Rockwell Automation T8019 Trusted Process Control Algorithm Software Package - Manual (Page 20)

Trusted
TM
Process Control Algorithms T8019
Issue 8 Sep 07 PD-T8019 20
2.10. Time Averaged Value
T_AVG
OUT
PV
Tp
Ts
EN
The average function calculates the average process variable (PV) value over the specified period
(Tp). The Ts specifies the minimum interval between samples. If the sample period is less than the
elapsed period between application program scans, Ts will be the application scan time. PV and OUT
are of type REAL; Tp and Ts are of type TIMER.
The enable input (EN) enables the use of the time averaged value; if this input is TRUE, OUT returns
the time averaged value, if this input is FALSE, out is set to the current process variable (PV) value.
The calculation of the time-averaged value continues irrespective of the EN state.
The function assumes that the input value (PV) changes linearly between samples, i.e. between
application program scans. The maximum number of samples taken must be less than 64, i.e.
64
Ts
Tp
. If the number of samples required to support
Ts
Tp
is greater than 64, then 64 samples will be
used and the time averaged value over these 64 samples returned 5 . The function is to include time
weighting of the sampled values to correct for variation in application scan times. If the sample period
(Tp) is less than the sample interval (Ts) the function will effectively average over a single sample, and
return OUT = PV.
The output value (OUT) will be updated only after a sample has been taken, i.e. after Ts.
If an underflow condition occurs during the execution of the function, the function is to set OUT to zero
and increment the “underflow” error counter.
Overflow conditions are to result in OUT = PV, and increment the “overflow” error counter.
On initialisation, i.e. first application program scan, the average value is set to the current input value,
and is assumed to have been stable at this value for greater than Tp.
Note: Tp and Ts are variable at run-time. Although the function should not normally be used in this
manner, it is important that the behaviour with varying values is predictable. Changes in Tp or
Ts are to result in OUT being held at its previous value until sufficient samples have been taken
to calculate the valid time weighted average, i.e. after Tp.
5 ISaGRAF does not support the verification of the values used for Tp and Ts within the
workbench. Also, note that these values are variables and may be changed at runtime, although this is
not recommended. Changing these values at runtime will result in the same behaviour as on
initialisation, i.e. the input will be assumed to have been stable for Tp and the output set to PV.