DE19905340A1 - Pre-adjustment method and dynamic correction of piezoelectric actuators e.g. for fuel-injection valve drive in motor vehicles - Google Patents

Abstract

A method of adjusting and correcting or compensating electric piezo-actuators (1) from a stack of piezo-elements placed one on the other, in which an idling stroke (ho) exists between the piezo-actuator (1) driven by drive-voltage (U1), and an actuating element (valve stem 3) with the drive voltage (U1) not applied. A DC voltage (Uv,Uvsol) is supplied for adjusting a given desired value (Ho',Hosol) of the idling stroke during installation of the piezo-actuator (1) and/or for adjusting the idling stroke during operation of the piezo-actuator (1), to at least one part of the piezo-element of the piezo-actuator (1) for effecting a change in length of the piezo-actuator independent of the drive voltage (U1).

Description

The invention relates to a method and an arrangement for Presetting and dynamic tracking of piezoelectric 7. Actuators according to the preamble of patent claims 1 and 7 respectively.

For the actuation of electromechanical or electrohydraulic shear or electropneumatic control elements are used in Increasingly used piezoelectric actuators. For this include in particular the drives for injection valves of internal combustion engines for motor vehicles. For the strength Fuel supply to internal combustion engines is increasing mend high-pressure accumulator injection systems are used. Such Injection systems, for example for diesel engines, are becoming common rail Called systems. With these injection systems, the power fabric with a high pressure pump in all cylinders of the Motors promoted common pressure accumulator, from which the Injectors are supplied to the individual cylinders. The opening and closing of the injection valves, which at Com mon-rail systems, called injectors, become electric controlled, for example, by means of the piezo actuators mentioned. The piezo actuator usually acts on a servo valve one that hydraulically controls the pressure on the nozzle del of the injector is exercised.

The essential technical requirements for such piezo drives are

• - high positioning power,
• - large adjustment range (stroke),
• - high electromechanical efficiency (low electri tax benefit),
• - low response time,
• - Long service life or number of switching cycles, and
• - low temperature dependence.

The combination of high positioning power with a large positioning range can be used with piezoelectric multilayer actuators (multilay stack (s) stacked on top of each other the plate-shaped piezo elements exist. Such actuators are already in different designs and designs known. In addition to actuators where the mechanical deflection only orthogonal to the layer plane (linear stacks), for example according to DE 196 15 695 or DE 196 46 676, designs are also known which tilt Carry out movement, e.g. B. so-called. Torque blocks according to DE 196 46 511.

Basically, the goal is for most applications in that approximately the entire stroke of the piezo actuator also used for the mechanical movement of the control element because the size increases with the required stroke. It follows that the game between the piezo actuator and the Control element, which is also referred to as an idle stroke, if possible should be small. A small idle stroke also serves the Redu decoration of the dynamic load on the piezo actuator and position lement, because at constant electrical control voltage Deflection speed of the actuator in the area of small deflections kungen grows with the hub. It also increases with increasing out deflect the actuating force approximately linearly.

On the other hand, there must always be a minimum play between the actuator and Control element to be guaranteed to malfunction in operation to avoid. That is why today in general one goes NEN so that the idle stroke when mounting the actuator and Adjusting element is set so that the required minimum game even under the most unfavorable conditions and known aging effects (shrinking, setting, etc.) so far are taken into account as possible. The idle stroke is therefore despite a high measurement and adjustment effort during assembly the system usually does not function optimally during operation. He can also un in the system behavior in operation  become small. This has the consequence, for example, that the servo valve of a common rail injector is not complete closes and therefore no defined fuel injection is possible.

The object of the present invention is a method and to create an arrangement in which the effort for setting the idle stroke during assembly is reduced in particular in series production, and in which or in a simple way a safe dynamic night guidance of the idle stroke in operation is possible.

This object is achieved with the in claim 1 or the measures specified in claim 7 solved. Preferred embodiments of the invention are in these Claims cited in each of the following subclaims.

The above object is therefore ge according to the invention triggers that the electrical control voltage of the actuator dynamic deflection an independent controllable DC voltage component is superimposed. This DC chip then determines a new rest position of the actuator and can thus be used to adjust the idle stroke and for tracking of the idle stroke can be used in operation. An advantage is there at that the piezo actuator has almost no electrical power the DC voltage source.

The size of the idle stroke can e.g. B. by measuring the actuator absorbed current and by measuring secondary size which can be derived from the system function, be be true, e.g. B. by the pressure curve at the injector rend of the injection process. This results in a closed its control loop. Since a change in the idle stroke in general runs very slowly compared to dynamic operation, this regulation is not time-critical.  

The measurement and adjustment of the Empty stroke through the integration of a separate adjustment algo rhythm in the starting process of an internal combustion engine gene.

In practice, the usable control range is approx. ± 10% of the Total stroke. With common rail injectors, for example a total stroke of the actuator of approx. 40 to 50 µm is aimed for. This results in a control range of approx. ± 5 µm. This value is rich for the compensation of aging and temperatu sufficient effects.

In a similar way, a separately switchable equal voltage used to clear the game between during assembly Adjust actuator and control element exactly. This is before partly proceed so that the actuator during assembly is supplied with a DC voltage value, which too a static deflection that leads to the Match or a value derived from it. As soon as during the assembly process the mechanical contact of Ak If the gate and actuator are set, the adjustment process will be completed det, and the components are fixed in relation to each other. To when the DC voltage is switched off, the actuator goes into its Ru helage back so that the required game sets in. There the relationship between electrical voltage and actuator steering is known very precisely, the game can be played with high Set precision (accuracy better than 1 µm).

In this way, error-critical geometric measurement and Mating operations or mechanical reset movements largely avoided. The mechanical contact of the actuator and Actuator can be detected very easily, for. B. by way measurement on the control element or by indirect variables, e.g. B. through a pneumatic pressure drop test.

Basically, the described task can also be done by ge triggers that the actuator consists of two independently  controllable, mechanically connected stacks of Piezo elements, one of the two stacks with egg ner DC voltage applied to adjust the idle stroke while the other stack is used to control the positioning or Actuator is used.

The advantages of the solution described are

• - simple circuitry feasibility,
• - high technical effectiveness,
• - Diagnostic functions for the state of the injector in operation derivable,
• - Cost savings and process reliability in series production supply.

Exemplary embodiments of the invention are described below the drawing explained in more detail. Show it:

1 shows a structure for a control valve with a piezoelectric actuator, said piezoelectric actuator 1a of the rest position and the valve is in the closed and Fig deflected in Figure 1b of the piezoelectric actuator and the valve is open...;

Fig. 2 shows schematically the idle stroke in the arrangement of Figure 1, wherein the idle stroke is shown in the rest position of the Piezoak gate in Figure 2a and in Figure 2b an idle stroke tracked by applying egg ner DC voltage.

Fig. 3 shows the time course of the electrical clamping voltage U, the stroke h and the time derivative dh / dt of the stroke;

FIG. 4 shows the transfer function h (U);

Figure 5 is a block diagram for the superposition of the dynamic excitation voltage U1 and the static bias voltage U _v .

Fig. 6 shows the time course of the dynamic clamping excitation voltage U1, the pressure P _v and the time derivative dp _v / dt of the pressure;

Fig. 7 is a timing diagram for the adjustment of the idle stroke when starting the engine;

Fig. 8, the operations during the assembly of a piezoelectric actuator, with FIG. 8a to the state at the beginning of the assembly, FIG. 8b a state during assembly, Fig. 8c the state after assembly, and Fig. 8D is a time chart for represents the assembly;

FIG. 9 shows a piezo actuator for generating a tilting movement, where in FIG. 9a the rest state and FIG. 9b the deflected state;

Fig. 10 shows schematically circuit arrangements for adjusting the empty tilt angle ϕ ₀ in the arrangement of Fig. 9, the

FIG. 10a and 10b represent voltage sources alternatives with one or two equal tension; and

. 11, an alternative arrangement is a linear divided Piezoak catalyst which separately controllable in two piezo element stack FIG.

Fig. 1 of the drawing shows schematically and by way of example the structure of a valve control with an electrical piezo actuator 1 , which consists of a stack of plate-shaped piezoelectric elements. In the embodiment shown, it follows the mechanical deflection when a voltage is applied perpendicular to the layer plane of the individual piezo element plates, that is to say in the longitudinal direction (z direction) of the piezo actuator. The individual piezo elements are electrically connected to terminals 2 via common outer stack contacts.

In the rest position, that is to say without an electrical voltage U being present, the piezo actuator 1 has the length l ₀ ( FIG. 1a). When an electrical voltage U = U _{1 is applied} to the terminals 2 of the piezo actuator 1 , the latter expands in the z direction and takes on the length l ₁ with l ₁ <l ₀ ( FIG. 1b). The difference l ₁ -l ₀ is referred to as stroke h. The distance between the un-expanded piezo actuator 1 of FIG. 1 a and the mechanical actuating or actuating element (here a valve piston 3 ) is the idle stroke h ₀ . The effective valve lift h _v , as shown in FIG. 1b, results in h _v = hh ₀ = l ₁ -l ₀ -h ₀ .

In the state shown in FIG. 1 a , since no electrical voltage U is applied, the piezo actuator 1 is in its rest position, the valve is closed, and a pressure p ₀ prevails in the control chamber 4 of the valve. In the state shown in FIG. 1b, the voltage U1 is applied to the connecting terminals 2 , the piezo actuator 1 is deflected, the valve piston 3 has a valve ball 5 lifted out of its seat against the action of a return spring 6 , the valve is open through it, and a pressure p ₁ is established in the control room.

The Fig. 2a again shows in detail the idle stroke h ₀ in the rest position of the piezoelectric actuator 1, when no voltage U is applied. There is now the possibility of reducing the idle stroke, that is, the position or the play between piezo actuator 1 and valve piston 3 , by applying a direct voltage U _v to the connecting terminals 2 of the piezo actuator 1 . The DC voltage U _v is less than the control voltage U ₁ of FIG. 1b. At the direct voltage U _v , the piezo actuator 1 therefore does not come into mechanical contact with the valve piston 3 , but only approximates it. The piezoelectric actuator 1 thus expands upon application of the DC voltage U _v to a length l ₀ 'which is smaller than the length a, in which the piezoelectric actuator 1 just comes into contact with the valve piston 3 without these to bewe gen In FIG. . 2b is set in accordance with the idle stroke h by Anle gene of the DC voltage U _v to the set value h0 'h0'_<0,.

FIG. 3 shows the time curve of the voltage U at the terminals 2 of the piezoelectric actuator 1, that is U (t), and the stroke of h (t) and its time derivative dh / dt (the moving speed). Also shown is the generally desired target area h _{0 opt of} the Leerhu bes, in which the value of dh / dt and thus the dynamic load on the valve piston 3 is still sufficiently low. The optimal range for the idle stroke h ₀ is (0.01 ... 0.1) * h.

Fig. 4 illustrates the transfer function h (U) schematically. By injecting the bias voltage U _v in the range of values .DELTA.U _v shifting the characteristic curve in the direction of the h-axis by the area .DELTA.h _0th The permissible range of values for ΔU _v is ± 0.1 * U ₁ . With conventional piezoceramic, the maximum electrical field strength U / d is approx. 2 kV / mm (d = thickness of a single piezo element in the piezo actuator stack). The lower limit of the electrical voltage is determined by the depolarization field strength.

FIG. 5 shows a schematic block diagram of a circuit arrangement for superimposing the dynamic (pulse-shaped) excitation voltage U ₁ and the static bias voltage U _v for dynamic tracking of the idle stroke during operation of the piezoelectric actuator 1. The circuit comprises a pulse generator 10 for generating the excitation voltage pulses, which are amplified in an amplifier 11 to the pulsed excitation voltage U ₁ . The excitation voltage U ₁ is guided via a summation circuit 12 to the piezo actuator 1 of the injector 14 . For example, pressure values x _{1 ... n} at the valve 15 are detected by the injector 14 and passed to a microprocessor μC 16, at the inputs of which other values such as the excitation voltage U ₁ , the excitation current i and the like are also present. The microprocessor 16 then calculates the DC voltage U _v required to set the idle stroke to the setpoint h ₀ target and sets a controllable DC voltage source 18 so that it outputs the DC voltage U _v to the summation circuit 12 , in which the DC voltage U _{v is} the excitation voltage U _{1 is} superimposed.

As shown in FIG. 6, the current value (actual value) of the idle stroke in the microprocessor 16 can be derived, for example, from the time offset Δt0 between the edges of the pulse-shaped electrical excitation voltage U ₁ (t) and the change over time in the pressure p _v at the valve 15 become. The Druckver run p _v (t) can be done with comparatively little technical effort, for. B. using an existing pressure sensor.

Fig. 7 shows another way of setting the idle stroke based on a timing diagram. This adjustment process can be carried out, for example, during the starting process of an internal combustion engine. Here, too, the pressure sensor which is generally present in any case in a common rail system can advantageously be used for measuring the pressure curve.

. The setting operation of Figure 7 is as follows: The on voltage U _v is increased starting from a minimum value -U _{v max} evenly, until time t ₁ at a Vorspan voltage U _v0, the valve opens and the pressure p in the fuel line falls off. The bias voltage U _{v is then} reduced to a value U _{v soll} which corresponds to the desired idle stroke h _{0 soll} .

The following applies to the determination of U _v :
U _{v soll} = U _v0 - h _{0 soll} / s, with s = Δh / ΔU.

Depending on the design of the overall system, the pressure p can be built up automatically immediately after the valve closes at time t ₂ (dashed line) or independently of the adjustment process at a later point in time (continuous line).

Fig. 8 shows the processes at an adjustment of the empty stroke during the installation of the piezo actuator 1 in an injection valve. The acted upon with a DC voltage U _{v should} Ak tor 1 is mechanically moved in the z direction towards the valve piston 3 during assembly ( Fig. 8a). At the moment of mechanical contact between actuator 1 and valve piston 3 , the test pressure p _v in the valve drops ( FIG. 8b, t ₁ in FIG. 8d). The mechanical feed in the z direction is then immediately interrupted and the actuator 1 is fixed in position with respect to the valve piston 3 . When the bias voltage U _{v should be switched off} at time t2 in FIG. 8d, the actuator 1 then returns to its rest position 10 , and the idle stroke h _{0 target} has exactly the value seen before, which was selected by means of the voltage U _{v target} .

Fig. 9 shows the torque block known from DE 196 46 511. Here, a Kippbe movement is generated instead of the linear movement when the voltages U _a and U _b applied to the two adjacent piezo element stacks 20 and 22 are different from one another. The idle stroke h ₀ corresponds here to the idle tilt angle ϕ ₀ . This angle ϕ ₀ can be preset by a difference in the control voltages U _a and U _b like the idle stroke h ₀ or adjusted during operation.

Fig. 10 shows schematically circuit arrangements for a set the empty tilt angle ϕ ₀ . In these arrangements, the same reference signs were used to designate corresponding components as in FIG. 5, which were added to distinguish the two parallel branches for the piezo element stack 20 and 22 with the letters A and B. In the scarf Fig processing. 10a is a summing circuit 12 in egg nem of the two parallel branches A and B, respectively, and a DC clamping voltage source 18 for a bias voltage U _v is provided, capable of delivering both positive and negative values to the summation circuit 12 '. In the circuit of FIG. 10b, a summation circuit 12 A, 12 B is provided in each of the two parallel branches and for each of the two summation circuits 12 A, 12 B a separate DC voltage source 18 A, 18 B for delivering the bias voltages U _va and U _vb . The advantage of this embodiment is that only positive voltages are required and the field strength at the piezoelectric elements always acts in the direction of polarization.

The Fig. 11, finally, shows a solution in which a piezoelectric actuator in linea rer two electrically separated and inde pendent controllable from one another but mechanically connected in series regions 30, 31 is divided. Each area has its own pair of terminals 32 , 33 . The upper area 30 only has the task of adjusting the idle stroke, for which purpose the bias voltage U _{v is} applied to the connection terminals 32 . With the lower area 31 , to which the control voltage U _{1 is} supplied, the dynamic control of the actuating element or valve takes place. The length ratio l _v / l _{1 of} the regions 30 , 31 can be approximately 1:10 with the same thickness of the piezoelectric layers in the two regions.

Claims (10)

1. A method for adjusting and tracking electrical piezo actuators (1) superposed in a stack of piezoelectric elements, wherein between the with a drive voltage (U ₁₎ controllable piezoelectric actuator (1) and an actuating element (spool 3) in non-fitting drive voltage (U ₁₎ There is an idle stroke (h ₀ ), characterized in that to set a predetermined idle stroke setpoint (h ₀ ', h ₀ target) during the assembly of the piezo actuator ( 1 ) and / or to adjust the idle stroke during operation of the piezo actuator ( 1 ) At least a part of the piezo elements of the piezo actuator ( 1 ) _is supplied with a DC voltage (U _v , U _{v soll} ), which causes a change in the length of the piezo actuator ( 1 ) that is independent of the control voltage (U ₁ ). 2. The method according to claim 1, characterized in that for setting the predetermined idle stroke setpoint (h _{0 set} ) when assembling the piezo actuator ( 1 ) with an actuating element (valve piston 3 ) with the DC voltage (U _{V set} ) acted upon piezo actuator in The direction of the actuating element is moved until the piezo actuator ( 1 ) and the actuating element (valve piston 3 ) come into mechanical contact, whereupon the piezo actuator ( 1 ) fixes its position to the actuating element and the DC voltage (U _{v soll} ) again is switched off. 3. The method according to claim 1, characterized in that the DC voltage (U _v ) is increased starting from a minimum value (-U _{v max} ) for setting the predetermined idle stroke setpoint (h ₀ target) in a permanently installed piezo actuator ( 1 ), until the actuating element (valve piston 3 ) is actuated, and then the DC voltage (U _v ) is reduced by a value corresponding to the desired value (h _{0 should} ) for the idle stroke. 4. The method according to claim 1, characterized in that for dynamic tracking of the piezo actuator ( 1 ) in operation, the current value of the idle stroke (h ₀ ) from the time offset (Δt ₀ ) between the pulse-shaped control voltage (U ₁ ) and the time course the effect triggered by the actuating element (valve piston 3 ) (pressure drop in p _v ) is determined and this time offset is set by superimposing the direct voltage (U _v ) in such a way that the desired value (h _{0 setpoint} ) results for the idle stroke. 5. The method according to claim 1, characterized in that the piezo actuator ( 1 ) comprises two areas ( 30 , 31 ) which can be controlled electrically separately, but mechanically connected in series, with one of the areas via the direct voltage (U _v ) the idle stroke is set and the other of the areas is supplied with the control voltage (U ₁ ). 6. The method according to claim 1, characterized in that the piezo actuator ( 1 ) consists of at least two adjacent piezo element stacks ( 20 , 22 ) to the respective different control voltages (U _a , U _b ) are applied, the idle stroke (h ₀ ) corresponds to an empty tilt angle (ϕ ₀ ), which can be set and adjusted by the difference of the control voltages (U _a , U _b ). 7. Arrangement elements for setting and updating electrical piezo actuators (1) from a stack of superimposed piezoelectric, wherein between a drive voltage (U ₁₎ controllable piezoelectric actuator (1) and an actuating element (spool 3) in non-fitting drive voltage (U ₁₎ There is an idle stroke (h ₀ ), and the arrangement comprises a pulse generator ( 10 ) with a downstream amplifier ( 11 ) for generating the control voltage (U ₁ ), characterized in that for setting a predetermined idle stroke setpoint (h ₀ ', h _{0 soll} ) during the assembly of the piezo actuator ( 1 ) and / or for tracking the idle stroke during operation of the piezo actuator ( 1 ), at least some of the piezo elements of the piezo actuator ( 1 ) receive a direct voltage (U _v , U _{v soll} ) from a direct voltage Supply source ( 18 ) is supplied, which is optionally superimposed on the control voltage (U ₁ ) in a summation circuit ( 12 ) and one of the Anste overvoltage (U ₁ ) causes independent length change of the piezo actuator ( 1 ). 8. Arrangement according to claim 7, characterized by a microprocessor ( 16 ) which detects the action triggered by the actuating element (valve piston 3 ) and which acts on the direct voltage source ( 18 ) in such a way that the desired direct voltage (U _v , U _v) ) is obtained. 9. Arrangement according to claim 7, characterized in that the piezo actuator ( 1 ) comprises two areas ( 30 , 31 ) which can be controlled electrically separately, but mechanically connected in series, with one of the areas via the direct voltage (U _v ) the idle stroke is set and the other of the areas is supplied with the control voltage (U ₁ ). 10. The arrangement according to claim 7, characterized in that the piezo actuator ( 1 ) consists of at least two adjacent piezo element stacks ( 20 , 22 ) to which each different control voltages (U _a , U _b ) can be applied, the idle stroke (h ₀ ) corresponds to an empty tilt angle (ϕ ₀ ), which can be adjusted and adjusted by the difference in control voltages (U _a , U _b ).