Chapter 4 How To Execute Process Control Instructions; Execution Cycle And Control Cycle - Mitsubishi MELSEC-Q series Programming Manual

CHAPTER 4
4.1

Execution Cycle and Control Cycle

(1) Execution cycle
(a) An execution cycle is an interval at which the process control instruction is executed.
(b) There are the following methods to execute the process control instruction in each execution cycle.
1) Method using timer
A timer is used to measure the execution cycle and the process control instruction is executed when the timer
times out.
2) Method using interrupt programs
Any of interrupt programs of I28 to I31 is run in each execution cycle.
3) Method using fixed scan execution type program
A fixed scan execution type program is run in each execution cycle.
(c) Specify in the special registers (SD1500, SD1501) the value of the execution cycle used for the process control
instruction as a real number.
(2) Control cycle
(a) A control cycle is an interval in which PID control is performed for an instruction such as S.2PID (2-degree-of-
freedom PID).
As the control cycle, specify an integral multiple of the execution cycle.
The S.2PID or similar instruction counts the execution cycle in each execution cycle and starts PID operation when
the specified control cycle is reached.
(b) Specify in the loop tag memory (See Page 27, Section 3.3.1) the control cycle used for the S.2PID or similar
instruction.
The S.2PID or similar instruction uses the value of the control cycle specified in the loop tag memory to perform PID
control.
Example) When monitoring is performed at intervals of 1s in 2-degree-of-freedom PID control and PID control is
carried out at intervals of 5s.
0
Execution cycle
Control cycle
When the control cycle is set to an integral multiple of the execution cycle, monitoring such as a PV check can be
performed in each execution cycle.
HOW TO EXECUTE PROCESS CONTROL
INSTRUCTIONS
1 2 3 4 5 6
1s 1s 1s 1s 1s
5s
(Execution cycle)
N
S.2PID instruction performs processing at intervals of 5s.
7 8 9 10 11 12 13
5s
(s)
14 15 16 17 18
5s
1
2
3
4
6
6
7
8
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