Chapter Three: Overview Front Panel Controls
25
GAIN
- The GAIN setting determines the overall gain of the controller and should be as high as
possible without making the system unstable. The higher the gain, the smaller the dead band. If
some overshoot is tolerable, better control (less dead band) can be achieved. Typical settings are
20 to 50%.
BIAS
- The BIAS control adjusts the minimum voltage or current applied to the valve in the
AUTO, EXT, and MANUAL positions. Proper adjustment of the bias will improve the settling
time at initial turn-on, and reduce undershooting during oscillations or large changes in flow
(refer to
How To Use the Bias Control
, page 31).
VALVE MODE SWITCH
- The VALVE MODE SWITCH selects the method of valve control.
In CLOSE, all signal is removed from the valve. The MANUAL control determines the valve
signal (voltage or current). In AUTO, the controller drives the valve according to the error signal
and the phase and gain controls. In EXT, the controller is in automatic unless one of the external
control lines (MANUAL or CLOSE) is connected to digital ground. If both MANUAL and
CLOSE are connected to digital ground, the “Close” operation will dominate. External
commands will not affect the 250 controller unless the CMAE switch is in the EXT position.
MANUAL
- The MANUAL control allows you to control the valve signal (voltage or current)
directly, without worrying about set point setting, process variables, etc. Note that with most
valves, the flow output is not proportional to valve signal.
A signal of from 40 to 90% is needed to start the valve opening and it will be completely open
with an additional 10%.
ERROR METER
- Meter indication of the difference between the set point and the actual
pressure, or flow. The ERROR METER is useful when tuning the PHASE or GAIN controls
because it shows overshoot and oscillation around the proper control level.
The ERROR METER is not provided on controllers with the DVM option.
SET POINT
- The SET POINT control is your input to the 250 controller. The dial represents
the fraction of the full scale pressure range. The SET POINT is set to the desired pressure, or
flow, and the controller maintains that pressure while in the AUTO or EXT mode. For example,
if the transducer’s full scale pressure is 1 Torr and the SET POINT is set for 0.432 (4/32
indicated on the dial), then the 250 controller maintains a pressure of 432 microns. The 1V range
can be used to increase the resolution at the lower 10% of a 10V range. In the example above, if
the 1V range is used (100 microns full scale), the controller maintains 43.2 microns.
When operating with external set point input (0 to 5V), the front panel SET POINT control
should be fully clockwise (CW).
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