User Guide - Eurotherm Ltda

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nanodac RECORDER/CONTROLLER: USER GUIDEB2.4.2 Loop ResponseIgnoring loop oscillation, there are three categories of loop performance viz Under damped, Criticallydamped and Over damped:UNDER DAMPEDIn this situation the parameters are set to prevent oscillation but lead to an overshoot of the Process Value(PV) followed by decaying oscillation until the PV finally settles at the Setpoint. This type of response cangive a minimum time to Setpoint but overshoot may cause problems in certain situations and the loop maybe sensitive to sudden changes in PV, resulting in further decaying oscillations before settling once again.CRITICALLY DAMPEDThis represents an ideal situation where noticeable overshoot to small step changes does not occur and theprocess responds to changes in a controlled, non oscillatory manner.OVER DAMPEDIn this situation the loop responds in a controlled but sluggish manner which results in a non-ideal and unnecessarilyslow loop performance.B2.4.3 Initial SettingsIn addition to the tuning parameters listed above, there are a number of other parameters which can affectloop response. These parameters must be correctly configured before tuning is initiated. Parameters include,but are not limited to:-SETPOINTBefore tuning, the loop conditions should be set as closely as practicable to the actual conditions which willbe met in normal operation. For example, in a furnace or oven application a representative load should beincluded, an extruder should be running, etc.OUTPUT HIGH, OUTPUT LOWThese Output menu heat and cool limits define the overall maximum and minimum power which may bedelivered to the process by the control loop. For a heat only controller the default values are 0 and 100%.For a heat/cool controller the defaults are -100 and 100%. Although most processes are designed to workbetween these limits there may be instances where it is desirable to limit the power delivered to the process.REM. OUTPUT LOW, REM. OUTPUT HIGHIf these Remote Output Limits parameters (Output menu) are used, they are effective only if they lie withinthe Heat/Cool Limits above.CH2 DEADBANDHeat/Cool Dead band If a second (cool) channel is configured, a parameter ‘Ch2 Deadband’ is also availablein the Output menu which sets the distance between the heat and cool proportional bands. The defaultvalue is 0% which means that heating will cease to be available at the same time as cooling becomes available.The dead band may be set to ensure that there is no possibility of the heat and cool channels operatingtogether, particularly when cycling output stages are installed.MINIMUM ON TIMEIf either or both of the output channels is fitted with a relay or logic output, the parameter ‘Min On Time’ appearsin the output menu. This is the cycling time for a time proportioning output and should be set correctlybefore tuning is started.FILTERThe ‘Filter’ parameter is found in the Channel ‘Main’ menu (section 4.4). It is used to remove noise from slowlychanging signals so that the underlying trend can be seen more clearly.Appendix BPage 306HA030554Issue 7 Nov 12
nanodac RECORDER/CONTROLLER: USER GUIDEB2.4.3 INITIAL SETTINGS (Cont.)RATESets the maximum PID rate-of-change. The output rate limit is active during tuning and can affect the tuningresults. Rate is useful in preventing rapid changes in output from damaging the process or heater elements.The parameter ‘Rate’ is found in the ‘Setpoint’ menu.CH1 TRAVEL TIME, CH2 TRAVEL TIMEValve Travel Time. If the output is a motor valve positioner the ‘Ch1 Travel Time’ and Ch2 Travel Time’ Outputmenu parameters must be set correctly. The valve travel time is the time taken for the valve to travel from0% (closed) to 100% (open). This may be different from the motor travel time limits because the mechanicallinkage between the motor and the valve, setting of limit switches etc. can modify behaviour. In a valve positionerapplication, the channel output is wired to the ‘PV’ input of relay 2A2B or 4AC. Configuring this relayas Type = Valve Raise causes the associated relay (3A3C or 5AC respectively) to be configured automaticallyas Type = Valve Lower, and the action of the relay pair is controlled by the single wire. In a heat/cool application,channel one is the heat valve and channel two is the cool valve.B2.4.4 Other tuning considerationsIf a process includes adjacent interactive zones, each zone should be tuned independently with the adjacentzones at operating temperature.It is recommended that a tuning process be initiated when the PV and setpoint are far apart. This allows startup conditions to be measured and cutback values to be calculated more accurately. Cutback is not set for‘Tune at setpoint’.In a programmer/controller tuning should only be attempted during dwell periods and not during rampstages. If a programmer/controller is tuned automatically the controller should be placed in ‘Hold’ duringeach dwell period whilst autotune is active.Note: Tuning, carried out in dwell periods which are at different extremes of temperature may givedifferent results owing to non linearity of heating (or cooling). This may provide a convenient way toestablish values for Gain Scheduling.If an auto tune is initiated there are two further parameters (High Output’ and ‘Low Output’) which need tobe set. These are found in the ‘Tune’ menu.High Output Sets a high output limit to be imposed during autotune. Must be ≤ Output High, set inthe Output menu.Low Output Sets a low output limit to be imposed during autotune. Must be ≥ Output Low, set inthe Output menu.The above values must be set correctly, otherwise sufficient power to achieve SP might not be available duringtuning, and the tune will eventually fail.B2.4.5 AutotuneAutotune automatically sets the following PID menu parameters:PBProportional band.TiIntegral time. If previously set to ‘Off’ Ti will remain off after an autotune.TdDerivative time. If previously set to ‘Off’ Td will remain off after an autotune.CBH, CBL Cutback high and low values. If either is set to ‘Auto’, it will remain so after auto tuning.In order that Autotune set the cutback values for the user, a value other than ‘Auto’ mustbe selected before Autotune is initiated. Autotune never returns cutback values lessthan 1.6 × PBR2GCalculated only if the unit is configured as Heat/Cool. Following an Autotune, R2G liesbetween 0.1 and 10. If the calculated value lies outside this range, a ‘Tune Fail’ alarm isset.LBTLoop break time. Following an autotune, LBT is set to 2 × Ti (if Ti was not previously set‘Off’), or to 12 × Td (if Ti was previously set to ‘Off’).HA030554Issue 7 Nov 12Appendix BPage 307
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nanodacUser Guidenanodac recorder/c
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User Guide - Eurotherm Ltda

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