User Guide - Eurotherm Ltda

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nanodac RECORDER/CONTROLLER: USER GUIDEB2.4.5 AUTOTUNE (Cont.)Autotune can be performed at any time, but normally it is performed only once, during the initial commissioningof the process. However, if the process under control subsequently becomes unsatisfactory (becauseits characteristics have changed), it may be necessary to tune again for the new conditions.The auto tune algorithm reacts in different ways depending on the initial conditions of the plant. The explanationsgiven later in this section are for the following example conditions:-1. Initial PV is below the setpoint and, therefore, approaches the setpoint from below for a heat/cool controlloop2. As above, but for a heat only control loop3. Initial PV is at the same value as the setpoint (tune at setpoint). That is, 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 process inputs or the thermocoupleor RTD range defined in section A3 for temperature inputs. If the PV is just outside the range statedabove the autotune will attempt a tune from above or below SP.AUTOTUNE AND SENSOR BREAKWhen the controller is autotuning and sensor break occurs, the autotune aborts and the controller outputsthe sensor break output power ‘Sbrk OP’ set up in the Output menu. Autotune must be re-started when thesensor break condition is no longer present.AUTOTUNE AND INHIBIT OR MANUALIf the Loop Inhibit is asserted or the controller is put into Manual Mode, any tune in progress will be abortedand will need to be restart once the condition has been removed. Note that it is not possible to start an autotunesequence if the loop is inhibited or in Manual control.AUTOTUNE AND GAIN SCHEDULINGWhen gain scheduling is enabled and an autotune is performed, the calculated PID values are written intothe PID set that is active, on completion of the tune. Therefore, the user may tune within the boundaries ofa set and the values will be written into the appropriate PID set. However, if the boundaries are close (becausethe range of the loop is not large), then, at the completion of the tune, it cannot be guaranteed thatthe PID values will be written to the correct set particularly if the schedule type is PV or OP. In this situationthe scheduler (‘Sched Type’) should be switched to ‘Set’ and the ‘active set’ chosen manually.INITIAL CONDITIONSConfigure the parameters described in sections B2.4.3 and B2.4.4, above.Notes:1. The ‘tighter’ power limit applies. For example, if ‘High Output’ is set to 80% and ‘Output High’ isset to 70% then the output power will be limited to 70%2. The PV must oscillate to some degree to allow the tuner to calculate the relevant values. The limitsmust be set so as to allow oscillation about the setpoint.INITIATING THE AUTOTUNEIn the Loop Tune menu for the relevant loop, set ‘TuneEn’ to ‘On’.Appendix BPage 308HA030554Issue 7 Nov 12
nanodac RECORDER/CONTROLLER: USER GUIDEB2.4.5 AUTOTUNE (Cont.)EXAMPLE 1: AUTOTUNE FROM BELOW SP (HEAT/COOL)The point at which Automatic tuning is performed (Tune Control Point) lies just below the setpoint at whichthe process is normally expected to operate (Target Setpoint). This ensures that the process is not significantlyoverheated or overcooled. The Tune Control Point is calculated as follows:-Tune Control Point = Initial PV + 0.75(Target Setpoint – Initial PV).The Initial PV is the PV measured after a 1 minute settling period (point ‘B’ in the figure below).Examples:If Target Setpoint = 500°C and Initial PV = 20°C, then the Tune Control Point is 380°C.If Target Setpoint = 500°C and Initial PV = 400°C, then the Tune Control Point is 475°C.This is because the overshoot is likely to be less as the process temperature approaches the target setpoint.Figure B2.4.5a shows the auto tune sequence.Target SPTrue control point1stovershootPeak topeakIt is not guaranteed thatPV will not exceed SPPVHysteresisHigh outputZero outputLow outputOPB - A = 1 minuteKEYAA to BB to DB to FFGHFigure B2.4.5a Autotune heat/cool processStart of AutotuneHeating and Cooling off for one minute allows steady state conditions to be established.First heat/cool cycle to establish first overshoot. Cutback low (CBL) value calculatedfrom the overshoot magnitude (unless CBL set to ‘Auto’).Two cycles of oscillation allow peak-to-peak value and oscillation period to be determined.PID terms are calculated.Heating is switched on.Heating (and cooling) are switched off allowing the plant to respond naturally. Measurementsover the period F to G are used to calculate the Relative Cool Gain (R2G). CutbackHigh is calculated from the equation (CBH = CBL × R2G).Autotune is turned off and the process is allowed to control at the target setpoint usingthe new control terms.Note: Autotuning from above SP is identical except that heating and cooling are reversed.HA030554Issue 7 Nov 12Appendix BPage 309
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User Guide - Eurotherm Ltda

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