IMPROVING THE ACCURACY OF THE ELECTRO HYDRAULIC SERVOMECHANISMS BY ADDITIONAL FEEDBACKS

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1 THE PUBLISHING HOUSE PROCEEDINGS OF THE ROMNIN CDEMY, Series, OF THE ROMNIN CDEMY Volue, Nuber /9, pp. IMPROVING THE CCURCY OF THE ELECTRO HYDRULIC SERVOMECHNISMS BY DDITIONL FEEDBCKS Nicolae VSILIU, Constantin CLINOIU, Daniela VSILIU, Dragoş ION-GUT University POLITEHNIC of Bucharest, Fluid Power Laboratory, Power Engineering Faculty, Independentei, Bucharest, 64, Roania, Phone: , Fax: Corresponding author: Nicolae VSILIU, E-ail: vasiliu@fluid-power.pub.ro The paper contains a report on soe theoretical and experiental researches aiing to create high accuracy electro hydraulic digital servoechaniss used in high tech applications as actuators, load siulators, fly control systes, dynaic test achines etc. Coputational ethods, control software, design probles, and experiental validation are shortly presented. The authors developed a new hardware and software solution in order to replace the old generation of two or three stage electro hydraulic servo valves, and to achieve axiu flexibility of the testing progras. By using one derivative feed forward path fro the syste input and one derivative path fro the syste output the authors developed a hardware and software configuration that eliinates steady-steady error for step, rap and parabolic input. The Keythley Dwin PRO DSP high perforance industrial coputer and DBasic prograing language were used. lso, a configuration with PXI Controller fro National Instruents and LabView developent environent were used as a very flexible cobination. The experiental researches were carried out on a general-purpose actuator designed and built in the Fluid Power Laboratory fro the University Politehnica of Bucharest. The sae ideas were ipleented in different kind of high perforance applications as power units speed governors. Key words: Digital electro hydraulic control systes; proportional valves; control algoriths.. INTRODUCTION clasical electrohydraulic servoechanis (figure ) contins a position feedback and a P copensator only. The dynaic perforance is liited by the load ass ainly. n efficient way of iproving the overal daping is the use of an aditional force feedback supplied by a force transducer placed on the piston rod (figure ). The transient force feedback is ore suitable for a general purpose servoechanis, loaded by a rando force (Lebrun and Claude, 994). This hardware additional feedback can be replaced by a software one based on soe additional derivative feedback suitable for high speed digital servocontrolers. The general predictive control by additional state variables feedback leads to good results even with low coputation speed (Mare, 994). Fig.. Basic electrohydraulic servoechanis

2 Nicolae VSILIU, Constantin CĂLINOIU, Daniela VSILIU, Dragoş ION-GUŢĂ Fig.. MESi siulation network with aditional force feedback in the loop. MTHEMTICL MODELING The basic set of equations describing the dynaic behavior of an electrohydraulic servo syste contains the following equations: Q = K i K P () qi qp dz V dp Q = + ap + dt 4E dt () d z dz P = M + f + F dt dt () P = P P (4) Siple transfer functions can be obtained by neglecting the fluid copresibility, and by accepting the practical condition Here, Z( s) K H ( s) = = (5) I( s) s( T s + ) Z( s) K v H v ( s) = = (6) F ( s) s( T s + ) f K ce << ce MK T = (8) is the equivalent tie constant of the hydraulic otor, including the overal pressure drop influence by the coefficient and K ce = a + K qp (9) (7)

3 Iproving the accuracy of the electro hydraulic servoechaniss by additional feedbacks K qi K = ; K K v = () are the transfer factors on the two control channels. Syste perforance can be derived by the aid of this odel. proportional copensator ensures a null steady-state error ( ε st = ) for a step input signal, y r = (t) only. The steady-state error for a rap input signal depends on the overal speed gain b ce ε st = () Kb K = K K K () R t The high level applications as aerospace ones need the eliination of the steady-state error both for step, rap and parabolic input signals. This goal can be achieved by the aid of soe derivative functional correction (figure ), which can be easy ipleented by the aid of a high speed IPC (Catana, Vasiliu and Calinoiu, 999). Fig.. Servoechanis with derivative corrections This new configuration is described by the equation ( K K + K Td s) Yr ( s) K vf ( s) Z( s) = () T + ( + K T ) s + K K K R s d The syste follow error expressed as voltages becoes ε(s) = Yr (s) Y(s) = Yr (s) Kt Z(s) (4) Hence [ Ts + ( + KTd Kt KTd) s] Yr (s) + Kt KvF (s) ε = (5) T s + + K T s + K K K For a P-copensator, and if we apply the condition ( d ) R t U ( s) H ( s) = = ε ( s) R K R T d R t (6) + K Td = (7) K K the iput rap steady-state error becoes null for y r (t) = t. The parabolic input signal, t

4 Nicolae VSILIU, Constantin CĂLINOIU, Daniela VSILIU, Dragoş ION-GUŢĂ 4 y r ( t) = t (8) is followed with the steady-state error By the notation a PI copensator, leads to the following equation: T ε st = (9) KK t K R a = + K T K K T () H ( T d t d U ( s) ( s) = = K R ( ) () ε ( s) T s R + s + a s) T sy ( s) + K K i r t i v T sf ( s) i ε ( s) = () TTi s + ( + K Td ) Ti s + K K R KtTi s + K K R Kt If a =, the rap input signal has no steady-state error, and the servoechanis rejects the force disturbances. t the sae tie, the syste stability depends on the condition T i > T + K Td which has to be always checked during the design process. PID copensator can lead to a better dynaic perforance, but the fine tuning is ore delicate. siple nuerical siulation can prove the above considerations. For a typical servo syste, 5 = 4 ; K qi =, / s; Kiu = 5 / V ; a =. /Ns; Kqp =. 5 / Ns; () Z ax =. ; K t = V / ; Ns f = ; M = 8 kg. The figures 4 7, show clear the advantages offered by the derivative corrections. Yr(V) Y(V) 4 e(v) Yr(V).5.5 t(s) Fig. 4. Clasical response t(s) Fig. 5. Clasical follow error

5 5 Iproving the accuracy of the electro hydraulic servoechaniss by additional feedbacks Yr(V) Y(V) 4 g p e(v) Yr(V) eroareadeurărire t(s) Fig.6. Optiised response t(s) Fig.7. Optiised follow error. DIGITL SYSTEM FETURES The best solution for fast real tie applications is to place a dedicated CPU close to the signal source and therefore having dedicated resources for the purpose of processing this data. Only this structure gives the ability of exact response ties with predictable delays. If the intelligence is not localized and dedicated, but centered on a host PC platfor, all calculations are under the control of PC s operating syste, Windows, and its available resources. s a consequence, there can be no guarantees for response ties to either an external event interrupt or an internal tier interrupt. Furtherore, processes executing based on tier feedback will becoe erratic, at best, due to inconsistencies in the syste tier. However, Windows offers cofortable user interfaces, ultitasking functionality, and great possibilities for network functionality. In order to take advantages of a Windows environent and run fast and stable real-tie processes, it is necessary to use the Dwin faily of products. Dwin real-tie systes are coplete process controllers with analog and digital I/O, a local CPU, and local eory (Vasiliu and Vasiliu, 4, 5). Dwin systes use DSP s, which guarantee response ties of as little as.5 icroseconds to an interrupt, while aintaining coplete software stability, even in a Windows environent. Since the local processor handles the process control and/or data acquisition, the PC processor is free to run a user interface progra, for exaple, a an-achine interface with data visualization, user input, data storage, etc., without regard for the effect the user front-end software has on PC resources. Since the progras runs on the processor of the Dwin board, up to processes can run siultaneously on one CPU, with priorities assigned where required. Processes can interact, exchanging paraeters and data. Thus, each process has its own independent tiing, but it is also possible to exchange paraeters and data with other running processes. Finally it is possible to develop coplex applications with processes, which interact ore or less. This is the case of ultiple issiles launcher, aerospace gunnery, ground defenses and other special applications. 4. EXPERIMENTL TEST The experiental researches were carried out on a full-scale general-purpose load siulator, and on a test bench designed and built in the Fluid Power Laboratory fro the University Politehnica of Bucharest. The test bench diagra is presented in figure 8. The digital copensators included in the overall loops are proportional or PI ones, but any other type like fuzzy can be used. Stanford signal generator supplies the input signal of the syste. The use of an analog proportional copensator only offers a good dynaics, but leads to a steady-state error, which strongly depends on the actuator load. This fact was systeatic checked by nuerical siulation and extensive experients. very siple control syste cannot solve the proble of the accuracy, even for low dynaic requireents. The ain characteristics of the test bench are the following: industrial process coputer type Dwin-Pro (KEITHLEY); servo solenoid valves with integrated aplifier OBE (BOSCH); analog speed/position servo controller VPC (BOSCH); high speed actuator or heavy hydraulic cylinder; inductive position transducers (PENNY & GILLES), industrial coputer IPC connected by an Ethernet interface with an industrial process coputer. The pressure supply is a constant one. The control syste diagra is presented in figure 8. The dual control syste structure designed for high speed applications is presented in figure 9 (Vasiliu, Calinou, and Vasiliu, 6).

6 Nicolae VSILIU, Constantin CĂLINOIU, Daniela VSILIU, Dragoş ION-GUŢĂ 6 Fig. 8. Test bench diagra: SG - signal generator; SVEH high speed proportional valve; SC analog servo controller; IPCindustrial process coputer; DS data acquisition syste; i- input signal; e-output signal; PT-position transducer; DWinPRO-DSP industrial process coputer Fig. 9. Dual test bench diagra The theoretical results are found in good agreeent with the experiental ones. The iproveent of the dynaic perforance of the low speed servo systes for a sine input signal can be identified on the figures and. The overall sine input aplitude was great enough to point out the iproveent introduced by the control algorith. The sapling period of the real tie control was s, good enough for a axiu speed of about 45 /s. The tuning paraeters of the digital error aplifier were correlated with the tuning paraeters of the analog aplifiers included in the systes. Soe restriction was introduced by the nuerical derivative corrections. good dynaics needs a close correlation of the T d and T d paraeters by theory aid. The step input response (fig.) also shows a good dynaics.

7 7 Iproving the accuracy of the electro hydraulic servoechaniss by additional feedbacks Generator No correction Speed correction U[V] T[s] Fig.. Typical experiental sine input response Fig.. Nuerical siulation by MESi of sine input response

8 Nicolae VSILIU, Constantin CĂLINOIU, Daniela VSILIU, Dragoş ION-GUŢĂ U[V] Generator --- No correction --- Speed corection t[s] Fig.. Typical experiental iproved step input response CONCLUSION The additional feedbacks are effective, and easy to ipleent by the aid of odern industrial realtie coputers based on the digital signal processing. This new control strategy was successfully ipleented in the new generation of speed governors for hydro power units (Vasiliu, Calinou, and Vasiliu, 6). REFERENCES. CTN, I., CLINOIU, C ND VSILIU, N., (999), High speed electrohydraulic servosystes, Scientific Bulletin of the Politechnical University of Tiisoara, Toe 44. LEBRUN, M., CLUDE, R., (997), How to create Good Models without Writing a Single Line of Code, Fifth Scandinavian International Conference on Fluid Power, Linköping. LEBRUN, M. (4), EH's Model Reduction Using ctivity Indexes. Recent dvances in erospace Hydraulics, INS Toulouse 4. MRE, J.-CH. (994), Dynaics of the Electrohydraulic Rotary Servoechaniss, Doctor Thesis, I.N.S.. Toulouse. 5. MRE, J.C., CREGUT, S. (), Electro Hydraulic Force Generator for the Certification of a Thrust Vector ctuator. Recent dvances in erospace Hydraulics, INS Toulouse. 6. VSILIU, N., VSILIU, D., (4), Electro Hydraulic Servoechaniss with Two Stages DDV for Heavy Load Siulators Controlled by DWIN. Recent dvances in erospace Hydraulics, INS Toulouse. 7. VSILIU, N., VSILIU, D. (5), Fluid Power Systes, Vol.I. Technical Publishing House, Bucharest. 8. VSILIU, N., CLINOIU, C., VSILIU, D. (6), Modeling, Siulation and Identification of the Electrohydraulic Speed Governors for Hydraulic Turbines by MESIM. 6 European MESi User Conference - 4 th Edition, Strasburg 9. VSILIU D. (997), Researches on the electrohydraulic servopup and servootors, Ph.D. Thesis, University POLITEHNIC of Bucharest. VSILIU N. ND NTONESCU I., (998), Hydrostatic Transission for ircraft Loaders. EROTEH Research Report. VSILIU, D., CLINOIU, C., VSILIU, N., (), Governing Hydraulic Turbines By Servo Solenoid Valves Nuerical Siulation nd Test, ESS CONFERENCE, The Society for Coputer Siulation, Haburg. ***IMGINE S.., (6). dvanced Modelling nd Siulation Environent, Release 4.. User Manual, Roanne. ***Servo Solenoid Valves (999), Technical Specification/, BOSCH - utoation Technology, Stuttgart Received February 6, 9