Communications: electrical – Condition responsive indicating system – Specific condition
Patent
1998-02-18
2000-01-18
Hofsass, Jeffery A.
Communications: electrical
Condition responsive indicating system
Specific condition
340635, 340438, 307 101, 180271, G08B 2100
Patent
active
060161043
DESCRIPTION:
BRIEF SUMMARY
FIELD OF THE INVENTION
The present invention relates to an arrangement for monitoring the resistance of a load connected to the secondary coil of a transformer. In particular, the present invention relates to monitoring the resistance of an airbag squib (firing pellet).
BACKGROUND INFORMATION
A conventional arrangement is described in German Patent 38 12 633 A1 in which a rotary transformer is used, for example, for transmitting signals between a controller and an airbag installed in the steering wheel of a vehicle. The operability of an airbag must be constantly monitored to ensure that it is ready for use in any emergency situation, so the resistance of the squib, which should typically be between 1.8 Ohm to 2.5 Ohm, is subjected to periodic measurement. In the conventional arrangement, the signal transmission between the controller and the airbag using inductively coupled coils of the rotary transformer depends on the distance between the two coils, i.e., the size of the air gap between the pot-type cores of the coils. Consequently, assembly and manufacturing tolerances of the steering wheel, in which the secondary pot-type core of the rotary transformer is installed, may result in distortions of the measured resistance of the squib. In order to avoid such distortions, the aforementioned background art provides an oscillator circuit on the secondary side of the rotary transformer, which is excited by a control signal from the primary side. After the control signal is turned off, the decay of the response signal of the oscillator circuit is retransmitted to the primary side and the resistance on the secondary side is calculated from the decaying response signal.
SUMMARY OF THE INVENTION
According to the present invention, the resistance of a load preferably an airbag squib connected to the secondary coil of a transformer is monitored for deviation from a given value by measuring exclusively the inductance of the transformer on the primary side. This procedure relies on the fact that the inductance measured on the primary side only depends on the resistance of the load connected on the secondary side. Therefore, if the measured inductance is found to deviate from a certain value, it can be deduced that the load resistance has changed. A shift in inductance may also be used to generate a signal indicating an undesired change in the load.
Useful embodiments of the invention are presented in the subclaims. In one embodiment, a measuring resistor that is much greater than the primary side input resistance of the transformer is installed in the primary circuit of the transformer. The primary side of the transformer is supplied with a pulsed control voltage, and the rise and/or decay time constant of a voltage drop across the measuring resistance is determined. Due to the considerable discrepancy in size between the resistance of the measuring resistor and the load resistance, the time constant is almost exclusively dependent on the primary side inductance of the transformer, thus establishing a direct relationship with the load resistance. Additionally, the transformer input resistance, which has no such direct dependence on the load resistance, has no effect on the measured time constant. Therefore, the time constant provides definite and reliable unambiguous information on whether the load resistance has changed due to a malfunction. It is preferable to select the pulse frequency of the pulsed control voltage so that the inductance of the transformer measured on the primary side has as little dependence as possible on the change in the coupling between the primary and secondary coils of the transformer when the load resistance operates properly.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 shows a transformer with a load connected thereto.
FIG. 2 shows an equivalent circuit of a transformer on the primary side with devices connected thereto for determining the inductance of the transformer.
FIG. 3 shows the voltage variation measured on the primary side of the transformer.
FIG. 4 shows the inductance o
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Dobler Klaus
Dukart Anton
Herrmann Thomas
Zabler Erich
Hofsass Jeffery A.
Huang Sihong
Robert & Bosch GmbH
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