Click the TDO Parameters tab.

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Figure 6-5. Timed Duration Output Parameters

The output pulse from the TDO function must be defined for proper engineering unit (EU) conversion. The minimum pulse width (0% Count) and the maximum pulse width (100% Count) define the minimum and maximum ON time of the output pulse. The values entered in the 0% Count and 100% Count are the number of seconds the output is ON.

Example:

A TDO is used to emulate a field instrument measuring flow. The TDO outputs a pulse width of 3 seconds for no flow and a pulse width of 12 seconds for 1000 MCF per day flow.

0% Count = 3 seconds 100% Count = 12 seconds

6.5.2 TDO – Defining the Engineering Units

The engineering units (EUs) are defined by entering range values that correspond to the 0% Count and the 100% Count. These values are entered in the Low Reading EU and High Reading EU fields.

Example:

A TDO is used to emulate a field instrument measuring flow. The TDO outputs a pulse width of 3 seconds for no flow and a pulse width of 12 seconds for 1000 MCF per day flow.

0% Count = 3 seconds 100% Count = 12 seconds Low Reading EU = 0 High Reading EU = 1000

6.5.3 TDO – Accuracy / Resolution

The accuracy of the output depends upon the sample rate of the ROC or FloBoss (typically 50 ms) and the width of the maximum pulse. For the preceding example, the accuracy (in percent) is:

sample rate 50 ms accuracy =

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6.5.4 TDO – Time On

The Time On field is located in the main Discrete Outputs screen. The Time On field is calculated from the entered EU Value and the previous definitions of 0% Count, 100% Count, Low Reading EU, and High Reading EU entered in the TDO Parameters screen. The calculation formulas are:

EU Span = High Reading EU Value - Low Reading EU Count Span = 100% Count - 0% Count

Entered EU Value × Count Span On Time =

EU Span + 0% Count

6.5.5 TDO Toggle – Cycle Time

Cycle Time is defined in the TDO Parameters screen for TDO Toggle configurations. The Cycle Time entry is used to define the OFF time in the TDO Toggle mode. The OFF time is calculated by the formula:

Off Time = Cycle Time - On Time

Example:

A TDO is used to emulate a field instrument measuring flow. The TDO outputs a pulse width of 3 seconds for no flow and a pulse width of 12 seconds for 100 MCF per day flow. The output is repeated every 15 seconds.

If the Cycle Time is less than, or equal to the On Time, the OFF time is set to one. Care must be taken in configuration (including other places, such as FSTs) to make sure that the Cycle Time remains greater than the calculated On Time for proper operation.

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6.6 PID

Closed-loop Proportional, Integral, and Derivative (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.

NOTE: To enable PIDs, select ROC > Information > Points. Enter the number of PIDs you desire in the Point PIDs Active field and click Apply.

In a given PID Loop point, either Single or Override control can be accomplished. The PID control loop can use either an Analog Output or a pair of Discrete Outputs to control the regulating device.

For ROC300-Series and FloBoss 407 units, the PID Override control algorithm has two

interdependent PID loops, each of which can have its own Analog Output, but not at the same time. The Override algorithm operates when the Process Variable input exceeds a predetermined switchover Setpoint, the Override (secondary) loop takes over control of the regulating device. When the Process Variable input no longer exceeds the switchover Setpoint, the Single primary loop regains control. One application of Override PID control in a ROC300-Series or FloBoss 407 unit is allowing pressure control to override flow control when the pressure exceeds a Setpoint value. For example: The Single primary flow control loop could be active until the pressure input reaches the Primary Setpoint of 700 psig. At this point, switchover to the Override pressure control loop would occur. Control would return to the Single primary flow control loop when the pressure falls to the Override Setpoint of 630 psig.

For FloBoss 103 and 500-Series units, the PID function acquires the current configured input and calculates the change in output required to maintain the Setpoint for both the Single primary and the Override (secondary) loops. The calculation result that is applied to the output depends upon whether the High or Low Override Type Select is chosen (Section 6.6.4). The PID Parameters screen lists the user-configurable parameters associated with PID control loops. Note that the type of PID control, Single primary or Override, is determined by the Control Type parameter. Override can be disabled, leaving only the Single primary PID function active. Up to three PID configurations can be saved and active in the FloBoss 500-Series unit. An I/O card must be installed in the FloBoss 500-Series unit to produce a control output. One PID configuration can be saved and active in the FloBoss 500-Series unit. The FloBoss 103 must have the optional I/O points installed on the termination board.

The current state of the PID algorithm displays in the Loop Status field as Disabled, Primary, or Override. The status of an enabled Single control loop will always be Primary. For an enabled Override loop, the status indicates which loop is currently active: Primary or Override.

Refer to:

♦ Section 6.6.1, Single PID Loop, on page 6-11.

♦ Section 6.6.2, Primary PID Loop in Override Control, on page 6-12.

♦ Section 6.6.3, Override PID Loop in Override Control, on page 6-14.

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6.6.1 Single PID Loop

To use a Single PID Loop: