Actuator driving circuit

Details Actuator driving circuit Actuator driving circuit

1999-05-18

2001-08-14

Barlow, Jr., John E. (Department: 2853)

Incremental printing of symbolic information

Ink jet

Controller

C347S009000, C347S068000

Reexamination Certificate

active

06273537

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an actuator driving circuit.
2. Description of Related Art
An ink jet printer is provided with an ink jet print head. In one type of the ink jet printer, the ink jet print head is formed form piezoelectric material. The print head is provided with a plurality of side walls for defining a plurality of ink chambers. Each ink chamber is filled with ink. A nozzle is provided in fluid communication with each ink chamber. Each side wall serves as an actuator. That is, when the side wall is applied with a drive voltage, the side wall is bent, thereby changing the volume of the corresponding ink chamber. Due to the change in the ink chamber volume, an ink droplet is ejected from the ink chamber through the corresponding nozzle. The thus ejected ink droplet will form characters and images on a sheet of paper.
SUMMARY OF THE INVENTION
FIG. 1
is a circuit diagram of a conceivable ink jet print head
1000
. Each side wall or actuator
1005
operates in a manner of a capacitor, and therefore is represented in
FIG. 1
as a capacitor C. As shown in
FIG. 1
, the print head
1000
is connected to a drive integrated circuit (IC)
2000
. The drive IC
2000
includes a plurality of driver circuits
1001
, each for driving a corresponding actuator (capacitor C)
1005
. More specifically, each actuator (capacitor C)
1005
is connected to a corresponding driver circuit
1001
via a resistor R.
The driver circuit
1001
is designed to apply a fixed voltage to each actuator (capacitor C)
1005
. When one actuator (capacitor C)
1005
is applied with the fixed voltage, the actuator (capacitor C)
1005
gradually charges up and deforms, thereby increasing the volume of a corresponding ink chamber. Ink is supplied into the ink chamber. When the driver circuit
1001
stops the application of the voltage to the actuator
1005
, the actuator
1005
gradually discharges, and returns to its original shape. As a result, the volume of the ink chamber reduces, and ink is ejected from the ink chamber via a corresponding nozzle.
Properties, such as viscosity, of the ink change according to an ambient temperature during printing. It is therefore desirable to perform a minute control for changing the amount of the drive voltage, according to the changes in properties of the ink, thereby changing the amount that the actuator deforms.
It is noted, however, that the actuator
1005
is formed from piezoelectric material and therefore capacitance of the actuator
1005
increases with increase in the drive voltage applied to the actuator
1005
. Accordingly, if the drive voltage is changed in accordance with changes in properties of the ink, then the capacitance of the actuator
1005
will also change, which affects the ink ejection property as described below.
Generally, when the actuator (capacitor C)
1005
is applied with a drive voltage form the corresponding driver circuit
1001
, a voltage is developed at the actuator (capacitor C)
1005
in a manner that the amount of the developed voltage changes in time as shown in FIG.
2
. In
FIG. 2
, a rising edge time Tr is defined as a time duration from when the voltage is first applied to the actuator
1005
to when the actuator
1005
deforms. A falling edge time Tf is defined as a time duration from when an application of the voltage stops to when the actuator
1005
returns to its original shape. More specifically, when the actuator (capacitor)
1005
is charged 100%, the actuator
1005
deforms at a maximum. In this example, the rising edge time Tr is defined as the time duration from when the actuator (capacitor)
1005
is charged 10% to when the actuator (capacitor)
1005
is charged 90%. The falling edge time Tf is defined as the time from when the actuator (capacitor)
1005
is discharged 10% to when the actuator (capacitor)
1005
is discharged 90%. Both the rising edge time Tr and the falling edge time Tf are important parameters in ejection characteristics of the print head
1000
.
When the amount of the drive voltage applied to the actuator (capacitor C)
1005
is increased, however, the capacitance of the actuator (capacitor C)
1005
also increases. Accordingly, both the rising edge time Tr and the falling edge time Tf will increase. It becomes impossible to maintain the desired ejection characteristics of the print head
1000
. In order to ensure that the ejection characteristics be as desired, the amount of the drive voltage can be changed only over a very narrow range.
It is conceivable to select the material forming the actuator
1005
so that the capacitance of the actuator
1005
will not change with the changes in the amount of the applied drive voltage. However, in this case, the actuator becomes incapable of producing a sufficient displacement amount required to eject ink. Accordingly, this method is not applicable to the print head.
In view of the above-described drawbacks, it is an objective of the present invention to provide an improved actuator drive circuit that is capable of maintaining the rising edge time and the lowering edge time of the actuator drive as substantially fixed, regardless of changes in the amount of the applied drive voltage.
In order to attain the above and other objects, the present invention provides a drive circuit for applying a drive voltage to an actuator whose capacitance increases with increase in an amount of the drive voltage, the drive circuit comprising: an output element for outputting a drive voltage in a driving waveform; and an element driving circuit for driving the output element, at least one of the output element and the element driving circuit having a characteristic to decrease a rising edge time period and a falling edge time period of the driving waveform in accordance with increase in the amount of the drive voltage.
Because the actuator has the characteristic that its capacitance increases with increase in the amount of the applied drive voltage, the time required for the actuator to be electrically charged and the time required for the actuator to be discharged increases as the amount of voltage applied to the actuator increases. The driving waveform applied to the actuator by the output element, that is driven by the element driving circuit, is controlled such that the rising edge time and the falling edge time are decreased as the amount of the voltage applied to the actuator increases. Accordingly, the characteristic of the actuator is cancelled out. Regardless of changes in the amount of the voltage applied to the actuator, the rising edge time and the falling edge time of the actuator drive can be maintained as substantially being fixed.
At least one of the output element and the element driving circuit may have a characteristic to increase a rate, at which the drive voltage rises to a predetermined driving level and falls from the predetermined driving level in the driving waveform, in accordance with increase in the value of the predetermined driving level.
The output element may include: a first transistor for electrically charging the actuator; and a second transistor for electrically discharging the actuator, each of the first and second transistors having an ON resistance that decreases according to increase in an amount of an output element driving voltage outputted form the element driving circuit to each of the first and second transistors.
Each of the first and second transistors has characteristics wherein its ON resistance decreases according to increase in an amount of the output element driving voltage applied thereto. Accordingly, as the amount of the output element driving voltage applied to each transistor increases, the amount of the electric current flowing through each transistor increases, whereby the time period required to electrically charge the actuator and the time period required to electrically discharge the actuator is decreased. Thus, the above-described characteristics of the actuator is cancelled out. As a result, the rising edge time and the falling edge time of the ac

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