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

Tuning Installation and Operation Handbook
4-8 2408 and 2404 Controller
C OMMISSIONING THE M OTORISED V ALVE C ONTROLLER
The commissioning procedure is the same for both bounded and boundless control modes,
except in bounded mode you must first calibrate the position feedback potentiometer, as
described in the section below.
Proceed as follows:
1. Measure the time taken for the valve to be raised from its fully closed to its fully open
position and enter this as the value in seconds into the ‘ tm’ parameter.
2. Set all the other parameters to the default values shown in Table 4-3.
The controller can then be tuned using any of the automatic, or manual, tuning procedures
described earlier in this chapter. As before, the tuning process, either automatic or manual,
involves setting the values of the parameters in Table 4-1. The only difference with
boundless control is that the derivative term ‘
td’, although present, will have no effect.
Adjusting the minimum on-time ‘ mp.t’
The default value of 0.2 seconds is satisfactory for most processes. If, however, after tuning
the process, the valve activity is excessively high, with constant oscillation between raise and
lower pulses, the minimum on-time can be increased.
The minimum on-time determines how accurately the valve can be positioned and therefore
the control accuracy. The shorter the time, the more precise the control. However, if the
time is set too short, process noise will cause an excessively busy valve.
Inertia and backlash settings
The default values are satisfactory for most processes, i.e. ‘
OFF’.
Inertia is the time taken for the valve to stop after the output pulse is turned off. If this
causes a control problem, the inertia time needs to be determined and then entered into the
parameter, ‘
In.t’. The inertia time is subtracted from the raise and lower output pulse
times, so that the valve moves the correct distance for each pulse.
Backlash is the output pulse time required to reverse the direction of the valve, i.e. the time
taken to overcome the mechanical backlash of the linkages. If the backlash is sufficient to
cause a control problem, then the backlash time needs to be determined and then entered into
the parameter, ‘
bac.t’.
The above two values are not part of the automatic tuning procedure and must be entered
manually.
C ALIBRATING THE P OSITION F EEDBACK P OTENTIOMETER
Before proceeding with the feedback potentiometer calibration, you should ensure, in
configuration level, that module position 2 (
2a), or 3 (3a), has its ‘id’ indicating ‘Pot.i’,
(meaning Potentiometer Input). Continue to scroll down the module configuration list.
‘
func’ should be set to ‘Vpos’, ‘VAL.L’ must be set to ‘0’ and ‘VAL.H’ to ‘100’.
Exit from configuration and you are now ready to calibrate the position feedback
potentiometer. Proceed as follows.
1. In Operator level, press the AUTO/MAN button to put the controller in Manual mode.
2. Drive the valve to its fully open position using
.
3. Press until you get to ‘ ip-List’.
4. Press to get to ‘ PCAL-OFF’.