[0001] The invention relates to an actuating device for a multi-nozzle ink jet printhead
comprising a linear array of electromechanical transducers some of which are configured
as actuator fingers associated with the nozzles of the printhead while others are
configured as support fingers intervening between the actuator fingers, wherein each
transducer has a first and a second electrode and is adapted to expand and contract
in accordance with a voltage applied between the first and second electrodes.
[0002] An actuating device of this type has been disclosed in EP-B-0 820 869. The electromechanical
transducers are formed by piezoelectric elements and are disposed on one side of a
channel plate in which a plurality of parallel ink channels are formed which each
lead to a nozzle of the printhead. Each of the transducers serving as an actuator
is disposed adjacent to one of the ink channels, so that, by contraction and expansion
of the actuator finger, ink is sucked into the ink channel from an ink reservoir and
is then expelled from the associated nozzle. The support fingers intervening between
the actuator fingers are connected to dam portions separating the individual ink channels.
The ends of the support fingers and actuator fingers opposite to the channel plate
are interconnected by a backing plate which, together with the support fingers, has
the purpose to absorb the reaction forces of the contraction and expansion strokes
of the actuator fingers.
[0003] While an alternating arrangement of actuator fingers and support fingers is possible,
the cited document proposes an arrangement with one support finger for two actuator
fingers. In a preferred embodiment disclosed in this publication, the support fingers
are passive. It is mentioned however that these support fingers may be formed also
by piezoelectric transducers which could then be controlled actively in order to compensate
the reaction forces of the actuator fingers. However, an electronic control system
permitting to control each of the active support fingers individually would considerably
add to the complexity of the system.
[0004] On the other hand, when passive support fingers are used, the backing plate is caused
to vibrate, especially when a large number of nozzles of the printhead are activated
simultaneously, and this leads to the production of noise, at a frequency in the order
of 10 kHZ for example, to an increased power consumption and to cross-talk phenomena
causing the volumes and velocities of the ink droplets expelled from the various nozzles
to become non-uniform,
[0005] It is generally known to actively compensate the cross-talk phenomena by modifying
the control signals applied to the actuator fingers in accordance with the activation
or non-activation state of the neighboring actuators, for example by means of a resistor
network, as described in US-A-4 381 515.
[0006] It is an object of the invention to provide an actuating device of the type indicated
in the opening paragraph of the present description, in which active control of the
support fingers can be achieved by simplified control means.
[0007] According to the invention, this object is achieved by the feature that the totality
of said transducers consists of at least one group which includes a plurality of actuator
fingers and a plurality of support fingers, and control means are associated with
each group for applying a voltage, that depends on the number of active actuator fingers
in this group, to the first electrodes of all support fingers in the group.
[0008] It has been found that, since the vibration of the backing plate caused by the reaction
forces of the actuator fingers becomes significant only when a large number of actuator
fingers is activated simultaneously, the disturbing effect of this vibration can largely
be eliminated when the actuator and support fingers are grouped, and all support fingers
belonging to the same group are controlled by one and the same control signal which
depends on the number of active actuator fingers in this group. As a result, the number
of control signals that have to be generated in real-time corresponds only to the
comparatively small number of groups and not to the comparatively large number of
individual support fingers, so that the control means can be simplified considerably.
[0009] More specific features of the invention are indicated in the dependent claims.
[0010] In the extreme, the totality of the transducers may form only a single group, so
that not more than one control signal is required for all actuator fingers.
[0011] In a particularly attractive embodiment, the first electrodes of all actuator fingers
and all support fingers belonging to the same group are interconnected with each other
and are held on a floating potential. Then, electrically, the actuator fingers and
the support fingers form a network of impedance elements with the actuator fingers
connected in parallel with each other and the support fingers also connected in parallel
with each other but with the actuator fingers and the support fingers connected in
series, with the floating potential between them. As a result, the support fingers
are actively controlled by the voltage drop between the common potential and their
respective second electrode, and the common potential will automatically depend on
the number of active actuator fingers in the group.
[0012] The impedances (i.e. capacitances in case of piezoelectric elements) of the support
fingers in relation to the impedances of the actuator fingers may be adjusted in order
to achieve an optimal compensation of the reaction forces.
[0013] If Ohmic resistances are neglected, the first electrodes of all support fingers within
a group are kept at exactly the same voltage. However, if impedances in the lines
interconnecting the first electrodes of the various support fingers are considered,
the voltages applied to the individual support fingers may deviate from one another.
If only a single actuator finger of the group is activated, then the voltages applied
to the first electrodes of the support fingers will decay with increasing distance
from the activated actuator finger. On the other hand, the deflection or bending stress
of the backing plate caused by the reaction force of the active actuator finger will
also decay with increasing distance from this actuator finger. As a result, it is
possible to adjust the impedances between the adjacent first electrodes of the transducers
so as to map the decay of the stresses in the backing plate. In this way, it is even
possible to attenuate a local deflection of the backing plate, although the support
fingers are not controlled individually.
Preferred embodiments of the invention will now be described in conjunction with the
accompanying drawings, in which:
- Fig. 1
- is a schematic cross-sectional view of an actuating device of a multi-nozzle ink jet
printhead;
- Fig. 2
- is a circuit diagram for the actuating device shown in figure 1; and
- Fig. 3
- is a circuit diagram for a modified embodiment of the invention.
[0014] As is shown in figure 1, a multi-nozzle ink jet printhead 10 comprises a channel
plate 12 with a large number of parallel ink channels 14 (shown in cross-section),
each of which leads to a nozzle 16 of the printhead. The ink channels 14 are covered
by a flexible plate 18 fixed to the top surface of the channel plate 12, and a piezoelectric
actuating device 20 is fixed on the top surface of the flexible plate 18.
[0015] The actuating device 20 has a comb-structure of piezoelectric material forming a
plurality of electromechanical transducers 22, 24 interconnected by a backing plate
26 at their ends remote from the channel plate 12. The transducers 22 serve as actuator
fingers and are each disposed right above one of the ink channels 14, whereas the
transducers 24 serve as support fingers and are disposed above dam portions 28 of
the channel plate. The backing plate 26 is fixedly connected to the assembly of the
flexible plate 18 and the channel plate 12 through the support fingers 24.
[0016] Each actuator finger 22 has a first electrode 30 and a second electrode 32, and the
piezoelectric material between them is polarized so that, when a voltage is applied
between the electrodes 30, 32, the actuator finger 22 expands or contracts, depending
on the polarity of the voltage. Although only one first electrode 30 and one second
electrode 32 are shown in figure 1, it is understood that the actuator finger 22 may
include a plurality of internal electrodes serving alternatingly as first electrode
and second electrode, as is well known in the art.
[0017] The support fingers 24 have the same electrode structure as the actuator fingers
22 and, thus, each comprise a first electrode 34 and a second electrode 36.
[0018] Of the four actuator fingers 22 shown in figure 2, the first and the fourth are inactive,
whereas the second and the third one have been activated so as to perform an expansion
stroke. Accordingly, the flexible plate 18 has been deflected downwardly into the
corresponding ink channels 14, so that the ink contained therein is compressed and
ink droplets are expelled from the corresponding nozzles 16. Due to the expansion
of the active actuator fingers 22, the backing plate 26 is subject to upwardly directed
reaction forces indicated by arrows A in figure 1. The backing plate 26 is supported
against these reaction forces by the support fingers 24. Since these support fingers
are also formed by electromechanical transducers, they may be energized to actively
counterbalance the reaction forces of the actuator fingers 22 by performing contraction
or expansion strokes opposite to the respective strokes of the actuator fingers. In
the example shown in figure 1, all three support fingers 24 are energized to perform
contraction strokes so as to counterbalance the reaction forces A by downwardly directed
forces B. As a result, the backing plate 26 as a whole will be held stable and will
be prevented from vibrating.
[0019] Electrically, the first and second electrodes 30, 32 of each actuator finger 22 may
be considered as a capacitor. The same applies to the first and second electrodes
34, 36 of the support fingers 24.
[0020] Figure 2 shows the electric circuit of the actuating device shown in figure 1, with
the actuator fingers 22 and the support fingers 24 being represented by capacitors.
The second electrodes 32 of the actuator fingers 22 are each connected to a terminal
38, so that they may be energized individually by applying a voltage pulse 40 which,
as is well known in the art, is generated by a control circuit in accordance with
the printing instructions. The second electrodes 36 of the support fingers 24 are
grounded. The first electrodes 30 and the first electrodes 34 of the actuator fingers
22 and the support fingers 24 are all interconnected by a common line 42. Ohmic resistances
and other impedances (capacitances and inductivities) between the neighboring first
electrodes 30, 34 are represented by impedance elements 44. If these impedances are
neglected, then all first electrodes 30, 34 of the actuator fingers and support fingers
are kept at a common potential which depends upon the balance between the voltage
drops at the parallel circuit formed by the various actuator fingers 22 on the one
hand and the parallel circuit formed by the various support fingers 24 on the other
hand. Thus, the potential of the common line 42 relative to ground increases in proportion
with the number of actuator fingers 22 to which energizing pulses 40 are applied,
and the potential of the line 42 and hence the potential of the first electrodes 30,
34 will always be between the potential of the second electrodes 32 of the active
actuator fingers and ground. The electric field generated between the first and second
electrodes 34, 36 of the support fingers 24 will always be opposite to the electric
field generated between the first and second electrodes 30, 32 of the actuator fingers
22. Accordingly, if the piezoelectric material of all transducers, i.e. of the actuator
fingers 22 and of the support fingers 24, has the same polarisation, an expansion
of the actuator fingers 22 will always be accompanied by a contraction of the support
fingers 24 and vice versa. In addition, since the first electrodes 30 and 34 of the
actuator fingers 22 and the support fingers 24 are disposed on the same level, these
electrodes may easily be interconnected by a conductor forming the common line 42.
[0021] In practice, the sections of the line 42 interconnecting the neighboring first electrodes
30, 34 will have a certain impedance (resistance, capacitance and inductivity), and
this will cause a certain drop or decay of the potential of the line 42 with increasing
distance from the actuator finger or fingers that have been energized. Due to a certain
flexibility of the backing plate 26, a similar decay will be observed in the reaction
forces transmitted from an active actuator finger 22 to the support fingers disposed
at increasing distances therefrom. Thus, by appropriately adjusting the impedances
of the impedance elements 44, it is possible to match the decay of the potential on
the line 42 with the decay of the forces transmitted through the backing plate 26,
so that the reaction forces A caused by individual actuator fingers 22 are compensated
with high accuracy over the whole length of the array of transducers.
[0022] While figure 1 shows an alternating arrangement of actuator fingers 22 and support
fingers 24, the invention is also applicable to other arrangements, in which the number
of actuator fingers is different from that of the support fingers 24.
[0023] Since there will only be a negligible amount of coupling or cross talk between actuator
fingers 22 and support fingers 24 that are separated by a large distance, it will
also be possible to divide the array of transducers 22, 24 into several blocks or
groups and to provide a separate line corresponding to the common line 42 in figure
2 for each of these groups.
[0024] Figure 3 shows an embodiment in which the array of transducers is subdivided into
groups 46, 48 which comprise each a certain number of adjacent transducers. In the
example shown, the group 46 comprises a total of six transducers, i.e. three actuator
fingers 22 and three support fingers 24. Here, the first electrodes 34 of the support
fingers 24 are interconnected by a line 50 the potential of which is not floating
but is actively controlled by an output of a control circuit 52 which is preferably
the same as the control circuit which applies the energizing pulses to the second
electrodes 32 of the actuator fingers 22. The first electrodes 30 of the actuator
fingers 22 are grounded in this embodiment.
[0025] Since all support fingers 24 of one group are commonly controlled by only one output
of the control circuit 52, the circuitry and/or the control algorithm of the control
circuit 52 may be simplified. Of course, in a practical embodiment, the number of
support fingers per group will be significantly larger than three.
[0026] Instead of grounding the first electrodes 30 of the actuator fingers 22, as in figure
3, it would also be possible to connect these first electrodes 30 to the common line
50. The circuit would then function in a similar way as the circuit shown in figure
2, with the only difference that the potential of the common line 50 is not floating
but is controlled actively.
1. Actuating device for a multi-nozzle ink jet printhead (10), comprising a linear array
of electromechanical transducers (22, 24) some of which are configured as actuator
fingers (22) associated with the nozzles (16) of the printhead while others are configured
as support fingers (24) intervening between the actuator fingers (22), wherein each
transducer (22, 24) has a first (30, 34) and a second electrode (32, 36) and is adapted
to expand and contract in accordance with a voltage applied between the first and
second electrodes, characterized in that the totality of said transducers (22, 24) consists of at least one group (46, 48)
which includes a plurality of actuator fingers (22) and a plurality of support fingers
(24), and control means (42, 44; 50, 52) are associated with each group (46, 48) for
applying a voltage, that depends on the number of active actuator fingers (22) in
this group, to the first electrodes (34) of all support fingers (24) of the group.
2. Actuating device according to claim 1, wherein all the transducers (22, 24) of the
linear array form only a single group.
3. Actuating device according to claim 1 or 2, wherein the second electrodes (32) of
the actuator fingers (22) are connected to be energized individually by energizing
pulses (40), and said control means comprise a common line (42) which is kept floating
and interconnects the first electrodes (30, 34) of all actuator fingers (22) and support
fingers (24) of the group.
4. Actuating device according to claim 3, wherein said control means comprise impedance
elements (44) intervening between each pair of first electrodes (30, 34) of neighboring
actuator fingers (22) and support fingers (24).
5. Actuating device according to claim 1 or 2, wherein said control means comprise a
control circuit (52) having one output for each group (46, 48) of transducers, said
output being connected to the first electrodes (34) of all support fingers of the
group through a common line (50).