This invention is a new simplified design of crash sensors for use with vehicle passive restraint systems such as air bags. In the above cross referenced patent applications, it is disclosed that a crash sensor can be constructed with a configuration of a square or rectangular flapper swinging inside a closed passage. It is also disclosed that such sensors can be made of plastic by a molding process. This present invention provides further simplifications and improvements on the previous designs and in particular with regard to undamped spring mass sensors for mounting in the passenger compartment and their use in a sensor system. This invention is also an improvement of above referenced U.S. Pat. No. 4,580,810 of Thuen and the entire contents of that patent are hereby included herein by reference. This patent disclosed an all mechanical air bag system using an air damped sensor. The present invention solves certain newly discovered problems of this all mechanical air bag system as explained below.
During a crash and in particular at the time that a crash sensor must decide whether to trigger the deployment of a passive restraint system such as an air bag, a vehicle can be divided into two parts; the crush zone which is that portion of the vehicle which has substantially changed its velocity and the non-crush zone which is the remainder of the vehicle. In a typical 30 MPH barrier crash, for example, the crash sensor must trigger deployment of the air bag in about 20 milliseconds for a typical full size American car. At this time the vehicle has typically crushed about 10 to 12 inches measured from the point on the vehicle which first contacted the barrier. A sensor designed to sense a crash in the crush zone will typically require a velocity change of about 10 MPH to trigger while non-crush zone mounted sensors must typically trigger on a 2 to 4 MPH velocity change in a 30 MPH barrier crash.
Crush zone sensors are typically mounted on the radiator support while non-crush zone sensors are typically mounted in the passenger compartment on the firewall, under the seat, or on the transmission tunnel, for example. In particular, crush zone sensors are usually mounted on the front surface of the radiator support and are actuated when struck by crushed materials which are forced rearward in the crash by the object being struck. For this reason, a crush zone sensor must be sufficiently large so that it will be struck by the crushed material with a high probability. A very small sensor, such as disclosed in this invention, could not be used in the crush zone since folds, wrinkles and voids in the crushed material could span the sensor delaying its functioning. Crush zone sensors typically project as much as 2 inches in front of the radiator which increases the response time of the sensor by about 4 milliseconds in a 30 MPH barrier crash. Since the radiator is frequently on the border of the crush zone for many crashes, this forward projection and resulting faster response time sometimes becomes important. If the sensor were small and were projected forward on a special bracket, for example, it would run the risk of being missed by the crushed material or of being rotated if not hit squarely. A prime advantage of spring mass sensors, particularly of the cantilevered and hinged mass type, is that they can be made very small which is of no value for crush zone applications but very useful for non-crush zone or passenger compartment mounting locations. Also, since the non-crush zone mounted sensor will not be impacted during the crash, it does not have to be protected by a metal can. It can be mounted on a printed circuit board for example and the sealing requirements are much less severe. For a more detailed discussion of the differences in crush zone and non-crush zone sensing, refer to Breed, D. S. and Castelli, V., Problems in Design and Engineering of Air Bag Systems, SAE Paper No. 880724 which is included herein by reference.
Crash sensors can be divided into three categories depending on their mounting location and intended function. Crush zone and non-crush zone mounted discriminating sensors determine that the vehicle is in a crash and that the restraint system should be deployed. Usually a sensor system also has an arming or safing sensor which functions to validate that the whole vehicle is decelerating at a rate in excess of that which accompanies braking. This is to prevent a momentary hammer blow on the crush zone sensor, for example, from deploying the air bag.
Spring mass sensors have been designed for use both in the crush zone and in the non-crush zone. However, for a variety of reasons arising out of the severe vibration environment, spring mass sensors have not proven successful for crush zone locations whereas they have had some limited success for non-crush zone locations particularly for arming or safing sensor applications where the sensor accuracy is not critical. Spring mass sensors of the flat spring cantilevered type, such as disclosed in U.S. Pat. No. 4,249,046, have been designed for the crush zone but not for the non-crush zone where round spring cantilevered sensors have been attempted. One reason that spring mass sensors are seldom used as discriminating sensors in the non-crush zone is that if they are not carefully designed, they can trigger late particularly in soft crashes. A more important reason, which has not been appreciated until now, is that most spring mass sensors, and particularly the cantilevered type, are sensitive to cross-axis vibrations which has now been shown to have a significant effect on the calibration of most electro-mechanical sensors. The focus of this invention is to provide both non-crush zone mounted spring mass discriminating and arming sensors of the hinged mass type which are insensitive to cross axis vibrations and which are primarily used in conjunction with crush zone mounted discriminating sensors.
The all mechanical air bag system (AMS) as disclosed in above referenced U.S. Pat. No. 4,580,810 uses an air damped ball-in-tube sensor which has recently also found to be significantly affected by cross axis vibrations. This sensitivity to cross axis vibrations was one factor causing a major automobile manufacturer to use an self contained air bag system employing an electronic sensor instead of the system of U.S. Pat. No. 4,580,810. Another focus of this invention, therefore, is to eliminate this sensitivity to cross axis vibration for AMS systems.
Current non-crush zone or passenger compartment mounted crash sensors can be classified into three categories: spring-mass, electronic, and damped. This invention is in primarily in the first category, however some aspects may be applicable to damped sensors as well. A sensing mass in the shape of a flapper is disclosed in the aforementioned cross referenced patent applications. The flapper, which is described below, is coupled with and arranged to move in a housing. The flapper is biased by a spring or magnet toward a first position in the housing. When the sensor is installed at an appropriate location on a vehicle and a crash occurs, the flapper moves toward a second position in the housing. If the crash pulse is of enough magnitude and duration, an electrical circuit is closed to initiate deployment of the protection apparatus associated with the sensing device. During the motion of the flapper, gas is forced to flow through the gap between the flapper and the housing which imparts a damping force on the motion of the sensing mass. This damping results in a fast acting sensor which is exceeded in response time only by some electronic sensors. In many applications, especially when the vehicle occupants are wearing seat belts or where the sensor serves as a backup to crush zone crash sensors, a slight delay in triggering can be tolerated and the damping disclosed in the previous patent applications can be eliminated resulting in a sensor which is simpler and less expensive to manufacture and where the sensing mass no longer must have a planar shape.
The configuration of some of the sensors disclosed in the above referenced patent applications consists of a rectangular flapper in a rectangular housing. A flapper, which is the mass for sensing the acceleration of the crash, is a planar member having a thickness in the sensing direction which is much less than its width or height and is arranged to rotate relative to the housing. The flapper is coupled with the housing by a thin hinge on the edge of the flapper, by a knife edge support or other means. The axis of the housing is parallel to or aligned with the desired crash detecting direction. For example, if the sensor is to be used for frontal impact sensing, the sensor should be installed to have the axis of the housing approximately parallel to the front-rear direction of the vehicle. The flapper is arranged to rotate along an axis perpendicular to the axis of the housing.
The electro-mechanical version of the sensor (EMS) of the present invention differs from the prior art in that the damping has been eliminated. The sensing mass is biased by, but not supported by, a cantilevered beam contact and does not necessarily have the flat rectangular or circular shape of the previous inventions. In this manner the simplest configuration results. In the preferred configuration, it is supported by a rod, interlocking hinge or by pivots. The all mechanical version (AMS) is similar except the biasing spring is not a contact and a firing pin is released in response to a rotation of the sensing mass.
Some previous designs of simple spring mass sensors as shown, for example, in U.S. Pat. No. 4,262,177 of Paxton et al, have used a wire for the mass support with the result that the sensing mass was responsive to cross-axis accelerations. The sensor disclosed in Paxton is also sensitive to lateral velocity changes which can result in an air bag deployment even though the longitudinal velocity change is below the deployment threshold. This event could happen, for example, in a side impact when the struck car exhibits a small forward velocity change. Such a deployment could result in the air bag not being available for protecting the occupant in a subsequent accident. Air bags are designed to cushion the forward impact of an occupant with the passenger compartment and the decision to deploy the air bag should not be affected by lateral accelerations or velocity changes.
Cross-axis accelerations are those accelerations perpendicular to the preferred motion of the sensing mass. For example, for a vehicle crash sensor for sensing frontal crashes, cross-axis accelerations would be in the vertical and lateral directions. One implementation of the present invention uses a flat beam or other construction as the first contact and to bias a pivoted sensing mass. This construction is superior to all other spring mass sensors except the flapper designs disclosed in the above referenced patent applications, in resistance to cross axis vibrations. The importance of cross axis vibrations has not been appreciated by those designing spring mass sensors and partially explains why the flapper design has not been used for passenger compartment mounted sensors. Particular attention must be paid to the support of the mass to render motion of the mass insensitive to cross axis vibrations which is a key feature of the flapper sensors and of the sensors of this invention.
Recent studies have shown that cross axis vibrations with magnitudes up to 80 to 90 G's are common in marginal crashes in the crush zone and up to 40 to 50 g's in the passenger compartment. For these marginal crashes, the average longitudinal acceleration in the crush zone is three to four times the value in the passenger compartment. Thus, cross axis vibrations are relatively more significant in the passenger compartment and the sensor design must take this environment into account to prevent performance deterioration.
The particular effect of cross axis vibrations on electro-mechanical sensors depends on the sensor type. These vibrations cause the ball in ball-in-tube sensors, such as disclosed in Breed U.S. Pat. No. 4,329,549 and Thuen U.S. Pat. No. 4,580,810, to orbit around inside the tube which can prevent the ball from rolling down the tube and result in sliding friction which can decrease the sensitivity of the sensor by 20% or more. This increases the risk of a late air bag deployment and injury to an occupant who has become out-of-position due to the crash, or of no deployment when an air bag is required. Also, these sensors have been shown to trigger on cross axis vibrations alone even in the absence of a longitudinal pulse.
Other sensors which have sliding masses similarly can be significantly influenced by increased friction forces. Still other sensors have masses which exhibit complicated vibratory motions when subjected to cross axis vibrations which can result in a late deployment or intermittent contact closures causing either an unwanted or a late deployment. For a further discussion of cross axis vibrations, refer to Breed, D. S. and Castelli, V., Are Barrier Crashes Sufficient for Evaluating Air Bag Sensor Performance, SAE Paper No. 900548, and Breed, D. S. and Castelli, V. A New Automobile Crash Sensor Tester, SAE paper No. 910655, which are included herein by reference.
Other attempts have been made to construct a spring mass sensor using a cantilevered beam. Representative of such efforts is a sensor construction disclosed in U.S. Pat. No. 4,249,046 of Livers et al, which describes a crush zone mounted sensor which, as discussed above, is required to be large in order to interact with the crushed material in the crush zone; This sensor differs from this invention in that it is intended for mounting in the crush zone; it is thus considerably larger and more complicated; the mass is not separately supported but is attached to the cantilevered beam rendering it sensitive to cross axis vibrations; and, the secondary contact is constructed in such a manner as to also render it sensitive to both cross axis and longitudinal shocks and vibrations. The sensors of this invention are intended for non-crush zone mounting; the sensor is very small; the sensing mass is separately supported; and, the design of the secondary contact, for the EMS case, is such as to also render it insensitive to cross axis vibrations. The motion of the secondary contact is also limited so that it cannot contact with the first contact when the sensor is subjected to shock and vibration unless the first contact moves the required distance.
The fact that cantilevered mass sensors are particularly sensitive to cross axis vibrations was not understood until the recent development of testing equipment which can simultaneously subject a sensor to both longitudinal accelerations and cross axis vibrations of the magnitudes found in crashes. During tests on an optimized cantilevered mass sensor, the unbalance in the mass relative to the cantilever mounting caused severe torsional vibrations which, in some cases, even caused the sensor to trigger from cross axis vibrations alone. This occurred even though that sensor was particularly designed to minimize this sensor unbalance and thus to minimize this effect.
The parts of the EMS sensor ofthis invention can be manufactured by the plastic injection molding processes in which both contact assemblies are insert molded into the housing in a single operation. A near hermetic seal is obtained using the metal treatment process disclosed in the above referenced patent applications and below.
In U.S. Pat. No. 4,580,810, it is mentioned that the placement of the sensor outside of the inflator housing results in a larger and heavier sensor due to the requirement that the sensor housing must be sufficiently strong to withstand the pressures of the burning propellant. This problem can be solved, as mentioned in that patent, by placing the entire sensor within the inflator housing. It can also be solved by placing the primer within the inflator which is impacted by the firing pin through a small hole. The small amount of propellant which leaks back through the firing pin hole can be made insignificant through the choice of hole size and firing pin spring.
It has also been found that the AMS sensor need not be mounted on the steering wheel axis, as taught in the above referenced patent, as long as the sensor is so mounted that vibrations caused by impacts to the steering wheel rim are weekly coupled to the sensor.