Electrical generator or motor structure – Non-dynamoelectric – Piezoelectric elements and devices
Reexamination Certificate
2000-05-23
2004-12-14
Mullins, Burton S. (Department: 2834)
Electrical generator or motor structure
Non-dynamoelectric
Piezoelectric elements and devices
Reexamination Certificate
active
06831392
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a piezoelectric element driving apparatus for driving a plurality of piezoelectric elements that use the piezoelectric effect, in particular, to a piezoelectric element driving apparatus applicable to small printer heads for use with an ink jet printer or the like.
2. Description of the Related Art
In recent years, ink jet printers have been commercially available. Each ink jet printer has ink nozzles from which ink droplets are sprayed to a sheet of paper so as to print characters and images thereon. The ink jet printer uses heating elements and piezoelectric elements that produce the ink droplets and spray them on a sheet of paper. As the piezoelectric elements vibrate, the ink droplets are sprayed. Conventionally, to prevent the printer nozzle from being clogged with ink, piezoelectric nozzles are multi-layered and the spraying of ink droplets is controlled.
The heads of the piezoelectric element driving type ink jet printers use electro-strictness of which mechanical distortion takes place with a crystal such as Rochelle salt or barium titanium in an electric field using piezoelectric effect wherein the dielectric value of a crystal varies as a function of an electric charge on the surface thereof corresponding to an applied mechanical distortion. Using the characteristic that a piezoelectric element is deformed with a voltage, ink droplets are sprayed from nozzles of heads. Since the slope of the voltage and the potential are proportional to the acceleration and the intensity of the deformation of the piezoelectric element, by controlling them, the velocity and diameter of the ink droplets can be varied. Thus, to accurately control the acceleration and size of sprayed ink droplets, it is necessary to properly apply a voltage to the piezoelectric element. to the acceleration and the intensity of the deformation of the piezoelectric element, by controlling them, the velocity and diameter of the ink droplets can be varied. Thus, to accurately control the acceleration and size of sprayed ink droplets, it is necessary to properly apply a voltage to the piezoelectric element.
FIG. 1
shows the structure of a piezoelectric element. Referring to
FIG. 1
, the piezoelectric element
10
is structured in a rectangular shape. The piezoelectric element
10
has piezoelectric lamination portions
13
and electrodes
11
that are alternately formed. By applying an electric field between the electrodes
11
, a vertical mechanical distortion takes place. By applying the mechanical distortion to an ink reservoir of a side
12
disposed adjacent to the electrodes
11
, ink droplets are sprayed from the nozzle of the ink reservoir.
When full colors are printed, a plurality of nozzles corresponding to a plurality of ink reservoirs for cyan ink, magenta ink, yellow ink, and black ink are used.
FIG. 2
is a schematic diagram showing the structure of a printer apparatus including a printer head peripheral portion using piezoelectric elements
10
. The printer apparatus comprises ink reservoirs
23
, a carrier
22
, a SP (spacing) motor
26
, a shaft
24
, an LF (line field) motor
25
, a platen
28
, and a flat flexible cable (FFC)
27
. The carrier
22
moves heads (not shown) in the main scanning direction. The SP motor
26
drives the carrier
22
. The shaft
24
is used to move the carrier
22
. The LF motor
25
feeds paper
21
in the sub-scanning direction. The FFC
27
bends as the carrier
22
travels.
In the structure shown in
FIG. 2
, the paper
21
is fed in the sub-scanning direction by the LF motor
25
, the platen
28
, a feed roller (not shown), and so forth. The carrier
22
is moved along the shaft
24
by the SP motor
26
. A drive signal and a control signal are supplied to the heads through the FFC
27
so that ink droplets are sprayed to the paper
21
at a predetermined timing.
In the carrier
22
, the ink reservoirs
23
and the heads are connected with respective tubes (not shown). Inks in the ink reservoirs
23
are supplied to the heads. When the piezoelectric elements
10
are driven, they are deformed. Thus, the heads are partly stressed and thereby ink in the heads are partly sprayed from the nozzles. Consequently, an image is formed on the paper
21
.
In a conventional piezoelectric element driving circuit, when a drive waveform signal amplified by a power amplifier is sent to a piezoelectric element
10
, an RC filter is formed by a total of the resistance of an FFC as a transmission path and the static capacitance of the piezoelectric element. Thus, since a high frequency component of the drive waveform signal is lost, the drive waveform signal cannot be transmitted to the piezoelectric element
10
that requires it.
In particular, as the number of piezoelectric elements becomes large, the capacitance component C of the time constant RC of which the resistance component R and the capacitance component C are multiplied becomes large. Thus, since the time constant ô=RC becomes large, only lower frequency components are transmitted to the piezoelectric elements. Consequently, the piezoelectric effect of the piezoelectric elements that should be driven at high speed is deteriorated. For example, when the piezoelectric elements are used for an ink jet printer, the velocity and size of ink droplets sprayed from the heads cannot be accurately controlled. Thus, the print quality of a print image is deteriorated.
Next, with reference to
FIG. 3
, a piezoelectric element driving circuit for use with a conventional printer apparatus will be described. The piezoelectric element driving circuit shown in
FIG. 3
comprises a drive waveform signal generating circuit
1
, a power amplifier
2
, a flexible flat cable (FFC)
3
, a plurality of head units
4
, a plurality of switch devices
5
, and a plurality of piezoelectric elements
6
. The drive waveform signal generating circuit
1
generates a drive waveform signal for driving a plurality of piezoelectric elements
6
. The power amplifier
2
amplifies the drive waveform signal. The FFC
3
connects the power amplifier
2
and the head units
4
. The switch devices
5
are disposed in the head units
4
. The piezoelectric elements
6
are connected to the switch device
5
of each of the head units
4
. The head units
4
are color head units for cyan c, magenta m, yellow y, and black b. Each of the head units
4
has, for example, 32 nozzles. Each piezoelectric element
6
can be represented as a capacitance on an equivalent circuit diagram. Thus, corresponding to 32 nozzles of each color head unit, there is a capacitance of 32 capacitors. By turning on/off switches of each switch device
5
through a controlling circuit (not shown), required piezoelectric elements are driven. In this example, it is assumed that the capacitance of one piezoelectric element
6
is 1 nF.
Next, with reference to
FIGS. 4A
,
4
B, and
4
C, the relation of an input waveform signal and an output waveform signal of a conventional piezoelectric element driving circuit will be described.
FIG. 4A
shows an output waveform signal of a piezoelectric element driving power amplifier. The output waveform signal of the piezoelectric element driving power amplifier becomes an input waveform signal of an RC filter composed of a resistance component R of an FFC and the capacitance component C of piezoelectric elements.
FIG. 4B
shows an output waveform signal in the case that the waveform signal shown in
FIG. 4A
is input to a load of R=1 ohm and C=10 nF. As is clear from
FIG. 4B
, the capacitance C of the piezoelectric elements as the load is small, the time content ô=RC=10 nsec, and the output waveform signal is almost the same as the input waveform signal.
FIG. 4C
shows an output waveform signal in the case that the waveform signal shown in
FIG. 4A
is input to a load of which R=1 ohm and C=10×32×4 colors=1280 nF.
As described above, since the value of the time constant ô=RC is large (name
Addison Karen
Fuji 'Xerox Co., Ltd.
Morgan Lewis & Bockius
Mullins Burton S.
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