Page 228 - Understanding Automotive Electronics

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SENSORS AND ACTUATORS 6

Figure 6.24
Knock Sensor

FPO

AUTOMOTIVE ENGINE CONTROL ACTUATORS

In addition to the set of sensors, electronic engine control is critically
dependent on a set of actuators to control air/fuel ratio, ignition, and EGR.
Each of these devices will be discussed separately.
In general, an actuator is a device that receives an electrical input (e.g.,
from the engine controller) and produces a mechanical or thermal (or other)
output. Examples of actuators include: various types of electric motors,
solenoids, and piezoelectric force generators. In automotive electronic systems
the solenoid is the most commonly used device because it is relatively simple
and inexpensive.
The solenoid is used in applications ranging from precise fuel control to
mundane applications such as electric door locks. A solenoid is in essence a
powerful electromagnet having a configuration generally similar to that
illustrated in Figure 6.25. The solenoid consists of a fixed steel (i.e.,
ferromagnetic) frame with a movable steel element. A spring holds the movable
element in position such that there is a gap between the end of the movable
element and the opening in the frame. A coil is wound around the steel frame,
forming a powerful electromagnet.
When a current passes through the coil, a magnetic field is created that
tends to pull the movable element toward the steel frame. When the magnetic
field, which is proportional to the current, is sufficient to overcome the force at
the spring holding the movable element, then it begins to move toward the
frame. As this element moves, the size of the gap is reduced, causing an increase
in the strength of the magnetic field. This increase causes the movable element
to accelerate toward the frame until it reaches the stop.
This abrupt motion of the movable element is essentially in the form of a
mechanical switching action such that the solenoid tends to be either in its rest
position (as held by the spring) or against the mechanical stop. The movable

UNDERSTANDING AUTOMOTIVE ELECTRONICS 215