Electrical transmission or interconnection systems – Vehicle mounted systems – Automobile
Patent
1998-06-23
1999-10-19
Elms, Richard T.
Electrical transmission or interconnection systems
Vehicle mounted systems
Automobile
180282, 180280, 180735, 180307, 180104, B60R 2132
Patent
active
059694327
DESCRIPTION:
BRIEF SUMMARY
FIELD OF THE INVENTION
The present invention relates to an arrangement for controlling a load connected to the secondary side of a transformer, preferably the squib (firing pellet) of an air bag, using the primary-side input voltage of the transformer.
BACKGROUND INFORMATION
A conventional arrangement is described in German Patent Application No. 38 12 633. In this conventional arrangement, a rotary transformer is used, for example, for signal transmission between a controller and an air bag built into the steering wheel of a motor vehicle. The operability of an air bag must be constantly monitored, so that it is ready to be used immediately in an emergency situation. For this purpose, the resistance of the squib, which should be typically between 1.8 ohm to 2.5 ohm, must be periodically measured. Signal transmission between 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 pot-type cores of the windings. Installation and manufacturing tolerances of the steering wheel, on which the secondary pot-type core of the rotary transformer is installed, can result in distortions in the measured resistance value of the squib, as can the operating temperature. In order to avoid such distortions, the above-described conventional arrangement 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 switched off, the decay of the 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.
The object of the present invention is to provide a device allowing reliable information to be derived from the primary-side input voltage of the transformer concerning the status (resistance) of the load connected to the secondary side.
SUMMARY OF THE INVENTION
This object is achieved by connecting a synchronous rectifier to the primary side of the transformer. The primary-side input voltage of the transformer and a control voltage with the same frequency as the input voltage is applied to the synchronous rectifier, and the control voltage phase is switched between 0.degree. and 90.degree.. At the 0.degree. phase the output voltage of the synchronous rectifier corresponds exclusively to the voltage drop component at the ohmic input resistance of the transformer, and at the 90.degree. phase the output voltage corresponds exclusively to the voltage drop component at the inductive input resistance of the transformer. Since the ohmic and inductive input resistances vary as different functions of the resistance of the load connected to the secondary side, the arrangement according to the present invention always allows the voltage component providing reliable information on the status of the load connected to the secondary side to be evaluated. Information on the load connected to the secondary side can also be derived from a comparison between the two voltage components.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 shows a circuit according to the present invention for determining either an ohmic voltage component or an inductive voltage component on a primary side of a transformer.
FIG. 2 shows an equivalent circuit of the transformer connected to a load.
FIG. 3 shows a primary-side ohmic resistance and a primary-side inductive resistance as qualitative functions of a load resistance connected to the secondary side.
DETAILED DESCRIPTION OF THE INVENTION
A load resistance connected to the secondary side of a transformer is monitored for changes from the primary side. FIG. 1 shows the diagram of a transformer UT and a load resistance Rz connected to its secondary side. Transformer UT may be a rotary transformer, for example, arranged between a steering wheel equipped with an air bag and the stationary steering column. Load Rz connected to the secondary side then corresponds to the resistance of an air
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patent: 5872460 (1999-02-01), Bennett et al.
U. Tietze et al., "Halbleiter-Schaltungstechnik", 10.sup.th edition, 1993, Springer-Verlag, pp. 880-885. Described in the specification of the above-identified application.
Dukart Anton
Zabler Erich
Elms Richard T.
Robert & Bosch GmbH
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