Answer Posted / d tilston
PID = Proportional Integral Derivative
My favourite example of PID control is the cruse control in
an automobile.
Proportional is the response of the controlled variable, the
amount of fuel supplied, to the process variable, the speed
of the car. When you set the cruse, the car has the required
fuel flow to maintain the speed of the car. When the car
starts going up hill, it will slow down. This error in the
measured variable will cause an increase in the controlled
variable. This increase in fuel flow is proportional to the
error. Using only proportional control, as the car goes up
the hill, it must be slower then the set speed. If it were
the same then there would not be any error to increase the
fuel flow.
Integral control is used to remove this error. It looks at
the amount of difference between the set point and the
measured variable. It incrementally adjusts the controlled
variable to eliminate the error.
Derivative control is not usually present in cruse control
systems, but for this example I will assume that it is
available. When the car encounters a steep hill, the car
will slow down dramatically. This rapid increase in error
will cause a rapid response. Also if the car is approaching
the set speed very quickly, it is likely to over shoot the
target speed. The derivative function compensates for this
and adjusts the controlled variable to moderate the
acceleration so as not to exceed the set point.
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