Page 424 - Power Electronics Handbook

Automobile control 4 13

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Figure 14.62 An electronic ignition system
Protection circuitry is usually incorporated into the system. For example if
the spark plug is disconnected for any reason then the energy stored in the
ignition coil is transferred back to the power drive circuit, and could damage
it unless these are protected.
Some other areas in which electronics are used within automobiles are
antilocking braking systems (ABS) and electrical power assisted steering
(EPAS). In a braking system when the brakes are applied the wheels slow to
a speed slightly below that of the vehicle, so that some slip occurs between
the tyres and ground, resulting in friction. The ratio between the difference
in wheel and vehicle speed, relative to the vehicle speed, is the friction
coefficient for the interface between the tyre and road, and this should be
maintained at about 0.15 and 0.30. Smaller ratios, caused as a result of heavy
braking, could result in the wheels locking and the vehicle going out of
control.
An ABS system continuously monitors the speed of each wheel and, when
in the braking mode, it calculates the rate of deceleration. If this exceeds the
limit which would result in the wheels locking it takes control and either
maintains the rate of braking or reduces it. When safe to do so control is
returned to the driver, but the wheel deceleration is still monitored and
control can again be assumed, if needed.
EPAS is fitted to automobiles in order to provide easier steering action so
reduce driver fatigue. It is therefore a driver convenience and a safety