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PID: Entering Parameters
Normally, you place the PID instruction on a rung without conditional logic. The output remains at its last value when the rung is false. The integral term is also cleared when the rung is false.
During programming, you enter the Control Block, Process Variable, and Control Variable addresses after you have placed the PID instruction on a rung.
The PID instruction does not allow floating-point values for any of its parameters. So, if you attempt to move a floating-point value to one of the PID parameters using ladder logic, a floating-point-to-integer conversion occurs.
Control Block - a file that stores the data required to operate the instruction. The file length is fixed at 23 words and should be entered as an integer file address. Do not write to control block addresses with other instructions in your program. Only the setpoint and following PID instruction flags can be set or cleared by your ladder program:
SP (setpoint) Control Block Word 2
TM (time mode bit) Control Block Word 0, bit 0
AM (auto/manual bit) Control Block Word 0, bit 1
CM (control mode bit) Control Block Word 0, bit 2
OL (output limiting enabled bit) Control Block Word 0, bit 3
Important: |
Do not alter the state of any PID control block value unless you fully understand its function and related effect on your process. Unexpected operation could result with possible equipment damage and/or personal injury. |
Note: |
Use a unique data file to contain your PID control blocks. This avoids accidental reuse of the PID control block addresses by other instructions in your program. |
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Control Block Length - Specify an integer file, for example N7:0. The file length is fixed at 23 words.
Process Variable PV - The element address that stores the process input value. This address can be the location of the analog input word where the value of the input A/D is stored. This value could also be an integer value if you choose to pre-scale your input value to the range 0-16383.
Control Variable CV - The element address that stores the output of the PID instruction. The output value ranges from 0-16383, with 16383 being the 100% ON value. This is normally an integer value, so that you can scale the PID output range to the particular analog range your application requires.