Fluid jet print head

A stimulation arrangement for a fluid jet printer. A pair of piezoelectric crystals are mounted on opposing surfaces of a high acoustic Q solid member and are excited for periodic lengthening at the frequency of desired stimulation. This creates shear waves in the surface of the high Q member. The high Q member is configured in such a fashion that it transforms the shear waves into stationary compression waves which drive an orifice plate and thereby stimulate fluid filaments being generated by the jet printer. The high Q member may be a rod-like stimulator supported for localized contact against a filament forming orifice plate or it may comprise support structure for the orifice plate.

BACKGROUND OF THE INVENTION 
The present invention relates to the field of jet drop printing and more 
particularly to an improved fluid jet print head and a method of operation 
therefor. 
Jet drop printers operate by generating streams of small drops of ink and 
controlling the deposit of the drops on a print receiving medium. 
Typically, the drops are electrically charged and then deflected by an 
electrical field. The drops are formed from fluid filaments which emerge 
from small orifices. The orifices may be formed in an orifice plate which 
communicate with a fluid reservoir in which fluid is maintained under 
pressure. Each fluid filament tends to break apart at its tip to form a 
stream of drops. In order to produce accurate printing it is necessary 
that the drops be generated at accurately timed intervals. This is 
accomplished by a process known as "stimulation". 
One prior art approach vibrates the entire print head, including the ink 
manifold structure and the orifice plate structure, together. This is 
shown in Beam et al U.S. Pat. No. 3,586,907. Such an arrangement 
necessarily fatigues the print head mounting structure, since the mounting 
structure experiences the same vibrations as are applied to the manifold 
and the orifice plate. Further, the amplitude and phase of the vibratory 
motion are difficult to control at the frequencies commonly used for jet 
drop printer operation. 
Another prior art stimulation technique, as shown in Lyon et al U.S. Pat. 
No. 3,739,393, provides the fluid orifices in a relatively thin, flexible 
orifice plate. The orifice plate is stimulated by causing a series of 
bending waves to travel therealong. This technique, known as traveling 
wave stimulation, results in substantially uniform drop size and spacing, 
but the timing of break up of the fluid filaments varies along the length 
of the orifice plate. 
Other prior art approaches have attempted to stimulate the filaments in a 
common phase by exciting coplanar movement of the orifices in the orifice 
plate, a typical example is disclosed in Cha U.S. Pat. No. 4,095,232. 
Using the technique disclosed in this patent, stimulators mounted in the 
upper portion of a fluid reservoir generate pressure waves which are 
transmitted downward through the fluid. Each stimulator includes a pair of 
piezoelectric crystals which vibrate in phase and which are mounted on 
opposite sides of a mounting plate which is coincident with a nodal plane. 
A reaction mass is positioned at the end of each stimulator opposite the 
stimulation member. The reaction mass ensures that the nodal plane is 
properly positioned. 
In British Patent Specification No. 1,293,980, and Cha et al U.S. Pat. No. 
4,198,643, print heads are disclosed in which a pair of piezoelectric 
crystals are bonded to opposite sides of a support plate. A print head 
manifold structure is bonded to one of the piezoelectric crystals and a 
counterbalance is bonded to the other of the crystals. The weight of the 
counterbalance is selected so as to offset the weight of the manifold 
structure. By this balanced arrangement, the support plate is placed in a 
nodal plane when the two piezoelectric transducers are energized in 
synchronism. 
Finally, in Keur U.S. Pat. No. 3,972,474, an ink drop writing system is 
shown in which a vibrating nozzle is used to produce a stream of drops. 
The length of the nozzle is selected so that its mechanical resonant 
frequency is much higher than the frequency at which it is driven. The 
nozzle, configured as a tube, is surrounded by a piezoelectric ring which, 
when electrically driven, provides radial contraction and expansion of the 
tube. 
Generally speaking, the prior art stimulation systems have employed 
piezoelectric crystals incorporated into mechanical arrangements of 
complex acoustical design. Each such arrangement has had to be 
individually tailored for resonant operation at the design frequency 
within its specifically associated print head. Such tailoring has required 
careful mechanical adjustment and/or trial and error selection of 
component parts. This has "tuned" the stimulation system for operation 
within an extremely narrow range of operating frequencies. For operation 
outside this range the performance is extremely degraded. 
In some applications it is desirable to adjust the frequency of the 
stimulation driving signal. A typical example is in precision printing of 
high resolution graphics. In such printing there are unavoidable 
variations in the transport speed of the substrate, and these variations 
tend to produce drop positional placement errors. This can be corrected by 
adjusting the stimulation drive, as shown for instance in Van Brimer et al 
U.S. Pat. No. 3,588,906. This results in stimulation at a frequency which 
deviates from the nominal design frequency. Such deviation cannot be 
accomodated satisfactorily by systems of the above described types. 
Thus it is seen that there is a need for an improved and simplified 
apparatus for effecting fluid jet stimulation and for accommodating 
adjustments in the frequency of the stimulation. 
SUMMARY OF THE INVENTION 
The present invention provides constructions for simpler and more effective 
stimulation of fluid jet printing streams. Moreover the invention is 
applicable to multi-orifice print head systems of the type wherein an 
orifice plate is excited by traveling bending waves as well as those 
wherein the orifice plate is excited for movement with its orifices 
coplanar. In either case the system may be provided with stimulation means 
comprising a high acoustic Q solid member having a major dimension 
substantially equal to an integral number of half wavelengths of vibration 
at the stimulation frequency and two other minor dimensions each 
substantially shorter than a half of such a wavelength. A pair of 
elongated strips of piezoelectric material are bonded to opposite surfaces 
of the metallic member and driven so as to elongate periodically at the 
stimulation frequency in a direction parallel to the major dimension of 
the high Q member. This induces corresponding shear stresses in the 
surfaces of the metallic member, and those shear stresses cause the 
desired vibration of the orifice plate. 
For application to traveling wave stimulation the metallic member may 
comprise a rod-like structure supported for localized contact against the 
orifice plate. For coplanar orifice movement the high Q member may 
comprise support structure integrally associated with the print head body. 
In one aspect the present invention provides an improved fluid jet print 
head comprising an elongated print head body, the length or major 
dimension of the body between first and second ends thereof being 
substantially greater than its other minor dimensions. The body defines a 
fluid receiving reservoir in its first end and at least one orifice 
communicating with the fluid receiving reservoir. Fluid is supplied to the 
reservoir under pressure by appropriate means such that it emerges from 
the reservoir to form a fluid stream. A transducer means is mounted on the 
exterior of the body and extends along the body in the direction of 
elongation toward both the first and second ends of the body. The 
transducer means is responsive to a stimulation driving signal for 
changing dimension in the direction of elongation of the body, thereby 
causing mechanical vibration of the body and break up of the fluid stream 
into a stream of drops. The major dimension of the print head is 
substantially equal to an integral number of half wavelengths of head 
vibration at the frequency of the stimulation driving signal. 
The transducer means comprises a pair of elongated strips of piezoelectric 
material bonded to opposite sides of the body and extending in the 
direction of elongation. The piezoelectric strips induce alternating shear 
stresses in the surfaces of the elongated print head body in the direction 
of elongation of the body. These surface shear stresses are converted into 
compression waves which travel in the direction of elongation and produce 
longitudinal vibration of the print head body at the stimulation driving 
frequency. 
The transducer means further comprises means for electrically connecting 
the pair of transducers in parallel, whereby the transducers operate in 
phase so as to produce vibration which is in a direction substantially 
parallel to the direction of elongation of the elongated print head body. 
A support means for the print head engages the print head body 
intermediate and substantially equidistant from its first and second ends. 
Alternatively, the transducer means may comprise means for electrically 
connecting the transducers so that they operate out of phase, thus 
producing flexure waves. The support means for the print head engages the 
print head body a distance from each end of the body approximatey equal to 
23 percent of the overall length of the body. 
The print head is provided with a fluid receiving reservoir and an orifice 
plate having a plurality of orifices communicating with the reservoir. The 
orifice plate may be mounted upon a face of the print head extending 
perpendicular to the major dimension of the head or, alternatively, upon a 
face extending parallel to the major dimension. Accordingly, the printing 
jets may be directed either parallel or perpendicular to the major 
dimension of the print head. 
The fluid jet print head may further include means for applying an 
electrical driving signal of a frequency substantially equal to f.sub.o 
=C/2L, where L is the dimension of the body in the direction of 
elongation, and C is the speed of sound through the body. In this case the 
fluid jet print head is driven at a frequency approximating its mechanical 
resonant frequency. 
For flexure wave vibration, the transducers are driven at a frequency 
F.sub.o .congruent..alpha.Ca/L.sup.2, where a is the transverse thickness 
of the print head body and .alpha..congruent.1 in MKS units. In this case, 
two nodal mounting axes are established a distance equal to approximately 
0.23 of the length of the print head body, centered between the 
transducers. 
The method for stimulating the break up of a fluid stream emanating from at 
least one orifice communicating with the fuid reservoir in a half 
wavelength fluid jet print head includes the steps of: 
(a) providing an elongated print head which defines the reservoir and 
orifice at one end thereof; 
(b) applying fluid under pressure to the reservoir so as to produce fluid 
flow through the orifice; 
(c) supporting the print head at points in a plane substantially 
equidistant from the ends of the elongated print head and normal to the 
direction of elongation of the print head; and 
(d) alternately elongating and contracting the print head substantially at 
the resonant frequency of the print head, whereby the print head is 
supported in at least one nodal plane and the stream is effectively 
stimulated to break up into drops. 
The resonant frequency of the print head may be substantially equal to the 
resonant frequency of the fluid stream. The print head may be elongated 
and contracted by means of piezoelectric transducers bonded to its 
exterior. 
The stream may also be stimulated by operating the transducers out of 
phase, thereby causing flexure of the print head. In this stimulation 
mode, the print head is mounted at points spaced from the ends by a 
distance approximately equal to 23 percent of the length of the print head 
when operated in its fundamental bending mode. 
In another aspect the invention provides improved traveling wave 
stimulation through use of an elongated stimulator member having a length 
which is substantially greater than its other dimensions and a pair of 
transducer means mounted on opposite exterior sides of the stimulator 
member. The transducer means extend in opposing relation a substantial 
distance in the direction of elongation of the stimulation member and are 
responsive to an electrical driving signal for applying surface shearing 
stresses to the stimulation member in the direction of elongation. 
In yet another aspect the present invention provides improved constructions 
for detecting the frequency and amplitude of print head stimulation for 
use in print head control. 
Accordingly, it is an object of the present invention to provide improved 
apparatus and method for fluid jet stimulation wherein a pair of 
transducers are mounted on opposite surfaces of a metallic member and are 
excited to produce surface shearing stresses and consequential vibration 
of an orifice plate through which a fluid jet is being directed. 
Other objects and advantages of the invention will be apparent from the 
following description, the accompanying drawings and the appended claims.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
A first embodiment of the print head of the present invention is shown in 
FIGS. 1-4. The print head generally includes an elongated print head body 
10, having a major dimension or length, L, which is substantially greater 
than its other dimensions a and b. The body 10 includes an orifice plate 
12 bonded to a block of high acoustic Q solid material 14. The body 10 
defines a fluid receiving reservoir 16 in its first end, and at least one 
and preferably a number of orifices 18 which are arranged in a row across 
orifice plate 12. Block 14 is preferably manufactured from stainless 
steel, but other high acoustic Q solid materials such as glass or ceramic 
may be used. Block 14 defines a slot 20 which, in conjunction with orifice 
plate 12 defines the reservoir 16. The block 14 further defines a fluid 
supply opening 22 and a fluid outlet opening 24, both of which communicate 
with the slot 20. 
The print head further includes means for supplying fluid to the reservoir 
16 under pressure such that fluid emerges from the orifices 18 as fluid 
filaments which then break up into streams of drops traveling in a 
direction parallel to the major dimension of the body 10. A pump 26 
receives fluid from a tank 28 and delivers it, via fluid conduit line 30, 
to the reservoir 16. A conduit 32 is connected to fluid outlet 24 such 
that fluid may be removed from the reservoir 16 at shut down of the print 
head or during cross-flushing of the reservoir 16. As will become 
apparent, the end of the print head to which conduits 30 and 32 are 
attached, as well as the opposite end of the print head, is subjected to 
mechanical vibrations which cause the fluid filaments to break up into 
streams of drops of uniform size and spacing. The conduits 30 and 32 are 
selected from among a number of materials, such as a polymeric material, 
which have a vibrational impedance substantially different from that of 
the stainless steel block 14. As a consequence, power loss through the 
conduits 30 and 32 and the resulting damping of the vibrations are 
minimized. The ink conduits may also be machined to the nodal plane, where 
vibrations are minimal and can then be connected to tubes having less 
critical acoustic properties. 
The print head further includes support means, such as mounting flanges 34. 
Flanges 34 are relatively thin and are integrally formed with the block 
14. The flanges 34 extend from opposite sides of the elongated print head 
body 10 and are substantially equidistant from the first and second ends 
of the body. As a result, the flanges may be used to support the body 10 
in a nodal plane. The flanges 34 are therefore not subjected to 
substantial vibration. 
The print head further comprises a transducer means, including thin 
piezoelectric transducers 36 and 38. The transducers are bonded to the 
exterior of the body of block 14 and extend a substantial distance along 
the body in the direction of elongation thereof, from adjacent the support 
means toward both the first and second ends of the body. The transducers 
36 and 38 respond to an electrical driving signal, provided by power 
supply 40 on line 42, by changing dimension, thereby applying shear 
stresses to the surfaces of the print head body. Due to the geometry of 
the print head body these shear stresses are converted into compression 
waves which travel along the body in a direction parallel to the direction 
of extent of the major axis. The resulting compression waves stimulate the 
fluid streams to break up into streams of drops. 
The piezoelectric transducers 36 and 38 have electrically conductive 
coatings on their outer surfaces, that is the surfaces away from the print 
head block 14, which define a first electrode for each such transducer. 
The metallic print head block 14 typically grounded, provides the second 
electrode for each of the transducers. The piezoelectric transducers are 
selected such that when driven by an A.C. drive signal, they alternately 
expand and contract in the direction of elongation of the print head. As 
may be seen in FIG. 3, transducers 36 and 38 are electrically connected in 
parallel. The transducers are oriented such that a driving signal on line 
42 causes them to elongate and contract in unison. 
If desired, an additional piezoelectric transducer 44 may be bonded to one 
of the narrower sides of the print head to provide an electrical output 
potential on line 46 which fluctuates in correspondence with the 
elongation and contraction of the print head block 14. The amplitude of 
the signal on line 46 is proportional to the amplitude of the mechanical 
vibration of the block 14. 
The mechanism by which the first embodiment of the print head of the 
present invention functions may be described as follows. The elongated 
print head body is somewhat analogous to an ordinary helical spring. If 
such a spring is compressed and then quickly released, it will oscillate 
about its center at a frequency f.sub.o, called its fundamental 
longitudinal resonant frequency. In this condition, both ends of the 
spring move toward and away from the center of the spring, while the 
center remains at rest. Therefore, if one fixes the center of the spring 
and repeats the above described operation, the spring will oscillate in 
the same manner at the frequency f.sub.o. 
The steel block 14 which forms a part of the print head body can be 
considered to be a very stiff spring. If properly mechanically stimulated, 
it may therefore be held at its center, as by flanges 34, while both ends 
of the block 14 alternately move toward and away from the center. Since 
the center of the block lies in a nodal plane, the flanges 34 are not 
subjected to substantial vibration and the support for the print head does 
not interfere with its operation. As the end of the print head body 10 
which defines the fluid receiving reservoir 16 is vibrated, the vibrations 
are transmitted to the fluid filaments which emerge from the orifices 16, 
thus causing substantially simultaneous uniform drop break up. Note that 
the reservoir 16 is small in relation to the overall size of the block 14 
and is centered in the end of the block. As a consequence, the reservoir 
16 does not interfere significantly with the vibration of the block 14, 
nor affect the resonant frequency of the print head substantially. The 
homogeneous nature of the solid block assures uniform amplitude of 
vibration along the ends whereby synchronous breakup of relatively long, 
dense ink jet arrays is possible. 
The fundamental resonant frequency of the block 14 can generally be said to 
be given by 
##EQU1## 
where C is the speed of sound through the print head block 14 material, L 
is the length of the print head body in the direction of elongation, E is 
the modulus of elasticity of the material forming block 14 and is the 
density of the material forming the block 14. Preferably the print head is 
designed to operate at or near its resonant frequency, and this frequency, 
in turn, is selected within an appropriate fluid jet stimulation frequency 
range, e.g., 50 KHz to 100 KHz; that is, the print head block is 
constructed of a material and with dimensions such that its fundamental 
longitudinal mode resonant frequency is approximately equal to the nominal 
jet droplet stimulation frequency for the printing system. The homogeneous 
nature of the solid block assures uniform amplitude of vibration along the 
ends whereby synchronous breakup of relatively long dense ink jet arrays 
is possible. As above described, print head block 114 has a length L which 
is equal to a half wavelength, where a wavelength is a distance determined 
by the equation: 
##EQU2## 
In general L may have a value substantially equal to any integral number 
of half wavelengths. Thus: 
##EQU3## 
By providing a pair of piezoelectric transducers 36 and 38 on opposite 
sides of the block 14, the block 14 is elongated and contracted without 
the flexure oscillations which would otherwise result if only one such 
piezoelectric transducer were utilized. Additionally, the use of two 
piezeoelectric transducers allows for a higher power input into the print 
head for a given voltage and, consequently, for a higher maximum power 
input into the print head, since only a limited voltage differential may 
be placed across a piezoelectric transducer without break down of the 
transducer. 
As is well know, E, .rho. and L are temperature dependent and, as a 
consequence, the resonant frequency of the print head varies with changes 
in temperature. The variation .DELTA.f in f.sub.o for a temperature change 
of .DELTA.T, at or near room temperature, is given by 
.DELTA.f=.DELTA.f.sub.o k.DELTA.T/2, where k is approximately 
4.times.10.sup.-4 /C..degree. for stainless steel. 
When the dimensions a and b are small as compared to L, the print head can 
be driven at a frequency off resonance. FIG. 5 illustrates the changes in 
the driving voltage applied to the transducers which are required in order 
to drive a single jet print head for a constant nominal filament length of 
16.5.times.10.sup.-3 in. In general, the nominal filament length is a 
function of both the driving voltage and the driving frequency. At any 
given driving frequency the nominal filament length decreases with 
increases in the driving voltage. 
From FIG. 5, it is clear that at resonance, 83 KHz, the print head requires 
a drive voltage of approximately 20 volts peak-to-peak. When driven by an 
oscillator at a frequency to either side of the resonant frequency, the 
driving voltage must be increased substantially in order to maintain the 
filament length at 16.5.times.10.sup.-3 in. On either side of the resonant 
frequency, the voltage required rises approximately linearly with 
frequency. There is, however, a maximum voltage which may be applied to 
the piezeoelectric transducers and, so long as the maximum voltage is not 
exceeded, the transducers may be driven on the positive slope portion of 
the curve of FIG. 5, or the negative slope portion of the curve. Assuming 
that the resonant frequency remains constant, the driving frequency may be 
varied in synchronization with fluctuations in speed of the print 
receiving medium upon which drops from the print head are to be deposited, 
thereby compensating for such fluctuations. In such an instance, the 
frequency of the drive signal is monitored, however, and the voltage of 
the drive signal adjusted accordingly in order to compensate for the 
frequency shift and thereby maintain the desired fluid filament length. 
If desired, the additional piezoelectric transducer 44 may be utilized to 
monitor the frequency of the drive signal and amplitude of vibration of 
the print head and provide a corresponding feedback signal. This feedback 
signal is plotted in FIG. 6 as a function of the frequency of the driving 
signal for the maintenance of a single jet print head nominal fluid 
filament of a length equal to 16.5.times.10.sup.-3 in., and a diameter of 
approximately 1.times.10.sup.-3 in. Assuming no change in the resonant 
frequency of the print head or the jet, a fluid filament of a desired 
length can be maintained by monitoring the output voltage and frequency on 
line 46 and adjusting the level of the driving signal as needed to 
maintain the output voltage on line 46 at a reference voltage level 
specified by the curve of FIG. 6. 
In a typical application it may be desirable to apply in the order of about 
2 percent frequency adjustment to the stimulation driving signal. In order 
to accommodate this, the minor dimensions of the print head preferably 
should be less than about one-fourth the major dimension, and the major 
dimension should be substantially equal to an integral number of half 
wavelengths at the driving frequency. 
It will be appreciated that numerous variations may be made in the 
disclosed print head within the scope of the present invention. For 
example, flanges 34 may be deleted. Another arrangement, such as support 
screws may be provided for attaching the print head body to appropriate 
support structure, as long as the point or points of attachment lie 
substantially in the nodal plane intermediate the ends of print head body 
10. Alternately ink supply tubes may serve as support members when 
connected to fluid conduits internal to the block extending from the ink 
reservoir to the nodal plane. 
Reference is made to FIG. 7 which illustrates a circuit which may be used 
for supplying a fixed frequency stimulation driving signal. The output of 
a fixed frequency oscillator 48 is supplied to transducers 36 and 38 via a 
voltage controlled attenuator circuit 50, a power amplifier 52 and a 
step-up transformer 54. The output from transducer 44 on line 46 is used 
to control the amount of attenuation provided by circuit 50. The signal on 
line 46 is amplified by amplifier 56, converted to a D.C. signal by 
converter 58, and then compared to a selected reference signal by summing 
circuit 60 to produce a signal on line 62 which controls the attenuation 
provided by circuit 50. By this feedback arrangement, the amplitude of the 
mechanical vibration of the print head is precisely controlled. For 
variable frequency stimulation a somewhat different stimulation driving 
circuit may be employed. 
FIG. 8 is a side view illustrating a second embodiment of the present 
invention, with elements corresponding to the print head of FIG. 1 being 
labeled with identical reference numerals. In this embodiment the 
transducers 36 and 38 are oriented on the print head body such that a 
positive driving signal on line 42 causes one of the transducers to 
elongate and the other transducer to contract, while a negative driving 
signal has the opposite effect. As a consequence, as an A.C. driving 
signal is supplied to line 42, the print head is caused to vibrate in its 
first flexure mode. This vibrational mode is illustrated in FIG. 8 by 
medial lines 64 which, although greatly exaggerated in flexure for 
purposes of clarity, indicate the extent of movement of the center of the 
print head body 14. It should be noted that lines 64 cross at points which 
are approximately 0.23L inward from each end of the print head body, thus 
indicating nodal points. Mounting holes 66 are drilled into body 14 at the 
nodal points and a second corresponding pair of mounting holes are drilled 
into the opposite side of the print head body. By providing mounting pins 
which extend into holes 66, pivot supports are provided which do not 
interfere with flexure of the print head. 
This flexure mode may be excited by driving the transducers at a frequency 
EQU f.sub.o .congruent..alpha.Ca/L.sup.2, 
where .alpha. is approximately 1 in MKS units. 
This is a simplification of the resonant frequency equation 
EQU f.sub.o .congruent.9.pi.CK/8L.sup.2, 
where K is the radius of gyration, which for the print head illustrated 
equals a/2. 
A third embodiment of the invention, as illustrated in FIG. 9, comprises a 
fluid jet print head 110 having a major dimension L and minor dimensions a 
and b corresponding to like designated dimensions for the embodiment of 
FIG. 1. Similarly, fluid jet print head 110 has a fluid receiving 
reservoir 116 provided with a supply opening 122 for reception of printing 
fluid from a fluid conduit 130. A fluid exit conduit 132 enables fluid 
removal from the print head. 
Print head 110 also has an orifice plate 112 provided with a series of 
orifices 118 in communication with reservoir 116 but mounted differently 
than the corresponding orifice plate 12 of print head 10. As illustrated 
in FIG. 9, orifice plate 112 is mounted on a side face of print head 110 
covering a sidewardly extending slot 120, so as to produce a series of 
jets 150 projecting in a direction perpendicular to the major dimension of 
the print head. These jets may be selectively charged by a series of 
electrodes 152, as is well known in the art. 
Stimulation of jets 150 is achieved by applying stimulation driving signals 
of appropriate frequency to a pair of piezoelectric transducers 136, 138 
bonded to the narrow sides of print head 110. The stimulation mechanism is 
the same as for the embodiment of FIG. 1. A stimulation driver (not 
illustrated) applies driving signals at near resonant frequency in common 
phase to both of transducers 136, 138. The transducers lengthen and 
shorten in unison, thereby applying shearing stresses to the surface of 
the print head. These stresses extend in a direction parallel to the major 
dimension of the print head and are converted to compression waves 
traveling in that direction. In order to minimize the power required for 
stimulation orifice plate 112 preferably should be located near the end of 
print head 110, as illustrated. Furthermore, the major dimension should 
again be substantially equal to an integral number of half wavelengths at 
the stimulation frequency, and the minor dimensions preferably should be 
less than about one-fourth the major dimension. 
Print head 110 also may be provided with a pair of mounting flanges 134, 
134 positioned for providing support at a nodal plane. A feedback 
transducer in the form of a strip of piezoelectric material 144 may be 
mounted on print head 110 as illustrated. Electrical connections to 
transducers 136, 138 and 144 may be made as shown in FIG. 7 for 
transducers 36, 38 and 44 respectively. 
FIGS. 10, 11 and 12 illustrate a fluid jet print head and stimulator 
therefor constructed according to a fourth embodiment of the present 
invention. The print head includes a manifold means consisting of an upper 
manifold element 210, a lower manifold element 212, and a gasket 214 
therebetween. The manifold means defines a fluid receiving reservoir 216 
to which fluid may be applied under pressure via fluid inlet tube 218. 
Fluid may be removed from reservoir 216 through outlet tube 220 during 
cleaning operations or prior to extended periods of print head shutdown. 
An orifice plate 222 is mounted on the manifold means. The plate is formed 
of a metal material and is relatively thin so as to be somewhat flexible. 
Orifice plate 222 is bonded to the manifold element 212, as for example by 
solder or by an adhesive, such that it closes and defines one wall of the 
reservoir 216. Orifice plate 222 defines a plurality of orifices 224 which 
are arranged in at least one row and which communicate with the reservoir 
216 such that fluid in the reservoir 216 flows through the orifices 224 
and emerges therefrom as fluid filaments. A stimulator means 226 mounted 
on contact with the orifice plate 222 vibrates the orifice plate to 
produce a series of bending waves which travel along the orifice plate 222 
in a direction generally parallel to the row of orifices. 
The stimulator means 226 includes a stimulator member 228, configured as a 
thin metal rod. The type of metal for the stimulator member 228 is 
selected to be compatible with the fluid supplied to reservoir 216. 
However, member 228 need not be made of metal, as other high acoustic Q 
solid materials such as glass or ceramic could be used. The stimulator 
member 228 is of a length L which is substantially equal to an integral 
number of half wavelengths of an acoustic wave traveling along the 
stimulator member 228. The distance L may be calculated by the formula set 
forth above in connection with the description of the embodiment of FIG. 
1. 
The end 230 of member 228 is tapered so that the member 228 contacts the 
orifice plate 222 in a localized region which is substantially a point. As 
is known, such point contact on the center line of the orifice plate 222 
insures that bending waves of a first order are generated in the orifice 
plate 222, and that satisfactory stimulation is obtained. 
The stimulator means 226 further includes piezoelectric crystal means, 
comprising piezoelectric crystals 232 and 234, which are mounted on the 
stimulator member 228. The crystals 232 and 234 each include a thin, 
electrically conductive layer on their outer surfaces to which conductors 
236 and 238 are electrically connected. The inner surfaces of the crystals 
are in contact with and are grounded by the member 228. Member 228, in 
turn, may be grounded through orifice plate 222 or through ground 
conductor 240. The crystals 232 and 234 are configured such that they tend 
to compress or extend in a direction parallel to the axis of elongation of 
the member 228 when a fluctuating electrical potential is placed across 
the crystals. As a consequence, when an A.C. electrical drive signal is 
applied to lines 236 and 238 by driver circuit means 240, the crystals 232 
and 234 produce acoustic waves in the stimulator member 228. The circuit 
240 supplies an electrical drive signal at a frequency f.sub.o, as 
specified above in relation to the length of the member 228. 
In the embodiment illustrated in FIGS. 10-12, the stimulator member is 
substantially equal in length to one wavelength, that is, n is equal to 2. 
The member 228 extends into the manifold means through an opening 244 
defined by element 210. The member 228 contacts the orifice plate 222 
inside the reservoir 216. A seal, such as O-ring 246 surrounds the member 
228, contacting the member 228 and element 210. 
The stimulator means is mounted by tapered pins 248 which engage generally 
conical detents 250 in the sides of member 228. The pins 248 and detects 
250 provide a pivotal mounting which restricts movement of member 228 
vertically. As may be noted, the detents 250 are positioned 1/4.lambda. 
from the upper end of the member 228, as seen in FIG. 11, while the O-ring 
246 contacts the member 228 substantially 1/4.lambda. from the lower end 
of the member 228. It will be appreciated that since crystals 232 and 234 
extend above and below the detents 250 by substantially equal distances, 
pins 248 support the stimulator means in a nodal plane. Since the ring 246 
contacts the member 228 1/2.lambda. below the pins 248, O-ring 246 also 
contacts the member 228 at a nodal plane. Thus substantial damping between 
the member 228 and the ring 246 does not occur. Additionally, the end of 
230 of the member 228 is 1/4.lambda. below a nodal plane and therefor at 
an anti-node, producing maximum amplitude mechanical stimulation for 
generation of the bending waves in the orifice plate 222. It will be 
understood that it is desirable to limit the length L.sub.c of the 
crystals 232 and 234 to 1/2.lambda. or less. If the length of the crystals 
is greater than this, their vibratory motion will tend to counteract 
formation of standing waves in the member 228 and the production of nodal 
planes. 
It will be appreciated that member 228 could be substantially longer than 
illustrated. The length of the member can be increased in multiples of 1/2 
wavelength with predictable harmonic progressions. In any event, however, 
it is desirable that the mounting for the member 228 be at a nodal plane 
and that sealing also occur at a nodal plane so that vibrational energy is 
not lost through the sealing or the mounting structures and that the 
member 228 contacts the orifice plate 222 at an anti-node. 
An additional pair of piezoelectric crystals 252 may also be mounted on the 
member 228. Crystals 252 act as sensors and provide an electrical feedback 
signal on line 254 which is proportional in frequency and amplitude to the 
frequency and amplitude of the acoustic waves traveling through the member 
228. The feedback signal on line 254 may be used by the driver circuit 240 
to control the frequency and amplitude of the drive signal applied on 
lines 236 and 238. 
FIG. 13 illustrates a fifth embodiment of the present invention in which 
the elements corresponding to the those in the fourth embodiment have been 
designated by the same numerals as those used in FIGS. 10-12. The 
stimulator member 228 of FIG. 13 is rectangular in cross-section and is 
substantially 1/2 wavelength long, that is, L equals 1/2.lambda.. 
Piezoelectric crystals 232 and 234 (not shown) are mounted on opposing 
faces of the member 228. 
A vibration transmission pin 256 is mounted on one end of the member and is 
preferably pressed into a hole in the end of the member or is machined on 
the end of the member. The pin 256 directly transmits the movement of the 
lower end of the member 228 to the orifice plate 222. The pin 256 has a 
cross-sectional area, taken in a plane substantially perpendicular to the 
direction of the elongation of member 228, which is substantially less 
than the cross-sectional area of the member. Thus, the acoustic waves in 
the member 228 do not pass through pin 256, but rather are reflected back 
toward the nodal plane which passes through pins 248. The length of pin 
256 is not related to the frequency of operation of the stimulator means, 
since the pin acts merely as a means of transmitting the vibrations from 
the anti-node at the end of member 228 to the plate 222. The pin 256 
passes through opening 244 and is engaged by a small diameter O-ring 258 
which prevents leakage of fluid from reservoir 216. Preferably, an 
automatic gain control in the driver circuit allows the stimulation 
amplitude to be held constant, regardless of the degree of damping 
provided by O-ring 258. 
A single piezoelectric transducer 260 is mounted on a side of the member 
228 other than the sides upon which the piezoelectric transducers 232 and 
234 are mounted. Transducer 260 provides a feedback signal on line 254 
which may be used by a driver circuit to control operation of the 
stimulator. 
It will be appreciated that in each of the above described embodiments of 
the invention there are provided surface mounted transducers which induce 
shear stresses therebelow. These shear stresses are converted to 
compression waves which in turn are coupled into the fluid filaments. 
While the method and the forms of apparatus herein described constitute 
preferred embodiments of this invention, it is to be understood that the 
invention is not limited to such precise method or forms of apparatus, and 
that changes may be made therein without departing from the scope of the 
invention which is defined in the appended claims.