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2.9.5. Tuning the Drive Closed Loop ControllerOnce the drive’s Closed Loop Controller has been set up, the performance of the controller should be tested. In many cases, its performance may beacceptable using the default values of PID Proportional Gain (par. 20-93) and PID Integral Time (par. 20-94). However, in some cases it may be helpfulto optimize these parameter values to provide faster system response while still controlling speed overshoot.2.9.6. Manual PID Adjustment1. Start the motor2. Set par. 20-93 (PID Proportional Gain) to 0.3 and increase it until the feedback signal begins to oscillate. If necessary, start and stop the driveor make step changes in the setpoint reference to attempt to cause oscillation. Next reduce the PID Proportional Gain until the feedback signalstabilizes. Then reduce the proportional gain by 40-60%.3. Set par. 20-94 (PID Integral Time) to 20 sec. and reduce it until the feedback signal begins to oscillate. If necessary, start and stop the driveor make step changes in the setpoint reference to attempt to cause oscillation. Next, increase the PID Integral Time until the feedback signalstabilizes. Then increase of the Integral Time by 15-50%.4. Par. 20-95 (PID Differentiation Time) should only be used for very fast-acting systems. The typical value is 25% of the PID Integral Time (par.20-94). The differentiator should only be used when the setting of the proportional gain and the integral time has been fully optimized. Makesure that oscillations of the feedback signal are sufficiently dampened by the lowpass filter for the feedback signal (par 6 16, 6 26, 5 54 or 5 59,as required).2.9.7. Ziegler Nichols Tuning MethodIn general, the above procedure is sufficient for Water applications. However, other, more sophisticated procedures can also be used. The Ziegler Nicholstuning method is a technique which was developed in the 1940s, but is still commonly used today. It generally provides acceptable control performanceusing a simple experiment and parameter calculation.NB!This method must not be used on applications that could be damaged by oscillations created by marginally stable control settings.Illustration 2.3: Figure 1: Marginally stable system1. Select proportional control only. That is, PID Integral Time (par. 20-94) is set to Off (10000 s) and PID Differentiation Time (par. 20 95) is alsoset to Off (0 s, in this case).2. Increase the value of the PID Proportional Gain (par 20-93) until the point of instability is reached, as indicated by sustained oscillations of thefeedback signal. The PID Proportional Gain that causes sustained oscillations is called the critical gain, Ku .3. Measure the period of oscillation, Pu .NOTE: Pu should be measured when the amplitude of oscillation is relatively small. The output must not saturate (i.e., the maximum or minimumfeedback signal must not be reached during the test).4. Use the table below to calculate the necessary PID control parameters.VLT® AQUA Drive Design Guide 2. Introduction to VLT AQUA DriveMG.20.N4.02 - VLT® is a registered Danfoss trademark 272