PID Parameter Configuration

The Proportional, Integral, and Derivative (PID) parameters are configured using the DO control mode of operation. Refer to Figure A-4 for the configuration examples.

NOTE: To enable PIDs, select Information from the System menu, and click the Advanced Features tab. Enter the number of PIDs you desire (cannot exceed the number available) in the PIDs Active field.

Closed-loop PID control is used to provide smooth and stable operation of the feedback control loops employing a regulating device, such as a control valve. The typical use for PID parameters is to control a process variable to a setpoint. Refer to Figure A-4.

Rev 05/03 PID with TDO Control A-5

Figure A-4. Primary PID Parameters – FloBoss 500-Series

2. Set Scanning to Enabled. Loop Status displays the current state of the PID algorithm.

3. Set the Mode to Auto.

4. Select Control Type to Single and Discrete in the Control Type lists.

5. Set the Loop Period as the amount of time between PID calculations in second intervals for the Primary loop. To provide the best timing, this parameter is typically set to one-fourth of the time required for the actuator to move the valve from fully open to fully closed. In this example, it takes 60 seconds to move the valve over its range, so the Loop Period is set to 15 seconds.

6. Set the Primary PID parameters to control a Process Variable to a Setpoint.

♦ Set the Setpoint around which the Primary Process Variable is controlled. Initially set to 0. After the loop is totally configured, set to the desired control Setpoint value.

♦ Set the Process Variable current value as measured by the input definition, used for comparison to the Setpoint. When in Manual Mode, you may enter a value to be in line with the Setpoint so there is not a harsh jump at startup.

A-6 PID with TDO Control Rev 05/03 7. Set the Discrete I/O Definitions when Single Loop Discrete is selected in the Control Type

field of the PID Parameters screen, the PID expects both outputs to be Discrete Outputs. The Discrete Output Control Type for the PID decides which Discrete Output changes to the calculated output. Ramping can be used if the Primary / Override Setpoint change maximum is set to allow for a slower change in the output.

♦ Select the Primary PV correct point type defined for the Primary loop. This is the input assigned as the primary input (Process Variable) for the PID algorithm. When in Manual Mode, you may enter a value to be in line with the Setpoint so there is not a harsh jump at startup. This is the Analog Input that you set up previously.

♦ Specify the DO Open Pt for the desired Discrete Output point to open the valve. This is the Discrete Output that you set up previously.

♦ Set the DO Closed Pt for the desired Discrete Output point to close the valve. This is the Discrete Output that you set up previously.

A.5.1 Tuning Primary PID Parameters

The Primary Loop and the Override Loop are configured independently.

1. Select Configure >Control > PID.

2. Click the Tuning button in the PID Parameters screen, as displayed in Figure A-4, to display the Tuning dialog.

3. Set Halt PID on Reset to Enabled. The PID control loop shuts down upon a reset of the ROC or FloBoss, such as a power restart or a Warm Start.

Rev 05/03 PID with TDO Control A-7 4. Set the Loop Period as the amount of time between PID calculations second intervals for the

Primary loop. To provide the best timing, this parameter is typically set to one-fourth of the time required for the actuator to move the valve from fully open to fully closed. In this example, it takes 60 seconds to move the valve over its range, so the Loop Period is set to 15 seconds.

5. Set the Gain (Proportional Gain) as the ratio of the change in output to the change in input (the Primary Process Variable), based on control action in which there is a continuous linear

relationship between the output and input. An initial proportional Gain setting of 0.2 is recommended to avoid loop instability. Initially set to these values and change later if tuning is required.

6. Set the Reset (Integral Gain) as the ratio of the change in output to the change in input (the Primary Process Variable), based on control action in which the output is proportional to the time integral of the input. An initial Reset setting of 3 is recommended to avoid loop

instability. This value is in terms of repeats per minute. Initially set to these values and change later if tuning is required.

7. Set the Rate (Derivative Gain) as the ratio of the change in output to the change in input (the Primary Process Variable), based on control action in which the output is proportional to the rate of change of the input. An initial Rate setting of 0 is recommended to avoid loop instability.

8. Set the Scale Factor as a number representing the ratio of the output span to input (Primary Process Variable) span. The sign of the number specifies the action of the loop: negative for reverse action (the default), or positive for direct action. Reverse action causes the PID loop point to produce a “decrease” in output (to close a valve, for example) when the Process Variable exceeds the Setpoint. Used to set direction and match the input/output scaling.

9. Set the Integral Deadband as a window around the Primary Setpoint. When the Primary Process Variable is within this window, the integral action (Reset) is disabled. If Discrete Output Control Type is enabled, no output pulses will be produced. For example, if you enter 5, there will be a region of 5 units above and 5 units below the Setpoint in which the Process Variable can move without affecting the output. Set the Integral Deadband to a value that is practical for the control loop, to eliminate constant cycling. If the Process Variable is within the Integral Deadband value of the Setpoint, no output corrections are sent to the TDOs.

10. Set the SP Ramp Rate as the Setpoint ramp is the maximum rate (per minute) at which the Setpoint and the Primary Process Variable are allowed to ramp to a new value.

11. When Manual Tracking is set to Enabled, the loop Setpoint tracks the Process Variable when the loop is in Manual mode. This is typically used to eliminate a “bump” when transferring back to the Automatic mode.

Rev 05/03 FST Editor B-1