Abstract:
A firing device is described and has an electronic unit which is connected upstream of a firing cap and generates a constant firing voltage. A bypass line is provided for bypassing the electronic unit. A test switching device can be actuated by a test signal and, when a test signal is present, connects a line, which conducts a supply voltage, to the bypass line. In this way, a Brouston test can be carried out.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application is a continuation of International Application PCT/DE00/03622, filed Oct. 13, 2000, which designated the United States. 
    
    
     BACKGROUND OF THE INVENTION 
     Field of the Invention 
     The invention relates to a firing device for a pyrotechnic vehicle-occupant protection device, in particular for motor vehicles. The firing device contains a terminal for an external supply line, an electronic unit with a voltage input and a control input that are connected to the terminal, a voltage output, and a firing output. A firing component is connected to the voltage output and to a firing switch that is connected to the firing output. The electronic unit generates a predetermined firing voltage at its voltage output when a supply voltage is applied to its voltage input, and when a firing signal is applied to its control input, generates a firing pulse at its firing output. The firing pulse actuates the firing switch with the result that the firing component fires. 
     Conventional firing devices for pyrotechnic vehicle-occupant protection devices, for example firing caps for gas generators of airbags or belt pretensioning devices are subjected to what is referred to as a “Brouston” test for quality assurance. In the test, a number of firing caps are extracted from a batch. A first voltage is then applied to a first firing cap. If the firing cap does not yet fire at the first voltage, the voltage is increased until firing occurs. After the firing of the firing cap, the voltage at which the first firing cap has fired is lowered again somewhat on a second firing cap. If the second firing cap does not fire, the voltage is slightly increased again until the firing cap fires, and so on. As a result, a firing distribution over firing voltages is obtained. The advantage of the initial Brouston test is that relatively reliable statistical data can be acquired with relatively few triggered and fired firing caps. The reason for the reliability of this data is that the voltage always fluctuates around the firing voltage. 
     Conventional firing caps have two terminals or terminal pins between which a firing bridge, for example a wire fuse is located. The firing voltage is applied to the terminals. 
     In modern firing devices that are actuated by a firing bus, an electronic unit, which generates a well defined and stabilized voltage and permits energy to be supplied to the firing cap only if a coded firing signal is detected at the terminal is connected between the cable-harness-end terminal, or the cable-harness end terminal pins, and the firing cap. This ensures a high degree of operational reliability. However, the intermediately connected electronic unit makes the Brouston test impossible. In such modern firing caps and firing devices there are no accessible connecting points between the electronic unit and the firing bridge. 
     SUMMARY OF THE INVENTION 
     It is accordingly an object of the invention to provide a firing device for a pyrotechnic vehicle-occupant protection device which overcomes the above-mentioned disadvantages of the prior art devices of this general type, in which there is a large degree of flexibility with respect to flexibility. 
     With the foregoing and other objects in view there is provided, in accordance with the invention, a firing device for a pyrotechnic vehicle-occupant protection device. The firing device contains a terminal for an external supply line and an electronic unit having a voltage input coupled to the terminal, a control input connected to the terminal, a voltage output, a firing output, and a test output. A firing switch is connected to the firing output. A firing component is connected to the voltage output and to the firing switch. A test switching device is connected between the terminal and the voltage input. The test switching device is connected to the test output of the electronic unit. A bypass line is connected between the test switching device and the voltage output. The test switching device disconnects a connection between the terminal and the voltage input and connects the terminal to the bypass line when a test signal is applied to the control input of the electronic unit. The electronic unit generates a predetermined firing voltage available at the voltage output when a supply voltage is applied to the voltage input, and when a firing signal is applied to the control input, the electronic unit generates a firing pulse available at the firing output. The firing pulse actuates the firing switch with a result that the firing component fires. 
     By using the switching device provided according to the invention it is possible to bypass the electronic unit and to test the firing device directly in a conventional way by applying an appropriate supply voltage or test voltage. 
     In accordance with an added feature of the invention, a charging resistor is connected in the bypass line for limiting a current flowing through the bypass line to a value at which the firing component does not fire. 
     In accordance with a further feature of the invention, the test switching device contains a MOSFET. 
     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 firing device for a pyrotechnic vehicle-occupant protection device, 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 firing device according to the invention; and 
     FIG. 2 is a block circuit diagram of a test switching device. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring now to the figures of the drawing in detail and first, particularly, to FIG. 1 thereof, there is shown a terminal of a firing cap  2  that is connected to a voltage output  6  of an electronic unit  8  via a line  4 . A voltage input  10  of the electronic unit  8  is connected via a test switching device  12  to a terminal  14  to which an external supply line  16 , for example a firing bus of the vehicle electric system, is connected. The terminal  14  is also connected to a control input  18  of the electronic unit  8  that has a control section  19  with a voltage section  20 . 
     A bypass line  22  with a charging resistor  24  leads from the test switching device  12  to the voltage output  6  parallel to the electronic unit  8 . 
     The firing cap  2  is connected via a firing switch  26  to a further terminal  28  of the firing device, which further terminal  28  can be connected, for example, to ground and to a corresponding conductor of the supply line  16 . 
     A charging capacitor  30  is located parallel to the firing cap  2  and to the firing switch  26 . 
     The electronic unit  8  has two control outputs  32  and  34  that are connected to corresponding control inputs  32   1  and  34   1  of the test switching device  12  or of the firing switch  26 . 
     A housing  36  within which the entire firing device is accommodated is shown by dashed lines, only the terminals  14  and  28  being accessible from the outside. It goes without saying that the degree of circuit integration may vary, for example the switches  12  and  26  and the bypass line  22  may be integrated into the electronic unit  8 . 
     The individual components or elements are known per se in terms of their configuration and their function so that only their interaction is explained below. It is assumed that there is a voltage, for example an alternating voltage with constant frequency, on the supply line  16 . The test switching device  12  is in the position illustrated in which the terminal  14  is connected to the voltage section  20 . A predetermined, constant firing voltage V Z  to which the charging capacitor  30  is charged is generated in the voltage section  20  from the supply voltage V S  of the supply line  16 . 
     If a firing signal that is coded in a predetermined fashion is then supplied via the supply line  16 , the firing signal is detected in the control section  19  of the electronic unit  8  and the control section  19  generates, at the control output  34 , a firing pulse which switches the firing switch  26  to the conductive position. The charging capacitor  30  discharges through the firing cap  2  and the firing switch  26 , as a result of which the firing cap  2  fires. 
     If a predetermined, coded test signal is present on the supply line  16 , this is detected by the control section  19  of the electronic unit  8  and a test pulse which is fed to the control input  32   1  of the test switching device  12  is generated at the control output  32 . The test switching device  12  then switches over so that the voltage input  10  is disconnected from the terminal  14 , and the supply line  16  is connected to the bypass line  22 . In this way, the supply voltage V S  on the supply line  16  is fed directly to the charging capacitor  30  so that the voltage section  20  is deactivated and the voltage V Z  at the voltage output  6  is equal to the supply voltage V S . The charging resistor  24  ensures that the charging current is limited so that the firing cap  2  cannot fire even when there is a defect in the firing switch  26 . 
     If the charging capacitor  30  is charged in each case to the supply voltage V S , the firing switch  26  can, as described above, be switched to the conductive position by a firing signal of the supply line  16  and a firing pulse which subsequently appears at the control output  34 , with the result that, as in the case of the Brouson test described at the beginning, it is possible to test whether the firing cap  2  fires at the respective supply voltage. 
     The test switching device  12  can be formed by any suitable components or switching elements. FIG. 2 shows such an example. 
     The supply line  16  is connected via a first switch  38 , for example a MOSFET (Metal-Oxide Semiconductor Field Effect Transistor), to the charging resistor  24  and to the voltage section  20  via a second switch  40 , which is also embodied, for example, as a MOSFET. The control input  32   1  is connected directly to the switch  38  in order to actuate it, and to the switch  40  via an inverter  42  in order to actuate the switch  40 . Depending on the polarity of the voltage at the control input  32   1 , the switch  38  connects the supply line  16  to the charging resistor  24  and the switch  40  disconnects, or the switch  40  connects the supply line  16  to the voltage section  20  and the switch  38  disconnects. 
     Of course, the firing device described can be modified in various ways. For example, it is possible to provide, in addition to the firing switch  26 , a saving switch which must be activated in addition to the firing switch  26  in the sense of a logic AND switching operation so that the charging capacitor  30  can discharge through the firing cap  2 . The supply line  16  may have a different configuration, for example may contain one conductor that conducts the supply voltage and another conductor that conducts the control signals. In this case, the control input  18  is connected to the control section, and the input of the test switching device  12  is connected to the voltage section.