Pneumatic stored energy systems have many applications. In transport safety they may be used to rapidly inflate various devices such as car air bags, aircraft passenger escape slides and helicopter flotation systems, in the case of an emergency incident. Due to their use in emergency incidents, rapid activation of the pneumatic stored energy system is necessary.
A pneumatic stored energy system may comprise, for example, a sealed vessel filled with a compressed helium-nitrogen (He/N) mixture, and some sort of firing mechanism to puncture a region of the vessel wall. A collection nozzle arranged about the vessel fluidly connects an inflatable device, folded in its uninflated state, to the vessel. In use, the firing mechanism causes a region of the vessel wall to puncture so as to supply the compressed He/N mixture via the nozzle to inflate the inflatable device.
Typically, pyrotechnic actuators have been used as a firing mechanism to trigger the pneumatic stored energy systems to inflate the specific device.
In use, an electric current is passed through a wire buried in a chemically reactive substance (the pyrotechnic), causing the pyrotechnic to burn. As the pyrotechnic burns, it produces a gas. The burn is contained within a sealed cavity of a container, and the pressure caused by the generation of exhaust gasses is applied to a piston fitted in the cavity at the end of the container. The piston moves due to the applied gas pressure and the linear movement can be used to shear through a diaphragm, or to bend and break a hollow pillar, thus releasing the stored pneumatic energy.
The use of pyrotechnic actuators is highly regulated. The potentially hazardous substances used in pyrotechnic actuators are difficult to handle and store. These potentially hazardous substances also require expensive and time consuming checks, for both the producer and end user. The potentially hazardous substances may also have a limited life-span, requiring replacement.
An alternate firing mechanism for activating a pneumatic stored energy system is through the use of shape memory alloys. The shape memory alloy may be caused to pull on a hollow break-off pillar and break it, thus releasing the stored pneumatic energy.
In this case the shape memory alloy is stretched during heat treatment such that it retains its stretched dimension during operation at normal, ambient, temperature. When the alloy is heated, for instance by passing an electrical current through it, it reverts to its unstretched dimensions. The change in length and resulting linear force can be used to break a hollow pillar.
However, shape memory alloy firing mechanisms often perform unreliably. Furthermore there remains the risk of unscheduled operation should the local environment conditions change.