PID Instruction

Specifications

 PID Special Program :

Open

Automatically created PID program by above dialog box uses two data tables for the loop control. As shown in the above picture,
one table (‘A’) is manipulated for PID initialization and the other table (‘B’) is used for the actual PID calculation. These two tables are summarized as followings. These device memory areas should not be overlapped by the other program memory area.

PIDINIT Instruction (A)

Instruction Operation

This is used to check the items set up for PID operation, and to initialize the devices for PID operation.
PIDINIT instruction is to be operated once initially in an initialization program. If an error occurs in a set value or etc.
while initializing, the PIDCAL instruction in a main program is not operated. But, in case of several loops, corresponding
loops only are not operated.

Instruction Form

PIDINIT " The start address among the devices where set values are stored "

Ex) If the set values of PID are stored from D0008 => PIDINIT D0008

Set Items

 Statuses of PIDINIT

1. Number of Loops
   This is used to set up the number of loops to operate PID. To control an object, one PID operation loop is used.

2. Number of Loops for a Scan
   This is used to set up the number of loops for one scan in case of several loops at sampling intervals.

3. Operation Expression
   Normal(0) : This is the operation to output a manipulated value(MV) to get a current value to a target value in case that the current value(PV) is less than the target value(SV).
   Reverse(1) : This is the operation to output a manipulated value(MV) to get a current value near to a target value in case the current value(PV) is greater than the target value(SV).

4. Sampling Interval(Ts)
   This is used to set up the interval at which PID is operated.

5. Proportional Integer (Kp)
   This, as the constant which is multiplied to a deviation(Sv - Pv) in the proportional operation
   " Mv = Kp * (Sv - Pv) ", is the constant for the proportional operation of PID control. In the above expression, a current value
   reaches a target value fast if a proportional integer(Kp) is big. But, if this value is too big, that gives bad influence on a stable control.

6. Integral Integer (Ki)
   This is the constant to add deviation to a manipulated value or subtract deviation from the manipulated value according
   to time continuously in the integral operation "Mv = P * E + P * 1/Ki * dE/dt" in order to clear the deviation, in case that the deviation(E)
   between a target value(Sv) and a current value(Pv) occurs. As proportional operation is not big influence on the variation of a manipulated
   value, in case of small deviation, the effect to clear the deviation can be gotten by integral operation. Integral time is the one for which the
   manipulated value in integral operation is to be the value in proportional operation after deviation occurs in the integral operation.

7. Differential Integer (Kd)
   This is the constant used to operate a manipulated value in proportion to variation speed, and to control the variation of the deviation in the differential operation "Mv = Kp * E + Kp * dE/dt" in order to clear the deviation, in case that the deviation occurs. Integral time is the one for which the manipulated value in differential operation is to be the value in proportional operation after deviation occurs in the differential operation.

8. Filter Coefficient (α)
   This is used to set up the range of the filtering effect on the measured value(PV), which is inputted from an A/D card. The closer it goes to 0, the less filtering effect is.

9. Min. Manipulated Value (MVLL)
   This is used to set up the min. manipulated value from the PID operation in auto mode. In case that a manipulated value(MV) is less than the min. manipulated value (MVLL), the value is applied as the minimum.

10. Max. Manipulated Value (MVHL)
    This is used to set up the max. manipulated value from the PID operation in auto mode. In case a manipulated value(MV) is greater than the min. manipulated value(MVHL), the value is applied as the maximum.

11. Variation Rate Limit of Manipulated Value (∆MVL)
    This is used to apply the set value for Variation Rate Limit of Manipulated Value if a variation value, comparing the manipulated value(MV) for the previous scan to the manipulated value(MV) for current scan, is greater than the set value for Variation Rate of Manipulated Value.

12. MV Transfer
    MV Transfer Use(1) : This is used to apply the manipulated value(MV) in auto mode to a manually manipulated value(MVMAN) in case that PID control is operated in auto mode and is converted to manual mode.
    MV Transfer Use(0) : This is used to apply the manipulated value(MV) in manual mode to a manually manipulated value(MVMAN).

13. SV Steadiness
    To control the occurrence of over shoot due to a sharply changed target value(SV) and to keep stable control, this is used to divide a variation value into 10 steps, and to increase or decrease the value progressively at sampling intervals(Ts) when a target value is changed.

14. States of PIDINIT
    This is used to indicate the status of error while PID operation is initialized.

PIDCAL Instruction(B)

Instruction Operation

This is used to execute PID operation.

Form of Instruction

PIDCAL   " The start address among the devices where the result values of PID operation are stored "

Ex) If the result values of PID operation are stored from D0100  => PIDCAL  D0100

Result Items

1. Set Value (SV : 0 ~ 16,000)
   This is used to input the target value controlled.

2. Measured Value (PV : 0 ~ 16,000)
   This is the current value of the object controlled, which is read from an A/D card. The value in the device where a measured
   value is stored, is to be read from the A/D card using "FROM" instruction and be updated periodically.

3. Manipulated Value (MV : 0 ~ 16,000)
   This is the result value of the PID operation, which is output to the object controlled through a D/A card.

4. Measured Value after Filtered (PVnf : 0 ~ 16,000)
   The filter is used to protect the instantaneous deviation of the measured value(PV) caused by noise.
   The measured value(PV) is filtered and stored in the device assigned to the measured value after filtered(PVnf).
   At this time, if the filter coefficient(a) is 0, the measured value(PV) is not filtered.

5. Manually Manipulated Value (MVMAN)
   The value set up as manually manipulated value (MVMAN) is outputted as manipulated value (MV).

6. Manual/Auto (0 / 1)
   This is used to select whether the object controlled is controlled by using PID or the value set up in a manually manipulated
   value (MVMAN ) is output to the object controlled.

7. Status Code
   Each of set values is initialized for PID operation. To check that, PID initialization instruction(PID INIT) is used. At this time,
   if there is an error in the set value or during PID operation, the error is stored in the device assigned to status code. Also,
   in case of an error, PID operation instruction(PIDCAL) cannot be operated.