EP3347584B1 - Detection method for detecting a gap size of a gap between an injector valve assembly and piezo stack, and actuation method for actuating an actuator unit in a piezo stack - Google Patents

Description

Detection method for detecting a gap size of a gap between an injector valve assembly and a piezo stack and driving method for driving an actuator in a piezo stack.

The invention relates to a detection method for detecting a gap size of a gap between an injector valve assembly and a piezo stack, which is intended to actuate the injector valve assembly. Furthermore, the invention relates to a driving method for driving an actuator unit in the piezo stack, which is used for actuating the Injektorventilbaugruppe.

An actuator unit in a piezo stack used to actuate an injector valve assembly in an internal combustion engine typically includes a stacked device having a plurality of electrode layers and a plurality of material layers responsive to application of an electric field. Each material layer is arranged between two of the electrode layers. When an electric field is applied to the actuator unit via the electrode layers, the material layers react by expanding, so that the actuator unit as a whole extends along an actuator unit longitudinal axis. This deflection can then be transferred to other components, such as an injector valve assembly of an internal combustion engine to lift an injector needle from a needle seat and thereby inject fuel into the combustion chambers of the internal combustion engine.

The injector needle in the injector valve assembly is opened and closed by direct or indirect Transmission of the longitudinal extent of the actuator unit to the Injektorventilbaugruppe, wherein for transmitting the longitudinal extent at any point between the piezo stack, which has the actuator unit, and the injector valve assembly is a force closure.

Out DE 10 2013 206 933 A1 It is known to construct a piezo stack modular, so that the piezo stack in addition to the described actuator unit also has a sensor unit which is non-positively coupled to the actuator unit. The sensor unit has at least one ceramic material layer, each with two electrode layers. As a result, changes in force that are transmitted to the piezo stack by the opening or closing of the injector needle can be detected, so that opening and closing times of the injector needle can be detected.

In the installed state, a gap exists between the piezo stack and the injector valve assembly, which changes over the life of the two elements due to, for example, wear or abrasion on the two elements or depolarization of the actuator unit.

As emission and fuel consumption requirements increase, fuel injection requirements increase in the combustion chambers, with higher pressures, higher temperatures, and multiple injections requiring greater accuracy in metering the injected fuel. In order to achieve the required accuracies, it is not sufficient to drive an injector into actuating mode, but regulation is unavoidable. Among other things, it is also important in this scheme to be able to compensate for the gap between injector valve assembly and piezo stack, including the knowledge of Gap size, especially over the lifetime changing gap size, is necessary.

The DE 10 2011 005 285 A1 discloses a method for detecting the gap size of a gap (idle stroke) of a piezoelectric injector having a piezoelectric actuator and a directly actuated by the piezoelectric actuator nozzle needle, wherein the piezoelectric actuator, a test pulse is applied, which causes a steady increase of the piezo stroke. The time interval between the start of the test pulse and the time when the idle stroke is overcome is measured. The idle stroke of the piezoelectric actuator is determined from the measured time span. Measured thereby are electrical signals of the piezoelectric actuator, which are caused by mechanical influences.

The DE 10 2008 023 373 A1 discloses a method of controlling a piezoelectrically actuated injector. The method is based on the fact that the force exerted by the piezoactuator force depends on the voltage applied to the piezoelectric actuator and the charge shifted by the piezoactuator and the position of the maximum of the force curve is correlated with a point in time at which the closing element actually opens and the injector releases a path for fuel injection. The determination of the position of the maximum of the force curve of the force exerted by the piezoelectric actuator allows a conclusion on the time of the actual opening of the injection valve.

The DE 10 2011 004 613 A1 discloses a method for monitoring the state of a piezo injector of a fuel injection system. In a partial stroke, the course of an electrical parameter over time is detected in order to draw conclusions about the state of the piezo injector.

The DE 199 01 711 A1 discloses a method of operating a fuel injector with piezoelectric actuator. Depending on a temperature-dependent linear expansion of the actuator is acted upon by a first actuating voltage, which is dimensioned such that a trained in a non-excited state of rest of the actuator in the operating path gap disappears or is reduced. With a second actuation voltage, the fuel injection valve is opened.

The object of the invention is therefore to propose a detection method for detecting this gap size.

This object is achieved by a detection method having the features of claim 1.

Another object is to propose a driving method for driving the actuator unit, with which the gap size can be compensated.

A driving method for driving an actuator unit in a piezo stack is the subject of the independent claim.

Advantageous embodiments of the invention are the subject of the dependent claims.

• Bereitstellen eines Piezostapels mit einer Aktoreinheit und einer Sensoreinheit, die kraftschlüssig miteinander gekoppelt sind, wobei die Sensoreinheit zum Erfassen von an der Aktoreinheit wirkenden Kräftegradienten ausgebildet ist;
• Bereitstellen einer Injektorventilbaugruppe, die im Betrieb über die Aktoreinheit betätigt wird, wobei die Injektorventilbaugruppe und der Piezostapel über einen Spalt mit unbekannter Spaltgröße voneinander beabstandet angeordnet sind;
• Erfassen eines Spannungssignals der Sensoreinheit;
• Beaufschlagen der Aktoreinheit mit einem definierten Spannungspuls, sodass sich die Aktoreinheit unter Verringerung des Spaltes entlang einer Aktoreinheitlängsachse auslenkt;
• Erfassen einer Zeitdauer der Spannungspulsbeaufschlagung der Aktoreinheit ausgehend von einem ersten Zeitpunkt, in dem die Spannungspulsbeaufschlagung beginnt, bis zu einem zweiten Zeitpunkt, in dem in dem erfassten Spannungssignal der Sensoreinheit ein Spannungsgradient auftritt;
• Ermitteln der Spaltgröße des Spaltes aus der erfassten Zeitdauer und dem definierten Spannungspuls.

A detection method for detecting a gap size of a gap between an injector valve assembly of an internal combustion engine and a piezo stack for actuating the injector valve assembly comprises the following steps:

• Providing a piezo stack with an actuator unit and a sensor unit, which are coupled to one another in a force-locking manner, wherein the sensor unit is designed to detect force gradients acting on the actuator unit;
• Providing an injector valve assembly that is actuated via the actuator unit in operation, wherein the injector valve assembly and the piezo stack are spaced apart from each other via a gap of unknown gap size;
• Detecting a voltage signal of the sensor unit;
• Actuation of the actuator unit with a defined voltage pulse, so that the actuator unit deflects while reducing the gap along an actuator unit longitudinal axis;
• Detecting a time duration of the voltage pulse application of the actuator unit from a first point in time when the voltage pulse application starts, to a second point in time in which a voltage gradient occurs in the detected voltage signal of the sensor unit;
• Determining the gap size of the gap from the detected time duration and the defined voltage pulse.

According to the invention, a second voltage gradient in the voltage signal of the sensor unit is detected in a third point in time in which an injector needle of the injector valve assembly lifts off from a needle seat. In this case, a negative voltage gradient is detected in the voltage signal of the sensor unit. In a fourth time, in which the injector needle comes into frictional engagement with the needle seat, a third voltage gradient is detected in the voltage signal of the sensor unit, wherein between the fourth time and the second time is the third time. At the fourth time a positive voltage gradient is detected.

In the detection method, the knowledge is used that a frictional connection between the piezo stack and the injector valve assembly leads to a force surge in the piezo stack. The force impulse corresponds to a force gradient, which is a charge in generated, so that, for example, a voltage can be tapped from the outside. The frictional connection and thus the force gradient occur in the moment in which the gap between the piezo stack and Injektorventilbaugruppe is overcome. Since the voltage with which the actuator unit is subjected to expansion is known, the gap size of the gap can be deduced over a measured period of time until the sensor unit detects the frictional connection with the injector valve assembly.

For this purpose, a previously determined characteristic field is advantageously stored which, for predefined voltage pulses, sets a gap size of the gap as a function of a time duration of the voltage pulse application.

In order later to be able to compensate the gap size of the gap, for example via a control, it is necessary to determine defined measured variables from the system, in order to calculate the corresponding control variables therefrom. In the present case, the modular construction of the piezo stack consisting of an actuator unit and a sensor unit is advantageously used in order to determine the gap size. Therefore, it is not necessary to provide further sensors, via which the gap size is to be determined, since already the existing sensor unit is used. The sensor unit detects an increase in force in the second time point in which the piezo stack reaches a force fit to the injector valve assembly.

By detecting a second voltage gradient in the voltage signal of the sensor unit in the third point in time in which an injector needle of the injector valve assembly lifts from a needle seat, it is advantageously possible to detect exactly the point in time at which an injector opens for injecting fuel. In particular, there is a negative voltage gradient in the voltage signal of the sensor unit detected. Accordingly, the signs can be used to detect whether a force gradient in the piezo stack has been caused by the fact that a frictional connection has taken place between the piezo stack and injector valve assembly, or in that the injector needle has lifted off the needle seat.

Also, by closing the injector and thus terminating the fuel injection, a force gradient occurs in the piezo stack, which can be detected by the sensor unit by a voltage gradient. Therefore, it can now be accurately detected via the sensor unit when there is a force fit with the injector valve assembly, when the injector needle opens, and also when the injector needle closes again. Thus, the injected fuel can be metered exactly to the respective combustion chamber. From the measured data, it is also possible to provide a control that can compensate for aging phenomena, so that an exact injection of the fuel into the respective combustion chamber remains possible.

Preferably, the gap size of the gap between the piezo stack and the injector valve assembly is detected at each actuation cycle of the injector valve assembly. Thus, it is possible to record further data on aging phenomena of the elements, for example a depolarization of the actuator unit or wear or abrasion phenomena of the elements, which are reflected in the life span of the gap.

In an advantageous embodiment, a positive voltage gradient in the voltage signal of the sensor unit is detected at the second time. Accordingly, if the signal of the sensor unit representing the voltage gradient is positive, it can be immediately recognized that the second time is present.

• Durchführen des oben beschriebenen Erfassungsverfahrens, um eine Spaltgröße eines Spaltes zwischen dem Piezostapel und der Injektorventilbaugruppe zu erfassen;
• Beaufschlagen der Aktoreinheit mit einem Prä-Spannungspuls zum Schließen des Spaltes zwischen dem Piezostapel und der Injektorventilbaugruppe.

In a driving method for driving an actuator unit in a piezo stack for actuating an injector valve assembly in an internal combustion engine, the actuator unit is acted upon by a needle valve seat with a predetermined opening voltage pulse for lifting an injector needle of the injector valve assembly. Before the actuation of the actuator with the Opening voltage pulse, the following steps are performed:

• Performing the detection method described above to detect a gap size of a gap between the piezo stack and the injector valve assembly;
• Actuation of the actuator unit with a pre-voltage pulse for closing the gap between the piezo stack and the Injektorventilbaugruppe.

Because the gap size is now known, it is possible to compensate for the gap by readjusting the actuator unit by applying a pre-voltage pulse to the actuator unit so that it deflects and overcomes the gap.

For this purpose, it is advantageous if a further characteristic field is deposited, from which a size of the pre-voltage pulse necessary for closing the gap can be read out.

The actuator unit can therefore be operated based on the highly accurate measurement in the detection method with said pre-voltage pulse, so that at the respective time at which the injection is to start, a reproducible gap-free state between the piezo stack and injector valve assembly is reached. The injection control can thus be completely independent of an absolute length of the piezo stack, of closure phenomena, etc. As a result, negative disturbance variables such as the absolute change in the length of the piezo stack and signs of wear, in particular at the needle seat, can be eliminated, resulting in a reproducible opening and closing behavior of the injector needle.

At the same time, it can also be detected by the required pre-voltage pulse when the gap can no longer be compensated by being subjected to the pre-voltage pulse, so that a maintenance is necessary. In this case, a signal can be output as a wear indicator to the outside.

Preferably, the pre-voltage pulse is determined from the gap size determined by the detection method, wherein the pre-voltage pulse is redetermined, in particular during each actuation cycle of the injector valve assembly. As a result, the gap size which changes over the service life can also be continuously compensated over the life of the arrangement.

Preferably, the detection method is carried out in a first actuation cycle of the injector valve assembly, wherein the actuation of the actuator unit with the pre-voltage pulse is performed at a second actuation cycle of the injector valve assembly that is temporally downstream of the first actuation cycle. Therefore, it is advantageously first detected with the detection method how large the gap size currently is, so that the necessary pre-voltage pulse can be determined. Only in the next cycle of operation, this pre-voltage pulse is used to compensate for the gap.

It is advantageous if the pre-voltage pulse is given to the actuator unit so early that a voltage pulse for opening the injector needle can be output as intended to the actuator unit without any time delay. For example, the pre-voltage pulse can also be given immediately after the detection process has been carried out, even if the actual subsequent injection should take place much later in time.

It is advantageous if the first actuation cycle and the second actuation cycle follow one another directly in time.

Overall, a total injector comprises an actuator, a valve assembly with a valve seat and a valve piston, and a nozzle with a nozzle seat and a needle.

An injector unit for injecting fuel into a combustion chamber of an internal combustion engine has an injector valve assembly with an injector needle, wherein the injector needle forms an injector valve with a needle seat. Furthermore, the injector unit has a piezo stack with an actuator unit and a sensor unit, which are coupled to one another in a force-locking manner. The sensor unit is designed to detect force gradients acting on the actuator unit, and the actuator unit is designed to actuate the injector valve assembly. Between the piezo stack and the Injektorventilbaugruppe a gap with an unknown gap size is formed. Furthermore, a control unit is provided, which is designed to detect a voltage signal of the sensor unit and to act on the actuator unit with a voltage pulse. The control unit is designed to carry out the detection method described above or to carry out the drive method described above.

For this purpose, the control unit has, for example, the two named maps, as well as means for detecting voltage gradients of the voltage signal of the sensor unit. Furthermore, the control unit advantageously has elements with which the gap size of the gap and the required size of the pre-voltage pulse for closing the gap can be determined from various parameters. In addition, the control unit advantageously has an output device in order to output voltage pulses to the actuator unit so that they can vary in their length along the actuator unit longitudinal axis.

Fig. 1

eine schematische Darstellung einer ersten Ausführungsform einer Injektoreinheit mit einem Piezostapel und einer Injektorventilbaugruppe, wobei die Injektoreinheit gemäß dem direkt betriebenen Funktionsprinzip funktioniert;

Fig. 2

eine schmematische Darstellung einer zweiten Ausführungsform einer Injektoreinheit mit einem Piezostapel und einer Injektorventilbaugruppe, wobei die Injektoreinheit gemäß dem Funktionsprinzip des Servobetriebes funktioniert;

Fig. 3

eine schematische Längsschnittdarstellung durch den Piezostapel aus Fig. 1 und Fig. 2 in größerem Detail;

Fig. 4

ein Flussdiagramm, das ein Erfassungsverfahren zum Erfassen einer Spaltgröße eines Spaltes zwischen dem Piezostapel und der Injektorventilbaugruppe in Fig. 1 und Fig. 2 darstellt;

Fig. 5

ein Flussdiagramm, das ein Ansteuerungsverfahren einer Aktoreinheit in dem Piezostapel aus den Fig. 1 - Fig. 3 zum Überwinden des gemäß Fig. 4 erfassten Spaltes darstellt, und

Fig. 6

eine schematische Darstellung einer Steuereinheit, die zum Ausführungen des Erfassungsverfahrens gemäß Fig. 4 bzw. des Ansteuerungsverfahrens gemäß Fig. 5 ausgebildet ist.

Advantageous embodiments of the invention will be explained in more detail with reference to the accompanying drawings. It shows:

Fig. 1

a schematic representation of a first embodiment of an injector with a piezo stack and an injector valve assembly, the injector operates according to the directly operated operating principle;

Fig. 2

a schematic representation of a second embodiment of an injector with a piezo stack and an injector valve assembly, the injector operates according to the operating principle of the servo operation;

Fig. 3

a schematic longitudinal section through the piezo stack Fig. 1 and Fig. 2 in greater detail;

Fig. 4

a flowchart illustrating a detection method for detecting a gap size of a gap between the piezo stack and the Injektorventilbaugruppe in Fig. 1 and Fig. 2 represents;

Fig. 5

a flowchart illustrating a driving method of an actuator in the piezo stack of the Fig. 1 - Fig. 3 to overcome the according to Fig. 4 recorded gap, and

Fig. 6

a schematic representation of a control unit, the embodiments of the detection method according to Fig. 4 or the driving method according to Fig. 5 is trained.

Fig. 1 and Fig. 2 show in each case schematic representations of an injector unit 10, which is used for injecting fuel into a combustion chamber of an internal combustion engine. The injector unit 10 has an injector valve assembly 12 and a piezo stack 14 with which the injector valve assembly 12 can be actuated. In the injector valve assembly 12, an injector needle 16 is arranged, which cooperates with a needle seat 18 so that an injector valve 20 is formed. If the injector needle 16 is lifted off the needle seat 18, the injector valve 20 is opened and fuel can be injected into the respective combustion chamber connected to the injector unit 10. However, if the injector needle 16 again comes into frictional connection with the needle seat 18, the injector valve 20 is closed, and the injection of fuel is terminated.

The piezo stack 14 has, as later with reference to Fig. 3 will be explained in more detail, an actuator unit 22 and a sensor unit 24. These are arranged one above the other in the piezo stack 14 along an actuator unit longitudinal axis 26, the sensor unit 24 being arranged above the actuator unit 22 (cf. Fig. 3 ) or under the actuator unit 22 may be arranged.

The piezo stack 14 is connected to a control unit 28, which on the one hand can detect voltage signals from the sensor unit 24, but on the other hand can also output voltage pulses to the actuator unit 22 so that it expands along the actuator unit longitudinal axis 26.

Such an expansion along the Aktoreinheitlängsache 26 causes the piezo stack 14, for example via a pin 30 attached thereto, to the Injektorventilbaugruppe 12 moves. In this case, a gap 32 is overcome, which in the installed state of Injektorventilbaugruppe 12 and des Piezo stack 14 is always present, and also changed over the life of the individual elements in its gap size 34. If the gap 32 is overcome and the actuator unit 22 continues to deflect along the actuator unit longitudinal axis 26, the injector needle 16 is lifted out of the needle seat 18 via an operating unit 36 by the force acting on the operating unit 36 from the piezo stack 14. If the voltage application of the actuator unit 22 is terminated, it contracts again along the actuator unit longitudinal axis 26, so that the contact between the injector valve assembly 12 and the piezo stack 14 is terminated, and the injector needle 16 can return to the needle seat 18 again.

In Fig. 1 In this case, a directly operated functional system is shown, in which the operating unit 36 lifts the injector needle 16 out of the needle seat 18 during a force application from the piezo stack 14 via lever 38.

Fig. 2 shows an alternative embodiment in which the injector 10 operates via a servo operation, wherein the operating unit 36 has a liquid-filled control chamber 40, which exerts a closing force by the existing in the control chamber 40 fluid pressure on the Injektornadel 16, and holds them in the needle seat 18 , Upon contact of the piezo stack 14 via the pin 30 with a valve element 42 of the operating unit 36, a fluid pressure in the control chamber 40 is reduced, so that the injector needle 16 can lift out of the needle seat 18.

Fig. 3 shows a schematic longitudinal sectional view of the piezo stack 14 from Fig. 1 and Fig. 2 in greater detail.

The piezo stack 14 has the actuator unit 22 and the sensor unit 24, which along the Aktoreinheitlängsachse 26 in the in Fig. 3 shown embodiment are arranged one above the other, in such a way that the sensor unit 24 is arranged on the side of the actuator unit 22, which faces away from the injector valve assembly 12. However, it is also possible a reverse arrangement of actuator unit 22 and sensor unit 24.

The actuator unit 22 includes a plurality of electrode layers and a plurality of reacting on application of an electric field material layers, which are arranged alternately stacked along the actuator unit longitudinal axis 26. The electrode layers and the material layers are in Fig. 3 not shown for reasons of clarity. The electrical contacting of the electrode layers takes place via external electrodes 44, which are electrically connected to the electrode layers via electrical conductors 46. However, a contacting of the outer electrode 44 can also be done differently. The outer electrodes 44 are connected to the control unit 28, which can deliver voltage pulses to the actuator unit 22 via the outer electrodes 44, so that it expands along the Aktoreinheitlängsachse 26. The actuator unit 22 is non-positively connected to the sensor unit 24. The sensor unit 24 also advantageously has a sensor body 48, which is formed, for example, from the same material that also forms the material layers of the actuator unit 22. On the sensor body 48 electrode layers 50 are arranged, in particular on two opposite side surfaces 52, which are arranged along the Aktoreinheitlängsachse 26. The electrode layers 50 are connected to a voltage measuring device 54, which forwards a voltage signal of the sensor unit 24 to the control unit 28.

Because the control unit 28 can detect voltage signals of the sensor unit 24, mediated via the voltage measuring device 54, all power gradients occurring within the piezo stack 14 can be detected by the control unit 28.

This makes it possible to carry out with the control unit 28, a detection method with which the gap size 34 of the gap 32 between the Injektorventilbaugruppe 12 and the piezo stack 14 can be reliably detected.

A flow chart for detecting this gap size 34 is shown in FIG Fig. 4 shown.

First, it is a first time t ₁ is detected, at which the charging of the actuator unit 22 is effected with a voltage pulse from the control unit 24th Thereafter, it is detected when a voltage gradient dU occurs at a second time t ₂ in a voltage signal, which is reported by the sensor unit 24 to the control unit 28. From the two time points t1, _t2 may be the length of time .DELTA.t are detected, the elapsed before the occurrence of the voltage gradient dU. Using a first characteristic diagram K ₁ , which sets the gap size 34 as a function of the time duration Δt, the gap size 34 present at the present time can then be determined. At the same time, the voltage signal of the sensor unit 24 is further detected by the control unit 28, so that a third time t ₃ can be determined, to which a further voltage gradient dU occurs, namely when the injector needle 16 lifts off from the needle seat 18. In order to distinguish the second time t ₂ from the third time t ₃ , the sign of the voltage gradient is used, which is positive at time t ₂ and negative at time t ₃ . in the Further course, especially in directly driven injector units, a further voltage gradient dU detected in a fourth time t4, which has a positive sign, which is due to a closing of the Injektornadel 16. In Fig. 5 a flow chart is shown, which shows a driving method schematically, with which the actuator unit 22 can be controlled via the control unit 28. At first, as with reference to Fig. 4 described, the gap size 34 of the gap 32 between the Injektorventilbaugruppe 12 and the piezo stack 14 determined. From a second map K ₂ , which sets a size of a necessary pre-voltage pulse for closing the gap 32 as a function of the determined gap size 34, then the size of the pre-voltage pulse is determined, which is necessary to close the gap 32.

In a subsequent step, the actuator unit 22 is then subjected to this pre-voltage pulse. Subsequently, the actuator unit 22 is then subjected to an opening pulse in order to lift the injector needle 16 from the needle seat 18.

The control unit 28 is configured both in Fig. 4 presented preliminary investigation as well as in Fig. 5 to perform illustrated driving method. For this purpose, the control unit 28, as in Fig. 6 is shown schematically, the maps K ₁ and K ₂ on. Further, detecting means 56 for detecting a voltage gradient dU in the voltage signal from the sensor unit 24 is provided. In addition, the control unit 28 comprises a time measuring device 58 and an output device 60, which comprises an opening pulse output device 62, from which an opening pulse is output to the actuator unit 22 for opening the injector needle 16. The opening pulse output device 62 gives a signal to the time measuring device 58 when it receives an opening pulse to the time Actuator 22 has issued. The detection device 56 sends a signal to the time measuring device 58 when a voltage gradient dU has been determined via the sensor unit 24. From this, the time measuring device 58 can determine the time duration Δt.

In the control unit 28, a determination unit 64 is further provided, which can determine the gap size 34. For this purpose, it is supplied from the time measuring device 58, the detected time duration .DELTA.t and the map K ₁ and the size of the opening pulse. From this data it is possible to determine the gap size 34, since the map K ₁ sets the gap size 34 in dependence on the time duration Δt and the size of the opening pulse.

Further, in the control unit 28, a determination unit 66 is provided to determine the size of the pre-voltage pulse, namely on the basis of the determined gap size 34 and the second map K ₂ , which sets the necessary pre-voltage pulse for closing the gap 32 in dependence the determined gap size 34. In addition to the opening pulse output device 62, the output device 60 also includes a pre-voltage pulse output device 68, to which the determined pre-voltage pulse from the determination unit 66 is supplied. The pre-voltage pulse output device 68 then outputs a signal to the actuator unit 22, which corresponds to the specific pre-voltage pulse, so that the actuator unit 22 can extend along the Aktoreinheitlängsachse 26 such that the gap 32 disappears.

Claims (8)

1. Detection method for detecting a gap size of a gap between an injector valve assembly (12) of an internal combustion engine and a piezo stack (14) for activating the injector valve assembly (12), having the steps of:

   - providing a piezo stack (14) having an actuator unit (22) and a sensor unit (24) that are coupled to one another with a force fit, wherein the sensor unit (24) is configured to detect force gradients acting on the actuator unit (22);

   - providing an injector valve assembly (12) that is activated during operation by means of the actuator unit (22), wherein the injector valve assembly (12) and the piezo stack (14) are arranged at a distance from one another across a gap (32) having an unknown gap size (34);

   - detecting a voltage signal of the sensor unit (24);

   - applying a defined voltage pulse to the actuator unit (22), so that the actuator unit (22) deflects along an actuator unit longitudinal axis (26) while decreasing the gap (32);

   - detecting a period (Δt) for which the voltage pulse is applied to the actuator unit starting from a first time (t₁), at which the voltage pulse begins to be applied, up to a second time (t₂), at which a voltage gradient (dU) occurs in the detected voltage signal of the sensor unit (24);

   - ascertaining the gap size (34) of the gap (32) from the detected period (Δt) and the defined voltage pulse; characterized in that

   - a second voltage gradient (dU) is detected in the voltage signal of the sensor unit (24) at a third time (t₃), at which an injector needle (16) of the injector valve assembly (12) lifts off from a needle seat (18), wherein a negative voltage gradient (dU) is detected in the voltage signal of the sensor unit (24), and

   - a third voltage gradient (dU) is detected in the voltage signal of the sensor unit (24) at a fourth time (t₄), at which the injector needle (16) comes into force-fitting contact with the needle seat (18), the fourth time (t₄) and the second time (t₂) enclosing the third time (t₃), wherein a positive voltage gradient (dU) is detected in the voltage signal of the sensor unit (24) at the fourth time (t₄).
2. Detection method according to Claim 1,
   characterized in that the gap size (34) of the gap (32) between the piezo stack (14) and the injector valve assembly (12) is detected for every activation cycle of the injector valve assembly (12).
3. Detection method according to either of Claims 1 and 2,
   characterized in that a positive voltage gradient (dU) is detected in the voltage signal of the sensor unit (24) at the second time (t₂) .
4. Actuation method for actuating an actuator unit (22) in a piezo stack (14) for activating an injector valve assembly (12) in an internal combustion engine, wherein the actuator unit (22) has a predetermined opening voltage pulse applied to it to lift off an injector needle (16) of the injector valve assembly (12) from a needle seat (18), wherein the application of the opening voltage pulse to the actuator unit (22) is preceded by the following steps being performed:

   - performing the detection method according to one of Claims 1 to 3 to detect a gap size (34) of a gap (32) between the piezo stack (14) and the injector valve assembly (12);

   - applying a preliminary voltage pulse to the actuator unit (22) to close the gap (32) between the actuator unit (22) and the injector valve assembly (12).
5. Actuation method according to Claim 4,
   characterized in that the preliminary voltage pulse is determined from the gap size (34) ascertained using the detection method, wherein the preliminary voltage pulse is particularly determined afresh for each activation cycle of the injector valve assembly (12).
6. Actuation method according to either of Claims 4 and 5,
   characterized in that the detection method is performed in a first activation cycle of the injector valve assembly (12), and in that the application of the preliminary voltage pulse to the actuator unit (22) is performed for a second activation cycle of the injector valve assembly (12) that occurs at a time after the first activation cycle.
7. Actuation method according to Claim 6,
   characterized in that the first activation cycle and the second activation cycle succeed one another directly in time.
8. Injector unit (10) for injecting fuel into a combustion chamber of an internal combustion engine, having:

   - an injector valve assembly (12) having an injector needle (16), wherein the injector needle (16) and a needle seat (18) form an injector valve (20);

   - a piezo stack (14) having an actuator unit (22) and a sensor unit (24) that are coupled to one another with a force fit, wherein the sensor unit (24) is configured to detect force gradients acting on the actuator unit (22), and wherein the actuator unit (22) is configured to activate the injector valve assembly (12);
   wherein the piezo stack (14) and the injector valve assembly (12) have a gap (32) having an unknown gap size (34) formed between them;

   - a control unit (28) that is configured to detect a voltage signal of the sensor unit (24) and to apply a voltage pulse to the actuator unit (22),
   wherein the control unit (28) is configured to carry out the detection method according to one of Claims 1 to 3 and/or to carry out the actuation method according to one of Claims 4 to 7.

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