ELECTRONIC CIRCUIT FOR HIGHLY EFFICIENT DRIVING OF PIEZOELECTRIC LOADS

Abstract: The invention relates to a circuit (1) for highly efficient driving of piezoelectric loads (2), comprising a linear driving circuit portion (3) connected to the load (2) through an inductive-resistive connection whereto a voltage waveform is applied. Advantageously, the circuit comprises further respective circuit portions (4, 5), structurally independent, connected in turn to the inductive-resistive connection through respective inductors (L1, L2) to supply a considerable fraction of the overall current required by the load (2) in the transient and steady state respectively.

Description: ELECTRONIC CIRCUIT FOR HIGHLY EFFICIENT DRIVING OF PIEZOELECTRIC LOADS DESCRIPTION Technical Field The present invention relates to an electronic circuit for highly efficient driving of piezoelectric loads, comprising a driver circuit portion connected to at least one load terminal by means of an inductive-resistive connection whereon a voltage waveform is applied. The invention relates also to a method for driving a piezoelectric load with inductive-resistive connection. The invention relates particularly, but not exclusively, to a driver circuit adapted to follow the current profile required by a piezoelectric printer head connected to the driver circuit by means of an inductive and resistive cable referred to as flat cable and similar to a resistance and to a series inductor. The following description is given with reference to this field of application simply for convenience of illustration. Background Art As is well known in this specific technical field, a wide range of transducers is provided, such as those described in the US patent No. 5,895,998, whose electric feature is similar to a piezoelectric load. Among said transducers it is possible to include also some types of printer heads. The head driving is performed by applying voltage waveforms generally formed by a series of ramps having a predetermined slew-rate. An example of such an application is disclosed in the US patent no. 4,767,959 in the name of Nippondenso Co. The special accuracy required for applying said voltage waveform to the load terminals, along with the high frequency of the driver signal, leads to the use of linear- mode driver circuits, involving high power dissipation. Moreover, the presence of a parasitic inductance in the flat cable for connection to the load, and consequently connected in series to the real capacitive load, allows the current required by the load to be filtered. The profile of this current is not rectangular and has a beveled pattern with over- and under-elongations, as shown in the attached figure 1. Therefore, with respect to an ideal case of merely capacitive load, it is necessary to manage adequately a current slew-rate at the ramp base and a current queue at the ramp end. The features of these fronts depend on the parasite parameters inserted by the flat cable. The structure conventionally used to apply a voltage waveform to a piezoelectric load with an inductive-resistive connection is shown in figure 1 which illustrates a linear driving example. In practise, the driver circuit of figure 1 comprises an operational amplifier comprising a power output stage sufficient for load driving. The piezoelectric load is substantially a non-dissipating capacitive load and the whole electric power is dissipated on the transistors incorporated in the linear driving stage. The linear driving solution is not particularly effective because of the considerable electric power dissipation. The technical problem underlying the present invention is to provide a driver circuit, particularly for piezoelectric loads, with such functional and structural features to allow an higlly efficient load driving without reducing the quality of the waveform generated at the load terminals. Disclosure of Invention The solution idea underlying the present invention is to provide a driver circuit with further driver portions if compared to the linear portions of the prior art, each further portion having to supply as much current as possible during both the transient and the steady condition. The linear driving stage is charged to supply a residual current portion required for following precisely the reference signal. In this manner it is possible to supply the highest amount of current required by the load avoiding in the meantime too frequent switching, which would be required in case of switching-mode driving. Based on this solution idea, the technical problem is solved by a driver circuit as previously described and defined in the characterizing portion of Claim 1. The invention relates also to a method for driving a piezoelectric load with inductive-resistive connection and wherein at least a linear driving of said load is provided by means of a driver circuit equipped with a linear circuit portion connected to the load by means of said connection, the method being characterised in that it provides two further different driving modes, in the transient and in the steady state, wherein further respective circuit portions, which are structurally independent and connected in turn to said connection by means of respective inductors, supply a considerable portion of the overall current required by the load. The characteristics and advantages of the circuit and method according to the invention will be apparent from the following description of an embodiment thereof given by way of non limiting example with reference to the attached drawings. Brief Description of Drawings Figure 1 is a schematic view of a piezoelectric load driver circuit according to the prior art; Figure 2 is a schematic view of a piezoelectric load driver circuit according to an embodiment of the invention; Figures 3, 4 and 5 are respective schematic views of the circuit of figure 2 in three different operating phases; Figure 6 is a diagram comparing the waveform of the current absorbed by the load with the voltage applied at the load terminals; Figure 7 is a current vs. time diagram for current signals present in the circuit of figure 2; Figure 8 is a schematic view of a different embodiment of the circuit of figure 2; Figures 9, 10, 11 and 12 are respective current vs. time diagrams indicating some time periods of activation of the circuit of the invention according to the driving method of the invention; Figures 13 and 14 are respective schematic views of circuit portions accompanying the circuit according to the invention. Modes for Carrying Out the Invention With reference to the drawings, and particularly to the example of figure 2, a driver circuit according to the present invention for driving a piezoelectric load 2 is generally and schematically indicated with 1. The circuit 1 according to the invention comprises an operational amplifier 3 having the output in feedback to its inverting input (-) and further connected to one terminal of the load 2, in a node X, to supply a current IA MP . The other non inverting (+) input of the amplifier 3 receives a reference signal from an input terminal IN of the circuit 1. This amplifier 3 can be considered as the core of the linear portion of the circuit 1. Advantageously, the circuit 1 according to the invention comprises at least a half-bridge circuit portion, including at least a switching device, connected to the node X through an inductance. More particularly, the circuit 1 comprises respective half-bridge circuit portions 4, 5, each portion being connected to the node X through a corresponding inductor LI, L2. The portions 4, 5 are structurally independent one from the other. The inductors LI and L2 have preferably different value, being however possible to use inductors having the same value. The half-bridge circuit portion 4, indicated with LF, is associated to the inductor LI with higher value. Viceversa, the half-bridge circuit portion 5, indicated with HF, is associated to the inductor L2 with lower value. A control block 7 is provided to drive the half-bridge circuit portions 4 and 5. Briefly, the invention provides a circuit diagram aiming at supplying the highest amount of current required by the load 2 by means of the two half- bridges 4 and 5, avoiding in the meantime too frequent switching. The linear portion 3 must therefore supply the current difference I AM P that is required to follow precisely the voltage reference signal, formed for instance by a series of predetermined slew-rate ramps. Advantageously, the half-bridge circuit portions 4 and 5 are therefore provided to supply with high efficiency, in the transient and in the steady state respectively, a considerable portion of the overall current required by the load 2, whilst the linear circuit portion 3 ensures the accuracy of the voltage waveform by supplying only the current difference IAMP, with considerable power saving. As just mentioned, in order to fill the load 2 current demands, the half-bridge portion LF comprises at least a switching device. More particularly, a pair of transistors Ml, M2 are interconnected together in a node XI. The half-bridge LF is powered between a first supply voltage reference V ALIM and a second ground reference GND. The inductor LI is inserted between the nodes XI and X. In the preferred embodiment herein described by way of non limiting example the half-bridge LF comprises MOS power transistors, it is also possible to use a bipolar transistor bridge or half-bridge. The control terminals of the half-bridge LF transistors Ml and M2 are all connected to the control block 7. The control block 7 acts on the transistors Ml and M2 to obtain a current profile as close as possible to the required profile. For this reason, the control block 7 requires information about the duration, the ramp slew-rate and the load CLOAD value. In a similar and symmetrical manner, the half-bridge HF comprises a pair of transistors M3, M4 interconnected together in a node X2. The half-bridge HF is powered, in turn, between the first supply voltage reference VALIM and the second ground reference GND. The inductor L2 is inserted between the nodes X2 and X. The control terminals of the half-bridge HF transistors M3 and M4 are all connected to a respective output of the control block 7. The use of two half-bridges 4 and 5 allows, by an appropriate strategy for closing the transistor switches incorporated therein, a good approximation of the profile of the signal in voltage applied to the load. The lower value inductor L2 is suitably sized as to be able to follow the initial transient and the current final queue. The half-bridge 4 with higher value inductor LI plays the function of following the waveform supplying the steady state current value without the need for too frequent switching which would be instead required by the use, during this phase, of the other half- bridge 5. The control strategy is highly important since it determines the efficiency which can be obtained in terms of power dissipation, as well as the switching frequency of half-bridge switches. The control method according to the invention is based on the measure of the current IAMP outputted by the operational 3 and is implemented by dividing into three phases the piezoelectric load charge period. These three method phases will now be described with reference to the schematic figures 3, 4 and 5. The diagram of the circuit 1 shown in these figures 3, 4 and 5 has been modified to use the current IAMP outputted by the linear portion 3 as variable. A sensor 8 is located downstream of the portion 3, upstream of the node X and connected to the control block 7 to detect the current IAMP value. The control block 7 comprises basically a logic interface coupled with the current sensor 8, a digital- technology logic network having analogue output stages connected to the control terminals of the half-bridge 4 and 5 transistors. Depending on the value of the current IAMP, the half-bridge devices are switched according to appropriate control strategies described hereinafter. Steady state transient, Tl: During this phase, the switch Ml of the half- bridge LF is closed for the time needed by the current on the inductor LI to reach the value Io required in the steady condition by the load 2. In this phase, the switch M3 of the half- bridge HF is conveniently switched so that the current injected by the system on the load approximates at best that demand, with the aim, once again, of minimising the amount of current supplied by the linear portion 3. The control flow is represented by the dot- line 9. Steady state, T2: Once the current value on the inductor LI has reached the steady state value Io, the half-bridge HF is deactivated, or the switch M3 is open, while the half-bridge LF, through the switch Ml control, provides to keep an output current close to the current required. The control flow is represented by the dot- line 11. Fall phase, T3: during this phase, it is necessary to shut the half-bridge LF off so that the inductor LI is not charged at a current value other than zero when the current is no more required by the load 2, current which would be otherwise absorbed by the linear stage in a dissipating manner. During this phase, the half-bridge HF is activated and, for sizing the components thereof, it succeeds at best to follow fast- changing signals having faster charge and discharge transistors. The control flow is represented by the dot- line 12. The systems substantially splits the current to be generated into three distinct parts: - two current peaks supplied by the half-bridge HF in correspondence with the fronts ILOAD; - most of the current in the central portion, in the steady state, supplied by the half-bridge LF; - a corrective current supplied by the linear portion 3. Figure 6 shows the patterns of the currents injected by the two half-bridges, ILF for the bridge LF and IHF for the bridge HF, of the current IAMP supplied by the operational and of ILOAD required by the load 2. For implementing the control method according to the invention, two procedures are substantially used: a feedback and a feedforward procedure. The times Tl and T3 are derived analytically and their assessment can therefore be performed in feedforward mode once the inductor LI value, the voltage supply VALIM, the load value CLOAD, the slew-rate and the initial and final ramp voltages are known. The feedback variable used in the diagram is represented, on the contrary, by the current I AMP outputted by the linear stage; depending on this current IAMP value, the stage LF is switched during the phase T2 and the stage HF during the phases Tl and T3, according to the criteria described hereinafter. The feedback variable I AMP may be controlled in several ways. For example, the transistor Ml can be closed at predetermined times and opened again when the current IAMP exceeds a predetermined threshold. A different control provides the use of an hysteresis; for example, when the current outputted by the linear portion 3 exceeds an appropriate threshold IHIGH, the switch M 1 or M3 of the half-bridge LF or HF connected to the supply is closed and, consequently, the current 5 outputted by the non-linear portion increases, whereas the current IAMP of the linear portion decreases until the latter reaches a lower threshold ILOW at which the switch Ml or M3 is opened, thus repeating the cycle. In this way the hysteresis type of control is: IHYST = IHIGH - ILOW .0 The outcome of this control and the corresponding waveforms are shown in figure 7 for a time period indicated with T2 at which the feedback variable acts on the half-bridge LF. During the times indicated with T* the switch M l is closed since the operational 3 current is lower than the value ILOW. This switch Ml is closed until the threshold IHIGH is .5 reached. The IHYST value choice is a compromise-choice. A small value involves a high half-bridge switching speed whereas a high value determines a higher dissipation by the linear portion. The previous comments are based on the use of a single ! 0 feedback variable acting on the two half- bridges LF and HF in time- distinct phases. Figure 8 shows schematically a different embodiment illustrating the possibility of measuring a second quantity, besides the linear portion current IAMP. At this purpose, for example, a second current sensor 9 !5 inserted between the inductor L2 and the node X is provided. Therefore, if another quantity is measurable through the second sensor 9, this quantity can be so used as to control in every instant both half-bridges. Knowing, for example, the current outputted by the half-bridge HF, IAMP could be used as variable for controlling the half- S 0 bridge HF switchings during the whole period and the sum IAMP + IHF (equal to ILOAD - ILF) to drive the LF stage. The way for determining the three times Tl, T2, T3 in an analytical manner will now be described. The following formulas, obtained analytically, allow a real time calculation of said times during the load 2 control phase to be performed. A different method consists in formulating in a discrete manner the differential equation which determines the time pattern of the inductor current. A finite difference equation is therefore obtained in which the current value in a precise instant is given by the sum of the value in the previous instant plus an increase depending on the voltage present at the inductor terminals. Through simple adding and accumulating circuit blocks it is possible to calculate the inductor current and assess (as clock stroke number) the time required by the current to reach the steady state value Io, thus obtaining Tl . Likewise, it is possible to calculate T3 by inverting the time scale: if the inductor starts from a current value equal to zero and reaches the steady state value Io, by using a voltage signal which is time-inverted with respect to the signal required it is possible to obtain a transient having the same duration as the discharge transient T3; in this case too, simple adding and accumulating circuit blocks are sufficient for the assessment. Figures 13 and 14 show respective embodiments of circuit networks comprising adding and accumulating blocks which can be used for the above purposes. Figure 13 shows a circuit for calculating the time Tl . The current In value is compared with the current Io value, Tl being reached when these values coincide. Figure 14 shows a circuit for calculating the time T3. The current In value is compared with the current Io value, T3 being reached when these values coincide. From the control point of view, four different situations shown in figures 9, 10, 11 and 12 can occur. The cases shown in figures 10 and 11 require a special control. In the case of figure 10, the time T3 is known later than the moment of its use for shutting the half-bridge LF off. This occurs because the algorithm does not produce the desired results in the instant of their application. In the case of figure 11, the current is so high that the sum of times Tl and T3 is higher than the ramp duration, the assessment thereof being therefore useless for control purposes. In both cases, it is useful to know a time Tx defined as the instant of intersection between the charge and discharge currents of the inductor LI : this time Tx allows the inductor LI charge to be interrupted so that no residual current is present therein at the ramp end, that current being otherwise recovered by the linear stage, with subsequent detrimental power dissipation. Steady state rise time for the inductor Li, i.e. the time required by Li to reach the current theoretic value (SRC): rise ramp: t x = γ fall ramp: t 1 = — IL ■ 2LC SR SR Discharge time required to let the inductor Li have no residual current at the ramp end: rise ramp: t , 3 = • v D + v F v D +v F y - 2EC SR SR r n < ■ V ΛTJIMM " F fall ramp: t AL * D + "ALIM * F I "3 = ■ 2EC SR SR where VD = direct voltage on the fedback diode; VF = final voltage; Vo = initial voltage; L = inductor Li; C = overall load capacity; SR = voltage ramp slew rate; VALIM = supply voltage TIME Tl CALCULATION: At the inductor LI terminals (L indicating the inductor LI value) the following differential equation applies: V r L =v r ALM -V y C =L- — J f by solving it in a discrete form, it becomes Δi V mM - V c ' CL by solving it in a discrete form also the voltage at the capacitive load terminals: V C =SR t + V n V c =SR-n-T CL +V 0 V -V where SR= F ° l ON V -V SR- F ° n ON ' ■ ' ■ CL with SR = ^÷ V ° and 'ON SR = — it becomes T 1 CL Cn ■■ SR-n + Vn and then: V Cn =V Cn +SR with V CO =V 0 L n+1 — L n ~r τ 1 CL through the temporary variable i n ~i n * ' + i =i n +V Aim -V 0 -SR-n TIME T3 CALCULATION: to calculate the discharge time, when the inductor passes from a current Io to a current equal to zero at the real end of the voltage ramp, assuming to be in said last instant and to go back in time till the instant T3 when the current Io flows through the inductor, it is evident that (by applying the variable replacement t=-τ) that the system is analogue to another one being advanced in time, but with negative voltage ramp, in formulas: L~ = -V D -V c {t) With V c {t) = V F +SR-t The previous equation with negative t is equivalent to the following one with positive t: L~ = V D +V c (t) With V c (t) = V F -SR-t By solving the latter in a discrete form, it becomes: Δi _ V D -V C by solving the voltage value in a discrete form at the capacitive load terminals: V c =SR-t + V 0 V c =V F -SR-n-T CL V -V where SR = - ∑ - ° ' ON sκ= v ' ■ v„ "ON ■ T, CL with SR = Vp V ° and n ON SR = it becomes V, Cn = V„-SR n and then: V D +V F -SR-n rr • • 71+1 = ln + CL through the temporary variable i n =i n 'CL Tx TIME CALCULATION: simple calculations generate the following formula: V„+V n V D + F x — ■ ' ■ ON v D + v DD From the foregoing it will be appreciated that, although specific embodiments of the invention have been described herein for purposes of illustration, various modifications may be made without deviating from the spirit and scope of the invention. Accordingly, the invention is not limited except as by the appended claims.

Claims: CLAIMS 1. Circuit (1) for highly efficient driving of piezoelectric loads (2), comprising a linear driving circuit portion (3) connected to the load (2) through an inductive-resistive connection whereto a voltage waveform is applied, characterised in that it comprises at least a further circuit portion (4, 5) connected in turn to said connection by means of an inductance (LI, L2) to supply a considerable fraction of the overall current required by the load (2) in the transient and steady state respectively. 2. Circuit according to claim 1, characterised in that it comprises respective circuit portions (4, 5) connected in a structurally independent manner to said connection by means of respective inductances (LI, L2). 3. Circuit according to claim 1, characterised in that it comprises a single control block (7) for driving said circuit portions (4, 5). 4. Circuit according to claim 1, characterised in that said at least further circuit portion (4, 5) comprises a half-bridge (LF, HF) circuit comprising at least a switching device powered between a first supply voltage (VALIM) reference and a second ground reference (GND). 5. Circuit according to claim 2, characterised in that each further circuit portion (4, 5) comprises a half-bridge (LF, HF) circuit including at least a switching device powered between a first supply voltage (V ALI M) reference and a second ground reference (GND) . 6. Circuit according to claim 2, characterised in that said inductances (LI, L2) have different values. 7. Circuit according to claim 5, characterised in that each half-bridge circuit (LF, HF) comprises a pair of transistors (Ml, M2; M3, M4) and is coupled with the load (2) by means of one of said inductors (LI, L2). 8. Circuit according to claim 7, characterised in that the control terminals of the half-bridge (LF, HF) circuit transistors are all connected to control block (7) outputs. 9. Circuit according to claim 3, characterised in that it comprises a current sensor (8) connected downstream of said linear portion (3) and to said control block (7). 10. Method for driving a piezoelectric load (2) with inductive- resistive connection wherein at least a linear driving of said load (2) is provided through a driver circuit (1) equipped with a linear circuit portion (3) connected to the load (2) through said connection, characterised in that two further distinct driving modes are provided, a transient and a steady state mode, wherein further respective circuit portions (4, 5), structurally independent and connected in turn to said connection through respective inductances (LI, L2), provide a considerable fraction of the overall current (I LOAD) required by the load (2) . 11. Method according to claim 10, characterised in that each further circuit portion (4, 5) is independently activated according to the value of a current (IAMP) outputted by said linear portion (3). 12. Method according to claim 10, characterised in that each further portion (4, 5) comprises a half-bridge (LF, HF) transistor circuit powered between a first supply voltage (VALIM) reference and a second ground reference (GND) coupled with the load (2) through one of said inductances (LI, L2). 13. Method according to claim 12, characterised in that said inductances (LI, L2) have different values. 14. Method according to claim 11, characterised in that said circuit portions (4, 5) are activated according to a time schedule (Tl, T2, T3) depending on the value of said current (IAMP) and corresponding to a transient state, a steady state and a discharge state respectively. 15. Method according to claim 10, characterised in that said current (ILOAD) required by the load (2) is spitted into three distinct parts: - two current peaks supplied by one circuit portion (5) at the transient phase starting; - most of the current of the steady state supplied by the other circuit portion (4); - a corrective current supplied by the linear portion (3) .

Inventors: BATTAGLIN LUCA, DIAZZI CLAUDIO, GALLINA PIETRO, PEDUTO VITTORIO, SABA GIANCARLO, ZINCO GIANCARLO, BATTAGLIN, LUCA, DIAZZI, CLAUDIO, GALLINA, PIETRO, PEDUTO, VITTORIO, SABA, GIANCARLO, ZINCO, GIANCARLO
Applicants: BATTAGLIN LUCA, DIAZZI CLAUDIO, GALLINA PIETRO, PEDUTO VITTORIO, SABA GIANCARLO, ST MICROELECTRONICS SRL, ZINCO GIANCARLO, BATTAGLIN, LUCA, DIAZZI, CLAUDIO, GALLINA, PIETRO, PEDUTO, VITTORIO, SABA, GIANCARLO, STMICROELECTRONICS S.R.L., ZINCO, GIANCARLO
Application year: 2001
Main classification: H01L41
IPC codes: B06B1, B41J2, H01L41, H02N2