1. Field of the Invention
The present invention relates to an ink-jet recording device which squirts droplets of liquid ink onto a recording medium to record an image, and more particularly to an ink-jet recording device which squirts droplets of liquid ink onto a recording medium by virtue of the pressure generated by ultrasonic beams emitted from piezoelectric elements.
2. Description of the Related Art
A so-called ink-jet printer has been put to practical use. This printer is a recording device which squirts droplets of liquid ink onto a recording medium, thereby to form ink dots thereon and recording an image thereon. It makes less noise than other recording devices. Nor does it require development or fixation of images recorded on the medium. The ink-jet printer is now popular as a device for recording data on ordinary paper. Many techniques for squirting ink-jet printer ink have been proposed to this date. Notable among them are:
(a) To apply the pressure of vapor generated by a heating element to squirt a droplet of ink; and PA1 (b) To apply a mechanical pressure pulse generated by a piezoelectric element to squirt a droplet of ink. PA1 where n is an integer greater than 0 and .lambda.m is the wavelength of the ultrasonic beams traveling in the member. The speed of the beams in the member is preferably an integral multiple of the speed of the beams in the liquid ink. PA1 where .lambda.i is the wavelength of the ultrasonic beams propagating in the liquid ink. If .lambda.m is substantially an integral multiple of .lambda.i, each thick portion and each thin portion of the member satisfy the equation (1), whereby acoustic matching is provided. As a result, the ultrasonic beams are effectively emitted into the ink from the thick and thin portions of the member, and an ink droplet flies onto a recording medium. PA1 where D is the aperture of the lens and .lambda. is the wavelength of the ultrasonic beams passing through the lens.
An ink-jet printer has a serial scanning head. The head is mounted on a carriage. It records data while moving in the direction (hereinafter referred to as "main-scanning direction") perpendicular to the direction in which recording paper is fed (hereinafter referred to as "sub-scanning direction"). Driven mechanically, the serial scanning head cannot move as fast as desired to accomplish high-speed recording. It is proposed that the serial scanning head be replaced by a line scanning head, because the line scanning head can record data faster since it is as long as a recording sheet is wide and need not move to record data on the recording sheet. However, it is difficult to use a line scanning head, for the following reasons.
In an ink-jet recording system, ink is liable to concentrate locally as the solvent evaporates. The concentrated ink clogs up the fine nozzles arranged in a density which determines the resolution of an image the system can form. If the pressure of vapor is applied to form an ink jet, insoluble matter is likely to accumulate in each nozzle as it thermally or chemically reacts with the ink. If the pressure generated by an piezoelectric element is used to form an ink jet, each ink passage needs to be complex in structure and the ink is liable clog the passage.
Nozzle clogging occurs at low frequency in a serial scanning head which has tens of nozzles to a hundred and odd nozzles. In a line scanning head having as many nozzles as several thousands, nozzle clogging takes place so frequently as to reduce the reliability of the head seriously.
Furthermore, a conventional ink-jet recording device does not help to increase the resolution of images recorded. If vapor pressure is used, the device can hardly produce an ink droplet having a size of 20 .mu.m or less (which will form on recording paper a dot having a size of about 50 odd .mu.m). To use pressure generated by a piezoelectric element, the recording head needs to have a complex structure and cannot be made by the existing manufacturing technology so as to record high-resolution images.
Various systems have been proposed which squirt ink droplets from a mass of ink, using the pressures of ultrasonic beams generated by an array of thin-film piezoelectric elements. Each is known as "nozzleless system" which has neither nozzles for forming dots on recording paper nor partitions for the ink passages. The nozzleless system can reliably prevent ink clogging and remedy nozzle clogging, if any. Moreover, the system can record high-resolution images since it form tiny ink droplets and squirts them stably.
The nozzleless system, however, needs to comprise a plurality of piezoelectric element arrays arranged in a staggered fashion. Only one piezoelectric element array does not suffice to record high-resolution images. This is because ultrasonic beams are applied to ink, after converged by acoustic lenses larger than pixels (e.g., lenses having a size 30 times as large as the size of pixels). The nozzle system with piezoelectric element arrays arranged in a staggered fashion is, however, disadvantageous in that the ink periodically changes in concentration and that adjacent dots shift with respect to one another.
The piezoelectric element arrays arranged in a staggered fashion may be replaced by a linear piezoelectric array which emits ultrasonic beams such that the beams interfere with one another in an ink reservoir and converge at a point, thereby achieving so-called phased array scanning.
One of phased array scanning technique is known as "linear scanning," in which the ultrasonic beams from a piezoelectric element are converged at a point in an ink layer. Linear scanning cannot be performed without many drive-signal sources capable of generating element-driving signals which have accurately controlled different phases. The linear scanning is employed in ultrasonic diagnosis apparatus. When the linear scanning is utilized in an ink-jet recording device, there will arise a problem.
The size of an ink droplet squirted when a pressure built up by ultrasonic beams is applied to liquid ink greatly depends on the frequency of the ultrasonic beams. For the ink-jet recording device to record images having a sufficient resolution, the ultrasonic elements must be driven by signals of a high frequency ranging from tens of magahertzes to hundreds of magahertzes, high frequency of the drive signals. To achieve phased array scanning by use of the such high-frequency signals, a drive circuit needs to delay the drive signals with high accuracy in the order of nanosecond (10.sup.-9 second), in view of the difference in length among the lines for supplying the signals from the drive circuit to the piezoelectric elements.
In the case where a sector electronic scanning is performed by using the phased array, i.e., acoustic beams are applied into liquid ink to accomplish phased array scanning, an ink droplet may fail to fly perpendicular to a recording medium if the ultrasonic beams are converged at a point other than the desired point. If ink droplets fly slantwise to the medium, ink dots will be formed on the medium at different pitches. This has been proven by experiments in which acoustic beams were converged, forming a single beam whose axis was inclined at a few degrees to the perpendicular to the surface of liquid ink.
To form ink dots at a regular pitch, the phases of the signals for driving piezoelectric elements must be controlled with high accuracy. In other words, the signal for driving a piezoelectric element needs to differ in phase very minutely from the signal for driving the immediately adjacent piezoelectric element. In order to control the phases of the drive signals so accurately, it is necessary to use a drive circuit complicated and thus expensive and a memory for storing a great amount of phase-correcting data.
The piezoelectric elements used to perform phased array scanning are discrete members made by cutting a piezoelectric layer. When the layer with a limited length is divided into many discrete piezoelectric elements juxtaposed at a small pitch, in order to record images of high resolution, the elements will be narrow and will likely to be broken. Consequently, the piezoelectric element array cannot be manufactured at high yield.
Assume that the piezoelectric elements are juxtaposed at a sufficiently small pitch to form ink dots in a high density. Then, noise will be generated due to the cross talk between the adjacent piezoelectric elements. The cross-talk noise greatly hinders the convergence of the ultrasonic beams emitted from the elements.
The cross-talk noise between the elements forming either end portion of the piezoelectric element array differs in magnitude from the cross-talk noise between the elements forming a middle portion of the array. This is because no discrete electrodes are provided for the elements forming either end portion, or less discrete electrodes are provided for them than for the other elements. The cross-talk noise between the elements forming either end portion must be controlled differently from the cross-talk noise between the other elements. The method of controlling the cross-talk noise is unavoidably complicated.
When phased array scanning is carried out to converge ultrasonic beams, forming a single beam which reach at a point in the surface of liquid ink, the axis of the single beam inevitably inclines to the ink surface, not extending perpendicular thereto. As a consequence, an ink droplet may fail to fly in a path perpendicular to the ink surface. To make matters worse, the ultrasonic beams are attenuated as they are reflected by the glass walls of the ink reservoir, decreasing the efficiency of squirting ink droplets. To prevent the reflection of beams, the piezoelectric element array may be processed to have a curved beam- emitting surface. If the array is so processed, the yield of the piezoelectric element array will lower.
An ink-jet printer is known which has an acoustic lens for converging the ultrasonic beams from the piezoelectric element array, at a point in the surface of liquid ink. The lens is a bulk lens with a convex surface having a predetermined radius of curvature or a Fresnel lens (designed on the Fresnel diffraction theory) for shifting the phase of one beam with respect to another. When used in combination with an acoustic lens, the piezoelectric element array need not have a curved beam-emitting surface and can, therefore, be made easily. However, the ultrasonic beams are attenuated as they travel through the acoustic lens, and each beam is partly reflected at the interface between the lens and the liquid ink. The ultrasonic energy applied to the ink is less than required to squirt an ink droplet. The drive signals applied to the piezoelectric elements of the array must have an energy high enough to compensate for the inevitable energy loss of the ultrasonic beams.
The piezoelectric element array may be formed into a curved beam-emitting surface so that the beams they emit may converge at a point in the surface of the ink, rendering it unnecessary to use an acoustic lens. In this case, the signals for driving the element need not have a high voltage, but the step of processing the array reduces the yield of the array.
As described above, a piezoelectric element array having a curved beam-emitting surface is used, or a piezoelectric element array having a flat beam- emitting surface is used together with an acoustic lens, in order to achieve phased array scanning, thereby to converge the ultrasonic beams in a plane perpendicular to the axis of the array (i.e., the main scanning direction). If the a piezoelectric element array having a curved beam-emitting surface is used, the yield of the array will decrease. If a flat piezoelectric element array is used together with an acoustic lens, the signals for driving the piezoelectric elements must have a high energy.
So-called sector electronic scanning is known which is one type of phased array scanning. In the sector electronic scanning, the piezoelectric elements juxtaposed and spaced in the main-scanning direction are driven by signals delayed with respect to one another. The elements emit ultrasonic beams which differ in phase. The beams are converged at a point is near the surface of liquid ink, whereby an ink droplet fly from that point.
The sector electronic scanning is advantageous in that the point from which an ink droplet flies can be changed, regardless of the pitch at which the piezoelectric elements are juxtaposed. However, accurate delay times must be imparted to the drive signals so that the elements may emit ultrasonic beams which converge at a desired point. Accurate delay times can be imparted to the signals by nothing but a drive circuit which is complicated and which is hence very expensive. Without such a drive circuit, the sector electronic scanning cannot be accomplished. Furthermore, when the ultrasonic beams converge at a point other than the point located right above the midpoint of the array, forming a single ultrasonic beam, the axis of the single beam inclines to the ink surface. An ink droplet will fly a path inclined to the recording medium, forming an ink dot at a position off the desired position on the recording medium.
(1) In the ink-jet recording technique, wherein piezoelectric element arrays arranged in staggered fashion are used to apply ultrasonic beams to ink to squirt an ink droplet, the ink periodically changes in concentration and that adjacent dots shift with respect to one another. Further, since the high-frequency signals for driving the piezoelectric elements must be phase-controlled accurately, phased array scanning cannot be effected without a drive circuit which is complicated and expensive.
(2) In order to form ink dots at a desired pitch on a recording medium, the signal for driving a piezoelectric element needs to differ in phase very minutely from the signal for driving the immediately adjacent piezoelectric element. To control the phases of the drive signals so accurately, it is necessary to use a drive circuit complicated and thus expensive and a memory for storing a great amount of phase-correcting data.
(3) With the ink-jet recording device which performs phased array scanning to apply ultrasonic beams at a point in liquid ink, squirting an ink droplet onto a recording medium, the piezoelectric elements can hardly arranged at a small pitch to record high-resolution images if each element comprises a discrete piezoelectric layer. If the elements are arranged at such a small pitch by all means, the yield of the device will lower.