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

Abstract

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 actuating the injector valve assembly (12), comprising the steps: - Providing a piezo stack (14) with an actuator unit (22) and a sensor unit (24) which are coupled to one another in a force-locking manner, wherein the sensor unit (24) is designed to detect force gradients acting on the actuator unit (22); - Providing an injector valve assembly (12) which is actuated during operation via the actuator unit (22), wherein the injector valve assembly (12) and the piezo stack (14) are arranged spaced apart from one another via a gap (32) with an unknown gap size (34); - detecting a voltage signal from 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 reducing the gap (32); - detecting a time duration (Δt) of the voltage pulse application to the actuator unit starting from a first time (t1) at which the voltage pulse application begins, up to a second time (t2) at which a voltage gradient (dU) occurs in the detected voltage signal of the sensor unit (24); - Determining the gap size (34) of the gap (32) from the recorded time period (Δt) and the defined voltage pulse.

Description

[0001] The invention relates to a detection method for detecting a gap size between an injector valve assembly and a piezo stack provided for actuating the injector valve assembly. Furthermore, the invention relates to a control method for controlling an actuator unit in the piezo stack used to actuate the injector valve assembly.

[0002] An actuator unit in a piezo stack used to actuate an injector valve assembly in an internal combustion engine typically comprises a stacked component having a plurality of electrode layers and a plurality of material layers that respond to the 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 is elongated along an actuator unit longitudinal axis. This deflection can then be transmitted to other components, for example, to an injector valve assembly of an internal combustion engine, in order to lift an injector needle from a needle seat and thereby inject fuel into the combustion chambers of the internal combustion engine.

[0003] The opening and closing of the injector needle in the injector valve assembly is effected by direct or indirect transmission of the longitudinal expansion of the actuator unit to the injector valve assembly, whereby a force connection is established at any point between the piezo stack comprising the actuator unit and the injector valve assembly to transmit the longitudinal expansion.

[0004] From DE 10 2013 206 933 A1, it is known to construct a modular piezo stack, so that in addition to the described actuator unit, the piezo stack also has a sensor unit that is frictionally coupled to the actuator unit. The sensor unit has at least one ceramic material layer, each with two electrode layers. This allows force changes transmitted to the piezo stack by the opening and closing of the injector needle to be recorded, allowing the opening and closing times of the injector needle to be detected.

[0005] When installed, there is a gap between the piezo stack and the injector valve assembly, which changes over the service life of the two elements due, for example, to wear or abrasion on the two elements or a depolarization of the actuator unit.

[0006] With increasing demands on emissions and fuel consumption, the requirements for fuel injection into the combustion chambers are also increasing. Higher pressures, higher temperatures, and multiple injections require greater precision in the metering of the injected fuel. To achieve the required accuracy, simply operating an injector in actuating mode is not sufficient; closed-loop control is essential. Among other things, this control system also requires the ability to compensate for the gap between the injector valve assembly and the piezo stack. This requires knowledge of the gap size, especially the gap size that changes over the service life.

[0007] DE 10 2011 003 751 A1 describes a method for determining an idle stroke between a piezo stack and an injector valve assembly, in which weaker voltage pulses are connected upstream of the actual control voltage pulse for deflecting the piezo stack in order to be able to read the contact time between the piezo stack and the injector valve assembly from a frequency spectrum fed back by the piezo stack.

[0008] DE 103 19 530 A1 discloses a method for determining an idle stroke between a piezo stack and an injector valve assembly, in which a test signal is modulated onto the actual control signal for deflecting the piezo stack.

[0009] DE 10 2005 046 933 A1 describes an active adjustment of an idle stroke between a piezo stack and an injector valve assembly.

[0010] The object of the invention is therefore to propose a detection method for detecting this gap size.

[0011] This object is achieved by a detection method having the features of claim 1.

[0012] A further task is to propose a control method for controlling the actuator unit with which the gap size can be compensated.

[0013] A control method for controlling an actuator unit in a piezo stack is the subject of the independent claim.

[0014] Advantageous embodiments of the invention are the subject of the dependent claims.

[0015] 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 which is actuated during operation via the actuator unit, wherein the injector valve assembly and the piezo stack are arranged spaced from each other via a gap of unknown gap size; - Detecting a voltage signal from the sensor unit; - Applying a defined voltage pulse to the actuator unit so that the actuator unit deflects along a longitudinal axis of the actuator unit while reducing the gap; - detecting a duration of the voltage pulse application to the actuator unit starting from a first point in time at which the voltage pulse application begins until a second point in time at which a voltage gradient occurs in the detected voltage signal of the sensor unit; - Determine the gap size of the gap from the recorded time period and the defined voltage pulse.

[0016] The detection method utilizes the knowledge that a frictional connection between the piezo stack and the injector valve assembly leads to a force impulse in the piezo stack. The force impulse corresponds to a force gradient that generates a charge in the stack, so that, for example, a voltage can be tapped from the outside. The frictional connection and thus the force gradient occur the moment the gap between the piezo stack and the injector valve assembly is overcome. Since the voltage applied to the actuator unit for expansion is known, the gap size can be deduced from a measured period of time until the sensor unit detects the frictional connection with the injector valve assembly.

[0017] For this purpose, a previously determined characteristic map is advantageously stored, which sets a gap size of the gap for predefined voltage pulses depending on the duration of the voltage pulse application.

[0018] In order to later compensate for the gap size, for example, via a closed-loop control system, it is necessary to determine defined measured variables from the system in order to calculate the corresponding controlled variables. In this case, the modular design of the piezo stack, consisting of an actuator unit and a sensor unit, is advantageously used to determine the gap size. Therefore, it is not necessary to provide additional sensors to determine the gap size, as the existing sensor unit is already used. The sensor unit detects a force increase at the second time point at which the piezo stack achieves a frictional connection with the injector valve assembly.

[0019] Preferably, the gap size between the piezo stack and the injector valve assembly is recorded during each actuation cycle of the injector valve assembly. This makes it possible to collect further data on aging phenomena of the elements, such as depolarization of the actuator unit or wear or abrasion of the elements, which are reflected in the gap size changing over the service life.

[0020] In an advantageous embodiment, a positive voltage gradient is detected in the voltage signal of the sensor unit at the second time point. Accordingly, if the signal from the sensor unit representing the voltage gradient is positive, it can be immediately recognized that the second time point has occurred.

[0021] Preferably, at a third point in time, at which an injector needle of the injector valve assembly lifts off a needle seat, a second voltage gradient is detected in the voltage signal of the sensor unit. This advantageously allows the precise point in time at which an injector opens to inject fuel to be detected.

[0022] In particular, a negative voltage gradient is detected in the voltage signal of the sensor unit. The sign can therefore be used to determine whether a force gradient in the piezo stack was caused by a frictional connection between the piezo stack and the injector valve assembly or by the injector needle lifting off the needle seat.

[0023] Particularly in injector units with a direct transfer of the longitudinal expansion from the actuator unit to the injector valve assembly, a third voltage gradient is recorded in the voltage signal of the sensor unit at a fourth point in time when the injector needle comes into frictional connection with the needle seat, with the third point in time being between the fourth point in time and the second point in time. At the fourth point in time, a negative voltage gradient is advantageously recorded. The closing of the injector and thus the termination of fuel injection also creates a force gradient in the piezo stack, which can be recorded by the sensor unit as a voltage gradient. The sensor unit can therefore now precisely record when there is frictional connection with the injector valve assembly, when the injector needle opens, and when the injector needle closes again.This allows the injected fuel to be precisely metered to the respective combustion chamber. The measured data also makes it possible to implement a control system that can compensate for aging phenomena, ensuring precise fuel injection into the respective combustion chamber.

[0024] In a control method for controlling an actuator unit in a piezo stack to actuate an injector valve assembly in an internal combustion engine, the actuator unit is subjected to a predetermined opening voltage pulse to lift an injector needle of the injector valve assembly from a needle seat. Before the actuator unit is subjected to 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; - Applying a pre-voltage pulse to the actuator unit to close the gap between the piezo stack and the injector valve assembly.

[0025] 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.

[0026] For this purpose, it is advantageous if a further characteristic map is stored from which a size of the pre-voltage pulse required for closing the gap can be read out.

[0027] The actuator unit can therefore be operated based on the high-precision measurement in the detection process with the aforementioned pre-voltage pulse, so that at the respective time at which injection is to start, a reproducible gap-free state is achieved between the piezo stack and the injector valve assembly. The injection control can thus be completely independent of the absolute length of the piezo stack, obstruction phenomena, etc. This eliminates negative disturbances such as absolute length changes of the piezo stack and wear phenomena, particularly on the needle seat, resulting in a reproducible opening and closing behavior of the injector needle.

[0028] At the same time, the required pre-voltage pulse can also be used to detect when the gap can no longer be compensated by applying the pre-voltage pulse, meaning maintenance is necessary. In this case, a signal can be emitted externally as a wear indicator.

[0029] Preferably, the pre-voltage pulse is determined from the gap size determined by the detection method, with the pre-voltage pulse being re-determined, in particular, for each actuation cycle of the injector valve assembly. This allows the gap size, which changes over the service life, to be continuously compensated for over the service life of the assembly.

[0030] Preferably, the detection method is performed in a first actuation cycle of the injector valve assembly, with the pre-voltage pulse being applied to the actuator unit during a second actuation cycle of the injector valve assembly, which follows the first actuation cycle. Therefore, the detection method advantageously first detects the current gap size so that the required pre-voltage pulse can be determined. This pre-voltage pulse is only used to compensate for the gap in the next actuation cycle.

[0031] It is advantageous if the pre-voltage pulse is sent to the actuator unit early enough so that a voltage pulse for opening the injector needle can be sent to the actuator unit as intended without any time delay. For example, the pre-voltage pulse can be sent immediately after the detection process has been carried out, even if the actual subsequent injection is scheduled to take place significantly later.

[0032] It is advantageous if the first actuation cycle and the second actuation cycle follow one another immediately in time.

[0033] Overall, an injector comprises an actuator, a valve assembly with a valve seat and a valve piston, as well as a nozzle with a nozzle seat and a needle.

[0034] 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. The injector unit also has a piezo stack with an actuator unit and a sensor unit, which are frictionally coupled to one another. 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. A gap with an unknown gap size is formed between the piezo stack and the injector valve assembly. Furthermore, a control unit is provided, which is designed to detect a voltage signal from the sensor unit and to apply a voltage pulse to the actuator unit. The control unit is designed to carry out the detection method ordesigned to carry out the control method described above.

[0035] For this purpose, the control unit has, for example, the two aforementioned characteristic maps, as well as means for detecting voltage gradients of the voltage signal from the sensor unit. Furthermore, the control unit advantageously has elements with which the gap size and the required size of the pre-voltage pulse to close the gap can be determined from various parameters. In addition, the control unit advantageously has an output device for outputting voltage pulses to the actuator unit, allowing the actuator unit to vary its length along the actuator unit's longitudinal axis.

[0036] Advantageous embodiments of the invention are explained in more detail below with reference to the accompanying drawings, in which: Fig. 1 is a schematic representation of a first embodiment of an injector unit with a piezo stack and an injector valve assembly, wherein the injector unit functions according to the directly operated functional principle; Fig. 2 is a schematic representation of a second embodiment of an injector unit with a piezo stack and an injector valve assembly, wherein the injector unit functions according to the functional principle of servo operation; Fig. 3 a schematic longitudinal section through the piezo stack from Fig. 1 and Fig. 2 in greater detail; Fig. 4 is a flowchart illustrating a detection method for detecting a gap size of a gap between the piezo stack and the injector valve assembly in Fig. 1 and Fig. 2 represents; Fig. 5 a flowchart showing a control method of an actuator unit in the piezo stack from the Fig. 1 - Fig. 3 to overcome the Fig. 4 represents the gap detected, and Fig. 6 a schematic representation of a control unit used to carry out the detection method according to Fig. 4 or the control procedure according to Fig. 5 is trained.

[0037] Fig. 1 and Fig. 2 each show schematic representations of an injector unit 10 used to inject 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. An injector needle 16 is arranged in the injector valve assembly 12 and interacts with a needle seat 18 to form an injector valve 20. If the injector needle 16 lifts off the needle seat 18, the injector valve 20 opens, and fuel can be injected into the respective combustion chamber connected to the injector unit 10. However, if the injector needle 16 comes into frictional connection with the needle seat 18 again, the injector valve 20 is closed, and the fuel injection is terminated.

[0038] The piezo stack 14 has, as will be shown later on the basis of Fig. 3, 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, with the sensor unit 24 being arranged above the actuator unit 22 (cf. Fig. 3) or can also be arranged under the actuator unit 22.

[0039] The piezo stack 14 is connected to a control unit 28, which can, on the one hand, 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.

[0040] Such an expansion along the actuator unit's longitudinal axis 25 causes the piezo stack 14 to move toward the injector valve assembly 12, for example, via a pin 30 attached thereto. In doing so, a gap 32 is overcome, which is always present when the injector valve assembly 12 or the piezo stack 14 is installed, and whose gap size 34 also changes over the service life of the individual elements. Once the gap 32 has been overcome and the actuator unit 22 continues to deflect along the actuator unit's longitudinal axis 26, the injector needle 16 is lifted out of the needle seat 18 via an operating unit 36 by the force acting from the piezo stack 14 on the operating unit 36. When the voltage is removed from the actuator unit 22, it contracts again along the actuator unit's 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.

[0041] In Fig. 1 shows a directly operated functional system in which the operating unit 36 lifts the injector needle 16 out of the needle seat 18 via lever 38 when a force is applied from the piezo stack 14.

[0042] Fig. 2 shows an alternative embodiment in which the injector unit 10 functions via servo operation, wherein the operating unit 36 has a fluid-filled control chamber 40 that exerts a closing force on the injector needle 16 through the fluid pressure present in the control chamber 40, thus holding it 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, the fluid pressure in the control chamber 40 is reduced, allowing the injector needle 16 to be lifted out of the needle seat 18.

[0043] Fig. 3 shows a schematic longitudinal section of the piezo stack 14 from Fig. 1 and Fig. 2 in greater detail.

[0044] The piezo stack 14 has the actuator unit 22 and the sensor unit 24, which are arranged along the actuator unit longitudinal axis 26 in the Fig. 3, are arranged one above the other, specifically such that the sensor unit 24 is arranged on the side of the actuator unit 22 facing away from the injector valve assembly 12. However, a reversed arrangement of the actuator unit 22 and the sensor unit 24 is also possible.

[0045] The actuator unit 22 comprises a plurality of electrode layers and a plurality of material layers that react to the application of an electric field, which are arranged alternately stacked one above the other along the actuator unit's longitudinal axis 26. The electrode layers and the material layers are arranged in Fig. 3 not shown for reasons of clarity. The electrical contacting of the electrode layers is achieved via external electrodes 44, which are electrically connected to the electrode layers via electrical conductors 46. However, contacting of the external electrode 44 can also be achieved in other ways. The external electrodes 44 are connected to the control unit 28, which can output voltage pulses to the actuator unit 22 via the external electrodes 44, so that the actuator unit 22 expands along the actuator unit's longitudinal axis 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. Electrode layers 50 are arranged on the sensor body 48, in particular on two opposite side surfaces 52, which are arranged along the actuator unit's longitudinal axis 26.The electrode layers 50 are connected to a voltage measuring device 54, which transmits a voltage signal from the sensor unit 24 to the control unit 28.

[0046] Because the control unit 28 can detect voltage signals from the sensor unit 24, mediated via the voltage measuring device 54, all force gradients that occur within the piezo stack 14 can be detected by the control unit 28.

[0047] This makes it possible to also carry out a detection method with the control unit 28 with which the gap size 34 of the gap 32 between the injector valve assembly 12 and the piezo stack 14 can be reliably detected.

[0048] A flow chart for recording this gap size 34 is shown in Fig. 4 shown.

[0049] First, a first time t1 is recorded, at which the actuator unit 22 is subjected to a voltage pulse originating from the control unit 24. Subsequently, it is recorded when, at a second time t2, a voltage gradient dU occurs in a voltage signal that is reported from the sensor unit 24 to the control unit 28. From the two times t1, t2, the time period Δt that has elapsed until the voltage gradient dU occurs can then be recorded. Using a first characteristic map K1, which sets the gap size 34 as a function of the time period Δt, the gap size 34 present at the current time can then be determined. At the same time, the voltage signal from the sensor unit 24 continues to be recorded by the control unit 28, so that a third time t3 can be determined, at which a further voltage gradient dU occurs, namely when the injector needle 16 lifts off the needle seat 18.To distinguish the second time t2 from the third time t3, the sign of the voltage gradient is used, which is positive at time t2 and negative at time t3. Further on, especially in the case of directly driven injector units, a further voltage gradient dU can also be detected at a fourth time t4, which has a positive sign, which is due to the closing of the injector needle 16.

[0050] In Fig. 5 is a flow chart schematically showing a control method with which the actuator unit 22 can be controlled via the control unit 28. First, as described with reference to Fig. 4, the gap size 34 of the gap 32 between the injector valve assembly 12 and the piezo stack 14 is determined. From a second characteristic map K2, which sets the size of a necessary pre-voltage pulse for closing the gap 32 as a function of the determined gap size 34, the size of the pre-voltage pulse required to close the gap 32 is then determined.

[0051] In a subsequent step, the actuator unit 22 is then subjected to this pre-voltage pulse. Subsequently, the actuator unit 22 is subjected to an opening pulse to lift the injector needle 16 from the needle seat 18.

[0052] The control unit 28 is designed to control both the Fig. 4 as well as the investigation procedure described in Fig. 5. For this purpose, the control unit 28, as shown in Fig.6, the characteristic maps K1 and K2 are shown schematically. Furthermore, a detection device 56 is provided for detecting a voltage gradient dU in the voltage signal from the sensor unit 24. 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 outputs a signal to the time measuring device 58 when it has output an opening pulse to the actuator unit 22. The detection device 56 outputs 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 period Δt.

[0053] The control unit 28 also includes a determination unit 64 that can determine the gap size 34. For this purpose, the detected time period Δt, as well as the characteristic map K1 and the size of the opening pulse, are fed to it from the time measuring device 58. From this data, it is possible to determine the gap size 34, since the characteristic map K1 makes the gap size 34 dependent on the time period Δt and the size of the opening pulse.

[0054] Furthermore, a determination unit 66 is provided in the control unit 28 to determine the size of the pre-voltage pulse, specifically based on the determined gap size 34 and the second characteristic map K2, which sets the necessary pre-voltage pulse for closing the gap 32 as a function of the determined gap size 34. In addition to the opening pulse output device 62, the output device 60 also comprises 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 corresponding to the determined pre-voltage pulse, so that the actuator unit 22 can expand along the actuator unit's longitudinal axis 26 in such a way that the gap 32 disappears.

Claims

[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 actuating the injector valve assembly (12), comprising the steps: - Providing a piezo stack (14) with an actuator unit (22) and a sensor unit (24) which are coupled to one another in a force-locking manner, wherein the sensor unit (24) is designed to detect force gradients acting on the actuator unit (22); - Providing an injector valve assembly (12) which is actuated during operation via the actuator unit (22), wherein the injector valve assembly (12) and the piezo stack (14) are arranged spaced apart from one another via a gap (32) with an unknown gap size (34); - detecting a voltage signal from 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 reducing the gap (32); - detecting a time duration (Δt) of the voltage pulse application to the actuator unit starting from a first time (t1) at which the voltage pulse application begins, up to a second time (t2) at which a voltage gradient (dU) occurs in the detected voltage signal of the sensor unit (24); - Determining the gap size (34) of the gap (32) from the recorded time period (Δt) and the defined voltage pulse. [2] Detection method according to claim 1, characterized by that the gap size (34) of the gap (32) between the piezo stack (14) and the injector valve assembly (12) is detected during each actuation cycle of the injector valve assembly (12). [3] Detection method according to one of claims 1 or 2, characterized bythat at the second time (t2) a positive voltage gradient (dU) is detected in the voltage signal of the sensor unit (24). [4] Detection method according to one of claims 1 to 3, characterized by that at a third time (t3), at which an injector needle (16) of the injector valve assembly (12) lifts off a needle seat (18), a second voltage gradient (dU) is detected in the voltage signal of the sensor unit (24), wherein in particular a negative voltage gradient (dU) is detected in the voltage signal of the sensor unit (24). [5] Detection method according to claim 4, characterized bythat at a fourth time (t4), at which the injector needle (16) comes into frictional connection with the needle seat (18), a third voltage gradient (dU) is detected in the voltage signal of the sensor unit (24), wherein the third time (t3) lies between the fourth time (t4) and the second time (t2), wherein at the fourth time (t4) in particular a positive voltage gradient (dU) is detected in the voltage signal of the sensor unit (24). [6] Control method for controlling an actuator unit (22) in a piezo stack (14) for actuating an injector valve assembly (12) in an internal combustion engine, wherein the actuator unit (22) is subjected to a predetermined opening voltage pulse for lifting an injector needle (16) of the injector valve assembly (12) from a needle seat (18), wherein the following steps are carried out before the actuator unit (22) is subjected to the opening voltage pulse: - Carrying out the detection method according to one of claims 1 to 5 for detecting a gap size (34) of a gap (32) between the piezo stack (14) and the injector valve assembly (12); - Applying a pre-voltage pulse to the actuator unit (22) to close the gap (32) between the actuator unit (22) and the injector valve assembly (12). [7] Control method according to claim 6, characterized by that the pre-voltage pulse is determined from the gap size (34) determined by the detection method, wherein the pre-voltage pulse is redetermined in particular during each actuation cycle of the injector valve assembly (12). [8] Control method according to one of claims 6 or 7, characterized bythat the detection method is carried out in a first actuation cycle of the injector valve assembly (12), and that the actuation of the actuator unit (22) with the pre-voltage pulse is carried out at a second actuation cycle of the injector valve assembly (12) which follows the first actuation cycle. [9] Control method according to claim 8, characterized by that the first actuation cycle and the second actuation cycle follow one another immediately in time. [10] Injector unit (10) for injecting fuel into a combustion chamber of an internal combustion engine, comprising: - an injector valve assembly (12) with an injector needle (16), wherein the injector needle (16) forms an injector valve (20) with a needle seat (18); - a piezo stack (14) with an actuator unit (22) and a sensor unit (24) that are frictionally coupled to one another, wherein the sensor unit (24) is designed to detect force gradients acting on the actuator unit (22), and wherein the actuator unit (22) is designed to actuate the injector valve assembly (12); wherein a gap (32) with an unknown gap size (34) is formed between the piezo stack (14) and the injector valve assembly (12); - a control unit (28) which is designed 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 designed to carry out the detection method according to one of claims 1 to 5 and / or to carry out the control method according to one of claims 6-9.

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