UnderstandingAutomotiveElectronics8th-Chapter10

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CHAPTER

ELECTRONIC SAFETY-RELATED

SYSTEMS

CHAPTER OUTLINE

Airbag Safety Device .......................................................................................................................... 505

Blind Spot Detection .......................................................................................................................... 512

Automatic Collision Avoidance System ................................................................................................ 515

Lane Departure Monitor ...................................................................................................................... 521

Tire Pressure Monitoring System ........................................................................................................ 522

Enhanced Vehicle Stability ................................................................................................................. 524

Safety aspects of any vehicle can be classified into two major areas. The first to be discussed in this

chapter involves occupant protection in the event of an accident, for example, collision with another

vehicle or a large object. The second area involves techniques to avoid such accidents including blind

spot object detection, collision avoidance, and enhanced vehicle stability (EVS).

The first topic discussed in this chapter is airbags, which are an important supplement to seat belts

for occupant protection. This section of the chapter reviews the basic theory of operation of airbag

systems and explains some of the significant improvements in these systems with respect to maximiz-

ing occupant protection.

AIRBAG SAFETY DEVICE

An airbag is one of the major electronics-based occupant protection devices. An airbag is a gasbag that

is stored at specific locations within the vehicle in an empty collapsed configuration. In its stored state,

it is covered by normal automotive interior panels that are readily ruptured upon airbag inflation. The

theory of operation is perhaps best begun with an explanation of the earliest airbags that were deployed

in some production automobiles in the 1970s. In such early vehicle deployment, the focus was on pro-

tection of the driver from impact with the steering wheel in the event of a frontal (or nearly frontal)

collision.

An airbag system consists of the airbag itself, an inflation devi ce, a digital control system, and one

or more sensors. During an essentially frontal collision, the vehicle experiences a very large deceler-

ation as the vehicle is crushed by the impact forces associated with a collision. The earliest sensors

employed by airbag system s were electromechanical swit ches, such as are depicted in Fig. 10.1 , that

were normally open but were clos ed whenever deceleration reached a predetermined level that was

Understanding Automotive Electronics. http://dx.doi.org/10.1016/B978-0-12-810434-7.00010-7

505

associated with collision dynam ics. At this level of deceleration, an electric signal was sent to the ig-

niter of the airbag inflator. In the earliest configurations, the airbag was fully inflated in 30–40 ms.

The concept of an airbag system is that an inflated airbag can act as a cushion that can isolate or

partially isolate occupants from an impact with various body parts with the internal vehicle structure.

Initially, during the early 1970s, it was believed by many in the industry that airbags might have be-

come a substitute or replacement for seat belts since data had suggested that seat belt use generally was

relatively low. At the present time, airbags are considered a supplementary safety system to the primary

system of seat belts by the automobile industry and are required by regulatory authorities.

Airbag deployment in vehicles has evolved from the initial steering wheel locations intended for

driver protection. Vehicles were later provided with airbags for front seat passengers and then alon g

vehicle sides including the side doors and the side curtain airbag to provide protection in the event of a

side impact. Other airbag locations and configurations tend to be somewhat manufacturer-specific, the

details of which are beyond the scope of this book. Rather, this book is intended to explain the details of

the electronic components and the theory of operation.

The practical implementation of the airbag has proved to be technically challenging. At car speeds

that can cause injury to the occupants, the time interval for a crash into a rigid barrier from the moment

the front bumper contacts the barrier until the final part of the car ceases forward motion is of the order

of a second. Table 10.1 lists required airbag deployment times for a variety of test crash conditions.

For an understanding of the conceptual framework of an airbag electronic system, it is, perhaps,

helpful to briefly review a somewhat simple example of an early airbag concept. The earliest concepts

involved protecting the driver and front seat passenger from essentially frontal-only collisions.

One of the early configurations that was intended to protect occupants from longitudinal axis de-

celeration employed a pair of acceleration switches SW1 and SW2 as depicted in Fig. 10.1. Each of

these switches is in the form of a mass suspended in a tube with the tube axis aligned parallel to the

Forward

(A)

(

)

Hollow tube

Spring

Car

battery

Airbag

squib

SW1 SW2

Mass

Switch

contacts

FIG. 10.1 Airbag deployment system. (A) Traditional airbag switch configuration. (B) Circuit for airbag activation.

506 CHAPTER 10 ELECTRONIC SAFETY-RELATED SYSTEMS

longitudinal car axis. Fig. 10.1A is a drawing of the accelerator switch configuration. Fig. 10.1B is a

circuit diagram for an early airbag system.

The two switches, which are normally open, must both be closed to complete the circuit for firing

the airbag. When this circuit is complete, a current flows through the igniter that activates the charge. A

gas is produced (essentially explosively) that inflates the airbag.

In early airbag systems, 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 com-

partment (some automakers located a switch under the driver’s seat on the floor pan).

Referring to the sketch in Fig. 10.1, the operation of the acceleration-sensitive switch can be un-

derstood. 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 contact s, thereby completing the circuit and

igniting the chemical compound to inflate the airbag.

An approximate dynamic model for the mechanical crash sensor is given below:

€

x + D

x + F

+ Kx ¼ 0 (10.1)

Table 10.1 Airbag deployment times

Test library event Required deployment time (ms)

9 mph frontal barrier ND

15 mph frontal barrier 50.0

30 mph frontal barrier 24.0

35 mph frontal barrier 18.0

12 mph left angle barrier ND

30 mph right angle barrier 36.0

30 mph left angle barrier 36.0

10 mph center high pole ND

14 mph center high pole ND

18 mph center high pole ND

30 mph center high pole 43.0

25 mph offset low pole 56.0

25 mph car to car 50.0

30 mph car to car 50.0

5 mph curb impact ND

20 mph curb drop-off ND

35 mph Belgian blocks ND

Note, ND ¼ nondeployment.

507AIRBAG SAFETY DEVICE

where M ¼mass of the movable element, D ¼viscous friction coefficient, F

¼coulomb friction force

(stiction), and K¼spring constant.

The acceleration of the mass

€

xðÞis related to vehicle acceleration (a) or deceleration (a) by the

following:

€

x ¼a

The motion of the movable mass is the solution to the following:

x + F

+ Kx ¼ Ma (10.2)

Whenever the mass displacement exceeds the spacing to the switch contact (x

) (i.e., x ¼ x

), the con-

tacts close, and the action described above proceeds.

Fig. 10.1 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.

The evolution of airbag sensing technology advanced to electronic sensor. 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. Fig. 10.2 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. As an illustrator of the characteristic waveform from an accel-

erometer, Fig. 10.3 presents measurements of a 3200 lb (curb weight) vehicle that was crashed into a

rigid barrier at 30 mph.

Under normal driving conditions, the acceleration at the accelerometer locations is <1 g. However,

during a collision at a sufficiently high speed, the signal increases rapidly. Signal processing can be

employed to enhance the collision signature in relation to the no rmal driving signal. Such signal pro-

cessing must be carefully designed to minimize time delay of the output relative to the colli sion de-

celeration signal. A comparison of the deceleration profile of Fig. 10.3 for this crash with the

deployment requirements of Table 10.1 illustrates the complexity of the signal processing necessary

to properly deploy the airbag.

As explained above, the original electromechanical deceleration sensors have been replaced with

solid-state accelerometers. In addition, airbag systems are equipped with other sensors including

FIG. 10.2 Accelerometer-based airbag system.

508 CHAPTER 10 ELECTRONIC SAFETY-RELATED SYSTEMS

sensors for side door pressure, wheel angular speed, brake pressure, seat occupancy, and gyroscopic

sensors for angular position/rate measurement and impact sensors. The electronic control for an airbag

system receives and processes signals from all of the associated sensors. The algorithms for this signal

processing are relatively complex since the central system must distinguish signals due to a collision

from signals that are produced during normal driving. In the latter case, the signals from the sensors

depend upon the vehicle speed, the condition of the road (or off-road terrain), and sometimes vehicle

maneuvers resulting from driver input. For example, a vehicle traveling over a road with significant

potholes or a vehicle striking a curb during an improperly executed turn can involve rapid vehicle dy-

namics that result in signals from the sensors that can emulate a collision.

Regulatory requirements for airbag deployment are usually specified in terms of a specific colli-

sion. For example, the frontal collision into a rigid barrier at a specific speed (e.g., 14 mph) requires

airbag deployment. Su ch a collision will cause a very specific set of signals from the various sensors for

a particular vehicle model and/or configuration. If the threshold for airbag deployment was set too low

and if the driver’s airbag was deployed when the vehicle hit a severe pothole (or curb), the occupant

safety would be reduced because the airbag would obstruct the driver’s visibility and could impact his/

her ability to control the vehicle.

As an illustration of the complexity of the signal processing required by the airbag control system,

Fig. 10.3 depicts the actual output signal waveform of an accelerometer during a frontal barrier col-

lision for an actual vehicle. In addition to signal waveform complexity, the detection of a collision must

occur within a few mill iseconds of the initial contact of the vehicle with the colliding object. The time

for full inflation of a representative airbag from the trigger (igniter) signal is 30–40 ms. The vector

Deceleration in Multiples of g

–5

0 1020304050

Time

(

)

60 70 80 90 100

FIG. 10.3 Acceleration data for 30 mph crash.

509AIRBAG SAFETY DEVICE

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