User Guide - Eurotherm Ltda

nanodac RECORDER/CONTROLLER: USER GUIDEB2.6.8 Feed forwardFeed forward is a method of adding an extra scalable component to the PID output, before any limiting. Itcan be used, for example, in the implementation of cascade loops and constant head control or it can beused to pre-load the control signal with a value close to that which is required to achieve the setpoint, thusimproving system response. Feed forward (FF) is applied such that the PID output is limited by trim limitsand acts as a trim on a FF value. The FF value is derived either from the PV or setpoint by scaling the PV orSP by the ‘FF Gain’ and ‘FF Offset’. Alternatively, a remote value may be used for the FF value, but this is notsubject to any scaling. The resultant FF value is added to the limited PID OP and becomes the PID outputas far as the output algorithm is concerned. The feedback value then generated must then have the FF contributionremoved before being used again by the PID algorithm. The diagram below shows how feed forwardis implemented.FF GainFF OffsetSPPVFF TypeGainRemoteFF TypeTrimHiFeedbackSP1PVPIDFF Trim LimOutputalgorithmOutputTrimLoFigure B2.6.8 Implementation of Feed forwardB2.6.9 Effect of Control Action, Hysteresis and DeadbandCONTROL ACTIONFor temperature control ‘Control Act’ should be set to ‘Rev’. For a PID controller this means that the heaterpower decreases as the PV increases. For an on/off controller, output 1 (usually heat) will be on (100%) whenPV is below the setpoint and output 2 (usually cool) will be on when PV is above the setpoint.HYSTERESISHysteresis applies to on/off control only and is set in the units of the PV. In heating applications the outputwill turn off when the PV is at setpoint. It will turn on again when the PV falls below SP by the hysteresis value.This shown in Figures B2.6.9a and B2.6.9b below for a heat and cool controller.Hysteresis is intended to prevent the output from repeated switching on and off ‘chattering’ at the controlsetpoint. If the hysteresis is set to 0 then even the smallest change in the PV when at setpoint will cause theoutput to switch. Hysteresis should be set to a value which provides an acceptable life for the output contacts,but which does not cause unacceptable oscillations in the PV.If this performance is unacceptable, it is recommended that PID control be used instead.DEADBANDDeadband ‘Ch2 Deadband’ can operate on both on/off control or PID control where it has the effect of extendingthe period when no heating or cooling is applied. In PID control the effect is modified by both theintegral and derivative terms. Deadband might be used in PID control, for example, where actuators taketime to complete their cycle thus ensuring that heating and cooling are not being applied at the same time.Deadband is likely to be used, therefore, in on/off control only. Figure B2.6.9b, below, adds a deadband of20 to the first example in figure B2.6.9a.HA030554Issue 7 Nov 12Appendix BPage 323