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2 proportional with integral control, 3 proportional w/ integral w/ derivative control, 4 output digital filter – Watlow 12LS Controller User Manual

Page 94

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5-3

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 tuned
by tuning the PB first and than the TI may be tuned. Many times preset values may be use to
shorten the time required for tuning.

The initial setting for the PB may be obtained from the PID TUNING CONSTANTS Table. A
useful range will be in the 5% to 20% of the SP. The initial TI should be set for 60 with TD set to
OFF. Set the Digital Filter to 2.
Use of TI below 30 seconds will most likely cause cycling and is not recommended for most
applications. Use of TI above 500 seconds is not recommended as it will give the effect of using
manual reset. 500 seconds and above should only be used when manual reset is desired for the
control 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 as
in the Quarter Wave Decay response of two-mode PI control. The PI must be tuned first before
attempting to tune the Derivative mode.

Initial setting of the PB may be selected from the PID TUNING CONSTANTS Table. Initial TI
setting 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. A
greater 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 signal
after the PID functions. It has a range of 0-255 levels that gives a time constant of 0-127.5
seconds. It is used to filter out erratic swings of the output due to extremely sensitive input
signals, 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. Some
processes may be very sensitive, requiring a wide PB, such that good control control is not
possible. By increasing the digital output filter to reduce the high and low output swings due to
the 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. It
may also be used to reduce output noise [control output cycling] when a large amount of
derivative action is required.