to said switching means for causing each of said switching means to selectively open thereby being effective to discharge said potential across said electrostrictive member and thereby causing said electrostrictive member to expand; and print means responsive to the expansion of said electrostrictive member for performing print operations.

8. In a line printer: a plurality of print characters; a cyclically movable member, having said print characters disposed thereto in such a manner as to have each character traverse the print line of the printer as said cyclical member performs its cyclical motion; a row of electrostrictive elements extending parallel to the print line of the printer with one such element individually located at each print position thereof; a row of print hammers extending parallel to the print line of the printer in interposed relation between said row of electrostrictive elements and said cyclically movable member, said print hammers being individually associated with each of said electrostrictive elements and capable of being impelled by said electrostrictive elements toward said print characters; power supply means for applying a potential difference across the opposed surfaces of said electrostrictive elements wherein said potential difference is effective to contract said electrostrictive elements; a switch means, one associated with each of said electrostrictive elements, connected to said power supply means, for discharging said potential difference across said associated electrostrictive element; a divider means, one for each of said switch means, connected to said power supply means and its said associated switch means, for reducing the potential across selected elements of said associated switch means; and a timing means, one for each of said switch means, connected to said associated switch means for causing each of said associated switch means to open thereby being effective to discharge said potential across said associated electrostrictive element causing said electrostrictive element to expand thereby impelling said print hammers toward said print characters.

9. The line printer of claim 8 wherein said power supply means includes a high voltage power supply that provides, in combination with each individual electrostrictive element, a charging time constant substantially longer than the discharging time thereby minimizing the maximum current to be provided by said high voltage power supply.

10. The line printer of claim 8 wherein each of said electrostrictive elements is an elongated piezoelectric crystal having electrode positions on opposite sides thereof for receiving said potential difference.