EP2084757A1

EP2084757A1 - Control device for an ultrasonic piezoelectric actuator

Info

Publication number: EP2084757A1
Application number: EP07823859A
Authority: EP
Inventors: Clément Nouvel
Original assignee: Renault SAS
Current assignee: Renault SAS
Priority date: 2006-10-16
Filing date: 2007-09-26
Publication date: 2009-08-05
Also published as: JP5086358B2; FR2907279B1; JP2010507360A; FR2907279A1; US20100033116A1; WO2008047018A1; US8154225B2

Abstract

The device comprises a first stage (1) supplied by a DC voltage source (Vbat) and provided with means for forming a second DC voltage (Vint) above that (Vbat) delivered by the source, and a second stage (2) provided with means for amplifying the second voltage (Vint) and chopping the voltage obtained by the excitation of the actuator with the chopped voltage, under the control of a computer (C), the amplifying means comprising an inductor (Lp) in electrical resonance with the piezoelectric actuator. According to the invention, the inductor (Lp) is connected to the first stage (1) so as to constitute, during formation of the second voltage (Vint), the secondary winding of a transformer forming part of a flyback voltage converter introduced into said first stage in order to develop the second voltage (Vint). Application to the control of a fuel injector in an internal combustion engine.

Description

Control device of an ultrasonic piezoelectric actuator

The present invention relates to a device for controlling at least one piezoelectric actuator controlled by a computer, comprising a first stage powered by a DC voltage source and provided with means for forming a second DC voltage greater than that delivered by this source, and a second stage provided with means for amplifying this second voltage and for chopping the voltage obtained for the excitation of the actuator with the chopped voltage, under the control of the computer, these amplification means comprising an inductance in electrical resonance with the piezoelectric actuator.

A device of this type is known from European Patent Application No. 1,422,764 filed in the name of the applicant, hereinafter referred to as the aforementioned patent. This known device is designed to provide excitation of an ultrasonic piezoelectric actuator incorporated in a fuel injector for an internal combustion engine. It is shown schematically in Figure 1 of the accompanying drawing. As shown in this figure, it comprises a first stage A powered by the battery of a vehicle powered by this engine, which battery thus establishes a voltage Vbat on a capacitor C ₁ placed at the entrance of this stage. This takes the form of a switching power supply, such as "boost" in the terminology of the skilled person _^ conventionally comprising an inductor L _1, a switching transistor T _A, a diode D and an output capacitor C ₂ . A computer Ca which manages the opening time of the fuel injectors I ₁ (i from 1 to 4) also controls the conductive duty cycle of the transistor T _A so as to establish a voltage across the capacitor C ₂ continues Vint. The level of this voltage is intermediate between that of the battery (commonly 12 volts) and that required to ensure the excitation of the piezoelectric actuators incorporated in the injectors I (currently of the order of 1 kilovolt). This voltage Vint feeds a second stage B comprising an inductance L ₂ calculated to be in electrical resonance with the capacity of each injector I ₁ , so that the output voltage of this stage reaches the aforementioned kilovolt at a capacitance of adaptation C ₃ . A hash transistor T ₆ , also controlled by the aforementioned computer, cuts the output voltage of the stage A to the ultrasonic frequency required by the piezoelectric actuators, during the opening times of the injectors determined by this computer. The sequential activation of the injector I ₁ selected is also provided by the computer, which controls for this purpose the conduction of a transistor T ₁ associated with this injector. Reference is made to the aforementioned patent for more details on the structure and operation of this known device. It follows from the foregoing description that this device comprises two separate inductive elements L ₁ and L ₂ whose size and cost can be significant within the electronic injection system associated with the engine. The present invention is specifically intended to provide a piezoelectric actuator control device, integrable including fuel injectors for internal combustion engine, wherein these disadvantages are removed or, at least, minimized.

This object of the invention, as well as others which will appear in the remainder of the present description, is achieved with a device for controlling at least one piezoelectric actuator driven by a computer, comprising a first stage powered by a DC voltage source and provided with means for forming a second DC voltage greater than that delivered by this source, and a second stage provided with means for amplifying the second DC voltage and for chopping the voltage obtained for excitation. of the actuator with the chopped voltage, under the control of the computer, these amplification means comprising an inductance in electrical resonance with the piezoelectric actuator, this device being remarkable in that this inductance is connected to the first stage so during the formation of the second DC voltage, constituting the secondary winding of a transformer forming part of a voltage converter n of the "flyback" type introduced in the first stage to develop the second DC voltage.

As will be seen in more detail below, the two inductors used in the device according to the invention are wound on the same core, and not on two separate cores, which achieves the goal announced above. According to other features of the present invention:

the "flyback" converter comprises a switching transistor in series with the primary winding of the transformer, an output capacitance under the intermediate voltage (Vint) and a rectifying diode, the rectifying diode being constituted by a free-wheeling diode associated with a transistor forming part of the chopping means of the amplified voltage,

the freewheeling diode is integrated in the transistor,

the transistor is of the MOSFET type,

the device comprises overvoltage limiting means on the primary of the transformer, the device comprises means for regulating the intermediate voltage available at the terminals of the capacitor,

the device comprises means for prohibiting simultaneous operation of the first and second stages, in the control of a plurality of fuel injectors in an internal combustion engine, the injectors being each equipped with an ultrasonic piezoelectric actuator the device comprises means for sequentially energizing the actuators with the chopped voltage delivered by the second stage, - the switching frequency chosen for the flyback converter of the first stage is separated by at least 1 kHz from the resonance frequency piezoelectric actuators fitted to the fuel injectors. Other characteristics and advantages of the present invention will appear on reading the following description and on examining the appended drawing in which: FIG. 1 schematically represents a device of the prior art, described in the preamble of the present invention; DESCRIPTION OF THE PREFERRED EMBODIMENTS FIG. 2 diagrammatically represents the structure of the first and second stages of the device according to the present invention, designed to be substituted for the corresponding stages of the device of FIG. 1, FIG. 2a illustrates a detail of FIG.

FIG. 3 represents a set of timing diagrams useful for describing the operation of the device according to the invention.

Referring to Figure 2 of the accompanying drawing to describe the structure of the first and second stages of the device according to the invention, which replace the stages A and B, respectively, of the device known from the aforementioned patent and shown in Figure 1. Thus, this first stage 1 and this second stage 2 are inserted into the environment of an internal combustion engine propelling a motor vehicle, for example. In this situation the first stage is supplied with electrical energy by the battery (not shown) of the vehicle which delivers a voltage Vbat, of the order of 12 volts for example. The control of stages 1 and 2 is provided by a computer C which manages the operation of fuel injectors installed downstream of the second stage, just like those shown in FIG. 1. The sequential excitation of these injectors takes place as described above in connection with this figure and as explained in more detail in the aforementioned patent. The following description therefore focuses on the structure and operation of the first and second stages of the device according to the invention, the reader can refer to the surplus to this patent.

The first stage of the device according to the invention generally takes the form of a switching power supply of the type well known to those skilled in the art under the name of "flyback" converter. This converter comprises a transformer comprising a primary winding L _f and a secondary winding L _p , wound on the same ferrite core schematized by the broken line referenced 3.

The primary winding, or inductance, L _f is placed in series with the conduction circuit of a transistor T _f , advantageously of the MOSFET type, between a line 4 connected to the battery of the vehicle, and the ground. The conduction of this transistor, chopping, is controlled by high frequency signals from the computer C, through a control stage E _d . The capacitor Ci placed at the input of the stage 1, between the line 4 and the ground, plays the same role as the corresponding capacitance of the known device of FIG. 1. According to a characteristic of the device according to the invention, the secondary winding is constituted by an inductance L _p also used in the second stage but, as will be seen later, in a different time.

It is known that in a flyback converter the energy stored in the magnetic core (in ferrite) is transferred into a capacitor connected between the terminals of the secondary winding, here the capacitor C _f , through a non-return diode which will be given the position further. It is at the terminals of this capacitance C _f that the intermediate voltage Vint formed by the first stage 1 is taken from the voltage Vbat delivered by the vehicle battery. This intermediate voltage is advantageously between 50 and 300 volts. The second stage 2 is similar in structure to the corresponding stage B of the known device of FIG. 1. It thus comprises an inductance L _p connected between the terminal at the intermediate voltage Vint of the capacitor C _f and the drain D of a transistor T _h , of the MOSFET type, for example, whose source is grounded, the conduction of this transistor being also controlled by the computer C, through a control stage E _h .

As seen above, this inductance L _p is however different from the independent inductance L ₂ of stage B of the known device of FIG. 1 in that it consists of the secondary winding of the transformer ( L _f , 3, L _p ) forming part of the flyback converter integrated in the first stage 1. With this essential difference, which benefits the present invention as will be seen below, the second stage 2 of the device according to the invention operates substantially like that of the known device. Thus, this stage amplifies the intermediate voltage Vint to deliver a voltage reaching in peak the level required for the excitation of the piezoelectric actuators, ie 1 kilovolt for example, this voltage being chopped at the ultrasonic frequency, also required, by a suitable control of the transistor T _h , developed by the computer C.

As described in the aforementioned patent to which we can refer for more details on this point, this amplification is obtained through an oscillating circuit formed by the inductance L _p and the capacity of each actuator, this inductance L _p being a function of the acoustic excitation resonance of the actuator. The chopped high voltage is delivered to the piezoelectric actuator of the selected injector by an output line 5 of the second stage 2, connected to the point common to the inductance L _p and to the drain of the transistor T _h . An adaptation capacitance C _p is connected between the line 5 and ground to play the same role as the capacitor C3 of the known device of FIG.

Before proceeding to the description of the operation of the device according to the invention, we come back to a point mentioned above and which remains to be clarified, namely the position of the rectifying diode necessarily present in the flyback converter of the first stage of this device . As is well known to those skilled in the art, the current MOSFET transistors are normally equipped with a freewheeling diode such as the diode D _r of the transistor T _h of the second stage of the device according to the invention. According to the present invention, in the coupling described above stages 1 and 2, this freewheeling diode D _r assumes the rectifying function of the flyback converter.

Reference is now made to the timing diagrams of FIG. 4 to describe the operation of the control device according to the invention, in its application to the control of fuel injectors in an internal combustion engine, these injectors being equipped with piezoelectric actuators. ultrasonic electric. These chronograms illustrate the evolution of the "on / off" commands of the transistors T _f and T _h respectively and the voltages across the capacitance C _f and the drain-source circuits of the transistors T _f and T _h, respectively.

The periods during which the transistor T _h is active in hash corresponds to the fuel injection times t, as determined by the computer C. The periods during which the transistor T _f is active in cutting correspond to the energy storage times in the inductance L _f , or recharge time t _r . The comparison of the chronograms of the "on / off" (closed / open) commands of the transistors T _f and T _h makes it clear that the periods of activity of these two transistors never overlap. This allows, according to a feature of the present invention, the pooling of the use of the inductance L _p by the two stages 1 and 2, even though the functions of these two stages are distinct.

The voltage rise supported by the first stage is effected by cutting the primary supply current L _f of the transformer (L _f , 3, L _p ) by means of a high-frequency switching (for example the order of 100 kHz or more) of the transistor T _f between its open and closed states, controlled by the computer C. This switching is effected with an adjustable duty cycle, also determined by this calculator, as is well known in the art. order a flyback converter. When the transistor closes, the primary is charged and at the next sudden opening of this transistor the energy stored in the primary passes through the secondary L _p of the transformer. The capacitance C _f then retrieves this energy through the free-wheeling diode D _r of the transistor T _h of the second stage.

The charge or recharge of the capacitance C _f is prolonged as long as the voltage at the terminals of this capacitance has not reached a suitable predetermined value such that Vint = 250 volts for example, as appears on the graph of this voltage represented in FIG. Figure 3.

As also appears in this figure, between two successive injection times t, the maintenance of the intermediate voltage at this level may require the presence of regulating means adapted to properly reactivate the switching operated by the transistor T _f flyback converter of the first stage 1 of the device according to the invention. This compensates for a partial discharge of the capacitance C _f . These regulating means may comprise a sensor of the voltage to be monitored across the capacitor C _f and a regulator sensitive to the signal delivered by this sensor for controlling the switching excitation of the transistor T _f so as to raise the voltage Vint to its setpoint. The installation of such control means does not present any difficulty for the skilled person and therefore does not require a more complete description in the context of the present patent application.

As has been seen above, during the charging of the capacitor C _f the transistor T _h of the second stage 2 remains open. It is obviously the same during the time spent regulating the load of this capacity. During a fuel injection control for a time t, determined by the computer C, it keeps open the transistor T _f of the first stage 1 and hashed the transistor T _h of the second stage 2. then operates substantially as the second stage of the device described in the aforementioned patent, which is therefore referred for a detailed description of this operation. The transformer is then used as a simple inductance L _p (the first winding L _f then having a very high impedance between its terminals), in electrical resonance with the piezoelectric actuator of the injector selected by the computer C. is thus obtained the high voltage (about 1 kilovolt) chopped at the ultrasonic frequency (40-50 kHz, for example) necessary for the excitation of this piezoelectric actuator.

As can be seen in the timing diagrams of FIG. 3, in the injection phase, the transistor T _f is subjected to a chopped voltage of the order of 100 volts, lower than the one (1 kV) supported by the transistor T _h because of the transformation ratio of the transformer. In the recharge phase of the capacitor C _f , the chopped voltages supported by the transistors T _f and T _h are respectively of the order of 50 and 250 volts. The drain-source isolation characteristics, or collector-emitter, of these transistors must allow these voltage levels to be supported. From the above it follows that the computer is properly programmed to control the transistors T _f and T _h so as to prohibit simultaneous operation of the first and second stages of the device according to the invention. Despite the precautions taken, interference such as the unintentional control of an injector during a recharge phase of the output capacity of the flyback converter can not be ruled out (a peak voltage during recharging may be sufficient to activate an injector).

According to the present invention, a first solution of this problem is to program the computer so that no load (injector or compensation capacitor) is connected during the voltage rise operated by the first stage of the device. Another solution is to use very different switching and chopping frequencies, separated by at least 1 kHz. This is clearly the case for the frequencies indicated above as an example (100 kHz at chopping and 40-50 kHz chopping). An improvement of this solution consists in using a switching frequency separated by more than 1 kHz resonance frequencies of the injectors and their harmonics. A feature of the device according to the present invention is that the dimensioning of the transformer of the flyback converter of the first stage must obviously take into account the value that must have the inductance L _p of its secondary winding when it occurs in the oscillating circuit constituting the second floor. As an illustrative example only, this inductance can be between 40 μH and 500 μH. To fix the transform ratio of the transformer, the conventional rule is applied that, in a flyback stage, this ratio is related to the ratio of the input voltages Vbat and the output Vint of the stage by the relation: Vint = M. α.Vbat,

With α = 0.5 in first approximation. This gives the value of M, equal to the transformation ratio, and therefore to the ratio of the number of turns of the primary and secondary windings.

Furthermore, the transformer primary must be protected against overvoltages resulting from the use of the flyback operating mode of the first stage of the device according to the invention. To do this, as shown in FIG. 2, transistor T _f can be protected by means of a diode Zener di mounted in antiparallel between the drain and the source of this transistor and with the aid of a other Zener diode d ₂ rising pass from the source to the gate of the transistor. It is also possible, as shown in broken lines in Figure 2, to have a circuit Z in parallel on the primary winding L _f . As illustrated by FIG. 2a, this circuit may comprise two diodes mounted head to tail in series, one being of the Zener type. It can also consist of a resistance and a capacitance in series, a diode being mounted in parallel on the resistor. These protection means are of types well known to those skilled in the art who know how to size without difficulty. However, care should be taken not to clog the signals that appear on the transformer primary during a fuel injection phase. This could indeed degrade the performance of the device due to a decrease in the primary load of the transformer. Thus, the Zener di di, see Figure 2, must have a Zener voltage lower than the maximum voltage found on the primary injection phase.

It now appears that the invention makes it possible to achieve the stated purpose of providing a device for controlling ultrasonic piezoelectric actuators of reduced size and cost by the fact that the two windings L _f and L _p are wound on the same ferrite core while the two windings used in the device of the aforementioned patent are wound on separate cores.

The transformer of the device according to the invention further establishes a galvanic isolation reassuring between the two stages of this device, insulation that does not exist in the device of the aforementioned patent.

Also, it is possible to install in the first stage of the device according to the invention a transistor with a lower insulating voltage than that placed in the corresponding stage of the device of the aforementioned patent. In the latter one typically installs a transistor with 600 volts insulation voltage, proportional to the intermediate voltage. In the device according to the invention, the transistor T _f must support the battery voltage Vbat and the return of the secondary voltage during the injection commands. This last voltage being lower, of the order of 50 volts, the necessary insulation voltage for T _f can be of the order of 150 volts only. It is then possible to choose a transistor T _f in a technology that guarantees better performance of the first stage.

Naturally, the invention is not limited to the embodiment described and shown which has been given only by way of example. It is thus possible to use in the invention transistors other than the MOSFET type, for example IGBT transistors. Also the invention is not limited to the control of fuel injectors and extends to the control of piezoelectric actuators in general and, more generally still, to the high voltage alternating control, in particular for frequencies of control below 500 kHz.

Claims

1. Device for controlling at least one piezoelectric actuator controlled by a computer, comprising a first stage fed by a DC voltage source (Vbat) and provided with means for forming a second DC voltage (Vint) greater than that (Vbat) delivered by said source, and a second stage provided with means for amplifying said second voltage (Vint) and for chopping the voltage obtained for the excitation of said actuator with said chopped voltage, under the control of said calculator, said amplification means comprising an inductance (L _p ) in electrical resonance with said piezoelectric actuator, characterized in that said inductor (L _p ) is connected to said first stage so as to constitute, during the formation of said second voltage (Vint), the secondary winding of a transformer forming part of a voltage converter of the flyback type introduced into said first stage to develop oppate said second voltage (Vint).

2. Device according to claim 1, wherein said converter

"flyback" comprises a switching transistor (T _f ) in series with the primary winding of said transformer, a capacitance (C _f ) output under said intermediate voltage (Vint) and a rectifying diode, characterized in that said diode rectification is constituted by a freewheeling diode (D ₁ -) associated with a transistor (T _h ) forming part of said means for chopping said amplified voltage.

3. Device according to claim 2, characterized in that said freewheeling diode (D ₁ -) is integrated in said transistor (T _h ).

4. Device according to claim 3, characterized in that said transistor (T _h ) is of the MOSFET type.

5. Device according to any one of claims 2 to 4, characterized in that it comprises overvoltage limiting means on the primary of said transformer (L _f , 3, L _p ).

6. Device according to any one of claims 2 to 5, characterized in that it comprises means for regulating said intermediate voltage (Vint) available at the terminals of said capacitance (C _f ).

7. Device according to any one of claims 1 to 6, characterized in that it comprises means for prohibiting simultaneous operation of said first and second stages.

8. Use of the device according to any one of claims 1 to 7, the control of a plurality of fuel injectors in a motor to internal combustion, said injectors being each equipped with an ultrasonic piezoelectric actuator, said device comprising means for sequentially energizing said actuators with the chopped voltage delivered by said second stage.

9. Use according to claim 8, characterized in that the switching frequency chosen for the flyback converter of said first stage is spaced at least 1 kHz from the resonant frequency of said piezoelectric actuators equipping said fuel injectors. .

10. Use according to claim 9, characterized in that said switching frequency is further separated by at least 1 kHz harmonics of said resonant frequency.