A vehicle may contain automatic safety restraint actuators that are activated responsive to a vehicle crash for purposes of mitigating occupant injury. Examples of such automatic safety restraint actuators include air bags, seat belt pretensioners, and deployable knee bolsters. One objective of an automatic restraint system is to mitigate occupant injury, thereby not causing more injury with the automatic restraint system than would be caused by the crash had the automatic restraint system not been activated. Generally, it is desirable to only activate automatic safety restraint actuators when needed to mitigate injury because of the expense of replacing the associated components of the safety restraint system, and because of the potential for such activations to harm occupants. This is particularly true of air bag restraint systems, wherein occupants too close to the air bag at the time of deployment--i.e. out-of-position occupants--are vulnerable to injury or death from the deploying air bag even when the associated vehicle crash is relatively mild. Moreover, occupants who are of small stature or with weak constitution, such as children, small adults or people with frail bones are particularly vulnerable to injury induced by the air bag inflator. Furthermore, infants properly secured in a normally positioned rear facing infant seat (RFIS) in proximity to a front seat passenger-side air bag are also vulnerable to injury or death from the deploying air bag because of the close proximity of the infant seat's rear surface to the air bag inflator module.
Air bag inflators are designed with a given restraint capacity, as for example, the capacity to protect an unbelted normally seated fiftieth percentile occupant when subjected to a 30 MPH barrier equivalent crash, which results in associated energy and power levels which can be injurious to out-of-position occupants. While relatively infrequent, cases of injury or death caused by air bag inflators in crashes for which the occupants would have otherwise survived relatively unharmed have provided the impetus to reduce or eliminate the potential for air bag inflators to injure the occupants which they are intended to protect.
Known deployment systems for vehicle safety devices such as an air bag require the host vehicle to actually collide with an obstacle or other vehicle before the deployment decision process begins. At that point in time, the sensors detect a deceleration in the host vehicle and deploy one or more safety systems. Thus, the crash is identified based solely on the characteristic of the acceleration versus time measure. The disadvantage with existing post-crash detection systems derives from the fact that the time available to deploy an active safety device is very short, particularly for side impact or high speed frontal collisions where occupant restraint systems can provide significant safety benefits. These short time frames lead to rates of inflation of the airbags that are so great that injury or death are possible if the occupant is not well aligned with the airbag.
One technique for mitigating injury by the air bag inflator to occupants is to reduce the power and energy levels of the associated air bag inflator, for example by reducing the amount of gas generant in the air bag inflator, or the inflation rate thereof. This reduces the risk of harm to occupants by the air bag inflator while simultaneously reducing the restraint capacity of the air bag inflator, which places occupants a greater risk for injury when exposed to higher severity crashes.
Another technique for mitigating injury by the air bag inflator to occupants is to control the rate of inflation rate or the capacity of the inflator responsive to a measure of the severity of the crash. The prior art teaches the use of multi-stage inflators having distinct independent compartmentalized stages and corresponding firing circuits, whereby the stages may be fired in delayed succession to control the effective inflation rate, or stages may be inhibited from firing to control the effective inflator capacity. The prior art also teaches the use of a hybrid inflator having a combination of stored gas and plural pyrotechnic gas generator elements which are independently fired. Furthermore, the prior art also teaches the use of control valves for controlling the gaseous discharge flow from the inflator. The inflation rate and capacity may be controlled responsive to the sensed or estimated severity of the crash, whereby a low severity would require a lower inflation rate or inflation capacity than a high severity crash. Since lower severity crashes are more likely than those of higher severity, and since such a controlled inflator would likely be less aggressive under lower severity crash conditions than those of higher severity, occupants at risk of injury by the air bag inflator because of their size or position will be less likely to be injured overall because they are more likely to be exposed to a less aggressive inflator. However, the risk of injury to such occupants would not be mitigated under the conditions of higher crash severity when the inflator is intentionally made aggressive in order to provide sufficient restraint for normally positioned occupants.
Ideally, the air bag would be inflated prior to any interaction with a normally seated occupant, and at a rate which is sufficiently slow that an out of position occupant would not be injured by the inflating air bag. For a crash of sufficient severity, this requires the crash sensing system to be able to predict immanent crashes because the time required to inflate the bag at an inflation rate which is sufficiently slow to be safe for out-of-position occupants may be greater than either that required for the occupant to move so as to commence interaction with an inflated air bag or to safely decelerate the occupant.
Current sensing technology uses accelerometers to detect the occurrence of the actual crash and therefore make it impossible to activate the safety devices prior to the crash. Radar sensors are currently being investigated for intelligent cruise control applications that merely provide a convenience to the operator of the vehicle in terms of maintaining a safe distance from other vehicles. Failure of such a system will only inconvenience the driver and force them to maintain their own distance. Collision prediction sensors, however, must operate with 100 percent effectiveness since the passenger safety is at risk. In light of this the system must operate in a reliable and robust manner under all imaginable operating conditions and traffic scenarios. Known automotive radar systems use either a dual frequency ranging method, or continuous linear frequency modulated (FM) signals. The dual frequency method uses two tones to derive range from the relative phase between the two signals. The linear FM approach uses a continuously swept ramped waveform of increasing frequency with time. This is then repeated over and over.
The dual tone method is useful for a single target within the radar beam for estimating the range. However, in a predictive collision sensing application, the radar needs to track multiple targets at varying ranges within a field of interest because each such target is a potential collision.
The linear FM approach is susceptible to corruption due to non-linearities in the frequency ramp of the signal. This in turn causes blurring of the point spread function and reduces the resolution and accuracy of the signals. In addition, since the linear FM approach will have all cars in a general vicinity operating at effectively the same ramp rate and frequency band of operation, there is a high probability of signal interference between with each individual radar unit. Only the time variation of the signals due to the differing start times of the radars will provide interference immunity. It is also very possible for two radars to be close enough in the start time of their ramps that they would completely overlap and interfere when the main lobes of the respective antennas of the two systems are directed at each other or when the main lobe of one system is in the other system's side lobes, which is a very likely occurrence for collision prediction systems having a very wide field of view (&gt;180 degrees). Such interference can cause a system to either miss detecting a target, or detect a false target when none is present.