3.4.5 Ziegler Nichols Tuning MethodIn order to tune the PID controls of the frequency converter, several tuning methods can be used. One approach is to use a technique which was developedin the 1950s but which has stood the test of time and is still used today. This method is known as the Ziegler Nichols tuning method.NB!The method described must not be used on applications that could be damaged by the oscillations created by marginally stable controlsettings.The criteria for adjusting the parameters are based on evaluating thesystem at the limit of stability rather than on taking a step response. Weincrease the proportional gain until we observe continuous oscillations (asmeasured on the feedback), that is, until the system becomes marginallystable. The corresponding gain(Ku ) is called the ultimate gain. The periodof the oscillation(Pu ) (called the ultimate period) is determined as shownin Figure 1.Illustration 3.4: Figure 1: Marginally stable systemPu should be measured when the amplitude of oscillation is quite small. Then we “back off” from this gain 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 *K u 0.833 *Pu -PID tight control 0.6 *Ku 0.5 *Pu 0.125 *PuPID some overshoot 0.33 *K u 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 closed loop response for many systems. The processoperator can do the final tuning of the control iteratively to yield satisfactory control.Step-by-step Description:Step 1: Select only Proportional Control, meaning that the Integral time is selected to the maximum value, while the differentiation time is selected tozero.Step 2: Increase the value of the proportional gain until the point of instability is reached (sustained oscillations) and the critical value of gain,K u , isreached.Step 3: Measure the period of oscillation to obtain the critical time constant,Pu .Step 4: Use the table above to calculate the necessary PID control parameters.FC 300 Design Guide 3 Introduction to AutomationDrive FC 300MG.33.BC.02 - VLT® is a registered Danfoss trademark 373
