Marathon Computer AACC 2000 - Manual (Page 60)

Marathon Monitors Inc.
AACC 2000 Carbon Nov. 1, 1997
60
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 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