Abstract:
A method for extracting a constant part and a variable part from a data signal that is determined. The values that are extracted in this way are transmitted with different timing over a transmission link that is present. Depending on the bandwidth of the composite signal, various methods can be applied for transmitting relatively low bandwidth signals with possibly higher resolution.

Description:
BACKGROUND OF THE INVENTION 
   FIELD OF THE INVENTION 
   The invention relates to a method and a configuration having a transmitter and a receiver for transmitting digital data in a motor vehicle. The use of data transmission systems is known in motor vehicles. In a large number of application areas, measurement signals have to be transmitted from sensors to a central processor unit within a motor vehicle. In many transmission systems the data rate is limited. In the case of airbag systems, for example, data transmissions with 7 bit data word width and a transmission frequency of 2 kHz are used. In such systems, the pressure values that are measured in a side airbag, for example, are transmitted as data values. Such pressure values are usually composed of the average ambient pressure, which lies in a range from 600 to 1300 mbar, and from a dynamic change in pressure that lies in the range from minus 50 to plus 200 mbar. 
   It is desirable to measure such data with a high resolution of, for example, 15 bits and further process it. However, the given transmission links do not permit such high resolution. 
   SUMMARY OF THE INVENTION 
   It is accordingly an object of the invention to provide a method and a configuration for digitally transmitting data in a motor vehicle that overcome the above-mentioned disadvantages of the prior art methods and devices of this general type. 
   With the foregoing and other objects in view there is provided, in accordance with the invention, a method for transmitting data in a motor vehicle having a transmitter and a receiver. The method includes splitting a signal that is to be transmitted, into a constant part and a variable part, and combining a partial value of the constant part with the variable part and transmitting a result signal to the receiver. 
   The advantage of the invention is that existing transmission systems can continue to be used and nevertheless a higher resolution can be obtained at least for a composite signal. The method according to the invention extracts a constant part and a variable part from the data signal that is determined. The values that are extracted in this way are transmitted over the transmission link that is present. Depending on the bandwidth of the composite signal, various methods can be applied for transmitting relatively low bandwidth signals with a possibly higher resolution. 
   In a first embodiment, the constant part is, for example, divided by a predefined value N and only this fraction is transmitted. The constant part can be reconstructed by N-fold transmission of the fraction. In one development errors that arise as a result of the division are compensated in that the error is minimized by a control loop. This embodiment can be used in both analog and digital systems. 
   In a second embodiment, a digitized value of the constant part is transmitted bit by bit. According to the invention, just a single bit is transmitted per transmitted data word, and the rest of the data word can be used for the variable value. 
   In accordance with an added mode of the invention, there are the steps of dividing the constant part by a predefinable value N resulting in N divided values, and summing each of the divided values with the variable part forming the result signal to be transmitted. 
   In accordance with an additional mode of the invention, there is the step of determining the constant part by a low-pass filtering of the signal that is to be transmitted. 
   In accordance with another mode of the invention, there is the step of applying a correction factor to the constant part. 
   In accordance with a further mode of the invention, there is the step of forming the correction factor by summing N values to be transmitted, minus the constant part. 
   In accordance with an added additional mode of the invention, there is the step of transmitting the constant part as a digital word, the constant part being divided into M identical word parts where M≧2, and in each case a word part of the constant part is transmitted in combination with the variable part so that after in each case M transmissions a value of the constant part is transmitted. 
   In accordance with a further additional mode of the invention, there is the step of adding the N divided values transmitted to the receiver in the receiver to determine the constant part. The constant part is divided by N in the receiver resulting in a further divided value and the further divided value is subtracted from the result signal received in order to obtain the variable value. 
   In accordance with an additional further mode of the invention, there is the step of forming the variable part by subtracting the constant part from the result signal transmitted. 
   With the foregoing and other objects in view there is provided, in accordance with the invention, a configuration for transmitting data in a motor vehicle. The configuration includes a transmitter having extraction means for extracting a constant part from a signal to be transmitted formed of a constant part and a variable part, a subtracting element having an output for subtracting the constant part from the signal to be transmitted and the subtracting element is connected to the extraction means, a divider having an output is connected downstream of the extraction means and divides the constant part by N, and an adding element connected to the output of the divider and to the output of the subtracting element. The adding element has an output where a data-reduced signal for transmission can be tapped. A receiver is provided for communicating with the transmitter. 
   In accordance with an added feature of the invention, the receiver includes a summing element having an output where the constant part of the data-reduced signal which is to be transmitted can be tapped, a divider connected downstream of the summing element and divides the constant part by N, and a subtracting element receiving the transmitted data-reduced signal and an output signal of the divider and having an output where the variable signal can be tapped. The subtracting element is connected to the divider. 
   In accordance with an additional feature of the invention, the transmitter has means for correcting the constant part. The means for correcting the constant part includes a summing element for summing N data-reduced signal values and outputting an output signal, a further subtracting element connected to the summing element and to which the output signal of the summing element and the constant part are fed, the further subtracting element outputting an output signal, and a further adding element connected to the further subtracting element and to the extraction means. The constant part and the output signal of the subtracting element are fed to the further adding element and the further adding element having an output connected to the divider. 
   In accordance with another feature of the invention, the extraction means includes a low-pass filter. 
   In accordance with a concomitant feature of the invention, the transmitter and the receiver form part of a microprocessor. 
   Other features which are considered as characteristic for the invention are set forth in the appended claims. 
   Although the invention is illustrated and described herein as embodied in a method and a configuration for transmitting data in a motor vehicle, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims. 
   The construction and method of operation of the invention, however, together with additional objects and advantages thereof will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a block circuit diagram of a first embodiment of a transmitter and receiving unit according to the invention; 
       FIG. 2  is a partial block circuit diagram of a second embodiment of the transmitting unit; 
       FIG. 3  is a block circuit diagram of a third embodiment of the transmitting and receiving unit; and 
       FIGS. 4 and 5  are flowcharts in accordance with the embodiment illustrated in FIG.  3 . 
   

   DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   In all the figures of the drawing, sub-features and integral parts that correspond to one another bear the same reference symbol in each case. Referring now to the figures of the drawing in detail and first, particularly, to  FIG. 1  thereof, there is shown a transmitter  100 , for example a satellite of an airbag system, and a receiver unit  200 , for example what is referred to as an electronic control unit (EMU) of an airbag system. Reference symbol  110  designates a sensor unit that supplies a pressure signal. The internal pressure is determined, for example, in a vehicle door by the pressure sensor  110 . Given an impact against the vehicle door, the internal pressure increases suddenly, which is reflected in the pressure signal. The unit  110  can also contain an analog/digital converter that converts the pressure signal determined in analog terms into a digital signal with a resolution of, for example, 15 bits. As a result, a resolution of 0.1 mbar can be achieved. However, the system can also be of completely analog construction. The frequency range of such a pressure signal usually lies in the range from 0 to 400 Hz. The digitized or analog signal is fed to a low-pass filter  140  and to a first input of a subtracting element  120 . An output of the low-pass filter  140  is connected to a second input of the subtracting element  120 . Furthermore, the output of the low-pass filter  140  is wired to an input of a divider  150 . The divider  150  divides the fed-in value by N. N can assume a value of, for example, 1024. The value that is divided in this way is fed to a first input of an adding element  130  whose second input is connected to an output of the subtracting element  120 . The output of the adding element  130  is connected to a transmission link  300 . At the receiver end, the transmission link  300  is wired to an input of a summing element  210  and to a first input of a subtracting element  230 . An output of the summing element  210  supplies a constant part p 0  at a terminal  250  after N summations. The output of the summing element  210  is also connected to an input of a further divider  220 . The divider  220  also divides the received value by N. An output of the divider  220  is connected to a second input of the subtracting element  230 . The variable value Δp of the pressure signal can be tapped at an output of the subtracting element  230  via a terminal  240 . 
   The ambient pressure or constant part p 0  is filtered out, by the low-pass filter  140 , from the pressure signal p 0 +Δp which is determined. The low-pass filter  140  can have a cutoff frequency of, for example, 0.3 Hz. The variable value Δp is determined by the subtracting element  120 . In doing this, the subtracting element  120  subtracts the constant part p 0  determined by the low-pass filter  140  from the digitized pressure signal. The constant part p 0  which is determined in this way at the output of the low-pass filter  140  is then divided by the divider  150 , for example by the value N=1024. The component value of p 0  that is generated in this way is added to the variable value Δp by the adding element  130  and transmitted over the transmission link  300 . 
   The component value p 0  that is generated in this way can be used in each of the following N transmissions as a summand, combined in each case with the current variable value Δp. Only after the N transmissions have taken place in this way is the current constant part, which is an element determined by the filter method described above, divided by the value N. Before the component value generated in this way is again the summand with respect to the variable value in the next N transmission steps. Alternatively, the constant part p 0  that is currently present at the output of low-pass filter  140  is always divided by the value N, and the current component value of p 0  that is determined in this way is always transmitted. In this way, the component value can vary with respect to each transmission, and provided that the constant part changes between two sampling times, different component values are then also summed in the receiver  200 . However, it can usually be assumed that in described application examples of the invention, fluctuations in the constant part occur only with a very large time constant so that as a result transmitted current component values do not differ significantly in their amplitude from the transmission of a component value in accordance with the aforementioned first alternative in which the component value of a previous sampling time is transmitted over N transmission cycles. 
   Both signals are reconstructed at the receive end. This is carried out by the summation of N=1024 successive values in the present case, by the summing element  210 . The output value that is present at the summing element  210  is in turn divided by N by the divider  220  and subtracted from the received value by the subtracting element  230 . As a result, the variable value Δp is available at the output of the subtracting element  230 , and the constant part p 0  is available at the output of the summing element  210 . 
   The summing element  210  sums both the component values of p 0  and the variable values Δp that are transmitted over the transmission link  300 . The variable values Δp usually fluctuate around the constant part p 0  as a result of which the errors which are caused by this for large values of N are averaged out. In order nevertheless to prevent small errors being amplified by the summation,  FIG. 2  shows a further embodiment of the transmitter  100  with an integrated error correction. For this purpose, the output signal of the low-pass filter  140  is fed to a first input of a further subtracting element  170 . An adding element  160  is inserted between the low-pass filter  140  and the divider  150 , a first input of the adding element  160  being connected to the output of the low-pass filter  140  and its output being connected to the input of the divider  150 . The output of the subtracting element  170  is connected to a second input of the adding element  160 . The output of the first adding element  130  is connected to the input of a further summing element  180 , which operates in the same way as the first summing element  220  in the receiver unit  200 . The output of the summing element  180  is connected to the second input of the subtracting element  170 . 
   By the correction unit described, the same sum which the summing element  220  forms in the receiving unit  200  is formed in the transmitter unit  100  via the further summing element  180 . The deviations Δp 0  are formed by forming the difference between the signal and the output signal of the low-pass filter  140  by the subtracting element  170 . The deviations Δp 0  of the transmitted signal from the actual value p 0  are then fed in by the adding element  160  in such a way that a control loop is produced that corrects the errors of the sum. 
   The configuration according to the invention does not always transmit the complete constant part p 0  that is required with a significantly lower bandwidth, but rather, together with the pressure changes Δp, only a fraction that is integrated at the receiver end and can thus be reconstructed. As a result, the data set which is to be respectively transmitted can be drastically reduced without information loss and at the same time a large error tolerance achieved because the disturbance of individual information packets affects only a fraction of the constant part but not the entirety of the information which is distributed among a large number of data items. The method can be applied both for analog and digital data processing. 
     FIG. 3  shows a further embodiment of the present invention. Block  400  shows in turn a transmitter unit  400 , for example a pressure sensor  410  with associated evaluation and transmission unit within a motor vehicle door. The pressure sensor  410  has an integrated analog/digital converter which converts the pressure p 0 +Δp into a digital value. An output of the sensor  410  is connected to a first input of a subtracting element  420  and to an input of a low-pass filter  470 . An output of the low-pass filter  470  is wired to the second input of the subtracting element  420 . Furthermore, the output value at the low-pass filter  470  is written into a register  460 . An output of the subtracting element  420  is wired to a shift register  430  whose output is connected to a transmission register  440 . A bit selecting device  450  is provided which is connected to the register  460  and which generates an output signal which is connected to a bit place of the register  440 , for example the least significant bit (LSB). The transmission register  440  is connected to the transmission link  600 . 
   A reception register  510 , which is connected to the transmission link  600 , is provided in the receiver  500 , for example a processing unit for the airbag controller. The upper six places of the register  510  form the pressure variable value Δp, which can be tapped at the terminal  520 . The LSB of the register  510  is connected to a bit selecting device  530 . The bit selecting device  530  describes, as a function of a control signal, the individual bits of the register  540  which is connected to a terminal  550 . The constant part p 0  of the pressure value can then be tapped at the terminal  550 . 
   The method which can be carried out by the embodiment described in  FIG. 3  will now be explained in more detail in conjunction with FIG.  4  and FIG.  5 . The method starts in step  40 . First, in step  41  a synchronization value is transmitted over the transmission link  600 . To do this, it is possible, for example, to transmit a sequence of synchronizing values that can be detected in the receiver  500 . As the simplest example, it is possible, for example, to transmit a sequence “0101010”, “1010101” for synchronization. In step  42 , a counter is set to N=1 and the pressure constant part p 0  is determined by the low-pass filter  470 . In step  43 , the pressure variable value Δp is determined by the subtracting element  420 . In step  45 , the pressure variable value Δp is then combined with bit  1  of p 0 . To do this, the bit selecting unit  450  selects, for example, the first bit from the register  460 . In step  46 , the contents of the register  440  are transmitted, over the transmission link  600 , with the combination of the pressure variable value Δp and in place of the pressure constant value p 0 . In step  47  it is checked whether N is equal to the number M of places of the register  460 . If the register  460  is a 15 bit register, it is checked whether N is equal to 15. If not, N is increased by 1 in step  44  and the step sequence  43 ,  45 ,  46 ,  47  is processed until N=M. In other words each place of the register  460  is transmitted individually with the pressure variable value. If the value of N=M, the system jumps back to step  41  of the procedure in which the synchronization signal is transmitted again. At each new pass, the bit place  2 ,  3 ,  4 , . . . ,  15  of the register  460  is sampled and transmitted. 
   The method for receiving the pressure values starts in step  50  according to FIG.  5 . In step  51 , the system waits for the synchronization values transmitted by the transmitter unit  400 . As soon as these values have been received, the counter is set to N=1 in step  52 . In step  53 , the first value is received in the register  510 . In step  54 , the upper six bits of the register  510  are output to the output terminal  520  at which the pressure variable value Δp can be tapped. In step  56 , the LSB of register  520  is written into the N-th place of the register  540  via the bit selecting device  530 . In step  57  it is checked whether N=M. If not, in step  55 , N is incremented by one place and the steps  53 ,  54 ,  56 ,  57  are repeated until N=M. If N=M, register  540  has been completely written to and now contains the complete value of the pressure constant part p 0 . In step  58 , this value is output via the connecting terminal  550 . 
   The variable pressure value is transmitted with only six bits. However, each bit can have a resolution of 0.1 mbar. The maximum pressure variable value can be approximately ±3.2 mbar. If the fluctuation width of the pressure variable signal is greater, the resolution can be correspondingly reduced. In this embodiment, it is possible to transmit the pressure variable value with the maximum time resolution of approximately 2 kHz and the pressure constant part with approximately 120 Hz (when two synchronization values are used). 
   In a further embodiment, a resolution of 12 bits can also be provided for the pressure variable value. However, in this case, two transmissions per pressure variable value and 15 transmissions per pressure constant part value are necessary. However, the transmitter and receiver units must be expanded by an appropriate evaluation logic. However, for the synchronization it is possible to use the same method as described above. Because the bandwidth of the pressure variable signal is restricted to approximately 400 Hz, the division of the pressure variable value into two component values with a transmission bandwidth of 2 kHz is still not a problem. 
   The method according to the invention can be expanded as desired within the scope of the partial transmission. It is essential that the constant part that has a very much lower bandwidth (for example 0.3 Hz) is transmitted by use of divided transmission.