ABB AX418 User Manual

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63APPENDIX BB3 Setting Up Three Term (PID) Control ParametersTo enable a process to be controlled satisfactorily, the followingconditions must apply:a) The process must be capable of reaching a natural balancewith a steady load.b) It must be possible to introduce small changes into thesystem without destroying either the process or the product.The Proportional Band determines the gain of the system. (thegain is the reciprocal of the proportional band setting, e.g. asetting of 20% is equivalent to a gain of 5). If the proportionalband is too narrow, the control loop may become unstable andcause the system to oscillate. With proportional band controlonly, the system normally stabilizes eventually but at a valuewhich is offset from the set point.The addition of Integral Action Time removes the offset but, if settoo short, can cause the system to go into oscillation. Theintroduction of Derivative Action Time reduces the time requiredby the process to stabilize.B4 Manual TuningBefore starting up a new process or changing an existing one:a) Select the Config. Control page and ensure that Controller isset to PID – see Section 5.7.b) Select the PID Controller page and set the following:Proportional Band – 100%Integral Time – 0 (off) – see Section 5.7.1Derivative Time – 0 (off)Notes.• If the system goes into oscillation with increasingamplitude (Fig. B4 Mode B), reset the proportionalband to 200%. If oscillation continues as in Mode B,increase the proportional band further until thesystem ceases to oscillate.• If the system oscillates as in Fig. B4 Mode A, or doesnot oscillate, refer to step c).c) Reduce the Proportional Band by 20% increments andobserve the response. Continue until the process cyclescontinuously without reaching a stable condition (i.e. asustained oscillation with constant amplitude as shown inMode C). This is the critical point.d) Note the cycle time 't' (Fig. B4 Mode C) and the ProportionalBand (critical value) setting.e) Set Proportional Band to:1.6 times the critical value (for P+D or P+I+D control)2.2 times the critical value (for P+I control)2.0 times the critical value (for P only control)f) Set Integral Time to:t2 (for P+I+D control)t1.2 (for P+D control)Response TimeProcess VariableTimeMode CCycle Time tResponse TimeProcess VariableTimeMode BResponse TimeProcess VariableTimeMode AFig. B4 Control Conditionsg) Set Derivative Time to:t8 (for P+I+D control)t12 (for P+D control)The analyzer is now ready for fine tuning by small adjustments tothe P, I and D terms, after the introduction of a small disturbanceof the set point.

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