User Guide - Eurotherm Ltda

nanodac RECORDER/CONTROLLER: USER GUIDEB2.3 LOOP PARAMETERSB2.3.1 Relative cool gain (R2G)This is the gain of channel 2 control output, relative to the channel 1 control output and is used to compensatefor the different quantities of power available to heat and to cool a process. For example, water coolingapplications might require a relative cool gain of 0.25 because cooling is 4 times greater than the heatingprocess at the operating temperature.By default, this parameter is set automatically when an Autotune is performed, but setting the Tune menuparameter ‘AT.R2G’ to ‘No’ causes the R2G value(s) entered in the PID menu to be used instead.B2.3.2 High and Low cutbackCutback high ‘CBH’ and Cutback low ‘CBL’ are values that modify the amount of overshoot, or undershoot,that occurs during large step changes in PV under start-up conditions, for example. They are independentof the PID terms which means that the PID terms can be set for optimal steady state response and the cutbackparameters used to modify any overshoot which may be present.Cutback involves moving the proportional band towards the cutback point nearest the measured valuewhenever the latter is outside the proportional band and the power is saturated (at 0 or 100% for a heat onlycontroller). The proportional band moves downscale to the lower cutback point and waits for the measuredvalue to enter it. It then escorts the measured value with full PID control to the setpoint. In some cases it cancause a ‘dip’ in the measured value as it approaches setpoint as shown in figure B2.3.2 but generally decreasesthe time to needed to bring the process into operation.The action described above is reversed for falling temperature.If cutback is set to Auto the cutback values are automatically configured to 3 × PB.Upper cutback point (CBH)SP0% outputTemperatureProportional band100% outputLower cutback point (CBL)TimeFigure B2.3.2 Cutback operationBriefly, if PV < CBL then the output is set to its maximum.If PV > CBH, then the output is set to its minimumIf PV lies within the range CBH-CBL, then PID calculations take control.B2.3.3 Manual ResetWith PID control, the integral term automatically removes the steady state error from the setpoint. With PDcontrol, the integral term is set to ‘OFF’, and the measured value will not settle precisely at the setpoint. TheManual Reset parameter (MR in the PID menu) represents the value of the power output that will be deliveredwhen the error is zero. This value must be set manually in order to remove the steady state error.HA030554Issue 7 Nov 12Appendix BPage 303