Presentation on theme: "Ch3 Feedback control system characteristics"— Presentation transcript:
1 Ch3 Feedback control system characteristics Main content:Open- and closed-loop control systemsSensitivity to Parameter variationsTransient response of control systemDisturbance in feedback control system
2 continue Main content: Steady-state error The cost of feedback Examples and simulation
3 3.1 Open-loop and closed-loop control systems Review the concepts and structureRefer to P
4 The roles of feedback Benefits: Reduce error (eliminating the error) Reduce sensitivity or Enhance robustnessDisturbance rejection or eliminationImprove dynamic performance or adjust the transient response (such as reduce time constant)
5 3.2 Sensitivity of system to parameter variations System are time-varying in its nature because of inevitable uncertainties such as changing environment , aging , and other factors that affect a control process.All these uncertainties in open-loop system will result in inaccurate output or low performance. However, a closed-loop system can overcome this disadvantage.
6 ContinueA primary advantage of a closed-loop feedback control system is its ability to reduce the system’s sensitivity to parameter variation. Sensitivity analysis Robust control
7 Effect of parameter variations If process is change asOpen-loop systemClosed-loop system
8 continueIn the limit, for small incremental changes, last formula is
9 Sensitivity comparison Open-loop systemClosed-loop system
10 Sensitivity to parameters If system TF isSystem sensitivity to is
11 Example of sensitivity Feedback amplifierGoal: Reduce the sensitivity to parameters variation, that is enhance the robustness to change in amplifier gain.Refer to
12 3.3 Transient response of system Transient response is the response of a system as a function of time. It is one of the most important characteristics of control system.If transient response is not satisfying, what shall we do?
13 Control of transient response (Take speed control system as example) Cascade controllerFeedback controllerRefer to P180 (Figure 4.6 and Figure 4.7)Refer to P181 (Figure 4.8)
14 3.4 Disturbance in a feedback control system Disturbance signal is an unwanted extraneous input signal that affects the system’s output signal, such as noise for amplifier,wind gusts for radar antennas,etc.Feedback control can completely or partially eliminate the effect of disturbance signal.
15 Example of steel rolling mill Refer to PLoad changes or disturbances+NoiseFeedback can alleviate the effects of disturbances and noise signal occurring within the feedback loop.If system exists noise at the input point, we can design a low-pass filter to improve SNR (signal-noise ratio)
16 System sensitivitySystem sensitivity is defined as the ratio of the percentage change in the system transfer function to the percentage change of the process transfer function. It is defined as
17 3.5 Steady-state errorSteady-state error is the error after the transient response has decayed,leaving only the continuous response.Feedback can reduce the steady-state error of control system
18 How to define the error ? From input point: From output point: E a (s)=R(s)-H(s)Y(s)E (s)=R(s)-Y(s)Only for unit feedback H(s)=1,We haveE a (s)= E (s)
19 Comparison of errorOpen-loop systemClosed-loop system
20 continue Open-loop system under unit step input Closed-loop system under unit step input
21 Example illustration Refer to P189-190 An example of first-order system
22 3.6 The cost of feedback Increase of complexity Loss of gain Instability
23 An unrealistic dream Why not simply set G(s)=Y(s)/R(s)=1? Transfer function represent the physical system or process, Therefore G(s)=1 is unrealizable.
24 3.7 Design examples English channel boring machines Mars rover vehicle PD controller, and how to select K ?To compare the sensitivity and steady-state error between open-loop and closed-loop system
25 3.8 Simulation using MATLAB Refer to PSelf-learning after class
26 Summary The fundamental reasons for using feedback are as follows: Decrease the sensitivity to parameter variationImprove transient or dynamic performanceEnhance the robustnessReduce the steady-state errorRefer to P