Electricity: measuring and testing – Impedance – admittance or other quantities representative of... – Lumped type parameters
Patent
1996-04-05
1998-03-31
Nguyen, Vinh P.
Electricity: measuring and testing
Impedance, admittance or other quantities representative of...
Lumped type parameters
324526, 361 93, G01R 2708, G01R 3108
Patent
active
057342695
DESCRIPTION:
BRIEF SUMMARY
TECHNICAL FIELD
The present invention relates to a bridge circuit fault monitoring apparatus, and in particular to fault monitoring technology which can accurately detect a bridge circuit fault in a sensor circuit utilizing a bridge circuit, even when multiple faults occur with the simultaneous malfunction of the bridge circuit and other constituent elements of the sensor.
BACKGROUND ART
Sensors which detect a change in an object of detection by detecting a change in an imbalance output signal of a bridge circuit which is provided on one side with a transducer for converting a change in the object of detection into a change in an electrical signal, are used in many types of measurement systems and the like. For this type of sensor, there is for example, the electromagnetic induction sensor which uses an electromagnetic induction coil for the transducer, the strain sensor which uses a strain gage for the transducer, or the temperature sensor which uses a temperature sensitive element (for example a thermistor or the like) for the transducer.
Fault modes of a bridge circuit incorporated in the sensor, are considered for the case where one side of the four sides has a disconnection fault so that the imbalance output signal increases, and for the case where a fault occurs in the power source input side of the bridge circuit and a disconnection fault occurs simultaneously in two of the four sides, so that the imbalance output signal becomes zero, and an output signal equivalent to the imbalance condition is produced.
As an example of a fail-safe sensor incorporating a bridge circuit fault monitoring function which takes into consideration such a fault mode in the bridge circuit, there is disclosed in U.S. Pat. No. 5,027,114, an electromagnetic induction sensor having an electromagnetic induction coil provided on one side of the bridge circuit, which detects a metallic movable body from an impedance change in the coil.
FIG. 1 illustrates the structural principle of the circuit of this sensor.
A bridge circuit 1 is made up of four sides with three resistors R.sub.1, R.sub.2, R.sub.3, and a resonance circuit comprising a resistor R4, a capacitor C and a coil L, with an alternating current power source supplied from a signal generator SG. The resonance circuit may be a parallel resonance circuit. An imbalance output signal from the bridge circuit 1 is input to an amplifier A via a transformer T. The amplified output signal is then rectified by a rectifying circuit 2, and input to a fail-safe window comparator WC. Here the fail-safe window comparator WC is a known device such as disclosed in U.S. Pat. No. 4,661,880.
The fail-safe window comparator has a threshold value with upper and lower limits relative to an input level, such that when the input level is within a range (window) of the upper limit threshold value and the lower limit threshold value, an output voltage (corresponding to a logic value "1") is produced. On the other hand, when the input level is a higher level than the upper limit threshold value or when the input level is a lower level than the lower limit threshold value, an output voltage is not produced (corresponding to a logic value "0"). Furthermore, the construction is such that when a fault occurs in the window comparator itself, the output is always zero (logic value "0").
FIG. 2 illustrates the operating principle of the sensor circuit of FIG. 1.
In FIG. 2, the left hand graph shows changes in an imbalance output signal e of the bridge circuit 1. The vertical axis shows the imbalance output signal e (rectified output signal from the amplifier A) while the horizontal axis shows the magnitude of the inductance L of the coil or the magnitude of the electrostatic capacity C of the capacitor. With the imbalance output signal e, the valley point close to zero (the point where L=Lr, or C=Cr) is the balance point of the bridge circuit 1 (e.apprxeq.0). The operating point for the bridge circuit 1 (e=es) is set to the position of inductance L.sub.S. Furthermore, an inductance change .DELTA.L.sub.
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patent: 5027114 (1991-06-01), Kawashima et al.
patent: 5081442 (1992-01-01), Ito et al.
patent: 5568131 (1996-10-01), Sakai et al.
patent: 5619110 (1997-04-01), Sugimoto et al.
Anzai Hiroji
Futsuhara Koichi
Sakai Masayoshi
Shirai Toshihito
Do Diep
Nguyen Vinh P.
The Nippon Signal Co. Ltd.
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