Presentation on theme: "Model-based PID tuning methods Two degree of freedom controllers"— Presentation transcript:
1Model-based PID tuning methods Two degree of freedom controllers PID Controller TuningIntroductionModel-based PID tuning methodsTwo degree of freedom controllersOn-line PID controller tuningPID tuning guidelines and troubleshootingSimulink example
2Introduction PID control law Transfer function Controller tuning Need to select PID parameters (Kc, tI, tD) that yield “good” closed-loop response to disturbances and setpoint changesOnly know: KKc > 0, tI > 0, tD >0Trial-and-error tuning difficult and time consumingNeed methods to determine good initial parameter valuesRefine parameter values by trial-and error fine tuning
4Closed-Loop Performance Criteria Stability of closed-loop systemMinimization of disturbance effectsRapid, smooth tracking of setpoint changesElimination of steady-state offsetAvoidance of excessive control actionRobustness to changes in process conditionsRobustness to errors in process model
5Model-Based PID Controller Tuning Often based on first-order-plus-time-delay modelIntegral error criteria
7General TrendsThe controller gain (Kc) is inversely proportional to the process gain (K).Kc decreases as the ratio of the time delay (q) and the dominant process time (t) constant increases.Both the integral time (tI) and the derivative time (tD) increase as q/t increases.A reasonable initial guess for the derivative mode is tD = 0.25tI.Kc decreases as integral control is added to a proportional controller. Kc increases as derivative control is added to a PI controller.
8ITAE Controller Tuning Example Process modelITAE tuning for disturbance (more aggressive)ITAE tuning for setpoint (less aggressive)
9Two Degree of Freedom Controllers Seek to tune the setpoint tracking response independent of the disturbance rejection responseSetpoint filteringController operates on Y*sp instead of YspTuning parameter tf determines speed of responseSetpoint weightingTuning parameter b determines speed of response
10On-Line PID Controller Tuning Often a process model is not available for controller tuningNeed to determine tuning parameters directly from plant dataContinuous cycling methodUtilizing a proportional controller, find the Kc value that produces sustained oscillations. This is the ultimate gain Kcu.Determine the period of the sustained oscillations. This is the ultimate period Pu.Kc and Pu can also be found from an available transfer function model by direct substitution.
12Shortcomings of Continuous Cycling Method Time consuming for processes with large time constantsProcess pushed to stability limitUltimate gain does not exist for first- and second-order systems without time delaysGenerally not applicable to integrating and open-loop unstable systems because stability is achieved only for intermediate Kc valuesFirst two shortcomings can be overcome using relay autotuning method to determine Kc and Pu (see text)
14Step Test Method cont. Model structure Calculation of q and t Determine times when output has reached 35.3% (t35) and 85.3% (t85)Calculate q and tThen use tuning method for FOTPD models
15Shortcomings of Step Test Method Experimental test performed under open-loop conditionsMethod is not applicable to open-loop unstable processesStep response can be sensitive to the direction and magnitude of the step change if the process is nonlinear
16Guidelines for Common Control Loops Flow rate control – aggressively tuned PI controller due to fast dynamics and few disturbancesLiquid level control – conservatively tuned P or PI controller depending on whether offset-free performance is requiredGas pressure control – moderately tuned PI controller with little integral action due to small time constants and stability concernsTemperature control – PID controllers with application dependent tuningComposition control – PID or more advanced controller due to sampling delay and measurement noise
17Troubleshooting Control Loops Surveys indicate that as many as 35% of industrial controllers are placed in manual at any given timeDetermine if control problem is attributable to:Transducer – degradation or complete failureController – poor tuning or inadequate designFinal control element – control valve stictionProcess – change in operating conditionTroubleshooting procedureCollect data on control system performanceDetermine if operating conditions have changedCheck individual control system componentsRetune or redesign the controller