DE10307461A1 - Method and device for monitoring a piezoelectric actuator - Google Patents

Abstract

A device and a method for monitoring a piezoelectric actuator for controlling an injector are described. A characteristic curve of the actuator is determined. The determined characteristic is compared with an expected characteristic. If the determined characteristic curve deviates from the expected characteristic curve, an error of the injector is recognized.

Description

The invention relates to a method and a monitoring device of a piezoelectric actuator according to the preambles of the independent claims.

A method and a device for measuring the voltage of a piezoelectric actuator is known from the EP 11 38 902 known. Such actuators are used in particular to control an injector, which are used in particular to control the fuel metering in an internal combustion engine.

In the area of such an injector or such a piezoelectric actuator can cause a variety of errors occur. For example, due to various causes, such as wear, an increased mechanical friction occur. This can go so far that individual Components of the injector no longer move. Furthermore can it happen that leaks occur or that currents of liquids to be controlled prevented or difficult. Furthermore, errors can occur that individual layers of the piezo actuator are damaged or no longer functional. Furthermore, the case may occur that the actuator is defective in electrical terms is contacted.

Advantages of invention

According to the invention it is provided that a characteristic curve of the actuator is determined and this with an expected characteristic is compared. The determined characteristic deviates from the expected Characteristic curve, an error is recognized according to the invention. With such an approach can numerous errors in the area of the actuator or in the area of the injector be recognized.

A force-displacement characteristic is preferably determined. This can be determined with little effort. Certain errors lead to characteristic courses. From these courses various errors can be reliably identified. The force-displacement characteristic can be in simply by capturing pairs of values for the load and the voltage can be determined.

According to the method and Device as part of the test the injector before the first start-up, at regular intervals, for example during maintenance work and / or constantly carried out while the injector is in operation.

The are still of particular importance Realizations in the form of a computer program with program code means and in the form of a computer program product with program code means. The computer program according to the invention has program code means for all steps of the method according to the invention perform, if the program on a computer, in particular a control device for an internal combustion engine of a motor vehicle becomes. In this case, the invention is thus stored in the control unit Program implemented so that this control unit provided with the program in the same The invention represents how the method, the execution of which Program is suitable. The computer program product according to the invention has program code means which are stored on a computer-readable data carrier are to carry out the method according to the invention if the program product on a computer, in particular a control unit for an internal combustion engine a motor vehicle becomes. In this case, the invention is implemented by a data carrier, so that the inventive method accomplished can be, if the program product or the data carrier in one control unit for one Internal combustion engine integrated especially of a motor vehicle becomes. As a data carrier or as a computer program product, in particular an electrical one Storage medium are used, for example a read-only memory (ROM), an EPROM or an electrical permanent memory such as for example a CD-ROM or DVD.

Advantageous and expedient configurations and further developments of the invention are characterized in the subclaims.

The extension is shown below of the embodiments illustrated in the drawing.

Show it

1 2 shows a block diagram of the device according to the invention,

2 2 shows a flow diagram of the device according to the invention,

3 to 7 different maps that are characteristic of different errors and

8th an injector.

description of the embodiments

The procedure according to the invention is as follows using the example of a so-called common rail system, in which a piezoelectric Actuator controls the amount of fuel metered by an injector, described.

In 1 the essential elements of a device for controlling a piezo actuator are shown. An output stage is labeled 100. This output stage applies voltage to the actuator (not shown) and charges it to a specific one th time by energizing to predetermined values. Correspondingly, the actuator is discharged again by energizing it in the opposite direction at a predetermined time. The final stage 100 includes a current measurement 110 and a voltage detection 120 , These are used to measure the current I which flows when charging into the actuator and when discharging from the actuator. It also detects the voltage detection 120 the voltage U, which characterizes the voltage at the actuator. A corresponding output stage is in the EP 11 38 902 described.

The output signal I of the current detection 110 arrives at an integrator 130 , At the output of the integrator 130 the signal Q is present, which characterizes the amount of charge with which the actuator was charged.

The output signal Q of the integrator and the output signal U of the voltage detection 120 arrive at a map determination 140 , The map determination is based on the quantities Q and U. 140 the current force-displacement characteristic. The force-displacement characteristic curve determined in this way is used for map analysis 150 , which she compares with a given warp line. Detects the map evaluation 150 a corresponding deviation, this becomes the final stage 100 reported back or alternatively or additionally by means of an error display 160 displayed to the driver. Furthermore, it can be provided that a fault of a higher-level control system, which influences fuel metering, for example, is also signaled.

The mode of operation of this device is explained in the following using the flowchart of 2 described. The method described below is preferably used while the internal combustion engine is in operation. The method described below is used for all controls of a piezo actuator, or only for a predetermined number of controls. Alternatively or additionally, it can also be provided that the method is carried out during maintenance or before the actuator is started up for the first time. This means that the method can be used both during operation and as part of maintenance or before or when the actuator is started up for the first time.

In a first step 200 the rail pressure P is measured at the beginning of the process. This pressure is the pressure against which the actuator works. With the start of the charging process, the crankshaft revolution in one step is at fixed time intervals or at fixed intervals 210 the voltage U on the actuator and the charge Q that is loaded into the actuator are recorded. The subsequent query 220 checks whether the actuator is fully charged. That is, it is checked whether the loading process has been completed. If this is not the case, step is repeated 210 , This means that the charge Q, which flows into the actuator, and the voltage U applied to the actuator are detected in discrete measuring points, which are preferably equidistant, during the entire charging process.

If the loading process is finished, then in step 230 the force-displacement characteristic curve is determined. In the subsequent step 240 the force-displacement characteristic curve is compared with an expected curve. The subsequent query 250 checks whether there is a discrepancy. If this is not the case, the program ends in step 260 , If this is the case, the step 270 detected for errors. Such a detected fault is preferably displayed to the driver or an appropriate emergency driving operation is initiated and / or an emergency shutdown is carried out.

Usually are three phases of the activation of an injection Actuator differentiated. The actuator is charged in a first phase and moves from its first position to a second position, in a second phase the actuator remains charged and remains there in his position, in a third phase, he is unloaded and takes hence its original first position again.

The charge measurement described Q and the voltage U occur in a preferred embodiment over all three phases. Various errors can be recognized at the end of the loading process become. Other mistakes can on the other hand only after completion of the second phase and / or after the Be recognized at the end of the discharge process.

In 3 the force-displacement characteristic is plotted. The force is labeled F and the path is labeled X. The force-displacement characteristic of an actuator for a low rail pressure P1 is dashed and a higher rail pressure P2 is plotted with a solid line. In addition, the lines with constant voltage are dashed in this diagram and the lines with constant charge are dotted. With increasing voltage U or with increasing charge Q, the corresponding lines of constant voltage and / or constant charge shift to the right.

Each point of the Kraft-Web characteristic clearly corresponds to a pair of values consisting of voltage U and charge Q. The one in step 210 measured value for the charge Q and the voltage U each result in a point in the force-displacement characteristic. By measuring several pairs of values for the charge Q and the voltage U during the charging process and / or during the unloading process, the force-web characteristic curve is determined in step 230 ,

As long as the force on the actuator due to the rail pressure is greater than the actuator force applied to the coupler, the control valve remains closed. The control valve is located in its lower seat US. The actuator force only acts against the elasticity of the individual elements of the injector. In accordance with the resulting balance of forces, the actuator executes a corresponding stroke without the control valve opening. This causes the Force-displacement characteristic has an almost linear course. If the actuator force applied via the coupler is greater than the force exerted by the rail pressure, the sealing element is released from the sealing seat and the actuator expands. This point is referred to as the separation point A1 in the force-displacement characteristic curve or as A2 at increased pressure. Up to this point, the force-displacement characteristic is almost linear.

In the following, the actuator expands continue until there is a balance of forces between actuator force and the force as a result of the rail pressure on the control valve. The new balance of power forms when the control valve in the OS point its upper Seat reached. Due to the sensory effect of the piezo actuator, one leads greater strength on the actuator to a greater voltage on the actuator. With the same load you get X in with the same stroke Follow higher Rail pressure a higher Voltage U.

If the sealing element of the control valve is stuck, the actuator only works against the elasticity of the injector parts. Such clamping of the sealing element can be caused, for example, by contamination and / or a mechanical defect thereof. A corresponding defect has the effect that the separation point along the in 3 straight line drawn in dash-dot lines moves.

With a sticking in the closed seat Control valve results in comparison to error-free operation same charge a higher one Voltage U, since the actuator only works against the elasticity of the injector parts and realized a smaller expansion than in error-free operation. With that leaves a simple measurement at a few operating points detect the corresponding defect. According to the invention, a pinching valve detected when a combination of voltage and charge values occurs that lies on the straight line shown and not with the measured rail pressure is plausible, i.e. at a pressure P1 is recognized for errors when the measuring pressure at correspondingly larger voltages or / and charges.

According to the invention, the detection of a jamming valve in the step 240 Checks whether the straight line goes beyond the permissible separation point, which depends on the rail pressure, or it is checked whether, with appropriate control, a characteristic point, i.e. a pair of values for the voltage and the charge, occurs above the expected separation point.

Another mistake is that the hydraulic coupler between the actuator's piston and the piston of the control valve is only incompletely filled between two meterings. Such an incomplete one filled hydraulic coupler means that the distance between the Control piston of the actuator and the piston of the control valve smaller than with one completely filled hydraulic coupler. As a result, the actuator must have the lower one Distance with a larger stroke compensate. The cause of an unfilled one Couplers are, for example, a filling pressure that is too low, or that the control valve remains in the open state and does not close.

In the case of an incompletely filled coupler, there is an offset to the right on the stroke axis X compared to the error-free operation, since the piezo actuator makes a so-called free travel before a corresponding actuator force builds up. This causes an area in the force-displacement characteristic in which the slope of the force-displacement characteristic is significantly smaller than expected. The slope preferably assumes a value of almost zero. A corresponding course is exemplified in 4 dash-dotted lines. Will in step 240 If a corresponding characteristic curve is detected, the device detects an insufficiently filled coupler.

The detection takes place, for example due to the fact that a lower voltage is present with the same charge or that to achieve the same voltage, more charge in the Actor must be invited. The gradient of the force-displacement characteristic is preferably determined and recognized for errors when the slope is almost zero, the is called is less than a certain value. The certain value corresponds the slope in perfect condition.

If individual layers of the piezoceramic of the piezo actuator fail, the straight line with constant charge or the straight line with constant voltage changes. These changed straight lines with constant charge and / or voltage are in 5 shown. Such failures of actuator layers are essentially caused by cracks in the layer or drop in the contacting electrode. The straight lines of constant tension are steeper with a constant force axis section, since more tension is necessary to achieve the same stroke as in the faultless case. The straight lines of constant charge are steeper with constant expansion, since there are fewer layers for charge absorption. This leads to the consumption of the force-displacement characteristic. In the event of a shift failure, the actuator voltage is higher compared to error-free operation with the same charge, since the piezo actuator requires more voltage to achieve the same stroke. For this purpose, the expected course of the force-displacement characteristic is compared with the determined characteristic. The error is then recognized on the basis of the determined deviations.

The likelihood that a Layer loss immediately after installation at the customer's end of the line occurrence is very low. Because of this, the same can be done Procedure a polarity reversed actuator can be recognized. Reverse polarity of the actuator to re-polarization of the ceramic, which in turn leads to a lower capacity of the actuator.

If a shift failure is detected, a occurs An entry in the fault memory of the control unit and / or the fault is signaled to the driver and causes the driver to seek service. As a rule, a major shift failure means that no fuel is injected into the associated cylinder through the control valve.

It is due to an error an elevated Friction in the knitting chain between actuator and control valve, so to overcome a higher friction force Actuator force needed. This manifests itself in that compared to faultless operation with the same charge higher Actuator voltage is required because the actuator is additional against the additional friction works and realizes a smaller stroke.

If the friction and thereby the Stroke loss too large, so opens the control valve no longer and there is no injection.

An additional friction force results in Compared to error-free operation with the same charge, a lower one Actuator voltage when charging, as the piezo actuator also counteracts the additional frictional force works and with a larger extension follows. In an extreme case the control valve no longer closes and it would for continuous injection and possibly unwanted acceleration of the vehicle.

If one considers the charging and discharging process, compared to the fault-free operation of the injector with the same charge, there is a greater voltage hysteresis, which is also expressed in a greater electrical power loss of the piezo actuator. In 6 a corresponding displacement-force diagram of an actuator with increased friction force is plotted. When loading, the force increases significantly steeper than with a fault-free actuator. This means that the slope of the force-displacement characteristic is larger than an expected value. This error is recognized when the slope of the force-displacement characteristic exceeds a certain value during the charging process. The determined value corresponds to the slope in the faultless state.

If an inadmissibly high frictional force is recognized, this is done preferably an entry in the error memory of the control unit. Further can additionally signaling to the driver by a signal lamp and / or others Activities, like a torque limitation. This can make one in time Total failure of the injector can be prevented. That it can be timely through which may continue increasing friction caused permanent injections or misfires in time be avoided. By reducing the available Moments the driver is forced to go to a workshop. at one slightly increased Friction within acceptable limits it can be provided that the changed opening and closing behavior of the injector by correcting the electrical activation starts and / or the electrical activation duration and / or the charging time and / or the actuator setpoint voltage is corrected.

In 7 the characteristic curve is plotted which results from a leaky coupler. If the coupler is leaking, the result is that the actuator continues to move even with constant charge and the stroke increases until the forces are balanced again. In the force-displacement characteristic of the actuator, this means that if the charge remains constant, the actuator voltage drops along the straight line for the constant charge. The change in the actuator voltage over time depends on the size of the leak. As the actuator stroke continues to increase, however, the actuator force also decreases until the force increases due to rail pressure and the sealing element moves back to the equilibrium of forces. This closes the nozzle needle again. According to the invention, it was recognized that in the case of a leaky coupler, compared to the fault-free case, there is a falling actuator voltage at the open seat position. If a leaky coupler is detected, an entry is made in the error memory of the control unit. The error is also signaled to the driver so that he can have the error rectified. For this purpose, it is provided that the characteristic curve is recorded over the entire injection, that is to say over the charging process and the unloading process.

8th shows an example of an injector in a schematic representation 1 with a central hole. There is an adjusting piston in the upper part 3 with a piezoelectric actuator 2 introduced into the central bore, the adjusting piston 3 with the actuator 2 is firmly connected. The actuating piston 3 closes a hydraulic coupler at the top 4 down while opening an opening with a connection channel to a first seat 6 is provided in which a piston 5 with a valve closing member 12 is arranged. The valve closing member 12 is designed as a double closing control valve. It closes the first seat 6 when the actuator 2 is at rest. When actuating the actuator 2 , that is, when a control voltage Ua is applied to terminals +, -, the actuator actuates 2 the actuating piston 3 and pushes over the hydraulic coupler 4 the piston 5 with the closure member 12 towards a second seat 7 , Below the second seat is a nozzle needle in a corresponding channel 11 arranged that the outlet in a high pressure channel (common rail pressure) 13 closes or opens, depending on the control voltage Ua. The high pressure is supplied by the medium to be injected, for example fuel for an internal combustion engine, via an inlet 9 supplied via an inlet throttle 8th and a flow restrictor 10 becomes the inflow amount of the medium in the direction of the nozzle needle 11 and the hydraulic coupler 4 controlled. The hydraulic coupler 4 has the task of, on the one hand, the stroke of the piston 5 to reinforce and on the other hand the control valve from the static temperature expansion of the actuator 2 to decouple. The again filling the coupler 4 is not shown here.

The mode of operation of this injector is explained in more detail below. Every time the actuator is activated 2 becomes the actuating piston 3 towards the hydraulic coupler 4 emotional. The piston also moves 5 with the closure member 12 towards the second seat 7 to. Part of the in the hydraulic coupler is via leakage gaps 4 medium, for example the fuel, pressed out.

The hydraulic coupler must therefore be used between two injections 4 be refilled in order to maintain its functional reliability.

Via the inlet channel 9 there is a high pressure, which can be, for example, between 200 and 2000 bar in the common rail system. This pressure acts against the nozzle needle 11 and keeps it closed so that no fuel can escape. Now becomes the actuator due to the control voltage Ua 2 actuated and thus the closure member 12 is moved in the direction of the second seat, the pressure in the high pressure area decreases and the nozzle needle 11 releases the injection channel. P1 is the so-called coupler pressure, as it is in the hydraulic coupler 4 is measured. Without control Ua, a stationary pressure P1 is set in the coupler, which is, for example, 1/10 of the pressure in the high-pressure part. After unloading the actuator 2 the coupler pressure P1 is approximately 0 and is raised again by refilling.

According to the invention in the course of control of an injector at fixed time intervals or at fixed angular intervals measured value pairs for the Voltage that is present at the injector and the charge that is in the injector is invited. The charge is recorded by a current measurement, the current value being integrated. These pairs of values are in the control unit entered in a map. This map thus determined is then compared with an expected map. Gives way to the measured Map from the expected map, so is recognized for errors. Based on the type of deviation, there are different errors recognized.

The map in which the expected The course of the Kraft-Web characteristic curve is preferred before the first start-up and / or the first start-up of the injector. It is particularly advantageous if this Reference map stored in application and learned during operation becomes. This means whenever the map is remeasured the map values are compared with the old map. If the deviation is smaller than a threshold value, the old and new values a new mean is determined. alternative can also be provided that in the event of a small deviation of the new value is used immediately. Give way to the old and the new pairs of values differ significantly from each other, i.e. by more than one Threshold, so it is recognized for errors.

Claims (12)

Method of monitoring a piezoelectric Actuator for controlling an injector, with a characteristic curve of the actuator is determined, the determined characteristic with an expected characteristic is compared and if the determined characteristic curve deviates an error of the injector is recognized by the expected characteristic. A method according to claim 1, characterized in that while a charging process and / or an unloading process and assigned voltage values are recorded. A method according to claim 2, characterized in that based on the charge values and assigned voltage values a force-displacement characteristic is determined. Method according to one of the preceding claims, characterized characterized that a mechanical blockage is detected, if the force-displacement curve is largely linear having. Method according to one of the preceding claims, characterized characterized that an incompletely filled coupler is recognized, if the force-displacement characteristic curve is not linear at the start of the charging process increases. Method according to one of the preceding claims, characterized characterized that a leaky coupler is detected when the Force-displacement characteristic curve drops at the end of the charging process. Method according to one of the preceding claims, characterized characterized that increased internal friction is detected when the force-displacement characteristic is stronger than expected increases. Method according to one of the preceding claims, characterized characterized that a layer failure and / or reverse polarity is recognized when the force-displacement characteristic is consumed. Monitoring device a piezoelectric actuator for controlling an injector, with Means that determines a characteristic curve of the actuator that determined Compare the characteristic curve with an expected characteristic curve and with a Deviation of the determined characteristic from the expected characteristic detect an error in the injector. Computer program with program code means to perform all steps of any one of claims 1 to 11 when the program on a computer, in particular a control unit for an internal combustion engine. Computer program product with program code means, which are stored on a computer-readable data medium in order to Perform the method of any of claims 1 to 11 when the program product on a computer, in particular a control device for an internal combustion engine, accomplished becomes. Digital storage medium, in particular floppy disk, with electronically readable control signals, which with a programmable Computer system can interact, the existence Method according to one of the claims 1 to 11 executed becomes.