Method of operating an on demand-type ink jet head and system therefor

Abstract: A method of operating an on demand-type ink jet head of the type comprising a pressure chamber (2) and a piezo-electric element (9) arranged to alter the volume of the chamber (2) in response to electrical signals applied thereto, comprising the steps of: applying a signal to the piezo-­ electric element (9) to increase the volume of the pressure chamber (2) and thereby causing ink (6) to be drawn into the chamber (2) from an ink source (7), and changing the signal at a time when the amplitude of vibration of a mechanical system formed by the piezo-electric element (9), a wall of the pressure chamber (5) and ink (6) within the pressure chamber (2) is near a maximum so that elastic energy stored in the said mechanical system is used to propel ink (6) out of a nozzle (3) and the pressure chamber (2).

Description: The present invention relates to a method of operating an on demand type ink jet head of the type which comprises a pressure chamber with an inlet which communicates with a supply of ink and an outlet which communicates with the atmosphere, and an electro-mechanical transducer which is arranged to alter the volume of the pressure chamber, comprising the steps of: applying an electrical signal to the electromechanical transducer so that the volume of the pressure chamber is increased whereby ink is drawn into the pressure chamber; and changing the said signal so that the volume of the pressure chamber is reduced whereby ink is propelled out of the said outlet. The present invention also relates to a system employing a head of the said type. A known method of driving an on demand type ink jet head is disclosed in U.S. Patent Specification No. 4,161,670. This head comprises a tubular drive element of polarised ceramic which contains printing ink and whose diameter is altered when a voltage is applied thereto. The tubular drive element thus forms a pressure chamber. In this known method a voltage of polarity opposite to that of the polarisation voltage of the piezo-electric ceramic element is applied to the element to deform, or maintain, the wall of the pressure chamber so that the volume of the pressure chamber is increased for a predetermined period of time after which the polarity of the voltage supplied to the piezo-electric element is reversed so that the volume of the pressure chamber is reduced and ink droplets are thereby jetted out of the tube. A voltage transducer is used to reverse the polarity of the voltage of the signal applied to the piezoelectric element, the secondary inductance of the voltage transducer forming an oscillatory circuit with a capacitance of the piezo-electric element. The resonance frequency of this oscillatory circuit is set equal to the mechanical resonance frequency of the column of ink in the drive tube and the duration of the primary current path applied to the voltage transducer is equal to half the period of the mechanical resonance frequency. In other words, the resonance frequency of the oscillatory circuit, constituted by the secondary inductance of the voltage converted and the capacitance of the piezo-electric element, is equivalent to the resonance frequency of the column of ink in the pressure chamber. For implementing such a driving method, a separate voltage transducer and control circuit are required for each nozzle. Therefore, in the case of a multi-nozzle ink jet head the total cost of the assembly is high as it is necessary to provide as many voltage transducers and control circuits as there are nozzles. In order to maximise the velocity at which ink droplets are propelled from an ink jet head whilst applying a relatively low voltage to the piezo-electric element, the duration of the primary current pulse should not simply be set equal to half the period of the resonance frequency of the column of ink for the following reasons. The oscillation of the column of ink is a transient response to the primary current pulse applied to the voltage transducer in a system which is formed by the wall of the pressure chamber, the piezo-electric element and the ink, and accordingly, the oscillation is a damped oscillation involving a phase lag related to the driving waveform applied to the piezo-electric element. Therefore, the time instant at which the volume of the pressure chamber should be decreased by changing the voltage applied to the piezo-electric element should be selected to occur in synchronisation with the phase of the damped oscillation and the phase lag of the column of ink so as to maximise the velocity of the ink droplets. In other words, when the duration of the primary current pulse applied to the voltage transducer is equal to half the period of the resonant frequency of the column of ink, it should coincide with the optimum phase of the damped oscillation of the column of ink in the pressure chamber and the nozzle of the chamber, so that ink droplets can be propelled from the head by application of low voltage signals to the piezoelectric element. Experiments have confirmed that the duration of the current pulse referred to above should preferably be longer than half of the period of the natural frequency of the column of ink. One advantage of decreasing the voltage required to jet ink from the head is that depolarisation of the piezoelectric element is reduced in circumstances where the voltage applied to the element has a polarity opposite to that of the polarisation voltage of the piezoelectric element. According to one aspect of the present invention there is provided a method of operating an on-demand type ink jet head, which comprises a pressure chamber with an inlet which communicates with a supply of ink and an outlet which communicates with the atmosphere, and an electro-mechanical transducer which is arranged in response to an electrical signal to alter the volume of the pressure chamber, the said method comprising the steps of applying a signal to the transducer so that the latter is in a stand-by condition in which the pressure chamber is in a contracted stand-by state; altering the said signal to the electro-mechanical transducer so that the volume of the pressure chamber is increased whereby ink is drawn into the pressure chamber; and changing the said signal so that the volume of the pressure chamber is reduced, whereby ink is propelled out of the said outlet;'the said signal being so changed at a time when the amplitude of oscillation of a mechanical system formed by the transducer, at least part of the pressure chamber and ink within the pressure chamber is substantially at a maximum, characterised in that, in order to draw in the ink, the voltage of the said signal is reduced so that the volume of the pressure chamber is increased to substantially its maximum value by means of oscillation of the said mechanical system; and the voltage of the said signal is directly increased at the said time to its value in the said stand-by condition without changing its polarity. Preferably, the said time occurs at a point which is in excess of half of the period of the signal, the waveform of the signal being such that the portion of the waveform corresponding to the reduction of the applied voltage is more gently curved than the portion of the waveform corresponding to the increase of the applied voltage. The said signal is preferably applied to the transducer by signal supply means which comprises a transistor drive circuit, the transducer comprising a piezo-electric element which is connected in parallel with a resistor, the parallel arrangement being connected to the collector of an output transistor of the said circuit. According to another aspect of the present invention, there is provided an on demand type ink jet head system comprising: an ink jet head having a pressure chamber with an inlet which communicates with a supply of ink and an outlet which communicates with the atmosphere; an electro-mechanical transducer which is arranged in response to an electrical signal to alter the volume of the pressure chamber; and means for supplying the said electrical signal to the electro-mechanical transducer; the signal supplying means being arranged, in use, to apply a signal to the transducer so that the latter is in a stand-by condition in which the pressure chamber is in a contracted stand-by state; to alter the said electrical signal to the electro-mechanical transducer so that the volume of the pressure chamber is increased with respect to its volume in the stand-by state, whereby ink is drawn into the pressure chamber; and to change the said signal so that the volume of the pressure chamber is reduced whereby ink is propelled out of the said outlet, the said signal supplying means being arranged, in use, to so change the said signal at a time when the amplitude of oscillation of a mechanical system formed by the transducer, at least part of the pressure chamber and ink within the pressure chamber is substantially at a maximum characterised in that the signal supplying means is arranged, in use, to reduce the voltage of the said signal so that the volume of the pressure chamber is increased substantially to its maximum value by means of oscillation of the said mechanical system in order to draw in the ink, and the voltage of the said signal is directly increased at the said time to its value in the said stand-by condition without changing its polarity. The present invention seeks to provide a method of operating an on-demand ink head in which the damped oscillation of the mechanical system formed by the piezo-electric element, the wall of the pressure chamber, and ink in the pressure chamber is utilized so that the ink jet head can be driven with a drive circuit of simple construction and low manufacturing costs, and so that a low drive voltage can be used to propel ink droplets out of the pressure chamber at the desired velocity. The present invention will be described, merely by way of example, with reference to the accompanying drawings, in which:- Figure 1 is a sectional side view of an ink jet head of the type to which the present is applied, Figure 2 is a top view, part cut away, of the ink jet head of Figure 1, Figure 3 is a circuit diagram showing a drive circuit for operating an ink jet head in accordance with the present invention, Figure 4A is a timing diagram showing an input signal to the drive circuit of Figure 3, Figure 4B is a timing diagram showing the waveform of a voltage across a piezo-electric element of an ink jet head such as that shown in Figures 1 and 2, Figure 5A is a sectional side view showing the ink jet head shown in Figures 1 and 2 when ink has been drawn into a pressure chamber in accordance with one method of the present invention, Figure 5B is a top view, part cut away, of the ink jet head of Figure 5A, Figure 6 is a sectional side view of the ink jet head of Figure 5A whilst jetting ink droplets, Figure 7A is a timing diagram showing a voltage across a piezo-electric element of an ink jet head such as that described above, Figure 7B is a timing diagram showing the damped oscillation of a wall of a pressure chamber and a piezo-electric element in an ink jet head such as that described above, Figure 7C is a timing diagram showing variations with time of the flow rate of air which is drawn in through the opening of the nozzle during the oscillation shown in Figure 7B, and Figure 7D is a diagram showing variations in velocity of ink droplets propelled from an ink jet head such as that described above with the variations of a pulse width T. The ink jet head shown in Figures 1 and 2 comprises a pressure chamber 2 having a nozzle 3 and a supply port 4. These are formed by recesses in a substrate 1. Ink 6 from an ink container 7 is introduced to the pressure chamber 2 through an ink supply tube 8 and the supply port 4 which forms a narrow path to the pressure chamber 2. In the opening, or mouth, 3 a of the nozzle 3, the surface tension of ink 6 balances with the negative pressure H between ink in the pressure chamber and ink in the container 7 so that ink does not leak or flow out of the nozzle 3. An electrode layer, or surface, 5 a is formed on a wall 5 of the pressure chamber 2 by means of a vacuum evaporation technique or the like. A piezo-electric element 9, which acts as an electro-mechanical transducer, is bonded to the electrode layer 5 a of the wall 5 in alignment with the pressure chamber 2 on the other side of the wall 5. Lead wires 10 are connected to the piezo-electric element 9 and the electrode layer 5 a . The polarities of the lead wires 10 are selected so that the piezo-electric element 9 contracts to cause the wall 5 to cave-in in such a manner that the wall 5 becomes substantially concave, thereby decreasing the volume of the pressure chamber 2. That is, the voltage applied to the piezo-electric element 9 is of the same polarity as that of the polarization voltage of the piezo-electric element 9. Figure 3 shows a drive circuit for supplying electrical pulses to the piezoelectric element 9. Figure 4A shows the waveform of an input signal 16 applied to the drive circuit. Before a time instant t₁ (see Figure 4B), a transistor 11 and a transistor 12 are rendered conductive (ON) as a result of which current flows in the direction of the arrow A to charge the piezoelectric element 9. The wall 5 of the pressure chamber 2 is, therefore, held in the concave position as shown in Figure 1. In this operation, the current flows through a charging resistor 13. The waveform of the voltage 19 applied to the piezo-electric element 9 is shown in Figure 48. At the time instant t₁, the input signal 16 rises, as shown by the reference numeral 17. In response thereto, a transistor 14 is rendered non-conductive (OFF) while a transistor 15 is rendered conductive (ON). As a result, the charge stored in the piezo-electric element 9 flows as a current in the direction of the arrow B through the transistor 15 and the resistor 13. The waveform of the voltage 19 across the piezo-electric element 9 is shown in Figure 4B, and this voltage corresponds to the voltage between the circuit points indicated by reference numeral 24 in Figure 3. When the input signal 16 falls (as shown by the reference numeral 18), at a time t₂, the transistors 14 and 15 are turned ON and OFF, and the transistors 11 and 12 are turned ON, causing an instantaneous current in the direction of the arrow A as a result of which the piezo-electric element 9 is charged. The voltage 24 across the piezoelectric element 9 thus becomes substantially equal to the source voltage 25. The mechanical operation which accompanies the above-described electrical operation will be described with reference to Figures 1, 5A, 5B and 6. As mentioned above, since the transistors 11 and 12 are conductive when the power source is connected to the circuit in Figure 3, the current flows in the direction of the arrow A and the piezo-electric element 9 begins to charge. When the charging is complete, the voltage 24 across the element 9 becomes substantially equal to the source voltage 25 and is held at this level. Therefore, the element 9 is held in the concave position so that the wall 5 of the pressure chamber 2 is also held in the concave state, as shown in Figure 1. At the time instant t₁, the piezo-electric element 9 starts to discharge and it is restored to its original state by the elastic energy stored in the wall 5 and the element 9. During this operation, ink 6 from the ink container 7 is drawn in through the supply port 4 to the pressure chamber 2 and air is drawn in through the opening 3 a in the nozzle 3. As a result of this, a state such as that shown in Figures 5A and 5B is reached. The time instant t₂ is set so that it occurs when the amount of air (shown by reference numeral 20) drawn into the nozzle is approximately at a maximum. By applying the voltage across the piezo-electric element 9 again at the time instant t₂, the piezo-electric element 9 is charged almost instantly and the element 9 is quickly deformed, as shown in Figure 6, so that ink 6 is propelled out of the opening 3 a in the nozzle 3 in the form of ink droplets 21. The manner in which the timing of the application of voltages to the piezo-electric element 9 are chosen will be described with reference to Figures 7 a to 7 d . If the pulse interval T between the time instants t₁ and t₂ is set to be relatively long, as indicated in Figure 7A, the wall 5 and the piezo-electric element 9 undergo damped oscillation 23 as indicated in Figure 7B. The damped oscillation 23 can be closely represented by the following expression:- X = 1 - βe -nt sin(ωt - ϑ), where X is the displacement of the wall 5 and the piezo-electric element 9 in the direction indicated in Figure 5, X = 0 represents the displacement of the wall 5 and the element 9 when the pulse width T is infinitely long, i.e., when no voltage is applied to the piezo-electric element 9, and X =-1 represents the displacement thereof when a voltage is applied to the piezo-electric element 9, t represents time, with the time instant t₁ representing zero time or the reference time, and β, n, ω and ϑ are constants which are determined by the elastic coefficients and internal resistances of the wall 5 and the piezo-electric element 9, the fluid mass, or impedance, in the vicinity of the nozzle 3 and the supply port 4, and the surface tension of the ink in the opening 3 a of the nozzle 3. Although the wall 5 and the piezo-electric element 9 reach the position at which X = 0 during the time period from t₁ to t₂ in which the volume of the pressure chamber 2 is increased the wall 5 and the element 9 undergo damped oscillation with reference to the position at which X = 0 as shown in Figure 78. The damped oscillation 23 is the transient resonance of a mechanical oscillation system formed by the piezo-electric element 9, the wall 5, and the ink in the pressure chamber 2 when a voltage having a waveform such as that shown in Figure 7A is applied to the piezo-electric element 9. The damped oscillation involves a time delay which is represented by the constant ϑ in the expression given above. As the wall 5 and the piezo-electric element 9 undergo the damped oscillation 23 described above, ink in the vicinity of the nozzle 3 undergoes a similar oscillatory movement. This can be observed through the variations with time of the amount 20 of air drawn in through the opening 3 a of the nozzle 3 as indicated in Figures 5A and 5B. The amount of air drawn-in undergoes a damped oscillation 22 as indicated in Figure 7C before the flow of air stops. At a time instant t₃ the amount of air drawn in is at a maximum and this substantially coincides with the time instant when the displacement X of the piezo-electric element 9 also reaches its maximum value 27 (see Figure 7B). If the supply voltage 25 (see Figure 3) applied to the piezo-electric element 9 is set at a predetermined value while the pulse width T in Figure 7A is gradually reduced, a plot can be made of the corresponding variations in the velocity at which ink droplets are propelled from the nozzle 3. The velocity curve 26 obtained is shown in Figure 7D. As Figure 7D shows, if the pulse width T is long, no ink droplets 21 are propelled from the nozzle 3. However, if the pulse width T is set near the time period (t₃ - t₀), so that the time instant t₂ at which the piezoelectric element 9 is re-charged substantially coincides with the time instant t₃, ink droplets 21 are propelled from the nozzle 3. The velocity of the ink droplets reaches a maximum when the pulse width T is set approximately, or slightly longer than, to the time period (t₃ - t₀). If the supply voltage 25 (Figure 3), which is relatively low, is applied to the piezo-electric element 9 when the damped oscillation 23 of the wall 5 and the piezo-electric element is at the point 33, or point 34 where X = 0, i.e. the pulse width T is shortened or increased, then the wall 5 and the piezo-electric element 9 are not returned to the position represented by X =-1 at a velocity which is sufficient to propel ink droplets from the nozzle 3. However, if the voltage is applied to the piezo-electric element 9 approximately at the time instant t₃, then the transition represented by X =-1 is such that after the time instant t₃ the energy of the damped oscillation 23, which is causing the volume of the pressure chamber 2 to decrease, is added to the energy supplied by the piezoelectric element to return the wall 5 to the position represented by X =-1. Accordingly, the wall 5 and the piezo-electric element 9 move to the position represented by X =-1 at a higher velocity and ink droplets are propelled from the nozzle 3. The pulse width T is thus set in accordance with the period of the damped oscillation 23 which occurs when the ink 6 is drawn into the pressure chamber as described above so that ink droplets are jetted at a desired predetermined velocity by the application of a low voltage to the piezo-electric element 9. It should be noted that since there is no damped oscillation 23 at the time when the power source is initially connected, no ink droplets are jetted from the pressure chamber 2 even when the wall 5 is deformed so as to reduce the volume of the pressure chamber 2. After ink droplets 21 have been jetted from the pressure chamber, the damped oscillation of the oscillation system composed of the wall 5, the piezo-electric element 9 and the ink 6 settles to the rest position because of the loss of ink from the nozzle 3 and the return of ink to the supply port 4. The next jetting of the ink droplets 21 is not, therefore, greatly affected by the damped oscillation of the previous jetting, so that the frequency response of the device is satisfactory. As described above, a voltage having the same polarity as that of the polarization voltage of the piezo-electric element 9 is initially applied to the piezo-electric element 9 in response to which the wall 5 is held displaced in the concave position, thereby decreasing the volume of the pressure chamber 2. In this case, application of the voltage to the piezo-electric element 9 is suspended when a printing operation is required, so that the volume of the pressure chamber 2 is abruptly increased whereby ink is drawn into the pressure chamber 2. The voltage is then applied again approximately at the time when the damped oscillation of the oscillation system composed of the piezoelectric element 9, the wall 5, and the ink 6 reaches its peak value 27, which occurs when the flow rate of ink 6 is drawn-ink is also approximately at a maximum. Accordingly, the droplets 21 can be jetted with a low voltage. The damped oscillation 23, being the transient response of the piezo-electric element 9, essentially involves a delay of time. Therefore, in order to ensure that the device operates efficiently, it is desirable that the pulse width T is set to end substantially at the time of occurence of the maximum value 27 of the oscillation 23. Accordingly, even if the pulse width T is set equal to half of the period of the resonace frequency of the mechanical system formed by the piezo-electric element 9, the wall 5 and the ink 6, a satisfactory operation can be provided so long as the period T ends at a time when the amplitude of the oscillation of the mechanical system is substantially at a maximum because of the time delay involved. With the method described above, which uses the properties of the damped oscillation 23, it is possible to drive the ink jet head highly efficiently. Since the polarity of the voltage to be applied to the piezo-electric element 9 is the same as that of the polarization voltage of the piezo-electric element 9, depolarisation of the element is avoided. Furthermore, since the voltage applied to the element 9 is always of the same polarity, the drive circuit can be considerably simplified and, thus, is not expensive to make. As described above, the ink jet head can be driven in a highly efficient manner by simply selecting a suitable pulse width T. Therefore, even if the oscillation system composed of the piezo-electric element, the wall 5, and the ink 6 is varied, and the time period at which the damped oscillation 23 has its peak value 27 is varied, it is still possible to efficiently drive the element by appropriately altering the pulse width T. This is an important advantage of the present invention over a device using a voltage converter which requires a complicated procedure for changing the primary winding and the secondary winding of the voltage converter in order to allow for such variations. In the method described above, the piezoelectric element 9 is initially deformed so that the volume of the pressure chamber 2 is decreased and it is then returned to its original state to draw in ink, after which the volume is again decreased in order to propel the ink out of the head.

Claims: A method of operating an on-demand type ink jet head, which comprises a pressure chamber (2) with an inlet (4) which communicates with a supply of ink (6) and an outlet (3) which communicates with the atmosphere, and an electro-mechanical transducer (9) which is arranged in response to an electrical signal to alter the volume of the pressure chamber (2), the said method comprising the steps of applying a signal to the transducer (9) so that the latter is in a stand-by condition in which the pressure chamber (2) is in a contracted stand-by state; altering the said signal to the electro-mechanical transducer (9) so that the volume of the pressure chamber (2) is increased, whereby ink is drawn into the pressure chamber (2); and changing the said signal so that the volume of the pressure chamber (2) is reduced, whereby ink (6) is propelled out of the said outlet(3); the said signal being so changed at a time when the amplitude of oscillation of a mechanical system formed by the transducer (9), at least part (5) of the pressure chamber and ink (6) within the pressure chamber (2) is substantially at a maximum, characterised in that, in order to draw in the ink, the voltage of the said signal is reduced so that the volume of the pressure chamber (2) is increased to substantially its maximum value by means of oscillation of the said mechanical system (9,5,6); and the voltage of the said signal is directly increased at the said time to its value in the said stand-by condition without changing its polarity. A method as claimed in claim 1 characterised in that the said time occurs at a point which is in excess of half of the period of the signal, the waveform of the signal being such that the portion of the waveform corresponding to the reduction of the applied voltage is more gently curved than the portion of the waveform corresponding to the increase of the applied voltage. A method as claimed in claim 1 or 2 characterised in that the said signal is applied to the transducer (9) by signal supply means which comprises a transistor drive circuit, the transducer comprising a piezoelectric element (9) which is connected in parallel with a resistor (13), the parallel arrangement (9,13) being connected to the collector of an output transistor (12) of the said circuit. An on demand type ink jet head system comprising an ink jet head having a pressure chamber (2) with an inlet (4) which communciates with a supply of ink (6) and an outlet (3) which communicates with the atmosphere; an electro-mechanical transducer (9) which is arranged in response to an electrical signal to alter the volume of the pressure chamber (2); and means for supplying the said electrical signal to the electro-mechanical transducer (9); the signal supplying means being arranged, in use, to.apply a signal to the transducer (9) so that the latter is in a stand-by condition, in which the pressure chamber (2) is in a contracted stand-by state; to alter the said electrical signal to the electro-mechanical transducer (9) so that the volume of the-pressure chamber (2) is increased with respect to its volume in the stand-by state, whereby ink (6) is drawn into the pressure chamber (2); and to change the said signal so that the volume of the pressure chamber (2) is reduced,whereby ink (6) is propelled out of the said outlet (3), the said signal supplying means being arranged, in use, to so change the said signal at a time when the amplitude of oscillation of a mechanical system formed by the transducer (9), at least part (5) of the pressure chamber (2) and ink (6) within the pressure chamber (2) is substantially at a maximum characterised in that the signal supplying means is arranged,in use, to reduce the voltage of the said signal so that the volume of the pressure chamber (2) is increased substantially to its maximum value by means of oscillation of the said mechanical system (9,5,6) in order to draw in the ink, and the voltage of the said signal is directly increased at the said time to its value in the said stand-by condition without changing its polarity.

Inventors: HANAOKA SEIJI, HANAOKA, SEIJI
Applicants: EPSON CORP, SUWA SEIKOSHA KK, EPSON CORPORATION, KABUSHIKI KAISHA SUWA SEIKOSHA
Application year: 1981
Main classification: B41J2
IPC codes: B41J2