Drop jet metering method and system

A dispensing system for metering precise quantitites of liquid to a use site in droplet streams. The system includes a reservoir containing a quantity of liquid to be dispensed, a droplet generator for producing a stream of uniformly sized droplets of the liquid in the reservoir; and a device for controlling the droplet generator to produce a droplet stream containing a predetermined number of droplets. The control device can include an operator-settable counter for selectively varying the quantity of dispensed liquid by adjusting the number of produced droplets or by adjusting the size of droplets comprising the droplet stream.

FIELD OF THE INVENTION 
The present invention relates to methods and systems for metering 
relatively small and precise quantities of liquid, for example, for uses 
in the scientific, industrial or medical fields; and, more particularly, 
to drop jet approaches for selectively supplying predetermined numbers of 
uniform volume droplets to accurately accumulate the desired quantity of 
liquid. 
BACKGROUND OF INVENTION 
There are a broad spectrum of technical and medical endeavors that require 
application of exact quantities of liquids For example, the dyeing of 
slides for microscope work, the applying of catalyst to cure epoxies and 
the administering of drugs, all require that precise amounts of a critical 
liquid(s) be metered to a predetermined use site. Present technology 
devices for accomplishing these tasks include syringes, brushes, pipets 
and "eye droppers". To provide accuracy of application within a requisite 
quantity range, it is often necessary to substantially dilute the use 
liquid in a carrier and supply more volume of the carrier/use liquid 
combination. 
A more sophisticated approach for controlled dispensing of liquids is 
described in U.S. Pat. No. 4,475,666, which uses a microcomputer 
controlled motor to operate the piston of a syringe. This system has a 
detection and feedback loop that monitors and controls the piston drive 
motor to assure accurate ejection of the quantities selected (i.e. entered 
into the system microcomputer) by the user. This system is relatively 
complicated structurally and requires servo systems that compensate for 
mechanical backlash and motor drive variations. 
SUMMARY OF THE INVENTION 
It is an object of the present invention to provide simple, but accurate, 
liquid metering methods and systems that utilize liquid droplet streams of 
the kind heretofore used in ink jet printing technologies. Thus, I have 
realized that by accurately controlling the number of minute liquid 
droplets eJected by such jetting system to a use site, very precise 
overall quantities of liquid can be conveniently and accurately metered. 
In one aspect the present invention constitutes a method of dispensing 
precisely metered quantities of a liquid to a use site by: (a) generating 
a stream of uniformly sized droplets of the liquid; and (b) controlling 
such droplet stream so that a predetermined number of droplets pass to the 
use site. 
In another aspect the present invention comprises a dispensing system for 
metering precise quantities of liquid to a use site. The system comprises 
a reservoir for containing a quantity of liquid to be dispensed; a droplet 
generator for producing a stream of uniformly sized droplets from liquid 
in the reservoir; and a circuit for controlling the generator to dispense 
a droplet stream containing a predetermined number of droplets.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
Referring to FIG. 1, a portable metering system constructed in accord with 
the present invention is shown schematically. The system, in general, 
includes a power and control assembly 10 and a droplet dispenser device 
20, connected by an electrical umbilical 7. The assembly 10 is contained 
in a housing 11 and has a drop count display 12, dials 13 for setting the 
desired number of drops to be dispensed, an on-off switch 14, drop 
amplitude, frequency adjustment knobs 15, 16 and start button 17. The 
dispenser device 20 comprises, in general, a hand-held housing 21, a 
liquid reservoir 22, a drop dispensing head 23 and electrodes 24, 25 for 
coupling the head 23 to the umbilical 7. 
One preferred construction for a thermal drop-on-demand dispensing device 
20 is shown in detail in FIGS. 2 and 3. There it can be seen that the 
device housing comprises an elongated cylindrical shell 31, a top cap 32 
having a hollow tip 33 and an end cap 34 having terminals 35, 36 coupled 
to the umbilical 7 and a stopper plug 37. Constructed to interfit within 
the housing portions, as shown in FIGS. 2 and 3, are a plurality of device 
elements for selectively thermally eJecting drops of a liquid. Thus 
electrodes 41, 42 are constructed to couple to terminals 36, 37 and have 
top portions 41a, 42a constructed to sandwich, and selectively energize, 
a resistive heater element 44. A flexible liquid reservoir 46 with an open 
top is constructed to fit within shell 31 and between electrodes 41, 42. A 
hollow spacer 48 is constructed to retain the top of reservoir bladder 46 
and to support an orifice plate 49 above the heater and electrode 
sandwich. The electrode tops 42a and 41a have, respectively, groove and 
slot openings which allow liquid to flow along a tortuous path from the 
bladder 46 to a location between the heater 44 and orifice 49. Upon 
energization of the heater element, a liquid bubble ejects a drop of the 
liquid in a known manner and the tortuous liquid supply path restricts 
back flow. 
The operation of the dispenser device to effect metering of desired 
quantities of liquid to a use site can be further understood by referring 
to the control system circuit shown in FIG. 4. Thus, to commence a 
dispensing operation, top cap 33 and spacer 48 are removed and a supply of 
the liquid to be dispensed is filled into bladder 46. When the cap and 
spacer are replaced, the power switch 14 is actuated and counter dials 13 
are set to a predetermined value representing the number of drops to be 
dispensed. The dial settings are converted to a digital signal by A/D 
device 51 and set the counter 52 to a pulse count total equal to the 
selected drop dispense number of the dials. 
Next, the tip of device 20 is placed on the use site and the start button 
17 is actuated to send a start signal, then via interrupt interface 53, to 
microprocessor 54. In accord with a drop dispense program stored in ROM 55 
and under the regulation of timing control 57, the microprocessor enables 
square wave oscillator 60 to produce a train of square pulses The 
oscillator 60 includes power amplifier states and outputs a drive pulse 
train to the dispense head, as well as a countdown pulse train to counter 
60. 
The drive pulse train provides a continuing series of energizations of 
heater 44, each with a corresponding liquid drop ejection to the use site. 
The drop ejections continue until counter 52 has been returned to a zero 
count by the countdown pulse train from oscillator 60. When the zero count 
occurs, counter 52 signals microprocessor 54 to disable oscillator 60, 
which stops the drop dispense sequence. If desired, a predetermined number 
of dispensing sequences of the same drop count can be selected by 
inputting the number of sequences into RAM 56. In this event, the original 
drop count selection is stored in RAM 56 along with the number of dispense 
repetitions and the counter is reset by the microprocessor automatically, 
e.g. after a predetermined time delay to provide a new use site for the 
dispense device 20. 
The physical mechanism of drop ejection used by the FIG. 2 dispense device 
20 is often referred to in the art as "bubble jetting". U.S. Pats. No. 
4,243,994 and 4,740,796 provide a further disclosure of this mechanism and 
of other structural configurations for effecting it. For practice of the 
present invention, the bubble jet approach is desirable for dispensing 
liquids that have the appropriate boiling point and decomposition point 
characteristics described in the '994 patent. An exemplary liquid that can 
be dispensed by the bubble mechanism is: 
A Liquid Useful for Bubble Jet 
X%: Material to be dispensed 
50%: Diethylene Glycol 
0.1-1%: Sodium Omadine (Sodium-2-Pyridinethiol-1-oxide) (for bacteria 
control) 
0.1-0.2%: Cobratec TT-50S (Tolyltriazole) (to control corrosion) 
0.1-2.0%: Triethanolamine (to control pH) 
Balance: Pure Solvent 
FIG. 5 shows an alternative drop dispensing head which can be used in 
practice of the present invention. In this embodiment drop eJection is 
effected by deflection of a piezoelectric element. Thus, the drop ejecting 
device 70 comprises a housing 71 forming a drop eJection chamber 72, a 
drop ejection nozzle 73 and a supply passage 74. A metal plate 75 at the 
rear of the drop eJection chamber has piezoelectric crystal 76 which is 
coupled by electrodes 78 to receive a sequence of voltage pulses in the 
same manners as described with respect to the current pulse applied to the 
heater of the FIG. 2 embodiment. In response to a voltage pulse, the 
crystal 76 deflects metal plate toward the orifices 73 causing a discrete 
drop eJection. Liquid to be dispensed is supplied to the inlet passage 74 
of the device 70 by a supply tube 79, e.g. by gravity or under fluid 
pressure. An inlet orifice 77 provides resistance to back flow during drop 
eJection. Further description of drop ejection devices such as shown in 
FIG. 5 is provided in U.S. Pat. No. 3,747,120. A number of other different 
piezoelectric drop on demand devices are useful for practice of the 
invention and are fairly versatile in regard to the characteristics of 
liquids handled. An exemplary liquid useful with piezoelectric DOD heads 
is: 
Liquid Useful for Piezoelectric DOD Heads 
X%: Material to be dispensed 
1-4%: Diethylene Glycol 
0.1-1.0%: Sodium Omadine (Sodium-2-Pyridinethiol-1-oxide) 
0.1-2.0%: Cobratec TT-50S 
0.1-2.0%: Triethanolamine 
Balance: Pure Solvent 
In certain dispensing applications, dispensing drop ejection systems 
similar to continuous ink jet printers is useful. FIG. 6 illustrates one 
approach for employing such continuous jet approaches to accurate metering 
for liquid dispensing. Thus, continuous Jet device 80 includes a resonator 
block 81 having an inlet 82 for receiving liquid under pressure and 
directing it to an orifice manifold region 83 and thence to a liquid 
return outlet 84. The liquid supplied to the manifold region discharges 
toward a use site as a liquid filament(s) through an orifice(s) in an 
orifice plate 86. 
The resonator block has a pair of piezoelectric strips 87, 88 which are 
energized by a voltage pulse train to vibrate the resonator and orifice 
plate at a predetermined frequency. The issuing liquid filament breaks 
into a stream of liquid drop of uniform size and a spacing which 
corresponds to the vibration frequency. A piezoelectric feedback tab 89 
provides a signal to servo the drop rate and the vibration signal. 
In accord with the present invention, a predetermined number of drops is 
dispensed by controlling the opening and closing of the drop stream 
passage 90 to the use site. Thus, a catch pan 91 and a slidable shutter 
member 92 are constructed between the orifice 86 and the use site. The 
shutter member is slidable between a position blocking the drop stream and 
the illustrated position opening the drop stream passage to the use site. 
The shutter is actuated between positions by a fast acting solenoid 95 
and, when closed, directs liquid drops to drain outlet 96 to be returned 
to the liquid supply for recirculation. 
The control system for the device of FIG. 6 operates in a manner similar to 
the system shown in FIG. 4. In this embodiment the oscillator signal is 
applied to the resonator and to the counter. After the Jet stream is 
stabilized the shutter is opened and the counter begins to count down in 
accord with the resonator pulse train When a predetermined drop count is 
reached (i.e. a known interval before the desired drop count), the 
microprocessor actuates solenoid 90 to commence closure. The shutter 92 is 
then moved to block the drop stream at the proper time to achieve the 
desired drop quantity to reach the use site. An exemplary liquid useful in 
a continuous jet dispensing system is: 
Liquid Useful for a Continuous Jet Dispensing System 
X%: Material to be dispensed 
1-4%: Polyethylene Glycol (Viscosity Modifier) 
0.1-1%: Sodium Omadine (Sodium-2-Pyridinethiol-1-oxide) (for bacterial 
control) 
0.1-0.2%: Cobratec TT-50S (Tolyltriazole) (for corrosion inhibition) 
1.0-2.0%: ION Solution Sodium Hydroxide 
Balance: High Purity Solvent 
The invention has been described in detail with particular reference to 
preferred embodiments thereof, but it will be understood that variations 
and modifications can be effected within the spirit and scope of the 
invention.