Electricity: measuring and testing – Fault detecting in electric circuits and of electric components – For fault location
Reexamination Certificate
2001-03-09
2002-12-24
Oda, Christine (Department: 2858)
Electricity: measuring and testing
Fault detecting in electric circuits and of electric components
For fault location
C324S502000, C280S735000, C180S282000
Reexamination Certificate
active
06498494
ABSTRACT:
BACKGROUND OF THE INVENTION
Field of the Invention
The invention relates to a diagnostic circuit for measuring the resistance and the leakage current of at least one electrical load, specifically a firing cap of a motor vehicle restraint system. With such electrical loads in the form of firing caps, there is frequently a requirement to measure the resistance and the leakage current for diagnostic purposes in order to ensure correct functional capability of the firing caps. This diagnosis can be carried out at regular or irregular intervals, for example when the motor vehicle is started. If it is determined during the diagnosis that the firing cap resistance lies outside the permissible tolerance range and/or an unacceptable leakage current occurs, a fault message can be promptly output and/or the driving parameters (driver voltage, driver current, driving time) for driving the firing cap can be adapted if required.
During the diagnostic measurement, the firing cap resistance and the firing cap leakage current are usually measured one after the other. However, if a multiplicity of firing caps are provided, which is frequently the case in modern motor vehicle restraint systems, a long period of time is required to test all of the firing caps.
German Patent DE 196 38 393 C1 discloses a diagnostic circuit in which the firing cap that is to be tested is supplied by a power source which is driven in broadband fashion. The voltage drop across the firing cap is applied, using a switching element, to a correlator that is also supplied in broadband fashion. In order to measure leaks, further separate power sources may be provided for measuring leakage currents to ground or battery voltage.
U.S. Pat. No. 5,640,095 describes a circuit for testing the leakage resistance of a switching point, in which a test current is fed into the switching point and the voltage occurring at the switching point is detected. In order to perform a bipolar leakage resistance measurement, two measuring currents in opposite directions can be fed into the switching point.
Published European Patent Application EP-A 0 486 114 discloses an apparatus for testing an electrical circuit with respect to the leakage current and the continuity. The circuit is part of a bridge circuit whose branches are supplied with identical currents. To detect the leakage current, the bridge voltage is evaluated.
SUMMARY OF THE INVENTION
It is accordingly an object of the invention to provide a diagnostic circuit for measuring the resistance and the leakage current of at least one firing cap which overcomes the above-mentioned disadvantageous of the prior art apparatus of this general type. In particular, it is an object to provide a diagnostic circuit that enables a precise diagnosis to be carried out quickly.
With the foregoing and other objects in view there is provided, in accordance with the invention, a diagnostic circuit for measuring resistance and leakage current for at least one firing cap of a motor vehicle occupant protection system. The diagnostic circuit includes: a terminal for receiving a reference potential; at least one load having two terminals, one of the two terminals of the at least one load connected to the terminal for receiving a reference potential; an activation circuit for feeding a current to the at least one load during a diagnostic measurement; a resistance measuring circuit for determining a resistance of the at least one load; a leakage current measuring circuit for determining a leakage current that may be flowing during the diagnostic measurement, the leakage current measuring circuit configured to measure a difference between the current fed to the at least one load by the activation circuit and a current flowing from the at least one load to the terminal for receiving a reference potential; and a plurality of electrical power supplies connected to the leakage current measuring circuit and to the two terminals of the at least one load.
In the diagnostic circuit, the electrical resistance of the at least one load, in particular of the firing cap, and the leakage current are measured by detection circuits which are preferably at least partially embodied separately. These measurements can be carried out simultaneously. The time interval which is required overall for measuring the resistance and leakage current is thus extremely short. In particular, if a multiplicity of loads are to be tested, for example, if more than 10 firing cap circuits, are present, the overall diagnostic time interval required can be shortened drastically. During the simultaneous determination of leakage currents, the resistance measurement result based on a current measurement can additionally be corrected immediately taking into account the leakage current, with the result that the measuring accuracy is increased.
The leakage current measurement can be determined easily by forming a difference between the current supplied on the input side and the current measured on the output side. This differential current is preferably detected by a resistive voltage divider which generates a voltage representing the leakage current at its tap. Here, two electrical power supplies which are connected to one another and which simulate the current supplied to the firing cap or flowing out of the firing cap are provided. A connection point between these electrical power supplies can be connected to the leakage current measuring circuit, preferably to the resistance voltage divider. The resistive voltage divider ensures that the measuring current occurring at the connection point lies between the reference voltages applied to the resistive voltage divider and increases or decreases depending on the direction of the leakage current. Instead of a resistive voltage divider, it is, however, also possible to provide a single resistor which is connected to ground, for example. In this case, the leakage current measuring circuit changes it's polarity depending on the direction of the leakage current.
In accordance with an added feature of the invention, a voltage regulator that regulates the voltage occurring at a load terminal to a specific value is connected to one of the electrical power supplies. In this way, the current and voltage conditions at the load are permanently predefined and the electrical power supply connected to this terminal is controlled, in contrast to the electrical power supply connected to the other load terminal, by an impressed voltage and not by an impressed current.
In accordance with an additional feature of the invention, the voltage is variable, with the result that, for example, it is possible to repeat leakage current measurements with different potential values. In this way, leakage resistances to potential values between ground and the supply voltage are reliably detected.
In accordance with an another feature of the invention, the diagnostic circuit is used in vehicle occupant protection systems for motor vehicles, in particular airbag systems. The diagnostic circuit can be integrated directly into the control unit of the restraint system. This permits rapid diagnosis with short transient recovery times.
In accordance with a further feature of the invention, an electrical power supply (current mirror circuit) is provided which exactly simulates or which provides a fixed proportion of the current flowing through the firing cap during the diagnosis. The current is preferably impressed by a power source, and conducts it to the resistance measuring circuit and leakage current measuring circuit via separate terminals. This permits current measurement without affecting and falsifying the current which is actually flowing through the firing cap. Reciprocally, undesired effects occurring during the measurement of the resistance and leakage current can also be prevented by the electrical power supply terminals.
In accordance with a further added feature of the invention, the currents supplied to the resistance measuring circuit and the leakage current measuring circuit correspond to just a fraction, for example a tenth,
Belau Horst
Hermann Stefan
Swart Marten
Greenberg Laurence A.
Mayback Gregory L.
Oda Christine
Siemens Aktiengesellschaft
Stemer Werner H.
LandOfFree
DIAGNOSTIC CIRCUIT FOR MEASURING THE RESISTANCE AND THE... does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with DIAGNOSTIC CIRCUIT FOR MEASURING THE RESISTANCE AND THE..., we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and DIAGNOSTIC CIRCUIT FOR MEASURING THE RESISTANCE AND THE... will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-2951525