SELECCIÓN DEL MEJOR PROCEDIMIENTO PARA CADA ACTIVIDAD

The following section shows the points to be noted when using a Self-Tuning PID Controller Block (PID-STC) for neutralization control.

Controlled fluid pHC Neutralizing agent pH transmitter pH 14 12 10 8 6 4 2 0 7 MV (neutralizing agent) Figure 1.13.8-4 Neutralization Control

In the neutralization control shown above, the pH process shows non-linear characteristics; i.e., the gain increases significantly near the neutralizing point (pH=7) and becomes small on both sides.

The pH controller is controlled by calculating optimum PID parameter values (P, I, D) near the neutralizing point. Therefore, the proportional band converges by several hundred percent, making it impossible to obtain appropriate control away from the neutralizing point. In this case, specify non-linear gain processing for the controller and use the STC function after ob- taining linear characteristics.

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Tank Level Control with Integral Characteristics

The following section shows the points to be noted when using a Self-Tuning PID Controller Block (PID-STC) in a process with integral characteristics, by using a level control process as an example: Constant flowrate LIC Constant-flow pump Fluid level H t Q1 H Q1 inflow volume Level gauge

Figure 1.13.8-5 Control with Integral Characteristics

The level control shown above is a process that maintains a constant outflow volume via a constant-flow pump, regardless of the fluid level. In this process, the fluid level (H) continues to rise linearly as the inflow volume (Q1) increases. In other words, this is an integral process that does not have a self-regulation function. When controlling an integral process, use pro-

portional and derivative (PD) control by setting a longer integral time, as the process becomes unstable if the integral time is short.

In this type of integral process, use the self-tuning function (STC function) by setting “1” for the process type (IP). When the process type (IP) is “1,” proportional and derivative (PD) con- trol is executed by setting a longer integral time.

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Controlling a Process with Phases of Varying Response Speeds

The following section shows the points to be noted when using a Self-Tuning PID Controller Block (PID-STC) in a process where the response speed varies depending on the phase, by using an example of temperature control.

PV Temperature

Heating phase Cooling phase

Time Time

MV Step input

Figure 1.13.8-6 Example of Step Responses in a Process with Phases of Varying Response Speeds In a heating furnace or heat exchanger process that requires temperature control, the process response time may vary with the heating phase and cooling phase. When using a Self-Tuning PID Controller Block (PID-STC) to control these processes, set the process 95 % response time (TR) based on the phase with a longer response time.

PID parameters changes between the optimum PID parameters (P, I, D) of the two phases in accordance with the direction of process variable response actions.

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Controlling a Process with Slower Actions at the Final Control

Element

The following section shows the points to be noted when using a Self-Tuning PID Controller Block (PID-STC) in a process whose final control element exhibits slower actions, by using an example of flowrate control with a motor-operated valve.

FIC

M Motor-operated _valve Flow gauge

SV + PV PID aTds 1+Tds -

a: First-order lead computation gain Td: First-order lead constant

+ -

MV

Figure 1.13.8-8 Controlling a Process with Slower Actions at the Final Control Element

When performing flowrate control using a motor-operated valve, the speed of response time control for the motor-operated valve may be affected, as the response time of the motor-oper- ated value is slower than that of the flow gauge. As the self-tuning function (STC function) es- timates process characteristics by taking into account delays at the final control element, set a larger proportional band than when there is no delay at the final control element.

If controllability needs to be further improved, consider using phase compensation by first-or- der lead computation, in order to compensate for the derivative actions.

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Program Pattern Control

The following shows points to be noted when applying a Self-Tuning PID Controller Block (PID-STC) to program control, by using an example of a program controlling process where the setpoint value of the temperature controller is changed in accordance with a specific tem- perature rise and fall pattern:

Temperature pattern

Time TPG

TIC

Temperature pattern generator

Fuel

Heat treatment furnace Temperature

Figure 1.13.8-9 Program Control

When applying a Self-Tuning PID Controller Block (PID-STC) to a program controlling proc- ess where the setpoint value of the temperature controller is changed in accordance with a specific temperature rise and fall pattern, note the following points:

• Select the PI-D type (PV derivative type) control algorithm. The I-PD type (PV proportion- al and derivative type) algorithm provides poor follow-up capability relative to changes in SV.

• If it requires that overshoot of the rising temperature must be minimized, set “0” for the control target type (OS) of the Self-Tuning PID Controller Block (PID-STC).

In general, when the setpoint value changes along a ramp pattern, as in the case of rising or falling temperature, the Self-Tuning PID Controller Block (PID-STC) generates an offset. If on- demand tuning is executed at this time, the MV Impulse amplitude (MI) is applied to the ma- nipulated output value (MV) in the direction of decreasing control deviation, then PID parame-

ters (P, I, D) are calculated and set automatically based on the response obtained. As a result, the offset is reduced.

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Batch Control Combining a Self-Tuning PID Controller Block (PID-

STC)

The following section shows the points to be noted when using a Self-Tuning PID Controller Block (PID-STC) for batch control:

SV

STC stop STC stop Time

Batch end Additional raw material input PV Temperature

Figure 1.13.8-10 Batch Control

During PID control, simple batch processes are subject to large disturbances beyond the cor- rectable range through feedback control, as a result of additional charging of raw materials or discharging of products in large quantities. In such cases, use the STC start and stop func- tions to build a sequence control process where STC actions stop temporarily when distur- bances occur.

If the process is left unattended after batch end, with the setpoint value (SV) maintained at a constant level in the automatic (AUT) mode, stop the STC function to prevent any unnecessa- ry action of the self-tuning function (STC function).

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Controlling Interacting Loops Whose Interaction cannot be

Eliminated

The following section shows the points to be noted when using Self-tuning PID controller blocks (PID-STC) to control interacting loops whose interaction cannot be eliminated, by us- ing an example of pressure control and flowrate control interacting with each other:

FC PC

Pressure Flowrate

Figure 1.13.8-11 Controlling Interacting Loops Whose Interaction cannot be Eliminated

If interaction exists between pressure control and flowrate control, as shown in the figure above, using the self-tuning function (STC function) in both control loops causes interacting oscillation when their PID parameters (P, I, D) approach optimum values. In this case, turn off the STC function of one Self-Tuning PID Controller Block (PID-STC) whose process variable (PV) is allowed to change, and set larger values for the current effective proportional band (PB) and current effective integral time (TI). (e.g.: PB=100 to 200 %, TI=30 to 80 seconds) After this, use the STC function of the other Self-Tuning PID Controller Block (PID-STC) only.

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Controlling Interacting Loops whose Interaction can be Eliminated