Abstract:
Digital data are transmitted from a measurement station to an air bag control unit in a motor vehicle. A direct signal of the air bag control unit which serves the purpose of supplying energy to the measurement station is current-modulated and thereby varied in accordance with the data to be transmitted. The variations in the current are controlled such that they cancel one another on average over time.

Description:
CROSS REFERENCE TO RELATED APPLICATION 
     This is a continuation of copending International Application PCT/DE98/01380, filed May 19, 1998, which designated the United States. 
    
    
     BACKGROUND OF THE INVENTION 
     Field of the Invention 
     The invention lies in the automotive field. More specifically, the present invention relates to a method and apparatus for transmitting digital data from a measurement station of an occupant protection system in a motor vehicle to an air bag control unit. The apparatus includes an air bag control unit, a measurement station connected to the air bag control unit, a direct current or direct voltage source in the air bag control unit for supplying the measurement station with a direct signal, first means in the measurement station for varying the direct signal in accordance with the data to be transmitted, and second means in the air bag control unit for detecting and evaluating changes in the direct signal as current changes. The measurement station is supplied with a direct signal from a direct current or direct voltage source of the air bag control unit and varies the direct signal in accordance with the data to be transmitted. The variations in the direct signal are then detected as current changes in the air bag control unit and evaluated. 
     For tripping circuits of occupant protection systems, which cause air bags or belt tensioners to be tripped in the event of an accident, for instance, signals must be transmitted from remotely located sensors (measurement stations) to a central control unit (air bag control unit). With increasing progress in the development of occupant protection systems, the associated sensors are being installed at more and more places in the vehicle. If the requisite short tripping times are to be adhered to, these remotely located sensors must be polled constantly, and the sensor signals must be transmitted reliably to the central control unit. 
     U.S. Pat. No. 5,712,784 to Fendt et al. (European patent application EP 0 693 401 A2) discloses a data transmission method in which pulse width modulated signals are generated by the affected remotely located sensors and are transmitted to the central control unit. The data information is encoded in the duty factor. 
     In a system for controlling an occupant protection device, such as an air bag, the entire system must be kept in operation for a minimum period after a collision in which the on-board electrical system and thus the energy supply to the system have been destroyed. To that end, in the air bag control unit, independent energy reserves, such as capacitors, are typically furnished for the individual circuit elements, so that this emergency operation can be assured for a minimum length of time. Such energy stores are not only expensive but also take up space. 
     SUMMARY OF THE INVENTION 
     It is accordingly an object of the invention to provide a method and an apparatus for a motor vehicle occupant protection system, which overcomes the above-mentioned disadvantages of the heretofore-known devices and methods of this general type and in which the energy reserves for emergency operation can be minimized. 
     With the foregoing and other objects in view there is provided, in accordance with the invention, a method of transmitting digital data from a measurement station of a motor vehicle occupant protection system to an air bag control unit, which comprises: 
     supplying a measurement station with a direct signal from a direct current or direct voltage source of an air bag control unit; 
     varying the direct signal with the measurement station in accordance with data to be transmitted; 
     transmitting data and thereby varying a current such that the variations cancel one another out on average over time; and 
     detecting and evaluating the variations in the direct signal as current changes in the air bag control unit. 
     According to the invention, the current consumption for data transmission from the measurement station to the air bag control unit is reduced. 
     In accordance with an added feature of the invention, the transmitting step comprises: 
     transmitting the digital data in a train of binary characters; 
     transmitting a logic zero by leaving the direct signal unvaried; 
     transmitting a logic one by selectively increasing and decreasing the direct signal by a predetermined amount, and thereby 
     increasing the direct signal if the direct signal had been decreased to transmit a preceding logic one; and 
     decreasing the direct signal if the direct signal had been increased to transmit the preceding logic one. 
     In accordance with an additional feature of the invention, an energy store is provided in the measurement station. The energy store is charged to increase the current and discharged to reduce the current. 
     In accordance with a preferred embodiment of the invention, the energy store is a capacitor. 
     In accordance with another feature of the invention, two switches are connected to the energy store. The switches control the charging and discharging of the energy store. The switches are thereby triggered in alternation upon successive logic one values of the data train. 
     In accordance with a further feature of the invention, a series resistor is connected at the air bag control unit in at least one supply line connected to the measurement station, and the method further comprises measuring a voltage drop at the series resistor for ascertaining the transmitted data. 
     With the above and other objects in view there is also provided, in accordance with the invention, an apparatus for transmitting digital data in a motor vehicle, specifically for performing the above-outline method. The apparatus comprises: 
     an air bag control unit and a measurement station connected to the air bag control unit; 
     a direct current or direct voltage source in the air bag control unit for supplying the measurement station with a direct signal; 
     a first device in the measurement station for varying the direct signal in accordance with data to be transmitted to the air bag control unit, the first device include an energy store to be charged in controlled fashion to increase the current and to be discharged in controlled fashion to decrease the current; and 
     a second device in the air bag control unit for detecting and evaluating changes in the direct signal in the form of current changes. 
     As noted above, the preferred energy store is a capacitor. 
     In accordance with again an added feature of the invention, the measurement station receives a supply voltage, the capacitor is connected in the measurement station such that, in an absence of data transmission, the capacitor is charged to an intermediate voltage between 0 V and the supply voltage, the first device includes a first switch and a second switch connected such that an actuation of the first switch causes the capacitor to increase the direct signal and an actuation of the second switch causes the capacitor to decrease the direct signal. 
     In accordance with again an additional feature of the invention, the measurement station includes a voltage regulator for deriving the supply voltage from the direct signal and a voltage divider subjected to the supply voltage, the voltage divider having a first resistor and a second resistor connected to one another at a node point acting as a center tap, the capacitor being connected between an input of the voltage regulator and the center tap of the voltage divider, the first switch being connected in parallel with the first resistor and the second switch being connected in parallel with the second resistor. 
     In accordance with again another feature of the invention, a common resistor connects the first and second switches to the center tap. 
     In accordance with again a further feature of the invention, the first device includes a trigger unit for triggering the first and second switches in accordance with the data train to be transmitted. 
     In accordance with a concomitant feature of the invention, the measurement station includes a sensor and a monitoring unit connected to the sensor, the monitoring unit generates the data trains to be transmitted in accordance with signals received from the sensor and forwards the data trains on to the trigger unit for transmission. 
     In the data transmission method known from the above-mentioned U.S. Pat. No. 5,712,784 (EP 0 693 401 A2), the sensor signal has two current level values, namely an H and an L level (also known as logic 1 and logic 0). The L level corresponds to the resting current of the sensor assembly. If the sensor signal is at the H level, then the current consumption is higher than at the L level. At a duty factor of 50%, the mean value of the transmitted sensor signal is then ½, if the H level is 1 and the L level is 0. The transmitted signal thus always has a direct current component. In resistors of the sensor assembly, this direct current component is consumed by being converted into thermal energy. On the other hand, in order to assure reliable signal transmission, the H level must be high enough. The high H level thus selected entails high current consumption on the part of the affected assembly for signal transmission. 
     As a result of the averaging, according to the invention, of the current changes, the direct current component required for the data transmission is zero. Thus the energy demand for the signal transmission is relatively slight, and accordingly the current supply to the external measurement stations in emergency operation, and thus also the energy reserves to be furnished, can be kept low. In particular, the energy furnished for the applicable measurement station then need not be furnished predominantly for the sake of communication with the air bag control unit. 
     Advantageously, signal transmission and the supply of electrical current are effected over a common line. 
     The terms air bag control unit and measurement station are used merely as examples. Thus the method of the invention can also be employed between two occupant protection control units of a motor vehicle, for instance. The measurement station may include a sensor for detecting a collision and can transmit sensor signals, or it may also be embodied as a firing station for an occupant protection means, which in the firing mode receives a trip command from the air bag control unit and thereupon heats up a firing element of a protection means. 
     The measurement station embodied as a firing station then transmits results of diagnostic measurements to the air bag control unit. 
     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 apparatus for transmitting digital data from a measurement station of an occupant protection system in a motor vehicle to an air bag control unit, 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 schematic block circuit diagram of an exemplary embodiment of an apparatus according to the invention; and 
     FIG. 2 is a timing diagram showing a) through f) pulse trains that occur in data transmission with the apparatus according to FIG.  1 . 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring now to the figures of the drawing in detail and first, particularly, to the block circuit diagram of FIG. 1, there is seen an apparatus according to the invention, in the form of an air bag system  10  for an air bag. The air bag system  10  includes an air bag control unit  12  and a measurement station  13 , which communicate with the supply lines  14  and  15  via a current interface  16 . Inside the measurement station  13 , a sensor  25  for collision detection for a non-illustrated, associated air bag is provided. The sensor is supplied with current from a direct current source, in the present case a car battery  11 , via the current interface  16 . The sensor  25  is connected to a monitoring unit  24 , such as a microprocessor provided with an A/D converter. The monitoring unit receives (analog) signals from the sensor  25 , converts them into a train of digital (binary) data, and transmits them to the air bag control unit  12  at predetermined time intervals. 
     The transmission of the data is effected directly via the current interface  16 , in that the direct current flowing through the current interface is modulated in accordance with the data train. In a series resistor R 4  inserted into the supply line  14 , the current modulation is detected as a voltage drop by two comparators CO 1  and CO 2  and sent on to a decoding unit  21 . The decoded data signal is then delivered to an evaluation unit  20 , where it is evaluated. The modulation of the current I by the resistor R 4  is effected such that the current changes caused by the modulation cancel one another out on average over time; that is, the direct current component resulting from the data transmission is zero. This is preferably attained by providing that in the manner of an AMI (alternate-mark inversion) encoding, every other 1 value, in a binary data train comprising 0 and 1 values that is to be transmitted and of the kind shown for instance in FIG. 2 a,  is converted into a −1 value as shown in FIG. 2 d ). Since on average over time, just as many positive (i.e., 1) as negative (−1) values always occur, the result—if the 1 value corresponds to a current increase and the −1 value corresponds to an equally great current decrease or current reversal—is a vanishingly small direct current value. 
     The current modulation, corresponding to the AMI encoding, in the current interface  16  is attained as follows, taking the circuitry of FIG. 1 as an example: In the measurement station  13 , a supply voltage Vcc, preferably 5 V, that is required for the sensor  25  and the other electronics is derived from the battery voltage U of the battery  11  by means of a voltage regulator  22  in series with it and is made available between the ground line  19  and the supply line  18 . For the current modulation, an energy store is provided, which for increasing the current is charged from the current interface  16  and for lowering the current is discharged into the current interface  16 . The energy store includes a capacitor C (e.g. 1 μF), for example, which is connected between the input of the voltage regulator and the center tap (node point K 1 ) of a voltage divider formed of two resistors R 1  and R 3 . The voltage divider is connected between the lines  18  and  19 . The two resistors R 1  and R 3  have the same resistance (e.g. 10 kΩ), so that the node point K 1  lies at a voltage of Vcc/2, or for instance 2.5 V. 
     For the current modulation, by means of two switches SW 1  and SW 2 , the node point K 1  can now be connected selectively, via a further, markedly smaller resistor R 2  (e.g. 100 Ω), either to the potential of the supply line  18  (Vcc), by closing the switch SW 1 , or to the potential of the ground line  19 , by closing the switch SW 2 . In the process, the potential at the node point K 1  shifts by ±2.5 V (Vcc/2). The potential at the node point K 4  on the other side of the capacitor C shifts accordingly. If the potential at K 4  is shifted upward by 2.5 V by closing the switch SW 1 , then the capacitor C, which is charged in the state of repose, discharges into the current interface  16 , and the current I through the resistor R 4  (e.g. 10 Ω) decreases or is reversed. If the potential at K 4  is lowered by 2.5 V by closing the switch SW 2 , then the capacitor C charges still further from the current interface  16 , and an additional charging current flows through the resistor R 4 . 
     In order now according to the invention to be able to transmit the exemplary data train of FIG. 2 a ) with a negligibly small direct current component via the current interface  16 , the binary signal train of FIG. 2 a ) is split into two partial trains shown in FIGS. 2 b ) and  2   c ). One partial train contains every other 1 signal, and the other partial train contains the remaining 1 signals, or in other words the ones in between. This splitting up of the signal train is done, in the circuit of FIG. 1, in a trigger unit  23 , which may also be part of the afore-mentioned microprocessor. The trigger unit  23  has two outputs, at each of which one of the two partial trains from FIG. 2, b) and c) appears and is sent onward to one of the switches SW 1  and SW 2  for triggering. If a 1 value appears in the partial train at the upper output, then the switch SW 1  is closed. If a 1 value appears in the partial train at the lower output, then the switch SW 2  is closed. Conversely, if a 0 value appears, then both switches remain open. 
     As a result of the alternating closure of the switches SW 1  and SW 2  upon successive 1 values, the aforementioned shift in potential occurs at the node point K 1 , and the result is a voltage course as shown in the timing graph e) of FIG.  2 . In accordance with this voltage course, the current I in the current interface  16  or through the resistor R 4  fluctuates about the resting current I 0 , as shown on line f) of FIG.  2 . 
     At the node points K 2  and K 3  at the series resistor R 4 , these current changes are converted into corresponding changes in the voltage drop. 
     The positive changes are detected by the first comparator CO 1 , and the negative changes are detected by the second comparator CO 2 . At the outputs of the comparators CO 1  and CO 2 , the partial signal trains b) and c) of FIG. 2 are then available again; in the following decoder or decoding unit  21 , they are reassembled into the original data signal a) and sent on to the evaluation unit. 
     The invention is preferably employed in arrangements for controlling an occupant protection system, since for this safety- critical application, reserve energy stores must be furnished for proper operation of the arrangement even after its disconnection from the on-board electrical system. In particular, the current interface of the invention is useful in data transmission between a remote collision sensor and a centrally disposed evaluation unit, but also between such an evaluation unit and a remote firing station, which is disposed in or at the occupant protection means and to which the evaluation unit sends an encoded command for firing the occupant protection means and causes the imposition of energy on the firing element of the passenger protection device.