An accelerometer is one of the primary sensors used in on-board automotive safety control systems and navigational systems, particularly crash sensing systems. Examples of such automotive applications include anti-lock braking systems, active suspension systems, supplemental inflatable restraint systems such as air bags, and seat belt lock-up systems. An accelerometer is a device which measures acceleration, or more accurately, accelerometers measure the force that is exerted by a body as the result of a change in the velocity of the body. A moving body possesses inertia which tends to resist the change in velocity. It is this resistance to any change in velocity that is the source of the force which is exerted by the moving body. This force is directly proportional to the acceleration component in the direction of movement when the moving body is accelerated.
In one form of a conventional type of accelerometer, a mass is suspended between two spring members which are coaxially attached on opposite sides of the mass. The mass is maintained in a neutral position so long as the system is at rest or is in motion at a constant velocity. When the mass-spring support system undergoes a change in velocity in the direction of the springs' axis, i.e. an acceleration or deceleration parallel the spring axis, the spring mounted mass will resist the movement because of its inertia. This resistance to the change in velocity will force one of the springs to be in tension while the other spring is compressed. Accordingly, the force acting on each spring is equal but opposite in magnitude to the force acting upon the other. From this, the acceleration component can be determined.
Analogously, another common type of accelerometer employs a cantilevered beam wherein the mass is disposed on the unsupported end of the beam. Upon acceleration of the mass in a direction perpendicular to the plane of the mass, the beam will deflect, causing one surface of the beam to be in compression and the other surface to be in tension. The acceleration component in the direction perpendicular to the plane of the beam can then be measured using various known means.
In many of these types of accelerometers it is desirable to limit the displacement of the mass, so as to maximize the integrity and life of the device. Current designs do not provide a satisfactory means for limiting this displacement, particularly when employing an accelerometer which utilizes a cantilevered beam.
In addition, although there are several different types of accelerometers currently available commercially, they are generally typified by the same shortcomings which make them unsatisfactory for high volume automotive use. In particular, many of the current accelerometers are characterized by extremely high production costs, without the requisite long life durability for highly rugged applications, such as in an automotive environment.
Therefore, it is desirable to provide an accelerometer which meets the requirements of reliability, accuracy, ruggedness and low cost, for an on-board automotive system, as well as other applications, and further which alleviates the shortcomings of the prior art, in particular, by providing an integral means for limiting the displacement of the proof mass.