User Guide - Eurotherm Ltda

nanodac RECORDER/CONTROLLER: USER GUIDEB2.3 LOOP PARAMETERS (Cont.)B2.3.4 Integral HoldIf ‘Integral Hold’ (Main menu) is set to ‘Yes’, the integral component of the PID calculation is frozen, that is, itholds its current value but does not integrate any disturbances in the plant. This is equivalent to switchinginto PD control with a manual reset value preconfigured.Integral Hold may be used, in a situation where the loop is expected to open. For example, it may be necessaryto turn heaters off for a short period or to switch into manual at low power. In this case it may be advantageousto wire Integral Hold to a digital input which activates when the heaters are turned off. When theheaters are switched on again, because the integral is at its previous value, overshoot is minimised.B2.3.5 Integral De-bumpThis feature is not accessible to the user. When changing from Manual to Auto control. the integral componentis forced to: (out put value – proportional component – derivative component) (I = OP – P – D).This ensures that no change occurs in output at the point of switch over, (‘Bumpless Transfer’). The outputpower then gradually changes in accordance with the demand from the PID algorithm.If manual mode = ‘Track’, bumpless transfer also occurs when changing from Auto to Manual control. At thepoint of changeover the output power remains the same as the demand in the auto state. The value can thenbe altered by the operator. For other modes, the output steps to the ‘Forced output’ or ‘Last MOP’ value asappropriate. See ‘Manual Mode in the Output menu for further detailsB2.3.6 Loop BreakLoop Break attempts to detect loss of restoring action in the control loop by checking the control output, theprocess value and its rate of change. Since response times vary from process to process, the Loop BreakTime (LBT) parameter (PID menu) allows a time to be set before a Loop Break Alarm (Loop Break - Diagnosticsmenu) becomes active. LBT is set automatically in Autotune.The Loop Break Alarm parameter has no direct effect on control. In order to define behaviour under LoopBreak conditions, the parameter must be wired, for example, to a relay, which can then activate an externalindicator.It is assumed that, so long as the requested output power is within the output power limits of a control loop,the loop is operating in linear control and is therefore not in a loop break condition. If, however, the outputbecomes saturated then the loop is operating outside its linear control region. If the output remains saturatedat the same output power for a significant duration, then this might be symptomatic of a fault in the controlloop. The source of the loop break is not important, but the loss of control could be catastrophic.Since the worst case time constant for a given load is usually known, a worst case time can be calculated overwhich the load should have responded with a minimum movement in temperature. By performing this calculationthe corresponding rate of approach towards setpoint can be used to determine if the loop can nolonger control at the chosen setpoint. If the PV was drifting away from the setpoint or approaching the setpointat a rate less than that calculated, the loop break condition would be met.If an autotune is performed the loop break time is automatically set to Ti × 2 for a PI or PID loop, or to 12 ×Td for a PD loop. For an On/Off controller loop break detection is based on loop range settings as 0.1 ×Span where Span = Range High – Range Low. Therefore, if the output is at limit and the PV has not movedby 0.1Span in the loop break time a loop break will occur.If the loop break time is 0 (off) the loop break time can be set manually. Then, if the output is in saturationand the PV has not moved by >0.5 × Pb in the loop break time, a loop break condition is considered to haveoccurred.Appendix BPage 304HA030554Issue 7 Nov 12