DE10228146A1 - Method for controlling and adjusting the length of a piezoelectric actuator (PA) and electronic unit for controlling the method - Google Patents

Abstract

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

Description

The invention relates to all directly driven high pressure injectors or valves without hydraulic power transmission element like warehouse or translator for direct fuel injection, where a working chamber with the high pressure necessary for operation filled and a piezoelectric multilayer actuator (PMA) in its electrical Control by elongation opens or opens a servo valve or a nozzle needle any other directly driven metering valve. By opening the Servo or control valve becomes a pressure drop in a control chamber which creates the opening of the high pressure injector.

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

When operating such high-pressure injection valves Problems sometimes arise when the valve gets hot at least once is. Although the valve and the PMA in terms of their thermal linear expansion are coordinated changes the length of the PMA is often too strong. One is sufficient length difference of a few μm and the control chamber of the high pressure injector or the like Injector is no longer securely closed. In the event of a leak As a result, the high pressure pump cannot operate in the control chamber build up the necessary pressure and the high pressure injector failed.

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

So far, people have trusted that through careful thermal tuning of the components and a so-called "thermal gap" the problem is manageable. A "thermal gap" means that between the end of the PMA and the servo valve, a distance remains from approx. 5 μm as a buffer for Reconciliation errors. For the cases in which this measure the problem is not yet solved.

To illustrate the problem of the different linear expansion of the piezo actuator and housing in the piezo diesel high-pressure injector:
1 shows schematically the function of the piezo actuator in the Piezo Diesel high pressure injection valve when opening the servo valve. 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 piezoceramic. 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, ie the safety distance for different thermal dilations, and then presses on the servo valve. As a result, the high-pressure injection valve opens because the pressure in the working chamber (no longer shown) of the high-pressure injection valve 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, on setting processes and on its history (hysteresis). These processes can also change the thermal expansion coefficient of the piezo actuator. It is therefore possible that the tolerance band for the thermal gap of 3–5 μm will be left over time. In the worst case, there is a pressure leak 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. 2 ).

But the function suffers beforehand of the Piezo Diesel high pressure injector because the dosage is smaller Injection quantities (e.g. in the so-called "pre-injection") outside the tolerance band for the thermal Gap of 3–5 μm loses precision.

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 electrical field, which acts against the direction of preferential polarization of the piezoceramic, with shortening, just as it reacts to a positive voltage with an extension. It is therefore possible to apply the ideal state of by applying a negative electrical voltage 1 restore.

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

This task is solved through the one or more combinations of features of the at least a main claim.

The operating range of the PMA, which has to perform an elongation for control, is additionally expanded to negative voltages so that its length can be varied in both directions (lengthening and shortening). This latitude 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 it being closed Reverse polarization in ceramics is coming.

An existing control electronics is about the possibility extended by an additional negative offset voltage, the thermal effects described above to be able to compensate.

Like the measurements in the exemplary embodiments show there is a usable scope, a negative tension exploit before depolarization and then the Reverse polarization of the ceramic begins.

This scope is large enough to shorten the PMA so far that the high pressure injector can be put back into operation.

• – Das Problem wird gelöst ohne mechanische Änderungen am Hochdruckeinspritzventil.
• – Die negative Spannung braucht nur bei Bedarf, wie beispielsweise Neustart nach starker Erwärmung oder ähnlichem angelegt werden. Bei allen andern Betriebszuständen kann die elektrische Ansteuerung des Hochdruckeinspritzventils in herkömmlicher Weise erfolgen.
• – Der Vorgang ist regelbar. Die negative Spannung kann bis zum Anspringen des Hochdruckeinspritzventils langsam erhöht werden. Das vermeidet unnötig große Feldstärken.

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 restarting 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 slowly increased until the high pressure injector starts. This avoids unnecessarily large field strengths.

The invention presents a simple solution which enables an offset voltage which can be regulated individually for each actuator without great effort and without influencing the functioning of the current control electronics. The solution described here provides for the circuit to be expanded by a capacitance, a resistor and a more controllable voltage source, as described in 3 is shown.

The capacitive voltage divider (C _injector ; C ₁ ) reduces the voltage that drops at the injector according to the following relationship:

But with a controllable, positive voltage source V ₁ , one is able to set a negative offset voltage on the injector, which is independent of the injector capacity. Resistor R ₁ determines the time constant with which the potential at node C ₁ , C _injector and R _{1 is} balanced.

In 4 it is made clear how the voltage profile of a current-controlled output stage can look for the procedure according to the invention.

• – Es ist keine negative Spannung erforderlich.
• – Der Schaltungsaufwand ist gering.
• – Die zusätzlichen elektrischen Verluste, die durch die Schaltung entstehen sind klein
• – Die Offsetspannung ist unabhängig vom Betriebszustand des Injektors, sie kann während der Startphase und während der Betriebs angelegt und verändert werden.
• – Die Kapazität C₁ benötigt nur eine eher geringe Spannungsfestigkeit.

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 C ₁ only requires a rather low dielectric strength.

The terminal behavior remains approximately the same, therefore there are no further changes to the control electronics necessary.

The following are schematic Embodiments, that do not limit the invention. The figures show in detail:

1 a functional group of the Piezo Diesel high pressure injector,

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

3 an electrical circuit for generating a regulated offset voltage,

4 2 shows a voltage curve of a current-controlled output stage which has been expanded by the function described in this invention,

11 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 that at –20 V the contraction is approx. –7 μm; the oval is not closed after the first cycle,

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

16 a first cycle with negative range down to -30 V; the contraction reaches –10 μm,

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

18 the double polarization of the diesel piezo stack,

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

20 the measurement order for step 4, accordingly 19 , can be found in the hysteresis figure shown.

Operation of a Diesel piezo stacks 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 just 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 also correct the length of the PMA. In normal operation, the PMA works 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. Similarly, by convention, one speaks of negative voltage if the external electric 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 thus 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:

• 1. Zyklus von 0 V bis 160 V und zurück; die übliche Betriebsweise
• 2. Zyklus zwischen –10 V und +160 V als erster Test für mögliche Verkürzungen
• 3. Zyklus zwischen –20 V und +160 V als zweiter Test für mögliche Verkürzungen
• 4. Zyklus zwischen –30 V und +160 V als dritter Test für mögliche Verkürzungen
• 5. Zyklus zwischen –160 V und +160 V als Test für die Grenzen möglicher Verkürzungen und zur Untersuchung der Umpolungsvorgänge.

A standard diesel actuator with an overall length of 30 mm 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 the 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 investigate the creep behavior and hysteresis of the PMA in more detail. Because the existing electronics are not designed for negative voltages is a tool has been applied to this voltage range to be able to represent. 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, around the connectors to swap. And the rest of the measurement was positive, i.e. unchanged polarity related to electronics, measured further. Because of the swapped connections however, this acts as a negative voltage on the PMA.

The results are shown in the 11 to 20 shown.

11 :
If only you 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 overall 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 approximately 2 μm. Due to the hysteresis, the PMA is still shortened by 1 μm at 0 V.

14 :
At –20 V, the shortening is –7 μm. The oval of the hysteresis curve is not closed down to –20 V on the first cycle.

15 :
The hysteresis now shows itself in the usual form of a closed oval curve.

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

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

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. 18 shows that a shortening of about 40 μm 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.

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 has changed –5 μm changed.

20 :
It will be the measurement order of 19 reproduced 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 order again leads to a closed hysteresis curve, which is already in 16 , according to the cycle 6 , is shown.

Claims (5)

Process for controlling piezoelectric Actuators (PA), in particular of piezoelectric multilayer actuators (PMA), which are regularly used in operation Elongation can be applied with positive tension, whereby for given operating conditions the PMA also with a limited one that does not polarize the PMA negative voltage or offset voltage is applied to this return to a definable starting position. The method of claim 1, wherein the in an operating state with insufficient structural compensation of thermal Extension effects executable Deferral, in the case of a diesel high-pressure injection valve, for example, the control valve can keep safely closed during the start-up process. Method according to claim 1 or 2, in which as a safety function The supervision of the high pressure injector always runs during operation and the Length of PMA adjusts when the high pressure injectors are in a critical Operating state comes. Electronic unit for executing the method according to a of claims 1, 2 or 3, in which to generate an individual for each actuator Offset voltage the one belonging to the actuator Switching by one capacity a resistor and a controllable voltage source is expanded. Electronic unit according to claim 4, wherein the Offset voltage is adjustable.