Sommerville-Software-Engineering-10ed

624 Chapter 21 ■ Real-time software engineering
Pedal pressure sensor
Brake 1
Pedal
Brake 2
monitor
Brake 1
process
Analysis
process
Brake 2
process
Figure 21.11 Control
system architecture
for an anti-skid
braking system
Brake 3
process
Wheel
monitor
Brake 4
process
Brake 3 Brake 4
Wheel sensors
from the equipment’s environment. For example, an anti-skid braking system in a
car monitors the car’s wheels and brake system (the system’s environment). It looks
for signs that the wheels are skidding when brake pressure is applied. If this is the
case, the system adjusts the brake pressure to stop the wheels locking and reduce the
likelihood of a skid.
Control systems may make use of the Environmental Control pattern, which is a
general control pattern that includes sensor and actuator processes. This pattern is
described in Figure 21.9, with the process architecture shown in Figure 21.10. A
variant of this pattern leaves out the display process. This variant is used in situations
where user intervention is not required or where the rate of control is so high
that a display would not be meaningful.
This pattern can be the basis for a control system design with an instantiation
of the Environmental Control pattern for each actuator (or actuator type) being
controlled. You then optimize the design to reduce the number of processes. For
example, you may combine actuator monitoring and actuator control processes,
or you may have a single monitoring and control process for several actuators.
The optimizations that you choose depend on the timing requirements. You may
need to monitor sensors more frequently than you send control signals, in which
case it may be impractical to combine control and monitoring processes. There
may also be direct feedback between the actuator control and the actuator monitoring
process. This allows fine-grain control decisions to be made by the actuator
control process.
You can see how this pattern is used in Figure 21.11, which shows an example of a
controller for a car braking system. The starting point for the design is associating an
instance of the pattern with each actuator type in the system. In this case, there are four
actuators, with each controlling the brake on one wheel. The individual sensor processes
are combined into a single wheel-monitoring process that monitors the sensors on all