Page 375 - Understanding Automotive Electronics

2735 | CH 10 Page 362 Tuesday, March 10, 1998 1:27 PM

10 DIAGNOSTICS

A typical airbag will require about 30 msec to inflate, meaning that the
crash must be detected within about 20 msec. With respect to the speed of
modern digital electronics, a 20 msec time interval is not considered to be
short. The complicating factor for crash detection is the many crashlike
accelerations experienced by a typical car that could be interpreted by airbag
electronics as a crash, such as impact with a large pothole or driving over a curb.
The configuration for an airbag system has also evolved from
electromechanical implementation using switches to electronic systems
employing sophisticated signal processing. One of the early configurations
employed a pair of acceleration switches SW1 and SW2 as depicted in Figure
10.18a. Each of these switches is in the form of a mass suspended in a tube with
the tube axis aligned parallel to the longitudinal car axis. Figure 10.18b is a
circuit diagram for the airbag system.
The two switches, which are normally open, must both be closed to
complete the circuit for firing the squib. When this circuit is complete, a
current flows through the squib ignitor that activates the charge. A gas is
produced (essentially explosively) that inflates the airbag.
The switches SW1 and SW2 are placed in two separate locations in the
car. Typically, one is located near the front of the car and one in or near the
front of the passenger compartment (some automakers locate a switch under
the driver’s seat on the floor pan).
Referring to the sketch in Figure 10.18a, the operation of the
acceleration-sensitive switch can be understood. Under normal driving
conditions the spring holds the movable mass against a stop and the switch
contacts remain open. During a crash the force of acceleration (actually
deceleration of the car) acting on the mass is sufficient to overcome the spring
force and move the mass. For sufficiently high car deceleration, the mass moves
forward to close the switch contacts. In a real collision at sufficient speed, both
switch masses will move to close the switch contacts, thereby completing the
circuit and igniting the squib to inflate the airbag.
Figure 10.18b also shows a capacitor connected in parallel with the
battery. This capacitor is typically located in the passenger compartment. It has
sufficient capacity that in the event the car battery is destroyed early in the
crash, it can supply enough current to ignite the squib.
In recent years there has been a trend to implement electronic airbag
systems. In such systems the role of the acceleration-sensitive switch is played
by an analog accelerometer along with electronic signal processing, threshold
detection, and electronic driver circuit to fire the squib. Figure 10.19 depicts a
block diagram of such a system.
The accelerometers a1 and a2 are placed at locations similar to where the
switches SW1 and SW2 described above are located. Each accelerometer
outputs a signal that is proportional to acceleration (deceleration) along its
sensitive axis.

362 UNDERSTANDING AUTOMOTIVE ELECTRONICS