Page 246 - Programmable Logic Controllers, Fifth Edition - Mobile version

L1 Inputs Ladder logic program Outputs L2
S1
MOV
S1 MOVE Heater
Source Thermocouple

Destination LED

B3:0/1 LED
Thermocouple LEQ
LESS THAN OR EQUAL
Source A Thermocouple (Internal)
LED Display
Source B 597
59 5

GEQ B3:0/2
GREATER THAN OR EQUAL
Source A Thermocouple (Internal)
Source B 603

High temp. Low temp.
S1 B3:0/2 B3:0/1 Heater

Heater

Figure 10-43 Set-point control program.

The operation of the program can be summarized as • Once the temperature increases to 598°F the LEQ
follows: instruction goes false but the heater output remains
on until the temperature rises to 603°F.
• Oven is to maintain an average set-point tempera- • At the 603°F point the GEQ instruction and
ture of 600°F with a variation of about 1 percent B3:0/2 will both be true and the heater will be
between the off and on cycles. switched off.
• The electric heaters are turned on when the temper-
ature of the oven is 597°F or less and will stay on Several set-point control schemes can be performed by
until the temperature rises to 603°F or more. different PLC models. These include on/off control, pro-
• The electric heaters stay off until the temperature portional (P) control, proportional-integral (PI) control,
drops to 597°F, at which time the cycle repeats and proportional-integral-derivative (PID) control. Each
itself. involves the use of some form of closed-loop control to
maintain a process characteristic, such as a temperature,
• Whenever the less than or equal (LEQ) instruction pressure, flow, or level, at a desired value. When a control
is true, a low-temperature condition exists and the system is designed such that it receives operating infor-
program switches on the heater. mation from the machine and makes adjustments to the
• Whenever the greater than or equal (GEQ) instruc- machine based on this operating information, the system
tion is true, a high-temperature condition exists and is said to be a closed-loop system.
the program switches off the heater. The block diagram of a closed-loop control system is
• For the program as shown the temperature is shown in Figure 10-44. A measurement is made of the
595°F so the LEQ instruction and B3:0/1 will variable to be controlled. This measurement is then com-
both be true and the heater output will be switched pared to a reference point, or set-point. If a difference
on and sealed-in through the heater examine-on (error) exists between the actual and desired levels, the
instruction. PLC control program will take the necessary corrective

Data Manipulation Instructions Chapter 10 227

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