Using PID Instructions

After entering the PID instruction and specifying the PID structure, use the configuration tabs to specify how the PID instruction should function.
Specify Tuning
Select the
Tuning
tab. Changes take effect as soon as you select another field, select
OK
, select
Apply
, or press
Enter
.
In this field:
Do the following:
Setpoint (SP)
Enter a setpoint value (.SP).
Set output %
Enter a set output percentage (.SO).
In software manual mode, this value is used for the output. In auto mode, this value displays the output %.
Output bias
Enter an output bias percentage (.BIAS).
Proportional gain (Kp)
Enter the proportional gain (.KP).
For independent gains, it’s the proportional gain (unitless). For dependent gains, it’s the controller gain (unitless).
Integral gain (Ki)
Enter the integral gain (.KI).
For independent gains, it’s the integral gain (1/sec).
For dependent gains, it’s the reset time (minutes per repeat).
Derivative time (Kd)
Enter the derivative gain (.KD).
For independent gains, it’s the derivative gain (seconds). For dependent gains, it’s the rate time minutes).
Manual mode
Select either manual (.MO) or software manual (.SWM).
Manual mode overrides software manual mode if both are selected.
Specify Configuration
Select the Configuration tab. You must select
OK
or
Apply
for any changes to take effect.
In this field:
Do the following:
PID equation
Select independent gains or dependent gains (.PE).
Use independent when you want the three gains (P, I, and D) to operate independently. Use dependent when you want an overall controller gain that affects all three terms (P, I, and D).
Control action
Select either E=PV-SP or E=SP-PV for the control action (.CA).
Derivative of
Select PV or error (.DOE).
Use the derivative of PV to reduce the risk of output spikes resulting from setpoint changes. Use the derivative of error for fast responses to setpoint changes when the algorithm can tolerate overshoots.
Loop update time
Enter the update time (.UPD) for the instruction.
CV high limit
Enter a high limit for the control variable (.MAXO).(1)
CV low limit
Enter a low limit for the control variable (.MINO).(1)
Deadband value
Enter a deadband value (.DB).
No derivative smoothing
Enable or disable this selection (.NDF).
No bias calculation
Enable or disable this selection (.NOBC).
No zero crossing in deadband
Enable or disable this selection (.NOZC).
PV tracking
Enable or disable this selection (.PVT).
Cascade loop
Enable or disable this selection (.CL).
Cascade type
If cascade loop is enabled, select either slave or master (.CT).
(1) When using the ladder-based PID instruction, if you set MAXO = MINO, the PID instruction resets these values to default. MAXO = 100.0 and MINO = 0.0
Specify Alarms
Select the
Alarms
tab. Select
OK
or
Apply
for any changes to take effect.
In this field:
Do the following:
PV high
Enter a PV high alarm value (.PVH).
PV low
Enter a PV low alarm value (.PVL).
PV deadband
Enter a PV alarm deadband value (.PVDB).
Positive deviation
Enter a positive deviation value (.DVP).
Negative deviation
Enter a negative deviation value (.DVN).
Deviation deadband
Enter a deviation alarm deadband value (.DVDB).
Specify Scaling
Select the Scaling tab. You must select
OK
or
Apply
for any changes to take effect.
In this field:
Do the following:
PV unscaled maximum
Enter a maximum PV value (.MAXI) that equals the maximum unscaled value received from the analog input channel for the PV value.
PV unscaled minimum
Enter a minimum PV value (.MINI) that equals the minimum unscaled value received from the analog input channel for the PV value.
PV engineering units maximum
Enter the maximum engineering units corresponding to .MAXI (.MAXS)
PV engineering units minimum
Enter the minimum engineering units corresponding to .MINI (.MINS)
CV maximum
Enter a maximum CV value corresponding to 100% (.MAXCV).
CV minimum
Enter a minimum CV value corresponding to 0% (.MINCV).
Tieback maximum
Enter a maximum tieback value (.MAXTIE) that equals the maximum unscaled value received from the analog input channel for the tieback value.
Tieback minimum
Enter a minimum tieback value (.MINTIE) that equals the minimum unscaled value received from the analog input channel for the tieback value.
PID Initialized
If you change scaling constants during Run mode, turn this off to reinitialize internal descaling values (.INI).
TIP: When using the ladder-based PID instruction, if you set MAXO = MINO, the PID instruction resets these values to default. MAXO = 100.0 and MINO = 0.0.
Use PID Instructions
PID closed-loop control holds a process variable at a desired set point. The illustration shows an example of a flow-rate/fluid level.
PID Diagram
In the above example, the level in the tank is compared against the setpoint. If the level is higher than the setpoint, the PID equation increases the control variable and causes the outlet valve from the tank to open; thereby decreasing the level in the tank.
The PID equation used in the PID instruction is a positional form equation with the option of using either independent gains or dependent gains. When using independent gains, the proportional, integral, and derivative gains affect only their specific proportional, integral, or derivative terms respectively. When using dependent gains, the proportional gain is replaced with a controller gain that affects all three terms. You can use either form of equation to perform the same type of control. The two equation types are merely provided to let you use the equation type with which you are most familiar.
Gains Option
Derivative Of
Dependent gains
(ISA standard)
Error (E)
Process variable (PV)
Independent gains
Error (E)
Process variable (PV)
Where:
Variable
Description
KP
Proportional gain (unitless) Kp = Kc unitless
Ki
Integral gain (seconds -1)
To convert between Ki (integral gain) and Ti (reset time), see Conversion Formula:
Kd
Derivative gain (seconds)
To convert between Kd (derivative gain) and Td (rate time), use: Kd = Kc (Td) 60
KC
Controller gain (unitless)
Ti
Reset time (minutes/repeat)
Td
Rate time (minutes)
SP
Setpoint
PV
Process variable
E
Error [(SP-PV) or (PV-SP)]
BIAS
Feedforward or bias
CV
Control variable
dt
Loop update time
Conversion Formula
If you do not want to use a particular term of the PID equation, just set its gain to zero. For example if you want no derivative action, set Kd or Td equal to zero.
Provide Feedback
Have questions or feedback about this documentation? Please submit your feedback here.