User Guide - Eurotherm Ltda

nanodac RECORDER/CONTROLLER: USER GUIDE4.5.2 Totaliser configurationTotalisers allow the user to maintain a running total of any input channel, or of any maths channel. Usingmaths channels, it is possible to totalise combinations of input channels so that, for example, the sum of twochannels or the difference between them could be totalised if required.A totaliser is configured using Virtual Channels. This is in essence a way to convert an input signal representinga rate of change of some parameter, such as a fuel flow being measured, for example, in litres/minuteinto a cumulative flow. If the fuel flow is constant then, of course, the conversion would be simple, just multiplythe flow rate by time and the answer comes out directly in litres. Provided, of course, that the time unitsof 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 theresults of the individual calculations must then be added together (Totalised). In order to get reasonableaccuracy it is important that the flow should be reasonably constant during each measurement period. Thismeans that the sampling time for the measurements should be sufficiently frequent that significant changesin flow rate are not missed. If the sampling frequency is high enough, the totalisation process is approximatelyequivalent to mathematical integration of the input signal.The totaliser block in the Nanodac is intended to automate this process. It uses the built-in sampling rate ofthe nanodac (125mSec) as the sampling period for the totalisation process. In addition, it provides two separateparameters which can be used to adjust the results of the totalisation process so that the output fromthe block is scaled in the correct units. Figure 4.5.2. shows the Main configuration parameter list when theVirtual Channel block is being configured as a totaliser.Wiring is carried out, either at the operator interface (Section 7), or in iTools (Section 6).The totaliser equation is:ma ttot t= tot t – 1+ ----------------------------- PSF × USFwhere,tot t = totaliser value this sampletot t-1 = totaliser value last samplema t = process value this samplePSF = Period Scaling Factor (Period)USF = Units Scaling Factor (Units scaler)Note: the time between samples is 125ms.HA030554Issue 7 Nov 12Page 91