Page 329 - Programmable Logic Controllers, Fifth Edition - Mobile version

Detergent that represents the weight of the container and
contents.
• The sensor signal is subtracted from the voltage sig-
nal or digital code that has been input to represent
Solenoid the desired weight.
gate • As long as the difference between the input signal
and feedback signal is greater than 0, the controller
Box keeps the solenoid gate open.
• When the difference becomes 0, the controller
Scale
outputs a signal that closes the gate.
Figure 14-10 Container-filling process. Virtually all feedback controllers determine their output
by observing the error between the set-point and a mea-
Output Actuator—The component that directly surement of the process variable. Errors can occur when
affects a process change. Examples are motors, an operator changes the set-point or when a disturbance
heaters, fans, and solenoids. or a load on the process changes the process variable. The
controller’s role is to eliminate the error automatically.
The process shown in Figure 14-10 is an example of a
closed-loop continuous control process used to automatically 14.3 On/Off Control
fill box containers to a specified weight of detergent. An
empty box is moved into position and filling begins. The With on/off controllers the final control element is either
weight of the box and contents is monitored. When the on or off—one for the occasion when the value of the
actual weight equals the desired weight, filling is halted. measured variable is above the set-point and the other for
Operation and block diagrams for the container-filling the occasion when the value is below the set-point. The
process are shown in Figure 14-11. The operation of the controller will never keep the final control element in an
process can be summarized as follows: intermediate position. Controlling activity is achieved by
the period of on-off cycling action.
• A sensor attached to the scale weighing the con-
tainer generates the voltage signal or digital code Figure 14-12 shows a system using on/off control in
which a liquid is heated by steam. The operation of the
process can be summarized as follows:
Operation
Error • If the liquid temperature goes below the set-point,
amplifier the steam valve opens and the steam is turned on.
Input + Filling Filled
desired Controller process container • When the liquid temperature goes above the set-point,
weight the steam valve closes and the steam is shut off.
–
• The on/off cycle will continue as long as the system
is operating.
Weight
Feedback sensor Set-point
temperature
On/o
13 6
Block Diagram controller
Thumbwheel
Container filling switch

Process

Final control Measurement
Solenoid Scale
element Set-point (sensor)
Steam Temperature
Valve sensor
Controller

Figure 14-11 Operation and block diagrams for the
container-filling process. Figure 14-12 On/off controlled liquid heating system.

310 Chapter 14 Process Control, Network Systems, and SCADA

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