Red Lion PAX2C - Manual (Page 48)

48
OperaTiOn OvervieW
CONTROLLER POWER-UP
Upon applying power, the controller delays control action and temperature
indication for several seconds to perform several self-diagnostic tests and
display basic controller information. Initially, the controller illuminates both
displays and all annunciators to verify that all display elements are functioning.
The controller then displays the unit model type on the top display as well as the
current firmware revision number on the bottom display. The controller then
checks for correct internal operation and displays an error message (E-XX) if an
internal fault is detected (see Troubleshooting for further information). Upon
completion of this sequence, the controller begins control action by displaying
the temperature/process value and updating the output(s) based on the PID
control calculation.
PROCESS START-UP
After starting the process, the controller’s PID settings must be initially
“tuned” to the process for optimum control. Minimal tuning consists of
adjusting the Proportional Band, Integral Time, and Derivative Time parameters
to achieve the optimum response to a process disturbance. The controller can be
tuned once, but must be re-tuned if the process has been changed significantly.
Several options exist for tuning these parameters:
A) Use the controller’s built-in Auto-Tune feature (see Auto-Tune).
B) Use a manual tuning technique (see Manual Tuning).
C) Use a third party tuning software package (generally expensive and not
always precise).
D) Use values based on control loop experience, calculated values or values
from a similar process.
If the controller is a replacement, the PID settings from the unit being
replaced may be used as good initial values. Be sure to consider any differences
in the units and the PID settings when replacing. The PID settings may be fine
tuned by using the techniques outlined in the PID Control section. After tuning
the controller to the process, it is important to power the load and the controller
at the same time for best start-up response.
CONTROLLER POWER-DOWN
At power down, all parameters and control modes are saved to provide a
quick and predictable process response on the next power-up. When powering
down the process, it is important to power down the controller at the same time.
This prevents the reset action of the controller from shifting the proportional
band while the temperature/process value is dropping and prevents excessive
overshoot on the next process start-up.
COnTrOl mOde eXplanaTiOns
ON/OFF CONTROL
The controller operates in On/Off Control when the Proportional Band is set
to 0.0%. In this control mode, the process will constantly oscillate around the
setpoint value. The On/Off Control Hysteresis (balanced around the setpoint)
can be used to eliminate output chatter. The Output Assignment can be set for
heating (reverse - output on when below the setpoint) or for cooling(direct -
output on when above the setpoint) applications.
ON/OFF CONTROL - FIGURES
Note: HYSt in the On/Off Control Figures is a user defined value in the PID
Configuration Parameters.
For heat and cool systems, one Digital Output is assigned as HEAt (reverse)
and another Digital Output is assigned as COOL (direct). The Proportional Band
is set to 0.0 and the Relative Gain in Cooling to 0.0. The Deadband in Cooling
sets the amount of operational deadband or overlap between the outputs. The
setpoint and the On/Off Control Hysteresis applies to both O1 and O2 outputs.
The hysteresis is balanced in relationship to the setpoint and deadband value.
ON/OFF CONTROL - HEAT/COOL OUTPUT FIGURES
OFF Digital Output :
SP
OFF
INPUT
SP - 1/2 HYSt
SP + 1/2 HYSt
ON
ON Cool Digital Output : ON
SP
OFF
INPUT
HEAT/COOL DEADBAND VALUE (dEAd) = 0
SP + 1/2 HYSt
SP - 1/2 HYSt
OFF OFF Heat Digital Output : ON
HYSt
SP
SP + 1/2 (dEAd) - 1/2 HYSt
SP + 1/2 (dEAd) + 1/2 HYSt
SP - 1/2 (dEAd) + 1/2 HYSt
SP - 1/2 (dEAd) - 1/2 HYSt
HEAT/COOL DEADBAND VALUE (dEAd) < 0
dEAd
HYSt
HYSt
ON
OFF
OFF
ON
Heat Digital Output :
Cool Digital Output :
ON
ON
INPUT
SP + 1/2 (dEAd)
SP - 1/2 (dEAd)
dEAd
Heat Digital Output :
Cool Digital Output :
SP - 1/2 (dEAd) + 1/2 HYSt
SP - 1/2 (dEAd) - 1/2 HYSt
OFF
SP
ON
OFF
OFF
ON
HYSt
OFF
HEAT/COOL DEADBAND VALUE (dEAd) > 0
SP + 1/2 (dEAd) - 1/2 HYSt
SP + 1/2 (dEAd) + 1/2 HYSt
HYSt
INPUT
SP + 1/2 (dEAd)
SP - 1/2 (dEAd)
OFF Digital Output : OFF
SP
ON
INPUT
SP + 1/2 HYSt
SP -1 /2 HYSt
REVERSE ACTING
DIRECT ACTING