Table of Contents
Advertisement
PRESYS
|
Instruments
PID control
PID - it associates the PID control mode with the related control block, 1 or 2
(or both).
ACTION - this option determines the direction of the control action (direct or
reverse).
REV (reverse) - whenever REV is selected for control action (ACTION), an
increase in the input signal will decrease the output signal. The error (E) between the
setpoint (SP) and the controlled variable (Y) is defined in the reverse action as:
E = SP - Y.
DIR (direct) - whenever DIR is selected for a control action (ACTION), an
increase in the input signal will decrease the output signal. The error (E) between the
setpoint (SP) and the controlled variable (Y) is defined in the reverse action as:
E = Y - SP.
The choice between direct and reverse action will depend on the system to be
controlled.
In the PID control mode the Controller output (U) relates to the setpoint (SP)
and to the controlled variable (Y) through the following equation described in
continuous time:
=
U
) (
t
ENGHIGH
In direct action, the positive sign before the derivative is valid and the error
(E(t)) is taken as the input (Y(t)) minus the setpoint (SP).
In reverse action, the negative sign is valid before the derivative and the error
(E(t)) is taken as setpoint (SP) minus the input (Y(t)).
The coefficients appearing in the above equation are selected within the PID
option and have the following meaning:
GAIN - it amplifies the error signal between the setpoint and the controlled
variable to establish the output signal.
RSTM - this is the Controller manual reset stated as an offset aggregated to
the Controller output signal. It will always be required when proportional control mode
(P) or proportional plus derivative control mode (P+D) are used, so as to eliminate
the offset between the setpoint and the controlled variable.
INT - this is the integral ratio expressed in terms of repeats per minute. It is
defined as the number of times the integral action will repeat the proportional action
resulting from the occurrence of a step in the controlled variable in 1 minute. The
integral action or automatic reset is the most important part governing the setpoint
control. While an error persists between the setpoint and the controlled variable the
integral action will actuate on the output signal until the error is brought to zero.
DER - this is the derivative time given in minutes. It is defined as the time
advance which the derivative action causes on the output signal in relation to the
DCY - 2057 Pressure Controller
GAIN
.
100.0
⋅
−
ENGLOW
⎡
∫
+
E
) (
t
INT
E
) (
t
dt
⎢
⎣
⎤
d
±
⋅
+
DER
Y
) (
t
RSTM
⎥
⎦
dt
Maintenance
Page 39
Advertisement
Table of Contents
