Electricity: measuring and testing – Fault detecting in electric circuits and of electric components – In an ignitor or detonator
Reexamination Certificate
1999-03-19
2001-01-09
Metjahic, Safet (Department: 2858)
Electricity: measuring and testing
Fault detecting in electric circuits and of electric components
In an ignitor or detonator
C324S603000, C324S550000, C340S638000
Reexamination Certificate
active
06172507
ABSTRACT:
BACKGROUND OF THE INVENTION
Field of the Invention
The invention relates to the field of electronic circuits, and specifically to a circuit configuration for measuring the resistance and/or leakage between two electrical nodes in an airbag control device.
Measurements of resistance and leakage resistance are frequently carried out by direct-current measurements. Since it is necessary to ensure an adequate signal
oise ratio, measurements are taken at high signal levels. This results in undesirably high electromagnetic radiation. Further, the required generators having a direct-current component and the required amplifiers are very expensive.
European Patent application EP A 0 689 058 discloses a circuit for monitoring an electrical circuit node for a fault condition in the form of a leakage resistance. The disclosed test configuration has a current source that is coupled to the node and an analyzer that has at least one comparator. A test current in the form of a direct current having a predetermined value is applied to one terminal of the circuit and only flows when the voltage between the node and the terminal drops below a predetermined limit value. Transgression of this voltage limit value by a leakage current leads to a change in the voltage at the comparator. A window comparator is preferably used to monitor the node for a leakage resistance with respect to each of two terminals of the circuit and a second current source having an opposite test current is provided.
German Patent Application DE-A 195 17 141 discloses a circuit for monitoring a priming-cap resistance of an airbag control device. A test current is alternately fed into the priming-cap resistance from mutually opposite directions. Each voltage value measured across the resistance is differentially amplified together with a positive reference voltage. A difference between the amplified voltages is calculated which is equal to a voltage value which would be caused by a test current having twice the level.
SUMMARY OF THE INVENTION
It is accordingly an object of the invention to provide a circuit for measuring resistance and leakage at low signal levels that can be constructed in a simple and inexpensive manner.
With the foregoing and other objects in view there is provided, in accordance with the invention, a circuit for measuring resistance between at least two circuit nodes in an airbag control device, comprising:
a current source for feeding a test current into one of two circuit nodes, the current source having a control input;
a circuit element connected between the two circuit nodes having an output dependent upon a voltage between the two circuit nodes resulting from the test current;
a broadband signal generator having an output electrically connected to the control input of the current source; and
a correlator having a first input electrically connected to the output of the circuit element and a second input electrically connected to the output of the broadband signal generator, the correlator providing an output signal dependent upon a correlation between the output of the circuit element and the output of the broadband signal generator.
In accordance with an added feature of the invention, the circuit element includes a voltage amplifier.
In accordance with an additional feature of the invention, the voltage amplifier includes an operational amplifier.
In accordance with another feature of the invention, the broadband signal generator includes a digital signal generator.
In accordance with a further added feature of the invention, the signal generator includes a feedback-type register having a register length and a clock frequency.
In accordance with a further additional feature of the invention, the circuit for measuring resistance between at least two circuit nodes in an airbag control device includes:
a further current source for feeding a test current into at least one additional circuit node, the further current source having a control input;
a further circuit element connected between one of the two circuit nodes and the additional circuit node, and having an output dependent upon a voltage between one of the two circuit nodes and the additional circuit node resulting from the test current; and
a further correlator having a first input electrically connected to the output of the circuit element, and a second input electrically connected to the output of the broadband signal generator, the correlator providing an output signal dependent upon a correlation between the output of the circuit element and the output of the broadband signal generator.
In accordance with yet an added feature of the invention, the signal generator is constructed to simultaneously provide uncorrelated signals to respective control inputs of the current source and the further current source and to respective second inputs of the correlator and the further correlator.
With the foregoing and other objects in view there is provided, in accordance with the invention, a circuit for obtaining a two-point measurement, comprising:
a multiplier having an output for obtaining a two-point measurement, the multiplier having a first input, and a second input provided with a signal representing a scaling factor; and
a signal generator having an output connected to the first input of the multiplier.
The inventive circuit provides a broadband signal generator that impresses an alternating-current signal into the current source. This dispenses with elaborate generators and amplifiers having a direct-current component and therefore, reduces circuit costs. In addition, measurement using a broadband signal prevents high electromagnetic radiation and provides low electromagnetic sensitivity.
It is possible to use an analog signal generator, for example a random-number generator with analog values and a digital/analog converter. Preferably, however, a digital signal generator is provided that has less circuit complexity and, for example, outputs +1 and −1 signal levels. One possible implementation is a feedback-type register having a length n and clock frequency F. With the aforementioned register, the bandwidth of the signal can be between F and F/(2n−1).
The clocked output signal of the signal generator is applied to the current source which corresponding outputs alternating current signals. The voltage drop generated by these alternating current signals is detected by a voltage amplifier connected between the two nodes and is supplied to a correlator.
The principle of the circuit configuration according to the invention is as follows. In the ideal case, where no leakage is present and no foreign signals are present in the circuit section investigated, the pure undamped test signal with the impressed generator signal is obtained at the amplifier. The correlator receives the output of the amplifier and an unaltered signal from the generator. Thus, a 100% correspondence is obtained between the signal measured by the amplifier and the unaltered generator signal applied to the correlator, and a signal having a maximum level at the output of the correlator is provided.
The correlator examines the time-shifted signal values and suppresses uncorrelated signal values. Foreign signals such as, for example, statistically independent noise signals are removed and only the signal component coming from the current source signal is taken into consideration. If pure interference signals are input into the correlator, this results in a correspondence of 0% with the generator signal and a signal having a minimum level at the output of the correlator is provided.
If there is a leakage and no interference signals, an output signal having a high level is obtained since there is a correlation with the current source signal. In the case of leakage and simultaneous interference signals, however, a signal with a low level is obtained at the output of the correlator.
The correlator, for example, can be constructed with a multiplier followed by a low-pass filter. The two input signals, the correlation of which is to be tested, are multiplied by the m
Greenberg Laurence A.
Lerner Herbert L.
Metjahic Safet
Siemens Aktiengesellschaft
Solis Jose
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