3-1
Section 3 Operation
Installation and Operation Manual
X-PR-SLA7800-PC-eng
Part Number: 541B047AAG
December, 2008
Model SLA7810 & SLA7820
3-1 Overview
This section contains the following information:
•
Proportional Band and Integration Time Constant Adjustment
3-2 Proportional Band and Integration Time Constant Adjustment
The Model SLA7810/SLA7820 Pressure Controller has proportional and
integral control action. The reset, or integral, control is combined with the
proportional control actions, commonly referred to as P.I. control. Both
proportional band and integration time are user adjustable by using special
commands over a DeviceNet connection or using Brooks Service Software.
The dynamic behavior of the control system is influenced by flow range,
pressure range, fluid type, orifice size and system volume. Therefore, the
pressure controller may need adjustment in the field to achieve the best
dynamic response.
The factory settings usually result in stable control. For most systems the
dynamic response is adequate with the factory settings. Improved response
can be had by final tuning the P. and I. software parameters in the intended
application by the user. The tuning of the P. and I. constants does not affect
the calibration of the instrument.
Definitions
Both proportional and integral control actions are defined with respect to a
step response error signal. The error signal is the difference between the
actual pressure and the setpoint. Since no control system is instantaneous,
an error signal will be present while the controller is attempting to reach
setpoint.
The SLA7810 and SLA7820 employ digital P.I. controls. There are no
potentiometers to adjust these values.
Proportional Control
Proportional control is a basic continuous control mode in which the
controller output is proportional to the error input signal to the controller. It
provides very rapid response and is relatively stable. Used alone,
proportional control has one disadvantage: at steady state; it exhibits an
offset. That is there is a difference between the desired value, or setpoint
and the actual value of the controlled variable. An integral action added to
the proportional mode eliminates offset.
Proportional control is sometimes called proportional gain. The gain of the
controller is high when the error is large, and low when the error is small.
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