2 Overall Course Objectives Develop the skills necessary to function as an industrial process control engineer.SkillsTuning loopsControl loop designControl loop troubleshootingCommand of the terminologyFundamental understandingProcess dynamicsFeedback control
3 Controller TuningInvolves selection of the proper values of Kc, tI, and tD.Affects control performance.Affects controller reliabilityTherefore, controller tuning is, in many cases, a compromise between performance and reliability.
4 Tuning Criteria Specific criteria General criteria Decay ratio Minimize settling timeGeneral criteriaMinimize variabilityRemain stable for the worst disturbance upset (i.e., reliability)Avoid excessive variation in the manipulated variable
6 Performance Assessment Performance statistics (IAE, ISE, etc.) which can be used in simulation studies.Standard deviation from setpoint which is a measure of the variability in the controlled variable.SPC charts which plot product composition analysis along with its upper and lower limits.
8 Classical Tuning Methods Examples: Cohen and Coon method, Ziegler-Nichols tuning, Cianione and Marlin tuning, and many others.Usually based on having a model of the process (e.g., a FOPDT model) and in most cases in the time that it takes to develop the model, the controller could have been tuned several times over using other techniques.Also, they are based on a preset tuning criterion (e.g., QAD)
9 Controller Tuning by Pole Placement Based on model of the processSelect the closed-loop dynamic response and calculate the corresponding tuning parameters.Application of pole placement shows that the closed-loop damping factor and time constant are not independent.Therefore, the decay ratio is a reasonable tuning criterion.
10 Controller Design by Pole Placement A generalized controller (i.e., not PID) can be derived by using pole placement.Generalized controllers are not generally used in industry becauseProcess models are not usually availablePID control is a standard function built into DCSs.
11 IMC-Based TuningA process model is required (Table 9.4 contain the PID settings for several types of models based on IMC tuning).Although a process model is required, IMC tuning allows for adjusting the aggressiveness of the controller online using a single tuning parameter, tf.
12 Controller Reliability The ability of a controller to remain in stable operation with acceptable performance in the face of the worst disturbances that the controller is expected to handle.
13 Controller Reliability Analysis of the closed loop transfer function for a disturbance shows that the type of dynamic response (i.e., decay ratio) is unaffected by the magnitude to the disturbance.
14 Controller Reliability We know from industrial experience that certain large magnitude disturbance can cause control loops to become unstable.The explanation of this apparent contradiction is that disturbances can cause significant changes in Kp, tp, and qp which a linear analysis does not consider.
15 Controller Reliability Example: CSTR with DCA0 Upsets
16 Controller Reliability Is determined by the combination of the following factorsProcess nonlinearityDisturbance typeDisturbance magnitude and durationIf process nonlinearity is high but disturbance magnitude is low, reliability is good.If disturbance magnitude is high but process nonlinearity is low, reliability is good.
19 Tuning Criterion Selection Procedure First, based on overall process objectives, evaluate controller performance for the loop in question.If the control loop should be detuned based on the overall process objectives, the tuning criterion is set.If the control loop should be tuned aggressively based on the overall process objectives, the tuning criterion is selected based on a compromise between performance and reliability.
20 Selecting the Tuning Criterion based on a Compromise between Performance and Reliability Select the tuning criterion (typically from critically damped to 1/6 decay ratio) based on the process characteristics:Process nonlinearityDisturbance types and magnitudes
21 Effect of Tuning Criterion on Control Performance The more aggressive the control criterion, the better the control performance, but the more likely the controller can go unstable.
22 Filtering the Sensor Reading For most sensor readings, a filter time constant of 3 to 5 s is more than adequate and does not slow down the closed-loop dynamics.For a noisy sensor, sensor filtering usually slows the closed-loop dynamics. To evaluate compare the filter time constant with the time constants for the acutator, process and sensor.
23 Recommended Tuning Approach Select the tuning criterion for the control loop.Apply filtering to the sensor readingDetermine if the control system is fast or slow responding.For fast responding, field tune (trail-and-error)For slow responding, apply ATV-based tuning
24 Field Tuning Approach Turn off integral and derivative action. Make initial estimate of Kc based on process knowledge.Using setpoint changes, increase Kc until tuning criterion is met
25 Field Tuning Approach Decrease Kc by 10%. Make initial estimate of tI (i.e., tI=5tp).Reduce tI until offset is eliminatedCheck that proper amount of Kc and tI are used.
26 An Example of Inadequate Integral Action Oscillations not centered about setpoint and slow offset removal indicate inadequate integral action.
27 Demonstration: Visual Basic Simulator Field Tuning Example
28 ATV Identification and Online Tuning Perform ATV test and determine ultimate gain and ultimate period.Select tuning method (i.e., ZN or TL settings).Adjust tuning factor, FT, to meet tuning criterion online using setpoint changes or observing process performance:Kc=KcZN/FT tI=tIZN×FT
29 ATV TestSelect h so that process is not unduly upset but an accurate a results.Controller output is switched when ys crosses y0It usually take 3-4 cycles before standing is established and a and Pu can be measured.
30 Applying the ATV Results Calculate Ku from ATV results.ZN settingsTL settings
31 Comparison of ZN and TL Settings ZN settings are too aggressive in many cases while TL settings tend to be too conservative.TL settings use much less integral action compared to the proportional action than ZN settings. As a result, in certain cases when using TL settings, additional integral action is required to remove offset in a timely fashion.
32 Advantages of ATV Identification Much faster than open loop test.As a result, it is less susceptible to disturbancesDoes not unduly upset the process.
33 Online TuningProvides simple one-dimensional tuning which can be applied using setpoint changes or observing controller performance over a period of time.
38 Critically Damped Tuning for CST Composition Mixer
39 Comparison Between 1/6 Decay Ratio and Critically Damped Tuning
40 Demonstration: Visual Basic Simulator ATV based tuning
41 PID Tuning ProcedureTune PI controller using field tuning or ATV identification with online tuning.Increase tD until minimum response time is obtained. Initially set tD=Pu/8.Increase tD and Kc by the same factor until desired response is obtained.Check response to ensure that proper amount of integral action is being used.
42 Comparison between PI and PID for the Heat Exchanger Model
43 Comparison of PI and PID The derivative action allows for larger Kc which in turn results in better disturbance rejection for certain processes.
44 Demonstration: Visual Basic Simulator PID Tuning Example
45 Initial Settings for Level Controllers for P-only Control Based on critically damped response.FMAX is largest expected change in feed rate.LMAX is the desired level change under feedback control.Useful as initial estimates for slow responding level control systems.
46 Initial Settings for Level Controllers for PI Control Ac is cross-sectional area to tank and r is liquid density.FMAX is largest expected change in feed rate.LMAX is the desired level change under feedback control.Useful as initial estimates for slow responding level control systems.
47 Initial Settings for Level Controllers Use online tuning adjusting Kc and tI with FT to obtain final tuning.Remember that Kc is expressed as (flow rate/%); therefore, height difference between 0% and 100% is required to calculate tI.
48 In-Class ExampleCalculate the initial PI controller settings for a level controller with a critically damped response for a 10 ft diameter tank (i.e., a cylinder placed on its end) with a measured height of 10 ft that normally handles a feed rate of 1000 lb/h. Assume that it is desired to have a maximum level change of 5% for a 20% feed rate change and that the liquid has a density corresponding to that of water.
49 Control Interval, DtDt is usually 0.5 to 1.0 seconds for regulatory loops and 30 to 120 seconds for supervisory loops for DCS’s.In order to adequately approach continuous performance, select the control interval such that: Dt < 0.05(qp+tp)For certain processes, Dt is set by the timing of analyzer updates and the previous formula can be used to assess the effect on control performance
50 Effect of Control Interval on Control Performance qp =0.5When the controller settings for continuous control are used with Dt=0.5, instability results.Results shown here are based on retuning the system for Dt=0.5 resulting in a 60% reduction in Kc.
51 OverviewController tuning is many times a compromise between performance and reliability.Reliability is determined by process nonlinearity and the disturbance type and magnitude.The controller tuning criterion should be based on controller reliability and the process objectives.
52 OverviewClassical tuning methods, pole placement and IMC tuning are not recommended because they are based on a preset tuning criterion and they usually require a process model.Tune fast loops should be tuned using field tuning and slow loops using ATV identification with online tuning.