Red-lion TCU User Manual download pdf (Page 34)

Output Power Dampening (OPdP)

The output power calculated by the PID controller can be dampened

(filtered) to reduce the controller output activity. Those processes with high

gain and/or derivative times or those processes with a relatively high noise

content can benefit from the dampening action.

OPdP - 0 to 250 seconds

The dampening parameter is expressed as a time constant in seconds.

Increasing the value increases the dampening or filtering effect. A value of

zero disables output power dampening. The amount of dampening to be used

depends primarily on the response time of the process and the amount of final

actuator activity desired. Generally, dampening times in the range of 1/20 to

1/50 of the controller’s integral time (or process time constant) prove to be

effective. Dampening times longer than these may cause controller instability

due to the added lag effect of too much filtering.

In the case where a relatively high dampening time is desired, the

controller’s proportional band may be increased to restore an adequate

stability margin. The Auto-tune procedure of the controller sets the

dampening value appropriate to the characteristics of the process.

ON/OFF Control Hysteresis Band (CHYS)

The controller can be placed in the ON/OFF control mode by setting the

proportional band to 0.0%. The control hysteresis value affects only the main

control output (OP1).

CHYS - 1 to 250 degrees

The hysteresis band should be set to a minimum value to eliminate output

chatter at the setpoint. Generally, 2° to 5° is sufficient for this purpose. Set the

hysteresis band to a sufficient level prior to invoking Auto-Tune. Internal

cascade controllers, secondary loop, have a fixed hysteresis of 1.5% of scaled

range.

Auto-Tune Dampening Code (tcod)

Prior to invoking Auto-Tune, the dampening code should be set to achieve

the desired dampening level under PID control. After Auto-tune is complete,

changes to “tcod” parameter have no effect until Auto-tune is re-started.

When set to 0, this yields the fastest process response with possible

overshoot. A setting of 4 yields the slowest response with the least amount of

overshoot. Dampening codes of 0 or 1 are recommended for most thermal

processes.

Figure 14, Dampening Code

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Red-lion TCU User Manual Page 34