Analysis of PID Control Design Methods for a Heated Airow Process


 Darren Wilson
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1 Helsinki University of Technology Faculty of Information and Natural Sciences Department of Mathematics and Systems Analysis Mat.8 Independent research projects in applied mathematics Analysis of PID Control Design Methods for a Heated Airow Process April 7, 8 Tapani Hyvämäki Instructors: Lic. Tech. Lasse Eriksson M. Sc. Antti Pohjoranta
2 Contents Introduction The Case Process. Process Identication Parameter Estimation PID Controller Tuning Methods 6. Ziegler Nichols Method Cohen and Coon Method AMIGO Method Lambda Tuning Method Internal Model Control Method Practical PID Controller Design Aspects Tools for Analysis of Tuning Methods. Performance of Tuning Methods Sensitivity to Disturbances Results. Process Model Parameter Estimation Controller Parameters Comparison of Controllers' Performance Comparison of Controllers' Sensitivity Applying Methods to the Heated Airow Process Educational Discussion 7 Summary and Conclusions 6 A PID Controllers 8 A. Formulations of Controllers A. Lambda Tuning Method Parameters B Performance Measure Formulations 9 C Additional Tables and Figures C. Controller Mvalues and Performances C. Closedloop System Output and Control Signals for Category One Controllers C. Performance and Robustness Comparison
3 Introduction The Department of Automation and Systems Technology is responsible for organizing a laboratory course on automation and control engineering for undergraduate students at the Helsinki University of Technology. One of the fundamental objectives of the course is to cover the basics of PID controllers and PID controllers' tuning methods. The current laboratory exercise on PID control included in the course has been found to be in need of improvements. The aim of this special assignment is to examine methods to renew and modernize the contents of the laboratory exercise and to give students more illustrative and practical exercise to systems' control. The case process under study is a heated airow process. It is described more precisely in Section. First we introduce the system model and the principles of parameter estimation. Dierent PI and PID control methods and also more complicated control methods as well as the tools for analysis and comparison of the controllers are described. Also, some aspects that have to be taken under consideration when implementing controllers are discussed. After familiarization with the used methods, the parameter estimation, simulations with the model and calculations of performance and robustness are performed. As a result, a thorough analysis of the performance and suitability of the controllers to the case process is performed. Comparison of the methods in the sense of performance and robustness is considered and a short discussion of the results from the educational point of view is included. The results obtained here are utilized in the development of the laboratory exercise and in its software implementation. However, the nal development and software implementation are not in the scope of this research project though the model implementation and scripts connected to the derivation of the results of this work will be used in the development of the laboratory course material and software. The Case Process The heated airow process that has been and will be still used as the case process in the laboratory exercise consists of a fan element, heater sink, tube and temperature sensor. The fan produces an airow owing through the tube with constant speed. Air is heated in the beginning of the tube with the heater whose heating power can be adjusted with a control signal. The air temperature is measured with a temperature sensor at dierent distances along the tube. The airow speed inside the tube can be manually controlled by a throttle control. The throttle control angle can be adjusted between 7. In this work we use constant throttle control value of. The temperature sensor position is also preserved at the distance of. cm. A schematic illustration of the process is shown in Figure. The control signal of the process is bounded to interval..v. It is approximately linearly proportional to heater power and can produce a temperature dierence up to 6 C between the inowing and Proportional Integral Derivative
4 Figure : A schematic gure of the case process [9] outowing air. The measurement signal is also bounded to interval..v and is linearly proportional to the airow temperature. [9]. Process Identication The heated airow process can be modeled starting from heat conducted from heater to owing air and again conducted from air into the temperature sensor. This leads to a linear distributed parameter model of rst order represented in [9]. This model can be simplied into a second order linear process, which has the transfer function G (s) = T s(s) P (s) = K (T a s + )(T b s + ) e sl, () where parameters T a and T b are system constants, L is output delay and K is static gain. Output T s is the simulated temperature and it is the temperature above the room temperature. From now on we denote the set of these model parameters as P = {K, T a, T b, L }, the subscript referring to the model order. In the future, we will also denote the system input P (t) with u(t) and output T s (s) with y(t). The model presented above does not take into account, for example, little turbulence in the airow or heat conduction from owing air into the tube before the temperature measurement and vice versa. However, these eects can be assumed to be minimal if the system is operating at some small temperature interval, i.e., varying input signal only at some small interval (and keeping the airow speed constant) keeps also the temperature at bounded interval and the heat conduction minimal. In many cases a rst order approximation of the system is accurate enough to model the process and as will be seen later, many PID tuning methods base on this approximation. Such a system has a transfer function G (s) = K T s + e L s, () where T is the constant, L is delay and K is the static gain of the system. Later on we will denote the set of these parameters as P = {K, T, L }.
5 .. K.8 y(t) / u(t).6. 6.% K K. L T. 6 8 Figure : Tangent method for rst order model parameter determination, static gain K = K /K.. Parameter Estimation In order to obtain proper results when using the system models introduced in previous section, the model parameters P and P need to be estimated. For our purposes it is sucient to estimate the parameters at each operating point at a or use the same estimated parameters in an operating interval as wide as possible. Estimation of parameters P is performed by tting the model output curve to measured process output values for a step input. The curve is tted using the least squares method. So, we minimize the sum of squares min P E(K, T a, T b, L ) = n k= ( Tm,k T s,k (K, T a, T b, L ) ), () where T m,k is the measured temperature value, T s,k is the estimated temperature value, k is the measurement index and n the total number of measurements. The minimization is performed with a Matlab optimization algorithm. Parameters P of the rst order model are determined with slightly dierent and simpler method called tangent method (explained in [] and [6]). The tangent method also utilizes process step response and it is based on sketching a tangent to the step response curve where the slope has its maximum value. Delay L is determined from the intersection point of tangent and taxis and constant T is determined from the value where step response reaches 6,% of it's nal value. Static gain K is the proportion of input and output signals at stationary state K /K. Illustration of the rst order model parameter estimation is shown in Figure.
6 PID Controller Tuning Methods PID controllers are the most widely used controllers in industry. One reason to such popularity is that usually PID controllers are very simple and easy to implement and tune. A basic PID controller consists of proportional, integral and derivative terms and it can be represented as u(t) = K p [ e(t) + T i t e(α)dα + T d d dt e(t)], () where e(t) is the error, u(t) the control signal and K p, T i and T d the controller parameters. The error signal e(t) can be written in terms of the reference signal y r (t) and the system output signal y(t) as e(t) = y r (t) y(t). Several other representations of () exist and certain of them are listed in Appendix A.. It is worth noting that in this report upper case letters K p, T i and T d are used when parameter K p aects all terms of the PID controller and lower case letters k p, k i and k d are used when each parameter aects only certain term of the PID controller. Most of the PID controller tuning methods assume that the controlled process behaves as a linear rst order system with constant output delay, see (). If the system is nonlinear it can be linearized at dierent operating points. Similarly, higher order systems can be approximated with lower order models, especially when the system has dominating poles. PID controllers are mainly used for two purposes in systems' control. The rst is load disturbance rejection which means that the controller is used to compensate the sudden and unexpected disturbances that may occur in process input. The second is reference signal tracking, which means that the controller is used to speed up the process ability to follow changes in the reference signal y r (t). In this work both cases are studied, using dierent PID tuning methods described in following sections. We ended up choosing the following methods mainly for two reasons. First of all, the methods have to be the simplest, because the course is the rst practical course of systems' control for its participants. And secondly as important is that the methods need to clearly illustrate the dierent characteristics as well as advantages and disadvantages of PID controllers. These subjects will be discussed more from the laboratory exercise point of view in Section 6. Other reasons for method selection are that they include the most well known methods (the Ziegler Nichols Methods) and also one fairly recent method (AMIGO) proposed by Åström and Hägglund, who are worldwide recognized researchers and pioneers of modern process control.. Ziegler Nichols Method Ziegler Nichols tuning methods (ZN tuning methods) are the principal methods used in PID controller tuning. The two methods are called Step Response Method and Ultimate Frequency Method. The step response method is based on the openloop step response of the system. The needed system parameters, delay L, constant T and gain K are determined similarly as explained in Section.. Tuning parameters are calculated using formulas in Table. Ultimate frequency method is based on determining the highest gain K kr that keeps the proportionally controlled closedloop system stable. By determining 6
7 T kr from the system output, as seen in Figure, we can calculate the PID controller parameters using formulas in Table. Both of these methods are designed using the quarter amplitude damping design criterion. [6] Table : PID parameters according to the Ziegler Nichols step response method K p T i T d P PI PID T KL  .9T KL L .T KL L.L Table : PID parameters according to the Ziegler Nichols ultimate frequency method K p T i T d P.K kr   PI.K kr.8t kr  PID.6K kr.t kr.t kr. (t) T kr Figure : Ziegler Nichols Ultimate Frequency Method parameters determination. Cohen and Coon Method The Cohen and Coon tuning method is based on the same assumptions on the rst order system as the Ziegler Nichols method. Nevertheless, the tuning criteria and parameters are determined in a slightly 7
8 dierent way. The PID controller parameters are calculated using the gain K, the constant T and the delay L of the rst order approximation of the process (), but using the rules shown in Table.[] Table : PID controller parameters according to the Cohen and Coon method K p T i T d P PI PID ( T +. )   K L ( T ). +.(L /T ) L K L +.(L /T ) ( T. +. ). +.6(L /T ) L K L +.6(L /T ) .7L +.9(L /T ). AMIGO Method AMIGO (Approximate Mconstrained Integral Gain Optimization) tuning method uses a dierent PID control algorithm compared to the previous methods. Here ltering is applied on all terms, not only on the derivative part. The AMIGO controller is given as u(t) = k p ( byr (t) y f (t) ) + k i t y r (α) y f (α)dα + k d ( c dy r (t) dt y f (t)), () dt where b and c are the setpoint weights and y f is the ltered process output signal. y f is obtained from the process output signal y by ltering, i.e. Y f (s) = G f (s)y (s). The lter transfer function is G f (s) = ( + st f ) n, (6) where T f is the lter constant and n =,,.. is the lter of order. The AMIGO PID controller actually has six tunable parameters k p, k i, k d, T f, b and c. The AMIGO tuning rules proposed by Åström and Hägglund in [8] are K p = K (. +. T L ) T i = L.L +.8T L +.T T d =.L T.L + T where parameters K, T and L are determined by the rst order model in (). Filter constant is set to T f.l and the set point weights are set to c = and b =, if L T +L <., if L T +L. Note that k p = K p, k i = K p /T i and k d = K p T d.. Lambda Tuning Method Lambda tuning method is principally a pole placement method. The process model is assumed to be the rst order transfer function (). The closedloop transfer function of the process is desired to be of rst 8 (7)
9 y r +  Q(s) u G(s) G ( s) d + + y  y + Figure : Block diagram of IMC controller order G cl (s) = K + λs e L s (8) where λ is the tuning parameter that determines the pole location. Smaller λ values increase the controller performance but take it closer to the instability region. Now by using the rst order Taylor approximation on the delay and by setting T i = T we get the following PI controller tuning parameters (see Appendix A.) T K p = K(λ + L ), T i = T. (9) Similarly using Padé approximation of the delay we obtain tuning parameters for a PID controller as [] K p = T + L / K(λ + L /), T i = T + L /, T d = T L L + T. (). Internal Model Control Method Internal Model Control (IMC) is a model based tuning method that utilizes a rst or higher order model of the process. It is the only model based tuning method we use that allows applying higher than rst order models. IMC controller consists of an estimator block G, a process G and the controller Q. A schematic block diagram of an IMC controller is illustrated in Figure. The system model G considered in this study was introduced in Section.. The model G(s) can be divided into two parts and written in form G(s) = G + (s) G (s), () where G + (s) is the noninvertible part of the transfer function and G (s) is the invertible part. Thus, all unstable zeros and delays of G(s) are included in G + (s). Now the controller Q(s) can be written in form Q(s) = where f is a lowpass lter of desired order n. For f we can write G (s)f(s), () f(s) = ( + T l s) n, () where T l is the constant of the lter and n is the lter order. The IMC control loop can also be represented in a classical control loop form with controller C(s) as illustrated in Figure. 9
10 y r +  C(s) u G(s) y Then, the controller transfer function becomes Figure : Block diagram of IMC controller Q(s) C(s) =. () G(s)Q(s) Dividing the heated airow process model transfer function () into invertible and noninvertible parts yield to K G + (s) = e sl, G (s) = (T a s + )(T b s + ). () Now, () can be written in the form Q(s) = Furthermore () can be written as G (s)f(s) = (T as + )(T b s + ) K( + T l s) n. (6) C(s) = (T a s + )(T b s + ) K ( + T l s) n, (7) sl e where n should be selected such that the degree of denominator is greater or equal to the degree of numerator. So, here n should be set to n. By approximating the delay with the Padé approximation we can write (7) in form e sl = L s + L s C(s) = L T a T b s + (T a T b + L (T a + T b ))s + (T a + T b + L )s + K L Tl s + (Tl + T l L )s + (T l + L )s (8). (9) It should be noted at this point that T l is the only tunable parameter in the controller. So the tuning of the controller reduces into nding the optimal value for parameter T l. The delay e Ls could also be approximated with a rst order Taylor polynomial which leads into Lambda PID controller introduced in previous Section. [].6 Practical PID Controller Design Aspects When embedding the PID controller into a physical process such as our case process, the heated airow system, some design restrictions must be taken into consideration. We focus on three of those restrictions called integrator windup, measurement noise and nonlinearity.
11 Integrator windup occurs because the control signal is in practice bounded. In our process the heater control signal is bounded to the interval..v. If the closedloop process is controlled with a reference signal y r that exceeds the process maximum output y max, a constant deviation remains in the error signal e(t). This causes the integral term, integrating the error signal to continuously increase, which directly causes the controller output signal to increase. While the actuator remains bounded, the integral term continues increasing and nally when the reference signal y r is set back to a feasible value it takes a relatively long for the controller integral term to get back to normal value. Integrator windup can be prevented using integrator antiwindup, which measures the dierence between the PID controller output and the bounds of output if the signal is out of bounds. Upon actuator saturation the integral term is aected by the antiwindup mechanism to prevent the undesired behavior. Measurement noise always occurs in physical measurements. The temperature measurement output signal of our process has substantially large noise. Measurement noise, especially noise with high frequency, inuences mostly on the PID controller derivative term, because it has an increasing frequency response. Hence, ltering needs to be applied at least on the controller derivative term as explained in Appendix A.. Nonlinearity in a system can be seen, for example, if the parameters of the process changes as a function of the operating point. In our process it can be seen that the static gain, for example, changes dramatically at high temperatures. Generally, the nonlinearities can be avoided by approximating the system to be linear at a small interval around certain operating point. All computeraided controllers are eventually discrete controllers. Discrete controllers are discrete approximations of the corresponding continuous controllers and thus contains always some error compared to the continuous controllers. However, with modern microprocessors and measurement electronics the sampling interval can be so small that the error is very small. [6] Tools for Analysis of Tuning Methods The educational purpose of the PID control laboratory exercise is to give a clear and illustrative picture of the basic behavior and characteristics of PID controlled systems, to familiarize students with advantages and disadvantages of PID controllers and to introduce the tuning methods of PID controllers. To meet these requirements some comparison and analysis of the PID control methods needs to be performed. Our target is to choose a few of the methods introduced in Section and use them rather than using all methods. It is also desired that the application area that is originally intended for each controller is illustrated. In order to be able to compare the methods, the performance criteria need to be set. In this work we focus on performance and robustness. This section briey reviews the chosen criteria.
12 . Performance of Tuning Methods There are several criteria that are used to measure controller performance. These methods more or less measure the total error between the system reference path and the real system path. The formulation of a performance measure can be represented in form J i (T ) = T h(e(t), u(t), t)dt, () where e(t) is the deviation from the path error, u(t) is the control signal and h is a weight function giving positive values. Dierent representations of performance measures are shown in Appendix B []. Another measure indicating system performance is the system settling. Generally, settling is determined as the from which the error remains less than % of the reference value after the step change in reference. Nevertheless, somes it can be quite dicult to determine the settling accurately. Especially, if the error signal oscillates or noise is present in the measurements, determining the instant when error is less than some xed value is very dicult and sensitive to variation of parameters. We measure the performance of each control method introduced in Section and compare the methods using the performance measures presented here. It has to be noted that we compare only PI controllers with other PI controllers and PID controllers with other PID controllers. In general, it is reasonable to compare only controllers of same order. This is because theoretically controllers with higher order can be tuned to give at least as good results as controllers of lower order.. Sensitivity to Disturbances The tuning methods might give good results in sense of the performance measures discussed in previous section, but no method is useful if the controlled system is sensitive to modeling errors, disturbances in control signal or measurement noise. System sensitivity analysis is another useful method to measure the quality and adequacy of the controllers. In this case we perform a sensitivity analysis based on the maximum sensitivity of the closedloop system. First we determine the sensitivity function and complementary sensitivity function for a closedloop system. Let the controller transfer function be G c (s) and process transfer function G(s). The sensitivity function of the system is then S(s) = + L(s), () where L(s) = G c (s)g(s) is the transfer function of the openloop system. The complementary sensitivity function can thus be formulated as T (s) = L(s) + L(s). () It should be noted that by denition T (s) + S(s) =.[] The sensitivity function describes the system response sensitivity to reference signal changes, whereas the complementary sensitivity function describes the system sensitivity to measurement noise. Moreover, they describe the system sensitivity to modeling errors which is important to notice in this study.
13 It is shown in [7] and [8] that if the Nyquist Diagram of the openloop system L(s) and a circle with radius r R and center on negative real axis in c r do not intersect, the system sensitivity function S(s) and complementary sensitivity function T (s) are less than or equal to M, where and r R = M M(M ) c R = M M +, () M(M ) where parameter M >. Thus, the parameter M can be interpreted as a parameter of sensitivity or equivalently robustness that expresses the maximum value of system sensitivity. Smaller values of M make the circle radius r R greater, which takes the Nyquist curve further o from the critical point +i and makes the system more robust. On the contrary, greater values of M decrease the radius r R and take the Nyquist curve closer to instability region. Typically M values of a robust system are at interval. < M <..[8] Results. Process Model Parameter Estimation The parameter estimation process of the system models described in Section. is performed with Matlab. Because of the nonlinearity of the real process and the inaccuracy of the utilized models it is desired that we choose a small bounded interval as operating interval as explained in Sections. and.. We start the parameter estimation by determining the model parameters' P dependence on the operation point. In the experiment setup we apply a step input of V into process and measure the response. This step response is determined at points in the input range and the parameters P are determined by solving (). The results are shown in Figure 6. Parameter values seem to vary quite signicantly at both ends of the range. At 9V there was a lot of drifting in the measurement data which explains the deviation in the parameter values. However, between input range 7V the parameter values seem to be quite stable and constant. So, it is reasonable to tune the controllers to operate at this interval. Thus, if we do not want varying parameters or operation point dependent parameters, this is the best interval to perform controller tuning with methods that utilize the linear process model. The average values of process parameters at interval 7V are shown in Table. These values are used in the controller tuning that is discussed in the following section. The parameters P of the rst order model (see () ) are estimated utilizing the second order model. Because the second order model approximates the process very well and the parameter values P are quite stable at the chosen interval, it is reasonable to assume that the rst order model also behaves in such manner. To perform the estimation process a suitable Matlab script was written. The script determines automatically the maximum of derivative function and calculates the parameters as described in Section.. The values for parameters P are shown in Table. ()
14 . Controller Parameters Table : Parameters P mean values and parameter P values P P parameter value parameter value K.888 K.888 T a.6 T.68 T b.76 L.87 L.67 Now, parameters for dierent controllers introduced in Section can be calculated. This is performed with the rst and second order models of the process and the values of parameters P and P determined in the previous section. For convenience, we divide the controller tuning methods into two categories. The rst category contains the tuning methods that have constant parameters and the second category contains the methods that have an adjustable tuning parameter. Thus, into the rst category we include Ziegler Nichols step response method and ultimate frequency method, Cohen and Coon method and AMIGO method. Into the second category we include the Lambda tuning method and the IMC tuning method. The controllers in the rst category are tuned rst. We concentrate on PI and PID controllers. The parameters for the controllers in the rst category are shown in Table. To illustrate the eciency of the tuning methods we performed simulations with the process model. This gives a rst hand information of the controller eects to the behavior of the actual process. We perform a more deeper comparison of the methods in the following section. We study the controller behavior in two. K T a [s] T b [s] L [s] Parameter value input operation point [V] Figure 6: P parameter values as function of operation point
15 Table : Parameter values of category one controllers ZN step ZN freq Cohen&Coon AMIGO PI: K p T i PID: K p T i T d...8. cases; the load disturbance and reference signal tracking as mentioned in Section. Response of the closedloop models to step change in reference signal are shown in Figure 7. Similarly, the closedloop system responses to a load disturbance are shown in Figure 8. In the second category, parameter values are determined only for Lambda controller. For IMC controller explicit parameter values cannot be determined, because the controller structure and parameter values depend signicantly on the process model structure. Parameter values for Lambda controller are determined such that we use four predetermined values for λparameter. These values have been selected with graphical inspection from system step response. The parameter values and corresponding λ values can be seen in Table 6. Table 6: Values of Lambda controller parameters Lambda λ =. λ =. λ =. λ =.6 PI: K p T i.68 PID: K p T i.796 T d.98 Determining the parameter values for IMC controller is more complicated. Even though the IMC controller reduces to PID controller when rst order process model is used, it is not practical to calculate the parameter values explicitly. The closedloop response of the model when a step reference signal is applied with Lambda tuning and IMC controller is shown in Figures 9 and (a). Here we also use few predetermined values for parameter T l value, which can be seen in the gures. Similarly, the behavior of the system, when a load disturbance (V step change in input signal) is applied is shown in Figures and (b).
16 . PI controllers reference ZN step ZN freq Cohen&Coon. PID controllers reference ZN step ZN freq Cohen&Coon AMIGO (t) (t) u(t) u(t) Figure 7: Output and control signals of the closedloop systems for dierent tuning methods when a step in reference signal is applied. Category one controllers. (t)..... PI controllers reference ZN step ZN freq Cohen&Coon (t)..... PID controllers reference ZN step ZN freq Cohen&Coon AMIGO u(t) u(t) Figure 8: Output and control signals of the closedloop systems for dierent tuning methods when a load disturbance is applied. Category one controllers. 6
17 .. =. =. =. =.6 y ref.. =. =. =. =.6 y ref (a) (b) Figure 9: Closedloop system output and control signals with Lambda tuned (a) PI (b) PID controller when a step of V is applied to reference at t=s..6.. =. =. =. =.6 y ref.6.. =. =. =. =.6 y ref (a) (b) Figure : Closedloop system output and control signals with Lambda tuned (a) PI (b) PID controller when a load disturbance of V is applied at t=s. 7
18 T l :. T l :. T l :. T l : t t (a) y ref T l :..8 T l :..6 T l :.. T l :.6. y ref 6 8 t t (b) Figure : Closedloop output and control signals of the IMC controlled system in case of (a) reference tracking (step at t=s) (b) load disturbance rejection (step at t=s). Comparison of Controllers' Performance We determine the controllers' performance with the methods introduced in Section. and Appendix B. As mentioned earlier, it is reasonable to compare only PI controllers with other PI controllers and PID controllers with other PID controllers. It is good to keep in mind that the performance measures determined here, loosely speaking, 'correlate' with the area between the reference signal and the output signal of the process seen in Figures 7 . First we determine the performance measures for category one PI controllers in the case of reference signal tracking and load disturbance rejection. Results are shown in Figures (a) and (b). The performance values are relative such that the best tuning method has value. and the other values are relative to this. Comparison of PI controllers in the case of reference signal tracking shows that Ziegler Nichols ultimate frequency method gives the best results in our performance tests. As seen in Figure (a) Ziegler Nichols ultimate frequency method beats all other methods with all performance measures. In case of load disturbance the Ziegler Nichols step response method is the best in sense of all performance measures except J opt. The performance measures for category one PID controllers in case of reference signal tracking and load disturbance rejection are shown in Figures (a) and (b). The best PID controller tuning methods in case of reference signal tracking are Ziegler Nichols ultimate frequency method and step response method as shown in Figure (a). They give almost equally good performance measures in case of all other performance measures except J opt. The AMIGO PID controller is the best in case of the J opt performance measure. In case of load disturbance rejection the situation is more complicated as seen in Figure (b). A clearly dominating controller in sense of all performance measures is dicult to determine among Ziegler Nichols and Cohen&Coon methods. Ziegler Nichols Step response method is best in case of absolute 8
19 J_itae:,,8 6,7 J_itae:,,,6 J_iae:,6,, J_iae:,,, J_ise:,,,78 J_ise:,,, J_itse:,,6, J_itse:,,8,6 J_opt:,,79,8 J_opt:,,,,,,,,, 6, 7, ZN step ZN freq Cohen&Coon (a),,,,,,, ZN step ZN freq Cohen&Coon (b) Figure : Relative performance values for category one PI controllers in case of (a) reference signal tracking (b) load disturbance rejection J_itae:,,,6,6 J_itae:,,,8, J_iae:,,,,6 J_iae:,,8,,79 J_ise:,,,,76 J_ise:,,,,99 J_itse:,,,77, J_itse:,,,6,68 J_opt:,,7,9 8,68 J_opt:,,,,,,,,,, 6, 7, 8, 9, ZN step ZN freq Cohen&Coon AMIGO (a),,,,,, 6, ZN step ZN freq Cohen&Coon AMIGO (b) Figure : Relative performance values for category one PID controllers in case of (a) reference signal tracking (b) load disturbance rejection 9
20 error performance measures, but Cohen&Coon seems to be the best in case of squared error performance measures, although, the dierences are small. AMIGO PID controller is not very good in sense of performance measures based on absolute and squared error, but it is dominating in case of the J opt performance measure. Settling s t s of the category one controllers are listed in Appendix C.. The shortest settling s seem to be achieved with Ziegler Nichols methods. In the worst case, the Cohen & Coon method seems to have twice as long settling s as Ziegler Nichols methods. For category two controllers we determined the performance measure values using dierent values for the tunable parameter. Thus, the performance measures were determined with small separation of tunable parameter and then plotted as function of the parameter to illustrate the performance behavior. Performance values for Lambda PI controller in case of reference signal tracking are shown in Figure (a) and the corresponding Lambda PID controller performance values are seen in Figure (b). Performance of the PID controllers seems to be slightly better at all values of λ in sense of all performance measures. Performance values for Lambda PI controller in case of load disturbance are shown in Figure (a) and the corresponding Lambda PID controller performance values are seen in Figure (b). Similarly Lambda PID controller performance values are slightly better for all values of λ. Performance values of the IMC controller are shown in Figures 6 (a) and (b). Compared to the Lambda controllers the performance values seem to increase very rapidly when parameter T l increases but at the same the decrease in the Mvalue is very gradual. This indicates that the IMC controller is more sensitive to variations of the tunable parameter T l. There also seem not be a similar minimum values for performance measures as the Lambda controllers have in case of reference signal tracking. A summary of performance numbers and settling s is shown in Appendix C... J itae J itae. J iae J iae J ise J itse. J ise J itse. J opt M value J opt M value M / J. M / J λ (a) λ (b) Figure : M values and cost function values (in case of reference signal tracking) as functions of λ for (a) PI controller (b) PID controller
21 6 6 J itae J itae J iae J ise J iae J ise J itse J itse M / J cost J opt M value M / J cost J opt M value lambda (a) lambda (b) Figure : M values and cost function values (in case of load disturbance rejection) as functions of λ for (a) PI controller (b) PID controller. 6 J itae J itae J iae J ise J iae J ise M / J cost.. J itse J opt M value M / J cost J itse J opt M value T f (a) T f (b) Figure 6: Mvalues and cost function values of IMC controller as functions of parameter T l in case of (a) reference signal tracking and (b) load disturbance rejection
22 . PI controllers. PID controllers Imag[L(jω)]. Imag[L(jω)].. ZN step, M=. ZN freq, M=. Cohen&Coon, M=.... Real[L(jω)] (a). ZN step, M=. ZN freq, M=. Cohen&Coon, M=.6 AMIGO, M= Real[L(jω)] (b) Figure 7: Nyquist diagrams and Mcircles for (a) PI and (b) PID controllers. Comparison of Controllers' Sensitivity Sensitivity of controllers is also analyzed separately for category one and category two controllers. For category one controllers the Nyquist diagrams and the Mcircles are plotted in Figure 7. Controllers with greater M values (smaller circles) are more sensitive to disturbances and measurement noise than ones with smaller M values (larger circles). For category two controllers the Mvalues were also determined as a function of tunable parameter. In Figures  the Mvalue is plotted as a function of λ in the same gure with controller performance function values that were determined in last section. Mvalues are naturally equal for PI controllers in both cases, the reference signal tracking and the load disturbance rejection. Similarly Mvalues are equal for PID controllers in both cases as the controllers and the plant are the same. Mvalues of the IMC controller are plotted together with the controller performance costs as a function of the parameter T f in Figure 6 (a)(b). The Mcurve is same in both gures (a) and (b) but the costs are dierent in the two cases. By plotting the controllers Mvalues and settling s into same gure gives good picture of the tradeo between robustness and performance. All controllers plotted in Mt s plane are shown in Figure 8. In case of tunable parameter controllers we have chosen the ones with best values by weighting Mvalue and settling with equal weights. It should be noted that only tuning methods that reach the area of good robustness ( < M ) are the AMIGO method and tunable parameter controllers. However, as mentioned earlier the settling can be considered only as an approximative performance indicator. Furthermore, to compare the theoretical goodness of tuning methods we have plotted the J iae
23 6 Cohen&Coon PI Cohen&Coon PID Mvalue ZN freq PID IMC Lambda PID ZN step PI ZN step PID ZN freq PI AMIGO Lambda PI Settling [s] Figure 8: Controllers' Mvalue versus settling. Better controllers are near origo measure in load disturbance rejection case versus the J ise in reference tracking case in Figure C. in Appendix. J iae and J ise are typically used to measure the performance in the case of load disturbance rejection and reference tracking, respectively. Thus, Figure C. illustrates if the controller is more suitable to reference tracking or load dsturbance rejection. The performance measures J iae, J ise and J opt are also plotted versus the M value in Figures C.  C.6 in Appendix. These gures show that in this case the Ziegler Nichols methods seems to be very good in both PI and PID controllers. On the contrary, the Cohen and Coon methods seem to be quite poor in all cases. The tunable methods seems to be better than other methods in case of reference tracking, but in load disturbance rejection they can only reach the performance of the Ziegler Nichols methods. The ranking of AMIGO method is quite tricky. In case of J opt performance measure it is superior and in load disturbance rejection it reaches quite good results, but in reference signal tracking it is very poor. A summary of the Mvalues of the category one controllers is shown also in Appendix C... Applying Methods to the Heated Airow Process After a thorough study of tuning methods and their applicability to the process model we lack only the knowledge of how the actual process can be controlled with the methods. Next, we perform tests with the process that are based on the results we have found with the process modeling. Implementing rst the controller algorithms into Matlab using xpc Target toolbox, a real process control and measurement system, we can test the behavior of the controlled process. The response given by the simulation of the second order model and the actual process are plotted in same gure in Appendix
24 C. for all controllers. For tunable parameter controllers we have chosen controllers that are good representatives or compromises of controllers with good performance and robustness. The actual process and simulation results seem to meet each other relatively well, especially in the case of load disturbance rejection. The gures show that the static gain of the system and the output delay were estimated very well. However, in case of reference signal tracking the actual process seems to settle to steady state value faster than the estimated model predicts. It is also noticed that in the case of PID controllers the measurement noise is not ltered very well in the feedback loop and hence the noise in the control signal u(t) is signicantly large. The noise could be reduced with increasing the measurement signal ltering but this would decrease the controller performance. 6 Educational Discussion In this section we discuss the essential guidelines and practical results we have found to be important in developing a laboratory course exercise on PID control for undergraduate students. From the laboratory exercise point of view it is important to give students knowledge about the basic behavior of the closedloop system dynamics and PID controllers and to give illustrative examples of their behavior. Furthermore, explaining the background and preconditions for usage of tuning methods as well as giving reasons for their usage is necessary to make students understand the conditions in which the usage of PID controllers is useful. The aim is to advise students to use the methods properly and to learn the advantages and drawbacks of the PID controllers. However, it should be kept in mind that introducing too many methods and controllers might be confusing and make it dicult to distinguish relevant issues from rather irrelevant issues. Students are expected to know the basics of the process dynamics and they most probably also know something about the PID controllers. For example the eects of measurement delay, especially in a closedloop process and the shortcoming of a simple P controller which leaves a constant error between process output and reference signal. Some student might also be familiar with the eects that appear in the closedloop system output when the controller parameters are varied. Particularly, greater Pterm makes the closedloop system more rapid, the Iterm can be used to overcome the problems related to simple P controller, but it increases the oscillation of the output signal and the oscillations (caused by Iterm and measurement delay) can be reduced with the Dterm. It is important to distinguish the variation of parameters K p, T i, T d and N and terms (P, I and D) because varying K p aects all terms as explained in Section. However, these eects can be demonstrated with the computer controlled process very easily and the eects of changing each parameter separately can be powerfully visualized. Also the possibility to vary certain process parameters, measurement delay and airow speed, makes the demonstration of process dynamics more versatile. The process identication is the rst part of the tuning methods' application. In this work we have studied rst and second order models. These models explain the process behavior suciently for
25 controller tuning purposes. But, it should be noted that these models are quite simple and do not take into account for instance process parameter variation. This means that the process model must be reestimated in case some of the process parameters change. The parameter tuning of a basic PID controller can be performed with the methods introduced in this work. These tuning methods provide a good starting point for students to learn the PID controller. However, it should be noticed that the PID tuning methods give only a good approximative values for controller parameters. These values should be used mainly as a good initial estimate for parameters rather than optimal or nal values. Hence the manual adjustment of parameters is at least as important part of the controller tuning as the application of the tuning methods. Actually the manual adjustment could be considered as the eventual ne tuning method that can be used to set the controller to behave as desired. Nevertheless, incorporating the robustness measures into the analysis in the simulation phase, as was done in this study, provides deeper understanding of dierent methods and may even decrease the need for manual ne tuning when implementing the controllers. As we found out in this project work, none of the basic tuning methods is superior to others in sense of both robustness and performance. Thus the relation between these properties requires clarication and students should understand the tradeo between the robustness and performance. In other words, it has to be emphasized that increasing the performance of a controller always makes the system more sensitive. This is because the performance is achieved by strengthening the feedback which makes the system behavior more rapid. With tunable parameter controllers it is easy to demonstrate the change in controller behavior, because the tuning can be reduced into changing one parameter. One of the most noticeable characteristics of the case process is the noise in temperature measurement. This is mainly caused by the turbulence in the owing air which is not included into the system model as mentioned in Section.. Noise is of course an undesired property in any part of a process, because it decreases the measurement accuracy and it has a destabilizing eect especially if the process includes feedbacks. Thus, dierent noise ltering methods can be used in the case process and the eects are very observable because the measurement noise of the case process is substantially large. The students are expected to have some knowledge of the concept of noise and probably the means to reduce it. At least they should know that noise is a high frequency variation in the measurement signal and they should be able to distinguish noisy measurement signal from a less noisy measurement. In the case of a simple PID controller () the noise aects the Dterm signicantly. It is easy to visualize that high frequency in signal means also high frequency in the derivative signal. So, including the Dterm into a well functional PI controller can cause a large noise in the controller output signal u(t) and can lead even to instability of the process. Lowpass ltering of the measurement signal provides a solution to this problem. The controllers introduced in this work contain the lowpass lter. However, the tuning methods do not determine any values to lter parameters except the AMIGO method that determines an approximative value for T f. This is because the strength of ltering depends on the existing noise and should be determined in all situations according to the characteristic process noise.
26 In tunable parameter controllers the ltering is included into controller and changes when adjusting the tunable parameter. So, including a proper lter increases the controller robustness but at the same it makes the system response slower. In Section. we already divided the studied controllers into two categories. This division is very good starting point in teaching the methods, although it is not yet necessary to decide the methods that should be nally used in the laboratory exercise. Category one tuning methods are very good to illustrate the behavior and dierences between the tuning methods. By illustrating both of the cases, the reference signal tracking and load disturbance rejection, it is evident for students that none of the methods is superior to others in sense of both performance and robustness. This could also be demonstrated with the robustnessperformance maps shown in Appendix C.. Category two controllers have a little more complex structure, but they are very useful in demonstrating the eects of controller tuning on the robustness and performance. This is because the controller tuning nally reduces to varying one parameter which determines the controller position in robustnessperformance map. 7 Summary and Conclusions In this research project we have performed a comprehensive study over the PID controller tuning on a heated airow process. Motivation for the study was the demand to develop a laboratory course exercise for undergraduate students. Several controllers and controller tuning methods described in this work were found out to be suitable for controlling the case process. Comparison of controllers performance and robustness proved that none of the controllers were superior to others. Thus, dierent controllers and tuning methods should be used in dierent situations. Measurement noise and process parameters variation also aect the controller performance and robustness. Controllers with high performance are usually sensitive to noise. Tradeo between robustness and performance makes the decision making among controllers more dicult. The prospective development of the laboratory exercise will concentrate on writing the exercise instructions and implementing the measurement software. Results of this work will be used to guide the determination of the exercise instructions. Especially this study gives basis for examination of the controllers robustness and performance. During the study of the tuning methods and process model estimation we have also implemented several Matlab scripts and Simulink models to perform the model parameter estimation automatically. These will be utilized in the measurement software development. 6
27 References [] Dorf R. C. and Bishop R. H., Modern Control Systems, Ninth Edition, Prentice Hall, [] Eriksson L., Control Design and Implementation of Networked Control Systems, Licentiate thesis, Department of Automation and Systems Technology, Helsinki University of Technology, May 7. [] Garcia C. E. and Morari M., Internal Model Control.. A Unifying Review and Some New Results, Industrial & Engineering Chemistry Process Design and Development, vol., pp. 8, 98 [] O'Dwyer, A., Handbook of PI and PID Controller Tuning Rules, Imperial College Press, [] Visioli A., Practical PID Control, Springer, 6 [6] Åström K.J. and Hägglund T., Automatic Tuning of PID Controllers, Instrument Society of America, 988 [7] Åström K.J., Panagopoulos H. and Hägglund T., Design of PI Controllers based on NonConvex Optimization, Automatica, Vol., No., pp. 86, 998 [8] Åström K.J. and Hägglund T., Revisiting the ZieglerNichols step response method for PID control, Journal of Process Control () 66, Elsevier, [9] Process Control Workshop Advanced Teaching Manual 7M, Feedeback Instruments Ltd. 7
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