DE19905340C2 - Method and arrangement for presetting and dynamic tracking of piezoelectric actuators - Google Patents

Abstract

In the method and the arrangement for setting and tracking electrical piezo actuators (1), a DC voltage (U¶) is set between the piezo actuator (1) and an actuating element (3) to set a desired idle stroke (h¶0¶ '). v¶), which may be superimposed on a pulsed control voltage (U¶1¶) and which causes a change in length of the piezo actuator (1) which is independent of the control voltage (U¶1¶).

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 C1 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. It can also be inadmissible if the system behavior differs in operation  grow 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.

A hydraulic device to compensate for play between Piezo element and plunger is known from DE 37 42 241 C2. There with the idle stroke or the play of a piezoelectric Injector are regulated.

Power supplies for piezo actuators are, for example, from a single kata log of the company PI Physik Instrumente (No. P236-04 / 89.3, 1989, p. 5.2, 5.45) or from a company letter from Dr. Lutz Pi ckelmann Piezomechanik, optics from 1982 ("analog amplifier, Computer controlled supplies,. . . for piezo actuators ", p. 3, 8) known. A manually or controlled adjustable change With these power supplies, the voltage can be one manually adjustable DC voltage (offset voltage) to be a control signal in a positive range to postpone.

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 component  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 U ₁ and the static bias voltage U _V.

Fig. 6 shows the time course of the dynamic clamping excitation voltage U _1, of 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 results, as shown in FIG. ₁ b, as h _V = h - h 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 U _{1 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 , and the valve is open therethrough , 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 to reduce the idle stroke, that is, from or the play between the piezo actuator 1 and the 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 a length l ₀ ', which is smaller than the length a, in which the piezoelectric actuator 1 just comes to the valve piston 3 is in contact, without this to bewe gen In FIG. . 2b corresponding to the idle stroke is set to ₀ 'with ₀ h'<h ₀ through Anle gene of the DC voltage U _V to the desired value h.

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 _{0opt 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 _V U in the range of values .DELTA.U _V characteristic curve in the direction of the h-axis by the area .DELTA.h ₀ is shifted. 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 an individual 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 for 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 piezo 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 . The injector 14 as pressure values x _{1. .n detected} at the valve 15 and led 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 target value h _0soll 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 of} 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, for example, from the time offset Δt ₀ between the edges of the pulse-shaped electrical excitation voltage U ₁ (t) and the temporal change in the pressure p _V at the valve 15 be derived. The pressure curve p _V (t) can be achieved with comparatively little technical effort, e.g. 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 smoothly from a minimum value -U _Vmax until the time t ₁ at a Vorspan voltage U _V0, the valve opens and the pressure p fuel line in the force drops . The bias voltage U _{V is then} reduced to a value U _Vsoll which corresponds to the desired idle stroke h _0soll .

For the determination of U _Vsoll applies:

U _Vsoll = U _V0 - h _0soll / 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 _Vsoll 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 fixed in its position with respect to the valve piston 3 . When the bias voltage U _{Vsoll is switched off} at time t ₂ in FIG. 8d, the actuator 1 then returns to its rest position l ₀ , and the idle stroke h _0soll has exactly the value seen before, which was selected by means of the voltage U _Vsoll .

Fig. 9 shows the torque block known from DE 196 46 511. Here, instead of the linear movement, a tilting movement is generated if the voltages U _a and U _b applied to the two adjacent piezo element stacks 20 and 22 are different from each other: The idle stroke h ₀ corresponds to the idle tilt angle ϕ _{0 here} . 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 provided that both positive and negative values can deliver 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 linea rer piezoelectric actuator into two electrically separate and independently controllable, but mechanically connected in series regions 30, 31 is divided. Each area has its own pair of terminals 32 , 33 . The upper region 30 only has the task of adjusting the idle stroke, for which purpose the bias voltage U _{V is} applied to the connecting 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 setting and tracking electrical piezo actuators ( 1 ) from a stack of stacked piezo elements, with an idle stroke (h ₀ ) between the with a control voltage (U ₁ ) controllable piezo actuator ( 1 ) and an actuating element (valve piston 3 ) at not applied control voltage (U ₁ ) is compensated, characterized in that for setting a predetermined idle stroke setpoint (h ₀ ', h _0soll ) during assembly of the piezo actuator ( 1 ) and / or for tracking the idle stroke during operation of the piezo actuator ( 1 ) at least a portion of the piezo elements of the piezo actuator ( 1 ) is supplied with a _DC voltage (U _V , U _Vsoll ), which causes a change in the length of the piezo actuator ( 1 ) 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 _0soll ) when assembling the piezo actuator ( 1 ) with an actuating element (valve piston 3 ) with the DC voltage (U _Vsoll ) applied to the piezo actuator in the direction 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 _Vsoll ) is switched off again . 3. The method according to claim 1, characterized in that for setting the predetermined idle stroke setpoint (h _0soll ) in a permanently installed piezo actuator ( 1 ), the DC voltage (U _V ) is increased from a minimum value (-U _Vmax ) until the _{Actuating element} (valve piston 3 ) is actuated, and then the DC voltage (U _V ) is reduced by a value which corresponds to the desired value (h _0soll ) 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 ) so that the desired value (h _0soll ) 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 between the control voltages (U _a , U _b ). 7. Arrangement for setting and tracking electrical piezo actuators ( 1 ) from a stack of stacked piezo elements, an idle stroke (h ₀ ) between the with a control voltage (U ₁ ) controllable piezo actuator ( 1 ) and an actuating element (valve piston 3 ) at not applied to control voltage (U ₁ ) is compensated, and wherein 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 _0soll ) during the assembly of the piezo actuator ( 1 ) and / or for tracking the idle stroke during the operation of the piezo actuator ( 1 ) at least some of the piezo elements of the piezo actuator ( 1 ) a DC voltage (U _V , U _Vsoll ) from a DC voltage source ( 18 ) is supplied, which is optionally to the control voltage (U ₁ ) in a summation circuit ( 12 ) superimposed and one of which Control voltage (U ₁ ) causes independent change in length 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 ) such that the desired direct voltage (U _V , U _Vsoll ) 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 ).