Rockwell Automation T8019 Trusted Process Control Algorithm Software Package - Manual (Page 22)

Trusted
TM
Process Control Algorithms T8019
Issue 8 Sep 07 PD-T8019 22
2.11. Rate of Change Detection
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The Rate of Change Detection function requires 4 parameters: the process variable to be monitored
(PV), the sample interval (Ts), the maximum rate of change parameter (R), and rate of change
hysteresis (H). The function detects when the rate of increase or decrease of the process variable (PV)
has exceeded the defined limit R. When the rate of change has exceeded R, the output (Q) is set to
TRUE. Once the output is TRUE, it is set to FALSE as soon as the rate of increase or decrease has
fallen below R by at least the hysteresis value (H).
The process variable is sampled at the specified sample interval; the sample period used will be the
greater of Ts and the application scan time
6 . The change in PV and the actual period between
samples is determined and used to establish the rate of change.
On initialisation, Q is set to FALSE and the previous PV value to the current PV value
PV, R and H must be of type REAL. Ts must be of type TIMER, the output (Q) is of type Boolean. The
rates of change parameters (R and H) are specified as rate of change per second.
i.e.
( ) ( ) (
)
1
1 2
1 2
1 2
1 2 .
t
UP
t t
t t
t t
t t
UP
Q H R
T T
PV PV
R
T T
PV PV
Q > > =
where:
PV
t1
is the input value on the previous sample, on the first sample this value is set to
the current value of the input, i.e. PV
t2
.
PV
t2
.is the current input value
T
t1
.is the time the previous sample was taken.
T
t2
is the current time.
R is the rate of change limit value
H is the rate of change hysteresis value
Q
1
is the previous value of Q
6 The actual sample interval will be NOT LESS THAN the specified sample interval. The
maximum period between samples is T plus the application scan time.