WO2004001794A2

WO2004001794A2 - Method for controlling and adjusting the length of a piezoelectric actuator (pa) and an electronic unit for regulating said method

Info

Publication number: WO2004001794A2
Application number: PCT/IB2003/004600
Authority: WO
Inventors: Georg Bachmaier; Bernhard Fischer; Bernhard Gottlieb; Andreas Kappel; Hans Meixner; Randolf Mock; Tim Schwebel
Original assignee: Siemens Aktiengesellschaft
Priority date: 2002-06-24
Filing date: 2003-06-20
Publication date: 2003-12-31
Also published as: DE10228146A1; WO2004001794A3

Abstract

The invention relates to a method for controlling piezoelectric actuators (PA), in particular piezoelectric multilayer actuators (PMA), which are regularly subjected to a positive voltage during operation to induce elongation. According to the invention, the PMA is subjected to a restricted negative voltage or offset voltage, which does not cause a polarization reversal, for predefined operating modes of the PMA, in order to return the latter to a definable initial position.

Description

description

Process for controlling and adjusting the length of a piezoelectric actuator (PA) and electronic unit for controlling the process

The invention relates to all directly driven ^high: pressure injection valves or valves with no hydraulic power transmission member such as bearings or translator for direct fuel injection, in which a working chamber is filled with the necessary for operating the high-pressure and a piezoelectric multilayer actuator (PMA) in the electrical control by elongation directly opens a servo valve or a nozzle needle or any other directly driven metering valve. By opening the servo or control valve, a pressure drop is generated in a control chamber, which causes the high-pressure injector to open.

This working principle is currently used in many high-pressure injection valves for diesel direct injection.

Problems sometimes arise in the operation of such high-pressure injection valves if the valve has become hot at least once. Although the thermal expansion of the valve and the PMA are matched, the length of the PMA often changes too much. A difference in length of a few μm is sufficient and the control chamber of the high-pressure injector or a similar injector is no longer securely closed. As a result, if there is a leak in the control chamber, the high pressure pump can no longer build up the necessary pressure and the high pressure injector fails.

If a sufficiently large pressure difference is built up between the valve seat and the working chamber (servo control), the injector itself may even leak. In In this case, fuel is injected into the combustion chamber in an uncontrolled manner.

So far, people have relied on the fact that the problem can be mastered through careful thermal coordination of the components and a so-called "thermal gap". A "thermal gap" means that there is a gap of approx. 5 μm between the end of the PMA and the servo valve as a buffer for tuning errors. In cases where this measure is insufficient, the problem has not yet been solved.

To illustrate the problem of the different linear expansion of the piezo actuator and housing in the piezo diesel high pressure injector:

FIG. 1 schematically shows the function of the piezo actuator in the piezo diesel high-pressure injection valve when the servo valve is opened. The servo valve is closed in the idle state. The piezo actuator lengthens when an external electrical field is applied, which acts in the direction of the preferred polarization of the piezo ceramic. This orientation of the external electric field is simply referred to as positive and the opposite orientation as negative voltage. When extending itself, the piezo actuator first overcomes the thermal gap, i. H. the safety distance for different thermal dilations, and then presses on the servo valve. In the

As a result, the high-pressure injector opens because the pressure in the working chamber (no longer shown) of the high-pressure injector drops.

Since the Piezo Diesel high pressure injector is used in the automotive sector, operating temperatures between -40 ° C and + 120 ° C are possible. The length of the piezo actuator and the housing for the piezo actuator can change by a few μm relative to one another. That is why the so-called "thermal gap" between the servo valve and the piezo actuator is left blank during production. The length of the piezo actuator depends not only on the temperature but also on the state of polarization, and from its history (hysteresis). These processes can also change the coefficient of thermal expansion of the piezo actuator. It is therefore possible that the tolerance band for the thermal gap of 3-5 μm is left over time. In the worst case, there is pressure leakage in the piezo diesel high-pressure injection valve because the piezo actuator presses the servo valve on even when it is not being activated. (see Fig. 2). However, the function of the Piezo Diesel high-pressure injector suffers beforehand because the metering of small injection quantities (eg in the so-called "pre-injection") loses precision outside the tolerance band for the thermal gap of 3-5 μm.

Figure 2 shows the worst case of pressure leakage on the servo valve. It can mean that the high pressure pump (not shown) can no longer build up the necessary pressure in the working chamber and the piezo diesel high pressure injector fails.

As described, the piezo actuator reacts to the application of an external electric field, which acts against the direction of preferential polarization of the piezoceramic, just as it reacts to a positive voltage with an extension. It is therefore possible to restore the ideal state of FIG. 1 by applying a negative electrical voltage.

The aim of the invention is to eliminate the disadvantages of the prior art.

This object is achieved by the one or more combinations of features of the at least one main claim.

The operating area of the PMA, which has to perform an elongation for shutdown, is additionally affected by negative voltages extended so that its length can be varied in both directions (lengthening and shortening). This scope in the characteristics of a piezoceramic is large enough to shorten the PMA to such an extent that the high-pressure injection valve can be put into operation again, or the servo valve with the control chamber can be securely closed without the polarities in the ceramic being reversed.

Existing control electronics are thus expanded by the possibility of being able to compensate for the thermal effects described above by an additional negative offset voltage.

As the measurements in the exemplary embodiments show, there is a usable scope to utilize a negative voltage before the depolarization and then the polarization of the ceramic begins.

This scope is large enough to shorten the PMA to such an extent that the high-pressure injector can be put into operation again.

Advantages of the invention:

• The problem is solved without mechanical changes to the high pressure injector.

• The negative voltage only needs to be applied if necessary, such as a restart after excessive heating or the like. In all other operating states, the electrical control of the high-pressure injection valve can take place in a conventional manner.

• The process can be regulated. The negative voltage can be increased slowly until the high pressure injector starts. This avoids unnecessarily large field strengths.

The invention presents a simple solution that can be used without great effort and without the functionality of the current hasten control electronics to enable an individually adjustable offset voltage for each actuator. The solution described here provides for the circuit to be expanded by a capacitance, a resistor and a more controllable voltage source, as shown in FIG. 3.

The capacitive voltage divider (cinjector; Ci) reduces the voltage that drops at the injector according to the following relationship:

But with an adjustable, positive voltage source Vi, one is able to set a negative offset voltage on the injector, which is independent of the injector capacity. The resistance R _x determines the time constant with which the potential at the node Ci, Cinjetor and Ri is balanced.

FIG. 4 shows how the voltage profile of a current-controlled output stage can look for the procedure according to the invention.

Advantages of this solution:

- No negative voltage is required.

- The circuitry is low.

- The additional electrical losses caused by the circuit are small - The offset voltage is independent of the operating state of the injector, it can be applied and changed during the start phase and during operation.

The capacitance Ci only requires a rather low dielectric strength. The terminal behavior remains almost the same, so no further changes to the control electronics are necessary.

In the following, schematic exemplary embodiments that do not restrict the invention are described. The figures show in detail:

1 shows a functional group of the piezo diesel high-pressure injection valve,

2 shows a pressure leak on the servo valve due to uneven thermal expansion of the piezo actuator and housing / housing,

3 shows an electrical circuit for generating a regulated offset voltage,

4 shows a voltage curve of a current-controlled output stage which has been expanded by the function described in this invention,

11 shows a hysteresis curve of a diesel PMA in its initial state and within the normal voltage range from 0 V to 160 V,

12 and 13 a first cycle in the negative range to -10 V,

14 shows that at -20 V the contraction is approximately -7 μm; the

Oval is not closed after the first cycle,

15 shows the second cycle down to -20 V; the hysteresis curve now shows the usual oval shape, 16 shows a first cycle with a negative range down to -30 V; the contraction reaches -10 μm,

17 shows a second cycle down to -20 V; the hysteresis curve now shows the usual oval shape,

18 shows the double polarization of the diesel piezo stack,

19 shows the measurement sequence for the fourth cycle, in which the PMA is operated for the first time with at least -20 V,

FIG. 20 shows the measurement sequence for step 4, corresponding to FIG. 19, in the hysteresis figure shown.

Operation of a diesel piezo stack with negative voltage

Motivation:

Current problems with internal pressure leakage in Piezo Diesel high-pressure injection valves make it clear that it is not enough to adjust the thermal expansion coefficients of the injector parts and the Piezo Multilayer Actuator (PMA). There are other effects that change the length of the PMA. If this length increases, the PMA relieves pressure on a valve seat and, in the worst case, even presses it open. This leads to an internal pressure leak and the high pressure pump cannot build up enough pressure. This causes the high pressure injector to fail. This can happen if the critical lengths of the PMA and valve differ by only a few μm. The Diesel Piezo HPDI injector (high pressure direct injection) only works optimally if a gap of 3-5 μm remains open between the PMA and the servo valve. Fortunately, with a critical effect, namely the piezo effect itself, you can correct the length of the PMA again. In normal operation the works PMA with positive voltage, typically between 0 and 160 volts. This means that the external electric field acts in the same direction in which the preferred polarization of the PMA points. Analogously, by convention, one speaks of negative voltage if the external electrical field acts in the opposite direction to the preferred polarization of the PMA. Negative voltages lead first to a shortening of the PMA, then to a polarity reversal and then to a renewed elongation. It is shown that it is possible to use the area that leads to the shortening of the PMA to correct the length without depolarizing the PMA.

Experimental results:

A standard diesel actuator with 30 mm length was used for the investigations. 5 test cycles were run:

1st cycle from 0 V to 160 V and back; the usual mode of operation 2nd cycle between -10 V and +160 V as a first test for possible reductions

3rd cycle between -20 V and +160 V as a second test for possible reductions

4. Cycle between -30 V and +160 V as a third test for possible reductions

5. Cycle between -160 V and +160 V as a test for the limits of possible reductions and for examining the polarity reversal processes.

Cycles 2-4 were repeated to further investigate the creep behavior and hysteresis of the PMA. Since the existing electronics are not designed for negative voltages, a tool was used to represent this voltage range. The measurement in the positive branch was started, e.g. B. at 0 V to +160 V and again to 0 V.

Then the cycle was briefly interrupted to swap the connections. And the rest of the measurement was positive, ie unchanged polarity in relation to the electronics. Because of the interchanged connections, however, this affects the PMA like a negative voltage.

The results are shown in FIGS. 11 to 20.

Fig. 11:

If one only looked at FIG. 11, one might think that the length of the piezo stack at 0 V and the same temperature is always the same. However, this is not always the case. For example, the polarization of the piezo stack can change. This can happen or be necessary if the piezo stack gets hot. This makes the individual areas flexible and rearranges them. Then a different total length results after cooling. Even the coefficient of thermal expansion can change itself. Once the length and the coefficient of thermal expansion have been coordinated, this does not have to be the case for all times.

12 and 13:

At -10 V the shortening is about 2 μm. Due to the hysteresis, the PMA is still shortened by 1 μm at 0 V.

Fig. 14:

At -20 V, the shortening is already -7 μm. The oval of the hysteresis curve is not closed on the first cycle down to -20 V.

Fig. 15:

The hysteresis now shows itself in the usual form of a closed oval curve.

Fig. 16: The first cycle down to -30 V is shown. The shortening reaches -10 μm.

Fig. 17: The second cycle to -0 V is shown. The hysteresis is again shown in the usual form of a closed oval curve.

Fig. 18: Double polarization of a diesel piezo stack:

The polarity reversal begins between -70 V and -80 V and, for reasons of symmetry, the polarity is reversed again at +70 V to +80 V, the piezo stack is not shortened further. The outer field and preferred polarization direction point in the same direction again, which again leads to an elongation of the piezo stack. The available area up to the polarity reversal can be used precisely to shorten the PMA. Figure 18 shows that a shortening of about 40 microns is possible. This is sufficient to solve the problem of pressure leakage in the diesel injector, which is known to be due to an adaptation error of just a few micrometers.

Fig. 19:

Measurement sequence for the fourth cycle. First, a ramp is traversed to +160 V and back to 0 V. The length of the piezo stack changes from -3 μm to approximately +40 μm. The starting value for the length of the piezo stack is not 0 μm, since two cycles to -10 V had already been run and a shortening of 3 μm remained due to the hysteresis. Back at 0 V, the PMA creeps back to its original length at -3 μm. When the polarity is finally changed, there is almost no change in length. Then the part negative for the PMA is run through. After that, the total length changed by -5 μm. Fig. 20:

The measurement sequence of FIG. 19 is shown in the hysteresis curve.

0 V are again present at the piezo stack. However, its overall length has become smaller. A new cycle with the same measurement sequence again leads to a closed hysteresis curve, which is already shown in FIG. 16, corresponding to cycle 6.

Claims

claims

1. Method for controlling piezoelectric actuators (PA), in particular piezoelectric multilayer actuators (PMA), which are regularly subjected to positive voltage during operation for elongation, whereby for predetermined operating states of the PMA the latter also does not have a limited one, the PMA polarizing negative voltage or offset voltage is applied in order to reset it to a definable starting position.

2. The method according to claim 1, in which the reset which can be carried out in an operating state with insufficient structural compensation of thermal expansion effects, for example in a diesel high-pressure injection valve, can keep the control valve securely closed during the starting process.

3. The method according to claim 1 or 2, in which the monitoring of the high-pressure injection valve always runs as a safety function and adjusts the length of the PMA when the high-pressure injection valve comes into a critical operating state.

4. Electronic unit for carrying out the method according to one of claims 1, 2 or 3, in which for generating an individual offset voltage for each actuator, the circuit belonging to the actuator is expanded by a capacitance, a resistor and a controllable voltage source.

5. Electronic unit according to claim 4, wherein the offset voltage is adjustable.