Incremental printing of symbolic information – Ink jet – Ejector mechanism
Reexamination Certificate
1998-08-27
2001-08-14
Barlow, John (Department: 2853)
Incremental printing of symbolic information
Ink jet
Ejector mechanism
Reexamination Certificate
active
06273551
ABSTRACT:
BACKGROUND OF THE INVENTION
This application is related to acoustic inkjet printing and more particularly to an acoustic inkjet print head with individual control circuits for the piezo-electric transducer of each aperture to provide an acoustic wave with single optimized frequency.
Referring to
FIG. 1
, there is shown a portion of a prior art acoustic inkjet print head
10
. Print head
10
has a housing
12
, which contains a sheet of glass substrate
14
and ink
16
over the glass substrate
14
. Housing
12
, has a plurality of apertures
18
, each of which is dedicated to a pixel. Under the glass substrate, there is a plurality of piezo-electric transducers
20
. For the purpose of simplicity, hereinafter, the “piezo-electric transducer” is referred to as “transducer”. Each transducer
20
is dedicated to one aperture
18
and is located directly across its respective aperture
18
. Once each transducer
20
is activated, it will oscillate and generate acoustic waves
22
. The acoustic waves
22
travel within the glass substrate
14
toward the ink
16
.
Over the glass substrate
14
, there is a plurality of Fresnel lenses
24
, each of which corresponds to one of the transducers
20
and is located across from its respective transducer
20
. The Fresnel lenses
24
receive the acoustic waves
22
from the transducers
20
and focus the acoustic waves onto their respective aperture
18
. The focused waves
22
cause the ink to be ejected from the apertures.
Transducers
20
receive an RF frequency signal from an oscillator
30
. Oscillator
30
generates an RF signal and sends it to an RF Amplifier
32
to be amplified. The amplified RF signal is sent to several RF power splitters
34
. Each output of power splitters
34
is distributed between the plurality of transducers
20
.
The output of each power splitter
34
is connected to a set of transducers
20
through individual switches S
1
for providing RF signal to respective transducers
20
. Switches Sare controlled by pixel information. Based on the pixel information, when a given pixel needs ink, switch S
1
of a respective transducer closes to send the RF signal to that transducer for activating the transducer and causing ink to be ejected from the respective aperture
18
.
In operation, the acoustic waves
22
, which are focused onto the apertures
18
will partially be reflected by the surface
50
. The reflected waves interfere with the original waves
22
. If the impedance of either the ink
16
or Fresnel lens
24
does not match that of the glass substrate
14
, the resulting stack of glass substrate
14
, Fresnel lens
24
, and the ink
16
will operate as a cavity. Hereinafter the combination of cavity (glass substrate
14
, Fresnel lens
24
and the ink
14
) and transducer will be referred to as resonant stack. Depending on the frequency of the original waves and the cavity length, the reflected waves can have a different phase than the phase of the original waves.
Referring to
FIG. 2
, there is shown the resonance distribution of a resonant stack.
FIG. 2
, shows the effect of the interference between the original acoustic waves and the reflecting acoustic waves. Referring to both
FIGS. 1 and 2
, if the reflected waves in the glass substrate
14
have opposite phase as that of original waves, then they will cancel the original waves
22
(cancellation C). However, if the reflected waves have the same phase as that of the original waves
22
, they will increase the amplitude of the original waves (spikes S). Any phase between the two extremes of in-phase or the opposite-phase will interfere constructively or destructively with the original waves depending on if the phase is closer to in-phase or to the opposite phase respectively.
In this approach, an external frequency from the oscillator
30
is applied to each transducer
20
to cause the transducer to oscillate. Typically in the absence of an external frequency, if each transducer
20
starts oscillating, it will oscillate at a resonance frequency which is defined by the resonant stack. Usually, the external frequency does not match the resonance frequency of the resonant stack and as a result, the transducers
20
generate acoustic waves which do not resonate with the resonant stack. In addition, manufacturing tolerances cause each resonant stack to oscillate at a unique frequency.
Since the transducers oscillate at different frequencies than the resonance frequencies of the resonant stack, spikes or cancellation can occur. As can be observed, the spikes S occupy a small percentage such as 5% of the distribution and the majority of distribution is cancellation. This reduces the efficiency of the transducers. In addition, depending on the acoustic waves generated by the transducers
20
, the intensity of the acoustic waves will vary strongly.
This problem is usually resolved in two ways. One approach is to deposit a matching layer over the glass substrate
14
. This layer compensates for the mismatched impedance of the ink
16
, the Fresnel lenses
24
and the glass substrate
14
and causes a reduction of amplitude of the reflected waves. Therefore, the reflected waves do not interfere as strongly with the original waves.
Another approach is to sweep or chirp the RF frequency to vary the frequency of the transducer's oscillation in order to generate acoustic waves with variable frequencies. Varying the frequency within a range gradually from one end of the range to the other end of the range is called “sweeping” or “chirping”. By chirping the frequency, the resulting waves will have the effect of the average of all the waves with different frequencies and therefore average out the resonance spike problem.
This configuration has several problems. The inefficiency of the transducers causes the control circuit
36
to dissipates a great amount of RF energy. The control circuit
36
has to be fully on regardless of the number of active transducers. In addition, the external frequency applied to the transducers has to be chirped, which in turn causes the acoustic waves generated at the transducer to have a varying frequency. With a varying frequency, at any given time, the reflected waves will have a different phase. Therefore, due to the varying phase of the reflected waves, the waves reaching each aperture will have an average frequency and amplitude.
It is an object of this invention to eliminate the high power dissipation. Also, it is another object of this invention to individually control and adjust each transducer to maximize the intensity of the acoustic waves when they reach their respective apertures and reduce the net amount of RF power required to eject a drop.
SUMMARY OF THE INVENTION
According to the present invention, an acoustic inkjet print head is disclosed which comprises a sheet of glass substrate, a plurality of transducers located on the glass substrate, and a plurality of control circuits each of which corresponds to one of the plurality of transducers. Each one of the of control circuits is electrically connected to a respective transducer. Each one of the transducers is responsive to a respective control circuit to oscillate at a resonance frequency which is defined by the respective transducer and the glass substrate.
REFERENCES:
patent: 5541628 (1996-07-01), Chang et al.
patent: 5790139 (1998-08-01), Umeno et al.
patent: 5798770 (1998-08-01), Nakayasu et al.
Guerin Jean-Michel
Wilson Eugene
Barlow John
Brooke Michael S
Virga Philip T.
Xerox Corporation
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