Air-bag control circuit

• US 5,787,377 A
• Filed: 09/23/1994
• Issued: 07/28/1998
• Est. Priority Date: 08/24/1990
• Status: Expired due to Fees

• Current Assignee: Kanto Seiki Company Limited
• Original Assignee: Kanto Seiki Company Limited
• Inventors: Matsumori, Satoru, Tsurushima, Shin-ichiro, Kon, Kazuyasu, Saito, Yasuo, Kaneko, Kunihiro, Ohneda, Katsushi, Watanabe, Kajiro
• Primary Examiner(s): Teska, Kevin J.
• Assistant Examiner(s): Walker, Tyrone V.
• Application Number:
  US08/311,741
• Time in Patent Office: 1,404 Days
• Field of Search: 364/424.05, 307/10.1, 180/268, 280/735, 340/436, 340/669, 340/438
• US Class Current: 701/45
• CPC Class Codes: B60R 2021/01122 Prevention of malfunction
• CPC Class Codes: B60R 2021/01311 monitoring the braking system, e.g. ABS
• CPC Class Codes: B60R 2021/01315 monitoring occupant displacement
• CPC Class Codes: B60R 21/013 including means for detecting collisions, impending collisions or roll-over
• CPC Class Codes: B60R 21/0133 by integrating the amplitude of the input signal
• CPC Class Codes: B60R 21/01336 using filtering
• CPC Class Codes: B60R 21/01526 using piezoelectric elements
• CPC Class Codes: B60R 21/01542 detecting passenger motion
• CPC Class Codes: B60R 21/01544 detecting seat belt parameters, e.g. length, tension or height-adjustment
• CPC Class Codes: B60R 21/01548 sensing the amount of belt winded on retractor
• CPC Class Codes: B60R 21/01558 monitoring crash strength
• CPC Class Codes: B60R 21/0173 Diagnostic or recording means therefor
• CPC Class Codes: G01P 21/00 Testing or calibrating of apparatus or devices covered by the preceding groups

Abstract

A control circuit for an air-bag mounted in a motor vehicle comprises an acceleration sensor for producing an acceleration signal indicative of acceleration of the vehicle, an ignition timing circuit for processing the acceleration signal to predict the time when a passenger'"'"'s head will reach a front surface of the air-bag in fully deployed condition and producing a first air-bag deploying signal in accordance with the predicted time, and a damage discrimination circuit for processing the acceleration signal and judging the need of activation of the air-bag when the cumulative effect of the acceleration exceeds a predetermined limit. The damage discrimination circuit produces a second air-bag deploying signal when judging the need of activation of the air-bag. A so-called AND means is further employed which actually activates the air-bag when receiving the first and second air-bag deploying signals concurrently.

Claims

1. A control circuit for an air-bag mounted in a motor vehicle, comprising:

an acceleration sensor mounted to the vehicle for producing an acceleration signal a(t) indicative of acceleration of the vehicle;

an ignition timing circuit which processes said acceleration signal to predict a time when a passenger'"'"'s head will reach a front surface of the air-bag in a fulls deployed condition and thereafter produces a timing-based ignition signal in accordance with the predicted time; and

wherein said ignition timing circuit includes circuit means which is operatively connected to said acceleration sensor, said circuit means performing the following functions;

carrying out partial integration of the acceleration signal a(t) to produce a signal V(t) indicative of velocity;

carrying out partial integration of the velocity indicative signal V(t) to produce a first signal x(t) indicative of displacement;

weighting the acceleration signal a(t) with a factor td² /2 to produce a second signal {1/2×

a(t)×

td² } indicative of displacement;

weighting the velocity indicative signal V(t) with a time factor (td) to produce a third signal {V(t)×

td} indicative of displacement;

adding the first, second and third displacement signals together to derive a predicted displacement signal {×

(t+td)};

comparing the predicted displacement signal with a reference value; and

issuing said timing-based ignition signal in the event that the predicted displacement signal exceeds the reference value.

2. A control circuit as claimed in claim 1, wherein said partial integration of the acceleration signal is achieved by utilization of a first-order lag integral of a form T1/(T1S+1), where T1 is a constant and S is a Laplace operator, and wherein said partial integration of the velocity indicative signal is achieved by utilization of the first-order lag integral of a form T2/(T2S+1), where T2 is a constant and S is the Laplace operator.

3. A control circuit for an air-bag mounted in a motor vehicle, comprising:

an acceleration sensor for producing an acceleration signal indicative of acceleration of the vehicle;

an ignition timing circuit which processes said acceleration signal to predict a position of a passenger'"'"'s head which will be taken after a given period from a given time and thereafter produces a timing-based ignition signal in accordance with the predicted position of the passenger'"'"'s head; and

an activation circuit which activates said air-bag in accordance with said timing-based ignition signal,

wherein said ignition timing circuit is operatively connected to said acceleration sensor for calculating the predicted position by using a prediction equation which includes partial integration.

4. A control circuit as claimed in claim,3, in which said partial integration is performed by utilization of the first-order lag integral of a form T/(TS+1), wherein T is a constant and S is the Laplace operator.

5. A control circuit as claimed in claim 4, in which said prediction equation is represented by:

space="preserve" listing-type="equation">x(t+td)={x(t)}+{V(t)×

td}+{1/2×

a(t)×

td.sup.2 }

wherein;

x(t+td) is a predicted displacement,

{x(t)} is a displacement value obtained by effecting partial integration of a velocity which is obtained by effecting partial integration of said acceleration signal,

{V(t)×

td} is a displacement value obtained by weighting the velocity, and

{1/2×

a(t)×

td² } is a displacement value obtained by weighting the acceleration,

and wherein said prediction equation does not include a derivative value of said acceleration.

6. A control circuit as claimed in claim 5, further comprising a low pass filter through which said prediction circuit is connected to said acceleration sensor.

7. A control circuit as claimed in claim 4, further comprising a G-level discrimination circuit which judges the level of acceleration sensed by said acceleration sensor and controls said ignition timing circuit in accordance with said level of acceleration.

8. A control circuit as claimed in claim 7, further comprising a switch which is disposed between said low pass filter and said ignition timing circuit, said switch being turned ON when the G-level discrimination circuit judges that said level of acceleration exceeds a predetermined reference value.

9. A control circuit as claimed in claim 8, in which said G-level discrimination circuit comprises:

a plurality of comparators, each comparator comparing the level of the sensed acceleration with a reference level; and

timer means which issues a reset signal to turn OFF said switch upon expiration of a set time,

wherein when the level of the sensed acceleration exceeds a predetermined reference level, said timer means starts to count the set time.

10. A control circuit as claimed in claim 4, further comprising:

a damage discrimination circuit which processes said acceleration signal and judges a need of activation of the air-bag when a cumulative effect of the acceleration exceeds a predetermined limit, said damage discrimination circuit producing a damage-based ignition signal when judging the need of activation of said air-bag; and

an AND gate circuit for activating said air-bag when receiving said timing-based and damage-based ignition signals concurrently.

11. A control circuit as claimed in claim 10, further comprising resetting means for resetting said damage discrimination circuit upon receiving a given instruction signal from a G-level discrimination circuit which Judges the level of acceleration sensed by said acceleration sensor and controls said ignition timing circuit in accordance with said level of acceleration.

12. A control circuit as claimed in claim 10, further comprising

a G-level discrimination circuit which judges when the level of acceleration exceeds a maximum reference value;

and

an OR gate circuit which issues an instruction signal to said AND gate circuit when said G-level discrimination circuit judges that the level of acceleration exceeds the maximum reference value or said damage discrimination circuit judges the need of activation of the air-bag.

13. A control circuit as claimed in claim 4, further comprising:

brake operation sensing means which issues a brake operation signal when a brake pedal of the vehicle has been depressed; and

control means which controls said timing-based ignition signal to be issued at a relatively slow timing when receiving said brake operation signal.

14. A control circuit as claimed in claim 13, further comprising:

a comparator circuit provided in said ignition timing circuit, said comparator circuit comparing said timing-based ignition signal with a reference value which corresponds to said predetermined degree, said comparator circuit issuing a comparator signal for controlling said reference value; and

a reference value controlling circuit which changes the reference value of said comparator circuit depending on whether the brake pedal is depressed or not.

15. A control circuit for an air-bag mounted in a motor vehicle comprising:

an acceleration sensor mounted on the vehicle for producing an acceleration signal indicative of acceleration of the vehicle;

an ignition-timing circuit which calculates a predicted displacement {x(t+td) } of a -passenger'"'"'s head by integrating said acceleration signal and issues a timing-based ignition signal when the predicted displacement {x(t+td) } reaches a predetermined degree (X) representing a time when the passenger'"'"'s head will reach a front surface of the air-bag in a fully deployed condition;

a damage discrimination circuit which processes said acceleration signal to issue a damage-based ignition signal when a cumulative effect of said acceleration exceeds a predetermined limit, said damage-based ignition signal being issued Prior to said timing-based ignition signal;

a date circuit which activates the air-bag with a timing decided by said timing-based ignition signal only when said damage discrimination circuit issues said damage-based ignition signal deciding the activation of the air-bag and said ignition timing circuit issues said timing-based ignition signal deciding the activation of the air-bag the ignition timing circuit performing the calculation of the displacement concurrently with the damage discrimination circuit determining whether or not to issue the damage-based ignition signal;

a G-level discrimination circuit which judges the level of acceleration sensed by said acceleration sensor and controls said ignition timing circuit in accordance with said level of acceleration;

a low pass filter through which said ignition timing circuit is connected to said acceleration sensor; and

switch means which is disposed between said low pass filter and said ignition timing circuit, said switch means being turned ON when the G-level discrimination circuit judges that said level of acceleration exceeds a predetermined reference level.

16. A control circuit as claimed in claim 15, further comprising resetting means for resetting said damage discriminating circuit upon receiving a given instruction signal from said G-level discrimination circuit.

17. A control circuit as claimed in claim 15, wherein said G-level discrimination circuit comprises:

a plurality of comparators, each comparator comparing the level of the sensed acceleration with a reference level; and

timer means which issues a reset signal to turn OFF said switch means upon expiration of a set time,

wherein, when the level of the sensed acceleration exceeds a predetermined reference level, said timer means starts to count the set time.

18. A control circuit as claimed in claim 15, further comprising:

seat position detecting means issuing a seat position signal indicative of a fore-and-aft position of a seat mounted in the vehicle; and

control means which, in accordance with said seat position signal, controls said timing-based ignition signal in such a manner that when said seat assumes a relatively front position, said timing-based ignition signal is issued at a relatively fast timing, and when said seat assumes a relatively rear position, said timing-based ignition signal is issued at a relatively slow timing.

19. A control circuit as claimed in claim 18, further comprising a comparator circuit possessed by said ignition timing circuit, said comparator circuit comparing said timing-based ignition signal with a reference value which corresponds to said predetermined degree, said comparator circuit issuing a comparator signal to control said reference value, said reference value of the comparator circuit being controlled by said control means in such a manner that when said seat assumes the relatively front position, said reference value is lowered, and when said seat assumes the relatively rear position, said reference value is increased.

20. A control circuit for an air-bag mounted in a motor vehicle, comprising:

an acceleration sensor mounted on the vehicle for producing an acceleration signal indicative of acceleration of the vehicle;

an ignition timing circuit which calculates a predicted displacement {x(t+td)} of a passenger'"'"'s head by carrying out partial integration of said acceleration signal and issues a timing-based ignition signal when the displacement {x(t) +td)} reaches a predetermined decree (X) representing a time when the passenger'"'"'s head will reach a front surface of the air-bag in a fully deployed condition;

a damage discrimination circuit which processes said acceleration signal to issue a damage-based ignition signal when a cumulative effect of said acceleration exceeds a predetermined limit, said damage-based ignition signal being issued prior to said timing-based ignition signal; and

gate means for activating the air-bag with a timing decided by said timing-based ignition signal only when said damage discrimination circuit issues said damage-based ignition signal deciding the activation of the air-bag and said ignition timing circuit issues said timing-based ignition signal deciding the activation of the air-bag,

wherein the ignition timing circuit performs the calculation of the displacement concurrently with the damage discrimination circuit determining whether or not to issue the damage-based ignition signal, and

wherein said ignition timing circuit comprises integrating circuit means, coefficient circuit means, and ADDER circuit means.

21. A control circuit as claimed in claim 20, further comprising a comparator circuit which receives output from said ADDER circuit means, said comparator judging the predicted position of the passenger'"'"'s head when the output from said ADDER circuit means exceeds a predetermined level.

22. A control circuit for an air-bag mounted in a motor vehicle, comprising:

an acceleration sensor mounted on the vehicle for producing an acceleration signal indicative of acceleration of the vehicle;

an ignition timing circuit which calculates a predicted displacement {x(t+td)} of a passenger'"'"'s head by integrating said acceleration signal without effecting a differentiation of the same and issues a timing-based ignition signal when the predicted displacement {x(t+td)} reaches a predetermined degree (X) representing a time when the passenger'"'"'s head will reach a front surface of the air-bag in a fully deployed condition;

a damage discrimination circuit which processes said acceleration signal to issue a damage-based ignition signal when a cumulative effect of said acceleration exceeds a predetermined limit, said damage-based ignition signal being issued prior to said timing-based ignition signal;

gate means for activating the air-bag with a timing decided by said timing-based ignition signal only when said damage discrimination circuit issues said damage-based ignition signal deciding the activation of the air-bag, and said ignition timing circuit issues said timing-based ignition signal deciding the activation of the air-bag, the ignition timing circuit performing the calculation of the displacement concurrently with the damage discrimination circuit determining whether or not to issue the damage-based ignition signal;

brake operation means which issues a brake operation signal when a brake pedal of the vehicle has been depressed;

control means which controls said timing-based ignition signal to be issued at a relatively slow or delayed timing when receiving said brake operation signal;

a comparator circuit provided in said ignition timing circuit, said comparator circuit comparing said timing-based ignition signal with a reference value which corresponds to said predetermined degree, said comparator circuit issuing a comparator signal for controlling said reference value;

a reference value controlling circuit which chances the reference value of said comparator circuit depending on whether the brake pedal is depressed or not,

wherein said reference value controlling circuit comprises;

first and second resistors which are arranged in series between an electric power line and ground;

a series circuit including a first switching transistor and a third resistor which is arranged in parallel with said first resistor; and

a second switching transistor which is turned ON upon depression of said brake pedal thereby turning said first switching transistor ON, and in which a potential at a junction point between said first and second resistors is used as the reference value of said comparator circuit.

23. A control circuit for an air-bag mounted in a motor vehicle, comprising:

an acceleration sensor mounted on the vehicle for producing an acceleration signal indicative of acceleration of the vehicle;

an ignition timing circuit which calculates a predicted displacement {x(t+td)} of a passenger'"'"'s head by integrating said acceleration signal without effecting a differentiation of the same and issues a timing-based ignition signal when the predicted displacement {x(t+td)} reaches a Predetermined degree (X) representing a time when the passenger'"'"'s head will reach a front surface of the air-bag in a fully deployed condition;

a damage discrimination circuit which processes said acceleration signal to issue a damage-based ignition signal when a cumulative effect of said acceleration exceeds a predetermined limit, said damage-based ignition signal being issued prior to said timing-based ignition signal;

gate means for activating air-bag with a timing decided by said timing-based ignition signal only when said damage discrimination circuit issues said damage-based ignition signal deciding the activation of the air-bag and said ignition timing circuit issues said timing-based ignition signal deciding the activation of the air-bag, the ignition timing circuit performing the calculation of the displacement concurrently with the damage discrimination circuit determining whether or not to issue the damage-based ignition signal;

seatback inclination angle detecting means which issues an inclination angle signal indicative of an inclination angle assumed by a seatback of a reclining seat mounted on the vehicle; and

control means which, in accordance with said inclination angle signal, controls said timing-based ignition signal in such a manner that when said inclination angle is smaller than a first predetermined inclination angle, said timing-based ignition signal is issued at a first timing, and when said inclination angle is larger than a second predetermined inclination angle, said timing-based ignition signal is issued at a second timing.

24. A control circuit as claimed in claim 23, further comprising a comparator circuit provided in said ignition timing circuit, said comparator circuit comparing said timing-based ignition signal with a reference value which corresponds to said predetermined degree, said comparator circuit issuing a comparator signal to control said reference value, said reference value of the comparator circuit being controlled by said control means in such a manner that when the inclination angle is smaller than the first predetermined inclination angle, said reference value is lowered, and when said inclination angle is larger than the second predetermined inclination angle, said reference value is increased.

25. A control circuit for an air-bag mounted in a motor vehicle, comprising:

an acceleration sensor mounted on the vehicle for producing an acceleration signal indicative of acceleration of the vehicle;

an ignition timing circuit which calculates a predicted displacement {x(t+td)} of a passenger'"'"'s head by integrating said acceleration signal without effecting a differentiation of the same and issues a timing-based ignition signal when the predicted displacement {x(t+td)} reaches a predetermined decree (X) representing a time when the passenger'"'"'s head will reach a front surface of the air-bag in a fully deployed condition;

a damage discrimination circuit which processes said acceleration signal to issue a damage-based ignition signal when a cumulative effect of said acceleration exceeds a predetermined limit, said damage-based ignition signal being issued prior to said timing-based ignition signal;

gate means for activating air-bag with a timing decided by said timing-based ignition signal only when said damage discrimination circuit issues said damage-based ignition signal deciding the activation of the air-bag and said ignition timing circuit issues said timing-based ignition signal deciding the activation of the air-bag, the ignition timing circuit performing the calculation of the displacement concurrently with the damage discrimination circuit determining whether or not to issue the damage-based ignition signal;

a seatbelt drawing degree detecting means which issues a belt drawing degree signal indicative of a degree by which a seatbelt is drawn from a seatbelt retracting shaft of a seatbelt retractor mounted in the vehicle; and

control means which, in accordance with said belt drawing degree signal, controls said timing-based ignition signal in such a manner that when the seatbelt is retracted by said seatbelt retracting shaft by a relatively small degree, said timing-based ignition signal is issued at a relatively fast timing, and when said seatbelt is retracted by said retracting shaft by a relatively large degree, said timing-based ignition signal is issued at a relatively slow timing.

26. A control circuit as claimed in claim 25, wherein said ignition timing circuit further comprises a comparator circuit, said comparator circuit comparing said timing-based ignition signal with a reference value which corresponds to said predetermined degree, said comparator circuit issuing a comparator signal to control said reference value.

27. A control circuit as claimed in claim 26, wherein said seatbelt drawing degree detecting means comprises a variable resistor which changes the reference value of the comparator circuit when actuated, a resistance of said variable resistor being varied in accordance with the length of the seatbelt which is drawn out from said seatbelt retracting shaft, and

wherein said seatbelt drawing degree detecting means has two blind zones within which the resistance value of the variable resistor is kept constant even when the seatbelt is drawn out from said seatbelt retracting shaft.

28. A control circuit for an air-bag mounted in a motor vehicle, comprising:

an acceleration sensor mounted on the vehicle for producing an acceleration signal indicative of acceleration of the vehicle;

an ignition timing circuit which calculates a predicted displacement {x(t+td)} of a passenger'"'"'s head by integrating said acceleration signal and issues a timing-based ignition signal when the predicted displacement {x(t+td)} reaches a predetermined degree (X) representing a time when the passenger'"'"'s head will reach a front surface of the air-bag in a fully deployed condition;

a damage discrimination circuit which processes said acceleration signal to issue a damage-based ignition signal when a cumulative effect of said acceleration exceeds a predetermined limit, said damage-based ignition signal being issued prior to said timing-based ignition signal; and

gate means for activating the air-bag with a timing decided by said timing-based ignition signal only when said damage discrimination circuit issues said damage-based ignition signal deciding the activation of the air-bag and said ignition timing circuit issues said timing-based ignition signal deciding the activation of the air-bag,

wherein the ignition timing circuit performs the calculation of the displacement concurrently with the damage discrimination circuit determining whether or not to issue the damage-based ignition signal, and

wherein said ignition timing circuit includes circuit means which is operatively connected to said acceleration sensor, said circuit means performing the following functions;

integrating the acceleration signal a(t) to produce a signal V(t) indicative of velocity;

integrating the velocity indicative signal V(t) to produce a first signal x(t) indicative of displacement;

weighting the acceleration indicative signal a(t) with a factor (t² d/2 to produce a second signal {1/2×

a(t)×

t² d} indicative of displacement; and

adding the first, second and third displacement signals together to derive a predicted displacement signal {x(t +td)},

wherein the integrating of the acceleration signal a(t) is performed by a first partial integration by utilization of a first-order lag integral of a form T1/(T1S+1), where T1 is a constant and S is a Laplace operator, and

wherein the integrating of the velocity indicative signal V(t) is performed by a second partial integration by utilization of the first-order lag integral of a form T2 /(T2S+1), where T2 is a constant and S is the Laplace operator.

29. A control circuit for an air-bag mounted in a motor vehicle, comprising:

an acceleration sensor mounted on the vehicle for producing an acceleration signal indicative of acceleration of the vehicle;

an ignition timing circuit which calculates a predicted displacement {x(t+td)} of a passenger'"'"'s head by integrating said acceleration signal without effecting a differentiation of said acceleration signal and issues a timing-based ignition signal when the predicted displacement {x(t+td)} reaches a predetermined degree (X) representing a time when the passenger'"'"'s head will reach a front surface of the air-bag in a fully deployed condition;

a damage discrimination circuit which processes said acceleration signal to issue a damage-based ignition signal when a cumulative effect of said acceleration exceeds a predetermined limit, said damage-based ignition signal being issued prior to said timing-based ignition signal; and

a gate circuit which activates the air-bag with a timing decided by said timing-based ignition signal only when said damage discrimination circuit issues said damage-based ignition signal deciding the activation of the air-bag, and said ignition timing circuit issues said timing-based ignition signal deciding the activation of the air-bag, the ignition timing circuit performing the calculation of the displacement concurrently with the damage discrimination circuit determining whether or not to issue the damage-based ignition signal,

a brake operation circuit which issues a brake operation signal when a brake pedal of the vehicle has been depressed;

a control circuit which controls said timing-based ignition signal to be issued at a relatively slow or delayed timing when receiving said brake operation signal;

a comparator circuit provided in said ignition timing circuit, said comparator circuit comparing said timing-based ignition signal with a reference value which corresponds to said predetermined degree, said comparator circuit issuing a comparator signal for controlling said reference value;

a reference value controlling circuit which changes the reference value of said comparator circuit depending on whether the brake pedal is depressed or not,

wherein said reference value controlling circuit comprises;

first and second resistors which are arranged in series between an electric power line and ground;

a series circuit including a first switching transistor and a third resistor which is arranged in parallel with said first resistor; and

a second switching transistor which is turned ON upon depression of said brake pedal thereby turning said first switching transistor ON, and in which a potential at a junction point between said first and second resistors is used as the reference value of said comparator circuit.

30. A control circuit for an air-bag mounted in a motor vehicle, comprising:

an acceleration sensor mounted on the vehicle for producing an acceleration signal indicative of acceleration of the vehicle;

an ignition timing circuit which calculates a predicted displacement {x(t+td)} of a passenger'"'"'s head by integrating said acceleration signal without effecting a differentiation of said acceleration signal and issues a timing-based ignition signal when the predicted displacement {x(t+td)} reaches a predetermined degree (X) representing a time when the passengers head will reach a front surface of the air-bag in a fully deployed condition;

a damage discrimination circuit which processes said acceleration signal to issue a damage-based ignition signal when a cumulative effect of said acceleration exceeds a predetermined limit, said damage-based ignition signal being issued prior to said timing-based ignition signal; and

a gate circuit which activates air-bag with a timing decided by said timing-based ignition signal only when said damage discrimination circuit issues said damage-based ignition signal deciding the activation of the air-bag and said ignition timing circuit issues said timing-based ignition signal deciding the activation of the air-bag, the ignition timing circuit performing the calculation of the displacement concurrently with the damage discrimination circuit determining whether or not to issue the damage-based ignition signal;

a seatback inclination angle detecting circuit which issues an inclination angle signal indicative of an inclination angle assumed by a seatback of a reclining seat mounted on the vehicle; and

a control circuit which, in accordance with said inclination angle signal, controls said timing-based ignition signal in such a manner that when said inclination angle is smaller than a first predetermined inclination angle, said timing-based ignition signal is issued at a first timing, and when said inclination angle is larger than a second predetermined inclination angle, said timing-based ignition signal is issued at a second timing.

31. A control circuit as claimed in claim 30, further comprising a comparator circuit provided in said ignition timing circuit, said comparator circuit comparing said timing-based ignition signal with a reference value which corresponds to said predetermined degree, said comparator circuit issuing a comparator signal to control said reference value, said reference value of the comparator circuit being controlled by said control circuit in such a manner that when the inclination angle is smaller than the first predetermined inclination angle, said reference value is lowered, and when said inclination angle is larger than the second predetermined inclination angle, said reference value is increased.

32. A control circuit for an air-bag mounted in a motor vehicle, comprising:

an acceleration sensor mounted on the vehicle for producing an acceleration signal indicative of acceleration of the vehicle;

an ignition timing circuit which calculates a predicted displacement {x(t+td)} of a passenger'"'"'s head by integrating said acceleration signal without effecting a differentiation of said acceleration signal and issues a timing-based ignition signal when the predicted displacement {x(t+td)} reaches a predetermined degree (X) representing a time when the passenger'"'"'s head will reach a front surface of the air-bag in a fully deployed condition;

a damage discrimination circuit which issues a damagebased ignition signal when a cumulative effect of said acceleration exceeds a predetermined limit, said damage-based ignition signal being issued prior to said timing-based ignition signal;

a gate circuit which activates the air-bag with a timing decided by said timing-based ignition signal only when said damage discrimination circuit issues said damage-based ignition signal deciding the activation of the air-bag and said ignition timing circuit issues said timing-based ignition signal deciding the activation of the air-bag, the ignition timing circuit performing the calculation of the displacement concurrently with the damage discrimination circuit determining whether or not to issue the damage-based ignition signal;

a seatbelt drawing degree detecting circuit which issues a belt drawing degree signal indicative of a degree by which a seatbelt is drawn from a seatbelt retracting shaft of a seatbelt retractor mounted in the vehicle; and

a control circuit which, in accordance with said belt drawing degree signal, controls said timing-based ignition signal in such a manner that when the seatbelt is retracted by said seatbelt retracting shaft by a relatively small degree, said timing-based ignition signal is issued at a relatively fast or advanced timing, and when said seatbelt is retracted by said retracting shaft by a relatively large degree, said timing-based ignition signal is issued at a relatively slow or delayed timing.

33. A control circuit as claimed in claim 32, wherein said ignition timing circuit further comprises a comparator circuit, said comparator circuit comparing said timing-based ignition signal with a reference value which corresponds to said predetermined degree, said comparator circuit issuing a comparator signal to control said reference value.

34. A control circuit as claimed in claim 33, wherein said seatbelt drawing degree detecting circuit comprises a variable resistor which changes the reference value of the comparator circuit when actuated, a resistance value of said variable resistor being varied in accordance with the length of the seatbelt which is drawn out from said seatbelt retracting shaft, and

wherein said seatbelt drawing degree detecting circuit has two blind zones within which the resistance value of the variable resistor is kept constant even when the seatbelt is drawn out from said seatbelt retracting shaft.

35. A control circuit for an air-bag mounted in a motor vehicle, comprising:

an acceleration sensor mounted on the vehicle for producing an acceleration signal indicative of acceleration of the vehicle;

an ignition timing circuit which calculates a predicted displacement {x(t+td)} of a passengers head by effecting partial integration of said acceleration signal without effecting a differentiation of said acceleration signal and issues a timing-based ignition signal when the predicted displacement {x(t) +td)} reaches a predetermined degree (X) representing a time when the passenger'"'"'s head will reach a front surface of the air-bag in a fully deployed condition;

a brake operation circuit which issues a brake operation signal when a brake pedal of the vehicle has been depressed;

a control circuit which controls said timing-based ignition signal to be issued at a relatively slow or delayed timing when receiving said brake operation signal;

a comparator circuit provided in said ignition timing circuit, said comparator circuit comparing said timing-based ignition signal with a reference value which corresponds to said predetermined degree, said comparator circuit issuing a comparator signal for controlling said reference value; and

a reference value controlling circuit which changes the reference value of said comparator circuit depending on whether the brake pedal is depressed or not,

wherein said reference value controlling circuit comprises;

first and second resistors which are arranged in series between an electric power line and ground;

a series circuit including a first switching transistor and a third resistor which is arranged in parallel with said first resistor; and

a second switching transistor which is turned ON upon depression of said brake pedal thereby turning said first switching transistor ON, and in which a potential at a junction point between said first and second resistors is used as the reference value of said comparator circuit.

36. A control circuit for an air-bag mounted in a motor vehicle, comprising:

an acceleration sensor mounted on the vehicle for producing an acceleration signal indicative of acceleration of the vehicle;

an ignition timing circuit which calculates a predicted displacement {x(t+td)} of a passenger'"'"'s head by effecting partial integration of said acceleration signal without effecting a differentiation of said acceleration signal and issues a timing-based ignition signal when the displacement {x(t+td)} reaches a predetermined degree (X) representing a time when the passenger'"'"'s head will reach a front surface of the air-bag in a fully deployed condition;

a seatback inclination angle detecting circuit which issues an inclination angle signal indicative of an inclination angle assumed by a seatback of a reclining seat mounted on the vehicle; and

a control circuit which, in accordance with said inclination angle signal, controls said timing-based ignition signal in such a manner that when said inclination angle is smaller than a first predetermined inclination angle, said timing-based ignition signal is issued at a first timing, and when said inclination angle is larger than a second predetermined inclination angle, said timing-based ignition signal is issued at a second timing.

37. A control circuit for an air-bag mounted in a motor vehicle, comprising:

an acceleration sensor mounted on the vehicle for producing an acceleration signal indicative of acceleration of the vehicle;

an ignition timing circuit which calculates a predicted displacement {x(t+td)} of a passenger'"'"'s head by effecting partial integration of said acceleration signal without effecting a differentiation of said acceleration signal and issues a timing-based ignition signal when the displacement {x(t+td)} reaches a predetermined degree (X) representing a time when the passenger'"'"'s head will reach a front surface of the air-bag in a fully deployed condition;

a seatbelt drawing degree detecting circuit which issues a belt drawing degree signal indicative of a degree by which a seatbelt is drawn from a seatbelt retracting shaft of a seatbelt retractor mounted in the vehicle; and

a control circuit which, in accordance with said belt drawing degree signal, controls said timing-based ignition signal in such a manner that when the seatbelt is retracted by said seatbelt retracting shaft by a relatively small degree, said timing-based ignition signal is issued at a relatively fast or advanced timing, and when said seatbelt is retracted by said retracting shaft by a relatively large degree, said timing-based ignition signal is issued at a relatively slow or delayed timing.