User Guide - Eurotherm Ltda

nanodac RECORDER/CONTROLLER: USER GUIDEB2.4.5 AUTOTUNE (Cont.)EXAMPLE 3: AUTOTUNE AT SP (HEAT /COOL)It is sometimes necessary to tune at the actual setpoint being used as shown below.For a tune at setpoint, autotune does not calculate cutback since there was no initial start up response to theapplication of heating or cooling. Cutback values of less than 1.6 × PB will not be returned.Peak topeakHysteresisTarget SPPVHigh outputZero outputLow outputOPB - A = 1 minuteFigure B2.4.5c Autotune at setpointAStart of Autotune. A test is done at the start of autotune to establish conditions for atune at setpoint. Conditions are that SP must remain within 0.3% of the range of thecontroller if ‘PB Units’ (Setup menu) is set to ‘Percent’, or +1 engineering unit (1 in 1000)if the ‘PB Units’ is set to ‘Eng’. Range is defined as ‘Range High’ – ‘Range Low’ for processinputs or the thermocouple or RTD range defined in section A3 for temperature inputs.A to BThe output is frozen at he current value for one minute, and the conditions are continuouslymonitored during this period. If the conditions specified above are met, then anautotune at setpoint is initiated at ‘B’. If PV drifts outside the condition limits at any timeduring this period, tuning at SP is abandoned, and tuning resumes as a ‘tune fromabove’ or ‘tune from below’, depending on the direction of drift. Since the loop is alreadyat setpoint, a Tune Control setpoint is not calculated; the loop is forced to oscillateabout the Target SP.C to GThe process is forced to oscillate by switching the output between the output limits.The oscillation period and the peak-to-peak response are determined, and the PIDterms calculated.G to H An extra heating stage is initiated, then all heating and cooling are switched off at H,allowing the plant to respond naturally. The relative cool gain (R2G) is calculated.IAutotune is switched off and the process is allowed to control at the target setpoint usingthe newly calculated terms.HA030554Issue 7 Nov 12Appendix BPage 311