Technical Field

This invention relates to a dot matrix printer and, in particular, to a driving mechanism for such a printer.

Background Art

In the field of non-impact printing, the most common types of printers have been the thermal printer and the ink jet printer. When the performance of a non- impact printer is compared with that of an impact printer, one of the problems in the non-impact machine has been the control of the printing operation. As is well known, the impact operation depends upon the move¬ ment of impact members such as wires or the like and which are typically moved by means of an electromechani¬ cal system which is believed to enable a more precise control of the impact members.

The advent of non-impact printing, as in the case of thermal printing, brought out the fact that the heating cycle must be controlled in a manner to obtain maximum repeated operations. Likewise, the control of ink jet printing in at least one form thereof must deal with rapid starting and stopping movement of the ink fluid from a supply of the fluid. In each case of non- impact printing, the precise control of the thermal elements and of the ink droplets is necessary to provide for both correct and high-speed printing.

In the ink jet printer the print head struc¬ ture may be a multiple-nozzle type with the nozzles aligned in a vertical line and supported on a print head carriage which is caused to be moved or driven in a hor¬ izontal direction for printing in line manner. Alterna¬ tively, the printer structure may include a plurality of equally-spaced, horizontally aligned single nozzle print heads which are caused to be moved in back-and-forth man¬ ner to print successive lines of dots in making up the lines of characters.

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There have been various ways and means devised for precisely controlling the deceleration and acceler¬ ation of a print head of a dot matrix printer wherein large amounts of energy are used in the printing oper- ation. It is well-known that a print head carriage may be connected with a pulley and* cable arrangement and driven by a separate motor for causing transverse move¬ ment of the carriage during printing operations. Another driving system utilizes a lead screw connected with the carriage and driven by a separate motor for back-and- forth movement of the carriage. A disadvantage of the motor driven carriage is the high cost of both the equipment and its operation.

In yet another driving system disclosed in the German Application No. 2,646,740, a carriage plate carrying a plurality of print heads is mounted on the free ends of the central legs of two comb-like leaf springs while the corresponding ends of two other legs are fixed to the frame of the printer. The reciprocating motion of the carriage plate is performed by a crank drive. The special structure and arrangement of the leaf springs bearing the carriage plate provides for parallel movement of the plate relative to the platen during the back-and-forth motion. A disadvantage of this arrange- ment is that the crank drive still requires substantial amounts of energy for the printing operation,- and that the printing operation is rather noisy.

Disclosure of Invention

It is an object of the present invention to provide a dot matrix printer having a substantially noiseless mechanism for moving a print head or platen in a side to side direction and requiring low energy for accelerating and decelerating the print head or platen. Another object of the present invention is to provide a driving mechanism for a dot matrix printer wherein the costs of manufacture and of operation are reduced.

Thus, according to the invention, there is provided a dot matrix printer including at least one print head having spring means operably connected there¬ with, and driving means for causing movement of said at least one print head in side to side direction relative to a record medium movable past said at least one print head so as to enable said at least one print head to print a line on said record medium, characterized in that said driving means is arranged to cause said spring means to oscillate at a frequency corresponding with the natural frequency of the assembly of said spring means and said at least one print head so as to bring about oscillating motion of said at least one print head in said side to side direction. According to another aspect of the invention, there is provided a dot matrix printer including a platen providing support for a record medium, and at least one print head for printing on said record medium, charac¬ terized by spring means operably connected with said platen, and driving means arranged to cause said spring means to oscillate at a frequency corresponding with the natural frequency of the assembly of said spring means and said platen so as to bring about oscillating motion of said platen in side to side direction relative to said at least one print head supported in a stationary manner.

Brief Description of the Drawing

Embodiment of the invention will now be des¬ cribed, by way of example, with reference to the accom- panying drawings, in which:

Pig. 1 is a diagrammatic view of spring means for causing horizontal movement of a print head;

Fig. 1A is a circuit diagram of a drive for the spring means; Fig. 2 is a diagrammatic view of a plurality of print heads on a support plate which is caused to be moved by spring means in a horizontal direction;

Fig. 3 is a view similar to portions of Fig. 1 and including circuit means for compensating the damping of the oscillating system;

Figs. 4A and 4B show a timing diagram of the voltage and deflection relationship;

Fig. 5 shows the current pulse through the drive circuit of Fig. 3;

Fig. 6 is a view of ratchet means rotatable by horizontal movement of the print head carriage; Fig. 7 is a view of drive means driven by the ratchet means of Fig. 6 for enabling vertical motion of the record media;

Fig. 8 is a diagrammatic view of a modified arrangement of spring means for causing horizontal move- ment of a platen;

Fig. 9 is a diagrammatic view showing the com¬ bination of horizontal oscillation of the print head and vertical movement of the record media;

Fig. 10 is a view showing another arrangement of spring means for causing horizontal movement of the print head, having straight line guide means with horizontal oscillating coil spring elements;

Fig. 11 shows still another arrangement of such spring means with the use of bent leaf springs performing the function of guide or bearing means along with the function of an oscillating drive;

Figs.. 12A, 12B and 12C are diagrammatic show¬ ings of different arrangements employing a leaf spring in providing the vertical oscillating mechanism for record media movement;

Fig. 13 shows a scanning arrangement for measuring the instantaneous speed of the oscillating print head;

Fig. 14 is a diagrammatic view of a still further spring means arrangement showing a bent leaf spring drive mechanism and a nozzle bar driven in oscil¬ lating manner;

Fig. 15 shows the use of a linear motor along with spring means for driving a print head in horizontal oscillating manner; and ^'

Fig. 16 is a circuit diagram of the linear motor.

Best Mode of Carrying Out the Invention

Fig. 1 is a diagrammatic view showing an ink jet print head 10 supported from the free end 12 of a leaf spring 14. The other end 16 of the leaf spring is secured to a fixed frame member 18. The print head 10 has a nozzle 20 for ejecting an ink droplet 22 therefrom in performing a printing operation. An electrodynamic drive system 24 for initiating or setting the leaf spring 14 into oscillation or swinging movement includes a permanent magnet 26 operable with an armature 28 over¬ lying a central core portion 30 of the magnet and wound with a drive coil 32. The drive coil 32 is energized by pulses amplified by a power transistor 31, as illus¬ trated in Fig. 1A. The armature 28 has a coupling member 34 in the form of a rod fixed thereto at one end and secured at the other end to the leaf spring 14 near the center thereof. The leaf spring 14 operates with a natural frequency wherein the ink jet print head 10 is swinging freely and no electrical power is required for acceleration and deceleration of the head 10. In this manner, the consumed electrical power is only that power which is converted into frictional work. A pulse is applied once for each forth movement of the leaf spring 14 in its back and forth movement and with the pulse frequency being equal to the frequency of spring oscil¬ lation.

Fig. 2 illustrates a plurality of ink jet print heads 40 carried on a support member 42 which, in turn, is supported from the free ends of a pair of leaf springs 44 and 46. The other ends of the leaf springs 44, 46 are secured to a frame member 48. The print heads 40 are positioned to print on paper 50 or like

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record media which is caused to be trained around a platen 52. The driving or transporting of the paper 50 in a vertical direction past the line of printing is performed by means to be described later. A drive module 54, similar in form to the electrodynamic drive system 24 of Fig. 1, is used to set the leaf springs 44 and 46 into oscillation with a natural frequency through a coupling member 56. It can be readily seen that the printing width is increased by a factor corresponding to the number of ink jet print heads 40 mounted on the support member 42.

Fig. 3 is a feed-back circuit for compensating the damping of the mechanically oscillating system of Fig. 1 wherein the ink jet print head 10 is supported from the free end 12 of the leaf spring 14 secured at its other end to the frame member 18. The armature 28 is secured to the leaf spring 14 by the coupling member 34_ and includes the drive coil 32 wound on the armature and connected to ground potential. The feed-back cir- cuit includes an operational amplifier 60 having compo¬ nents wired for use as an integrator and connected in series with a power amplifier 62 which energizes the drive coil 32. The operational amplifier 60 acts as an integrating element by reason of its feedback capacity to effect a phase shift by ninety degrees so that energy is supplied in phase to the drive coil 32 by the power amplifier 62. A measuring or sensing winding 64 is dis¬ posed around the drive coil 32 and is connected at one end to the operational amplifier 60 with the other end thereof connected to ground potential. The positive feed-back compensates the damping so that the mechanical spring system oscillates with its natural frequency. The damping compensation is required to maintain the oscil¬ lation at a constant amplitude wherein the natural fre- quency of spring oscillation is determined by the char¬ acteristics of the spring, such as mass, material, dimensions and design.

Figs. 4A, 4B and 5 show a timing relationship between the pulsating electrical energization of the drive system 24 of Fig. 1 and the mechanical oscillation of the spring 14. The relationship of Figs. 4A and 4B is shown by the formula V=-dD. The induced voltage V is proportional to the speeα of oscillation and the deflection D is shown in time relation for a range of positive or plus current pulse and for a range of negative or minus current pulse. The pulses of Fig. 5 are initiated at predetermined times which are not precisely fixed so that, for certain times or instants during the forward oscillating phase, an electrical pulse must be applied wherein the movement of the free mass spring oscillator is sustained by the electrical energization. More particularly, the pulses are applied, as indicated in Figs. 4A and 5, immediately after the maximum amplitude at a frequency equalling the natural frequency of oscillation. Pulses of opposite polarity are applied wherein the polarity corresponds to the respective direction of movement _* Generally, the drive system 24 is pulsed or energized once for each cycle of printing.

A simple drive means for performing vertical movement of the paper to be printed is possible through use of a simple low power motor which is continuously energized in order to produce a constant paper speed. It is also a simple matter to change the paper speed by using different means for operating the motor in order to perform a maximum paper feed condition whenever required. To overcome the need for such a separate drive. Figs. 6 and 7 illustrate means for obtaining a vertical drive for the paper from the longitudinal movement of the ink jet print head, wherein a ratchet gear 70 is positioned adjacent the platen 52 around which the paper 50 is trained in a path past the line of printing. An oscillating horizontal push rod 72, suit¬ ably connected to the leaf spring 14, drives a feed rod . 74 against the teeth of the ratchet gear 70 and operates

with a ratchet pawl 76 secured to a frame member 78 and engaging with the teeth of the gear 70. The ratchet gear 70 is carried on a shaft 75 along with a gear 77 which meshes with a gear 79 on the end of a worm gear 80 which, in turn, meshes with and drives a drive gear 82 for rotating the platen 52. The stepping mechanism enables driving the paper 50 by using the horizontal drive motion of the ink jet print head drive means and thereby eliminates the use of an electric motor. Fig. 8 shows an ink jet print head 86 sup¬ ported in stationary manner from a member 88 for print¬ ing on the paper 50 trained around the platen 52. The platen 52 is supported for rotation on a shaft 90 which is slidably jou naled for axial movement in bearing blocks 92 and 94 and which has a small plate 96 secured at the right end thereof associated with the armature 28 and the drive coil 32. A coil spring 98 is- placed on the shaft 90 between the plate 96 and the block 94. In this arrangement, the print head 86 is stationary and the platen- 52 along with the paper 50 is caused to be moved by action of the spring 98 in a horizontal oscil¬ lating direction as shown by the arrow.

A single print head 40 is illustrated in Fig. 9 as being carried on the support member 42 and oscil- lating in the horizontal direction, as in the manner illustrated in Fig. 2. At the same time, the paper 50 around the platen 52 is caused to be regularly moved in incremental manner in a vertical direction. In this combined horizontal oscillating and vertical incrementing drive arrangement the most advantageous operating point is at the resonance of the spring mass system.

Fig. 10 illustrates a straight line guide ar¬ rangement for an ink jet print head 100 supported from a shaft 102 slidably journaled in bearing blocks 104 and 106 on a frame member 108. The shaft 102 has a small plate 110 secured at the left end and a small plate 112 secured at the right end thereof. The plate 110 is

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drivingly associated with the armature 28 and the drive coil 32. A coil spring 114 is placed on the shaft 102 between the plate 110 and the block 104 and a coil spring 116 is placed on the shaft between the plate 112 and the block 106 to provide horizontal oscillation of the print head 100.

In Fig. 11 there is shown another arrangement wherein a pair of print heads 120 and 122, similar to the previously mentioned print heads 10, 40 and 86, are carried on a support member 124 having plates 126 and 128 at the ends thereof. A pair of bent or folded leaf springs 130 and 132 are operably connected with the plates 126 and 128 in a manner to provide guide means as well as horizontal oscillation of the print heads 120 and 122. In like manner, the plate 126 is connected to the armature 28, drive coil 32 assembly through a shaft or rod 131. The outer leg or portion of each spring is fixed or secured to a support member, as 133 for spring 132. Figs 12A, 12B and 12C show different ar¬ rangements of using a leaf spring for the vertical- oscillating mechanism employed in transporting or driving the paper. In Fig. 12A, the paper 50 is trans¬ ported and trained around a stationary roller 134, a pair of movable rollers 136 and 138 and then around a stationary roller 140. A leaf spring 142 is secured at one end and is connected with a member 144 coupling the movable rollers 136 and 138 to provide the vertical movement for the paper. Fig. 12B has the paper 50 trained around a stationary roller 146, a movable roller 148 and then around a stationary roller 150. A leaf spring 152 is secured at one end and is connected to the movable roller 148 for vertical movement thereof in relation to the print head 154. In each case the leaf spring performs the function of the spring member as well as the bearing or guide function. Fig. 12C shows a leaf spring 156 secured at one end and connected to a movable roller 158 for vertical movement. A pair of

guide members 160 and 162 are provided to be slidable or displaceable along a horizontal path to and from the roller 158 so as to allow changing the natural frequency of the spring 156. Fig. 13 shows the armature 28, as illustrated in Figs. 1 and 3, with the drive coil or operating winding 32 and with the addition of the sensor winding 64 which is within the magnetic field in similar man¬ ner as is shown in Fig. 3. The sensor winding is used for ascertaining the instantaneous speed of the oscil¬ lating ink jet print head. It should be here noted that the natural frequency of the spring mass oscillating system is very stable so that no accurate determination of the position of the ink droplets is required. It is only necessary that once for each oscillating p-eriαd a simple synchronization of the ink jet with the mechan¬ ical movement should be performed. This synchronization can be performed by the use of known position detectors, such as by optical scanning of the locating marks by capacitive measurement of an air gap wherein the capaci¬ tance is changed, or by the magnetic detection of the alteration of the magnetic flux. An electrodyn.amic scanning arrangement for such synchronization in the present invention is provided by use of the sensor winding shown in Fig. 13 as a measuring element for the speed of movement of the print head. The voltage, having a magnitude which is proportional to the speed of the plunger or armature, is induced in the second or sensor winding 64 as a result of the movement. Since a fixed phase relationship exists between the displacement and the speed of movement for a sine—shaped wave move¬ ment, a synchronizing signal may be derived from the speed measuring signal by means of a zero voltage detec¬ tor and this synchronizing signal can be applied to an associated electronic control unit.

Fig. 14 illustrates the -use of a nozzle bar 166 having a plurality of ink jet nozzles 168 and car¬ ried on a support member 170 which has end plates 172

and 174 operably connected with bent or folded leaf springs 176 and 178. The spring 176 is fixed or secured at the left side and the spring 178 is fixed or secured at the right side. The spring 176 has an aperture 181 through which a coupling member 180 is movable and having one end secured to the end plate 172 and being connected at the other end to the oscillating assembly or electrodynamic drive system 24, similar to the unit shown in Fig. 1. It is here noted that the width of an area to be printed is generally defined by the amplitude of the oscillation in connection with the number of ink nozzles so that it is possible to reduce the oscillating amplitude by increasing the number of nozzles. The limiting case is to provide as many ink nozzles as there are ink spots or dots required to print one line on the paper, thus making a horizontal drive not required or necessary in such case. Another arrangement is to pro¬ vide as many nozzles as there are characters to be printed on a line with one nozzle assigned only for the width of each character so that the horizontal oscilla¬ tion does not cover the spaces between the characters. A smaller amplitude of oscillation along with the same oscillation frequency results in a lower acceleration which is favorable for longer life of the print head. Figs. 15 and 16 illustrate an arrangement wherein the amplitude of the horizontal drive is in¬ creased or is larger as a result of the decreased number of nozzles, as may be the* case in a cost reduction scheme for the ink jet print heads. If the amplitude of the oscillation exceeds 2 centimeters, it is reasonable to use a magnet distributed over the entire length of oscillation and a meander-shaped coil arranged on a circuit board. Fig. 15 shows a nozzle bar 182 having three ink nozzles 184 spaced at a distance apart and carried on the shaft 102 which is slidably journaled in bearing blocks 104 and 106 on the frame member 108, all in the manner as previously shown in Fig. 10. Shaft 102

has the small plates 110 and 112 secured at the ends thereof along with the coil springs 114 and 116 to provide horizontal oscillation. The magnet and coil arrangement of Fig. 15 is known in the art as a linear motor and limitations in the amplitude of the horizontal oscillation for this type of drive are not an important _ concern. The linear motor 186 is diagrammatically shown with a magnetic field 188 and current flow to the line 190. Fig. 16 shows the linear motor 186 in more de- tailed form with the magnetic field 188 along with current flow in the several lines 190. The horizontal drive is in the direction of the arrow. The drive or the energization can be realized by the above described electrodynamic principle and also by means of a simple electromagnet. .This type of drive requires a stroke¬ like energization in the proper time relationship with respect to* the mechanical oscillation.

Since the use of a linear motor enables large amplitudes of oscillation, it must be taken into con- sideration that for utilizing such a spring mass system the spring used as the energy storage means likewise •has to be capable of a large amplitude of movement. One solution for enabling a large amplitude involves the use of a conical spiral spring which has advantages for the present invention, since a spiral spring has a good working volume relationship wherein the volume of the extended spring is contrasted with the volume of the compressed spring. The use of a spiral spring makes it possible to construct a long stroke oscillator in small installations.

If the amplitude of the oscillation of the horizontal oscillation is large, it may be possible that the oscillation of the vertical drive alone or by itself is insufficient to achieve a satisfactory print quality. In this case it may be useful to perform the vertical oscillation along the width of the paper in a dephased manner or where the vertical oscillation is combined

with the feed movement of the paper. The electro¬ magnetic exitation of two vertical oscillators is in phase opposition wherein the vertical movement at one side of the paper is downward and the vertical movement at the other side of the paper is upward. A tilting oscillation of the paper feeding is accomplished which in combination with the horizontal oscillation produces a satisfactory print pattern.