EP1485895B1 - Method for transmitting data from a sensor to a control unit, and a corresponding sensor and control unit - Google Patents

Description

State of the art

The invention relates to a method for data transmission from a sensor to a control unit and to a corresponding sensor and a corresponding control unit for restraint systems.

German patent applications 101 14 504.7 dated 23.03.2001 (published 02.10.2002 and thus no prior art) and 101 49 332.0 of 06.10.2001 (published on 30.04.2003 and thus no prior art) are methods for the transmission of Data from at least one sensor to a control unit, in particular in connection with restraint systems known in which the data is transmitted by means of current modulation via a two-wire line from the sensors to a control unit in accordance with a predetermined format. The format of the data transmission provides a fixed allocation of parts of the available value range for the data transmission to the sensor values, wherein a first part of the value range for sensor values, ie useful data, a second part for status and error messages and a third part for sensor identification data used becomes. These parts are separated from each other and follow each other during transmission.

Furthermore, the use of pressure sensors is known in a variety of applications, especially in connection with restraint systems, which are distributed in the vehicle and are connected via such or another interface with a central control unit.

Out US 5,691,714 is a data transmission from a peripheral sensor to an evaluation known. Absolute values and incremental change values are transferred from the sensor to the evaluation unit. The evaluation unit uses the absolute value and the incremental change values to restore absolute values for further evaluation.

Advantages of the invention

By transmitting an absolute pressure value in pressure sensors, which transmit useful pressure as a differential pressure, it is advantageously possible to perform a function verification of these pressure sensors and thus an error detection in a system which consists of at least two sensors and a central control unit.

It is of particular advantage that an error-free functioning of all pressure sensors in the system is provided with cost-effective software implementation without additional hardware expenditure. An introduction of error detection in such a sensor system is thus possible without system change solely by software modification.

It is particularly advantageous to transmit the transmission of the absolute pressure value instead of the differential pressure value during the initialization phase of the system. In this way, a check of the pressure sensors is made possible before starting the operation of the system.

In a particularly simple manner, the known current-based two-wire interface is used for data transmission.

In a particularly advantageous manner, according to a further aspect of the invention, a check of pressure sensors in a sensor system with a central control unit during operation is made possible when the transmission of absolute pressure values during operation of the system be mixed with the transmission of differential pressure values.

It has been found to be particularly advantageous to carry out the absolute pressure values in an interface whose range of values for the data transmission is divided into at least two parts in that part of the range of values which is not suitable for the sensor measured values, i. the differential pressure values, is available. As a result, a mutual influence of measured value and absolute pressure value is effectively avoided.

It is particularly advantageous to encode the absolute pressure values in the data format described in the aforementioned prior art into a value range which lies outside the useful signal value range, wherein identification codes are additionally assigned to the absolute pressure values. Both the identification code and the data word are outside the range of the differential pressure, so that advantageously a confusion of the individual pressure values can not take place.

Advantageously, a transmission of the absolute pressure values takes place only as long as no significant signal change of the differential pressure is present. As soon as such occurs, the current absolute pressure transmission is stopped and switched to differential pressure transmission. As a result, especially when used in conjunction with restraint systems, the system operation and the intended purpose of success is not affected.

Further advantages will become apparent from the following description of exemplary embodiments or from the dependent claims.

drawing

The invention will be explained in more detail below with reference to the embodiments shown in the drawing. FIG. 1 shows an overview image of a sensor system with central control unit, while in the FIGS. 2 to 4 Based on flow charts, the procedure for data transmission in conjunction with pressure sensors, preferably in restraint systems, outlined.

Description of exemplary embodiments

FIG. 1 shows a central control unit 10 which is connected to decentralized sensors 12, 14, 16, 18, 20, 22 connected. The sensors in each case consist of a sensor element (compare 12a, etc.) and an interface module (compare 12b, etc.), which in the preferred exemplary embodiment is designed as an ASIC and in which the procedure described below for data transmission is used as program or sequence control is implemented. In the preferred exemplary embodiment, unidirectional data transmission from the sensors to the central control unit takes place via two-wire lines 24 to 34, which are connected in a point-to-point connection to the central control unit 10 and the decentralized sensors. In other embodiments, a bus connection is used.

A preferred application of the in FIG. 1 illustrated system, wherein the number of sensors of the in FIG. 1 shown is in connection with restraint systems for automobiles. The sensors are pressure sensors that sense a pressure applied to them. On the basis of this measured absolute pressure, it is not possible for a single pressure sensor to evaluate whether the obtained pressure information is correct. On the other hand, at a like in FIG. 1 designed system, the central control unit via multiple pressure information from multiple pressure sensors connected to each other do not communicate, but transmit all their pressure values to the central control unit. Therefore, it is able to compare these on the basis of the absolute pressure values of the individual sensors for plausibility and to derive malfunctions. Since the pressure gradient within the system, in particular within a motor vehicle, is negligibly small, the absolute pressure values of the individual sensors must lie in a defined tolerance band with one another when the function is correct. Based on this, the absolute pressure values in the central control unit can be compared and checked for plausibility in restraint systems.

A possible procedure for the realization of this plausibility check is based on the flowchart of FIG. 3 shown. This outlines the program of a microcomputer included in the central control unit 10. The program is run through in a predetermined cycle. First, in step 100, the absolute measured values PABS1 to PABSn (n is the number of sensors) supplied by the sensors are read. Thereafter, in step 102, a print reference value Pref is formed. Depending on the design, this is either one of the sensor pressure values or an average of sensor pressure values, etc. In the following step 104, the individual pressure sensor values are compared with the reference value. Subsequently, it is checked in step 106 whether there is an impermissible deviation between a sensor value and the reference value. If this is the case, an error in the affected pressure sensor is determined according to step 108, while in the absence of impermissible deviation, that is, when all sensor signals are within a predetermined tolerance band, an error-free operation is assumed.

Other procedures, eg comparisons of pressure values among each other, for plausibility checking are also used in other versions.

In many applications it has been found suitable not to transmit absolute pressure values for carrying out the control tasks, but differential pressure values, for example the difference value between a reference pressure and the current measured pressure. In this way, environmental parameters are already taken into account in the sensor, so that the transmitted pressure measurements in the central control unit need not be additionally evaluated. In these cases, as part of the above-described functional check of the sensors, care must be taken to ensure that, in addition to or instead of the differential pressure transmission, the absolute pressure values are transmitted.

In a first embodiment, it has proven to be suitable to carry out the absolute pressure transmission in the initialization phase, to proceed to a differential pressure transmission after the completion of the initialization phase. The checking of the sensors in this case therefore takes place in the initialization phase on the basis of the transmitted absolute pressure values.

This embodiment is based on the flowchart of FIG. 2 outlined. This describes the process in the interface module of a pressure sensor. When the system is switched on (INIT phase), the pressure measurement value is read in in the first step 200. This is then transmitted to the central control unit according to step 202. Thereafter, it is checked in step 204 whether the initialization phase has ended. If this is not the case, steps 200 and 202 are repeated. In the other case, the pressure measurement value is read in step 206 and the pressure difference value Pdiff is formed in step 208. This is done, for example, by forming the difference of the pressure measurement value and a reference pressure value, for example an average value on previously measured pressure values. Thereafter, in step 210, the differential pressure value is transmitted. Steps 206 to 210 are repeated until the operating cycle has ended, for example by switching off the supply voltage. As a result, a continuous transmission of the differential pressure value is ensured.

Depending on the exemplary embodiment, a point-to-point interface to each sensor or a bus system connected to all components is provided as an interface between sensors and central control unit. In a point-to-point interface, in a preferred embodiment, it has proved suitable to provide a current-based two-wire interface as outlined in the aforementioned prior art.

The first embodiment outlined above describes the transmission of absolute pressure values in the initialization phase. A review of the sensor function during operation is not possible in such an implementation. Therefore, as part of a second exemplary embodiment, it is alternatively or additionally provided to permit a transmission of the absolute pressure value during operation and thereby to enable error detection in the sensor system in the manner indicated above, also during ongoing operation of the system.

If, as mentioned above, a transmission of pressure difference values or normalized differential pressure values is carried out in order to be independent of the ambient pressure and thus independent of the current altitude or weather-related pressure fluctuations, measures must be taken to allow absolute pressure values to be transmitted in addition to these differential pressure values. Normalized pressure values are to be understood as pressure values which are normalized in such a way that a signal not equal to 0 is transmitted only in the case of dynamic pressure fluctuations. That means with stationary pressure values is the Signal value 0. Again, errors in the sensor, which are characterized by a temporally constant output signal, can not be detected.

It is thus provided to additionally mix absolute pressure values during the normal operation of the system during the transmission from the sensor to the central control unit in the transmission of the normalized differential pressure value. The transmitted absolute pressure values are then compared with one another by the central control unit as shown above, and from this, indications for a correct or erroneous function of the individual sensors are derived.

A transmission of absolute pressure values takes place only as long as there is no significant signal change on the differential pressure. As soon as such a differential pressure change is detected, the sensor immediately stops the possibly running absolute pressure transmission and switches to differential pressure transmission. This measure ensures that no system performance is lost due to the additional transmission of absolute pressure values.

Furthermore, data mixing or confusion is not possible, since the different data types (absolute pressure, differential pressure) can be uniquely assigned over their value range.

In the preferred embodiment, the interface between sensor and central control unit represents a current-based two-wire interface as known from the aforementioned prior art. Here, the distinction between absolute pressure data and differential pressure data is made once by a corresponding identification of the data with different identification codes. On the other hand, the absolute pressure value is coded into a data word whose value range outside of the value range of the user data (differential pressure).

In other embodiments, one of the measures shown suffices.

The absolute pressure values therefore consist of a combination of identification code and data word, whereby both the identification code and the data word are outside the data range of the differential pressure values. This ensures that the absolute pressure values are not confused with regular differential pressure values.

Here, in the preferred embodiment, the value range for the data transmission as in the aforementioned prior art is essentially divided into three parts, wherein a middle region comprises the payload, in the present case, the differential pressure data, the areas outside the payload data status information, identification data, etc. include. To transmit the absolute pressure values and to exclude collisions, provision is made for the absolute pressure values to be preceded by a separate identifier and the absolute pressure values to be transmitted as a data word which has a value range which lies outside the useful data signal, for example in the status information range. Due to the small word length in these areas, the absolute pressure data word is divided and is sent with a corresponding identifier sent in several portions.

Absolute pressure value is understood to be the current measured pressure value, while for forming the differential pressure value an average value of past measured pressure values, which is related to the currently measured pressure value, is used.

FIG. 4 shows a flowchart in which the sensor-side sequence is sketched for the transmission of differential pressure values and absolute pressure values during operation. The in FIG. 4 outlined sequence takes place at predetermined time intervals. First, in the first step 300, the current pressure value PMess is read. Subsequently, the reference pressure value PREF is formed in step 302 on the basis of past values, and the differential pressure value PDIFF is formed in step 304 on the basis of the currently measured and reference pressure values. In an exemplary embodiment, this can also be supplied to an additional standardization, which normalizes the differential pressure value, for example to an average value from past differential pressure values. Thereafter, in step 306, it is determined whether there is a significant signal change based on the current differential pressure value and one or more past differential pressure values. If this is the case, an optionally ongoing transmission of absolute pressure values is stopped in step 308, the current differential pressure value in the first value range available for the data transmission is coded as data word, the corresponding identifier is selected and identifier and data word are sent in accordance with step 310.

If no significant signal change was detected in step 306, the absolute measured value PMESS is encoded in a second value range, which is available for the data transmission, as a data word according to step 312. If the word width is restricted in the respective application, the data word is split into several subsections, the corresponding identifiers are selected and the data transmission in step 314 consisting of identifier and data word is performed in several sections. Thereafter, the process described is repeated in the next time interval.

The illustrated procedure is used in particular in connection with restraint systems, but can also be used in other systems with distributed pressure sensors use.

Furthermore, the application of the procedure described using the example of pressure sensors is not limited to pressure sensors, but is used wherever decentralized sensors of the same type transmit differential values as useful data to a common control center without the possibility of self-testing.

Claims (9)

1. Method for transmitting data from a sensor (12, 14, 16, 18, 20, 22) to a control unit (10), the variable measured by the sensor (12, 14, 16, 18, 20, 22) being transmitted to the control unit (10) in the form of difference values, absolute measured values of the same variable also additionally being transmitted in at least one operating state, characterized in that the current transmission of the absolute measured values is stopped and the method changes over to the transmission of the difference values on the basis of a signal change in the difference values.
2. Method according to Claim 1, characterized in that the operating state is the initialization phase of the system comprising the sensor (12, 14, 16, 18, 20, 22) and the control unit (10).
3. Method according to one of the preceding claims, characterized in that the measured variables are pressure values.
4. Method according to one of the preceding claims, characterized in that the range of values provided for the purpose of transmitting data is subdivided into a plurality of ranges of values, the difference data being coded in a first range of values, and the absolute data being coded in a second range of values.
5. Method according to one of the preceding claims, characterized in that identifiers which are placed in front of a respective data word and differ in the case of difference data and absolute data are provided for the purpose of transmitting data.
6. Method according to one of the preceding claims, characterized in that transmission is effected using a current-based two-wire interface.
7. Method according to one of the preceding claims, characterized in that absolute values and difference values are transmitted during ongoing operation.
8. Sensor (12, 14, 16, 18, 20, 22) having an interface (12b, 14b, 16b, 18b, 20b, 22b) for transmitting data to a central control unit (10), the sensor (12, 14, 16, 18, 20, 22) having first means for detecting the measured values, the sensor also containing a module having second means which relate the detected measured values to a reference value in order to form a difference measured value, the module (12b, 14b, 16b, 18b, 20b, 22b) also having third means for transmitting the difference values and, in at least one predefined operating state, additionally the absolute measured values, characterized in that the module (12b, 14b, 16b, 18b, 20b, 22b) has a sequence controller which stops the current transmission of the absolute measured values and changes over to the transmission of the difference values on the basis of a signal change in the difference values.
9. Sensor according to Claim 8, characterized in that it is a pressure sensor (12, 14, 16, 18, 20, 22) in a restraint system for automobiles.

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