Controller Actions:

o Direct Acting: Increase in PV causes an increase in controller output. o Reverse Acting: Increase in PV causes a decrease in controller output. o Process Action: This is not to be confused with controller action. Defines the

relationship between changes in the valve and changes in the measurement.

 Increase in valve position causes an increase in the measurement. (Reverse Output = NO)

 Increase in valve position causes a decrease in the measurement. (Reverse Output = YES)

Note: 0% always means closed to the operator, and 100% always means open to the operator.

o PID Control: Reference Section 18 (Loop Tuning)

o Cascade Control: Two controllers are used but only one process variable “M” is manipulated. The primary controller (master) maintains the primary variable “C1” at its setpoint by adjusting the setpoint “R2” of the secondary controller (slave). The secondary controller in turn, responds both to the output of the primary controller and to the secondary controlled variable “C2”. The inner loop is tuned 1st, then the outer loop is tuned. There are two distinct advantages gained with cascade control:

 Disturbances affecting the secondary variable can be corrected by the secondary controller before a pronounced influence is felt by the primary controller.

 Closing the control loop around the secondary part of the process reduces the phase lag seen by the primary controller, resulting in increased speed of response.

Requirements for cascade control:

 Secondary loop process dynamics must be at least four times as fast as primary loop process dynamics.

 Secondary loop must have influence over the primary loop.

o Feed Forward: The traditional PID controller takes action only when the PV has been moved from set point, SP, to produce a controller error, e(t) = SP – PV. Thus, disruption to stable operation is already in progress before a feedback controller first begins to respond. From this view, a feedback strategy simply starts too late and at best can only work to minimize the upset as events unfold. In contrast, a feed forward controller measures the disturbance, D, while it is still distant. As shown on the next page, a feed forward element receives the measured D, uses it to predict an impact on PV, and then computes preemptive control actions, CO feedforward, that counteract the predicted impact as the disturbance arrives. The goal is to maintain the process variable at set point (PV = SP) throughout the disturbance event.

o Split Range: Split range control in which the output of a controller is split to two or more control valves.

 Controller output 0% Valve A is fully open and Valve B fully closed.  Controller output 25% Valve A is 75% open and Valve B 25% open.  Controller output 50% Both valves are 50% open.

 Controller output 75% Valve A is 25% open and Valve B 75% open.  Controller output 100% Valve A is fully closed and Valve B fully open.

OR

 Controller output 0% Both valves are closed.

 Controller output 25% Valve A is 50% open and Valve B still closed.  Controller output 50% Valve A is fully open and Valve B closed.  Controller output 75% Valve A is fully open and Valve B 50% open.  Controller output 100% Both valves are fully open.

OR

o Ratio: Ratio control is used to ensure that two or more flows are kept at the same ratio even if the flows are changing.

Applications of ratio control:

 Blending two or more flows to produce a mixture with specified composition.  Blending two or more flows to produce a mixture with specified physical

properties.

 Maintaining correct air and fuel mixture to combustion.

The controlled flow is increased and decreased to keep it at the correct ratio with the wild flow.

The "wild flow" is the flow not controlled by this loop. It may be controlled by some other control loop.

The "controlled flow" is controlled by this loop with a setpoint equal to the measured wild flow multiplied by some value (FF-102).

The measured wild flow is multiplied by a value that may be fixed or may be adjustable by the operator. The result of the multiplication becomes the setpoint of the controlled flow controller.

o Dynamic Response:

Dead Time: Usually associated with the physical movement of mass or energy. An

example would be a well insulated flowing pipeline where the temperature is measured at two points separated by a considerable distance. The temperatures recorded from the two measuring points would be identical except they would be separated by the time required for the fluid to move from the upstream to the downstream point of measurement. This process can be described by a single parameter model that represents the dead time.

1   s Ke s



K = process gain; θ = dead time;  = time constant

First Order Lag: A dynamic system will come to equilibrium in five time constants.

The system will reach 63.2% of equilibrium in one time constant, 63.2% of the remaining amount in one more time constant, and so on.

Time since Percentage of

Step Input Change Steady-State Change

1 Time Constant 63.2% 2 Time Constants 86.5% 3 Time Constants 95.0% 4 Time Constants 98.2% 5 Time Constants 99.6% ) ( ) ( ) (t c t K r t c dt d  



o Time Constant: In general terms, the time constant, , describes how fast the PV moves in response to a change in the output The time constant must be positive and it must have units of time. For controllers used on processes comprised of gases, liquids, powders, slurries and melts,  most often has units of minutes or seconds. compute  in five steps:

1. Determine ΔPV, the total change that is going to occur in PV, computed as “final minus initial steady state”.

2. Compute the value of the PV that is 63% of the total change that is going to occur, or “initial steady state PV + 0.63(ΔPV)”.

3. Note the time when the PV passes through the 63% point of “initial steady state PV + 0.63(ΔPV)”.

4. Subtract from it the time when the “PV starts a clear response” to the step change in the output.

5. The passage of time from step 4 minus step 3 is the process time constant, .

Summarizing in one sentence, for step test data,  is the time that passes from when the PV shows its first response to the output step, until when the PV reaches 63% of the total ΔPV change that is going to occur.

o Override: Override control is used to take control of an output from one loop to allow a more important loop to manipulate the output. (similar to high / low select)

The output from two or more controllers are combined in a high or low selector. The output from the selector is the highest or lowest individual controller output.