JPH0456656A - Trouble detecting device - Google Patents

Abstract

PURPOSE:To provide a trouble detecting device which prevents the occurrence of an erroneous detection due to failure in operation of a resistor and has the high reliability by alternately switching the contact point voltages of first and second Wheatstone bridges to input them to a DC amplifying circuit and measuring the output voltage difference of the circuit. CONSTITUTION:A second series circuit comprising resistors 10 and 11 is connected in parallel to a first series circuit comprising a resistor 32 and a monitoring resistor 4 to form a first Wheatstone bridge. A fourth series circuit comprising resistors 10' and 11' is connected in parallel to a third series circuit to form a second Wheatstone bridge. First - fourth switch circuits 12, 13, l2', and 13' are individually connected to connection points A, B, A', and B', respectively, and fifth and sixth switch circuits 14 and 14' are individually connected to the contacts Band B', respectively. A common output terminal D for the first - fourth switch circuits is connected to the comparing input terminal of a DC amplifying circuit 15, a common output terminal D' for the fifth and sixth switch circuits is connected to a bias input terminal, and the presence of failure in operation is decided by a deciding circuit 19 based on an output voltage Eo of the DC amplifying circuit 15.

Description

[Detailed Description of the Invention] [Industrial Application Field] The present invention relates to a failure detection device for a starting resistor used in an automobile airbag device, etc., and particularly relates to an inexpensive and highly reliable failure detection device. It is something.

[Conventional technology] Conventionally, airbag equipment for automobiles! In order to instantaneously deploy an airbag in the event of a collision, an activation heater called a squib, which has a relatively low resistance value, is used. If a failure occurs in such a starting resistor, human life is at stake, so it is constantly monitored to see if there is an abnormality in the resistance value.

FIG. 5 is a circuit diagram showing a conventional failure detection device for an automobile airbag device, which is described in, for example, Japanese Patent Publication No. 61-57219.

In the figure, (1) is a battery mounted on an automobile, that is, a DC power source, and (2) is an ignition switch connected to the DC power source (1) for starting the engine.

(3) is an acceleration sensor (G sensor) connected to the DC current R (1) via the ignition switch (2), and is composed of a parallel circuit of a normally open contact (31) and a resistor (32). . (4) is the squib (monitored resistor) for airbag deployment connected to the connection point A"("G sensor (3)), and the first It forms a series circuit.

(5) is another G sensor connected to the monitored resistor (4) at connection point C, and like G sensor (3), it has a normally open contact (
51) and a resistor <52) in parallel, and the other end is grounded. Resistor (52) in G sensor (5)
constitutes a third series circuit together with the monitored resistor (4).

(6) is a failure detection circuit that is connected to both ends of the G sensors (3) and (5) and the monitored resistor (4) to detect a failure of the monitored resistor (4), and connects points A and C. A DC differential amplifier circuit (7) connected to the
The comparator circuit (8) is connected to the output terminal of (7).

The differential amplifier circuit (7) has a resistor for determining the degree of amplification.
71) to (74) and an operational amplifier (75), the resistor (71) is connected between the connection point A and the non-inverting input terminal of the operational amplifier (75), and the resistor (72) is connected between the ground and the operational amplifier. The resistor (73) is connected between the connection point C and the inverting input terminal of the operational amplifier (75),
A resistor (74) is inserted between the output terminal and the inverting input terminal of the operational amplifier (75).

The comparison circuit (8) includes series resistors (81) to (83) for dividing the DC power supply (1) to determine a reference voltage, and the connection points of the resistors (81) and (82) are non-inverted. An operational amplifier (84) is connected to the input terminal and the output terminal of the operational amplifier (75) is connected to the inverting output terminal, and an operational amplifier (84) is connected to the inverting input terminal at the connection point of the resistors (82) and (83). an operational amplifier (85) whose output terminal is connected to a non-inverting input terminal; and operational amplifiers (84) and (85).
and an AND gate (86) that performs the AND of the outputs of the .

(9) is the output terminal of the comparison circuit (8), that is, the AND gate (
This is an alarm lamp connected to the output terminal of 86).

Next, the operation of the conventional failure detection device shown in FIG. 5 will be explained.

When the ignition switch (2) is closed by starting the car, the G sensors (3), (5), the monitored resistor (4), and the fault detection circuit (6) are powered by the DC power source (1). Since the normally open contacts (31) and (51) are open, the voltage V of the DC power supply (1) is present across the monitored resistor (4).
1 to resistors (32), (52) and monitored resistor (4)
A voltage divided by is generated.

At this time, the resistance values R1 and R5 of the resistors (32) and (52) are each several hundred ohms or more, whereas the resistance value R4 of the monitored resistor (4) is several ohms, and the power supply voltage ■ Since l is about 12V, the voltage difference between connection points A and 0 is V
A c is several tens of mV. For example, R,=PL5=
When lkΩ R1=2Ω, the voltage VAC across the monitored resistor (4) is v
Ao=12x2/(1o00+1ooo+2) 12mV.

Here, if the G sensor (3) is short-circuited, the VA
c=12 In order to make a failure determination based on this, it is necessary to adjust the resistance values of the resistors (71) to (74) so that the amplification degree of the differential amplifier (75) is approximately 100.

As a result, the output voltage (7) of the differential amplifier circuit (7) is normally 1.2V, 2.4V when the G sensor (3) or (5) is short-circuited, and the monitored resistor (4) is short-circuited. At the time of failure, it becomes OV.

Therefore, the resistors (81) to (83) in the comparator circuit (8)
is the operational amplifier (
Both the outputs of 84) and (85) are "H" level, and when the G sensor (3) or (5) is short-circuited (2,4V), the output of operational amplifier <84) is "L" level, and the monitored resistance Vessel (4
) is adjusted so that the output of the operational amplifier (85) becomes "L" level when a short-circuit failure occurs.

As a result, the output of the AND gate (86) is at the rH level under normal conditions, which turns the lamp (9) off.
When a failure occurs, the level goes to "L" and the lamp (9) lights up to alert the driver of the abnormality.

If a car crash occurs when the G-sensors (3), (5) and the monitored resistor (4) are normal, the normally open contacts (31) and (5]) will close. Monitoring resistor (
4) generates heat and deploys the airbag to protect the driver.

[Problem to be solved by the invention] As described above, the conventional failure detection device
), the failure of the monitored resistor (4) is detected based on the voltage VAo across the series resistor (32) or (52).
), there is a risk of erroneously detecting a failure in the monitored resistor (4), resulting in a lack of reliability.

In addition, in order to detect fluctuations in the voltage across both ends, the amplification degree of the DC differential amplifier circuit (7) must be set to about 100, and because of this, the high amplification degree makes it vulnerable to noise. There was a problem.

Furthermore, in the case of DC differential amplification, errors are likely to occur due to the input offset voltage of the amplifier circuit, so it is necessary to use a high-precision amplification element, and fine adjustments are required at the manufacturing stage, which increases costs. There was a problem in that it invited

This invention was made to solve the above-mentioned problems, and aims to provide an inexpensive and highly reliable failure detection device.

[Means for Solving the Problems] A failure detection device according to the present invention includes a first series circuit including a resistor to be monitored and a resistor connected to the resistor to be monitored at a connection point A; A first Wheatstone bridge consisting of a pair of resistors connected to each other at B and having the same resistance ratio as the resistance ratio in the first series circuit, and connected in parallel to the first series circuit to form a balanced first Wheatstone bridge. 2 series circuit, the monitored resistor, and the connection point A' to this monitored resistor.
and a pair of resistors connected to each other at connection point B' and having the same resistance ratio as the resistance ratio in the third series circuit; a fourth series circuit connected in parallel to the series circuit to constitute a balanced second Wheatstone bridge; a DC power source for supplying power to the first and second Wheatstone bridges; First and second switch circuits connected and alternately opened and closed, and connection points A' and B'
third and fourth switch circuits that are individually connected to and alternately opened and closed; fifth and sixth switch circuits that are individually connected to connection points B and B' and are alternately opened and closed; ~
The output voltage of the DC amplifier circuit connected to the common output terminal of the sixth switch circuit and the DC amplifier circuit that is switched in synchronization with the opening and closing of the first and second switch circuits and the third and fourth switch circuits. and a determination circuit that determines whether or not there is a failure in the resistor to be monitored based on the difference.

Further, a failure detection device according to another aspect of the present invention includes a voltage adjustment circuit that is feedback-connected to the DC amplifier circuit to adjust the output voltage of the DC amplifier circuit, and a voltage adjustment circuit that is connected in feedback to the DC amplifier circuit to adjust the output voltage of the DC amplifier circuit; The device further includes a seventh switch circuit that is inserted and opened and closed in synchronization with the second and fourth switch circuits.

[Operation] In this invention, the voltages at the connection points A and B and A' and B' of the balanced first and second Wheatstone bridges are alternately switched and inputted to one DC amplifier circuit, and the DC By measuring the output voltage difference of the amplifier circuit, the effect of resistor failure in the first and third series circuits is eliminated. Furthermore, the influence of the input offset voltage of the DC amplifier circuit and variations in the circuit constants and elements of the measurement system is eliminated, and furthermore, the transition of the resistance value of the monitored resistor is accurately detected without the need for fine adjustment at the manufacturing stage.

Further, in another aspect of the present invention, when the voltage at the connection point B or B' on the reference side is applied by opening the second or fourth switch circuit, the seventh switch circuit is closed;
A voltage adjustment circuit is connected to adjust the output voltage of the DC amplifier circuit to the center value of the input voltage range of the determination circuit. Furthermore, when the voltage at the connection point A or A' on the measurement side is applied by closing the first or third switch circuit, the seventh switch circuit is opened and the voltage adjustment circuit is switched to direct current while maintaining the feedback voltage. Disconnect from the amplifier circuit. This eliminates the influence of characteristic fluctuations of elements other than the measurement system on the output voltage of the DC amplifier circuit, and enables more accurate failure detection.

[Example] Below, an example of the present invention will be described with reference to the drawings. 1st
The figure is a circuit diagram showing one embodiment of this invention, (68)
corresponds to the failure detection circuit (6), and (1) to (5)
and (9) are the same as above.

In addition, the failure detection circuit (6^) performs the following (10) to (24).
).

(10) and (11) are a second series circuit consisting of a pair of resistors connected via connection point B, and each resistor (
The resistance values R1o and R11 in 10) and (11) are adjusted to have the same resistance ratio as the resistance ratio of the first series circuit consisting of the resistor (32) and the monitored resistor (4). . The second series circuit is connected in parallel to the first series circuit to form a balanced first Wheatstone bridge.

(10') and (11') are a fourth series circuit consisting of a pair of resistors connected via a connection point B', and the resistance value R1o of each resistor (10') and (11') ' and R
is adjusted to have the same resistance ratio as the resistance ratio of the third series circuit consisting of the monitored resistor (4) and the resistor (52). The fourth series circuit is connected in parallel to the third series circuit to form a balanced second Wheatstone bridge.

(12) and (13) are first and second switch circuits that are individually connected to connection points A and B and are alternately opened and closed;
12') and (13') are third and fourth switch circuits that are individually connected to connection points A' and B' and are opened and closed alternately, and (14) and (14') are connection points B and B. The fifth and sixth switch circuits are individually connected to the switch circuits 1 and 2 and are alternately opened and closed, each of which is composed of an FET.

(15) is a DC amplifier circuit, and its comparison input terminal is connected to the first to fourth switch circuits (12), (13), (12).
') and (13'), and the bias input terminal is connected to the common output terminal D' of the fifth and sixth switch circuits (14) and (14'') via an amplifier (described later). It is connected to the.

(19) is a determination circuit that determines the presence or absence of a failure based on the output voltage Eo of the DC amplifier circuit (15), and is composed of, for example, a microcomputer, and is connected to the first to sixth switch circuits (12), ( 13), (12'), (13')
, (14) and (14'), it outputs control signals F to F6 for opening and closing the gates, and also outputs a drive signal H for lighting the lamp (9) when a failure is determined. ing.

(20) is a voltage adjustment circuit that is connected in feedback to the DC amplification circuit B (15) to adjust the output voltage Eo of the DC amplification circuit (15), and includes an operational amplifier (21);
It consists of a capacitor (22) inserted between the input and output terminals of an operational amplifier (21).

(23) is a DC amplifier circuit (15) and a voltage adjustment circuit (20
) and the seventh switch inserted between F! @(FET
), and the second and fourth switch circuits (13) and (13') are controlled by the control signal F7 from the determination circuit (19).
It is designed to open and close in sync with the

(24A) is an amplifier with an amplification degree of 1, and its input terminal is connected to the fifth and sixth switch circuits (14) and (14').
The output terminal is connected to the bias terminal of the DC amplifier circuit (15).

(24B) is the reference voltage ■8 for the operational amplifier (21).
It is a reference power supply that generates a voltage, and provides an appropriate offset voltage to the output voltage Eo of the DC amplifier circuit (15), which serves as a comparison input of the operational amplifier (21).

The DC amplifier circuit (15) includes operational amplifiers (17^) and (17B>) connected in series, and these operational amplifiers (1
Resistors (18^) to (78) and (17B) related to
18H), the resistor (18A) is connected between the common output terminal and the non-inverting input terminal of the operational amplifier (17A), and the resistor (18B) is connected between the output terminal of the amplifier (24, A) and the non-inverting input terminal of the operational amplifier (17A). A resistor (18C) is connected between the non-inverting input terminal of the operational amplifier (17^) and a resistor (18C) is connected between the output terminal of the amplifier (24^) and the inverting input terminal of the operational amplifier (17^).
8D) is between the output terminal of the operational amplifier (17^) and the inverting input terminal, and the resistor (18E) is between the output terminal of the operational amplifier (17^) and the non-inverting input terminal of the operational amplifier (17B). The resistor (18F> is connected between the reference power supply (24B) and the non-inverting input terminal of the operational amplifier (17B).
) is the output terminal of the amplifier (24^) and the operational amplifier (17B
), and a resistor (18H) is inserted between the output terminal and the inverting input terminal of the operational amplifier (17B), respectively.

The output voltage EO of the DC amplifier circuit (15) is determined by the determination circuit (1
9) and the seventh switch circuit (23).
The reference voltage 8 of the reference power supply (24B) is applied to the inverting input terminal of the operational amplifier (21) through the operational amplifier (21).
21) is applied to the non-inverting input terminal.

In addition, the feedback voltage (2) output from the operational amplifier (21) in the voltage adjustment circuit (20) is connected to the capacitor (2).
2) to the inverting input terminal of the operational amplifier (21) itself, and the resistor (
18F) to the non-inverting input terminal of the operational amplifier (17B).

Next, the operation of the embodiment of the present invention shown in FIG. 1 will be described with reference to the timing chart shown in FIG. 2 and the flow chart shown in FIG.

In FIG. 1, the DC amplifier circuit (15) has a voltage of ■ at the connection point. and the output voltage (2) of the amplifier (24A) are input. Here, the output voltage ■1 is the common output terminal D'
Voltage ■. ', and this voltage 'W' is equal to the voltage '8 or V, r at the connection point B or B'.

Therefore, the output voltage EO of the DC amplifier circuit (15) is determined by the resistance values of the resistors (188) to (18H), respectively.
11, E o-fR11/(RA+R-)lf(RC÷RD)
/RoHRF/(Rt+R,))x ((Rc+R++
)/Rc)(vo-vi)+((RJc-RiRa)
/Rc(Rt+Rr))Vi+ (Re/(R,C+R
D)++(RE+RF)/R11VF4VOF...(1
). Here, each resistance value RA to RH is RA=
Ro=R,=Rc-R11-(2)RB=R,=R
, -R□=RP...(3), then
Equation (1) is E o” (Rp/R1)2(V D-V E)÷V,
+VoF..., (23). However, V OF is the input offset voltage of the operational amplifier (17^) and (17B), and the resistance values R5 and R given by equations (2) and (3), and the resistors (18A) to (18H) This value also includes error components caused by deviations from each resistance value.

Here, the control signal F1 output from the determination circuit (19)
~F7 shall vary as shown in Figure 2, and the first, second and fifth switch times associated with the first Wheatstone bridge! Control signals F for (12) to (14),
, F2 and F5. That is, control signals F3, F4 for the third, fourth and sixth switch circuits (12') to (14') associated with the second Wheatstone bridge.
and F6 are all turned off.

At this time, the control signal F causes the fifth switch circuit (
14) is always on, and each switch circuit (1
The operation modes of 2), (13) and (23) are (1) reference voltage (E o2) mode, the first switch circuit (12) is off, and the second and seventh switch circuits (13) and (23) is on (II) measurement voltage (Eo+) mode There are two modes: the first switch circuit (12) is on, and the second and seventh switch circuits (13) and (23) are off.

First, time t. ~t1, control signal F1 is off,
When the control signals F2, FS and F7 are turned on, the first
The switch circuit (12) is turned off, and the second, fifth and seventh switch circuits (13), (14) and (23) are turned off.
is turned on (step Sl in FIG. 3).

At this time, the operation mode is the reference voltage mode (1), and the output voltage Eo becomes the output voltage EO□ corresponding to the connection point B on the reference side (step S2).

Moreover, the second and seventh switch circuits (13) and (23)
Due to the conduction of V, =V, =V.

Therefore, from equation (4), the output voltage Eo2 is E o
2 ” V F + V ov ・
...(5). Furthermore, a seventh switch circuit (23)
Due to the conduction of the output voltage Eo2, the voltage adjustment circuit (20)
In the equation (24), the operational amplifier (21) receives VF 10V. Adjust the feedback voltage ■ so that F = V R (6).

Thereby, the output voltage Eo2 is accurately adjusted to the reference voltage V, which is the center value of the input voltage range of the determination circuit (18).

Next, when the control signals F1, F2, and F are inverted at times t to t2, the first and fifth switch circuits (1
2) and (14) are turned on, and the second and seventh switch circuits (13) and (23) are turned off (step S3).

At this time, the operation mode becomes the measurement voltage mode (II), and the output voltage Eo becomes the output voltage Eo corresponding to the connection point A on the measurement side (step S4). Also, due to the conduction of the first and fifth switch circuits (12) and (14), V,
=V.

V t-■- Therefore, from equation (4), the output voltage EO□ is E 0
l-(Rp/Rt,)2(V-V B)+V F+V
. F - (7).

At this time, the seventh switch circuit (23) is turned off,
Although the input of the operational amplifier (21) cannot be obtained, the feedback voltage VF in the reference operation mode (1) is maintained by the capacitor (22), so the formula (6) % formula %
(8) becomes. Therefore, the output voltage Eo is expressed by equation (7) and (8)
) From the formula, Eo, = (Rp/Rs)2(V, -v,)+V* -(
9).

As is clear from equation (9), the output voltage Eo in the measurement voltage mode is always centered around the reference voltage ■8, and the voltage difference (V, -
The value is a value that fluctuates by the amplified value of V[l). Therefore, if the reference voltage ■8 is set to the center value of the input voltage range of the judgment circuit (19), fluctuations in the power supply voltage ■1, fluctuations in the resistance ratio of the G sensors (3) and (5), etc. can be avoided. The voltage difference (VA Va) can be measured without being affected, and the change in resistance value of the monitored resistor (4) can be detected.

For example, if the power supply voltage is 1, the resistors (32), (4), (1o) and (1
If the resistance values of 1) are R32, R1, RIO, and R, respectively, the voltage VA and the seven colors at each connection point A and B when the ignition switch (2) is closed are V, -(V+- Vc
)R</(Rs2+R+)-, (io)V,-(V,
-Vc)Rz/ (Rio"R+1) ・'・<11
) is given by

Here, the connection point A in the first Wheatstone bridge is grounded via the resistor (10') and <11') of the second Wheatstone bridge, and each resistance value Rh
o' and R11' are usually on the order of R, o' = 300 Ω R, '' = 100 Ω, and the resistance values R3, 1 ( -1, kΩ> and R, (=3Ω). Therefore, (10
) and (11), resistor (10') and (1
The influence of 1') can be ignored.

By the way, the resistance value of the monitored resistor (4) during normal operation is R-
5 Letting the resistance value deviation of the monitored resistor (4) be ΔR4,
The resistance value R4 when the resistance value deviation ΔR1 changes from the normal value R4″ is expressed as R,=R4″+ΔR1 (12). In addition, as a balance condition for the first Wheatstone bridge, the resistor (10
) and (11), the resistance values R8° and R11 are set, so R4 wealth/R32-Rl1/R1o-α = 413) where α: resistance ratio. From equations (10) to (13), each connection point A and B
The voltage V and ■8 are as follows: ■8=[α(V, -Vc)/<1+α)]X(
1+ΔR</R4')+Vc...(
14) Ve-α(V,-Vc)/(1+α)+Vc
- given by (15). As is clear from equations (14) and (15), R, -R4' (i.e., △R, = O)
If so, V,=V.

becomes.

From equations (9), (14), and (15), the output voltage Eo in measurement voltage mode is E O+ = Q (V + V c) (Rp/R
s) 2ΔR1: [(1+α)R4”]+V, ,
・116). Here, R3z=1, Ω, R
♂-3Ω, R5゜=100Ω, R,, -300Ω, R
p/R, = 10, V, = IOV, and resistor (
52) resistance value R62, R1o>>R52(=
R12) >> R.

Then, ■1-■c-■1/2 5■ The output voltage Eo becomes Eo+=1.5ΔR4/R4y+VR. or,
Since the output voltage E O2 in the reference voltage mode is equivalent to the case of ΔR, -0 in the above equation, E O2=V R . Here, if ΔR,=0.1Ω, Eo, -
Eo□=0.05V was obtained, and it can be seen that this is a level that can be handled with sufficient margin by ordinary electronic circuits.

In the above explanation, we focused on the first Wheatstone bridge, but the resistor (4) in the second Wheatstone bridge
, (52), (10') and (11''), the third, fourth and sixth switch circuits (12'), (13'
) and (14') perform similar operations.

That is, from time t2 to time t, the third switch circuit (
12') is turned off, and the fourth, sixth and seventh switch circuits (
12'') and (23) (step S5), the voltage EO2 corresponding to the reference connection point B' is obtained as the output voltage EO (step S6), and at time t,
~t, the third and sixth switch circuits (12'
) and (14'') and turn off the fourth and seventh switch circuits (13') and (23) (step S7), the voltage EO3' at the connection point A' to be measured becomes the output voltage. Eo is obtained (step S8).

In this case, the output voltage Eol' in the measurement voltage mode is (
From formula 9), Eo+'=(Rp/Rs)2(VA'-Vp')+V
p ・−(17). Also, each connection point A' and B
'The voltages VA' and ■8' are the voltages VA' and
), (10') and (11') resistance values R4, R5□,
R1゜' and R11' and the voltage vA of connection points A and C'
and Vc', VA'-(VA-Ve')Rs
z/(R<”R5z)+Ve'-(18)V-
'=(VA-Vc')R1+'/(R+ o "R
+ l') + Vc' (19).

In this case as well, the influence of the resistance values R4° and R1 of the resistors (10) and (11) in the first Wheatstone bridge can be ignored.

Here, R4'/ R52= R+o'/ R+ t'= a
Then, equations (18) and (19) become VA'= (
VA-VC! ')/[12(1+1/(24AR4/R
4")]+VC'...(20) V-= (VA-VC!')/[(2(1"l/α)
]4Vc'-(21).

Therefore, from equations (17), (20), and (21),
The output voltage Eo l' for measurement is Eo'--α(V, -Vc')(Rp/Rs)2×
It is expressed as ΔR,/(1+α)R,'+V, (22). This is a similar equation to the above-mentioned equation (16), although the sign is opposite.

Here, RS 2 = lkΩ, R4' = 3Ω, R3
゜' = 300Ω, R1+ '-100, Ω, V, -V
, /225V, RP/R, = 10, then Eo'=-1,5ΔR</R4''10V.

becomes. Also, the output voltage E02' in the reference voltage mode is
As mentioned above, it is equivalent to the case when ΔR1=0, so E o
2'=V R .

Next, the determination circuit (19) determines whether the differential voltage ΔEO obtained by ΔEo=Eo−EO2 ΔEo−EO′−E02′ is greater than or equal to the preset allowable variation range ΔE or not (step S9). ), when Eol-Eo2≧E or EO1′ EO2′l≧ΔE, a drive signal H is output to turn on the alarm lamp (9) (step 510).

On the other hand, if it is determined in step S9 that the differential voltage ΔEo is within the allowable fluctuation range ΔE, step S9
Return to step 1 and repeat the same operation.

FIG. 2 shows a case where it is detected that the output voltage Eo exceeds the permissible fluctuation range ΔE around time t4, and the lamp drive signal H is turned on at time t after a delay time of the processing operation.

In this way, the output voltages for measurement E o + and Eo,′
By determining the failure of the monitored resistor (4) using both of the above, highly reliable and accurate failure detection becomes possible.

That is, (9) regarding the first Wheatstone bridge
As is clear from equation (11) and equations (17) to (19) regarding the second Wheatstone bridge, the output voltages Eo and Eo,' for measurement are determined only by the resistance value R4 of the resistor to be monitored (4). A resistor (3
It is also affected by changes in the resistance values R)2 and R62 of (2) and (52). Therefore, in the abnormality determination result based on only one Wheatstone bridge, the monitored resistor (4
) or resistor (32) or (52) cannot be determined.

For example, if the determination result of the output voltage Eo+ by the first Wheatstone bridge indicates a decrease in the resistance value R4 of the monitored resistor (4), there is a possibility that the resistance value R12 of the resistor (32) has increased. be. However, at this time, if the determination result of the output voltage E o + ' by the second Wheatstone bridge indicates a decrease in the resistance value R1 of the monitored resistor (4), then the monitored resistor (4) is actually It can be determined that it is a resistance value decrease fault. On the other hand, if the determination result of the output voltage E01' by the second Wheatstone bridge indicates that the resistance value R4 is normal, the resistance value R3
It can be seen that 2 is an increasing failure.

FIG. 4 is an explanatory diagram summarizing the causes of failure for each determination result of the output voltages E01 and E01' by the first and second Wheatstone bridges. As is clear from Fig. 4, when the judgment results of each output voltage Eol and Eo+' match, the state of the monitored resistor (4) is judged to be as per the judgment result, and only one judgment result is normal. , the series-connected resistor (32) or (5
2) can be determined to be a failure.

At this time, if the resistance value of any one of the resistors (4), (32), and (52) changes, the output voltages Eo and Eo,' of each Wheatstone bridge will necessarily change.

This is because it is actually impossible for the resistance values of each Wheatstone bridge to change while the resistance ratio of each Wheatstone bridge remains constant.

However, if the judgment results for each output voltage Eo and Eol' are in the opposite direction, this can only occur when a failure occurs at two or more points, not just one point, and it is difficult to identify the point of failure. It is difficult. However, for example, a resistor (
Both 32) and (52) are faulty at the same time (abnormal resistance value)
Since the possibility of this occurring is extremely low, accurate detection can always be achieved by using two Wheatstone bridges to detect failures in the monitored resistor (4).

Also, the output voltages Eo and E in the measurement voltage mode.

is always a value that varies by the position where the voltage difference at the connection point of each Wheatstone bridge is amplified, with the reference voltage ■R as the center, so set the reference voltage ■8 to the center value of the input voltage range of the judgment circuit (19). By doing so, the voltage difference can be measured without being affected by fluctuations in the power supply voltage V1, fluctuations in the resistance ratio of the G sensors (3) and (5), etc. Therefore, accurate detection that is not affected by variations in the characteristics of the measurement system can be achieved, and furthermore, even if conditions such as the power supply voltage (1) other than the measurement system vary, the output voltage Eo can be stabilized.

In the above embodiment, the voltage adjustment circuit (20), the seventh switch circuit (23), and the reference power supply (24B) are provided to remove the offset of the output voltage Eo, but there must be no particular problem in use. For example, the voltage adjustment circuit (20), the seventh switch circuit (23), and the reference power supply (24B) may not be provided.

In addition, although the case where the monitored resistor (4) is a heater for starting an automobile airbag device is shown, it is equally applicable to other monitored resistors as long as they have a relatively low resistance value. Needless to say, it has the following effects.

[Effects of the Invention] As described above, according to the present invention, a first series circuit consisting of a resistor to be monitored and a resistor connected to the resistor to be monitored at a connection point A, and a first series circuit consisting of a resistor to be monitored and a resistor connected to the resistor to be monitored at a connection point B, A second series circuit comprising a pair of resistors connected in the first series circuit and having the same resistance ratio as the resistance ratio in the first series circuit, and connected in parallel to the first series circuit to form a balanced first Wheatstone bridge. The connection point A' is connected to the circuit, the monitored resistor, and this monitored resistor.
and a pair of resistors connected to each other at connection point B' and having the same resistance ratio as the resistance ratio in the third series circuit; a fourth series circuit connected in parallel to the series circuit to constitute a balanced second Wheatstone bridge; a DC power source for supplying power to the first and second Wheatstone bridges; First and second switch circuits connected and alternately opened and closed, and connection points A' and B'
third and fourth switch circuits that are individually connected to and alternately opened and closed; fifth and sixth switch circuits that are individually connected to connection points B and B' and are alternately opened and closed; ~
The output voltage of the DC amplifier circuit connected to the common output terminal of the sixth switch circuit and the DC amplifier circuit that is switched in synchronization with the opening and closing of the first and second switch circuits and the third and fourth switch circuits. and a determination circuit that determines whether there is a failure in the resistor to be monitored based on the difference, and alternates the voltages at the connection points A and B and A' and B' of the balanced first and second Wheatstone bridges, respectively. Switch to 1
Since the input voltage is input to two DC amplifier circuits and the output voltage difference of the DC amplifier circuits is measured, it is possible to prevent false detection due to failure of a resistor connected in series with the resistor to be monitored.
This has the effect of eliminating the effects of the input offset voltage of the DC amplifier circuit and variations in the circuit constants and elements of the measurement system, thereby providing an inexpensive and highly reliable failure detection device.

According to another aspect of the present invention, there is provided a voltage adjustment circuit that is feedback-connected to the DC amplifier circuit and adjusts the output voltage of the DC amplifier circuit, and a voltage adjustment circuit that is inserted between the DC amplifier circuit and the voltage adjustment circuit. A seventh switch circuit is further provided which is opened and closed in synchronization with the second and fourth switch circuits, and when the second or fourth switch circuit is closed, the voltage at the connection point B or B' on the reference side is applied. When the voltage is applied to the connection point A or A' on the measurement side by closing the first or third switch circuit, the seventh switch circuit is opened and the measurement is started. Since the offset of the output voltage of the DC amplifier circuit is removed in the voltage mode, there is no effect of characteristic fluctuations of elements other than the measurement system, and a more accurate and reliable failure detection device can be obtained.

[Brief explanation of the drawing]

FIG. 1 is a block diagram showing an embodiment of the present invention, FIG. 2 is a timing chart for explaining the operation of FIG. 1, and FIG. 3 is a flowchart showing the operation of the determination circuit in FIG. 1. , FIG. 4 is an explanatory diagram showing the determination result by the determination circuit in FIG. 1, and FIG. 5 is a circuit diagram showing a conventional failure detection device. (1)・DC power supply (4)・Monitored resistor (6
A) Failure detection circuit (1,0), (11) to (10'), (11'), (3
2), (52)...Resistor (12)...First switch circuit (13)...Second switch circuit (12')...Third switch circuit<13')...・Fourth switch circuit (14)...Fifth switch circuit (14')...Sixth switch circuit (15)...DC amplifier circuit (19)...Judgment circuit (20)・... Voltage adjustment circuit (23) - Seventh switch circuit A, B, A', B'... Connection point D-D'... Common output terminal Eo... Output voltage Note that the same reference numerals in the figure Indicates the same or equivalent part.

Claims (2)

[Claims] (1) A first series circuit consisting of a resistor to be monitored and a resistor connected to the resistor to be monitored at a connection point A, and a resistance ratio in the first series circuit connected to each other at a connection point B. a second series circuit consisting of a pair of resistors having the same resistance ratio and connected in parallel to the first series circuit to form a balanced first Wheatstone bridge; a third series circuit consisting of a resistor connected to the monitored resistor at a connection point A', and a resistor connected to each other at a connection point B' and having the same resistance ratio as the resistance ratio in the third series circuit; A balanced second series circuit consisting of a pair of resistors is connected in parallel to the third series circuit.
a fourth series circuit constituting a Wheatstone bridge; a DC power supply for supplying power to the first and second Wheatstone bridges; and a first series circuit connected to the connection points A and B individually and opened and closed alternately. and a second switch circuit, and third and fourth switch circuits that are individually connected to the connection points A' and B' and are alternately opened and closed.
a fifth and sixth switch circuit connected to the connection points B and B' individually and opened and closed alternately, and a common output terminal of the first to sixth switch circuits. of the monitored resistor based on the difference between the output voltages of a DC amplifier circuit and the DC amplifier circuit that is switched in synchronization with opening and closing of the first and second switch circuits and the third and fourth switch circuits. A failure detection device comprising: a determination circuit that determines the presence or absence of a failure; (2) a voltage adjustment circuit that is feedback-connected to the DC amplifier circuit to adjust the output voltage of the DC amplifier circuit; and second and fourth switches inserted between the DC amplifier circuit and the voltage adjustment circuit. The seventh circuit opens and closes in synchronization with the circuit.
A failure detection device according to claim 1, comprising: a switch circuit;