WO2019101518A1 - Method and device for determining the opening time of the servo valve of a piezo injector - Google Patents

Abstract

The invention relates to a method and to a device for determining the opening time of a servo valve of a piezo injector, which has a piezo actuator, the servo valve and a nozzle needle. The progression of the force acting on the piezo actuator during a charging operation of the piezo actuator is determined taking account of the capacitance of the piezo actuator, and the time of the maximum force acting on the piezo actuator is identified as the opening time of the servo valve. Changes in the capacitance of the piezo actuator occurring during the charging operation of the piezo actuator are determined and the force acting on the piezo actuator during the charging operation is taken into account in the determination of the progression of the force.

Description

description

Method and device for determining the opening time of the servo valve of a Piezoinj ector

The invention relates to a method and apparatus for determining the opening time of the servo valve of a Piezoinj ector.

It is already known to precisely adjust Piezoinj ectors using a so-called closed-loop control. An important component of the control of a Piezoinj ectors are as accurate as possible control of the timing of the servo valve opening and the most accurate possible control of the opening stroke of the piezo drive. The prerequisite for such an accurate control is the most accurate determination of the opening time of the servo valve.

There are a number of factors that affect the accuracy of the electrical return measurement used for servo valve opening detection. These factors include, but are not limited to, the movement of the piezo actuator during charging and its effect on the electrical feedback signal, i. on the measured current and the measured voltage.

So far, a servo valve opening detection using linear piezo equations and a determination of the maximum of the force curve at the piezoelectric actuator from the measured voltage and the charge was performed.

The following equations were used for this servo valve opening detection: Dΐ / 1 = -S33-F / A + d33 ·U / d

and

 Q / (A-N) = -d33 * F / A + s33-U / d.

This designates

 D1 the stroke of the piezo actuator,

 1 the active length of the piezoelectric actuator,

 F the force acting on the piezoelectric actuator,

 A is the active area of the piezo actuator,

 U the voltage applied to the piezoelectric actuator,

d the thickness of a piezo layer,

 N is the number of piezo layers,

 Q is the charge of the piezo actuator,

s33 the inverse elasticity of the piezoactuator,

d33 is the electromechanical coupling constant of the

 Piezoelectric actuator,

e33 the dielectric constant of the piezoelectric actuator.

By means of a calibration procedure based on these piezo equations, a corrected, constant value for the piezo capacitance Ccor was calculated, with the help of which the force at the piezoactuator is calculated by means of the following equation:

F = K * (U-Ccor-Q), where K is a constant.

The time position of the maximum of this force F corresponds to the time of servovalve opening.

The aforementioned calibration method uses the following equation to calculate a corrected, constant value of the piezocapacitance:

1.25 * (F ² / E50) * Ccor = (Ccor-U-Q) ² .

This designates F the known force at the time of servovalve opening,

E50 a material constant of the piezoelectric actuator,

 U at the piezoelectric actuator at the time of Sevoventilöffnung to lying voltage and

 Q the charge of the piezoelectric actuator at the time of the servo valve opening.

The object of the invention is to increase the accuracy of the determination of the timing of the servo valve opening.

This object is achieved by a method having the features specified in claim 1. Advantageous embodiments and further developments of the invention are specified in the dependent claims 2 and 3. The claims 4 to 6 have a device for detecting the opening time of the servo valve of a Piezoinj ector subject.

The advantages of the invention are, in particular, that due to consideration of the increase in the piezoelectric capacitance in the course of the charging process of the piezoelectric actuator depending on the energy or the charge or the voltage using the equation

F = K * (U-Ccor (E (t)) - Q) the force curve at the piezoelectric actuator is reproduced in more detail than in the known method and therefore the accuracy of Be mood of the timing of the servo valve opening is increased.

The change in the piezoelectric capacitance during the charging process is advantageously determined on the basis of the hysteresis behavior in a charge-voltage hysteresis diagram of the piezoelectric actuator and in the form of the equation

Ccor (E (t)) Ceoc · (1 a · e ^{_b (E (t)} ).

This designates Ceoc the capacity at the end of the piezo holding phase,

a is a scale size and

b is a scale size.

 For example, these scaling values have the following values: a ~ 0.6 and b ~ 0.75 / mJ.

Further advantageous features of the invention will become apparent from the following explanation with reference to FIGS. It shows

1 shows a block diagram of a device for determining the opening time of a servo valve of a piezo injector,

2 shows a diagram for illustrating the course of the force acting on the piezoelectric actuator during a charging process of the piezoelectric actuator for different idle strokes in the prior art

3 shows a diagram for illustrating the course of the force acting on the piezoelectric actuator during a charging operation of the piezoelectric actuator for different idle strokes taking into account occurring during the charging process of the piezoelectric actuator changes in the piezoelectric capacitance,

Figure 4 is a diagram for illustrating the Leerhubabhängigen

 Changing the Servo valve opening time with already corrected start of the current supply for four different Piezoinj ectors in the prior art and

Figure 5 is a diagram for illustrating the Leerhubabhängigen

Change in the Servoventilöffnungszeitpunkt be already corrected start of the current for four different Piezoinj ectors in consideration of during the charging of the piezoelectric actuator on passing changes in the piezoelectric capacitance. In internal combustion engines fuel injection systems are used which have a pressure accumulator, at least one injection valve and a control unit. For injection valve include a piezoelectric actuator, a servo valve and a nozzle needle. The piezoactuator actuates the servo valve against the force of a closing spring and the pressure in the control chamber to achieve that the servo valve opens and the nozzle needle is actuated such that it releases an injection opening of the piezoelectric injector communicating with the pressure accumulator via a fuel line Fuel is injected into a combustion chamber of the internal combustion engine.

To trigger an opening of the servo valve, the control unit controls a charging of the piezoelectric actuator with electrical energy. By this application of electrical energy, the piezoelectric actuator expands, so that the servo valve lifts off from its seat to actuate the nozzle needle.

In this control of the Piezoinj ector, it is of great importance to perform the most accurate control of the timing of Servöentilöffnung. The prerequisite for such an accurate control is the most accurate possible determination of the opening time of the servo valve.

1 shows a block diagram of a device according to the invention for determining the opening time of a Piezoinj ector. This device has a control unit 1, a memory 2 and a piezoelectric injector 3. For Piezoinj ector 3 includes a piezoelectric actuator 4, a servo valve 5 and a nozzle needle 6. The control unit 1 is connected to a memory 2 in connection in which already the manufacturer's data are stored, corresponding to the material constants of the piezoelectric actuator. Furthermore, further data are stored in the memory 2, which correspond to the number of layers of the piezoelectric actuator and the thickness of the layers of the piezoelectric actuator. In addition, 2 data are stored in memory 2, which correspond to empirically determined diagrams. Among other things, these charts include a charge-voltage hysteresis diagram. The above data will be from the control unit 1 for determining the opening time of the servo valve of the piezoelectric injector required. Furthermore, the control unit input signals el and e2 are supplied, which are determined during operation of Piezoinj ector using sensors or suitable taps of the power electronics used to drive the piezoelectric actuator. These input signals are measured actual values of the current flowing through the piezoelectric actuator and the voltage applied to the piezoelectric actuator.

The control unit 1 is designed to determine the time of servovalve opening as precisely as possible using the input signals e1 and e2 supplied to it and the data stored in the memory 2 and a work program likewise stored in the memory 2.

In this determination of the timing of the servo valve opening is also carried out with the aid of the above-mentioned linear piezo equations a determination of the maximum of the force curve using the measured voltage and the charge.

The following equations are used:

Al / l = -s33 * F / A + d33 * U / d

Q / (A-N) -d33 * F / A + e33 * U / d.

This designates

 D1 the stroke of the piezo actuator,

 1 the active length of the piezoelectric actuator,

 F the force acting on the piezoelectric actuator,

 A is the active area of the piezo actuator,

 U the voltage applied to the piezoelectric actuator,

d the thickness of a piezo layer,

 N is the number of piezo layers,

 Q is the charge of the piezo actuator,

s33 the inverse elasticity of the piezoactuator, d33 the electromechanical coupling constant of the piezoactuator, e33 the dielectric constant of the piezoactuator.

These quantities are partly determined by the manufacturer and stored in the memory 2 and measured or calculated in part during the charging process of the piezoelectric actuator.

With the aid of these piezo equations, a value Ccor (E (t)) of the piezo capacitance which changes during the charging process of the piezoelectric actuator is calculated, with the aid of which the force is calculated on the piezoelectric actuator using the following equation:

F = K * (U-Ccor (E (t))) - Q).

The time position of the maximum of this force F also corresponds to the time of the servo valve opening.

The value of the piezoelectric capacitance Ccor (E (t)) which changes during the charging process is determined on the basis of the hysteresis behavior in the stored charge-voltage hysteresis diagram of the piezoactuator according to the following relationship:

Ccor (E (t)) = Ceoc- (1-a * e ^{_b (E (t)} ), where Ceoc is the piezo capacitance at the end of the piezo hold phase, a is a scaling quantity and b is another scaling quantity.

For example, these scaling values have the following values: a ~ 0.6 and b ~ 0.75 / mJ.

By taking into account the value of the piezocapacitance that changes during the charging process, which is an increasing value, when determining the force acting on the piezoactuator, the force curve occurring during the charging process is reproduced with a higher accuracy than in the known procedure. In particular, it is avoided that disturbing influences occurring at low pressure affect the exact Effect of the detection of the opening timing of the servo valve negative impact.

The procedure described above corresponds to a use of the piezo equations in differential form. It follows that in the equation

F = K * (U-Ccor (E (t))) - Q) capacitance Ccor (E (t))) of the differential capacitance to be used

Cor (E (t)) = dQ / dU. For the determination of the differential capacitance, characteristic, stored in the memory 2 em empirically determined charge voltage hysteresis diagrams are evaluated to determine the scaling quantities a and b, which in the equation

Ccor (E (t)) = Ceoc- (l-ae ^{_b (E (t)} ).

The described method represents a continuous transition from the small signal capacity to the large signal capacity. As a result, the force curve on the piezoelectric actuator can be described in more detail, in particular at low pressure in the pressure accumulator. This also makes it possible to increase the accuracy of determining the opening timing of the servo valve.

2 shows a diagram for illustrating the course of the force acting on the piezoelectric actuator during a charging process of the piezoelectric actuator for different idle strokes in the prior art, in which the force profile was determined using a corrected, constant value for the piezoelectric capacitance. In this diagram, the charge difference (U-Ceoc-Q) is plotted in As and to the right the time t in ms, where Ceoc is the capacitance of the piezoelectric actuator at the end of the charging phase of the piezoelectric actuator. The curve Kl shows the course of the force acting on the piezoelectric actuator force for a Leerhubreferenzspannung of 90 V, the curve K2 the course of the force acting on the piezoelectric actuator force for a Leerhubreferenzspannung of 80 V, the curve K3 the course of the force acting on the piezoelectric actuator for a Leerhubreferenzspannung of 60 V, the curve K4 the course of the force acting on the piezoelectric actuator for a Leerhubreferenz voltage of 40 V, the curve K5 the course of the force acting on the piezoelectric force for a Leerhubreferenzspannung of 30 V and the curve K6 the course of the on the Piezo actuator acting force for a Leerhubreferenzspannung of 20 V. These curves were each determined at a comparatively low operating pressure of 300 bar.

From the illustrated courses of the force acting on the piezoactuator during the charging phase of the piezoactuator, it can be seen that the force maxima along the time axis are spread relatively broadly. The rise of the curves results in particular in the presence of a large Leerhubreferenzspannung of 80 V and 90 V from the start of energization of each piezoelectric actuator. These curves do not correctly reflect the actual force distribution in this area.

3 shows a diagram for illustrating the course of the force acting on the piezoelectric actuator during a charging of the piezoelectric actuator force for different Leerhubreferenzspannungen in consideration of occurring during the charging of the piezo actuator changes in the piezoelectric capacitance.

In this diagram, the charge difference (U-Cscaling - Q) is plotted in As and the time t in ms to the right, where Cscaling is the variable capacitance of the piezo actuator during the charging process. The curve Kl 'shows the course of the force acting on the piezoelectric actuator force for a Leerhubreferenzspannung of 90 V, the curve K2' the course of acting on the piezoelectric actuator Force for a Leerhubreferenzspannung of 80 V, the curve K3 'the course of the force acting on the piezoelectric actuator force for a Leerhubreferenzspannung of 60 V, the curve K4' the course of the force acting on the piezoelectric actuator force for a Leerhubreferenz voltage 40 V, the curve K5 ' the course of the force acting on the piezoelectric actuator for a Leerhubreferenzspannung of 30 V and the curve K6 'the course of the force acting on the piezoelectric actuator force for a Leerhubreferenzspannung of 20 V. These curves were each determined at a comparatively low working pressure of 300 bar.

It can be seen from the curves of the force acting on the piezoactuator during the charging phase of the piezoactuator that the force maxima lie relatively close to one another along the time axis. The increase of the curves, in particular in the presence of a comparatively large idle reference voltage of 80 V or 90 V, is practically no longer coupled to the start of the current of excitation. The curves in this area, the actual force curve significantly improved again.

4 shows a diagram illustrating the Leerhubabhängigen change the Servo valve opening time with already corrected start of the current for four un ferent Piezoinj ektoren IA, IB, IC and ID in the prior art, in which the force curve using a corrected, constant value for the Piezocapacity was determined.

In this diagram, upwards the time difference AT_TT1 between the time of the detected servo valve opening and the time of the start of the movement of the nozzle needle in ps and to the right the Leerhubreferenzspannung in V is plotted. The curves shown in this figure were each determined at a comparatively low working pressure of 300 bar.

From the courses shown it can be seen that, in particular in a Leerhubreferenzspannungsbereich between 30 V and 60 V, a comparatively strong dispersion of the course of Time difference between detected servo valve opening and the beginning of the needle movement occurs. In this Leerhubrefe renzspannungsbereich the curves shown have a comparatively large slope. A precise estimation of the time difference between the time of the detected servo valve opening and the time of the beginning of the respectively associated needle movement is not possible.

5 shows a diagram illustrating the leerhubabhängigen change the Servo valve opening time at already corrected start of the current for the four different Piezoinj ectors IA, IB, IC and ID in Be taking into account occurring during the charging of the piezoelectric actuator changes in the piezoelectric capacitance.

In this diagram, upwards the time difference AT_TT1 between the time of the detected servo valve opening and the time of the start of the movement of the nozzle needle in ps and to the right the Leerhubreferenzspannung in V is plotted. The curves shown in this figure were each determined at a comparatively low working pressure of 300 bar.

From the courses shown it can be seen that in particular special in a Leerhubreferenzspannungsbereich between 30 V and 60 V due to the consideration of the change in the piezoelectric capacitance during the charging of the piezoelectric actuator, the variation of the course of the time difference between the servovalve opening and the beginning of the respective associated needle movement is reduced. In this area, the curves IA ', IB', IC 'and ID' are relatively flat. This allows a more accurate estimation of the time difference between the time of the detected servo valve opening and the time of commencement of the respective associated needle movement.

Claims

claims 1. A method for determining the opening time of a servo valve of a piezoelectric actuator, the servo valve and a nozzle needle having Piezoinj ector, wherein the course of the force acting on the piezoelectric actuator during a charging of the piezoelectric actuator force is determined taking into account the capacity of the piezoelectric actuator and the time of the maximum acting on the piezoelectric actuator force is detected as opening time of the servo valve, characterized in that During the charging of the piezoelectric actuator occurring changes occurring the capacitance of the piezoelectric actuator determined and taken into account in the determination of the course of the force acting on the piezoelectric actuator during the charging process. 2. The method according to claim 1, characterized in that the changes in the capacitance of the piezoelectric actuator during the La devorganges of the piezoelectric actuator are determined based on the hysteresis in a charge-voltage hysteresis diagram of the piezoelectric actuator. 3. The method according to claim 2, characterized in that the changes in the capacitance of the piezoelectric actuator during the charging of the piezoelectric actuator are determined using the following relationship: Ccor (E (t)) = Ceoc- (1-a * e ^{_b (E (t)} ), where  Ccor (E (t)) the changed capacity of the piezo actuator,  Ceoc the capacity of the piezo actuator at the end of the hold phase, a a scale size, b is a scale size and  E (t) the energy introduced into the piezoelectric actuator is. 4. Device for determining the opening time of a servo valve of a piezoelectric actuator, the servo valve and a nozzle needle having Piezoinj ector, which a control unit (1), which determines the course of the during a charging of the piezoelectric actuator (4) acting on the pie zoaktor force taking into account the capacity of the piezoelectric actuator and for detecting the time of the maximum of the force acting on the piezoelectric actuator force as opening time of the servo valve ( 5) is formed, characterized in that the control unit (1) is further adapted to determine during the charging of the piezoelectric actuator (4) occurring changes in the capacitance of the piezoelectric actuator and taken into account in determining the course of the force acting on the piezoelectric actuator during the charging force. 5. The device according to claim 4, characterized in that the control unit (1) is adapted to determine changes in the capacitance of the piezoelectric actuator during the charging process of the piezoelectric actuator based on the hysteresis behavior in a charge voltage hysteresis diagram of the piezoelectric actuator , 6. Control unit according to claim 5, characterized in that the control unit (1) is designed to to determine the changes in the capacitance of the piezoactuator (4) during the charging process of the piezoactuator using the following relationship: Ccor (E (t)) Ceoc · (1 a · e ^{_b (E (t)} ), where  Ccor (E (t)) the changed capacity of the piezo actuator,  Ceoc the capacity of the piezo actuator at the end of the hold phase, a a scale size, b is a scale size and  E (t) the energy introduced into the piezoelectric actuator is.