JPH11354853A - Method and apparatus for charging and discharging piezoelectric element - Google Patents

Abstract

PROBLEM TO BE SOLVED: To charge and discharge a piezoelectric element at desired values by already ending the charge or discharge process before specified time at which desired voltage in the piezoelectric element is reached. SOLUTION: The charging and discharging timings of a piezoelectric element 1 are controlled, i.e., switch control signals outputted from a closed loop controller 10 are fed through drivers 11, 12 to a charging and discharging switches 3, 5, so that the charging and discharging switches 3, 5 repeatedly close and open in the charging or discharging processing period. When the charging and discharging switches 3, 5 repeatedly close and open by predetermined no. of times or the piezoelectric element 1 reaches desired charge or discharge state, the charge and discharge of the piezoelectric element 1 are ended by keeping the charging and discharging switches 3, 5 open.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a method according to the preamble of claim 1 and a device according to the preamble of claim 8, ie a method and a device for charging and discharging a piezoelectric element.

[0002]

2. Description of the Related Art The piezoelectric elements discussed in detail here are in particular, but not necessarily, those used as actuators or adjusting elements.
Piezoelectric elements are used for this type of purpose. This is because, as is well known, they have the characteristic of contracting or expanding depending on the voltage applied thereto.

[0003] The realization of an adjusting element by means of a piezoelectric element has proven to be advantageous when the adjusting element has to carry out rapid and / or frequent movements.

The use of piezoelectric elements as adjusting elements has been found to be particularly advantageous in fuel injection nozzles for internal combustion engines. The use of piezoelectric elements in fuel injection nozzles is described, for example, in EP 0 371 469 and EP 03
Reference is made to 79182.

[0005] The piezoelectric element is, as already mentioned above,
It is a capacitive load that contracts and expands depending on the respective state of charge or the voltage present or applied thereto.

The charging and discharging of the piezoelectric element can take place, inter alia, via an element having an inductive characteristic, for example a coil, which firstly has
It is used to limit a charging current generated during charging and a discharging current generated during discharging. This type of device is shown in FIG.
It is shown in FIG.

[0007] The piezoelectric element to be charged or discharged is designated by the reference numeral 101 in FIG. It is a component part of a charging current circuit that can be closed via a charging switch 102 and a discharging current circuit that can be closed via a discharging switch 106, wherein the charging current circuit comprises: a charging switch 102; A diode 103;
The discharge current circuit includes a series circuit of a charging coil 104, a piezoelectric element 101, and a current source 105, and includes a discharge switch 106, a diode 107, a charging coil 108, a piezoelectric element 101, and a current source 10
5 and a series circuit.

The diode 103 in the charging current circuit prevents a current for discharging the piezoelectric element from flowing in the charging current circuit. On the other hand, the diode 1 of the discharge current circuit
07 prevents a current for charging the piezoelectric element from flowing in the discharge current circuit.

A normally open charging switch 102
Is closed, a charging current flows through the charging current circuit, whereby the piezoelectric element 101 is charged. The electric charge stored in the piezoelectric element 101 and the resulting voltage, and thus the current outer dimensions of the piezoelectric element 101, remain substantially unchanged after the element has been charged.

When the normally open discharge switch 106 is closed, a discharge current flows through the discharge current circuit, and the piezoelectric element 101 is discharged. The state of charge of the piezoelectric element 101 and thus the voltage generated thereby, and thus the current outer dimensions of the piezoelectric element 101, remain substantially unchanged after discharge of this element.

This type of charging and discharging of piezoelectric elements is advantageous. This is because the charging current circuit and the discharging current circuit do not have significant ohmic resistance and can be operated with little power loss and very little heat generation.

However, on the other hand, the degree and time course of charging and discharging is often not ideal. In particular, time-varying charging and discharging rates, more or less considerably pronounced build-up processes and charging and / or discharging of the piezoelectric element which are only partially or too strong are disadvantageous, so that during discharging, Charging with the opposite polarity may occur.

In order to avoid such drawbacks, the timing of charging and discharging of the piezoelectric element is controlled, that is, the charging switch is repeatedly opened and closed during the charging period or the discharge switch is repeatedly opened and closed during the discharging period. And implement it.

The functions and operations of the charging coil 104 and the discharging coil 108 change depending on the charging or discharging controlled in timing.

[0015] In the case of untimed charging or discharging, the charging coil 104 and the discharging coil 108 act as inductive elements of an LC series oscillating circuit formed in cooperation with the piezo element, whereby the inductance of the inductive element and the piezo The capacity of the element determines the course and magnitude of the charging and discharging (the oscillation of the first oscillation circuit is violated because the subsequent oscillation of the oscillation circuit is interrupted by the diodes contained in the charging and discharging current circuits). It is only charged and discharged in each case only by the first half-wave of current).

On the other hand, when the charging and discharging are timed, the charging coil 104 and the discharging coil 108 are used as a primary energy storage memory, which can be supplied from a current source (during discharge) or from a piezoelectric element. After storing the supplied electrical energy (in the form of magnetic energy) and operating the corresponding switch, the stored energy (in the form of magnetic energy) can be stored in the form of electrical energy in the form of a piezoelectric element (during charging) or another energy memory or It is delivered to an electrical load (during discharge), the energy storage and the time and duration (and thus also the magnitude) of the energy delivery determined by the switch operation.

Thus, the number of piezoelectric elements is arbitrary,
It can be charged and discharged in a desired size in successive steps at any size and at any time interval.

As a result, the degree of charge and / or discharge and the course of time can be controlled as desired, and is in this case largely independent of the technical data of the inductive element and the piezoelectric element.

However, the accuracy of bringing the piezoelectric element to the desired voltage by this type of charging and discharging is not always sufficient in all cases.

When a piezoelectric element is used as an actuator in a fuel injection nozzle of an internal combustion engine, for example, even higher precision is required throughout. That is, the voltage at which the piezoelectric element is brought about by charging and discharging,
For each injection operation, the fuel quantity to be injected must be determined and, for optimal functioning of the internal combustion engine, this fuel quantity must correspond to a predetermined target fuel quantity with high accuracy. They have hitherto not always been sufficiently successful, especially when the fuel quantity is small (as occurs, for example, in pilot injections).

[0021]

The object of the invention is therefore to provide a method according to the preamble of claim 1 and a device according to the preamble of claim 8 in which the piezoelectric element has the desired value in a simple manner. To be able to be charged and discharged.

[0022]

According to the present invention, this object is achieved by a structure (method) according to the characterizing part of claim 1 to a structure (device) according to the characterizing part of claim 8. Will be resolved.

According to this, the charging process or the discharging process is completed before a predetermined time to reach a desired voltage at the piezoelectric element (characteristic part of claim 1) or the charging process or the discharging process is completed. A charge or discharge control device is provided which is configured to end before a predetermined time to reach the desired voltage at (Characteristic part of claim 8).

By not terminating the charging or discharging process when the desired voltage on the piezoelectric element is reached, it is possible to prevent the piezoelectric element from being charged or discharged above a desired value. it can.
That is, the piezoelectric element is continuously charged or discharged for a predetermined time after the end of the charging or discharging process by a charging or discharging current that does not return to zero at the end of the charging or discharging process. If the charging or discharging process is completed at the right time before reaching the desired voltage,
As a result, a desired voltage can be accurately realized in the piezoelectric element.

The piezoelectric element can thus be easily charged and discharged to a desired value.

Advantageous embodiments of the invention are evident from the dependent claims, the following description and the figures.

[0027]

BRIEF DESCRIPTION OF THE DRAWINGS FIG.

The piezoelectric element, whose charging and discharging will be described in detail below, can be used, for example, as an adjusting member in a fuel injection nozzle of an internal combustion engine (in particular, a so-called common rail injector). However, it is not restricted to this type of use of piezoelectric elements.
Piezoelectric elements can basically be used in any device for any purpose.

The piezoelectric element starts out expanding in response to charging and contracting in response to discharging. However, the invention can of course also be used when this is exactly the opposite.

Referring now to FIG. 1, a device is described which can charge and discharge one or more piezoelectric elements exactly as desired.

The piezoelectric element that corresponds to charging in the example considered is designated by the reference numeral 1 in FIG.

As can be seen from FIG. 1, the piezoelectric element 1
Is permanently connected to ground via a (measurement) resistor 15 (connected to the first pole of a voltage source via a resistor 15), whereas The other of the connection terminals is connected to the second voltage source via a coil 2 (acting simultaneously as a charging and discharging coil) and a parallel circuit consisting of a charging switch 3 and a diode 4 (formed by transistors in the example considered). Connected to the first pole of the voltage source via a coil 2 and a parallel circuit consisting of a discharge switch 5 and a diode 6 (also formed by a transistor in the example considered). Have been.

The voltage source comprises a battery 7 (for example, a battery of a vehicle), a DC voltage converter 8 connected to the battery 7 and a capacitor 9 connected to the DC voltage converter 8 and used as a buffer capacitor. ing. With this device, the battery voltage (e.g. 12V) is converted to virtually any other DC voltage and provided as a supply voltage.

The charging and discharging of the piezoelectric element 1 are carried out with timing control in the example considered. That is, the charge switch 3 and the discharge switch 5 are repeatedly closed and opened during the charging process or the discharging process.

The state that occurs at this time will be described below with reference to FIGS. 2 and 3 show the charging of the piezoelectric element and FIGS. 4 and 5
Indicates the discharge of the piezoelectric element. 2 to 5
In the description, the resistor 15 was not considered. But resistance 1
The effect of 5 is negligible for this consideration.

The charge switch 3 and the discharge switch 5
The piezoelectric element 1 is open unless charging or discharging is performed and not performed. In this state, the circuit shown in FIG. 1 is in a steady state. That is, the piezoelectric element 1 maintains its charged state substantially unchanged, and no current flows.

When the charging of the piezoelectric element 1 is started,
The charging switch 3 is repeatedly closed and opened. In this case, the discharge switch 5 remains open.

When the charging switch 3 is closed, the state shown in FIG. 2 occurs. That is, a series circuit including a closed current circuit including the piezoelectric element 1, the capacitor 9, and the coil 2 is formed. In this current circuit, a current i _LE (t) indicated by an arrow in FIG. 2 flows. This current acts so that energy is stored in the coil 2. The energy flow in the coil 2 is affected by the positive potential difference between the capacitor 9 and the piezoelectric element 1.

When the charging switch 3 is opened immediately after the charging switch 3 is closed (for example, several μs later), the state shown in FIG. 3 occurs. That is, the piezoelectric elements 1
A closed current circuit consisting of the diode 6 and the coil 2 is formed. In this current circuit, a current i _LA (t) indicated by an arrow in FIG. 3 flows. This current acts so that the energy stored in the coil 2 completely flows through the piezoelectric element 1. Piezoelectric element 1
In response to the energy supply to the element, the voltage generated here and the outer diameter of the element are increased. After the energy transfer from the coil 2 to the piezoelectric element 1, the already described steady-state state of the circuit of FIG. 1 is achieved again.

At some point before or after that (depending on the desired duration of the charging process), the charging switch 3 is newly closed and opened again, with the above-described process being repeated. With the new closing and opening of the charging switch 3, the energy stored in the piezoelectric element 1 increases (the energy already stored in the piezoelectric energy and the newly supplied energy are added) and correspondingly As a result, the voltage generated on the piezoelectric element and the outer diameter of the element increase.

When the above-mentioned closing and opening of the charging switch 3 is repeated many times, the voltage applied to the piezoelectric element and the expansion of the piezoelectric element increase stepwise (for this reason, FIG. 6 which will be described in more detail later). (See curve A).

When the charging switch 3 has been closed and opened a predetermined number of times and / or the piezoelectric element 1 has reached the desired state of charge, charging of the piezoelectric element is terminated by keeping the charging switch 3 open. .

When the piezoelectric element 1 is to be discharged again, this is effected by repeated closing and opening of the discharge switch 5. In this case, the charging switch 3
Remains open.

When the discharge switch 5 is closed, the state shown in FIG. 4 occurs. That is, a current circuit composed of a series circuit composed of the piezoelectric element 1 and the coil 2 is formed. In this current circuit, a current i indicated by an arrow in the drawing is shown.
_EE (t) flows. This current acts so that the energy (part thereof) stored in the piezoelectric element is transferred to the coil 2. Piezoelectric element 1 to coil 2
The transfer of energy to the electrodes reduces the voltage developed on the piezoelectric element and the outer diameter of the element.

When the discharge switch 5 is opened immediately after the discharge switch 5 is closed (for example, after several μs), the state shown in FIG. 5 occurs. That is, the piezoelectric elements 1
A closed current circuit consisting of the capacitor 9, the diode 4 and the coil 2 is formed. In this current circuit, the current i _EA (t) indicated by the arrow in FIG.
Flows. This current acts so that the energy stored in the coil 2 is completely supplied back to the capacitor 9. After the energy transfer from the coil 2 to the capacitor 9, the steady state already described above of the circuit of FIG. 1 is realized again.

Already or before or afterwards (depending on the desired duration of the discharge process), the discharge switch 5 is newly closed and opened again, with the above-described process being repeated. With the new closing and opening of the discharge switch 5, the energy stored in the piezoelectric element 1 is further reduced and, correspondingly, the voltage developed on the piezoelectric element and the outer diameter of the element are likewise reduced.

When the above-mentioned closing and opening of the discharge switch 5 is repeated many times, the voltage generated at the piezoelectric element and the expansion of the piezoelectric element are reduced stepwise.

When the discharge switch 5 has been closed and opened a predetermined number of times and / or the piezoelectric element 1 has reached the desired discharge state, the discharge of the piezoelectric element is terminated by keeping the discharge switch 5 open. .

The degree and course of the charging and discharging can be determined by the frequency and duration of the opening and closing of the charging switch 3 and the discharging switch 5, the time during which each switch is closed and the respective time. The time during which the switches are open can be the same or different and can be varied at will, even within the respective charging or discharging process.

The operation of the charge switch 3 and the discharge switch 5 is performed by the closed loop control device 10. That is, the switch operation signal output from the closed loop control device 10 is supplied to the charge switch 3 and the discharge switch 5 via the drivers 11 and 12.

The closed-loop control device 10 opens and closes the charging switch 3 and the discharging switch 5 depending, inter alia, on the voltage developed on the piezoelectric element 1 and the magnitude of the charging or discharging current. That is, the voltage generated in the piezoelectric element 1 is supplied to the closed-loop controller 10 from the voltage measuring device 13, and the magnitude of the charging current or the discharging current is supplied from the current measuring device 14.

The charge switch 3 and the discharge switch 5
The charging current or the discharging current is the maximum value i _maxAnd minimum
i_minIt is operated to vibrate between. At that time
Value i _maxAnd the minimum value i_minIs the piezoelectric element 1
Is charged or discharged to a specified voltage within a specified time.
It is decided to be done.

When the piezoelectric element is used as an actuator in a fuel injection nozzle of a common rail injector of an internal combustion engine, as in this case, the predetermined time and the predetermined voltage vary, in particular, depending on: Do: (1) the amount of fuel to be injected per injection process, (2) engine speed, (3) pressure at the rails, and (4) engine temperature.

The course of the charging current and the resulting voltage at the piezoelectric element during charging of the piezoelectric element are shown in FIG. 6, the charging current course being represented by curve B and the voltage course being represented by curve B. Represented by A.

[0055] From FIG. 6, seen conditions caused when the charging process is terminated when t _A by the opening and an open maintenance of charging switch 3.

The end of the charging process acts to return the charging current to zero. But this time, as can be seen from FIG. 6, the reduction is not nearly jump to the charging current, not only achieved gradually in a time interval t _0. As a result, the voltage developed at the piezoelectric element will continue to increase after the end of the charging process, with the subsequent increase continuing until the charging current drops to zero. That is, the voltage generated in the piezoelectric element, the time interval t _0, the time t _A at taking have values u _A, rises to no longer change value u _{En d.}

The same applies to the discharge of the piezoelectric element.

In the example of this consideration, the closed-loop control device 10 controls charging of the piezoelectric element (opening and keeping the charging switch 3 open) or discharging of the piezoelectric element (opening and keeping the discharge switch 5 open) of the charging or discharging process. It ends when the voltage u _End resulting from the subsequent charging or discharging of the piezoelectric element, which is performed after termination, can start exactly from the fact that the piezoelectric element is at the desired voltage to be withdrawn by charging or discharging the element. It is stipulated that

This can be explained as follows in the example of this consideration.
And the instantaneous (discharge-) charge current i (t) and
And the voltage u generated instantaneously in the piezoelectric element_pConsider (t)
In consideration of this, the (discharge-)
Instantaneous termination by opening and keeping the switch open
(Final) voltage u that should occur when
_EndAnd this (final) voltage u_EndBut piezoelectric
The element should be brought by (discharge-) charging.
As soon as the (target) voltage is reached, the (discharge-) charging process
finish. That is, what is usually done in such cases
Unlike the instantaneous actual voltage,
Final voltage u_EndIs compared with the target voltage.

How to determine the final voltage u _End is based on the recognition that the piezoelectric element has approximately a capacitive characteristic in the example of this discussion. Therefore u
_End is given by the following equation.

[0061]

(Equation 1)

Where C _p is the capacitance of the piezoelectric element, i (t) is the charge current i _L (t) or discharge current i _E (t), and _up0 is )
An opening and an open maintenance time t _A occurs in the piezoelectric element to a voltage of the charging switch (voltage u _A in FIG. _6), t _A is (discharge -) and the time of opening and the open maintenance of the charging switch, and t ₀ is the time interval during which the (discharge-) charge current returns to zero within this time interval after opening and maintaining the (discharge-) charge switch.

[0063] Since the subsequent opening of the charging switch the charge state shown in FIG. 3 occurs, the charging current i _{L (t)} and generated in the piezoelectric element voltage u _{p (t)} the following equation

[0064]

(Equation 2)

Where i ₀ is the charging current at the time of opening and maintaining the charge switch, and L 0
Is the inductance of coil 2 and

[0066]

(Equation 3)

The following holds.

The time interval t ₀ during which the charging current drops to zero during this time is calculated as follows:

[0069]

(Equation 4)

I _L (t) in equation (2) and u _L in equation (3)
Substituting _p (t) and t ₀ of equation (5) into equation (1), the final voltage u _End sought is:

[0071]

(Equation 5)

The final voltage u _End is a function of the charging current and the voltage developed at the piezoelectric element at the time of opening and maintaining the charging switch. That is, it can be calculated in each case according to equation (6) at the actual time or can be taken from the corresponding correspondence table.

The calculation of the final voltage generated when the piezoelectric element is discharged is performed similarly to the calculation of the final voltage generated when the piezoelectric element is charged.

In the case of discharging after opening the discharging switch, FIG.
Since the state shown in the figure occurs, the voltage u _{p (t)} resulting discharge current i _{E (t)} and the piezoelectric elements are calculated as follows:

[0075]

(Equation 6)

[0076] In the above formula C _p is the capacitance of the capacitor 9, i ₀ is the discharge current at the time of opening and the opening sustain discharge switches, L is the inductance in it and u _B0 of coil 2, The voltage developed across the capacitor 9 at the time of opening and maintaining the discharge switch, and

[0077]

(Equation 7)

Holds.

The time interval t ₀ during which the discharge current drops to zero is calculated as follows:

[0080]

(Equation 8)

I _E (t) in equation (7) and u _E in equation (8)
_By substituting _p (t) and t ₀ of equation (10) into equation (1), the final voltage expected at the time of discharge can be accurately obtained in advance.

As in the case of the charging of the piezoelectric element, the final voltage to be determined can be calculated in real time or taken from a corresponding correspondence table.

The final voltage generated during discharging is different from the final voltage generated during charging, and additionally, the voltage u _{B 0} generated at the capacitor 9 at the time of opening and maintaining the discharge switch, ie, a total of 3 Relies on two independent variables. However, the dependence of the voltage u _B0 can be neglected. This is because this voltage usually only changes in a very narrow voltage range and this type of change is the final voltage u
_{This is because} it hardly affects _End . Therefore, when discharging the piezoelectric element, the discharge current and the voltage "only" generated in the piezoelectric element may be considered.

The (discharge-) charging current of the final voltage, the voltage generated in the piezoelectric element after the (discharge-) charging process by opening and maintaining the (discharge-) charge switch, and the Occurs, (discharge-)
The dependence of the charge switch on the voltage at the time of opening and holding open is shown in FIGS. 7 to 10 as a characteristic curve region, FIGS. 7 and 8 relating to the charging of the piezoelectric element and FIGS. FIG. 10 relates to the discharge of the piezoelectric element.

FIGS. 4 to 7 show that the resulting final voltage is
It depends very much on the (discharge-) charge current at the time of opening and maintaining the (discharge-) charge switch and, as a result, the time of opening and maintaining the (discharge-) charge switch on the piezo element It clearly shows that it is very important to rely not only on the instantaneously occurring voltage, but also on the instantaneous (discharge-) charging current.

In this connection, it should be taken into account that it is not advisable to make the (discharge-) charge current less variable by reducing the width in which it changes up and down. This means that the (discharge-) charge switch must be operated relatively frequently and at relatively short time intervals, which results in a significant increase in power loss.

The above-described charging and discharging of the piezoelectric element was performed using a closed-loop controller (closed-loop controller 10). Instead of a closed-loop control device, instead of a closed-loop control device, the (discharge-) charging current and the voltage developed on the piezoelectric element can be ascertained without these actual measurements, ie, for example, by calculation or using a table. Can also be used. In that case, the voltage measuring device 13 and the current measuring device 14
Can be omitted.

According to the described method and the described apparatus, irrespective of the details of their actual realization, the piezoelectric elements to be charged or discharged using them continue to be charged and discharged exactly as desired. Will be able to do it.

[Brief description of the drawings]

FIG. 1 is a schematic circuit diagram showing one embodiment of the device of the present invention for charging and discharging a piezoelectric element.

FIG. 2 is a circuit diagram for explaining a state occurring in the device of FIG. 1 during a first charging phase (charging switch 3 is closed).

FIG. 3 is a circuit diagram for explaining a state occurring in the device of FIG. 1 during a second charging phase (charging switch 3 is opened again).

FIG. 4 is a circuit diagram for explaining a state occurring in the device of FIG. 1 during a first discharge phase (the discharge switch 5 is closed).

FIG. 5 is a circuit diagram for explaining a state occurring in the device of FIG. 1 during a second discharge phase (the discharge switch 5 is opened again).

FIG. 6 is a waveform diagram showing a lapse of time of a charging current and a voltage generated in the piezoelectric element when the piezoelectric element is charged by the device of FIG. 1;

FIG. 7 is a characteristic curve region diagram for explaining a relationship generated between various electric quantities when the piezoelectric element is charged by the device of FIG. 1;

8 is another characteristic curve area diagram for explaining a relationship that occurs between various electric quantities when the piezoelectric element is charged by the device of FIG. 1. FIG.

FIG. 9 is a characteristic curve area diagram for explaining a relationship generated between various electric quantities when the piezoelectric element is discharged by the apparatus of FIG. 1;

FIG. 10 is another characteristic curve area diagram for explaining a relationship generated between various electric quantities when the piezoelectric element is discharged by the apparatus of FIG. 1;

FIG. 11 is a circuit schematic diagram of a conventional device for charging and discharging a piezoelectric element.

[Explanation of symbols]

Reference Signs List 1 piezoelectric element, 2 coil, 3 charge switch, 5 discharge switch, 9 capacitor, 10
Closed loop controller

 ──────────────────────────────────────────────────続 き Continued on the front page (72) Inventor Jörg Reineke, Germany Stuttgart Kalestraße 6 (72) Inventor Alexander Hock Stuttgart, Germany Knittlinger Strasse 1

Claims (8)

[Claims] 1. A method for charging and discharging a piezoelectric element to a desired voltage, characterized in that the charging or discharging process is terminated before a predetermined time to reach a desired voltage at the piezoelectric element. A method for charging and discharging a piezoelectric element. 2. A charging switch (3) provided in a charging circuit for charging or discharging the piezoelectric element (1).
2. The method for charging and discharging a piezoelectric element according to claim 1, wherein the method is performed by repeatedly opening and closing a discharge switch provided in a discharge circuit. 3. The method for charging and discharging a piezoelectric element according to claim 2, wherein the charging step or the discharging step is terminated by opening or maintaining an open state of the charging switch (3) or the discharging switch (5). Wherein the charging process or discharging process, the still exactly the desired voltage for the occurring voltage (u _p) charging to discharging current does not drop to zero in jumping manner in the piezoelectric element (1) 4. The method according to claim 1, wherein the method ends at a point where it can start from ascending or descending until reaching. 5. When the charging or discharging process is instantaneously terminated during the charging or discharging of the piezoelectric element (1), the piezoelectric element should still be subsequently charged or discharged. Final voltage (u _End )
5. The method for charging and discharging a piezoelectric element according to claim 4, wherein s is continuously determined. 6. The continuously determined final voltage (u
_{6. The} method as claimed in claim 5, wherein _End ) is continuously compared with a voltage at which the piezoelectric element (1) is to be taken up by charging or discharging. 7. The method for charging and discharging a piezoelectric element according to claim 5, wherein the final voltage (u _End ) is determined in consideration of an instantaneous charging or discharging current and a voltage instantaneously generated in the piezoelectric element. . 8. A device for charging and discharging a piezoelectric element (1) to a desired voltage such that the charging or discharging process is terminated before a predetermined time to reach a desired voltage at the piezoelectric element. A device for charging and discharging a piezoelectric element, comprising a designed open-loop or closed-loop controller (10).