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PID Loop Operation
Maintenance
8–49
PID Loop Operation
Time-Proportioning Control
The PID loop controller in the DL05 CPU generates a smooth control output signal
across a numerical range. The control output value is suitable to drive an analog
output module, which connects to the process. In the process control field, this is
called
continuous control , because the output is on (at some level) continuously.
While continuous control can be smooth and robust, the cost of the loop components
(such as actuators, heater amplifiers) can be expensive. A simpler form of control is
called
time-proportioning control . This method uses actuators which are either on or
off (no in-between). Loop components for on/off-based control systems are lower
cost than their continuous control counterparts.
In this section, we will show you how to convert the control output of a loop to
time-proportioning control for the applications that need it. Let’s take a moment to
review how alternately turning a load on and off can control a process. The diagram
below shows a hot-air balloon following a path across some mountains. The desired
path is the
setpoint . The balloon pilot turns the burner on and off alternately, which is
his control output . The large mass of air in the balloon effectively averages the effect
of the burner, converting the bursts of heat into a continuous effect: slowly changing
balloon temperature and ultimately the altitude, which is the
process variable .
Time-proportioning control approximates continuous control by virtue of its
duty-cycle – the ratio of ON time to OFF time. The following figure shows an example
of how duty cycle approximates a continuous level when it is averaged by a large
process mass.
Desired
Effect
On/Off
Control
Off
On
period
If we were to plot the on/off times of the burner in the hot-air balloon, we would
probably see a very similar relationship to its effect on balloon temperature and
altitude.