After little-Suzy-homemaker loses control of her fully-loaded-by-110-pounds-of-groceries shopping cart, will it set off the side airbags on your 2010 Ford Taurus at a speed of 10 MPH? Let's find out.
This is a problem Ford had to answer when it began using a new type of crash detection sensor. Traditional airbag sensors are small accelerometers mounted to the B-pillar next to the drivers shoulder, and in the event of a crash, they send a signal to the computer warning of a crash event. Only problem is they aren't fast enough anymore. Tough new safety requirements laid down by the Feds and the Insurance Institute for Highway Safety are forcing airbag deployment speeds to get much quicker.
So what to do. Ford decided to start using pressure wave detection. In this method, the sensor is placed inside the door on the outer skin of the car, it monitors the ambient air pressure in the door cavity and sends a signal to the crash computer. The crash computer interprets the data every few miliseconds, confirming it with what the other sensors scattered around the car tell it. What's the advantage? Fidelity. The signal coming from the pressure sensor has a much higher resolution than an accelerometer, which means it can tell the difference between a car hitting your door and say a shopping cart loaded with 110 lbs, hitting the door at 10 MPH. But before it can do that, engineers have to calibrate it to be able to tell the difference. This is where the shopping cart test comes in.
The strike from a shopping cart produces a short spike signal with relatively low amplitude. If it were a car striking the door panel, the crash computer would see a massive, sustained rise in pressure. This kind of simulation is also done with things which hit cars all the time in real life; a bicycle tire, a ball, you get the drift. Yes it smashes a very nice new Taurus door, put they just take it off and put on a new one to do it all over again, and because of this kind of testing, out in the real world, you have a door to fix and a soccer mom to yell at, but the expensive airbags stay thankfully unexploded.