User manual

PID Loop Operation
(DL450 Only)
Maintenance
and Troubleshooting
8--4
PID Loop Operation (DL450 only)
DL405 User Manual, 4th Edition, Rev. A
Introduction to PID Control
In this discussion, we will explain why PID control is used in process control instead
of trying to provide control by simply using an analog input and a discrete output.
There are many types of analog controllers available, and the proper selection will
depend upon the particular application. There are two types of analog controllers
that are used throughout industry:
S 1. The ON--OFF controller, sometimes referred to as an open loop
controller.
S 2. The PID controller, sometimes called a closed loop controller.
Regardless of type, analog controllers require input signals from electronic sensors
such as pressure, differential pressure, l evel, flow meter or thermocouples. As an
example, one of the most common analog control applications is located in your
house for controlling either heat or air conditioning, the thermostat.
You wish for your house to be at a comfortable temperature so you set a thermostat
to a desired temperature (setpoint). You then select the “comfort“ mode, either heat
or A/C. A temperature s ensing device, normally a thermistor, is located within the
thermostat. If the thermostat is set for heat and the setpoint is set for 69_, the furnace
will be turned on to provide heat at, normally, 2_ below the setpoint. In this case, it
wouldturnonat67_. When the temperature reaches 71_,2_ above setpoint, the
furnace will turn off. In the opposite example, if the thermostat is set for A/C (cooling),
the thermostat will turn the A/C unit on/off opposite the heat setting. For instance, if
the thermostat is set to cool at 76_, the A/C unit will turn on when the sensed
temperature reaches 2_ above the setpoint or 78 _, and turn off when the
temperature reaches 74_. This would be considered to be an ON--OFF controller.
The waveform below shows the action of the heating cycle. Note that there is a slight
overshoot at the turn--off point, also a slight undershoot at the turn--on point.
SETPOINT
TIME
OFF
ON
ONON
OFF
71_
69_
67_
The ON--OFF controller is used in some industrial control applications, but is not
practical in the majority of industrial control processes.
The most common process controller that is used in industry is the PID controller.
What is PID
Control?