1. Field of the Invention
The present invention relates to an inkjet printing apparatus, comprising a print head, the print head comprising an ink duct, a piezoelectric element operatively coupled to the ink duct, and a control device configured to control ink drop ejection from the ink duct by actuation of the piezoelectric element.
2. Description of Background Art
Inkjet printers comprising piezoelectric elements are well known in the background art. In such printers, each ink duct is operatively connected to a piezoelectric element. A control device controls actuation of a piezoelectric element, so that it deforms and a volume change is achieved in the ink duct associated with the piezoelectric element. A resulting pressure wave that is thereby generated in the ink duct, leads to a drop of ink being ejected from a nozzle of the duct. Each point of time that the piezoelectric element, having properties comparable with a capacitor with capacitance C, is actuated by charging and discharging by a current source, an amount of power equal to ½ CV2 is dissipated in the driver circuit for each charging of the piezoelectric element and an amount of power equal to ½ CV2 is dissipated for each discharging of the piezoelectric element, wherein V represents a voltage present over the current source.
In an inkjet printer with, for example, 128 nozzles per print head such power dissipation results in a substantial loss of energy. When using a higher jet frequency or a larger number of nozzles per print head the energy loss becomes even larger.
From U.S. Pat. No. 7,049,756, a capacitive load driving circuit is known for charging and discharging a capacitive load. The capacitive load driving circuit is provided with a voltage divider for dividing a power supply voltage VA into a plurality of different voltages and a plurality of capacitors with capacitance CA. With this circuit, it is possible to collect and reuse energy accumulated in a capacitive load. However, the dissipated power is still large since the power supply voltage is constantly present over the plurality of capacitors causing power dissipation during charging and discharging of the piezoelectric element of each equal to the amount of ½ CAVA2.
An object of the present invention is to reduce the power dissipation during charging and discharging of a piezoelectric element in the print head. According to the present invention, this object is achieved by an inkjet printing apparatus, comprising a print head, the print head comprising an ink duct, a piezoelectric element operatively coupled to the ink duct, and a control device configured to control an ink drop ejection from the ink duct by actuation of the piezoelectric element. The control device comprises a current source, a number of power supplies, and a switch, connected between the current source and the number of power supplies, wherein the current source is configured to generate a current for actuating the piezoelectric element by charging and discharging, and the switch is configured to connect the current source and one of the number of power supplies, the one of the number of power supplies being selected such that a lowest voltage difference over the current source exists.
The applicant has recognized that power dissipation released by the current source may be reduced by reducing the voltage difference over the current source. Therefore a low voltage difference is created over the current source. The current source is provided to generate a current for actuating the piezoelectric element by charging and discharging. A load voltage over the piezoelectric element is thereby generated. A switch and a number of power supplies are provided, the current source being connected between the piezoelectric element and the switch. A voltage difference over the current source is equal to a voltage difference between the load voltage and a voltage of one of the power supplies, the one power supply being connected through the switch. A low voltage difference over the current source is established by switching the switch in such a position that the lowest available supply voltage above the load voltage over the piezoelectric element is selected.
The term “switch” has spread to a variety of digital active devices such as transistors and logic gates whose function is to change their output state between two logic levels or connect different signal lines. Each kind of transistor may be used as a switch. Examples of transistors often used as a switch are a Bipolar Junction Transistor (BJT) or Field Effect Transistors (MOSFET or JFET). For reasons of convenience, the switch as meant in the present invention is represented in the figures belonging to the present application as an electromechanical device with one or more sets of electrical contacts. Each set of contacts can be in one of two states: either ‘closed’ meaning the contacts are touching and electricity can flow between them, or ‘open’, meaning the contacts are separated and non-conducting.
The figures do not indicate that the switch as meant in the embodiments of the present invention is implemented as such an electromechanical device. Moreover, in the embodiments in the present invention, electronic switches are preferred.
The switch comprises a number of input terminals and at least one output terminal. The number of input terminals are connected to the number of power supplies. The at least one output terminal is connected to the current source.
Each position of the switch corresponds to an input terminal of the switch and supplies a different voltage level. Such a voltage level may depend on the configuration of the number of power supplies which are corresponding to the engaged input terminal. Dependent on to which one of the number of input terminals the switch is switched, a predetermined supply voltage level is supplied at the output terminal of the switch that is connected to the current source.
Dependent on the number of input terminals, in case switching positions, of the switch, the level of the supply voltage may be selected out of a number of discrete levels of supply voltage between 0 V and a predetermined maximum supply voltage. When no power supply is switched into the circuit, the supply voltage will be 0 V and if a number of power supplies are switched into the circuit, the supply voltage may be higher than 0 V up to the predetermined maximum supply voltage. The supply voltage may be high enough to establish an actuation of the piezoelectric element resulting in an ink drop ejection.
By switching the power supplies into the circuit as described above, power dissipation released by the current source during the charging and discharging of the piezoelectric element may be significantly reduced. It is noted that a reduction of power dissipation of the current source may depend on the number of input terminals. In general, the reduction is larger in case that the number of input terminals is larger.
In the case of equidistant voltage levels at the input terminals of the switch, a significant reduction may be achieved corresponding to the number of input terminals.
In an embodiment of the present invention, the charging and discharging of the piezoelectric element is done by a current source, for example a linear current source. The current from the linear current source is controlled. The current source is switched at the output terminal of a switch, which has the lowest available voltage difference with the load voltage of the piezoelectric element. This may be done by a known control device, e.g. based on a voltage drop across the current source as switching criterion. If the voltage drop gets below a certain minimum (e.g. about zero), the switch may be put one switch position higher to the next input terminal, which supplies a higher voltage.
During the charging phase, the switch may start at an input terminal supplying a predetermined voltage level, stepping from one switch position to another switch position to at last an input terminal supplying the highest available voltage during the last part of the charging.
Discharging may be controlled in an opposite way. A first terminal to discharge to, may be an input terminal supplying a second highest voltage, a last input terminal may supply a voltage level of 0 V.
Each piezoelectric element may have its own control device and may operate independently of other control devices, giving this approach a large flexibility. Deviations and ripple at the voltage input terminals do not disturb the functioning, because the control device uses the voltage drop across the current source as switching criterion. A voltage ladder may be constructed by selecting a plurality of adequate voltage levels. The voltage ladder may be constructed only once per piezoelectric element.
In an embodiment, the current source may be a controlled current source, such as a voltage controlled current source or a current controlled current source. This may be advantageous since the current produced by the controlled current source may determine the waveform of the output voltage. In the case that the current is held constant during charging of the piezoelectric element, a linear growth of the voltage level at the piezoelectric element will be achieved. However, for arbitrary waveforms the current may be controlled such that a desired waveform is established over the piezoelectric element.
In an embodiment, the number of power supplies may be a number of direct voltage sources (DC). By applying a direct voltage source (DC) like a switched-mode power supply (SMPS), the dissipation power loss and heat in the direct voltage source is minimized as well as the power dissipation produced by the current source. A good design may have an efficiency of up to 95%.
In an embodiment, the number of power supplies is connected in series. The number of serialized power supplies may generate a voltage difference level up to the sum of the voltages of the number of power supplies. Power dissipated by the current source may be reduced during the charging and discharging of the piezoelectric element. In case that each power supply is able to generate a same voltage, a power dissipation reduction may be achieved with a factor approximately equal to the number of power supplies. If, for instance four power supplies, being connected in series, with the same voltage level are used, during charging of the piezoelectric element a power dissipation of for instance 400 W may reduce to 100 W. In general, the reduction is larger in case that the number of power supplies is larger.
In an embodiment, an inkjet printing apparatus may comprise a number of piezoelectric elements and a number of current sources. The number of power supplies and the number of current sources may be operatively connected by means of a multiplexer. The multiplexer may comprise a number of input terminals and a number of output terminals. Each power supply may be connected to a respective input terminal and each current source may be connected between a respective output terminal and a respective one of the number of piezoelectric elements. In this manner, only one switching device, the multiplexer, is embodied and this may be advantageous in the case that each piezoelectric element needs to be charged according to an approximately same voltage ladder by switching for each step on the voltage ladder to a next higher voltage level by connecting to a corresponding input terminal of the multiplexer. For discharging the same advantage may be achieved.
In an embodiment, the inkjet printing apparatus may comprise a number of piezoelectric elements, wherein each piezoelectric element has a respective switch. In opposition to the previous embodiment, this embodiment may be advantageous in the case that for each piezoelectric element a different type of charging is required. This may for example depend on the arrangement of the piezoelectric elements on the print head.
Further scope of applicability of the present invention will become apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description.
The present invention will now be explained further with reference to the accompanying drawings, wherein:
a-4c is a diagrams showing voltages on a piezoelectric element in a process of charging and discharging of the piezoelectric element;
a is a schematic illustration showing a control device according to an embodiment of the present invention, in which embodiment more than one piezoelectric element has been arranged;
b is a schematic illustration showing a control device according to an embodiment of the present invention, the control device comprising a switch for each piezoelectric element;
a is a schematic illustration showing an embodiment of the present invention in which a switch terminal and power supply is connected to a first terminal of the piezoelectric element and a second terminal of the piezoelectric element to a control circuit; and
b is a diagram showing voltages on a piezoelectric element in a process of charging and discharging of the piezoelectric element according to the illustration of
An inkjet printing apparatus is shown in
The roller 1 may rotate around its own axis as indicated by arrow A. In this manner, the receiving medium may be moved in a sub-scanning direction C relative to the carrier 5 parallel to an axis 9, and therefore also relative to the print heads 4a-4d. The carriage 3 may be moved in reciprocation using suitable drive mechanisms (not shown) in a direction indicated by double arrow B, substantially parallel to roller 1. To this end, the carrier 5 is moved across a guide rod 6. This direction is generally referred to as the main scanning direction. In this manner, the receiving medium may be fully scanned by the print heads 4a-4d.
According to the embodiment as shown in this figure, each print head 4a-4d may comprise a number of internal ink ducts (not shown), each with its own exit opening (nozzle) 8. The nozzles in this embodiment form one row per print head perpendicular to the axis of roller 1 (i.e. the row extends in the sub-scanning direction C). According to a practical embodiment of an inkjet printer, the number of ink ducts per print head is greater and the nozzles are arranged over two or more rows. Each ink duct comprises a piezoelectric element (not shown) that may generate a pressure wave in the ink duct so that an ink drop is ejected from the nozzle of the associated duct in the direction of the receiving medium. The piezoelectric elements may be actuated image-wise via an associated control circuit (not shown). In this manner, an image built up of ink drops may be formed on receiving medium 2.
An ink duct 13 is shown in
A second terminal of the current source 36 is connected to an output terminal of a switch 35. The switch 35 is connected to a number of power supplies 31, 32, 33, 34, each delivering a voltage of x V. The power supplies 31, 32, 33, 34 are connected in series. The switch 35 has five input terminals 35a, 35b, 35c, 35d, 35e. A first input terminal 35a is connected to ground, supplying a voltage level of 0 V. A second input terminal 35b is connected to a terminal of the first power supply 31, supplying a voltage level of x V. A third input terminal 35c is connected to a terminal of the second power supply 32, supplying a voltage level of 2x V. A fourth input terminal 35d is connected to a terminal of the third power supply 33, supplying a voltage level of 3x V. A fifth input terminal 35e is connected to a terminal of the fourth power supply 34, supplying a voltage level of 4x V.
To establish ink drop ejection from the ink duct the piezoelectric element 37 needs to be actuated. Actuation is established by charging the piezoelectric element 37 via the current source 36. A pressure wave due to the actuation is strong enough to eject an ink drop from the nozzle of the ink duct as described herein-above with reference to
At the start of the actuation, the piezoelectric element 37 may not be charged and the switch 35 may be switched towards the first input terminal 35a. Then the current source 36 is starting to charge the piezoelectric element 37 and at the same time the switch 35 is switched towards the second input terminal 35b such that a voltage difference over the current source 36 of x V is established. A voltage difference over the piezoelectric element 37 increases. The voltage difference over the current source 36 results in power dissipation. The voltage difference over the current source 36 decreases to a level of 0 V due to the voltage difference over the piezoelectric element 37 reaching x V. As soon as the voltage difference over the current source 36 reaches a level of 0 V, the switch 35 alters the switch position from the second input terminal 35b towards the third input terminal 35c. The third input terminal 35c is supplying a voltage of 2x V By doing so, the voltage difference over the current source 36 is increased towards approximately x V and power is dissipated over the current source 36 directly after the moment of altering the switch position. The power dissipation may start to decrease again, if the voltage difference over the piezoelectric element increases further.
The current from the current source 36 is still charging the piezoelectric element 37 towards a higher voltage difference over the piezoelectric element. Power is starting again to be dissipated by the current source 36, since a voltage difference over the current source 36 is established. When the voltage difference over the piezoelectric element 37 has increased to a level of 2x V and the voltage difference over the current source 36 has thereby decreased to a level of 0 V, the switch 35 alters the switch position from the third input terminal 35c towards to fourth input terminal 35d. The fourth input terminal 35d is supplying a voltage of 3x V. By doing so, the voltage difference over the current source 36 is increased towards approximately x V, thereby dissipating power over the current source 36. Analogue to the above description, the switch 35 may be switched towards the fifth input terminal 35e supplying a voltage of 4x V. By switching towards the fifth input terminal 35e, the voltage difference over the current source will be approximately x V and the voltage difference over the piezoelectric element 37 increases to a level of 4x V. At a voltage difference of 4x V over the piezoelectric element 37, the ejection of the ink drop takes place. During a short time period the voltage difference will stay at this maximum voltage difference of 4x V.
Before a next actuation of the piezoelectric element 37, the piezoelectric element 37 needs to be discharged. To establish discharging of the piezoelectric element 37, the current from the current source 36 is altered into an opposite direction indicated by a second arrow 39 towards the switch 35. The process of discharging the piezoelectric element 37 is reversible with respect to the process of charging the piezoelectric element 37. After discharging is started, the voltage difference over the piezoelectric element 37 decreases. The voltage difference over the current source 36 is increasing and power is dissipated again. As soon as the voltage difference over the piezoelectric element 37 has decreased to a level of 3x V, the switch 35 is switched towards the fourth input terminal 35d. Since the fourth input terminal supplies a voltage of 3x V, the voltage difference over the current source 36 becomes approximately 0 V.
The switch 35 is further switched towards the third input terminal 35c, when the voltage difference over the piezoelectric element 37 has decreased to 2x V, towards the second input terminal 35b, when the voltage difference over the piezoelectric element 37 has decreased to x V, and finally towards the first input terminal 35a, when the voltage difference over the piezoelectric element 37 has decreased to 0 V.
By switching to an input terminal 35a, 35b, 35c, 35d, 35e which supplies a voltage which has a low voltage difference with the voltage present over the piezoelectric element 37, the voltage difference over the current source 36 remains below a level of the x V during charging and discharging. Thus the voltage difference over the current source 36 is limited such that the power dissipation during charging and discharging of the piezoelectric element 37 is significantly reduced. The calculation of the amount of power dissipation reduction during charging and discharging as described above is explained on the basis of
The current source 36 is connected between the switch and the piezoelectric element. In a known circuit, a voltage difference over a piezoelectric element at actuation time is applied at once onto a current source. This is illustrated in
b-4c show diagrams according to embodiments of the present invention, each diagram comprising a graph of the voltage level on a vertical axis against time on a horizontal axis, output through an output terminal of the switch 35 (see
b shows, according to an embodiment of the present invention, a graph of two voltage ladders 44, 45, each voltage ladder comprising two voltage level steps. The voltage at an output terminal of the switch 35 is represented by bold line 48 which follow in discrete steps a dashed trapezoidal curve 49. At the beginning of a first step, at a first point of time t0, a first voltage V1 is set, for example by switching the switching device to the third output terminal 35c. During the time period between the first point of time t0 and a second point of time t1 the piezoelectric element 37 is charged and the voltage difference over the piezoelectric element 37 increases from 0 V towards V1 V. At the beginning of a second step, at the second point of time t1 a second voltage Vmax is set, for example by switching the switching device to the fifth output terminal 35e. The first and second voltage are selected such that V1=½ Vmax. During the time period between the second point of time t1 and a third point of time t2 the piezoelectric element 37 is charged and the voltage difference over the piezoelectric element 37 increases from V1 V towards Vmax V. The dashed trapezoidal curve 49 represents the voltage over the piezoelectric element 37 during the actuation cycle. Since the total surface of the hatched areas 44a, 44b, 45a, 45b is a measure for power dissipation in the current source 36 during the actuation cycle, the power dissipation in the current source 36 is approximately halved in the case of two voltage level steps as may be calculated when comparing the total surface of the hatched areas 43a, 43b in
c illustrates an embodiment comparable to the embodiment illustrated in
In general, it may be easily calculated that the original amount of power dissipation as shown in
Another embodiment may be contemplated, which may be preferred over the embodiments as shown in
In general, it may be easily calculated that the original amount of power dissipation as shown in
In
In
In another embodiment, a control circuit comprising a plurality of switches, as shown in
Although the power supplies are connected in series according to
Another embodiment is shown in
b corresponds to
Before charging the piezoelectric element 77, the switch position of switch 75 is connected to output terminal 75a, which is connected to ground. The switch terminal 74 is also connected to ground. When the current source 76 is starting to charge the piezoelectric element 77 at a first point of time t0, the switch position is switched to output terminal 75b and a voltage of 20 V is supplied to the current source 76 and to the second terminal of the piezoelectric element 77. At the same first point of time t0, the switch terminal 74 is switched to the first power supply 73, which supplies a voltage of 10 V on the first terminal of the piezoelectric element 77. The voltage difference over the current source 76 is therefore less than or equal to 10 V during the time period from point of time t0 towards point of time t1.
As soon as the voltage over the piezoelectric element 77 reaches 10 V at point of time t1, the switch terminal 74 is connected to ground. The voltage over the piezoelectric element 77 increases to 20 V at the point of time t2. Again the voltage difference over the current source 76 is therefore less than or equal to 10 V during the time period from point of time t1 towards point of time t2.
As soon as the voltage over the piezoelectric element 77 reaches 20 V at point of time t2, the switch terminal 74 is connected to the first power supply 73 supplying a voltage of 10 V. At the same point of time t2, the switch position of the switch 75 is connected to output terminal 75c and a voltage of 40 V is supplied to the current source 76 and to the second terminal of the piezoelectric element 77. At point of time t3, the voltage over the piezoelectric element 77 reaches a level of 30 V. Again, the voltage difference over the current source 76 is therefore less than or equal to 10 V during the time period from point of time t2 towards point of time t3.
As soon as the voltage over the piezoelectric element 77 reaches 30 V at point of time t3, the switch terminal 74 is connected to ground. When the voltage over the piezoelectric element 77 reaches a voltage of 40 V at point of time t4, the piezoelectric element is actuated.
Discharging of the piezoelectric element 77 may be started at point of time t5 and controlled analogously to controlling the charging of the piezoelectric element 77 by switching at time moments t6, t7, t8, t9 corresponding to switching to subsequent voltage levels of 30 V, 20 V, 10 V and 0 V, respectively.
It is noted that during the charging of the piezoelectric element 77 the voltage difference over the current source 76 is less than or equal to 10 V. In this manner, a block voltage is supplied to the first terminal of the piezoelectric element 77. The surface of the hatched areas 86a, 86b, 86c, 86d, 87a, 87b, 87c, 87d is a measure of power dissipation of the current source 76. Power dissipation over the current source 76 is approximately halved again with respect to an embodiment in which the first terminal of the piezoelectric element is not connected to a switch terminal, but is directly connected to ground, in which case the voltage would follow a voltage line 81 during charging and a voltage line 82 during discharging.
An embodiment in which the switch terminal 74 and the first power supply 73 are situated between the second power supply 71 and ground and the first terminal of the piezoelectric element 77 is connected directly to ground may reach the same power dissipation reduction. Variations on the number of power supplies connected to either the first terminal or the second terminal of the piezoelectric element 77 may be evident to a skilled person. The first power supply 73 may be a common power supply in the case of a control circuit with a plurality of piezoelectric elements to be controlled.
The invention being thus described, it will be obvious that the same may be varied in many ways. Such variations are not to be regarded as a departure from the spirit and scope of the invention, and all such modifications as would be obvious to one skilled in the art are intended to be included within the scope of the following claims.
Number | Date | Country | Kind |
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08168333.6 | Nov 2008 | EP | regional |
This application is a Continuation of copending PCT International Application No. PCT/EP2009/063894 filed on Oct. 22, 2009, which designated the United States, and on which priority is claimed under 35 U.S.C. §120. This application also claims priority under 35 U.S.C. §119(a) on Patent Application No. 08168333.6, filed in Europe on Nov. 5, 2008. The entire contents of each of the above documents is hereby incorporated by reference into the present application.
Number | Date | Country | |
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Parent | PCT/EP2009/063894 | Oct 2009 | US |
Child | 13099520 | US |