Patent ID: 6840539

Claim:
An airbag and a deployment sensor comprising: an airbag cushion, defining a cushion interior having an inside surface; a structure to which the airbag cushion is attached; a sensor mounted to the structure, and mounted internal to the airbag cushion, the sensor further comprising: a cartridge having portions defining a storage space within the cartridge, the storage space containing a quantity of string, the cartridge having portions forming a string outlet, wherein one end of the string is attached to a portion of the inside surface of the airbag cushion, so that deployment of the airbag cushion draws string out of the storage space in the cartridge, through the string outlet, and the string leaving the string outlet defines a direction of string withdrawal; a structure positioned within the cartridge to cause string drawn out of the storage space to move transverse to the direction of string withdrawal, wherein the transverse motion caused has a periodicity which is proportional to the length of string withdrawn; at least one string sensor positioned in the cartridge between the storage space and the string outlet, the at least one string sensor positioned to detect the transverse motion of the string, so that the sensor output varies with a frequency proportional to the rate string is withdrawn from the cartridge.