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6.5.2 Totalizer configuration
Totalizers allow the user to maintain a running total of any input channel, or of any maths channel. Using maths
channels, it is possible to totalize combinations of input channels so that, for example, the sum of two channels
or the difference between them could be totalized if required.
A totalizer is configured using Virtual Channels. This is in essence a way to convert an input signal representing
a rate of change of some parameter, such as a fuel flow being measured, for example, in litres/minute into a
cumulative flow. If the fuel flow is constant then, of course, the conversion would be simple, just multiply the
flow rate by time and the answer comes out directly in litres. Provided, of course, that the time units of the flow
rate and the time measurement are in the same units. Both need to be in Seconds, Minutes, Hours, etc. in order
to get the correct answer.
If the flow rate is variable, the calculation has to be done repeatedly over the time period required and the results
of the individual calculations must then be added together (totalized). In order to get reasonable accuracy it is
important that the flow should be reasonably constant during each measurement period. This means that the
sampling time for the measurements should be sufficiently frequent that significant changes in flow rate are not
missed. If the sampling frequency is high enough, the totalization process is approximately equivalent to
mathematical integration of the input signal.
The totalizer block in the FEC920 is intended to automate this process. It uses the built-in sampling rate of the
FEC920 (125mSec) as the sampling period for the totalization process. In addition, it provides two separate
parameters which can be used to adjust the results of the totalization process so that the output from the block
is scaled in the correct units. Figure 76 shows the Main configuration parameter list when the Virtual Channel
block is being configured as a totalizer.
Wiring is carried out either at the operator interface (Section 10), or in iTools (Section 9).
The totalizer equation is:
where,
tot
= totalizer value this sample
t
tot
= totalizer value last sample
t-1
ma
= process value this sample
t
PSF
= Period Scaling Factor (Period)
USF
= Units Scaling Factor (Units scaler)
Note: The time between samples is 125ms.
September 18
FEC920: USER GUIDE
ma t
------------------------------ -
tot t
tot t 1
=
+
–
PSF
A5E45696052A Rev-AA
USF
Page 83
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