Danfoss VLT AutomationDrive FC 300 Design Manual

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Danfoss VLT AutomationDrive FC 300 Design Manual Manual pdf 43 page image

3.4.10 Optimisation of the ProcessRegulatorAfter the basic settings have been made, optimise thefollowing:• Proportional gain• Integration time• Differentiation timeIn most processes, this can be done by following thesesteps:1. Start the motor.2. Set 7-33 Process PID Proportional Gain to 0.3 andincrease it until the feedback signal begins tovary continuously. Then reduce the value untilthe feedback signal has stabilised. Now lower theproportional gain by 40-60%.3. Set 7-34 Process PID Integral Time to 20 s andreduce the value until the feedback signal beginsto vary continuously. Increase the integrationtime until the feedback signal stabilises, followedby an increase of 15-50%.4. Only use 7-35 Process PID Differentiation Time forvery fast-acting systems only (differentiationtime). The typical value is 4 times the setintegration time. The differentiator should only beused when the setting of the proportional gainand the integration time has been fullyoptimised. Make sure that oscillations on thefeedback signal are sufficiently dampened by thelow-pass filter on the feedback signal.NOTICEIf necessary, start/stop can be activated a number oftimes to provoke a variation of the feedback signal.3.4.11 Ziegler Nichols Tuning MethodSeveral tuning methods can be used to tune the PIDcontrols of the frequency converter. One approach is touse the Ziegler Nichols tuning method.NOTICEThe method described must not be used on applicationsthat could be damaged by the oscillations created bymarginally 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. The proportional gain isincreased until continuous oscillations are observed via thefeedback, meaning the system is marginally stable. Theperiod of the oscillation (Pu) is determined as shown inIllustration 3.25.130BA183.10y(t)tPuIllustration 3.25 Marginally Stable SystemMeasure Pu when the amplitude of oscillation is quitesmall. Then “back off” from this gain again, as shown inTable 3.12.Ku is the gain at which the oscillation is obtained.Type 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 3.12 Ziegler Nichols Tuning for Regulator,Based on a Stability BoundaryAccording to the Ziegler Nichols rule, experience hasshown that the control setting described in the stepsbelow provides a good closed loop response for manysystems. The process operator can do the final tuning ofthe control repeatedly to yield satisfactory control.Step-by-Step Description1. Select only Proportional Control (Integral time isselected to the maximum value, while the differ-entiation time is selected to zero).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 3.12 to calculate the necessary PIDcontrol parameters.Product Introduction VLT® AutomationDrive FC 300 Design Guide 90-1200 kWMG34S202 - Rev. 2013-08-19 413 3