Controllers are designed to eliminate the need for continuous operator attention.
Controllers are used to automatically adjust some variable to hold the measurement
(or process variable) at the set point. The set point is where you would like the
measurement to be and the error is defined as the difference between set point and
measurement. Engineers often concern themselves with how to control such processes.
Often we take this concept of control for granted, and don't even consider that anything
is really happening at all. If we consider the cruise control on a car is very familiar
and easy to overlook, but the exact method by which the car is able to maintain a
constant (or almost constant) speed can be mysterious. Along the same lines, how does
your refrigerator keep your food at a constant cold temperature no matter how your house
temperature may change? How do modern radio receivers lock onto radio stations and adjust
the tuning as we drive, keeping our music playing clear. All of these are control
systems, and require good understanding of engineering principles to appreciate fully.
As an introduction to the concept of a control system, we'll start with a basic
and familiar example. Consider the furnace in your house. If you return from
vacation, you've probably had the house set at a low temperature, say 60°F.
As soon as you walk in it feels cold, and immediately turn the thermostat up
to 70°F. But, it takes a while for your house to warm up. What is happening?
In your house, the thermostat is connected to the furnace and acts as a switch.
Your familiar with how it works. If the temperature you've set in the thermostat
is less than the actual temperature in the house, the thermostat turns the furnace
on to add heat to your house.
PID –Graphic Response
In our example, when you turned the thermostat setting up to 70°F, the furnace
turned on because the actual temperature in the house was lower (60°F). As the
house warms up, the temperature rises. When the temperature in the house finally
passes 70°F, the thermostat automatically shuts off the furnace. Then the thermostat
waits until the temperature drops a couple of degrees below the 70°F you set when you
walked in the door. This couple of degrees is also known as a dead band, to avoid a
continuous start/stop cycle of the furnace.
When the temperature drops enough, around 68°F, the thermostat once again kicks
on the furnace and the cycle repeats. This type of control is called On/Off control.
No surprise there. This type of control system works by adjusting a controlled variable
(in our example the furnace changing the air temperature) to achieve a set point (70°F).
It does it by simply turning on the furnace if it is too cold, or by turning off the
furnace when it warms up.
The control action allows the process control to accurately maintain set point by
adjusting the control outputs. In this technical note we have attempted to explain
what PID is in practical terms.
