622 JOURNAL OF COMPUTERS, VOL. 8, NO. 3, MARCH 2013 Fuzzy PID Position Control Approach in Computer Numerical Control Machine Tool Guoyong Zhao Department of Mechanical Engineering, Shandong University of Technology, Zibo, China Email: zgy709@126.com Yong Shen and Youlin Wang Department of Mechanical Engineering, Shandong University of Technology, Zibo, China Email: ssy706@163.com, wangyoulin_1006@sdut.edu.cn Abstract—The traditional PID control approach is often adopted in CNC machine tool position controller. The approach is simple and easy to implement, but can’t achieve rapid adjustment and less overshoot at the same time. Consequently, on the basis of analyzing fuzzy control theory and design method, the CNC machine tool fuzzy PID position control approach is developed in the paper. The approach makes full use of the advantages of fuzzy control and PID control. The experiment results show that in contrast to traditional PID position controller, the developed approach can achieve better rapidity, shorter adjust time and satisfied overshoot. It is much significant to highprecision CNC machining. Index Terms—fuzzy PID position control, membership function, fuzzy control rule, adjust time, overshoot I. INTRODUCTION The traditional proportional integral differential (PID) control approach is often adopted to compute controlled quantity in the computer numerical control (CNC) machine tool position controller. The strongpoint of the approach is simple and easy to implement [1-2]. However, with the increasing requirement of practical manufacture speed, precision and stability, the traditional PID approach shows its disadvantage gradually. For example, the poor adaptability and anti-jamming ability, and the weakness that can’t achieve rapid adjustment and less overshoot at the same time [3-5]. Aimed to the weakness of traditional PID approach, the researchers all over the world study how to enhance PID controller performance from two aspects mainly [68]. For one thing, improve the classical PID control arithmetic. Y.X.Su combines nonline-differential tracker and traditional PID controller in series, and process signals with two nonline-differential trackers in order to improve anti-jamming ability [9]. Guo Yanqing analyzes each PID portion change trend with step input, and This project is supported by the National Natural Science Foundation of China (No.51105236), and the Shandong Province Promotive research fund for excellent young and middle-aged scientists of China(No.BS2011ZZ014). Corresponding author: Guoyong Zhao, zgy709@126.com © 2013 ACADEMY PUBLISHER doi:10.4304/jcp.8.3.622-629 develops a universal nonlinear PID control model [10]. K.Z.Tang combines the traditional PID and self-adaption nonlinear control, and compensates the linear model warp with nonlinear model [11]. Hu Ligang devises the nonline-differential tracker to receive corresponding integral and differential signals, and process the signals properly to build up nonlinear PID controller [12]. For another thing, study PID control with intelligence approach [13-15]. Ning Wang introduces the nonlinear PID control approach based on nerve cell network, which achieves nonlinear control through nerve cell network parameters online adjustment [16]. Fuzzy control is an intelligence approach, and its process flow is as follows. Above all, sample information with computer; Secondly, transform the information into fuzzy controlled quantity after fuzzy process; Thirdly, compare the fuzzification information with repository data, and process approximately the fuzzy information with rule library; Finally, compute clear controlled quantity according to fuzzy conclusion. The inner information data are fuzzy quantity, so it is easy to make use of human experience with natural language [17]. The fuzzy controller framework is shown in Fig.1. fuzzy controller input fuzzifi -cation reasoning machine rule library eliminate fuzzifi -cation output database repository Figure 1. The fuzzy controller framework In order to improve the traditional PID controller performance, the CNC machine tool fuzzy PID position control approach is developed in the paper, on the basis of analyzing fuzzy control theory and design method. The approach makes full use of the advantages of fuzzy control and PID control. In the end, the contrast experiments are done to validate introduced fuzzy PID position control approach. JOURNAL OF COMPUTERS, VOL. 8, NO. 3, MARCH 2013 623 II. THE FUZZY CONTROLLER FRAMEWORK DESIGN described with natural language and carried out with computer [18]. The fuzzy control elementary principle is shown in Fig.2. Fuzzy control approach is an automatic control technology, which is based upon experience summing-up, Fuzzy controller input fuzzifi -cation fuzzy reasoning fuzzy control rule sensor output eliminate fuzzfication actuating machanism control object Figure 2. The fuzzy control elementary principle The fuzzy controller can be divided into singlevariable fuzzy controller and multi-variable fuzzy controller according to input and output variables, and can be divided into self-adapting fuzzy controller, selforganizing fuzzy controller, self study fuzzy controller and expert fuzzy controller according to function [19]. Especially, the single-variable fuzzy controller can be divided into one-dimension fuzzy controller, twodimension fuzzy controller and three-dimension fuzzy controller according to input variable amount. The input variable amount is corresponding to fuzzy controller dimension. The single-variable fuzzy controller framework is shown in Fig.3. The two-dimension fuzzy controller is easy to implement with computer, so the two-dimension fuzzy PID controller is adopted in the CNC machine tool position control, which connects fuzzy control and traditional PID controller. In the traditional controller, the proportional coefficient, integral coefficient and differential coefficient are invariable. So the controller can’t adjust automatically when loads change or outer disturbances appear. It is a feasible approach to regulate the PID coefficients automatically in order to enhance position controller precision. The developed fuzzy PID position controller is shown in Fig.4. e u fuzzy reasoning (a) e fuzzy reasoning e& d/dt u (b) e& fuzzy reasoning e&& e d/dt u d/dt (c) Figure 3. The single-variable fuzzy controller framework (a) one-dimension fuzzy controller (b) two-dimension fuzzy controller (c) three-dimension fuzzy controller The strongpoint of fuzzy PID position controller shown in Fig.4 is that the PID coefficients are reasoned with three fuzzy controllers. And each fuzzy controller adjusts quantification factor according to practical requirement. fuzzy PID controller Ki fuzzy controller r(t) 1/s Kp e control object fuzzy controller u(t) Kd fuzzy controller du/dt position feedback Figure 4. The developed fuzzy PID position controller framework Ⅲ. CNC MACHINE TOOL FUZZY PID POSITION CONTROL APPROACH A. Quantification Factor and Proportional Factor The input signals are quantified with quantification factor, then sent to fuzzy controller to fuzzification © 2013 ACADEMY PUBLISHER process. After fuzzy controller reasoning and eliminate fuzzification, the precise output signals are achieved, and are magnified or reduced so as to match adjacent module well [20]. (1) Quantification factor Suppose the input signals of fuzzy controller be e and ec = de / dt in the two-dimension fuzzy controller. 624 JOURNAL OF COMPUTERS, VOL. 8, NO. 3, MARCH 2013 They are clear numerical values, obtained by compute sampling. Define x component span as elementary field. The input signal x component turns to fuzzy quantity after mapping transformation, and sends to fuzzy reasoning module. Suppose input component x elementary field be X i = [− x, x]( x > 0) Suppose fuzzy field be N i = [− n, n](n > 0) Define the transformation coefficient ki from X to N be quantification factor ki = n / x . (1) When elementary field isn’t symmetrical, such as X i = [a, b] ( a ≠ b ), the quantification factor be ki ' = 2n / b − a . (2) (2) Proportional factor The clear quantity fuzzy field isn’t consistent with implement machine elementary field sometimes, so the field transformation is required. Define the transformation coefficient from fuzzy field to elementary field be proportional factor. Suppose the fuzzy field be N ' = [−n, n](n > 0) Suppose the elementary field be U = [−u, u ](u > 0) Then the proportional factor ku can be obtained ku = u / n . (3) Except for field transformation, the quantification factor and proportional factor are important for system control performance. If quantification factor of error e is on the high side, the rise velocity will turn quick, the overshoot will increase, which may lead to unsteadiness. On the contrary, if quantification factor of error e is on the low side, the rise velocity will turn slow, which may destroy system steady-state precision. If quantification factor of error change ec increases, the restrain force to error change will augment, which will improve system stability; On the contrary, if quantification factor of error change ec decreases, the overshoot will increase, which will lead to system instability. Proportional factor is corresponding to the total system magnify multiple. If proportional factor increases, the system respond speed will turn quick; On the contrary, if proportional factor decreases, the system transition time will turn longer. B. The Fuzzification of Input and Output Variables in Fuzzy Controller Quantify the fuzzy controller input signal with quantification factor, then do fuzzification process. In essence, fuzzification process is to confirm the language variable in the field for a certain x in field X. © 2013 ACADEMY PUBLISHER The language variable words amounts is important to system performance. If the language variable words is more, the input and output variables will be described more detailedly, and the fuzzy reasoning process turns complicated; On the contrary, if the language variable words is few, the input and output variables will be described more roughly and poorly. The language variable words amounts are selected from five to seven commonly. For example, the language variable including seven words is: A = {NB, NM , NS , ZO, PS , PM , PB} Where each word is defined as follows NB---negative big NM--- negative medium NS--- negative small ZO---zero PS--- positive small PM--- positive medium PB--- positive big The fuzzy field and language variable are connected through membership function. The triangle-shape function, Gauss-shape function, Bell-shape function, Zshape function and S-shape membership function are in common use. Except the field boundary, the membership function is centrosymmetric. The sharp membership function curve leads to high distinguishability and sensitivity; While the mild membership function curve leads to low sensitivity. The triangle-shape membership function is adopted to study fuzzification process in the paper. Suppose the input variable fuzzy field after fuzzification process is Ak ∈ [−6,−5,−4,−3,−2,−1,0,1,2,3,4,5,6] . Suppose the language variable muster is A = {NB, NM , NS , ZO, PS , PM , PB} . The language variable value is shown in Fig.5. As shown in Fig.5, the field is divided into thirteen grades corresponding to seven language variables. PB: about +6 PM: about +4 PS: about +2 ZO: about 0 NS: about -2 NM: about -4 NB: about -6 Seven words are adopted to describe error e, error change rate ec, three output coefficient k p , ki and k d . And the field is {NB, NM , NS , ZO, PS , PM , PB} . Fuzzy subset NB and PB adopt Z-shape membership function and S-shape membership function. Fuzzy subset PM, PS, ZO, NS and NM adopt triangle-shape function. The input variable membership function is shown in Fig.6, output variable membership function is shown in Fig.7, and coefficient k p fuzzy control system is shown inFig.8. JOURNAL OF COMPUTERS, VOL. 8, NO. 3, MARCH 2013 625 μ NB -6 NS NM -5 -3 -4 -2 ZO 1.0 -1 0 PM PS 1 2 3 PB x 6 5 4 Figure 5. The language variable value (1) k p fuzzy control rule To increase k p may reduce steady state error and enhance system response velocity. However, the oversize k p will lead to big overshoot, even instability; To decrease small Figure 6. The input variable e and ec membership function k p may reduce overshoot. However, the very k p will lead to long adjust time. On the basis of analyzing the connection between error e, error change rate ec and parameter k p , the k p fuzzy control rule is built up shown in Table 1. TABLE 1 THE kp FUZZY CONTROL RULE ec Kp NB NM NS ZO PS PM PB PB PB PM PM PS ZO ZO PB PB PM PS PS ZO NS PM PM PM PS ZO NS NS PM PM PS ZO NS NM NM PS PS ZO NS NS NM NM PS ZO NS NM NM NM NB ZO ZO NM NM NM NB NB e Figure 7. The output variable k p , ki and kd NB NM NS ZO PS PM PB membership function (2) ki fuzzy control rule The integral control mainly play role on steady state error elimination. If parameter ki is on the high side, the system may be instable; If parameter Figure 8. The coefficient kp fuzzy control system C. The Fuzzy Control Rule Design The fuzzy control rule is based on expert experience knowledge and practical data analyzed result. The essential principle of fuzzy control rule is to achieve optimal dynamic and static state system performance. The fuzzy PID control approach reasons the three PID coefficients with fuzzy controller. Firstly, measure error e and error change rate ec from time to time; Secondly, adjust control parameters online according to the fuzzy connection between e, ec and control parameter. © 2013 ACADEMY PUBLISHER ki is on the low side, the system can’t be effective to reduce steady state error. On the basis of analyzing the connection between error e, error change rate ec and parameter ki , the ki fuzzy control rule is built up shown in Table 2. (3) k d fuzzy control rule The differential control can forecast the change trend of error in advance, and output corresponding restrain signal. The oversize k d will lead to long adjust time; While the very small k d will lead to unsatisfied overshoot when error changes sharply. On the basis of analyzing the connection between error e, error change 626 JOURNAL OF COMPUTERS, VOL. 8, NO. 3, MARCH 2013 rate ec and parameter k d , the k d fuzzy control rule is built up shown in Table 3. PS)( TABLE 2 THE ki (32) If (e is PS) and (ec is ZO) then ( k p is NS)( ec PS)( Ki NS ZO PS PM PB NB NB NM NM NS ZO ZO NB NB NM NS NS ZO NS NM NM NS NS ZO PS PS NM NM NS ZO PS PM PM NM NS ZO PS PS PM PM The relevant fuzzy contain connection are as follows: ZO ZO PS PS PM PB PB R32 (k p ) = PS (e) ∧ ZO(ec) ∧ NS (k p ) ZO ZO PS PM PM PB PB PS PM PB kd FUZZY CONTROL RULE ec Kd NB NM NS ZO PS NS NB NB NB NM PS PS NS NB NM NM NS ZO ZO NS NM NM NS NS ZO ZO NS NS NS NS NS ZO ZO ZO ZO ZO ZO ZO ZO PB NS PS PS PS PS PB PB PM PM PM PS PS PB e NB NM NS ZO PS PM PB D. The Fuzzy Reasoning Process The fuzzy approximate reasoning and synthesis principle is U * = ( A* )T o R . (4) Where fuzzy contain connection fuzzy condition propositions: R consists of 49 49 . = U Rj (5) j =1 It can be obtained with Mamdani reasoning arithmetic: U * = ( A* )T o R = ( A* )T o U R j . (6) j =1 =U j =1 49 k d is PS) (1) (40) If (e is PM) and (ec is PS) then ( k p is NM)( PM)( ki is k d is PS) (1) R32 (ki ) = PS (e) ∧ ZO (ec) ∧ PS (ki ) R32 (k d ) = PS (e) ∧ ZO (ec) ∧ ZO (k d ) R33 (k p ) = PS (e) ∧ PS (ec) ∧ NS (k p ) R33 (ki ) = PS (e) ∧ PS (ec) ∧ PS (ki ) R33 (k d ) = PS (e) ∧ PS (ec) ∧ ZO(k d ) R39 (k p ) = PM (e) ∧ ZO (ec) ∧ NM (k p ) R39 (ki ) = PM (e) ∧ ZO (ec) ∧ PS (ki ) R39 (k d ) = PM (e) ∧ ZO (ec) ∧ PS (k d ) R40 (k p ) = PM (e) ∧ PS (ec) ∧ NM (k p ) R40 (ki ) = PM (e) ∧ PS (ec) ∧ PM (ki ) R40 (k d ) = PM (e) ∧ PS (ec) ∧ PS (k d ) According to the above-mentioned fuzzy rules, the kp , ki and k d fuzzy reasoning output results can be obtained as follows, which are shown in Fig.9, Fig.10 and Fig.11 respectively. (1) From Fig.6, it can be obtained: Consequently, the fuzzy output is U 32 (k p ) = ( A* )T o R32 = ( PS (e = 1.5) ∧ ZO(ec = 0.75)) o R32 (k p ) = UU j = 0.5 ∧ 0.25 ∧ NS (k p ) = (0.25 NS )k p * j =1 Suppose the clear input quantity e=1.5, ec=0.75 on some period. According to Fig.6, e is corresponding to fuzzy subset PS and PM; The ec is corresponding to ZO and PS. According to Table 1, Table 2 and Table 3, the following principles are activated by the two input quantities. © 2013 ACADEMY PUBLISHER PS)( ki is = PS (e = 1.5) ∧ ZO(ec = 0.75) ∧ NS (k p ) . (7) 49 (( A* )T o R j ) (39) If (e is PM) and (ec is ZO) then ( k p is NM)( PS (e = 1.5) = 0.5 ZO (ec = 0.75) = 0.25 R = R1 U R2 U R3 ... U R49 49 k d is ZO) (1) NM TABLE 3 THE ki is NB e NB NM NS ZO PS PM PB k d is ZO) (1) (33) If (e is PS) and (ec is PS) then ( k p is NS)( FUZZY CONTROL RULE ki is U 32 (ki ) = ( A* )T o R32 = ( PS (e = 1.5) ∧ ZO (ec = 0.75)) o R32 (ki ) . (8) = (0.25PS )ki JOURNAL OF COMPUTERS, VOL. 8, NO. 3, MARCH 2013 627 U 32 (k d ) = ( A* )T o R32 U 40 (ki ) = ( A* )T o R40 = ( PS (e = 1.5) ∧ ZO (ec = 0.75)) o R32 (k d ) . (9) = ( PM (e = 1.5) ∧ PS (ec = 0.75)) o R40 (ki ) . (17) = (0.25ZO)k d = (0.5PM )ki (2) From Fig.6, it can be obtained: PS (e = 1.5) = 0.5 PS (ec = 0.75) = 0.75 Consequently, the fuzzy output is as follows: U 40 (k d ) = ( A* )T o R40 = ( PM (e = 1.5) ∧ ZO (ec = 0.75)) o R40 (k d ) . (18) = (0.5PS )k d U 33 (k p ) = ( A* )T o R33 = ( PS (e = 1.5) ∧ PS (ec = 0.75)) o R33 (k p ) . (10) = (0.75 NS )k p U 33 (ki ) = ( A* )T o R33 = ( PS (e = 1.5) ∧ PS (ec = 0.75)) o R33 (ki ) . (11) = (0.75PS )ki U 33 (k d ) = ( A* )T o R33 Figure 9. The fuzzy subset after kp fuzzy reasoning Figure 10. The fuzzy subset after ki fuzzy reasoning Figure 11. The fuzzy subset after kd fuzzy reasoning = ( PS (e = 1.5) ∧ PS (ec = 0.75)) o R33 (k d ) . (12) = (0.75ZO )k d (3) From Fig.6, it can be obtained: PM (e = 1.5) = 0.5 ZO(ec = 0.75) = 0.25 Consequently, the fuzzy output is as follows: U 39 (k p ) = ( A* )T o R39 = ( PM (e = 1.5) ∧ ZO (ec = 0.75)) o R39 (k p ) .(13) = (0.25 NM )k p U 39 (ki ) = ( A* )T o R39 = ( PM (e = 1.5) ∧ ZO (ec = 0.75)) o R39 (ki ) .(14) = (0.25PS )ki U 39 (k d ) = ( A* )T o R39 = ( PM (e = 1.5) ∧ ZO(ec = 0.75)) o R39 (k d ) .(15) = (0.25PS )k d (4) From Fig.6, it can be obtained: PM (e = 1.5) = 0.5 PS (ec = 0.75) = 0.75 Consequently, the fuzzy output is as follows: U 40 (k p ) = ( A* )T o R40 = ( PM (e = 1.5) ∧ PS (ec = 0.75)) o R40 (k p ) . (16) = (0.5 NM )k p E. The Output Elimination Fuzzification The elimination fuzzification process is to substitute reasoning fuzzy subset with an appropriate numerical value. The elimination fuzzification approach in common use includes area gravity model approach, weighted average method and so on. The area gravity model approach is adopted to eliminate fuzzification in the paper. IV. POSITION CONTROL SIMULATION EXPERIMENT © 2013 ACADEMY PUBLISHER 628 JOURNAL OF COMPUTERS, VOL. 8, NO. 3, MARCH 2013 A. The Feeding Servosystem Structure The position servo control modes of CNC machine can be divided into open control, half-closed loop control and closed loop control according to framework. The modes in high precision CNC machine include half-closed loop control and closed loop control mainly. Three-loop cascade control scheme is adopted on modern CNC machine mostly, which consist of position loop, velocity loop and current loop from outside to inside. Every control loop has its own feedback signal and connects each other. The screw pair drives are adopted in CNC feeding system usually. Neglecting the influence of mechanicallydriven nonlinear factors such as screw pairs, the approximate structure model of closed loop feeding servosystem is shown in Fig.12. instruction signal position controller D/A speed control unit output displacement position controller can achieve shorter adjust time and satisfied overshoot. (2) If change the input unit step signal to 1.2(t) at t=5s, the traditional PID controller step response curve and the developed fuzzy PID position controller step response curve are shown in Fig.14. From Fig.14 it can be seen, in contrast to the traditional PID controller, the developed fuzzy PID position controller has quicker transition process, better rapidity and satisfied overshoot. (3) If add a step disturbance signal at t=5s whose amplitude is 0.2, the traditional PID controller step response curve and the developed fuzzy PID position controller step response curve are shown in Fig.15. From Fig.15 it can be seen, in contrast to the traditional PID controller, the developed fuzzy PID position controller can spend lesser adjust time to come back to steady-state. 1/S position measurement Figure 12. The feeding servosystem structure B. The Contrast Experimentation on Position Control The common medium CNC machine tool feeding system mathematic model is introduced [21], the transfer function is G ( s) = 10 s + 9s + 23s + 15 3 2 The servo system simulation modules are set up with Matlab/Simulink to compare the traditional PID controller with the developed fuzzy PID position controller. The traditional PID controller coefficients can be obtained with Z-N approach: K p = 11.52 , Ti = 0.6575 , Td = 0.1644 . Figure 13. The traditional PID controller step response curve and the developed fuzzy PID position controller step response curve The fuzzy PID position controller coefficients can be obtained after NCD toolbox optimization: K e1 = 4.5301 , K ec1 = 1.6418 , K up = 0.2321 , K e 2 = 2.5207 , K ec 2 = 1.2610 , K ui = 10.1362 , K e 3 = 8.4617 , K ec 3 = 2.8396 , K ud = 0.7395 . (1) Aimed to the unit step input signal, the traditional PID controller step response curve and the developed fuzzy PID position controller step response curve are shown in Fig.13. From Fig.13 it can be seen, in contrast to the traditional PID controller, the rise time with the developed fuzzy PID position controller increases from 0.33s to 0.40s; The adjust time decreases from 3.69s to 1.59s markedly; And the overshoot decreases from 59.35% to 5.1%. In conclusion, the developed fuzzy PID © 2013 ACADEMY PUBLISHER Figure 14. The traditional PID controller step response curve and the developed fuzzy PID position controller step response curve with mutation signal JOURNAL OF COMPUTERS, VOL. 8, NO. 3, MARCH 2013 Figure 15. The traditional PID controller step response curve and the developed fuzzy PID position controller step response curve with a step disturbance signal V. CONCLUSIONS The traditional PID control approach is often adopted to compute controlled quantity in the CNC machine tool position controller. The approach is simple and easy to implement, but can’t achieve rapid adjustment and less overshoot at the same time. In order to improve the traditional PID controller performance, the CNC machine tool fuzzy PID position control approach is developed in the paper, which makes full use of the advantages of fuzzy control and PID control. The experiment results show that in contrast to traditional PID position controller, the developed approach can achieve better rapidity, shorter adjust time and satisfied overshoot. ACKNOWLEDGMENT The authors are grateful to the Project of the National Natural Science Foundation of China (No.51105236), and the Shandong Province Promotive research fund for excellent young and middle-aged scientists of China(No.BS2011ZZ014). REFERENCES [1] Ye Hongtao, Luo Fei, Xu Yuge, “Decoupling control of wastewater treatment system based on immune optimization PID neural network,” Application Research of Computers, vol. 28, pp. 4097-4099, April 2011. [2] Li Yuxian, “Research on PID Parameters Optimization Based on Immune Genetic Algorithm,” Computer Simulation, vol. 28, pp. 215-218, August 2011. 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[19] Meza J.L., Santibáñez V., “Fuzzy self-tuning PID semiglobal regulator for robot manipulators,” IEEE Transactions on Industrial Electronics, vol. 59, pp. 27092717, June 2012. [20] Navale, Rahul L., Nelson, Ron M., “Use of genetic algorithms and evolutionary strategies to develop an adaptive fuzzy logic controller for a cooling coil Comparison of the AFLC with a standard PID controller,” Energy and Buildings, vol. 45, pp. 169-180, February 2012. [21] Xiong Xiaojun, “Auto-control theory experiment tutorial (Hardware simulation and Matlab simulink),” China Machine PRESS, May 2009. Guoyong Zhao was born in Shandong, China in 1976. He has a Ph.D. in Mechanical and Electronic Engineering (2007) from Dalian University of Technology, Dalian, China. His main interest is mechanical manufacturing and automation technology. Yong Shen was born in Shandong, China in 1987. He is a master postgraduate majored in mechanical manufacturing and automation in Shandong University of Technology, Zibo, China. His main interest is mechanical manufacturing and automation technology. Youlin Wang was born in Heilongjiang, China in 1955. He is a professor worked in Shandong University of Technology. His main interest is mechanical manufacturing and automation technology.