Presentation on theme: "CHE 185 – PROCESS CONTROL AND DYNAMICS"— Presentation transcript:
1 CHE 185 – PROCESS CONTROL AND DYNAMICS TUNING FOR PID CONTROL LOOPS
2 Controller TuningInvolves selection of the proper values of Kc, τI, and τD.Affects control performance.Affects controller reliabilityin many cases controller tuning is a compromise between performance and reliability.
3 Available Tuning Criteria Specific criteriaDecay ratioMinimize settling timeGeneral criteriaMinimize variabilityRemain stable for the worst disturbance upset (i.e., reliability)Avoid excessive variation in the manipulated variable
4 Control 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.
6 TUNING CRITERIA error CONTROLLED VARIABLE PERFORMANCE AVOID EXCESSIVE VARIATIONMINIMIZE THE INTEGRAL ABSOLUTE ERROR:MINIMIZE THE INTEGRAL TIME ERROR:
7 TUNING CRITERIA error MANIPULATED VARIABLE AVOID EXCESSIVE SPIKES IN RESPONSE TO SYSTEM DISTURBANCES OR SETPOINT CHANGESMAINTAIN PROCESS STABILITY WITH LARGE CHANGESMINIMAL INTEGRAL SQUARE ERROR:AND INTEGRAL TIME SQUARE ERROR:OBTAIN ZERO STEADY-STATE OFFSETMINIMAL RINGING (EXCESSIVE CYCLING)
8 SUMMARY OF GOALS FOR TUNING DECAY RATIO APPROACHING QUARTER AMPLITUDE DAMPING, QAD
9 Decay Ratio for Non-Symmetric Oscillations Reference figure (c)
10 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)
11 Classical Tuning Methods Cohen and Coon methodTARGET THE VALUES SHOWN IN TABLE 9.2BASED ON MINIMIZING ISE, QAD AND NO OFFSET
12 Classical Tuning Methods CIANCONE AND MARLINDIMENSIONLESS CORRELATIONS BASED ON A TERM CALLED FRACTIONAL DEADTIME: 𝜃 𝑝 𝜃 𝑝 + 𝜏 𝑝RESULTING PARAMETERS ARE PLOTTED IN FIGURE 9.3.2
13 Classical Tuning Methods CIANCONE AND MARLINTHE SEQUENCE OF CALCULATION OF TUNING CONSTANTS:CERTIFY THAT PERFORMANCE GOALS AND ASSUMPTIONS ARE APPROPRIATEDETERMINE THE DYNAMIC MODEL USING AND EMPIRICAL METHOD TO OBTAIN Kp, θp AND τpCALCULATE THE FRACTION DEADTIMEUSE EITHER THE DISTURBANCE (FIGURES a - c) OR SETPOINT (FIGURES d - f) FOR SYSTEM PERTURBATIONS.
14 Classical Tuning Methods CIANCONE AND MARLINTHE SEQUENCE OF CALCULATION OF TUNING CONSTANTS:DETERMINE THE DIMENSIONLESS TUNING PARAMETERS FROM THE GRAPHS: GAIN, INTEGRAL TIME AND DERIVATIVE TIMECALCULATE THE ACTUAL TUNING VALUES FROM THE DIMENSIONLESS VALUES: (e.g.):
15 Classical Tuning Methods STABILTY-BASED METHOD - ZIEGLER-NICHOLSUSES THE ACTUAL SYSTEM TO MEASURE RESPONSES TO PERTURBATIONSAVOIDS THE LIMITS IN MODELING PROCESSESTARGET VALUES ARE IN TABLE 9.3
16 Classical Tuning Methods BASED ON A QAD TUNED RESPONSEBASED ON PROPORTIONAL-ONLY VALUESULTIMATE VALUESGAIN:PERIOD
17 Controller Tuning by Pole Placement (discussed previously) 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.Note eqn should be𝐹= 2𝜁 𝜏 𝑝 𝜏` 𝑝 −1
18 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.
19 Internal model control (IMC)-Based Tuning A 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, τf.
20 RECOMMENDED TUNING METHODS TUNING ACTUAL CONTROL LOOPS DEPENDS ON PROCESS CHARACTERISTICSPROCESSES CAN BE CATEGORIZED AS HAVING SLOW OR FAST RESPONSE, RELATED TO PROCESS DEAD TIME AND THE PROCESS TIME CONSTANTSEE TABLE 9,4 FOR TYPICAL TUNING PARAMETERS FOR PROCESS TYPES.
21 LIMITATIONS ON SETTING TUNING CONSTANTS FOR ACTUAL SYSTEMSIT IS VERY DIFFICULT TO DEVELOP A RIGOROUS MODEL FOR A PROCESS.THERE MAY BE MANY COMPONENTS THAT NEED TO BE INCLUDED IN THE MODEL.NONLINEARITY IS ALSO A FACTORPRESENT IN ALL PROCESSESCAN RESULT IN CHANGE IN PROCESS GAIN AND TIME CONSTANT
22 LIMITATIONS ON SETTING TUNING CONSTANTS ACTUAL PROCESSES MAY EXPERIENCE A RANGE OF OPERATIONS, BUT CONTROL IS TYPICALLY OPTIMIZED FOR ONE SET OF CONDITIONSTABLE 9.5 SHOWS HOW A CONTROL SYSTEMS CAN BECOME UNSTABLE DUE TO CHANGES IN FEED CONCENTRATIONS TO A REACTORTABLE 9.6 SHOWS THE SYSTEM REMAINS STABLE UNDER THE SAME LEVELS OF CONCENTRATION CHANGES IF A REACTION PARAMETER (ACTIVATION ENERGY) IS CHANGED
23 LIMITATIONS ON SETTING TUNING CONSTANTS CHANGES IN CONTROL CAN ALSO AFFECT DOWNSTREAM PROCESSESCHANGING RESIDENCE TIME IN A REACTOR CAN CHANGE THE FEED CONCENTRATIONS TO A DISTILLATION PROCESSCHANGING FEED RATES TO DISTILLATION COLUMNS CAN ALSO IMPACT THE HEAT BALANCE AND PRODUCT CONCENTRATIONS IN THE COLUMNIT MAY NOT BE PRACTICAL TO ACTUALLY INTRODUCE TRACERS OR PERTURBATIONS INTO OPERATING SYSTEMS IN ORDER TO OBTAIN TUNING DATA