Electricity: measuring and testing – Fault detecting in electric circuits and of electric components – In an ignitor or detonator
Patent
1998-10-13
2000-10-17
Metjahic, Safet
Electricity: measuring and testing
Fault detecting in electric circuits and of electric components
In an ignitor or detonator
324509, 324547, 324705, G01R 2726
Patent
active
061337412
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND INFORMATION
The present invention relates to a method for measuring the resistance of a load--preferably the firing pellet of an airbag--connected on the secondary side of a rotary transformer, by analyzing an electrical variable that can be sensed on the primary side of the transformer.
An arrangement is known from German Published Patent Application No. 38 12 633 A1 in which a rotary transformer is used, for example, for signal transmission between a control unit and an airbag built into a steering wheel of a vehicle. In order to continuously check the operability of an airbag so that it is immediately ready to operate in any emergency situation, the resistance of the firing pellet, which should lie in the range of, for example, 1.8.OMEGA. to 2.5.OMEGA., is subjected to a continuously repeating measurement. Transmission of signals by means of the inductively coupled windings of the rotary transformer depends on the spacing between the two windings, i.e. the size of the air gap between the dished cores of the windings. Installation and production tolerances of the steering wheel on which the secondary-side dished core of the rotary transformer is installed can lead to falsification of the measured resistance of the firing pellet. To prevent such measured-signal falsifications, the aforesaid existing art provides for an oscillator circuit on the secondary side of the rotary transformer that is excited from the primary side by a control signal. After the control signal is shut off, the decaying response signal of the oscillator circuit is transmitted back to the primary side, and the secondary-side resistance is determined from the time constant of the decaying response signal. With this measurement method, the resistance measured on the secondary side depends greatly on the radial and axial clearance between the primary and secondary windings of the rotary transformer. The accuracy with which the firing pellet resistance is measured is therefore not very high.
SUMMARY OF THE INVENTION
It is therefore an object of the present invention to provide a method according to which a load connected on the secondary side of a rotary transformer can be measured with the highest possible accuracy.
The aforesaid object is achieved, in that initially, a capacitor inserted into the primary circuit of the transformer is charged. In a first discharge operation, the capacitor is partially discharged for a predefined time period through the transformer, and then the residual charge voltage of the capacitor is determined. The capacitor is then charged again and subsequently, in a second discharge operation, partially discharged through the transformer with a pulse train, in steps. The residual charge voltage of the capacitor is then determined again. Lastly, the difference between the two residual charge voltages after the first and the second discharge operations is obtained; this provides an indication of the resistance of the firing pellet. Obtaining the difference between the two residual charge voltages mutually compensates for temperature influences and for influences of the radial and axial clearance between the two windings of the rotary transformer, so that the voltage difference depends, to the greatest extent possible, only on the actual resistance of the firing pellet.
The pulse train frequency at which the capacitor is discharged corresponds approximately to the resonant frequency of the resonant circuit which is made up substantially of the inductivity of the transformer and the capacitance of the capacitor.
The voltage occurring at the capacitor after it has been completely charged can be compared to a setpoint; a deviation between the measured voltage and the setpoint indicates a short-circuit at the primary-side terminals of the transformer, or an insulation fault in the capacitor.
A measured value for the capacitance of the capacitor can be derived from a charge voltage reached after a predefined capacitor charging time.
In an arrangement for carrying out the method according to the present in
REFERENCES:
patent: 4810950 (1989-03-01), Young
patent: 5278512 (1994-01-01), Goldstein
Henne Ralf
Mattes Bernhard
Schumacher Hartmut
Metjahic Safet
Nguyen Vincent Q.
Robert & Bosch GmbH
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