Method and apparatus for maintaining constant drop size mass in thermal ink jet printers

A Thermal ink jet printer has a rotatable platen with an oscillator circuit mounted therein which includes a resonant vibratory device on which the ink droplets ejected from the printhead nozzles by electrical pulses are received and the mass thereof are measured. A piezoelectric sensor, such as a quartz crystal, serves as an environment for measuring the mass of ink droplets deposited on the crystal face. The difference in frequency before and after drop deposition is exactly proportional to the ink drop mass. Frequency change is measured to provide a feedback signal to the printer controlled for adjustment of the droplet ejecting pulses to control the drop size.

BACKGROUND OF THE INVENTION 
This invention relates to drop-on-demand ink jet printing systems and more 
particularly, to a thermal ink jet printer having a rotatable platen 
having circuitry mounted therein including a resonant vibratory device on 
which the ink droplets ejected from the printhead nozzles are received and 
the droplet mass measured. 
Thermal ink jet printing is generally a drop-on-demand type of ink printing 
system which uses thermal energy to produce a vapor bubble in an ink 
filled channel that expels a droplet. A thermal energy generator or 
heating element, usually a resistor, is located in the channels near the 
nozzle a predetermined distance therefrom. The resistors are individually 
addressed with an electric pulse to momentarily vaporize the ink and form 
a bubble which expels an ink droplet. As the bubbles grows, the ink bulges 
from the nozzle and is contained by the surface tension of the ink as a 
meniscus. As the bubble begins to collapse, the ink in the channel between 
the nozzle and the bubble starts to move toward the collapsing bubble, 
causing a volumetric contraction of the ink at the nozzle and resulting in 
separation of the bulging ink as a droplet. The acceleration of the ink 
out of the nozzle while the bubble is growing provides the momentum and 
velocity of the droplet in a substantially straight line towards a 
recording medium, such as paper. 
Thus, thermal ink jet devices operate by pulsing heating elements in 
contact with ink so that bubbles are nucleated, ejecting ink droplets 
toward the paper. It has been found during print tests that print quality 
is affected as the device heats up. This is because the volume of the 
droplet and therefore the printed spot or pixel increases as a function of 
printhead temperature. Through study of this problem, it has been found 
that both the mass and velocity of the droplet increase with device 
temperature, and that both the mass and velocity contribute to increase 
pixel size on the paper. For the carriage type ink jet printer with 
sufficiently high printing density, the spot size increases as the 
carriage traverses the page. Then, as it pauses at the end of travel and 
reverses direction, it cools slightly, so that the next line or swath 
printed on the way back has increasing pixel sizes in the opposite 
direction. This gives rise to light and dark bands, which are most 
pronounced at the edges of the paper. Similarly, other patterns of high 
and low density printing are degraded by undesired change in pixel size 
with device temperature. 
U.S. Pat. No. 4,788,466 to Paul et al discloses a Q-loss compensation 
apparatus for a piezoelectric sensor such as a quartz crystal microbalance 
or other vibratory device wherein the vibration amplitude of the device is 
controlled by negative feedback in a manner to obviate the effect of of 
energy loss associated with viscous damping of a large liquid drop on the 
quartz crystal face serving as an environment for an experiment to measure 
mass deposited on the crystal. The specific apparatus includes an 
oscillator circuit for the vibratory device in which two similar variable 
gain amplifiers provide the regenerative feedback for maintaining 
oscillation. The negative feedback amplitude control circuit serves to 
maintain constant the output from the variable gain amplifier following 
the quartz crystal in the oscillator loop, and it thus of a near constant 
value equal to the product of the crystal vibration amplitude and the 
square root of the total gain in the oscillator loop. This results in 
stable operation of the quartz crystal with little influence from changing 
conditions such as temperature, viscosity of the fluid, evaporation of the 
fluid, etc., at the same time producing a linear frequency change 
dependent on the quantity of mass deposited on the crystal face from the 
liquid environment. Frequency change is measured in a conventional manner 
with accuracy of about one part per ten million, thereby permitting 
determination of minute mass amounts on the order of one nanogram. 
U.S. Pat. No. 5,036,337 to Rezanka discloses a method and apparatus for 
controlling the volume of ink droplets ejected from thermal ink jet print 
heads. The electrical signals applied to heating elements for generating 
droplet ejecting bubbles thereon are composed of packets of electrical 
pulses. Each pulse and spacing there between are varied in accordance with 
one or more whole, clock or timing units. The number of pulses per packet 
and width of pulses and spacing there between are controlled in accordance 
with the manufacturing tolerance variations, the location of the addressed 
heating heating element in the printhead, the number of parallel heating 
elements concurrently energized, and optionally the temperature of the 
printhead in the vicinity of the heating elements to maintain the desired 
volume of the ejected droplets. 
U.S. Pat. No. 5,107,276 to Kneezel et al discloses a thermal ink jet 
printer which has a printhead that is maintained at a substantially 
constant operating temperature during printing. Printing on demand is 
accomplished by the ejection of ink droplets from the printhead nozzles in 
response to energy pulses selectively applied to heating elements located 
in ink channels upstream from the nozzles which vaporize the ink to form 
temporary bubbles. To prevent printhead temperature fluctuations during 
printing, especially in translatable carriage printers, the heating 
elements not being used to eject droplets are selectively energized with 
energy pulses having insufficient magnitude to vaporize the ink. 
SUMMARY OF THE INVENTION 
It is the object of the present invention to provide an improved thermal 
ink jet printer which maintains a substantially constant spot size in ink 
droplets ejected from the printhead while printing. 
It is another object of the invention to maintain the spot size of the ink 
droplets ejected from the printhead constant during a printing mode by 
periodically measuring the mass of a predetermined plurality of ink 
droplets between each printed copy. 
In the present invention, a thermal ink jet printer with a printhead of the 
type having an ink supply manifold and a plurality of parallel ink 
channels with each channel having a nozzle and a heating element, is 
mounted on a translatable carriage and has a rotatable cylindrical platen. 
The printhead is mounted on a carriage which confronts and is 
reciprocatingly translated along the platen. An oscillator circuit is 
mounted inside the platen and includes a resonant vibratory device on 
which the ink droplets ejected from the printhead nozzles by electrical 
driving pulses selectively applied to the heating elements, are received 
and the mass thereof is measured. The resonant vibratory device such as a 
piezoelectric sensor or a quartz crystal, provides the means for measuring 
the mass of ink droplets deposited thereon. The difference in frequency 
before and after drop deposition is proportional to the droplet mass. The 
frequency change of the sensor is measured to provide a signal 
representative of the mass of the droplet and comparison of this signal to 
a desired value generates a comparison signal in the printer controller 
that adjusts the driving pulses which expel the ink droplets from the 
printer, so that the drop mass and therefore the droplet size is 
maintained constant. 
A more complete understanding of the present invention can be obtained by 
considering the following detailed description in conjunction with the 
accompanying drawings, wherein like parts have the same index numerals.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
While the present invention will hereinafter be described in connection 
with a preferred embodiment thereof, it is not intended to limit the 
invention to that embodiment. On the contrary, it is intended to cover all 
alternatives, modifications and equivalents that may be included within 
the spirit and scope of the invention as defined by the appended claims. 
In FIG. 1, a multicolor thermal ink jet printer 10 is shown containing 
several disposable ink supply cartridges 12, each with an integrally 
attached printhead 14. The ink cartridge and printhead combination are 
removably mounted on a translatable carriage 20. During the printing mode, 
the carriage reciprocates back and forth on, for example guide rails 22, 
parallel to the recording medium 24 as depicted by arrow 23. The 
end-to-end travel distance of the carriage and printheads is shown as 
distance B. The carriage is driven back and forth across the length of a 
cylindrical platen 16 by well known means such as, for example, by cable 
and pulley with a reversible motor (not shown). The recording medium, such 
as, for example, paper is mounted to platen 16. The platen has a diameter 
of between 10 and 20 cm and is constructed, for example, out of aluminum 
sleeve 17 with end caps 13 containing a shaft 13A there through which has 
a pulley 33 mounted on one end and driven via a stepper motor (not shown) 
by belt 32. The platen is rotatively mounted in frame sides 21 which also 
contain the ends of guide rails 22. The paper is held stationary by the 
platen while the carriage is moving in one direction. Prior to the 
carriage moving in the reverse direction, the paper is stepped by the 
platen in the direction of arrow 19 a distance equal to the height of the 
swath of data printed thereon by the printheads 14 during transversal in 
one direction across the paper. The width of the recording medium is the 
printing zone or region during the carriage transversal and is indicated 
as distance A. To enable printing by all of the plurality of printheads 
and to accommodate printhead priming and maintenance stations (not shown), 
the overall travel distance B is larger than the printing region A. Thus, 
an encoder 50 (see FIG. 6) must be used to monitor the position of the 
carriage 20 when the printheads are in the printing region. The droplets 
are ejected on demand from nozzles (not shown) located in the front faces 
(not shown) of the printheads along the trajectories 15 to the paper. The 
front face of the printhead is spaced from the paper a distance of between 
0.01 and 0.1 inch, with the preferred distance being about 0.02 inches. 
The stepping tolerance of the platen drum 16, the paper, and the linear 
deviation of the printheads are held within acceptable limits to permit 
contiguous swaths of information to be printed without gaps or overlaps. 
Each cartridge 12 contains a different ink, one black and one to three 
cartridges of different selected colors. The combined cartridge and 
printhead is removed and discarded after the ink supply in the cartridge 
has been depleted. In this environment, some of the nozzles do not eject 
droplets during one complete carriage traversal and generally, none of the 
nozzles eject droplets as the printheads move beyond the edge of the 
platen. While at this end of the carriage traversal, there is a small 
dwell time while the platen is being stepped one swath in height in the 
direction of arrow 19. A maintenance and priming station (not shown) is 
located on one side of the platen where the lesser used nozzles may fire 
nozzle-clearing droplets, and/or where the nozzles may be capped to 
prevent them from drying out during idle time when the printer is not 
being used. Also located on one side of the platen 16 where the 
maintenance and priming operations take place is a cavity 18. A spring 
loaded door 25, as shown in FIG. 3, is mounted over the cavity. The door 
is opened by a tab 27 fixed to the translatable carriage 20 indicated in 
FIG. 3. The door opens only as the printheads pass by whereby tab 27 
pushes against a mating tab 26 located on the door 25. When the carriage 
travel reverses and moves away from the cavity, the door 25 
correspondingly closes. 
Referring now to FIG. 2, a piezoelectric sensor 72 preferably in the form 
of an AT cut quartz crystal plate is located behind the door 25 (see FIG. 
3) to form the resonant vibratory device on which the mass of the ink 
droplets ejected from the printhead nozzles are measured as the printheads 
pass by. A plurality of ink droplets are fired onto the piezoelectric 
sensor 72 at the start up of printing once the nominal operating 
temperature of the printer has been reached or during the page interspace 
areas between multiple page printing operations. Without a sheet of 
recording medium on the platen 16, the platen shaft 13A and platen are 
rotated at a predetermined number of revolutions per minute (RPM), so that 
the voltage generating piezoelectric strip 102 may be vibrated, as 
discussed later, to produce the required voltage to operate the mass 
measuring circuitry 110. The droplets are ejected into the platen opening 
or cavity 18 when the printheads are in alignment therewith. As the platen 
is rotated, the droplets are ejected in synchonism with the alignment of 
the revolving cavity 18 as it rotates pass the printheads, so that all of 
the droplets enter the platen through the cavity and land on the 
piezoelectric sensor 72. When a sheet of recording medium, such as paper, 
is being printed, the recording medium is held stationary on the platen as 
a swath of information is printed, then stepped a distance equal to the 
printed swath. The printing of swaths and stepping is continued until the 
entire sheet is printed. 
Typically the natural mechanical resonant output of the crystal plate 72 
shown in FIG. 2 is due to its mechanical vibration, which will have a 
frequency of approximately 10 megahertz (Mhz). The vibration is in a 
thickness-shear mode parallel to the crystal face so that all parts on the 
crystal surface will be equally sensitive to frequency change. The change 
in the natural mechanical frequency (.DELTA.f) is proportional to the 
change in mass (.DELTA.M) of the crystal plate whereby 
.DELTA.f/f=2(.DELTA.M/M). Upon disassembling a standard crystal housing, 
the following parameters were obtained: mass=230 milligrams, frequency of 
vibration=5.5 Mhz., and crystal plate size=1.5 centimeters square. It is 
therefore extrapolated that a mass of 50 milligrams would result in a 
crystal frequency of 10 megahertz. Assuming an ink jet droplet mass of 
10.sup.-7 grams, the frequency change would be approximately 20 hertz. The 
value for the change in frequency is compared to a table of values called 
a software look-up table 57 (FIG. 6) that resides in a control circuitry 
module 48 (FIG. 4) used to drive the printhead. If additional ink droplets 
are deposited on the crystal plate, the change in frequency would be 
proportional to the number of drops accumulated. Thus, for example, 
firings 10 drops would cause a change in frequency of 200 hertz, thereby 
increasing the accuracy of the measurement. 
As shown in FIGS. 4 and 5, the crystal plate 72 forms part of a 
conventional crystal-controlled oscillator circuit 74 that is mounted on a 
11/2 inch by 1 inch printed wiring board assembly (PWBA) 73. The PWBA is 
located inside cavity 18 as illustrated in FIGS. 2 and 3. The crystal 
plate 72 is arranged so that ink drops can be deposited upon it. Referring 
specifically to FIG. 3, a layer of sticky substance 28, for example, such 
as a non-drying adhesive similar to that used on an adhesive tape is fixed 
around the interior periphery of the cavity 18 to prevent foreign material 
from falling around the inside of the platen interior, but instead adhere 
to the sticky substance. If moisture and/or other entrained contaminants 
is considered a problem, air could optionally be withdrawn from or 
filtered air blown into the interior of the platen by, for example, small 
passageways (not shown) in the shaft 13A. Heat also may be optionally 
directed to the interior of the platen by the shaft passageways or 
separate internal heaters (not shown) so as to more speedily evaporate the 
water content of the ink droplets. The evaporation subsequently disposes 
of the increased mass placed upon the crystal plate 72 by the ink droplets 
and does not affect the accuracy of the measurement. A total drop count of 
50 milligrams.times.0.01/10.sup.-7 grams or 5000 drops is equal to a 1% 
change in mass of the crystal plate. A mass increase of 1% will not cause 
the crystal plate to cease oscillation. Since 90% of an ink droplet is 
water, the water evaporates away and the accumulated drop count can exceed 
50,000 drops because evaporation reduces the additional mass deposited on 
the crystal face by a factor of 90%. The accuracy of the measurement is 
controlled as discussed later on. 
Referring to FIG. 5, the circuit comprising crystal plate 72 and oscillator 
74 is a closed loop system composed of an amplifier 81 and a feedback 
network containing the crystal plate 72 and capacitor 84 discussed in more 
detail later. For an ordinary piece of quartz, in which molecules are 
randomly arranged, physical pressure will move the molecules to new 
positions in a random manner, and no net change in electric charge between 
opposite sides occur. In crystalline quartz, however, molecules and atoms 
are arranged in exact symmetry, and, if physical pressure is applied which 
causes deformation along a mechanical axis, an electric charge will be 
observed between faces which are along an electrical axis. Conversely, if 
a voltage is applied to these faces, a physical deformation along the 
mechanical axis will occur. This is known as the piezoelectric effect. 
Quartz plates ordinarily vibrate in synchronism with the frequency of an 
applied voltage. They will vibrate at a vastly increased amplitude when 
the applied frequency corresponds with the natural mechanical resonant 
frequency of the plate. Thus, crystal-controlled oscillators oscillate at 
a mechanically resonant frequency or its multiples thereof called 
overtones. In the operation of a crystal-controlled oscillator, the 
amplitude of oscillation builds up to the point where circuit non 
linearity decrease the loop gain to unity. The frequency of oscillation 
adjusts itself so that the total phase shift around the loop is 0 or 360 
degrees. The crystal plate, which has a large reactance-frequency slope, 
is located in the feedback network where it has the maximum influence on 
the frequency of oscillation. The crystal-controlled oscillator is unique 
in that the impedance of the crystal plate changes so rapidly with 
frequency that all other circuit components are considered to be of a 
constant reactance at a frequency equal to the natural mechanical resonant 
frequency of the crystal plate. The frequency of oscillation will adjust 
itself so that the crystal plate presents a reactance to the circuit which 
will satisfy the phase requirement. 
The crystal-controlled oscillator 74 of the present invention is shown in 
FIG. 5 in schematic form. The circuit is that of a transistorized 
Pierce-type, crystal-controlled oscillator where the A.C. ground is at the 
emitter of amplifier transistor 81. Resistors 75 and 76 are base-biasing 
resistors that supply a fixed bias for easy starting of oscillation. 
Capacitors 85 and 86 provide a phase shift network. Capacitor 84 and 
crystal plate 72 form the feedback loop. The values for capacitors 85 and 
86 are selected to effectively swamp out the transistor output and input 
impedances as well as to provide feedback amplitude control by reducing 
the amplitude of the feedback so that it is not excessive. The phase shift 
through the transistor is 180 degrees and the total phase shift around the 
amplifier feedback loop is 0 or 360 degrees. The condition of a loop with 
unity gain is also provided by the capacitive voltage divider formed by 
the ratio of the values of capacitor 86 and capacitor 85. The oscillator 
output signal is taken across resistor 80 and coupled to the next stage by 
capacitor 79. Values and type designation for circuit elements are given 
in Table I below. 
______________________________________ 
Component 
Reference Nos. Type Value 
______________________________________ 
72 Crystal 10 Mhz. 
75 Resistor 470 Kohm 
76 Resistor 50 Kohm 
79, 84 Capacitor 1000 pfd 
80, 109 Resistor 5 Kohm 
81 Transistor 2N3904 
85 Capacitor 39 pfd 
86 Capacitor 10 pfd 
90 Transistor MPF102 
93 Coil 1 mh., 
10 turns, 
3/4 in. dia. 
111 Diode 1N4148 
112 Capacitor 1.0 mfd 
114 Variable 0-25 pfd 
Capacitor 
94 Coil 1.5 mh., 10 
turns, 1/2 in. 
dia. 
106, 110 Op Amp LF357 
107 Diode 1N4148 
108 Capacitor .01 mfd. 
______________________________________ 
FIG. 4 illustrates electronic circuitry 150 for measuring the mass of the 
ink droplets ejected from a linear array of printhead nozzles. The 
electronic circuitry includes a transmitter subsystem 55 and a receiver 
subsystem 56. Transmitter subsystem 55 consists of: crystal plate 72 and 
crystal-controlled oscillator 74; a 2-input AND gate 77; a frequency 
counter 87; a shift register 89; an electronically programmable logic 
device (EPLD) 88; an output field effect transistor 90; and a coupling 
coil 93. With the exception of the coupling coil 93, all other components 
comprising the transmitter subsystem 55 are mounted, utilizing surface 
mount technology to the PWBA 73 located inside the platen 16 illustrated 
in FIGS. 2 and 3. The transmitter subsystem 55 generates a transmission 
signal responsive to the output of the frequency counter 87. Frequency 
counter 87 is used to measure changes in frequency which are directly 
proportional to the changes in mass caused by ink droplets deposited on 
the crystal face. The output from the crystal-controlled oscillator 74 is 
provided to the frequency counter in the following manner: A wire 78 
interconnects the output of the crystal oscillator to one input of the 
2-input AND gate 77. The output of the AND gate is applied the input of 
frequency counter 87 via a wire 82. The remaining input of the 2-input AND 
gate 77 is connected to the GATE output of the EPLD 88 by a wire 78A. AND 
gate 77 allows some pulses for each cycle of the input frequency, to pass 
through to the counter and then close, preventing other pulses from 
entering. A counter GATE signal from the EPLD 88 is applied to one input 
and the pulse train of the unknown frequency at the other. When both 
signals are present at the inputs of the AND gate, a signal identical to 
the pulse train of the unknown frequency will appear at the output of the 
AND gate. If either the pulse train or the counter GATE signal go away, 
there will be no output at the AND gate. Therefore, for every input pulse 
at the AND gate there is an identical pulse presented to the frequency 
counter at the output, but only during the time the counter Gate signal is 
present. 
The frequency counter 87 is an up-down counter with a direct CLEAR 
capability. It can either count up or count down depending on the mode of 
its input. The up-down counter eliminates the problem of remeasuring an 
accumulation of residual ink droplets previously deposited on the crystal 
face. Ink droplets previously deposited are not detected when the 
oscillator frequency is measured before and after the firing of an ink 
jet. In actual operation, the frequency counter is triggered to perform a 
count-up sequence prior to depositing ink droplets on the crystal face and 
then triggered to perform a count-down sequence after the deposit so that 
the new measurement of frequency taken during the count-down sequence is 
representative of the change in mass for the new ink droplets deposited on 
the crystal face. Frequency counter 87 is controlled by the EPLD 88. The 
CLEAR input is an asynchronous input that causes the count output of the 
frequency counter to be in the logic low state of 0 whenever it is HIGH. 
The CLEAR input resets all the flip-flops (not shown) internal to the 
frequency counter 87. A CLEAR input to the frequency counter is applied 
from the EPLD through an interconnecting wire 97. The UP/DOWN input is yet 
another asynchronous input. When the UP/DOWN input is HIGH, the counter 
will increment on each pulse of the INPUT line. Similarly, when the 
UP/DOWN is LOW, the counter will decrement on each pulse of the INPUT 
line. An UP/DOWN signal to the frequency counter is applied from the EPLD 
through an interconnecting wire 92. The timing diagram of FIG. 7 has been 
prepared to illustrate the operation of frequency counter 87. Referring to 
FIG. 7, there is shown a continuous train of pulses labeled XTAL OSC 
OUTPUT which represent one pulse for each cycle of input frequency fed to 
one input of the AND gate circuit 86. As illustrated, the XTAL OSC OUTPUT 
signal has been squared up by a conventional input wave-shaping circuit 
(not shown). The output count (not shown) is undetermined until TIME=1, 
when the leading edge of the CLEAR pulse resets all the internal 
flip-flops (not shown) to 0. At TIME=3, the UP/DOWN line is set HIGH. A 
gate signal, GATE is set HIGH at TIME=4 and is applied to the other input 
of AND gate circuit 77. During the interval from TIME=5 to TIME=16, some 
of the XTAL OSC. OUTPUT pulses are transferred from the AND gate circuit 
77 to the INPUT of frequency counter 87. The positive transitions of these 
pulses increment the counter to an initial value (not shown) that occurs 
at TIME=16 when the gate signal, GATE is reset LOW preventing any further 
pulses from passing through AND gate circuit 77. For the interval of time 
between TIME=16 and TIME=20 ink droplets are deposited on the crystal 
plate 72. The UP/DOWN line is pulled LOW at TIME=18. At TIME=20, the GATE 
is set HIGH. This set of input conditions cause some of the XTAL OSC 
OUTPUT pulses to again transfer from the AND gate 77 to the INPUT of 
frequency counter 87 until TIME=32 when the gate signal, GATE is again 
reset LOW preventing further pulses from passing through. However, during 
this gate period the positive transitions of the pulses decrement the 
counter to a final value (not shown). Since there has been an amount of 
mass added to the crystal plate, there is a proportional change in 
frequency as indicated by a new value at the output of the frequency 
counter. 
The output of frequency counter 87, as shown in FIG. 4, is a binary number 
which is presented to the input of the shift register 89 on a data bus 98. 
The shift register is composed of a group of internal flip-flops (not 
shown) connected so each flip-flop transfers its bit of information to the 
next flip-flop when a clock pulse occurs. There are two modes of operation 
for the shift register: a LOAD mode and a SHIFT mode. During the load 
mode, the shift register has the ability to parallel load data 
simultaneously from the data bus 98. In the shift mode, the shift register 
serially transfers the data to the right so that the binary number 
contained in the shift register is presented to the input of transistor 
90. Shift Register 89 is controlled by the EPLD 88. The CLOCK input causes 
the register to shift data and the LOAD input controls the mode of 
operation. The CLOCK input is supplied by the EPLD through an 
interconnecting wire 99. In a similar fashion, a LOAD input is supplied by 
the EPLD through an interconnecting wire 100. The timing diagram of FIG. 7 
illustrates the operation of the shift register. 
Referring to FIG. 7, there is shown a pulse labeled LOAD at TIME=70. From 
TIME=70 to TIME=72, the shift register is in the LOAD mode as indicated by 
its HIGH logic state. During this period, the data present at the output 
of frequency counter 87 is transferred into the parallel inputs of the 
internal register flip-flops (not shown) comprising the shift register 89. 
The data loaded is equal to the new frequency of the drop mass measurement 
where, for example, the new frequency may be equal to 9,050,675 cycles per 
second. At TIME=72 the shift register returns to the shift mode as 
indicated by its LOW logic state on the LOAD input. The EPLD now sends out 
a burst of synchronous clock pulses labeled CLOCK from TIME=74 to TIME=90. 
The first shift clock occurs at TIME=74 and the data begins to form at the 
output labeled SR OUTPUT. With each succeeding CLOCK pulse, the data is 
serially shifted to the right through the shift register until the last 
bit of data reaches the output, SR OUTPUT at TIME=90. 
Referring again to FIG. 4, the combination of the transistor 90 and the 
coupling coil 93 form a final output stage for the transmitter subsystem 
55. The final stage transmits the signal generated by the frequency 
counter to the corresponding receiver subsystem 56. The signal is 
transmitted by a form of amplitude modulation whose carrier frequency is 
switched on and off. This form transmission of is known by those skilled 
in the art as interrupted continuous wave (ICW) or on-off keying. The 
coupling coil 93 is part of a resonant circuit comprising coil 94 and 
capacitor 118 employed in the receiver subsystem 56 which is discussed 
later. The natural resonant frequency of the coupling coil 93 is equal to, 
for example, 23 Mhz. which is the carrier frequency of the final stage of 
the transmitter subsystem 55 when the transistor 90 is in its conducting 
or ON state. Transistor 90 is a junction field effect transistor that is 
biased as a switch so that there is zero drain current when the input 
drive signal applied to the gate is cut off. The input drive signal from 
the shift register is supplied to the gate of transistor 90 through an 
interconnecting wire 101. The coupling coil 93 is connected across the 
output of the transistor 90 by an appropriate set of conductors 103. As 
the transistor is alternately keyed on and off by the output data from the 
shift register, there is a corresponding change in impedance across the 
coupling coil 93. Values and type designation for transistor 90 and the 
coupling coil 93 are given in Table I. 
The receiver subsystem 56 is located outside the cavity 18 as illustrated 
in FIG. 2. Coils 93 and 94 are mounted exteriorly of platen 16 along the 
axis of shaft 13A between the end cap 13 and the pulley 33. Additionally, 
the coils 93 and 94 are insulated by air from the surface of the shaft 
13A. The end-to-end spacing between the coils is shown as distance C, with 
the preferred distance being about 0.5 inches. Coil 93 is mounted to the 
transmitter subsystem 55 on PWBA 73 by wire leads 103 extended through 
holes 105 in the end cap 13 so as to rotate with the platen drum. Coil 94 
is mounted to the receiver subsystem 56 by wire leads 106 and is 
geometrically fixed. The entire receiver subsystem 56 is shown in FIG. 4 
where the receiver subsystem 56 is comprised of: a coil 94; a capacitor 
118; an oscillator 104; a preamplifier 122; a semiconductor diode 107; a 
capacitor 108; a resistor 109; and a post-amplifier 123. Oscillator 104 is 
a LC oscillator that uses the inductance of coil 94 and capacitor 118 
along with the inductance of coil 93 when transistor 90 is turned on as 
the frequency-determining components, coil 93 and coil 94 are mutually 
coupled by inductance field 152. The oscillator is a standard 
Colpitts-type oscillator that has a frequency of oscillation identical to 
the carrier frequency of the transmitter subsystem 55. The receiver 
subsystem 56 functions to detect the changes in impedance of the coupling 
coil 93. The change in impedance across the coupling coil 93, causes the 
resonant frequency of the combined components consisting of the coupling 
coil 93, coil 94, and the capacitor 118 to change. As the resonant 
frequency changes, the voltage drop across the coil 94 correspondingly 
changes to form a string of pulses whose total count is proportional to 
the ink drop mass. The voltage across coil 94 is amplified by preamplifier 
122. The combination of semiconductor diode 107, capacitor 108, and 
resistor 109 form a standard demodulation stage to recover from the 
modulated sine wave a pulsating D.C. voltage that varies in accordance 
with the modulation present on the wave. Thus, diode 107 rectifies the 
modulated wave. Capacitor 108 is a small value capacitor and resistor 109 
is a relatively high resistance so that the combination of capacitor 108 
and resistor 109 form the load impedance across which the rectified output 
voltage of the diode 107 is developed. At each positive peak of the 
radio-frequency cycle, the capacitor 108 charges up to a potential that is 
substantially equal to the peak of the applied voltage. Between peaks, 
some of the charge on the capacitor 108 leaks off through resistor 109, to 
be replenished by an appropriate new charge at the peak of the next 
radio-frequency cycle. The result of this situation is that the voltage 
developed across the load impedance of capacitor 108 and resistor 109 
varies in accordance with the input to reproduce the modulation envelop of 
the applied signal. The current that flows through the diode is in the 
form of pulses occurring at the peak of the radio-frequency cycle. The 
pulses assume whatever amplitude is necessary to charge capacitor 108 up 
to a voltage that is substantially equal to the the peak of the applied 
radio-frequency voltage. The average value of the pulses of current 
flowing through the diode 107, that is, the rectified current, is a 
pulsating direct current. The output voltage is the voltage that the 
rectified current produces across the load impedance when flowing through 
the impedance formed by capacitor 108 and resistor 109 in parallel. The 
recovered pulsating D.C. voltage is presented to the input of the 
post-amplifier 123 so as to obtain an output of greater magnitude which is 
compatible with the printer controller 48. Values and type designation for 
the components comprising the receiver subsystem 56 are given in Table I. 
As disclosed in U.S. Pat. No. 5,107,276, incorporated herein by reference, 
the operating temperature of the printhead is maintained constant because 
the drop sizes or drop volumes vary with temperature. Since each ejected 
droplet by an electrical pulse adds a known amount of heat to the 
printhead, a lookup table was used to adjust the drop ejecting pulses 
based upon the number of droplets ejected. Other prior art techniques to 
maintain constant drop sizes involve monitoring the printhead temperature 
with a temperature sensor and adjusting the drop ejecting pulses in 
accordance with the sensor printhead temperature. In contrast, the droplet 
ejecting pulses of this invention periodically measures the mass of the 
droplets and compares this mass measurement with the desired mass. The 
printing controller adjusts the droplet ejecting electrical pulses in 
accordance with values in a lookup table based upon the measured drop 
mass. 
In the preferred embodiment of FIG. 6, the logic controller 58 within the 
printer controller 48 receives data to be printed in the form of digitized 
date signals. The encoder 50 provides signals indicative of the location 
of the printheads 14, relative to the printing region "A" of FIG. 1, to 
the logic controller. The drop size mass measurement circuitry 110 sends 
measurement signals representative of the mass of the droplets to the 
receiver subsystem 56, in a manner discussed relative to FIG. 4, which in 
turn sends a series of pulses representative of the mass of droplets to a 
pulse counter 61. The pulse counter 61 sends a signal representative of 
the total pulse count received from the receiver subsystem to the drop 
mass lookup table which accepts the total count signal and compares the 
total pulse, a representation of the mass measurement, with the desired 
mass measurement, then submits signals to the logic controller to modify 
the pulse width AND AMPLITUDE given by the ejection pulse controller 62 to 
the heating elements 34 in the printheads 14. The power supply 52 provides 
a VARIABLE voltage V.sub.o to the common bus 36 and the heating elements 
are pulsed within this voltage through drivers 49 with one connected to 
the printhead addressing electrodes 35 and to ground. Thus, the electrical 
pulses applied to the heating elements or resistors 34 have a VARIABLE 
amplitude and width to eject a droplet. Clock 53 provides the TIMING for 
the logic controller 58. Accordingly, the droplet size or volume is 
maintained constant based upon the actual droplet volumes measured, 
instead of by using printhead temperature, measures directly or indirectly 
to adjust the droplet ejecting pulses applied to the heating elements. 
Referring to FIG. 6, the printer controller 48 contains a look up table 57 
which receives input signals representative of the drop size mass 
measurement from the drop size mass measurement circuitry 110. Based upon 
the mass size of the ink droplet, the subthreshold pulse width controller 
56 signals the logic controller 58 to adjust heat generating electrical 
pulses sufficient to eject constant volume droplets. 
FIG. 2 and FIG. 4 illustrate the method of electrically powering the 
transmitter subsystem 55 which is located inside the rotating platen. As 
shown in FIG. 2 a transducer 102 is also located inside the rotating 
platen drum and fixed to the shaft 13A that turns the rotatable platen. 
The transducer 102 is a vibrating strip of piezoelectric ceramic material 
such as, for example, a modified lead zirconate titanate (PZT) 
composition. Attached to the other end of the transducer 102 is a freely 
suspended weight 113. As the shaft 33 rotates, the bending moments 
indicated by the bi-directional arrow 114 and caused by the freely 
suspended weight 113 deflect the transducer 102 so that an electromotive 
force (EMF) is generated. The amount of EMF so obtained is proportional to 
the amplitude of the deflection multiplied by the frequency of rotation 
squared. However, the EMF generated is not a steady nonfluctuating 
voltage. Subsequently, the A.C. voltage is presented to the PWBA 73 by a 
set of interconnecting wires 115 and 116 for rectification, filtering, and 
regulation. 
Referring specifically to FIG. 4, the interconnecting wires 115 and 116 
attach the output the transducer 102 to the input of a power supply 130. 
The power supply 130 is comprised of a half-wave rectifier diode 111, a 
filter capacitor 112, and a voltage regulator 120. In operation, the diode 
111 conducts each time its anode goes positive. When the anode goes 
negative, the diode 111 cuts off, and, except for a slight leakage 
current, there is no output. The output is therefor a pulsating D.C. 
voltage equal to the peak amplitude of the A.C. voltage generated by the 
transducer 102. The output of rectifier diode 111 is filtered by a 
capacitor input filter 112. The filter capacitor 112 charges to the peak 
value of the A.C. input voltage. When the input voltage begins to decrease 
below the voltage across the capacitor, then the capacitor begins to 
discharge through the input resistance of the voltage regulator 120 
connected to the capacitor. The capacitor discharge current that flows 
through the load resistance of the voltage regulator prevents the voltage 
from dropping to zero, as it normally would without the presence of the 
capacitor. Load regulation is provided by connecting the filter capacitor 
112 to a voltage regulator 120. The voltage regulator 120 provides a 
constant voltage to the transmitter subsystem 55 despite deviations in the 
output voltage across the capacitor 112. Thus a DC voltage is applied to 
the appropriate components in the transmitter subsystem from the output of 
the voltage regulator via a power bus 124 as illustrated in FIG. 4. Values 
and type designation for diode 111, capacitor 112, and voltage regulator 
120 are given in Table I. 
In recapitulation, it is clear that the present invention relates an ink 
jet printer having a resonant vibratory device to measure the mass of the 
ink droplets ejected from the printhead nozzles. A rotatable platen with 
an oscillator circuit mounted therein has a quartz crystal on which ink 
droplets are deposited. The difference in frequency before and after 
deposition is proportional to the ink drop mass. Changes in drop mass are 
controlled by printer controller varying the droplet ejecting pulses to 
the heating elements is the printhead in response to input from the mass 
measurement circuitry 110 and lookup table 57. This provides a control of 
the spot size. 
It is, therefore, evident that there has been provided in accordance with 
the present invention, a drop size mass measurement system that fully 
satisfies the aims and advantages hereinbefore set forth. While this 
invention has been described in conjunction with a preferred embodiment 
thereof, it is evident that many alternatives, modifications, and 
variations will be apparent to those skilled in the art. Accordingly, it 
is intended to embrace all such alternatives, modifications and variations 
as fall within the spirit and broad scope of the appended claims.