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Chapter 5:
Tuning and Control
This chapter describes the different methods of control available with the D8. This chapter covers
control algorithms, control methods, PID control, starting PID values and tuning instructions to help
appropriately set control parameters in the D8 system.
Control Algorithms
This section explains the algorithms available for controlling a loop.
The control algorithm dictates how the controller responds to an input signal. Do not confuse control
algorithms with control output signals (for example, analog or pulsed dc voltage). There are several
control algorithms available:
• On/off
• Proportional (P)
• Proportional and integral (PI)
• Proportional with derivative (PD)
• Proportional with integral and derivative (PID)
P, PI or PID control is necessary when process variable cycling is unacceptable or if the load or set
point varies.
NOTE!
For any of these control algorithms to function, the loop must be in automatic
mode.
On/Off Control
On/off control is the simplest way to control a process. The controller turns an output on or off when
the process variable reaches limits around the desired set point. This limit is adjustable.
For example, if the set point is 1000°F and the control hysteresis is 20°F, the heat output switches
on when the process variable drops below 980°F and off when the process rises above 1000°F. A
process using on/off control cycles around the set point. Figure 5.1 illustrates this example.
