IDEC MicroSmart - Manual (Page 374)

21: PID I NSTRUCTION
21-18 « FC4A M ICRO S MART U SER ’ S M ANUAL »
Notes for Using the PID Instruction:
• Since the PID instruction requires continuous operation, keep on the start input for the PID instruction.
• The high alarm output (S2+4) and the low alarm output (S2+5) work while the start input for the PID instruction is on.
These alarm outputs, however, do not work when a PID instruction execution error occurs (S1+2 stores 100 through 107)
due to data error in control data registers S1+0 through S1+26 or while the star t input for the PID instruction is off. Pro-
vide a program to monitor the process variable (S4) separately.
• When a PID execution error occurs (S1+2 stores 100 through 107) or when auto tuning is completed, the manipulated vari-
able (D1) stores 0 and the control output (S2+6) turns off.
• Do not use the PID instruction in program branching instructions: LABEL, LJMP, LCAL, LRET, JMP, JEND, MCS, and MCR.
The PID instruction may not operate correctly in these instructions.
• The PID instruction, using the difference between the set point (S3) and process variable (S4) as input, calculates the
manipulated variable (D1) according to the PID parameters, such as propor tional gain (S1+7), integral time (S1+8), and
derivative time (S1+9). When the set point (S3) or process variable (S4) is changed due to disturbance, overshoot or
undershoot will be caused. Before putting the PID control into actual application, per form simulation tests by changing the
set point and process variable (disturbance) to anticipated values in the application.
• The PID parameters, such as proportional gain (S1+7), integral time (S1+8), and derivative time (S1+9), determined by the
auto tuning may not always be the optimum values depending on the actual application. To make sure of the best results,
adjust the parameters. Once the best PID parameters are determined, per form only the PID action in usual operation
unless the control object is changed.
• When a feedback control is executed using the control output (S2+6), the optimum control may not be achieved depending
on the controlled object. If this is the case, use of the manipulated variable (D1) in the feedback control is recommended.