DE102016204363A1 - Method of operating a piezoelectric element, apparatus for carrying out the method, control unit program and control unit program product - Google Patents

Abstract

The invention relates to a method for operating a piezoelectric element (18) which is charged by means of a charging switch (24) to a piezoelectric voltage (u) and discharged by means of a discharge switch (26), wherein the charging switch (24) and / or discharge switch ( 26) is operated clocked during the charging / discharging process, wherein the charging switch / discharging switch (24, 26) is switched on and off in chronological order, in which the charging current (34) and discharge current (60) via an inductive component (32) out is, with a configured as two-point control current control, in which the current value (48) of the charging current / discharge current (34, 60) is measured and compared with a lower and upper current threshold (50, 52). The method according to the invention is characterized in that the piezoelectric voltage (u) applied to the piezoelectric element (18) is measured and that a threshold difference (d) between the upper and lower current thresholds (50, 52) in dependence on the Piezo voltage (u) is set such that at smaller piezo voltage (u) a smaller threshold difference (d) is specified. Alternatively or additionally, the threshold difference (d) in dependence on the voltage difference between the intermediate circuit voltage (16) and the piezoelectric voltage (u) is set such that at smaller voltage difference, a smaller threshold difference (d) is specified.

Description

The invention relates to a method for operating a piezoelectric element, which is preferably used in a fuel injection valve, and to an apparatus for carrying out the method.

The invention further relates to a control unit program, with the aid of which the piezoelectric element is operated, as well as a control unit program product with a program code stored on a machine-readable carrier for carrying out the method.

State of the art

In the patent DE 600 18 549 T2 there are described a fuel injection system and a method of operating a fuel injection system, wherein the fuel injection system includes a piezoelectric element. A diagnosis is provided in which the charging current flowing to the piezoelectric element and the discharge current flowing away from the piezoelectric element are each directly evaluated. The diagnosis allows a statement about which voltage is applied to the piezoelectric element, which determines the dimensions of the piezoelectric element. If the piezoelectric element determines the position of an actuating element in a valve, for example in a fuel injection valve, the diagnosis can be used to detect the position of the actuating element in the valve, which is essential for exact metering of the injected fuel quantity. The described diagnosis makes it possible, in particular, to detect a short circuit to an electrical circuit ground. Depending on the position of the short circuit in the circuit, a short circuit to the electrical circuit ground may result in the piezoelectric element being unable to be charged and / or discharged in a defined manner.

The patent DE 198 41 002 C1 describes a method for diagnosing a short circuit on a piezoelectric element. A calculation is made of a charge amount flowing out of a buffer capacitor, which is compared with an integral of the current flowing during the charging process. If deviations are detected in a subsequent comparison of the charge quantities, a faulty operating state, such as a short circuit, is assumed.

A method and apparatus for charging and discharging a piezoelectric element is disclosed, for example, in the publication DE 198 14 594 A1 in which, instead of a continuous charging or discharging operation, a clocked operation is provided, in which a buffer capacitor is connected to the piezoelectric element during the charging process or during the discharging process in each case in rapid time sequence. The voltage applied to a buffer capacitor is called the intermediate circuit voltage. This term will also be used in the following description of embodiments of the invention. As an energy buffer an inductive component is provided. The current flow through the inductive component is maintained during the shutdown phases of the charging process or the discharge process in each case via a freewheeling circuit. The method described allows charging or discharging the piezoelectric element with a predetermined number, predetermined size and predetermined time intervals of successive charge levels. In particular, it is provided that the charging process or the discharging process is already completed a certain time before reaching the desired voltage on the piezoelectric element. This can prevent the piezoelectric element from being charged or discharged further than desired. This case can occur because the piezoelectric element is still charged or discharged for a certain time after completion of the charging or discharging process due to the charging or discharging current, which does not suddenly drop to zero. The described method therefore makes it possible to set the voltage at the piezoelectric element with high accuracy.

In the non-prepublished publication DE 10 2015 216 854 A1 there is provided a method of operating a piezoelectric element which is charged by means of a charging switch and discharged by means of a discharge switch in which the charging switch and / or discharge switch is operated in a clocked manner during the charging / discharging process in which the charging current and the discharging current are conducted via an inductive component is performed in which the charging current / discharge current is switched off during the charging / discharging during charging / discharging over a freewheel and at which the voltage applied to the piezoelectric element during the charging / discharging process is detected when the charging switch / discharging switch. The method described is characterized in that the actual value of the charging current / discharge current is measured, that a current controller regulates the actual value of the charging current / discharge current to a desired value and that the nominal value of the charging current / discharge current is corrected as a function of the detected voltage at the piezoelectric element , In this case, the current regulation can be carried out as a two-point control, in which the charging current / discharge current is compared with a lower and an upper current threshold value.

It is an object of the invention to provide a method and an apparatus for operating a piezoelectric element, which ensure the lowest possible loading of electronic components.

The object is achieved by the features specified in the independent method claim as well as in the independent device claim.

Disclosure of the invention

The inventive method for operating a piezoelectric element, which is charged by means of a charging switch to an operating voltage or piezoelectric voltage and discharged by means of a discharge switch, assumes that the charging switch and / or discharge switch is operated clocked during the charging / discharging process, the charging switch / Discharge switch is turned on and off in chronological order, that the charging current and discharge current is passed through an inductive component and that designed as a two-point control current regulation is provided, wherein the actual value of the charging current / discharge measured and measured with a lower and upper stream Threshold value is compared.

According to a first embodiment of the method according to the invention it is provided that the voltage applied to the piezoelectric element piezoelectric voltage is measured and that the threshold difference between the upper and lower current threshold depending on the piezoelectric voltage is set such that at smaller piezoelectric voltage, a smaller threshold Difference is specified.

According to a second embodiment of the method according to the invention it is provided that the voltage applied to the piezoelectric element piezoelectric voltage is measured and that the threshold difference between the upper and the lower current threshold depending on the voltage difference between an intermediate circuit voltage and the piezoelectric voltage is set such that at smaller voltage difference, a smaller threshold difference is specified. The DC link voltage has already been defined at the outset as the voltage applied to a buffer capacitor.

The procedures according to the invention make it possible to set an at least approximately constant switching frequency of the clocked operation of a piezo output stage. Due to the constant switching frequency, an optimum can be set between the losses in the charging switch or discharge switch and the loss in the inductive component.

Advantageous developments and refinements of the procedure according to the invention are each objects of dependent claims.

An advantageous embodiment provides that the threshold difference is set as a function of the smaller value of either the piezo voltage or the voltage difference between the intermediate circuit voltage and the piezo voltage. The embodiment therefore relates to a combination of the first and the second embodiment of the invention. As a result, without an extrapolation, for example, an at least approximately constant switching frequency in the entire region of the charging process or the discharging process of the piezoelectric element can be specified in a particularly simple manner.

The device according to the invention for carrying out the method provides a specially designed control unit which is able to carry out the individual method steps and to initiate the required actions.

The control unit program according to the invention provides that all steps of the method according to the invention are carried out when it runs in a control unit.

The control unit program product according to the invention with a program code stored on a machine-readable carrier executes the method according to the invention when the program is executed in a control unit.

Further advantageous developments and refinements of the method and the device according to the invention will become apparent from the description.

An embodiment of the invention is illustrated in the drawing and explained in more detail in the following description.

drawing

1 shows a piezo output stage during a charging process of a piezoelectric element with the charging switch turned on,

2 shows the piezo amplifier during charging with the charging switch open,

3 shows a block diagram of at least part of a control unit, by means of which a method according to the invention is carried out,

4 shows the piezo output stage during a discharging operation of the piezoelectric element with the discharge switch turned on,

5 shows the piezo output stage during the discharge process with the discharge switch open,

6 shows in the upper part of a time diagram of a charging current and in the lower part of a time chart of a piezoelectric voltage according to the prior art,

7 shows in the upper part of a time diagram of a charging current and in the lower part of a time course of a piezoelectric voltage also according to the prior art,

8th shows in the upper part of a time diagram of a charging current and in the lower part of a time chart of a piezoelectric voltage according to the invention and

9 shows in the upper partial image a time course of a charging current and in the lower part of a time course of a piezoelectric voltage according to a development of the invention.

Detailed description

1 shows a piezo amplifier 10 during a charge. The from a voltage source 12 provided voltage is by means of a DC / DC converter 14 to a DC link voltage 16 brought to operate a piezoelectric element 18 is sufficiently high. For buffering the DC link voltage 16 is a buffer capacitor 20 intended.

The piezoelectric element 18 is for example provided in a valve, in particular a fuel injection valve, and serves to actuate a valve element. The piezoelectric element 18 expands or contracts depending on the piezoelectric element 18 adjacent piezo voltage u or on the piezoelectric element 18 together.

Provided, for example, a two-quadrant power amplifier, at its output 22 can provide a unipolar voltage and a bipolar current. The piezo amplifier 10 contains a first switch 24 which in the following as charging switch 24 referred to as. The piezo amplifier 10 also contains a second switch 26 in the following as a discharge switch 26 referred to as. The charging switch 24 is from a first switching signal S1 and the discharge switch 26 controlled by a second switching signal S2. The charging switch 24 is a first freewheel 28 assigned, which is realized in the embodiment shown as a freewheeling diode. Provided may equally be an actively connected freewheel. The discharge switch 26 is a second freewheel 30 assigned, which is also realized in the embodiment shown as a freewheeling diode. Also the second freewheel 30 may alternatively be an actively connected freewheel.

Since the piezoelectric element 18 forms a capacitive load is usually at least one inductive component 32 in series with the piezoelectric element 18 connected.

During charging, the charging switch becomes 24 switched on and off in rapid time sequence by means of the first switching signal S1. In 1 are the conditions when the charging switch is switched on 24 shown. A charging current 34 flows through the charging switch 24 , the inductive component 32 , the piezoelectric element 18 back to the buffer capacitor 20 , In 2 are the conditions when the charging switch is switched off 24 shown.

Those in 2 parts shown with the in 1 match shown parts are each provided with the same reference numerals. This agreement also applies to the following figures.

With the charging switch switched off 24 the charging current flows 34 continue via the inductive component 32 , the piezoelectric element 18 and over the second freewheel 30 ,

The first switching signal S1 is specified in the context of a clocked operation, so that the first switching signal S1 corresponds to a clock signal of the clocked operation. The clock signal or the first switching signal S1 can be defined either in controlled operation or in the context of current regulation. The following is based on a current control. The current controller is realized as a two-position controller, which is measured by the piezoelectric element 18 comparing current i with upper and lower current thresholds. The difference of the current thresholds corresponds to a given hysteresis. For detecting the current i is a current sensor 36 provided, which may be realized for example as a low-impedance shunt resistor or as an inductive current sensor.

A control unit 40 which provides the first switching signal S1 and also the second switching signal S2 described later is in 3 shown. The control unit 40 contains a current regulator 42 that has a current setpoint 44 , which of a setpoint generator 46 is provided in a comparator with a current actual value 48 compares that corresponds to the measured current i. The current setpoint 44 can correspond to a medium current. The current- setpoint 44 consists of two current thresholds 50 . 52 together between which a threshold difference d occurs, which is referred to as hysteresis.

If the current actual value 48 the lower current threshold 50 falls below, sets the current regulator 42 the first switching signal S1 ready. When the rising current i is the upper current threshold 52 exceeds, the current controller takes 42 the first switching signal S1 back. The first switching signal S1 for the charging switch 24 puts the current regulator 42 only ready when a charging signal 54 is present, which is provided for example by a motor controller not shown in detail.

The current controller sets in the same way 42 the second switching signal S2, which indicates the discharging operation of the piezoelectric element 18 sets. The current setpoint 44 or the two current thresholds 50 . 52 are negative values in this case. The second switching signal S2 for the discharge switch 26 puts the current regulator 42 only ready when a discharge signal 56 is present.

The further function blocks as well as the further signals, which are in 3 are discussed below.

4 shows the piezo amplifier 10 during the discharging operation of the piezoelectric element 18 when the discharge switch is switched on 26 and 5 shows the piezo amplifier 10 during discharging when the discharge switch is off 26 , During discharging, the discharge switch becomes 26 controlled in the context of the clocked operation with the second switching signal S2.

According to 4 flows when the discharge switch is closed 26 a discharge current 60 from the piezoelectric element 18 via the inductive component 32 and via the discharge switch 26 back to the piezoelectric element 18 ,

5 shows the conditions with the discharge switch switched off 26 , In this case, the discharge current flows 60 instead of the discharge switch 26 over the first freewheel 28 back to the buffer capacitor 20 ,

6 shows in the upper part of a current waveform diagram and in the lower part of a waveform, as in the previously described Piezoendstufe 10 occur in the prior art, if no further action is taken. Therefore, the current controller realized as a two-position controller is required 42 with a fixed hysteresis. The loading process is shown as an example. The current i does not only oscillate between the upper and lower current thresholds 50 . 52 but shows due to switching delay times and signal propagation times in the current controller 42 distinct overshoots 70 or undershooters 72 , The amount of overshoot or undershoot depends among other things on the piezo voltage u at each time. The piezo voltage u determines according to the electromagnetic induction by the inductance 32 the current gradient and thus at a given switching delay time and signal propagation time, the amount of overshoot / undershoot.

As the piezo voltage u changes during the charging process, the amount of overshoot / undershoot changes accordingly. In this case, that at the beginning of the charging process, the overshoot 70 are higher than at the end of the charging process, while the undershoots 72 to behave contrary.

7 shows in the upper part of a current waveform diagram and in the lower part of a waveform, as in the piezo amplifier according to the unpublished published patent application DE 10 2015 216 854 A1 occur. There is a compensation of the reaction of the piezoelectric voltage u on the overshoot / undershoot 70 . 72 by adjusting current thresholds 74 . 76 intended. In 7 are externally specified current limits 78 . 80 entered, between which the current i oscillates. Furthermore, in 7 as a function of the piezo voltage u calculated current thresholds 74 . 76 registered, which increase with increasing piezo voltage u in each case over time. When the current regulator 42 the measured current i with the calculated current thresholds 74 . 76 compares and at a threshold exceeding the first switching signal S1 provides or withdraws, although continue to occur the overshoot 70 or undershooters 72 on, but are the overshoots 70 or undershooters 72 within the externally specified current limits 78 . 80 ,

The externally specified current limits 78 . 80 could according to 3 the setpoint generator 46 are made available, the two current thresholds due to the known switching times and signal transit times ts 74 . 76 according to the prior art further in dependence on the piezoelectric voltage u or alternatively of the on the piezoelectric element 18 calculate charge C and the current regulator 42 for comparison with the measured current actual value 48 could provide.

About the hysteresis, ie the threshold difference d between the lower and the upper current threshold 74 . 76 can the Switching frequency of the piezo output stage 10 be adjusted according to the prior art. In determining the switching frequency in particular the switching losses in the switches 24 . 26 as well as in the inductive element 32 occurring ripple current, because of the expected switching losses and the expected ripple current depends on the selection of components, as an overload of the components leads to a malfunction or complete failure of the components. The higher the switching frequency, the higher the switching losses in the switches 24 . 26 However, the expected ripple current becomes smaller. By setting the hysteresis or threshold difference d between the upper and lower current thresholds, there can be an optimum between those in the switches 24 . 26 as well as in the inductive element 32 occurring electrical power losses can be achieved.

A rating of the in the upper panels of the 6 and 7 shown in each case temporal current profile that the switching frequency at the beginning and at the end of the charging of the piezoelectric element 18 is significantly lower than in the middle. This is because the piezo voltage u is near the ground potential at the beginning of the charging process and near the intermediate circuit voltage at the end of the charging process 16 lies. According to the self-induction of the inductive element 32 It follows that at these times, a smaller current gradient occurs, so that a comparatively longer period elapses until the upper or lower current threshold 74 . 76 is exceeded or fallen below, so that there is a low switching frequency. Where: u (t) = L (di / dt).

A switching frequency which is as constant as possible over at least part of the charging process or discharging process in the context of the pulsed operation of the piezo output stage 10 According to the invention is achieved in that the hysteresis or the threshold difference d between the upper and lower current threshold value set according to the invention 50 . 52 is specified variably.

According to a first embodiment of the invention, it is provided that the threshold difference d between the upper and lower current threshold 50 . 52 is determined as a function of the piezoelectric voltage u, wherein the determination is made such that at a comparatively small piezoelectric voltage u a smaller threshold difference d and at a rising piezo voltage u an increasingly larger threshold difference d between the upper and lower current threshold 50 . 52 is provided. Without further intervention, the switching frequency can be kept at least approximately constant until at least approximately half of the charging process or from half of the discharging process until the end with this measure.

However, since the starting voltage of at least approximately 0 volts and the piezoelectric voltage u, to which the piezoelectric element 18 is to be brought, are known, in the case of charging when reaching about half the DC link voltage 16 the threshold difference d are again reduced. Accordingly, in the case of the discharge process until reaching about half the intermediate circuit voltage 16 the threshold difference d are increased. Overall, therefore, over the entire time charging or discharging an at least approximately constant switching frequency of the clocked operation of the piezo amplifier 10 be adjusted.

According to a second embodiment of the invention, it is provided that the threshold difference d between the upper and lower current threshold 50 . 52 depending on the voltage difference between the DC link voltage 16 and the piezo voltage u is set, wherein the determination is made such that at a comparatively high voltage difference between the intermediate circuit voltage 16 and the piezo voltage u a larger threshold difference d and at a decreasing voltage difference between the DC link voltage 16 and the piezo voltage u increasingly has a smaller threshold difference d between the upper and lower current thresholds 50 . 52 is given.

Without further intervention, the switching frequency of the piezo output stage can be achieved with this measure 10 be kept at least approximately constant in the context of the clocked operation at least from about half of the charging process or to about half of the discharging process. However, since it is known that the threshold difference d between the DC link voltage 16 and the piezo voltage u at the beginning of the charging process is maximum, can at the beginning of the charging process until approximately to reach half the DC link voltage 16 the threshold difference d are increased and, correspondingly, during the discharging process, from reaching approximately half of the intermediate circuit voltage 16 the threshold difference d can be reduced. Overall, therefore, even in the second embodiment of the invention over the entire time charging or discharging an at least approximately constant switching frequency of the clocked operation of the piezo amplifier 10 be adjusted.

A particularly advantageous embodiment provides that the threshold difference d between the upper and the lower Strom- threshold 50 . 52 depending on the smaller value of either the piezo voltage u or the voltage difference between the intermediate circuit voltage 16 and the piezo voltage u is set. By selecting the minimum of the two values, the optionally provided extrapolation described above in each case determines the threshold difference d, so that the threshold difference d directly as a function of the piezo voltage u or the voltage difference between the intermediate circuit voltage 16 and the piezo voltage u can be set, depending on which of the two values is smaller.

8th shows in the upper part of the diagram according to the invention the determination of the upper or lower current threshold value 50 . 52 , In 8th the case of a complete compensation is shown, in which the switching frequency of the piezo output stage 10 is at least approximately constant throughout the charging process. 8th shows at the same time that overshoot 70 above the upper current threshold 50 or undershoot 72 below the lower power threshold 52 occur, with the peak values are not specified.

According to an advantageous development is therefore beyond the invention, a compensation of the upper and lower current threshold 50 . 52 depending on the piezoelectric voltage u provided to a result according to 7 in which the peak values of the current i are within the externally set upper current limit 78 or lower current limit 80 lie.

QUOTES INCLUDE IN THE DESCRIPTION

This list of the documents listed by the applicant has been generated automatically and is included solely for the better information of the reader. The list is not part of the German patent or utility model application. The DPMA assumes no liability for any errors or omissions.

Cited patent literature

• DE 60018549 T2 [0003]
• DE 19841002 C1 [0004]
• DE 19814594 A1 [0005]
• DE 102015216854 A1 [0006, 0046]

Claims (10)

Method for operating a piezoelectric element ( 18 ), which by means of a charging switch ( 24 ) is charged to a piezo voltage (u) and by means of a discharge switch ( 26 ), in which the charging switch ( 24 ) and / or discharge switch ( 26 ) is clocked during the charging / discharging process, the charging switch / discharging switch ( 24 . 26 ) is switched on and off in chronological order, in which the charging current ( 34 ) and discharge current ( 60 ) via an inductive component ( 32 ), with a current control designed as a two-point control, in which the current actual value ( 48 ) of the charging current / discharge current ( 34 . 60 ) and with a lower and upper current threshold ( 50 . 52 ), characterized in that the piezoelectric element ( 18 ) and that a threshold difference (d) between the upper and lower current thresholds ( 50 . 52 ) as a function of the piezoelectric voltage (u) is set such that at a smaller piezoelectric voltage (u) a smaller threshold difference (d) is specified. Method for operating a piezoelectric element ( 18 ), which by means of a charging switch ( 24 ) is charged to a piezo voltage (u) and by means of a discharge switch ( 26 ), in which the charging switch ( 24 ) and / or discharge switch ( 26 ) is clocked during the charging / discharging process, the charging switch / discharging switch ( 24 . 26 ) is switched on and off in chronological order, in which the charging current ( 34 ) and discharge current ( 60 ) via an inductive component ( 32 ), with a current control designed as a two-point control, in which the current actual value ( 48 ) of the charging current / discharge current ( 34 . 60 ) and with a lower and upper current threshold ( 50 . 52 ), characterized in that the piezoelectric element ( 18 ) and that a threshold difference (d) between the upper and lower current thresholds ( 50 . 52 ) as a function of the voltage difference between a DC link voltage ( 16 ) and the piezo voltage (u) is set such that at smaller voltage difference, a smaller threshold difference (d) is specified. Method according to Claims 1 and 2, characterized in that the threshold difference (d) is dependent on the smaller value of either the piezo voltage (u) or the voltage difference between the intermediate circuit voltage ( 16 ) and the piezo voltage (u) is set. A method according to claim 1, characterized in that the threshold difference (d) in dependence on an externally predetermined upper and lower current limit ( 78 . 80 ) is calculated. A method according to claim 4, characterized in that the calculation in dependence on the switching time of the charging switch ( 24 ) and discharge switch ( 26 ) he follows. Method according to claim 4 or 5, characterized in that the current threshold value ( 50 . 52 ) is corrected in such a way in dependence on the piezo voltage (u) that with higher piezo voltage (u) a magnitude higher current threshold ( 50 . 52 ) is given. Device for operating a piezoelectric element ( 18 ), characterized in that a specially designed control unit ( 40 ) is provided for carrying out the method according to one of the preceding claims. Device that the piezoelectric element ( 18 ) Driving part of a fuel injection valve is. Control unit program, which carries out all the steps of a method according to one of claims 1 to 8, when it is in a control unit ( 40 ) expires. A control program product having a program code stored on a machine-readable carrier for carrying out the method according to one of claims 1 to 8, when the program is stored in a control device ( 40 ) is performed.

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