User`s manual

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MELSEC-Q
4 HOW TO EXECUTE PROCESS CONTROL INSTRUCTIONS
4 HOW TO EXECUTE PROCESS CONTROL
INSTRUCTIONS
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 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
12
3
4
56
7
8910
11 12
13
14
15 16
17
18
1s 1s 1s 1s 1s
5s 5s 5s
Control cycle
(s)
Execution cycle
(Execution cycle)
N
S.2PID instruction performs processing at intervals of 5s.
POINT
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.
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