Page 332 - Programmable Logic Controllers, Fifth Edition - Mobile version

• The derivative action responds to the speed at which
the error signal is changing—that is, the greater the used type of process controller. When combined into a
single control loop the proportional, integral, and deriva-
error change, the greater the correcting output. The tive modes complement each other to reduce the system
derivative action is measured in terms of time. error to zero faster than any other controller. Figure 14-18
shows the block diagram of a PID control loop, the opera-
Proportional plus integral (PI) control combines the
characteristics of both types of control. A step change in tion of which can be summarized as follows:
the set-point causes the controller to respond proportion- • During setup, the set-point, proportional band, reset
ally, followed by the integral response, which is added (integral), rate (derivative), and output limits are
to the proportional response. Because the integral mode specified.
determines the output change as a function of time, the • All these can be changed during operation to tune
more integral action found in the control, the faster the process.
the output changes. This action can be summarized as • The integral term improves accuracy, and the
follows: derivative reduces overshoot for transient upsets.

• To eliminate the offset error, the controller needs • The output can be used to control valve positions,
to change its output until the process variable error temperature, flow metering equipment, and so on.
is zero. • PID control allows the output power level to be varied.
• Reset integral control action changes the controller • As an example, assume that a furnace is set at 50°C.
output by the amount needed to drive the process • The heater power will increase as the temperature
variable back to the set-point value. falls below the 50°C set-point.
• After the reset integral control action a new equilib- • The lower the temperature the higher the power.
rium point is established. • PID has the effect of gently turning the power down
• Since the proportional controller must always as the signal gets close to the set-point.
operate on its proportional band, the proportional
band must be shifted to include the new equi- The long-term operation of any system, large or
librium point. small, requires a mass-energy balance between input and
• A controller with reset integral control does this output. If a process were operated at equilibrium at all
automatically. times, control would be simple. Because change does
occur, the critical parameter in process control is time, that
Rate action (derivative control) acts on the error signal is, how long it takes for a change in any input to appear in
just like reset does, but rate action is a function of the the output. System time constants can vary from fractions
rate of change rather than the magnitude of error. Rate of a second to many hours. The PID controller has the
action is applied as a change in output for a selectable ability to tune its control action to specific process time
time interval, usually stated in minutes. Rate-induced constants and therefore to deal with process changes over
change in controller output is calculated from the deriva- time. PID control changes the amount of output signal
tive of the error. Input change, rather than proportional in a mathematically specified way that accounts for the
control error change, is used to improve response. Rate amount of error and the rate of signal change.
action quickly positions the output, whereas proportional
action alone would eventually position the output. In Integral
effect, rate action puts the brakes on any offset or error by
quickly shifting the proportional band. Proportional plus Proportional
derivative (PD) control is used in process control systems Set-point + + Error + Process
with errors that change very rapidly. By adding deriva- –
tive control to proportional control, we obtain a controller
output that responds to the error’s rate of change as well Derivative
as to its magnitude.
PID control is a feedback control method that com-
bines proportional, integral, and derivative actions. The PID controller
proportional action provides smooth control without
hunting. The integral action automatically corrects off-
set. The derivative action responds quickly to large exter- Figure 14-18 PID control loop.
nal disturbances. The PID controller is the most widely Source: Photo courtesy Omron Industrial Automation, www.ia.omron.com.

Process Control, Network Systems, and SCADA Chapter 14 313

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