JPH0743407B2 - Failure detection device - Google Patents

Description

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a failure detecting device for a starting resistor used in an automobile air bag device or the like, and more particularly to an inexpensive and highly reliable failure detecting device. is there.

[Prior Art] Conventionally, in an automobile airbag device, a startup heater called a squib having a relatively small resistance value is used in order to instantly deploy the airbag at the time of a collision. Since such a starting resistor is fatal to human life when a failure occurs, it is constantly monitored whether or not the resistance value is abnormal.

FIG. 7 is a circuit diagram showing a conventional failure detection device in an automobile air bag device, which is described in, for example, Japanese Patent Publication No. 61-57219.

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

(3) is an acceleration sensor (G sensor) connected to the DC power supply (1) through the ignition switch (2), and is composed of a parallel circuit of a normally open contact (31) and a resistor (32). .
(4) is a squib (monitored resistor) for airbag deployment, which is connected to the G sensor (3) at the connection point A, and together with the resistor (32) in the G sensor (3), the first series circuit Are configured.

(5) is another G sensor connected to the monitored resistor (4) at the connection point C, and is a parallel circuit of the normally open contact (51) and the resistor (52) as in the G sensor (3). The other end is grounded.

Reference numeral (6) is a failure detection circuit connected to each end of the G sensors (3) and (5) and the monitored resistor (4) to detect a failure of the monitored resistor (4). And a comparison circuit (8) connected to the output terminal of the differential amplification circuit (7).

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

The comparison circuit (8) has a non-inverting connection point between the series resistors (81) to (83) for dividing the DC power supply (1) to determine the reference voltage, and the resistors (81) and (82). The operational amplifier (84) connected to the input terminal and the output terminal of the operational amplifier (75) connected to the inverting output terminal, and the connection point of the resistors (82) and (83) connected to the inverting input terminal and the operational amplifier ( An operational amplifier (85) whose output terminal is connected to the non-inverting input terminal,
It is composed of an AND gate (86) which takes the logical product of the outputs of the operational amplifiers (84) and (85).

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

Next, the operation of the conventional failure detection device shown in FIG. 7 will be described.

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

At this time, the resistance values R ₃ and R ₅ of the resistors (32) and (52) are several hundred Ω or more, respectively, whereas the resistance value R ₄ of the monitored resistor (4) is several Ω, and , The power supply voltage V ₁ is about 12
Since it is V, the voltage difference V _AC between the connection points A and C is several tens of mV. For example, when _{R 3 = R 5 = 1kΩ R} 4 = 2Ω, the voltage across V _AC of the monitored resistor (4) _{is, V AC = 12 × 2 /} (1000 + 1000 + 2) is a ≒ 12 mV.

Here, if the G sensor (3) has a short circuit failure, V _AC = 12 × 2 / (1000 + 2) ≈24 mV, and if the monitored resistor (4) has a short circuit failure, V _AC = 0V Become. In order to judge the failure based on the voltage value that varies in the range of 0 to several tens of mV, the resistor (71) is set so that the amplification degree of the differential amplifier (75) is about 100.
It is necessary to adjust the resistance value of (74). This allows
The output voltage V ₇ of the differential amplifier circuit (7) is normally 1.2V, 2.4V when the G sensor (3) or (5) has a short circuit failure, and 0V when the monitored resistor (4) has a short circuit failure. .

Therefore, the resistors (81) to (83) in the comparison circuit (8) are
When the output voltage V ₇ is normal (1.2V), the outputs of the operational amplifiers (84) and (85) are both "H" level, and when the G sensor (3) or (5) has a short circuit failure (2.4V), the operational amplifier The output of (84) is adjusted to "L" level, and the output of the operational amplifier (85) is adjusted to "L" level when the monitored resistor (4) is short-circuited.

As a result, the output of the AND gate (86) becomes "H" level under normal conditions and the lamp (9) is turned off.
At the time of a failure, the level becomes "L" and the lamp (9) is turned on to alert the driver of the abnormality.

When the car has a collision accident while the G sensors (3), (5) and the monitored resistor (4) are normal, the normally open contacts (31) and (51) are closed. Resistor (4)
Can generate heat and deploy and activate the airbag to protect the driver.

[Problems to be Solved by the Invention] As described above, in the conventional failure detection device, it is necessary to set the amplification factor of the DC differential amplifier circuit (7) to about 100 in order to detect the voltage fluctuation due to the failure. Therefore, there is a problem in that it is weak against noise due to the large amplification degree.
Also, in the case of DC differential amplification, an error is likely to occur due to the influence of the input offset voltage of the amplification circuit, so it is necessary to use a highly accurate amplification element, and fine adjustment at the manufacturing stage is required, which results in reliability. However, there is a problem in that the cost is increased as well as the lack.

The present invention has been made to solve the above problems, and an object thereof is to obtain 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 monitored resistor and a resistor connected to the monitored resistor at a connection point A, and a connection point. A second series that comprises 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 that is connected in parallel to the first series circuit to form a balanced Wheatstone bridge. Circuit, a DC power source for supplying power to the Wheatstone bridge, first and second switch circuits that are individually connected to the connection points A and B and are alternately opened and closed, and are connected to a common output terminal of these switch circuits. And a determination circuit for determining whether or not there is a failure in the monitored resistor based on the difference between the output voltages of the DC amplification circuit that is switched in synchronization with the opening and closing of the first and second switch circuits. Things is there.

Further, a failure detection device according to another invention of the present invention further comprises
A voltage adjusting circuit feedback-connected to the direct current amplifying circuit to adjust the output voltage of the direct current amplifying circuit; and a voltage adjusting circuit inserted between the voltage adjusting circuit and the direct current amplifying circuit and opened / closed in synchronization with the second switch circuit. 3 switch circuits.

[Operation] In the present invention, the voltages from the connection points A and B of the balanced Wheatstone bridge are alternately switched to 1
Input to two DC amplification circuits and measure the output voltage difference of this DC amplification circuit to eliminate the influence of the input offset voltage of the DC amplification circuit, the circuit constant of the measurement system, and the variation of the element. Fine adjustment is unnecessary, and the transition of the resistance value of the monitored resistor is accurately detected.

In another aspect of the present invention, when the voltage at the connection point B on the reference side is applied by closing the second switch circuit, the third switch circuit is closed and the voltage adjusting circuit is connected. The output voltage of the DC amplification circuit is adjusted to the center value of the input voltage range of the determination circuit, and when the voltage at the connection point A on the measurement side is applied due to the closing of the first switch circuit, the third switch circuit is opened. , The voltage adjustment circuit is disconnected from the DC amplification circuit while holding the feedback voltage. As a result, the dynamic range of the output voltage of the DC amplification circuit is not affected by the characteristic variations of elements other than the measurement system, and more accurate failure detection is possible.

[Embodiment] An embodiment of the present invention will be described below with reference to the drawings. First
The drawing is a circuit diagram showing an embodiment of the present invention, in which (6A) corresponds to the failure detection circuit (6), and (1) to (4) and (9) are the same as those described above. . The failure detection circuit (6A) is composed of the following (10) to (18).

(10) and (11) are a second series circuit composed of a pair of resistors connected via a connection point B, and each resistor (10)
The resistance values R ₁₀ and R ₁₁ of (11) and (11) are adjusted to have the same resistance ratio as the resistance ratio of the first series circuit including 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 Wheatstone bridge.

(12) and (13) are first and second switch circuits that are individually connected to the connection points A and B and are alternately opened and closed,
In this case, each is composed of a FET. (14) is a reference power source that generates a reference voltage V _RE .

(15) is a DC amplifier circuit connected to the common output terminal D of each switch circuit (12) and (13), and has a common output terminal D
The voltage V _D and the reference power adder for adding the reference voltage V _RE from (14) (16) is constructed from an amplifier for amplifying the input voltage Ei from the adder (16) (17) .

Reference numeral (18) is a determination circuit for determining the presence / absence of a failure based on the output voltage E _O of the amplifier (17), which is composed of, for example, a microcomputer, and includes switch circuits (12) and (1
3), the control signals F ₁ and F ₂ for opening and closing the gate are output, and when a failure is determined, the drive signal H for lighting the lamp (9) is output.

Next, the operation of the embodiment of the present invention shown in FIG. 1 will be described.

First, if the power supply voltage is V ₁ and the resistance values of the resistors (3), (4), (10) and (11) in the Wheatstone bridge are R ₃ , R ₄ , R ₁₀ and R ₁₁ , respectively, ignition The voltages V _A and V _{B at the} connection points A and B when the switch (2) is closed are V _A = V ₁ R ₄ / (R ₃ + R ₄ ) ... (1) V _B = V ₁ R ₁₁ / (R ₁₀ + R ₁₁ ) ... It is given by (2). Here, assuming that the resistance value of the monitored resistor (4) in a normal state is R ₄ ^* and the resistance value deviation is ΔR ₄ , the resistance value R ₄ when the resistance value deviation ΔR ₄ changes from the normal value R ₄ ^* Is represented by R ₄ = R ₄ ^* + ΔR ₄ (3) Further, as a balance condition of the Wheatstone bridge, the resistance values R ₁₀ and R ₁₁ of the resistors (10) and (11) are set so that the resistance ratio of each series circuit becomes equal when the monitored resistor (4) is normal. Is set, R ₄ ^* / R ₃ = R ₁₁ / R ₁₀ = α (4) However, α: resistance ratio. (1) than to (4) below, the voltage V _A ~V _B of the connection points A and _{B, V A = [αV 1 /} (1 + α)] (1 + ΔR 4 / R 4 *) ... (5) V B = ΑV ₁ / (1 + α) (6) As is clear from the equations (5) and (6),
If R ₄ = R ₄ ^* , that is, ΔR ₄ = 0, then V _A = V _B.

Now, the first switch circuit (12) is closed (turned on),
If the second switch circuit (13) is opened (OFF), V _D = V _A , and the voltage V _{A at the} connection point _A is input to the DC amplification circuit (15). Therefore, assuming that the input voltage and output voltage of the amplifier (17) at this time are Ei ₁ and E _O1 , respectively, Ei ₁ = V _A + V _RE E _O1 = G (V _A + V _RE ) (7) where G; It is the amplification degree of the amplifier (17).

On the contrary, when the first switch circuit (12) is opened and the second switch circuit (13) is closed, V _D = V _B, and the voltage V _{B at the} connection point _B becomes the DC amplification circuit. It is input in (15). Therefore, assuming that the input voltage and the output voltage of the amplifier (17) at this time are Ei ₂ and Eo ₂ , respectively, Ei ₂ = V _B + V _RE Eo ₂ = G (V _B + V _RE ) (8)

The determination circuit (18) takes in the output voltages Eo ₁ and Eo ₂ switched by opening / closing the switch circuits (12) and (13) in synchronization with the switching timing, and calculates the difference voltage ΔEo between them. This differential voltage ΔEo is given by the equations (5) to (8) as follows: ΔEo = Eo ₁ −Eo ₂ = αGV ₁ ΔR ₄ / (1 + α) R ₄ ^* … (9) and proportional to the resistance deviation ΔR ₄ . It will be the value. Where R ₃ = 1kΩ, R ₄ ^* = 3Ω, R ₁₀ = 100kΩ, R ₁₁ = 300Ω,
V ₁ = 10V, α = 3/1000, G = 100, and resistance deviation is Δ
Assuming that R ₄ = 1Ω, the differential voltage ΔEo is calculated from the equation (9).
Is ΔEo = 100 (3/1000) / 3 (1 + 3/1000) ≈ 0.997V. That is, the deviation Δ of the resistance value R ₄ of the monitored resistor (4)
It can be seen that a difference voltage ΔEo (≈1V) occurs with respect to R ₄ (= 1Ω). From equation (9), when ΔR ₄ = 0Ω, ΔEo =
Since it is 0V, when ΔR ₄ = 0.1Ω, ΔEo≈0.1V,
The difference voltage ΔEo is a level that can be handled with a sufficient margin in an ordinary electronic circuit.

On the other hand, the reference power supply (14) adds the reference voltage V _RE to the adder (16).
To correct the input voltage Ei of the amplifier (17) so that the output voltage Eo of the DC amplification circuit (15) falls within the voltage range (for example, 0 to 5V) that can be judged by the judgment circuit (18). I have to. This judgment voltage range varies depending on the semiconductor element used and the like.

At this time, the output voltage Eo ₁ on the measurement side given by equation (7)
Fluctuates depending on the resistance value R _{4 of the} monitored resistor (4), but it is easy to determine that it is an abnormal value even if it fluctuates greatly and exceeds the voltage range of 0 to 5V, so there is no particular problem. However, the output voltage Eo ₂ on the reference side given by equation (8) is
It is always necessary to be within the voltage range of 0 to 5 V, and the reference voltage V _RE takes this point into consideration. For example, under the same conditions as above, assuming that G = 100, the output voltage Eo ₂ is Eo ₂ = 100 (V _B + V _RE ) from the equation (8), and Eo ₂ = from the equation (1). It becomes 100 [V ₁ R ₁₁ / (R ₁₀ + R ₁₁ ) + V _RE ]. This output voltage Eo ₂ is the middle of the other output voltage Eo ₁ ,
That is, 2.5V is desirable, and the variable range (tolerance) is about 2.5 ± 1V. From the above, 1.5 ≦ 100 (0.003V ₁ + V _RE ) ≦ 3.5 (10) holds. Considering that the reference voltage V _RE is the sum of the fixed voltage V _RO and the voltage (= 0.003V ₁ ) driven by the power supply voltage V ₁ , V _RE = V _RO −0.003V ₁ (11) Then, from the expressions (10) and (11), the condition that the fixed voltage V _RO should satisfy is 15 mV ≤ V _RO ≤ 35 mV (12). By transforming the equation (12), it can be seen that V _RO = 25 mV ± 10 mV, and the reference voltage V _RE has a sufficient margin in terms of circuit technology.

Although only one G sensor (3) is used in the above embodiment, the same effect can be obtained by using two G sensors (3) and (5) as shown in FIG.

FIG. 2 is a circuit diagram showing another embodiment of the present invention,
(14A): The input terminal is connected to the connection point C and the gain is 1
This is an amplifier for the DC amplification circuit (15B), and is designed to apply a bias voltage to the DC amplification circuit (15B). (14B) is a reference power source provided to the fault detection circuit (6B) in conjunction with an amplifier (14A) for generating a reference voltage V _R, giving appropriate offset voltage to the output voltage Eo of the DC amplifying circuit (15B) There is.

In this case, the DC amplification circuit (15B) includes operational amplifiers (17A) and (17B) connected in series and resistors (19A) to (19).
H) and the resistor (19A) is between the common output terminal D and the non-inverting input terminal of the operational amplifier (17A), and the resistor (19B) is the output terminal of the amplifier (14A) and the operational amplifier. The resistor (19C) is between the non-inverting input terminal of (17A), the output terminal of the amplifier (14A) and the inverting input terminal of the operational amplifier (17A), and the resistor (19D) is the operational amplifier (17A). Between the output terminal and the inverting input terminal of the operational amplifier (17A) and between the output terminal of the operational amplifier (17A) and the non-inverting input terminal of the operational amplifier (17B). ) And the non-inverting input terminal of the operational amplifier (17B), the resistor (19G) operates between the output terminal of the amplifier (14A) and the inverting input terminal of the operational amplifier (17B), and the resistor (19H) operates. They are respectively inserted between the output terminal and the inverting input terminal of the amplifier (17B).

Here, set the resistance values of resistors (19A) to (19H) to R _A respectively.
As to R _H, when the _{R A = R C = R E} = R G = R S ... (13) R B = R D = R F = R H = R P ... (14), each switch circuit (12) Output voltage Eo ₁ and Eo ₂ when (13) and (13) are respectively closed are Eo ₁ = (V _A −V _C ) (R _P / R _S ) ² + V _R + V _OF … (15) Eo ₂ = (V _B −V _C ) (R _P / R _S ) ² + V _R + V _OF (16) However, V _C is the voltage at the connection point C. or,
V _OF is the input offset voltage of the operational amplifiers (17A) and (17B), and is the error component caused by the deviation of each resistor (19A) to (19H) from the value given by the equations (13) and (14). It is a value that also includes.

Further, in the equations (15) and (16), V _AC = V _A −V _C = (V ₁ −V _C ) R ₄ / (R ₃ + R ₄ ) ... (17) V _BC = V _B −V _C = (V ₁ −V _C ) R ₁₁ / (R ₁₀ + R ₁₁ ) ... (18), which are the above-mentioned equations (3) and (4), that is, R ₄ = R ₄ ^* + ΔR ₄ R ₄ ^* It can be modified in consideration of / R ₃ = R ₁₁ / R ₁₀ = α. For example, when making a slight variation in the resistance deviation ΔR ₄ a problem, ΔR ₄ ≈ 0Ω, so equations (17) and (18) are approximately V _AC = α (V ₁ −V _C ). (1 + ΔR ₄ / R ₄ ^* ) / (1 + α) ...
(19) V _BC = α (V ₁ −V _C ) / (1 + α) (20)

Therefore, the difference voltage ΔEo calculated by the judgment circuit (18) is
From Eqs. (15), (16), (19), and (20), ΔEo = α (V ₁ −V _C ) (R _P / R _S ) ² ΔR ₄ / (1 + α) R ₄ ^* …
It is represented by (21). Here, in the same manner as described above, each resistance value is set to R ₃
= 1kΩ, R ₄ ^* = 3Ω, R ₁₀ = 100kΩ, R ₁₁ = 300Ω, R _P
/ R _S = 10. Further, the resistance R ₅ of the resistor _{(52), R 10 >> R} 5 (= R 3) >> if _{R 4, V 1 -V C =} V 1/2 = 10V , and the (21) The formula is ΔEo≈3ΔR ₄ / R ₄ ^* = ΔR ₄ .

Therefore, for ΔR ₄ = 0.1Ω, ΔEo = 0.1V is obtained,
It can be seen that the failure can be detected with the same sensitivity as described above.

Thus, from the difference voltage ΔEo between the output voltage Eo ₂ corresponding to the voltage V _B at the connection point B on the reference side and the output voltage Eo ₁ corresponding to the voltage V _A at the connection point A on the measurement side, the monitored resistor is By detecting the change in (4), accurate detection that is not affected by the characteristic variation of the measurement system is realized.

However, the power supply voltage V ₁ other than the measurement system fluctuates,
If the voltage V _{C at the} connection point C fluctuates due to fluctuations in the resistor (52) in the G sensor (5), the reference side voltage V _BC (= V _B −V _C ) expressed by the equation (10) becomes It fluctuates. As a result, the output voltage Eo ₂ deviates from the center value (2.5V) of the input voltage range (0 to 5V) of the judgment circuit (18), and the output voltage Eo ₁ on the connection point A side to be detected can be measured. It may run out.

Next, another invention of the present invention which can stabilize the output voltage Eo even if the conditions of the power supply voltage V ₁ other than the measurement system or the resistor (52) of the G sensor and the like change will be described.

FIG. 3 is a circuit diagram showing another embodiment of the present invention, and (1) to (18) are the same as those in FIG. However, in this case, the input terminal of the amplifier (14A) is connected to the connection point B on the reference side.

Reference numeral (20) is a voltage adjustment circuit for adjusting the output voltage Eo of the DC amplification circuit (15B) by being feedback-connected to the DC amplification circuit (15B). The voltage adjustment circuit (20) is connected to the operational amplifier (21) and the operational amplifier (21). Capacitors inserted between output terminals (22)
It consists of and.

(23) is a third switch circuit inserted between the voltage adjusting circuit (20) and the DC amplifying circuit (15B), and the second switch circuit is controlled by the control signal F ₃ from the judging circuit (18). It is designed to be opened and closed in synchronization with (13).

The output voltage Eo of the DC amplifying circuit (15B) is an inverting input terminal of the operational amplifier via a third switch circuit (23) (21) (-) is applied to the reference voltage V _R of the reference power source (14B) is ,
It is applied to the non-inverting input terminal (+) of the operational amplifier (21).

The feedback voltage V _F output from the operational amplifier (21) in the voltage adjustment circuit (20) is applied to its own inverting input terminal (−) via the capacitor (22) and the direct current amplification circuit (15B). It is applied to the non-inverting input terminal (+) of the operational amplifier (17B) through the resistor (19F) inside.

Next, the operation of the embodiment of another invention of the present invention shown in FIG. 3 will be described with reference to the flow chart of FIG. 4 and the waveform diagram of FIG.

The voltage input to the DC amplifier circuit (15B) is the voltage V _{D at} the connection point _D and the output voltage V _E (= V _B ) of the buffer amplifier, that is, the amplifier (14A).

Therefore, the output voltage Eo from the DC amplification circuit (15B) is Eo = {R _B / (R _A + R _B )} {(R _C + R _D ) / R _C } {R _F / (R _E
+ R _F )} × {(R _G + R _H ) / R _G } (V _D −V _E ) + {(R _F R _G −R _E R _H ) / R _G (R _E + R _F )} V _E + { R _C / (R _C + R _D )} {(R _E + R _F ) / R _E } V _F + V
_It is expressed as _OF … (22). Here, the resistance values R _{A to} R _H are (13),
(14) Since satisfies the equation (22) is, Eo = become ^{_{(R P / R S) 2}} (V D -V E) + V F + V OF ... (23).

On the other hand, from the judgment circuit (18), as shown in FIG.
F _{1 to} F ₃ are output, and the operation modes of the switch circuits (12), (13) and (23) are: (I) Reference voltage (Eo ₂ ) mode First switch circuit (12) is off Second , The third switch circuit (13), (23) is on (II)
Measurement voltage (Eo ₁ ) mode The first switch circuit (12) is turned on and the second and third switch circuits (13) and (23) are turned off.

First, at time t _{0 to} t ₁ , when the control signal F ₁ is turned off and the control signals F ₂ and F ₃ are turned on, the first switch circuit (12)
Is turned off and the second and third switch circuits (13) and (23) are turned on (step S1 in FIG. 4).

At this time, the operation mode is the reference voltage mode (I), and the output voltage Eo becomes the output voltage Eo ₂ corresponding to the connection point B on the reference side (step S2).

Also, since V _D = V _E = V _B due to the conduction of the second switch circuit (13), the output voltage Eo ₂ is Eo ₂ = V _F + V _OF (24) from the formula (23). Become. Furthermore, the output voltage Eo ₂ is input to the voltage adjustment circuit (20) by the conduction of the third switch circuit (23), and the operational amplifier (21) is expressed as V _F + V _OF = V _{R in the} equation (24). The feedback voltage V _F is adjusted so that As a result, the output voltage Eo ₂ is accurately adjusted to the reference voltage V _R which is the center value of the input voltage range of the determination circuit (18).

Next, at times t _{1 to} t ₂ , each control signal F _{1 to} F ₃ is inverted,
When the first switch circuit (12) is turned on and the second and third switch circuits (13) and (23) are turned off (step S
3), the operation mode becomes the measurement voltage mode (II), and the output voltage Eo becomes the output voltage Eo ₁ corresponding to the connection point A on the measurement side (step S4).

Further, since V _D = V _A V _E = V _B due to the conduction of the first switch circuit (12), the output voltage Eo ₁ is Eo ₁ = (R _P / R _S ) from the formula (23). ² (V _A −V _B ) + V _F + V _OF (25)

At this time, the third switch circuit (23) is turned off and the input of the operational amplifier (21) cannot be obtained, but the feedback voltage V _{F in the} reference operation mode (I) is held by the capacitor (22). Therefore, the more _{(24), V F = Eo 2 -V OF} = V R -V OF ... (26). Therefore, the output voltage Eo ₁ is Eo ₁ = (R _P / R _S ) ² (V _A −V _B ) + V _R (27) from the equations (25) and (26).

As is clear from the equation (27), the output voltage Eo ₁ in the measurement voltage mode is always the reference voltage V _R as the center, and the voltage difference (V _A −V _B ) between the connection points A and B of the Wheatstone bridge is amplified. The value will be changed by the specified value. Therefore, the reference voltage V _R
If is set to the center value of the input voltage range of the judgment circuit (18), the fluctuation of the power supply voltage V ₁ and the G sensors (3) and (5)
Of the voltage difference (V _A
-V _B ) can be measured, and as in the case of FIG.
A change in the resistance value of the monitored resistor (4) can be detected.

That is, it is determined whether or not the difference voltage ΔEo (= Eo ₁ −Eo ₂ ) is equal to or greater than the preset allowable fluctuation width ΔE (step S
5) When | Eo ₁ −Eo ₂ | ≧ ΔE, the drive signal H is output to turn on the alarm lamp (9) (step S 6).

On the other hand, in step S5, the difference voltage ΔEo is equal to the allowable fluctuation width Δ
If it is determined to be within the range of E, the process returns to step S1 and the same operation is repeated.

In FIG. ₅ , it is detected that the output voltage Eo ₁ exceeds the allowable fluctuation width ΔE in the section from time t _{3 to} t ₅ , and the drive signal H is turned on at time t ₄ after the delay time of the processing operation. Indicate the case.

In the above embodiment, the case where the resistance value variation of the monitored resistor (4) of one airbag system is detected has been described, but the resistance value variation of a plurality of monitored resistors can be detected simultaneously.

FIG. 6 is a circuit diagram of another embodiment of another invention of the present invention, showing an example applied to failure detection of two airbag systems.

In this case, the second airbag system consisting of the G sensors (3 '), (5') and the monitored resistor (4 '),
It is provided in parallel with the first airbag system consisting of (3) to (5).

Further, in the failure detection circuit (6D), resistors (10 ') and (for forming a second Wheatstone bridge together with the resistor in the G sensor (3') and the monitored resistor (4 '). 11 ') and first and second switch circuits (12') and (13 ') corresponding to the second Wheatstone bridge,
Further provided are fourth switch circuits (24) and (24 ') for selectively connecting the connection points B and B'of the first and second Wheatstone bridges to the amplifier (14A).

Therefore, the determination circuit (18) alternately outputs two kinds of signals to each Wheatstone bridge according to the control signals F _{1 to} F ₄ , F ₁ ′, F ₂ ′ and F ₄ ′ corresponding to each switch circuit. Switch to operation mode.

That is, when the monitored resistor (4) is monitored, the control signal F ₄ for driving the fourth switch circuit (24) is turned on because the first Wheatstone bridge is selected, and the control signal F ₁ ', F _2' and F ₄ 'are all turned off. At this time, the control signal F ₁ to F ₃ switches the operation mode as in the fifth diagram (I) and (II).

Further, the monitored resistor (4 ') when monitoring is to select the second Wheatstone bridge, the fourth switching circuit (24') the control signal F ₄ 'for driving is turned on, The control signals F ₁ , F ₂ and F ₄ are all off. At this time, the control signals F ₁ ′, F ₂ ′ and F ₃ operate in the same manner as F _{1 to} F _{3 in} FIG. 5, and there are two operation modes (I ′) and (II ′).
Switch to.

In any case, in the reference voltage mode (I) or (I '), the output voltage Eo ₂ corresponding to the connection point B or B'is obtained, and in the measurement voltage mode (II) or (II'), An output voltage Eo ₁ corresponding to the connection point A or A'is obtained.

At this time, the resistor (3 ') and the monitored resistor (4') in the second Wheatstone bridge and the resistor (1
Even if the voltages V _A ′ and V _B ′ at the connection points A ′ and B ′ are different from V _A and V _B due to the difference in the resistance ratio with respect to 0 ′) and (11 ′), the output voltage Eo ₁ is Since it fluctuates around the voltage V _R , no trouble occurs.

In each of the above embodiments, the control signal for switching and driving each switch circuit is generated by the determination circuit (18), but may be generated by another timing circuit (not shown).

Also, although the case where the monitored resistor (4) is a starting heater for an automobile airbag device is shown, the same effect can be obtained even if it is applied to another resistor as long as it has a relatively low resistance value. It goes without saying that you play.

[Effects of the Invention] As described above, according to the present invention, the first series circuit including the monitored resistor and the resistor connected to the monitored resistor at the connection point A and the mutual connection at the connection point B are connected to each other. A second series circuit formed of a pair of resistors having the same resistance ratio as the resistance ratio in the first series circuit, which is connected in parallel to the first series circuit to form a balanced Wheatstone bridge; DC power supply for supplying power to the Wheatstone bridge, first and second switch circuits individually connected to the connection points A and B and alternately opened and closed, and DC amplification connected to a common output terminal of these switch circuits Since a circuit and a determination circuit for determining the presence or absence of a fault in the monitored resistor based on the difference in the output voltage of the DC amplification circuit that is switched in synchronization with the opening and closing of each switch circuit, the input offset of the DC amplification circuit is provided. In addition to eliminating the influence of the voltage variation, circuit constants of the measurement system, and variations in the elements, fine adjustments in the manufacturing stage are not required, and an inexpensive and highly reliable fault detection device can be obtained.

Further, according to another invention of the present invention, a voltage adjusting circuit, which is feedback-connected to the DC amplifying circuit and adjusts the output voltage of the DC amplifying circuit, is inserted between the voltage adjusting circuit and the DC amplifying circuit. And a third switch circuit that opens and closes in synchronization with the second switch circuit, and when the second switch circuit is closed, the output voltage of the DC amplifier circuit is determined by the feedback of the voltage adjustment circuit. The input voltage range is adjusted to the center value, and when the first switch circuit is closed, the voltage adjustment circuit is disconnected from the DC amplification circuit while maintaining the feedback voltage. Is not affected by characteristic variations of elements other than the measurement system, and there is an effect that a more accurate and highly reliable failure detection device can be obtained.

[Brief description of drawings]

FIG. 1 is a block diagram showing an embodiment of the present invention, FIG. 2 is a block diagram showing another embodiment of the present invention, and FIG. 3 is a circuit diagram showing an embodiment of another invention of the present invention. FIG. 4 is a flow chart showing the operation of the decision circuit in FIG.
FIG. 7 is a waveform diagram for explaining the operation of FIG. 3, FIG. 6 is a circuit diagram showing another embodiment of the present invention, and FIG. 7 is a circuit diagram showing a conventional failure detection device. . (1) ... DC power supply, (4) ... Monitored resistor (6A), (6B), (6C), (6D) ... Fault detection circuit (10), (11), (32) ... Resistor (12 ) ... 1st switch circuit (13) ... 2nd switch circuit (23) ... 3rd switch circuit (15), (15B) ... DC amplification circuit (18) ... Judgment circuit, (20) ... Voltage adjustment circuit a, B ... connection point, D ... common output terminal Eo ... output voltage, the voltage of V _a ... voltage V _B ... connection point B of the connection point a in the figure, the same reference numerals denote the same or corresponding parts.

Claims (2)

[Claims] 1. A first series circuit comprising a monitored resistor and a resistor connected to the monitored resistor at a connection point A; and a first series circuit connected to each other at a connection point B in the first series circuit. A second series circuit, which comprises a pair of resistors having the same resistance ratio as the resistance ratio, and is connected in parallel to the first series circuit to form a balanced Wheatstone bridge; and for supplying power to the Wheatstone bridge. A direct current power source; first and second switch circuits that are individually connected to the connection points A and B and are alternately opened and closed; a direct current amplification circuit connected to a common output terminal of these switch circuits; And a determination circuit that determines whether or not there is a failure in the monitored resistor based on a difference in output voltage of the DC amplification circuit that is switched in synchronization with opening and closing of the second switch circuit. 2. A voltage adjusting circuit feedback-connected to the DC amplifying circuit to adjust the output voltage of the DC amplifying circuit, and a second switch circuit inserted between the voltage adjusting circuit and the DC amplifying circuit. The failure detection device according to claim 1, further comprising a third switch circuit that is opened and closed in synchronization with the third switch circuit.