Danfoss FC 300 Design Manual

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Danfoss FC 300 Design Manual Manual pdf 35 page image

Optimisation of the process regulatorThe basic settings have now been made; all that needs to be done is to optimise the proportional gain,the integration time and the differentiation time (par. 7-33, 7-34, 7-35). In most processes, this can bedone by following the guidelines given below.1. Start the motor2. Set par. 7-33 (Proportional Gain) to 0.3 and increase it until the feedback signal again begins tovary continuously. Then reduce the value until the feedback signal has stabilised. Now lower theproportional gain by 40-60%.3. Set par. 7-34 (Integration Time) to 20 sec. and reduce the value until the feedback signal againbegins to vary continuously. Increase the integration time until the feedback signal stabilises,followed by an increase of 15-50%.4. Only use par. 7-35 for very fast-acting SYSTEMs only (differentiation time). The typical value isfour times the set integration time. The differentiator should only be used when the setting ofthe proportional gain and the integration time has been fully optimised. Make sure that oscilla-tions on the feedback signal is sufficiently dampened by the lowpass filter on the feedback signal.NB!If necessary, start/stop can be activated a number of times in order to provoke a variation ofthe feedback signal.Ziegler Nichols Tuning MethodIn order to tune the PID controls of the frequency converter, several tuning methods can be used. Oneapproach is to use a technique which was developed in the 1950s but which has stood the test of timeand is still used today. This method is known as the Ziegler Nichols tuning.NB!The method described must not be used on applications that could be damaged by the oscil-lations created by marginally stable control settings.The criteria for adjusting the parameters arebased on evaluating the SYSTEM at the limit ofstability rather than on taking a step response. Weincrease the proportional gain until we observecontinuous oscillations (as measured on the feed-back), that is, until the SYSTEM becomes margin-ally stable. The corresponding gain (K u) (calledthe ultimate gain) and the period of the oscillation(Pu) (also called the ultimate period) are deter-mined as shown in Figure 1. Figure 1: Marginally stable SYSTEMPu should be measured when the amplitude of oscillation is quite small. Then we ìback offî from thisgain again, as shown in Table 1.Ku is the gain at which the oscillation is obtained.Type of Control Proportional Gain Integral Time Differentiation TimePI-control 0.45 * Ku 0.833 * Pu -PID tight control 0.6 * K u 0.5 * Pu 0.125 * PuPID some overshoot 0.33 * Ku 0.5 * Pu 0.33 * PuTable 1: Ziegler Nichols tuning for regulator, based on a stability boundary.Experience has shown that the control setting according to Ziegler Nichols rule provides a good closedloop response for many SYSTEMs. The process operator can do the final tuning of the control iterativelyto yield satisfactory control.FC 300 Design GuideIntroduction to FC 300MG.33.B7.02 - VLT is a registered Danfoss trademark 35