Airbag and a deployment sensor

An airbag cushion deployment sensor comprising:

• a cartridge having portions defining an interior storage space;
• portions of the cartridge which define a string outlet communicating with the interior storage space;
• a spool fixed within the cartridge interior storage space;
• a string stored in a coil encircling the spool, and having a first end which extends through the string outlet for connection to the airbag cushion;
• at least one light source mounted to the cartridge;
• and
• at least one light detector mounted to receive light from the at least one light source, the at least one light detector being positioned such that extraction of the string from the coil causes the string to only intermittently pass between the at least one light detector and the at least one light source and to thus generate a fluctuating signal corresponding to the speed of withdrawal of the string from the cartridge.

A method of monitoring airbag deployment comprising the steps of:

• deploying an airbag cushion having a string mounted to an interior surface of the airbag, wherein the string is drawn from a cartridge which does not move in response to the deploying airbag cushion;
• drawing the string from the cartridge, wherein portions of the cartridge are arranged to cause the string passing through the cartridge to perform an oscillating motion at a frequency proportional to the rate at which the string is being withdrawn;
• and
• detecting the frequency of the oscillating motion and determining the rate at which string is being withdrawn.