Danfoss VLT AutomationDrive FC 360 Design Manual

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Danfoss VLT AutomationDrive FC 360 Design Manual Manual pdf 38 page image

2.5.5 Optimisation of the Process RegulatorAfter configuring the basic settings as described inchapter 2.5.5 Programming Order, optimise the proportionalgain, the integration time and the differentiation time(7-33 Process PID Proportional Gain, 7-34 Process PID IntegralTime, 7-35 Process PID Differentiation Time). In mostprocesses, complete the following procedure:1. Start the motor.2. Set 7-33 Process PID Proportional Gain to 0.3 andincrease it until the feedback signal again beginsto vary continuously. Reduce the value until thefeedback signal has stabilised. Lower the propor-tional gain by 40-60%.3. Set 7-34 Process PID Integral Time to 20 s andreduce the value until the feedback signal againbegins to vary continuously. Increase theintegration time until the feedback signalstabilises, followed by an increase of 15–50%.4. Only use 7-35 Process PID Differentiation Time forfast-acting systems only (differentiation time). Thetypical value is 4 times the set integration time.Use the differentiator when the setting of theproportional gain and the integration time hasbeen fully optimised. Make sure that oscillationson the feedback signal are sufficiently dampenedby the lowpass filter on the feedback signal.NOTICEIf necessary, start/stop can be activated a number oftimes in order to provoke a variation of the feedbacksignal.2.5.6 Ziegler Nichols Tuning MethodTo tune the PID controls of the frequency converter,Danfoss recommends the Ziegler Nichols tuning method.NOTICEDo not use the Ziegler Nichols Tuning method inapplications that could be damaged by the oscillationscreated by marginally stable control settings.The criteria for adjusting the parameters are based onevaluating the system at the limit of stability rather thanon taking a step response. Increase the proportional gainuntil observing continuous oscillations (as measured onthe feedback), that is, until the system becomes marginallystable. The corresponding gain (Ku) is called the ultimategain, and is the gain at which the oscillation is obtained.The period of the oscillation (Pu) (called the ultimateperiod) is determined as shown in Illustration 2.28 andshould be measured when the amplitude of oscillation isquite small.1. Select only proportional control, meaning thatthe integral time is set to the maximum value,while the differentiation time is set to 0.2. Increase the value of the proportional gain untilthe point of instability is reached (sustainedoscillations) and the critical value of gain, Ku, isreached.3. Measure the period of oscillation to obtain thecritical time constant, Pu.4. Use Table 2.10 to calculate the necessary PIDcontrol parameters.The process operator can do the final tuning of the controliteratively to yield satisfactory control.130BA183.10y(t)tPuIllustration 2.28 Marginally Stable SystemType ofcontrolProportionalgainIntegral time DifferentiationtimePI-control 0.45 * Ku 0.833 * Pu –PID tightcontrol0.6 * Ku 0.5 * Pu 0.125 * PuPID someovershoot0.33 * Ku 0.5 * Pu 0.33 * PuTable 2.10 Ziegler Nichols Tuning for RegulatorProduct Overview Design Guide36 Danfoss A/S © 09/2014 All rights reserved. MG06B40222