Presentation on theme: "Controller Design Based on Transient Response Criteria"— Presentation transcript:
1Controller Design Based on Transient Response Criteria Chapter 12
2Desirable Controller Features 0. Stable1. Quick responding2. Adequate disturbance rejection3. Insensitive to model, measurement errors4. Avoids excessive controller action5. Suitable over a wide range of operating conditionsImpossible to satisfy all 5 unless self-tuning. Use“optimum sloppiness"Chapter 12
6Alternatives for Controller Design 1.Tuning correlations – most limited to 1st order plus dead time2.Closed-loop transfer function - analysis of stability or response characteristics.3.Repetitive simulation (requires computer software like MATLAB and Simulink)4.Frequency response - stability and performance (requires computer simulation and graphics)5.On-line controller cycling (field tuning)Chapter 12
7Controller Synthesis - Time Domain Time-domain techniques can be classified into two groups:(a) Criteria based on a few points in the response(b) Criteria based on the entire response, or integral criteriaApproach (a): settling time, % overshoot, rise time, decay ratio (Fig can be viewed as closed-loop response)Chapter 12Process modelSeveral methods based on 1/4 decay ratio have been proposed: Cohen-Coon, Ziegler-Nichols
10Chapter 12Graphical interpretation of IAE. The shaded area is the IAE value.
11Chapter 12 Approach (b) 1. Integral of square error (ISE) 2. Integral of absolute value of error (IAE)3. Time-weighted IAEPick controller parameters to minimize integral.IAE allows larger deviation than ISE (smaller overshoots)ISE longer settling timeITAE weights errors occurring later more heavilyApproximate optimum tuning parameters are correlatedwith K, , (Table 12.3).Chapter 12
14Chapter 12 Summary of Tuning Relationships 1. KC is inversely proportional to KPKVKM .2. KC decreases as / increases.3. I and D increase as / increases (typically D = 0.25 I ).4. Reduce Kc, when adding more integral action; increase Kc, when adding derivative action5. To reduce oscillation, decrease KC and increase I .Chapter 12
15Chapter 12 Disadvantages of Tuning Correlations 1. Stability margin is not quantified.2. Control laws can be vendor - specific.3. First order + time delay model can be inaccurate.4. Kp, t, and θ can vary.5. Resolution, measurement errors decrease stability margins.6. ¼ decay ratio not conservative standard (too oscillatory).Chapter 12
16Example: Second Order Process with PI Controller Can Yield Second Order Closed-loop Response Chapter 12orPI:Let tI = t1, where t1 > t2Canceling terms,Check gain (s = 0)
17Chapter 12 2nd order response with... and Select Kc to give (overshoot)Chapter 12Figure. Step response of underdamped second-order processesand first-order process.
19Chapter 12 Direct Synthesis ( G includes Gm, Gv) 1. Specify closed-loop response (transfer function)Chapter 122. Need process model, (= GPGMGV)3. Solve for Gc,(12-3b)
20Chapter 12 Specify Closed – Loop Transfer Function (first – order response, no offset)Chapter 12But other variations of (12-6) can be used (e.g., replace time delaywith polynomial approximation)
21Chapter 12 Derivation of PI Controller for FOPTD Process Consider the standard first-order-plus-time-delay model,Chapter 12Specify closed-loop response as FOPTD (12-6), butapproximateSubstituting and rearranging gives a PI controller,with the following controller settings:
22Chapter 12 Derivation of PID Controller for FOPTD Process: let (12-3b) (12-30)
23Chapter 12 Second-Order-plus-Time-Delay (SOPTD) Model Consider a second-order-plus-time-delay model,Use of FOPTD closed-loop response (12-6) and time delay approximation gives a PID controller in parallel form,Chapter 12where
25Example 12.1Use the DS design method to calculate PID controller settings for the process:Consider three values of the desired closed-loop time constant: τc = 1, 3, and 10. Evaluate the controllers for unit step changes in both the set point and the disturbance, assuming that Gd = G. Perform the evaluation for two cases:Chapter 12The process model is perfect ( = G).The model gain is = 0.9, instead of the actual value, K = 2. This model error could cause a robustness problem in the controller for K = 2.
26The IMC controller settings for this example are: 3.751.880.6828.334.171.51153.33Note only Kc is affected by the change in process gain.
27The values of Kc decrease as increases, but the values of and do not change, as indicated by EqChapter 12Figure 12.3 Simulation results for Example 12.1 (a): correct model gain.
28Figure 12.4 Simulation results for Example 12.1 (b): Chapter 12Figure Simulation results for Example 12.1 (b):incorrect model gain.
29Chapter 12 Controller Tuning Relations How to Select tc? Model-based design methods such as DS and IMC produce PI or PID controllers for certain classes of process models, with one tuning parameter tc (see Table 12.1)How to Select tc?Chapter 12Several IMC guidelines for have been published for the model in Eq :> 0.8 and (Rivera et al., 1986)(Chien and Fruehauf, 1990)(Skogestad, 2003)
30Chapter 12 Tuning for Lag-Dominant Models First- or second-order models with relatively small time delays are referred to as lag-dominant models.The IMC and DS methods provide satisfactory set-point responses, but very slow disturbance responses, because the value of is very large.Fortunately, this problem can be solved in three different ways.Method 1: Integrator ApproximationChapter 12Then can use the IMC tuning rules (Rule M or N) to specify the controller settings.
31Chapter 12 Method 2. Limit the Value of tI Skogestad (2003) has proposed limiting the value of :where t1 is the largest time constant (if there are two).Chapter 12Method 3. Design the Controller for Disturbances, RatherSet-point ChangesThe desired CLTF is expressed in terms of (Y/D)d, rather than (Y/Ysp)dReference: Chen & Seborg (2002)
32Chapter 12 Example 12.4 Consider a lag-dominant model with Design three PI controllers:IMCIMC based on the integrator approximation in EqIMC with Skogestad’s modification (Eq )
33Evaluate the three controllers by comparing their performance for unit step changes in both set point and disturbance. Assume that the model is perfect and that Gd(s) = G(s).SolutionThe PI controller settings are:Chapter 12ControllerKcIMC0.5100(b) Integrator approximation0.5565(c) Skogestad8
34Figure Comparison of set-point responses (top) and disturbance responses (bottom) for Example The responses for the integrator approximation and Chen and Seborg (discussed in textbook) methods are essentially identical.Chapter 12
35On-Line Controller Tuning Controller tuning inevitably involves a tradeoff between performance and robustness.Controller settings do not have to be precisely determined. In general, a small change in a controller setting from its best value (for example, ±10%) has little effect on closed-loop responses.For most plants, it is not feasible to manually tune each controller. Tuning is usually done by a control specialist (engineer or technician) or by a plant operator. Because each person is typically responsible for 300 to 1000 control loops, it is not feasible to tune every controller.Diagnostic techniques for monitoring control system performance are available.Chapter 12
36Controller Tuning and Troubleshooting Control Loops Chapter 12
37Chapter 12 Ziegler-Nichols Rules: These well-known tuning rules were published by Z-N in 1942:controllerKctItDPPIPID0.5 KCU0.45 KCU0.6 KCU-PU/1.2PU/2PU/8Chapter 12Z-N controller settings are widely considered to be an "industry standard".Z-N settings were developed to provide 1/4 decay ratio -- too oscillatory?
38Chapter 12 Modified Z-N settings for PID control controller Kc tI tD originalSome overshootNo overshoot0.6 KCU0.33 KCU0.2 KCUPU/2PU/3PU/8Chapter 12