Switched digital drive system for an ink jet printhead

A switched digital drive system is used to actuate an ink jet printhead having a spaced, parallel series of internal ink receiving channels opening outwardly at front ends thereof through ink discharge orifices formed in the printhead body. The channels are laterally bounded by a spaced series of piezoelectrically deflectable internal sidewall sections of the printhead body interdigitated with the channels. The drive system includes a series of electrical actuation leads each connected to a different one of the sidewall sections, and dual transistor switch structures connected in the leads with each switch, in turn, connected to positive and negative DC voltage sources. To actuate a selected channel, the switches associated with its opposite bounding sidewall sections are operated in a manner sequentially (1) deflecting the sidewall sections outwardly away from initially undeflected positions thereof by imposing constant, opposite polarity voltages thereon, (2) reversing the polarities of the constant voltages to deflect the sidewall sections into the channel, and then (3) imposing a series of voltage pulses of sequentially opposite polarities on each of the inwardly deflected sidewall sections to controllably return them to their initial undeflected positions.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention generally relates to ink jet printhead apparatus and 
more particularly relates to a digital drive system for piezoelectrically 
driving an ink jet printhead. 
2. Description of Related Art 
A piezoelectrically actuated ink jet printhead is a relatively small device 
used to selectively eject tiny ink droplets onto a paper sheet operatively 
fed through a printer, in which the printhead is incorporated, to thereby 
form from the ejected ink droplets selected text and/or graphics on the 
sheet. In one representative configuration thereof, an ink jet printhead 
has a horizontally spaced parallel array of internal ink-receiving 
channels. These internal channels are covered at their front ends by a 
plate member through which a spaced series of small ink discharge orifices 
are formed. Each channel opens outwardly through a different one of the 
spaced orifices. 
A spaced series of internal piezoelectric wall portions of the printhead 
body separate and laterally bound the channels along their lengths. To 
eject an ink droplet through a selected one of the discharge orifices, the 
two printhead sidewall portions that laterally bound the channel 
associated with the selected orifice are piezoelectrically deflected into 
the channel and then returned to their normal undeflected positions. The 
driven inward deflection of the opposite channel wall portions increases 
the pressure of the ink within the channel sufficiently to force a small 
quantity of ink, in droplet form, outwardly through the discharge orifice. 
According to a recently proposed drive method for this type of ink jet 
printhead, top sides of the internal channel dividing wall portions are 
commonly connected to ground, and the bottom sides of the wall portions 
are individually connected to a series of electrical actuating leads. Each 
of these leads, in turn, is connected to a drive system operable to 
selectively impart to the lead an electrical waveform that sequentially 
changes (1) from ground to a first driving polarity, (2) from the first 
polarity to the opposite polarity, and (3) from the opposite polarity back 
to ground. 
When this electrical waveform is imparted to a piezoelectric wall portion 
bounding one side of a selected, and a second electrical waveform of 
opposite polarity sequence is simultaneously imparted (via another one of 
the actuating leads) to the opposite channel wall portion, the opposite 
channel wall portions, by piezoelectrical action, are sequentially 
deflected (1) outwardly away from the channel that they laterally bound, 
(2) into the channel to discharge an ink droplet therefrom, and (3) back 
to their starting or "neutral" positions. 
Both analog and digital type drive systems have been suggested for 
providing the above drive method in an ink jet printhead. As analog type 
drive systems which utilize analog circuitry, for example, operational 
amplifiers (or "op-amps"), to deliver the desired electrical waveform to 
the ink jet printhead are linear in nature, however, such drive systems 
tend to produce unacceptably high levels of power dissipation and have, 
therefore, proven inefficient in use. Furthermore, such analog type drive 
systems require excessive space on the printhead, thereby adversely 
affecting driver density for the printhead. 
Proposed digital type drive systems, on the other hand, while avoiding the 
aforementioned deficiencies relating to power dissipation, also have 
several built-in limitations and disadvantages. More specifically, such 
digital type drive systems utilize switching type circuits, also referred 
to as "digital drivers". Since the proposed drive method requires a 
bipolar voltage waveform with an active return to ground, digital type 
drive systems have heretofore required three separate drivers--one for 
each of the three channel wall drive portions described above. This 
requirement substantially increases the complexity of the drive system, 
thereby undesirably increasing its overall cost. Additionally, it 
undesirably increases the overall space requirement for the drive system. 
It can be readily seen from the foregoing that it would be desirable to 
provide an improved ink jet printhead drive system that eliminates, or at 
least substantially reduces, the above-mentioned limitations and 
disadvantages associated with the drive systems described above. It is 
accordingly an object of the present invention to provide such an improved 
ink jet printhead drive system. 
SUMMARY OF THE INVENTION 
In carrying out principles of the present invention, in accordance with a 
preferred embodiment thereof, a specially designed digital drive system is 
incorporated in an ink jet printhead having a spaced, parallel series of 
internal ink receiving channels opening outwardly, at front ends thereof, 
through a spaced series of small ink discharge orifices formed through a 
front end section of the printhead body. Each of the channels is laterally 
bounded along its length by an actuation portion of the printhead body 
that may be piezoelectrically deflected relative to the channel to force a 
quantity of ink therefrom outwardly through its associated ink discharge 
orifice. 
The digital drive system is operative to actuate a selected one of the ink 
receiving channels and includes first means for piezoelectrically 
deflecting the selected channel actuation portion, from an initially 
deflected position thereof, inwardly into the selected channel, and second 
means for piezoelectrically returning the inwardly deflected actuation 
portion to its initially undeflected position by imparting to the inwardly 
deflected actuation portion a series of driving voltage pulses of 
sequentially reversed polarities. 
In an illustrated preferred embodiment of the drive system it is 
representatively incorporated in an ink jet printhead in which, for each 
channel, the actuation portion is defined by an opposed pair of 
piezoelectrically deflectable internal sidewall sections of the printhead 
body that define opposite side portions of the channel along its length. 
The drive system comprises a series of electrical actuation leads each 
operatively connected to a different one of the internal sidewall 
sections, and a series of switch means each connected to a different one 
of the electrical actuation leads and to positive and negative DC voltage 
sources. 
Each of the switch means is operative to create a positive voltage in its 
associated electrical actuation lead in response to receipt of a first 
control signal, and further operative to create a negative voltage in its 
associated electrical actuation lead in response to receipt of a second 
control signal. Means are provided for selectively transmitting said first 
and second control signals to predetermined ones of the switch means. 
Each of the switch means preferably include a first transistor having a 
base portion operative to receive the first control signal, a collector 
portion connected to the positive voltage source, and an emitter portion 
connected to one of the electrical actuation leads; and a second 
transistor having a base portion operative to receive the second control 
signal, a collector portion connected to the electrical actuation lead, 
and an emitter portion connected to the negative voltage source. 
In its preferred embodiment, the digital drive system of the present 
invention is operative to sequentially (1) laterally outwardly deflect, 
from their initial undeflected positions, the opposing pair of internal 
body sidewall sections of the selected channel by simultaneously imposing 
opposite polarity DC voltages thereon, (2) laterally inwardly deflect the 
outwardly deflected sidewall sections, into the selected channel, by 
simultaneously reversing the polarities of the DC voltages imposed 
thereon, and then (3) controllably return the inwardly deflected sidewall 
sections to their initial undeflected positions by simultaneously imposing 
on each of the inwardly deflected sidewall sections a series of 
sequentially reversed polarity DC voltage pulses. 
The switched digital drive system of the invention provides several 
advantages over prior digital type printhead drive systems that require 
three separate drivers--one for each of the three sidewall movement 
segments described above. For example, the digital drive system of the 
present invention requires only two drivers. Additionally, the digital 
drive system is considerably less complex and is thus less expensive. 
Moreover, the digital drive system requires appreciably less space.

DETAILED DESCRIPTION 
Referring initially to FIGS. 1 and 2, the present invention provides an ink 
jet printhead 10 incorporating therein a specially designed switched 
digital drive system 12 embodying principles of the invention. Printhead 
10 has a body 14 having upper and lower rectangular portions 16 and 18, 
with an intermediate rectangular body portion 20 secured between the upper 
and lower portions 16 and 18 in the indicated aligned relationship 
therewith. A front end section of the body 14 is defined by an orifice 
plate member 22 having a spaced series of small ink discharge orifices 24 
extending rearwardly therethrough. As shown, the orifices 24 are arranged 
in horizontally sloped rows of three orifices each. 
In a left-to-right direction as viewed in FIG. 1, the printhead body 
portions 16,20 are shorter than the body portion 18, thereby leaving a top 
rear surface portion 26 of the lower printhead body portion 18 exposed. 
For purposes later described, a spaced series of electrical actuation 
leads 28 are suitably formed on the exposed surface 26 and extend between 
the underside of the intermediate body portion 20 and a controller portion 
30 of the drive system 12 mounted on the surface 26 near the rear end of 
the body portion 18. 
Referring now to FIG. 2, a plurality of vertical grooves of predetermined 
width and depth are formed in the printhead body portions 18 and 20 to 
define within the printhead body 14 a spaced, parallel series of internal 
ink receiving channels 32 that longitudinally extend rearwardly from the 
orifice plate 22 and open at their front ends outwardly through the 
orifices 24. The channels 32 are laterally bounded along their lengths by 
opposed pairs of a series of internal actuation sidewall sections 34 of 
the printhead body. 
Sidewall sections 34 have upper parts 34a defined by horizontally separated 
vertical sections of the body portion 20, and lower parts 34b defined by 
horizontally separated sections of the body portion 18. The underside of 
the body portion 16, the top and bottom sides of the actuation sidewall 
section parts 34a, and the top sides of the actuation sidewall section 
parts 34b are respectively coated with electrically conductive metal 
layers 36, 38, 40 and 42. 
Body portions 16 and 20 are secured to one another by a layer of 
electrically conductive adhesive material 44 positioned between the metal 
layers 36 and 38, and the upper and lower actuator parts 34a and 34b are 
intersecured by layers of electrically conductive material 46 positioned 
between the metal layers 40 and 42. The metal layer 36 on the underside of 
the upper printhead body portion 16 is connected to ground 48. 
Accordingly, the top sides of the upper actuator parts 34a are 
electrically coupled to one another and to ground 48 via the metal layers 
38, the conductive adhesive layer 44 and the metal layer 36. 
Each of the channels 32 is filled with ink received from a suitable ink 
supply reservoir 50 (see FIG. 1) connected to the channels via an ink 
delivery conduit 52 connected to an ink supply manifold (not shown) 
disposed within the printhead body 14 and coupled to rear end portions of 
the internal channels 32. In a manner subsequently described, each 
horizontally opposed pair of the sidewall actuators 34 is 
piezoelectrically deflectable into and out of their associated channel 32, 
under the control of the digital drive system 12, to force ink (in droplet 
form) outwardly through the orifice 24 associated with the actuated 
channel. 
Referring now to FIGS. 1 and 3A, as previously mentioned, the digital drive 
system 12 includes the controller 30 which is operatively connected to 
rear ends of the electrical actuation leads 28. The front ends of the 
leads 28 are individually connected to the metal layers 40 (see FIG. 2) on 
the undersides of the top sidewall actuator parts 34a. Within the 
controller 12 are a series of switching structures 54 each of which is 
connected to one of the leads 28 as schematically depicted in FIG. 3A. 
Each switching structure 54 includes first and second switching elements 
56, 58. It is contemplated that various switching circuits, for example, a 
bipolar transistor or a field effect transistor, are suitable for use as 
the switching elements 56, 58. The first switching element 56 has a 
control input line 60 connected to a first drive signal 66, a supply 
voltage input line 62 connected to a positive DC voltage source and an 
output line 64 connected to lead 28. Similarly, the second switching 
element 58 has a control input line 61 connected to a second drive signal 
68, a supply voltage input line 63 connected to a negative DC voltage 
source and an output line 65 connected to lead 28. In operation, the first 
drive signal 66 is asserted during a first time interval to produce a 
positive pulse as the output at lead 28 which would drive a piezoelectric 
wall portion electrically associated therewith in a first direction. Then, 
during a second time interval, the first drive signal 66 is deasserted and 
the second drive signal 68 is asserted, thereby causing the output at lead 
28 to transition from positive to negative, thereby driving the 
piezoelectrical wall portion electrically associated therewith in the 
opposite direction. Finally, during a third time interval, the second 
drive signal 68 is deasserted and the output is returned to ground to 
allow the piezoelectrical wall portion to return to its rest position. 
While the output will return to ground over time, it is contemplated that 
the drive signals 66, 68 may be used in various drive methods during the 
third time interval to drive the output to ground. One such technique will 
be described below with reference to FIG. 4. 
With respect to each of the dual switching elements 56, 58, the controller 
30 is operative to selectively transmit a first control signal 66 to the 
control input 60 of the switching element 56, or a second control signal 
68 to the control input 61 of the second switching element 58. Receipt of 
the first control signal 66 by the switching structure 54 creates a 
positive DC voltage in its associated electrical actuation lead 28, while 
receipt of the second control signal 68 by the switching structure 54 
creates a negative DC voltage in the lead 28. Via the lead 28, this 
positive or negative DC voltage is transmitted to the upper actuation 
sidewall portion metal layer 40 to which the lead is operatively 
connected. 
Using the switched digital drive system 12 of the present invention a 
selected one or more of the ink receiving channels 32 may be actuated to 
drive a quantity of ink therein, in droplet form, outwardly through the 
associated ink discharge orifice(s) 24. To illustrate the operation of the 
digital drive system 12, the actuation of a representative channel 32a 
will be described shortly in conjunction with FIGS. 2 and 4. 
Referring now to FIG. 3B, a specific embodiment of the switching structure 
54 will now be described in greater detail. In this embodiment, each 
switching element 54' includes a pair of transistors 56' and 58', each 
having a base portion 60', a collector portion 62', and an emitter portion 
64'. As illustrated, the collector portion 62' of transistor 56' is 
connected to a positive DC voltage source, the emitter portion 64' of 
transistor 58' is connected to a negative DC voltage source, and the 
emitter portion 64' of transistor 56' and the collector portion 62' of 
transistor 58' are connected to the lead 28'. 
With respect to each of the dual transistor switches 54', the controller 30 
is operative to selectively transmit a first control signal 66' to the 
base portion 60' of the transistor 56', or a second control signal 68' to 
the base portion 60' of the transistor 58'. Receipt of the first control 
signal 66' by the switching structure 54' creates a positive DC voltage in 
its associated electrical actuation lead 28', while receipt of the second 
control signal 68' by the switching structure 54' creates a negative DC 
voltage in the lead 28'. Via the lead 28', this positive or negative DC 
voltage is transmitted to the upper actuation sidewall portion metal layer 
40 to which the lead is operatively connected. 
Referring now to FIGS. 2 and 4, the operation of the digital drive system 
12 incorporating a switching circuit 54 such as that illustrated in FIG. 
3A, will now be described. Prior to the actuation of the channel 32a, its 
horizontally opposed left and right sidewall actuators 34.sub.L and 
34.sub.R are (at time T.sub.o in FIG. 4) in initial, laterally undeflected 
positions indicated by solid lines in FIG. 2. To initiate the channel 
actuation cycle, the switching structure 54 associated with the left 
sidewall actuator 34.sub.L is operated to impose thereon a constant 
positive DC voltage pulse 70 during the time interval T.sub.1 -T.sub.2 
shown in FIG. 4. Simultaneously, the switching structure 54 associated 
with the right sidewall actuator 34.sub.R is operated to impose thereon an 
equal constant negative DC voltage pulse during the time interval T.sub.1 
-T.sub.2. These opposite polarity DC voltage pulses transmitted to the 
sidewall actuators 34.sub.L and 34.sub.R outwardly deflect them away from 
the channel 32a being actuated and into the outwardly adjacent channels 
32b and 32c as indicated by the dotted lines 72 in FIG. 2. 
Next, at time T.sub.2, the positive voltage pulse 70 transmitted to 
sidewall actuator 34.sub.L and the corresponding negative voltage pulse on 
the sidewall actuator 34.sub.R are terminated, and the two switching 
structures 54 are operated to simultaneously impose a constant negative DC 
voltage pulse 74 on the left sidewall actuator 34.sub.L, while imposing an 
equal constant positive DC voltage pulse on actuator 34.sub.R, during the 
time interval T.sub.2 -T.sub.3. These opposite polarity constant DC 
voltage pulses inwardly deflect the sidewall actuators 34.sub.L and 
34.sub.R past their initial undeflected positions and into the channel 32a 
as indicated by the dotted lines 76 in FIG. 2. Such inward deflection of 
the actuators reduces the volume of channel 32a, thereby elevating the 
pressure of ink therein to an extent sufficient to force a quantity of the 
ink, in droplet form, outwardly through the orifice 24 associated with the 
actuated channel 32a. 
Finally, at time T.sub.3, these second opposite polarity DC voltage pulses 
are terminated and the switching structure 54 associated with sidewall 
actuator 34.sub.L is operated to piezoelectrically drive the actuator 
34.sub.L back to its initial undeflected position by imposing thereon an 
alternating series of positive and negative voltage pulses 78 and 80 
during the final time interval T.sub.3 -T.sub.4. As illustrated in FIG. 4, 
the total time duration of the positive voltage pulses 78 is greater than 
the total time duration of the negative pulses 80. Accordingly, the net 
lateral deflection force on the sidewall actuator 34.sub.L is outwardly 
directed. 
Also during the time interval T.sub.3 -T.sub.4 the switching structure 54 
associated with the right sidewall actuator 34.sub.R is used to 
piezoelectrically drive such actuator back to its initial undeflected 
position by imposing thereon an alternating polarity DC voltage pulse 
pattern which is a mirror image of the T.sub.3 -T.sub.4 pulse pattern 
shown in FIG. 4, with the total time duration of the negative voltage 
pulses being equal to the time duration of the positive voltage pulses 78 
in FIG. 4. 
The unique use of these alternating polarity voltage pulses to return the 
actuators 34.sub.L, 34.sub.R from their inwardly deflected positions to 
their initial undeflected positions during the representative time 
interval T.sub.3 -T.sub.4 improves the ability to control and "smooth out" 
their final return stroke as desired. This enhanced control of their 
return strokes may easily be adjusted to provide variable return stroke 
characteristics by adjusting the time interval T.sub.3 -T.sub.4 and/or the 
relative negative and positive pulse time duration for the voltage pulse 
series imposed on each actuator during time interval T.sub.3 -T.sub.4. 
Compared to other analog or digital drive systems used to actuate 
selectively variable internal ink receiving channels in an ink jet 
printhead, the switched digital drive system 12 of the present invention 
provides several desirable advantages. For example, other digital drive 
systems typically require three separate drivers--i.e., (1) a 
ground-to-positive driver, (2) a positive-to-negative driver, and (3) a 
negative-to ground driver. In contrast to this three driver requirement, 
the digital drive system 12 subject of the present invention requires only 
two drivers. This reduction in the number of drivers required 
substantially reduces the complexity of the drive system. In turn, this 
materially lessens the overall cost of the drive system. Additionally, the 
reduced driver requirement advantageously reduces the overall space 
requirement for the drive system. Likewise, when compared to analog drive 
systems, the disclosed digital drive system has achieved a significant 
reduction in space requirements. 
The foregoing detailed description is to be clearly understood as being 
given by way of illustration and example only, the spirit and scope of the 
present invention being limited solely by the appended claims.