12LS Controller Installation and Operation Manual, Rev 3 ... - Watlow

5.2.2 PROPORTIONAL WITH INTEGRAL CONTROL:The Two-mode PI is the most common of the control modes in use in industry. It must be tunedby tuning the PB first and than the TI may be tuned. Many times preset values may be use toshorten the time required for tuning.The initial setting for the PB may be obtained from the PID TUNING CONSTANTS Table. Auseful range will be in the 5% to 20% of the SP. The initial TI should be set for 60 with TD set toOFF. Set the Digital Filter to 2.Use of TI below 30 seconds will most likely cause cycling and is not recommended for mostapplications. Use of TI above 500 seconds is not recommended as it will give the effect of usingmanual reset. 500 seconds and above should only be used when manual reset is desired for thecontrol action.5.2.3 PROPORTIONAL W/ INTEGRAL W/ DERIVATIVE CONTROL:The Three-mode PID control is used primarily when overshoot of the PV cannot be tolerated asin the Quarter Wave Decay response of two-mode PI control. The PI must be tuned first beforeattempting to tune the Derivative mode.Initial setting of the PB may be selected from the PID TUNING CONSTANTS Table. Initial TIsetting may be set to 60. Adjust as above in the PI tuning. After tuning PI than a setting of 15%of the TI may used for the TD setting.A small amplitude and short time period of the PV cycling is characteristic of a narrow PB. Agreater amplitude and longer time period is characteristic of too fast of a TI.5.2.4 OUTPUT DIGITAL FILTER:The OUTPUT FILTER used by the ANAFAZE Controllers is a digital filter on the output signalafter the PID functions. It has a range of 0-255 levels that gives a time constant of 0-127.5seconds. It is used to filter out erratic swings of the output due to extremely sensitive inputsignals, such as open air T/C in a dry air gas oven or a turbine flow signal.It can be used also to allow the 12LS to function more effectively than with PID alone. Someprocesses may be very sensitive, requiring a wide PB, such that good control control is notpossible. By increasing the digital output filter to reduce the high and low output swings due tothe process, the PB may be narrowed (lower number -- higher gain) to obtain good control.The filter can also be used to forgive badly tuned PID loops and poorly designed processes. Itmay also be used to reduce output noise [control output cycling] when a large amount ofderivative action is required.5-3