Method for operating a common-rail system of a motor vehicle having a redundant common-rail-pressure sensor

Abstract

A method for operating a common-rail system of a motor vehicle that includes a common-rail-pressure sensor configuration having at least two signal paths, and that can be operated at a maximally permissible common-rail pressure and at a minimally permissible common-rail pressure. Sensor signals are read out in each case in response to a pressure measurement in a common rail of the common-rail system via the at least two signal paths, and a signal deviation value is ascertained that characterizes a deviation between the pressure values that are each determined on the basis of the sensor signals. The method includes reducing the maximally permissible common-rail pressure by a correction value to a maximally permissible emergency common-rail pressure and/or increasing the minimally permissible common-rail pressure by a correction value to a minimally permissible emergency common-rail pressure in response to the signal deviation value exceeding a predefined value.

Claims

1. A method for operating a common-rail system of a motor vehicle, the method comprising: reading out sensor signals in each case in response to a pressure measurement in a common rail of the common-rail system via at least two signal paths of a common-rail-pressure sensor configuration, wherein the common-rail system includes the common-rail-pressure sensor configuration having the at least two signal paths, and wherein the common rail system is operable at a target maximally permissible common-rail pressure and at a target minimally permissible common-rail pressure; ascertaining a signal deviation value that characterizes a deviation between pressure values that are determined based on the sensor signals; and in response to the signal deviation exceeding a predefined value, at least one of: reducing the target maximally permissible common-rail pressure by a correction value to another target maximally permissible emergency common-rail pressure, and increasing the target minimally permissible common-rail pressure by the correction value to another target minimally permissible emergency common-rail pressure; wherein a differential amount is ascertained as the signal deviation value between the two pressure values that are determined based on the sensor signals of two of the signal paths, in each case, half of the differential amount between the two pressure values being used as the correction value.

2. The method as recited in claim 1, where the sensor signals of two of the signal paths are determined in the form of two mutually inverted voltage signals, which each indicate a pressure in the common rail and from which the pressure values are ascertained.

3. The method as recited in claim 2, where the two voltage signals are received in each case using evaluation circuits associated with the signal paths.

4. The method as recited in claim 1, where the common-rail-pressure sensor configuration has exactly two signal paths.

5. The method as recited in claim 1, where a common-rail-pressure sensor having the two signal paths is used as the common-rail-pressure sensor configuration.

6. The method as recited in claim 1, where the sensor signals are at least one of averaged and linearized.

7. The method as recited in claim 1, where the motor vehicle is switched to an emergency operation in response to the signal deviation value exceeding the predefined value.

8. The method as recited in claim 1, wherein the method is used for a single-or two-actuator common-rail system.

9. A common-rail system of a motor vehicle, comprising: a common rail; a common-rail-pressure sensor configuration; at least two signal paths, the system being operable at a target maximally permissible common-rail pressure and at a target minimally permissible common-rail pressure; an arrangement for reading out sensor signals, in each case in response to a pressure measurement in the common rail via the at least two signal paths; and an arrangement for ascertaining a signal deviation value that characterizes a deviation between pressure values that are each determined based on the sensor signals; and an arrangement for, in response to the signal deviation value exceeding a predefined value, at least one of: reducing the target maximally permissible common-rail pressure by a correction value to another target maximally permissible emergency common-rail pressure, and increasing the target minimally permissible common-rail pressure by the correction value to another minimally permissible emergency common-rail pressure; wherein a differential amount is ascertained as the signal deviation value between the two pressure values that are determined based on the sensor signals of two of the signal paths, in each case, half of the differential amount between the two pressure values being used as the correction value.

10. The common-rail system as recited in claim 9, where each of the at least two signal paths of the common-rail-pressure sensor configuration includes a measuring bridge, the measuring bridges of at least two of the at least two sensor signals being configured on the same or different sensor elements.

11. A processing unit for operating a common-rail system of a motor vehicle, comprising: a processing arrangement configured for performing the following: reading out sensor signals in each case in response to a pressure measurement in a common rail of the common-rail system via at least two signal paths of a common-rail-pressure sensor configuration, wherein the common-rail system includes the common-rail-pressure sensor configuration having the at least two signal paths, and wherein the common rail system is operable at a target maximally permissible common-rail pressure and at a target minimally permissible common-rail pressure; ascertaining a signal deviation value that characterizes a deviation between pressure values that are determined based on the sensor signals; and in response to the signal deviation exceeding a predefined value, at least one of: reducing the target maximally permissible common-rail pressure by a correction value to another target maximally permissible emergency common-rail pressure, and increasing the target minimally permissible common-rail pressure by the correction value to another target minimally permissible emergency common-rail pressures; wherein a differential amount is ascertained as the signal deviation value between the two pressure values that are determined based on the sensor signals of two of the signal paths, in each case, half of the differential amount between the two pressure values being used as the correction value.

12. The processing unit as recited in claim 11, wherein the processing unit includes a control unit for the common-rail system.

13. A computer readable medium having a computer program, which is executable by a processor, comprising: a program code arrangement having program code for operating a common-rail system of a motor vehicle that includes a common-rail-pressure sensor configuration having at least two signal paths, and that can be operated at a target maximally permissible common-rail pressure and at a target minimally permissible common-rail pressure, by performing the following: reading out sensor signals in each case in response to a pressure measurement in a common rail of the common-rail system via the at least two signal paths; ascertaining a signal deviation value that characterizes a deviation between pressure values that are determined based on the sensor signals; and in response to the signal deviation exceeding a predefined value, at least one of: reducing the target maximally permissible common-rail pressure by a correction value to another target maximally permissible emergency common-rail pressure, and increasing the target minimally permissible common-rail pressure by the correction value to another target minimally permissible emergency common-rail pressure; wherein a differential amount is ascertained as the signal deviation value between the two pressure values that are determined based on the sensor signals of two of the signal paths, in each case, half of the differential amount between the two pressure values being used as the correction value.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 shows a block diagram of the essential elements of a common-rail system, as may underlie the present invention.

(2) FIG. 2 shows a block diagram of the essential elements of a common-rail pressure configuration that may be used in accordance with the present invention.

(3) FIG. 3 illustrates sensor signals that are obtainable using the common-rail-pressure sensor configuration in accordance with FIG. 2.

(4) FIG. 4 illustrates a recognition of a drift of a common-rail-pressure sensor configuration in accordance with one specific embodiment of the present invention.

(5) FIG. 5 illustrates a compensation of a drift of a common-rail-pressure sensor configuration in accordance with one specific embodiment of the present invention.

DETAILED DESCRIPTION

(6) Denoted by 100 and illustrated as a block diagram, FIG. 1 shows the essential elements of a common-rail system that may underlie the present invention. Common-rail system 100 includes a high-pressure zone 120 and a low-pressure zone 130 in which fuel is present in each case at a different pressure. In the high-pressure zone, for example, a pressure of 1,500 bar-2,000 bar is customary, whereas a pressure of up to 10 bar may prevail in the low-pressure zone.

(7) Components of high pressure zone 120 are essentially a high-pressure delivery line 150 (what is generally referred to as the common rail, respectively rail) and injectors 151, 152 and 153 for metering the highly pressurized fuel into one or a plurality of cylinders (not shown) of a combustion engine.

(8) To regulate the high pressure (line pressure), inter alia, a processing unit configured as an engine control unit 170 is provided that actuates a control element 110 for controlling line pressure P by an actuation signal A. Control element 110 may be a pressure-regulating valve (PRV) that connects high-pressure zone 120 to low-pressure zone 130, and/or a controllable high-pressure pump that delivers the fuel from low-pressure zone 130 into high-pressure zone 120. By appropriately actuating a solenoid valve (generally referred to as a metering unit) provided on the high-pressure pump, the delivered quantity and thus the pressure prevailing in the high-pressure zone may be controlled. Low-pressure zone 130 (for example, in the fuel tank, main filter or in the high-pressure pump) is equipped with a temperature sensor 162 that measures the temperature of the fuel.

(9) Common-rail-pressure sensor 14 records current value P of the pressure prevailing in the high-pressure zone, also referred to here as common-rail pressure. A signal indicative thereof from common-rail-pressure sensor 14 arrives at control unit 170. As a function of various other signals (not shown), the control unit calculates actuation signals to act upon injectors 151, 152 and 153. These injectors meter a specific fuel quantity to the combustion engine as a function of the particular actuation signal at a particular point in time. The injectors are connected to low-pressure zone 130 via return lines through which excess fuel flows off. Merely three injectors and three cylinders are shown in the figure. However, the described procedure may be used for any desired number of injectors and/or cylinders.

(10) Moreover, a pressure-regulating valve 160 is provided that connects high-pressure zone 120 to low-pressure zone 130 via a return flow 161. Normally, this valve is closed, and the connection is interrupted. In response to the pressure prevailing in high-pressure zone 120 (i.e., the common-rail pressure) increasing beyond an activation pressure value (of 2,000 bar, for example), pressure limiting valve 160 opens, and the common-rail pressure drops to a holding pressure (for example, 800 bar).

(11) FIG. 2 shows a block diagram of the essential elements of a common-rail pressure sensor configuration that may be used in accordance with the present invention and is denoted as a whole by 140. Common-rail-pressure sensor configuration 140 is connected to a control unit 170 whose function was already previously clarified.

(12) In the principle of operation thereof and the measuring principle used, common-rail-pressure sensor configuration 140 may correspond to known common-rail-pressure sensors 14. Common-rail-pressure sensor configuration 140 has a housing, for example, that is schematically illustrated here and is denoted by 143. In the case of conventional common-rail-pressure sensors, one single sensor element having a metal diaphragm, for example, is provided in housing 143. The fuel pressure acts on the metal diaphragm. A semiconductor pressure sensor is mounted on the side of the metal diaphragm opposing the acting fuel pressure. It may be designed as a piezoelectric sensor, for example. A known measuring bridge is associated with the pressure sensor.

(13) In common-rail-pressure sensor configuration 140, that may be used in accordance with the present invention, a corresponding sensor element (i.e., a metal diaphragm) or a corresponding measuring bridge is provided in duplicate. The signal paths resulting herefrom are denoted here by 141a and 141b. Thus, signal paths 141a and 141b each include at least one measuring bridge that is configured in the form of a full bridge, for example. As explained, two measuring bridges may be configured on one sensor element.

(14) The raw signals of signal paths 141a and 141b are preprocessed through an A/D conversion, data processing and subsequent D/A conversion, for example. The preprocessed raw signals are subsequently transmitted as sensor signals 144a and 144b, preferably in analog form, to control unit 170 and further processed there. To preprocess the raw signals, as explained, signal paths 141a and 141b are connected via corresponding lines to evaluation circuits 142a and 142b where it may be a question of application-specific, integrated circuits (ASIC), for example. Evaluation circuits 142a and 142b are adapted for generating corresponding signals 144a and 144b where, as explained, it may preferably be a question of analog signals. In this regard, common-rail-pressure sensor configuration 140 is connected via corresponding lines to control unit 170. A further line pair 145 is provided that includes a supply line and a ground line. It is understood that common-rail-pressure sensor configuration 140 may alternatively also have another ground connection.

(15) Thus, common-rail-pressure sensor configuration 140 that may be used in accordance with the present invention altogether features two signal paths having corresponding full bridges and two evaluation circuits. It is preferably provided that output sensor signals 144a and 144b be generated as mutually inverted signals. Corresponding sensor signals 144a and 144b may be recorded in control unit 170. Pressure values may be ascertained from sensor signals 144a and 144b. A value that has been averaged accordingly from sensor signals 144a and 144b, respectively from corresponding pressure values may be used for regulating pressure and for calculating the actuation duration.

(16) In a diagram 300, FIG. 3 illustrates sensor signals that are obtainable using common-rail-pressure sensor configuration 140 in accordance with FIG. 2. In diagram 300, a voltage U is plotted in volts on the abscissa relative to a pressure p in bar on the ordinate. For the sake of clarity, the two sensor signals 144a and 144b are shown in a linear representation; it is understood, however, that such signals do not necessarily have to be in linear form. Therefore, at least one of the axes of diagram 300 may also be present in logarithmic, respectively in another non-linear form. At a minimum pressure p, sensor signal 144a yields a minimum voltage u and, at a maximum pressure p, a maximum voltage U. Converselyin this sense, sensor signals 144a and 144b are invertedat a minimum pressure p, sensor signal 144b yields a maximum voltage U and, at a maximum pressure p, a maximum voltage U.

(17) In the context of the present invention, an asymmetric output stage is advantageously used to pull the corresponding sensor signals to a preferred potential. The diagnosis is preferably made following a respective linearization based on the level of the pressure. This makes it possible to immediately discern in response to both signal voltages being identical, that a cable harness error must be present. Since the potential is defined on the basis of the evaluation stage, the pressure signal may be robustly used up to half of the characteristic curve (compare FIG. 3, region 310). This makes it possible to ensure a pressure regulation and a proper metering. In other words, the present invention advantageously functions with pressure values that are ascertained from particular sensor signals 141a and 141b. However, other values derived from the sensor signals may also be used. For such derived values (for example, pressure values), variables, respectively reference numerals a and b are briefly used in the following.

(18) FIG. 4 illustrates the recognition of a sensor drift in accordance with a specific embodiment of the present invention. In this case, two diagrams A and B are shown, in each of which a pressure p in bar is plotted on the ordinate over a time t on the abscissa. In diagram A, both signal paths function properly, respectively the evaluation circuits associated therewith function properly; thus there is also no cable error. In FIG. 4 and subsequent FIG. 5, a denotes a pressure value that may be ascertained from a signal 144a (compare FIG. 3). In response to constant common-rail pressure, pressure value a is constant over time. Correspondingly, b denotes a pressure value that is derived from a sensor signal 144b. It is also constant in response to a constant common-rail pressure. m denotes the mean pressure value of these two pressure values a and b. In the illustrated example, mean pressure value m corresponds in the idealized representation to exactly real pressure value r that is present in the common rail. In the context of an ideal measuring quality of the signal paths, real pressure value r would correspond exactly to the corresponding individual pressure values a and b, which would then be identical. However, since this is never the case in reality, it may be assumed that pressure values a and b are only exactly identical when an error is present. In addition, in reality, average value m represents exactly real value r only in exceptional cases, since sensor signals a and b hardly exhibit an identical deviation from real value r (positive and negative).

(19) In diagram B, a situation is shown where pressure value b deviates considerably from real pressure value r. In this case, pressure value b lies appreciably below real pressure value r. On the other hand, value a corresponds (with a deviation that is not shown) to real pressure value r. If merely a mean value is generated in this case between pressure values a and b (mean pressure value m), and this mean pressure value m is used for regulating the common-rail system, damage could possibly be caused because real pressure value r, which acts upon the common-rail system, lies above the supposedly correct pressure value (indicated by mean pressure value m).

(20) For that reason, the present invention provides for compensating for such a sensor drift, as is shown in greater detail in FIG. 5. Diagrams and signal designations in FIG. 5 thereby essentially correspond to the diagrams and signal designations in FIG. 4. There is no further description of real pressure value r here because real pressure value r is not known in real systems, where merely sensor signals of a corresponding common-rail-pressure sensor configuration 140 are available. Pressure value a, pressure value b, and mean pressure value m are shown in diagram A. However, it is not known whether mean pressure value m, pressure value a, or pressure value b corresponds to a real value. Therefore, a plausibility checking method is provided that is clarified in greater detail in the following:

(21) If a signal deviation value A, in this case a differential amount of between pressure values a and b, =|ab| resides outside of a permissible range, a common-rail-pressure signal is recognized as being implausible. It is initially not possible to ascertain information here as to which of the two underlying sensor signals 144a, 144b, respectively which pressure value a, b, respectively which signal path is incorrect. For clarification, reference is again made to FIG. 4 including diagrams A and B. In this case, diagram A shows a case where signal deviation value =|ab| still resides within the permissible range; whereas diagram B illustrates the case where signal deviation value =|ab| resides outside of the permissible range.

(22) Depending on the signal deviation of the incorrect pressure value (positive or negative) from real pressure value r (that is not known), real pressure r in the common-rail system is greater or lower than pressure mean value m=|ab|/2. Since it is not known which pressure value is incorrect, the system must be placed in a secure state. This means that the maximally permissible system pressure must not be exceeded; at the same time, however, in the case of an error, the minimum system pressure must be ensured in order to permit a best possible availability, at least, however, a limp home, thus an emergency operation until the nearest service station is reached.

(23) In this case, the maximally permissible common-rail pressure is reduced by half of the difference of signal deviation value in order not to produce any system overpressure. The maximally permissible common-rail pressure is denoted here by p.sub.max, a correspondingly reduced pressure in the case of an error (referred to here as maximally permissible common-rail pressure) by p.sub.max,E. It holds here that p.sub.max,E=p.sub.max|ab|/2.

(24) Accordingly, the minimally permissible common-rail pressure is reduced by half of the difference of signal deviation value in order to ensure the valve opening pressure of the injectors. The minimally permissible common-rail pressure is denoted here by p.sub.min; a correspondingly reduced pressure in the case of an error (referred to here as minimally permissible common-rail pressure) by p.sub.min,E. It holds here that p.sub.min,E=p.sub.min+|ab|/2.

(25) A corresponding drop in pressure is illustrated in diagram B of FIG. 5. The maximally permissible common-rail pressure is lowered here to the point where corresponding pressure values, denoted here by a, b and m are no longer able to exceed the maximally permissible pressure value. Even if real pressure value r is supposed to correspond to pressure value a in the extreme case, it is ensured that maximally permissible common-rail pressure is not exceeded. This also applies correspondingly to the minimally permissible common-rail pressure.