CA1090862A - Method and apparatus for controlling droplet velocity in non-impact printing - Google Patents

Abstract

ABSTRACT OF THE DISCLOSURE
A recording apparatus and method is disclosed which includes a writing fluid source feeding drop projection means which asynchronously ejects a series of droplets of writing fluid from an integrally constructed, multiple nozzle printing head in a discontinuous stream with sufficient velocity to traverse a substantially straight trajectory to a recording medium. The volume and velocity of each droplet is individually controlled by electrical pulses applied to the projection means from an electronic driver. The projection means may be employed and connected to an electronic, alpha-numerical character generator to form predetermined graphical intelligence patterns on the recording medium.

Description

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This invention relates generally to a non-impact printing apparatus and method and more particularly to an apparatus and method in which the printing fluid is ejected from a nozzle.
Historically printing has been done by applying ink to a specially configured key or carrier and mechanically impacting the key or carrier on a recording medium such as paper to form an impression on the medium.
More recently, non-impact printing devices have been developed, where intelligence patterns (alpha-numeric characters, graphical displays, etc.) are deposited on a recording medium.
This invention is used in a method of controlling droplet velocity in an ink jet printer in which droplets are ejected from a battery of nozzles, each droplet ejected responsive to an electronic drive pulse. The improvement of the present invention comprises adjusting the rise time for each electronic drive pulse to give -the same velocity to each droplet ejected from the battery, no matter which nozzle it is ejected from.
In its apparatus aspect the invention is used in an ink jet printer using a battery of piezoelectric crystals, each having inherent capacitance, to displace volume in corresponding ink chambers, each cooperating with a nozzle, in which deformation of the crystal displaces ink in the ink chamber to asynchronously eject a single droplet of ink from the nozzle. The improvement of the present invention comprises a plurality of variable mb/J~

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magnitude resistive elements each in series with a piezoelectric crystal to form an R-C circuit for varyin~ resistance so that the velocity of each droplet ejected from each noz~le in the battery may be matched.

.~' mb/,u - la -" ~90~62 BRIEF DESCRIPTION OF THE DRAWINGS
Figure 1 is an exploded perspective view of the components of the ink ~et printing head in accordance with the invention.
Figure 2 is a plan view of a portion of a multi-chamber printing head in accord~nce with the invention.
Figure 3 is a circuit diagram of an ink control circuit for use with the printing head depicted in Figure lo Figure 4 is a schematic diagram of a system incorporating the ink jet printer of the present inve,ntion.
Figure 5 is a circuit diagram for the driver circuit for use in the systam depicted in Figure 4.
Figure 6 is a partial, horizontal cross-section of an alternative embodiment of a nozzle configuratlon.

DESCRIPTION OF THE PRE ERRED EMBODIMENT
Figures 1 and 2 show a m~ltinozzle battery 10 of seven ink ~e- pvinters constructed according to the inventitn. S-ven db -2-- 10908~2 printers are compatible with commercially available character generators using a matrix of five dots wide and seven dots high. However, the system operates well using other dot matrices and a battery oE between five and sixteen printers for the vertical component of the matrix. Figure

2 shows ~he lower portion of the battery comprising a ceramic base plate 11 having cavities etched into it to form the elements of the printer. Thus, base plate 11 has seven nozzles 12, seven necks 13 and seven ink chambers 14.
lG Over the ink chambers, on the upper portion of the battery shown in Figure 1, is a cover slip 18 on which are mounted seven piezoelectric crystals 21-27. Plate 11 also has etched therein a pulse trap chamber 16 communicating with each ink chamber opposite to the nozzles 12. The pulse trap chamber 16 serves, when filled with ink, to absorb back pressure of the i~k. As will hereinafter be explained in greater detail, when the cover slip 18 is caused by one of the crystals 21-27 to deflect into an ink chamber 14, a droplet 7 is ejected from the corresponding nozzle 12, while at the same time ink is forced back toward pulse trap chamber 16. Thus, the volume displaced within ink chamber 14 must necessarily exceed the volume of ink ejected as a droplet.
For accurate recording of information on a recording medium 15, a substantially straight trajectory is followed from the nozzles 12 on printing head 10 to the recording medium 15.
In this manner, careful positioning of the recording medium relative to the printing head 10, or vice versa, results in impingement of droplets in a predictable pattern according to signals generated by an electronic driver 106 (see Figure ., s~/r~

~091~8~;2 4), which is determined by the information to be printed.
For the best recording of information, the droplet should be of a precise and predictable shape and volume. That is, each droplet must closely follow the electronic signals from the driver so that equally spaced, uniform signals give equally spaced, uniform droplets.
Each droplet is discharged from the head 10 by the sudden reduction of volume in a selected one of the chambers 14. This sudden reduction in volume is accomplished by contracting the overlying crystal upon electrical signal, thereby deflecting the cover slip plate 18 into the selected-chamber 14 to displace a sufficient ink to form a droplet.
The deflection must be sudden enough to impart sufficient kinetic energy to the fluid in the associated no~zle 12 to accelerate a portion of the droplet beyond the escape ~elocity. Escape velocity is the minimum velocity which will cause a plug of ink extending from the nozzle to separate from the nozzle and form a free droplet.
The escape velocity can be determined by equating the kinetic ener~y of the droplet to the energy required to form the surface of the droplet:

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where a is the surface tension constant of the fluid, p is the density of the fluid and D is the diameter of the droplet. As an example, the escape velocity of a droplet of water .015 centimeters in diameter is 167 centimeters per second.
In order to achieve this, the crystal overlying deflected cover 18 must generate a pressure in the fluid Sb/r~, ~ L0~08~Z
capable of accelerating the fluid in the nozzle from rest to a velocity in excess of escape velocity by the time that the fluid has moved to extend a plug approximately one droplet diameter from the orifice. This minimum pressure is givcn by Minimum Pressure = 6Ka where K is a dimensionless constant for the given geometry of the device, ~ is the surface tension constant, and D is the droplet diameter. K usually is between 4 and 40. For a .015 cm. water droplet, fired from a head with K = 10, the minimum pressure is 2.8 x 10 Dynes/cm.2, or approximately 4 pounds per square inch. The deflected cover plate 18 must also displace an amount of fluid greater than the volume of the droplet to be ejected, since a portion of the fluid displaced by the pressure plate flows backward to the pulse trap chamber 16.
After a droplet has been ejected, the plate returns to its normal position and the miniscus of the liquid is drawn back in the orifice approximately one droplet diameter.
This liquid must be replaced before the printer can be activated again to eject another droplet, and the capillary action of the fluid in the orifice provides the required force. Due to this replacement process, the maximum speed at which droplets can be ejected is approximately Maximum Dot Frequency ~ [ - 3~

Dot rates as high as 50,000 drops per second may be achieved , .
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in this system, depending on the value of K, which, in turn, depends on the geometry of the device.
The action of the crystal on cover 18 is such that as the crystal returns to its rest position, a negative pressure pulse is generated that is approximately equal in magnitude to the positive pressure pulse. The negative pressure pulse reverses the direction of fluid flow in the nozzle, which assists the separation of the plug extending from the nozzle, generating a free droplet. The maximum pressure at which the head may be operated is determined by the onset of cavitation during the corresponding negative pressure pulse. The onset of cavitation is very difficult to express analytically, being dependent on frequency and viscosity, among other parameters. A practical upper limit for the print head pressure may be taken to ~e 3.5 x 106 Dynes per square centimeterj or approximately 50 pounds per square inch.
The opening 17 is connected between an ink source 5 and a valve assembly presently to be described. Referring again to Figure 1, the base plate 11 has a cover slip 18 thereover which is bonded to it to enclose the chambers~
nozzles and necks described previously. Unlike plate 11 which has chambers etched in it, the cover slip 18 has only two openings: a large one defining the pulse trap chamber 16 and a valve opening 19. Each of the ceramic plates 11 and 18 may conveniently be made of "Photoceram," a trade name of Corning Glass Corp., Corning, New York. Overlying the cover slip 18 and bonded thereto are the seven piezoelectric crystals 21-27. On the underside of lower plate 11 is a Sb/r rn ~ 09(~86~
tube fitting 28 which fits in opening 17 (Figure 2). A
tube 6 connects the ink source 5 to the fitting 28.
On top of the battery, at valve opening 19, a valve seal 29 is affixed. Overlying cover slip 18 at the two openings 16 and 19 is a diaphragm 31. Diaphragm 31 is prefer-ably made of a flexible material such as Saran plastic (a trademark of Dow Chemical Co., Midland, Michigan). Diaphragm 31 forms the upper wall of the pulse trap chamber 16.
Overlying the diaphragm 31 is a pressure regulator frame 32, preferably made of steel. Frame 32 is conveniently made in an outline generally corresponding to that of diaphragm 31, since both cover the chamber 16 as well as the valve opening 19. Frame 32 has an opening punched out to accommodate opening 19 on the plate 11 and another U-shaped cut to form a long tongue 33. Tongue 33 is formed by folding up its sides to make a channel having a long moment arm. The move-ment of tongue 33 i5 limited by a bar 34 attached to the frame 32. At the base of the tongue 33 are two strain gauges 36 and 37 one of which serves to measure the strain at the point at which tongue 33 is attached to the remainder of frame 32 and the second provides a control reference.
- As will now be explained, the strain gauges sense the pressure within pulse trap chamber 16. As the ink flows into the pulse trap chamber 16 under great~r pressure, it raises the diaphragm 31 and the tongue 33 lying thereover.
As tongue 33 raises, it creates a greater strain on strain gauge 36 which, by comparison with the control strain gauge 37 serVes to electrically indicate the pressure within pulse trap chamber 16.

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As strain gauge 37 exhibits strain corresponding to a change in pressure, it generates an electrical signal which, as will be explained in greater detail hereinafter, causes the opening of a gate valve in relation to the pressure being sensed. The gate valve regulates the ink flow through the opening 19. The gate valve is comprised of a plug 38 secured to a valve beam 39, the diaphragm 31 and a seal 29. When the plug 38 is raised from the opening 19, ink is permitted to flow out of the opening 19 into the pulse trap chamber 16 under the diaphragm 3I.
The control plug 38 is governed by the beam 39.
Beam 39 is preferably a stainless steel reaction plate which is mounted directly on the cover slip 18. Bonded to the beam 39 is a piezoelectric crystal 41 which is electrically controlled by the strain gauges 36 and 37 by means of the circuit shown in Figure 3. As the strain gauge 36 experiences a change in strain because of the raising or lowering of tongue 33, it generates an electrical signal to the circuit which causes the piezoelectric crystal 41 to 2Q contract or expand in response to the strain being exhibited which action opens or closes the gate valve. Set screw 40 serves to ad~ust beam 39 so that there is no flow of in~
through opening 19 when there is no voltage applied to crystal 41.
Figure 3 is a circuit diagram for the pressure regulator circuit. The strain gauges 36 and 37 are connected in series across a +5 volt DC source and their junction point is connected to one input of a differential amplifier circuit 30. A varia~le resistance 20 in series with a fixed sb/r r~

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resistance 35 i5 connected in parallel with the strain gauges 36 and 37 across the ~5 volt DC source. The junction of the resistances 20 and 35 is connected to the other input of the differential amplifier circuit 30. The output signal of the amplifier circuit 3t) is supplied to the crystal 41 and is responsive to the difference of the inputs to the amplifier circuit 30. The variable resistance 20 is thus the control which enables the system to be calibrated for the normal rest condition of pressure within the system as measured in pulse trap chamber 16 (Figure 1). Ordinarily, we prefer a slight negative pressure within pulse trap chamber 16 in order to have an inwardly directed meniscus at each of the nozzles. However, any pressure is suitable so long as ink remains in the nozzle and does not drip from the nozzle.
~hen resistance 20 is adjusted to the desired setting, the strain gauge 36 is calibrated to measure the strain caused by deflection of the diaphragm 310 The output of the pressure regulator circuit shown in Figure 3 is to piezoelectric crystal 41 which operates beam 39 to open and close the ball valve 38 against the valve seal 29.
The seven crystals 21-27 are each bonded on top o~
the cover slip 18 and are aligned over their respective chambers withi~ plate 11 of the battery. The crystals 21-27 are electrically connected to a printed circuit board 42 by appropriate leads ~not shown). Similarly, leads from board 42 are passed to strain gauges 36 and 37 as well as to piezoelectric crystal 41 in order to control the system.
Printed circuit board 42 is connected by a fourteen conductor flat cable 43 to a resistor printed circuit board 44, and .

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then to a fourteen pin dip socket 46. The entire assembly illustrated in Figure 1 forms one print head assembly having a battery of seven ink jet printers.
To be compatible with commercially available character generators (5 x 7 dot arxay) and commonly accepted print sizes (each character 0.25 cm. high), the seven individual orifices in the battery of printers must be spaced 0.036 cm. apart. Of course, other type fonts can be accommodated by the printing method of this invention, and the number of nozzles and the size of the droplets can be changed within limits dictated by the various components.
Nozzle spacings can be taken to be from 0.050 centimeters ~the maximum practical dot size) to 0.012 centimeters (the minimum nozzle spacing compatible with fabrication techniques for the system shown in Figure 1). For manufacturing convenience, the series of seven nozzles 12 in the array may be made with the nozzles between .005 and 0.025 centimeters in diameter. The distance between the first and last nozzles in the series should not exceed 0.25 centi-meters ln order to accommodate the usual print character size.
Mos~ typewriters, for example, have a character height of approximately 0.25 centimeters, and the present device approximates that size.
The crystals may be made of any suitable material capable of deforming upon electrical impulse. One suitable material is a lead zirconate - lead titanate ceramic available commercially. Others include Rochelle salt, ammonium dihydrogen phosphate, lithium sulfate and barium titanate. Upon application of a voltage across one of the sb/r ~

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crystals 21-27, the crystal contracts, and the action of the contracting crystal on the cover slip 18 causes it to deflect inward into the chamber 14 beneath the contracting crystal, reducing the volume of the chamber sufficiently to eject a droplet from the corresponding nozzle 12. The diameter of each projected droplet should be between 1 and 3 times the average cross-sectional dimension of the orifice.
Since the crystal and cover slip must interact to proyide the volume displacement and pressure necessary for droplet ejection, there are several explicit relationships that must be met for optimum functioning. It is desirable that the neutral axis ~point of zero strain) of the crystal-cover slip assembly be at the interface between them. This condition îs met provided that ~Et ~ crystal = ~Et2) cover slip where E is the modulus of elasticity and t is the thickness of the respective components~
It will ~e seen that the crystals 21-27 are smaller in ~idth than the rectangular pressure chambers 14, and this difference in dimensions is chosen to maximize the dis-placement attainable for a given width. The crystals 21-27 should be centered over the chambers 14 so as to equalize the gaps at both sides. The pressure chambers 14 and the crystals 21-27 are chosen in the form of long rectangles.
In this configuration, the width ~smaller dimension of the rectangle) is chosen so as to provide the necessary pressure, and the length ~larger dimension) is chosen so as to provide the necessary volume displacement. If the length exceeds 20 times the width, then the geometric factor K becomes sb/r ~

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undesirably larger, which lowers the maximum dot frequency and raises the required minimum pressure. On the other hand, the length should be greater than twice the width to minimize the area of less than maximum deflection at the ends of the crystal. In volume each of the pressure chambers 33 should be between 1.6 x 10 3 and 1.6 x 10 2 cubic centimeters.
If "A" represents the length of a shorter side of this hypothetical rectangle, and "a" and "b" the width and length, respectively, of the overlying crystal, then the dimensions of the crystal should be selected to satisfy the conditions:
0.5A-< a < 0.9A and 2a < b < 20a The diameter of the nozzle orifice is preferably - small to enhance capillary actioned ink replacement, and to determine the size of the droplet. The nozzle 12 must be long enough to assure the drop follows a substantially straight trajectory parallel and coaxial with the orifice, but ~et not excessively long, which would increase the geometric constant K, increase the pressure required, and decrease the maximum print rate. We have found that the nozzle length should be between 2 and 4 times the diameter of the nozzle orifice.
Using a volume displacement means for an ink jet printer requires that the ink system be entirely free of air, and that no cavitation occur. Because of this, it is desirable to remove the gasses dissolved in the ink that would be capable of coming out of solution during the negatiye pressure pulseO This can be accomplished by boiling sb/~r~

~ 1~9~862 the ink or by a process of vacuum deairing. If such "deaired" ink is exposed to atmospheric conditions, it will quickly reabsorb gasses to regain its original equilibrium state. It is preferred to use a self-contained ink reservoir and ~ressure-generating system that isolates the ink from the atmosphere.
Figure 4 is a schematic diagram for the control of the printers. A source 101 of alphanumeric characters is fed into a decoder 102. The source may be a key from an electric typewriter, a computer readout, or other source of alphanumeric characters. Decoder 102 provides the information required to define the specific alphanumeric character in a six bit code. The six bits o information or address are fed to a character generator 103. Dot matrix character generators having sixty-four characters using five by seven bits are commercially available from many sources. The character generator also has fed into it a strobe pulse train from a clock 104. rrhe pulse train also goes to driver circuit 106 as do the seven outputs of character generator 103. Driver 106 instructs the various crystals to fire for the printing of the desired dots to form alphanumeric characters. The seven outputs from the driver circuit 106 go to the cxystals 21-27 overlying the ink chambers.
Figure 5 illustrates the driver circuit for each of the crystals. The seven outputs from the character generator are connected to one input of seven separate AND gates 105 whose other inputs are supplied with a contr~l signal - ~oyerned by the strobe pulses. The AND gate outputs are : - 13 -sb/~

~L090~2 supplied through separate resistors 110 to the base electrodes of separate NPN transistors 111. The emitter electrodes of the transistors 111 are grounded. Each of the crystals 21-27 is grounded at one side and has its other side connected in series with a separate variable resistance RC 61-RC 67, respectivel~, to the collector electrode of a separate one of the transistors 111. Each collector electrode is also connected to a 130-volt source through a separate lOK resistance 112. The transistors 111 are all suitable high voltage, low power transistors.
The si2e of the dot and the velocity at which it is ejected can be controlled electronically by various means, such as by varying the energy content of the driving pulse, or by changing the shape o~ the pulse while having the energy content essentially unchanged. The circuit illustrated in Figure 5 has been designea to control dot size and velocity by means of changing the shape of the driving pulse, and more specifically by changing the rise time of the driving pulse, that is, the rate at which voltage is applied to the cr~stals. The crystals have an inherent capacitance due to their construction and geometry. Thus, when the 130 volt "source" voltage is applied by means of the switching action of one of the transistors 111, the circuit responds like an R-C circuit, with the resistances RC 61-67 being the variable by which control is achieved. The circuit shown in Figure 5 is capable of controlling the ejection velocity of droplets ~rom zero to the maximum attainable with the source voltage supplied. The resisting elements 61 to 67 ma~ be varied to give a time constant in the range between sb/r~

~ 09o8Gz 5 microseconds and 100 microseconds. This is particularly advantageous for character printer applications, where the velocities of the seven droplets should be equal, while small volume or size variations are unimportant. The size o the dots can be controlled by changing the magnitude of the driving voltaye supplied to the crystals. An increased drive voltage generates larger dots, and decreased drive yields smaller drops. In this manner, the darkness of the printed characters can be controlled. As an example, the size of the dot printed on paper from a nozzle with a diameter of 0.010 cm. can be varied from approximately 0.010 cm. to 0.060 cm.
In o~eration of the device, the information deter-mining which character is to be printed comes from the source 101, is decoded into a six bit code which selects the correct fiye by seven matrix in the character generator 103. The character generator 103 causes the driver circuit 106 to fire the appropirate crystal as the battery moves relative to the medium.
2Q Each time a drive pulse is fired, ink volume within one ore more of the confined chambers 14 is reduced by the flexing of the associated crystal to suddenly expel a single droplet asynchronously towards the recording mediu~. This gives a recording system which is entirely on call, and does not fire during idle periods, either between characters when printing or when the printer ;s inactive. The desired dots can be ormed or not as the printing head is stepped along relative to the recording medium. A stepper motor may sb/ ~ ~

conveniently be used to move the printing head to form each character in the X axis while dots can be formed in the Y axis by selectively causing specific drive crystals to be fired.
We have found that the droplets upon impact with a printing surface produce printed spots two to four times the diameter of the droplet in flight. It then becomes obvious that the smaller the droplets, the more efficient the printing process becomes in terms of ink required to cover a given area. In addition, there is an increase in resolution, and an increase in the maximum print rate attainable when less ink is expelled in each droplet.
Figure 6 shows an alternative preferred embodiment of a nozzle configuration, where two independen~ orificies 83 and 84 formed by a divider 85 are fed by a common pressure chamber 14' within base plate 11. With each pulse of the pressure plate (not shown), two droplets 86 and 87 are ejected simultaneously in exactly the same manner as a single droplet would be ejected from a single nozzle 12.
Using the device of the present invention a computer print-out, for example, can be produced at high speeds (up to 1000 characters per second) without the cost, noise and power requirements associated wlth impact printers currently in use. The device is light in weight and highly reliable.

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Claims (3)

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. The method of controlling droplet velocity in an ink jet printer in which droplets are ejected from a battery of nozzles, each droplet ejected responsive to an electronic drive pulse, the improvement comprising adjusting the rise time for each electronic drive pulse to give the same velocity to each droplet ejected from the battery, no matter which nozzle it is ejected from.

2. In an ink jet printer using a battery of piezoelectric crystals, each having inherent capacitance, to displace volume in corresponding ink chambers, each cooperating with a nozzle, in which deformation of the crystal displaces ink in the ink chamber to asynchronously eject a single droplet of ink from the nozzle, the improvement comprising a plurality of variable magnitude resistive elements each in series with a piezoelectric crystal to form an R-C circuit for varying resistance so that the velocity of each droplet ejected from each nozzle in the battery may be matched.

3. In an ink jet printer as in claim 2 wherein the resistive elements can be varied to give a time constant of the R-C circuit in the range between 5 microseconds and 100 microseconds.