Rockwell-automation T8019 Trusted Process Control Algorithm Software P User Manual download pdf (Page 20)

Trusted

Process Control Algorithms T8019

Issue 8 Sep 07 PD-T8019 20

2.10. Time Averaged Value

T_AVG

OUT

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

. If the number of samples required to support

is greater than 64, then 64 samples will be

used and the time averaged value over these 64 samples returned

. 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.

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.

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Rockwell-automation T8019 Trusted Process Control Algorithm Software P User Manual Page 20