Page 45 - Understanding Automotive Electronics

2735 | CH 2 Page 32 Tuesday, March 10, 1998 10:55 AM

2 THE SYSTEMS APPROACH TO CONTROL AND INSTRUMENTATION

block are identified. In electronic systems, these input and output variables are
electrical signals, except for the system input and system output. One benefit of
this approach is that the subsystem operation can be described by functional
relationships between input and output. There is no need to describe the
operation of individual transistors and components within the blocks.
Figure 2.1a depicts the architecture or configuration for a control
application electronic system. In such a system, control of a physical subsystem
(called the plant) occurs by regulating some physical variable (or variables)
through an actuator. An actuator has an electrical input and an output that
may be mechanical, pneumatic, hydraulic, chemical, or so forth. The plant
being controlled varies in response to changes in the actuator output. The
control is determined by electronic signal processing based on measurement of
some variable (or variables) by a sensor in relationship to a command input by
the operator of the system (i.e., by the driver in an automotive application).
In an electronic control system, the output of the sensor is always an
electrical signal (denoted e in Figure 2.1). The input is a physical variable in
1
the plant being controlled. The electronic signal processing generates an output
electrical signal (denoted e in Figure 2.1) that operates the actuator. The signal
2
processing is designed to achieve the desired control of the plant in relation to
the variable being measured by the sensor. The operation of such a control
system is described later in this chapter. At this point, we are interested only in
describing the control system architecture. A detailed explanation of electronic
control is presented later in this chapter.
The architecture for electronic measurement (also known as
instrumentation) is similar to that for a control system in the sense that both
structures incorporate a sensor and electronic signal processing. However, instead
of an actuator, the measurement architecture incorporates a display device. A
display is an electromechanical or electro-optical device capable of presenting
numerical values to the user (driver). In automotive electronic measurement, the
display is sometimes simply a warning light with a fixed message rather than a
numeric display. Nevertheless, the architecture is as shown in Figure 2.1b. It
should be noted that both control and instrumentation electronic systems use one
or more sensors as well as electronic signal processing.
Figure 2.1c depicts a block diagram for a communication system. In
such a system, data or messages are sent from a source to a receiver over a
communication channel. This particular architecture is sufficiently general
that it can accommodate all communication systems, from ordinary car
radios to digital data buses between multiple electronic systems on cars.
Communication systems are described in greater detail later in this chapter.

ANALOG SYSTEMS
Although digital electronic systems are rapidly replacing analog systems in
automotive electronics, it is simpler to describe analog systems first since they
can generally be understood more intuitively than digital systems. Considering
control and instrumentation applications, the sensor converts the input variable

32 UNDERSTANDING AUTOMOTIVE ELECTRONICS