Thermcraft XST-6-0-36-3V - Manual (Page 80)

Tuning Installation and Operation Handbook
4-6 2408 and 2404 Controller
Integral action and manual reset
In a full three-term controller (that is, a PID controller), the integral term ‘ti’ automatically
removes steady state errors from the setpoint. If the controller is set up to work in two-term
mode (that is, PD mode), the integral term will be set to ‘OFF’. Under these conditions the
measured value may not settle precisely at setpoint. When the integral term is set to ‘OFF’
the parameter manual reset (code ‘
rES’) appears in the ‘Pid LiSt’ in ‘FuLL’ level. This
parameter represents the value of the power output that will be delivered when the error is
zero. You must set this value manually in order to remove the steady state error.
Automatic droop compensation (Adc)
The steady state error from the setpoint, which occurs when the integral term is set to ‘
OFF’
is sometimes referred to as ‘droop’. ‘ Adc’ automatically calculates the manual reset value in
order to remove this droop. To use this facility, you must first allow the temperature to
stabilise. Then, in the autotune parameter list, you must set ‘
Adc’ to ‘cALc’. The controller
will then calculate a new value for manual reset, and switch ‘ Adc’ to ‘mAn’.
‘
Adc’ can be repeated as often as you require, but between each adjustment you must allow
time for the temperature to stabilise.
Tune Error
If any one stage of the automatic tuning process is not completed within two hours a
diagnostic alarm will occur. The display shows
tU.Er - Tune Error.
This alarm could occur if:
1. The process to be tuned has a very slow response time
2. The sensor has failed or is incorrectly aligned
3. The loop is broken or not responding correctly