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1. THE PUBLISHING HOUSE PROCEEDINGS OF THE ROMANIAN ACADEMY Series A OF THE ROMANIAN ACADEMY Volume10 Number 3 2009 pp 000 000 IMPROVING THE ACCURACY OF THE ELECTRO HYDRAULIC SERVOMECHANISMS BY ADDITIONAL FEEDBACKS Nicolae VASILIU Constantin CALINOIU Daniela VASILIU Dragos ION GUTA University POLITEHNICA of Bucharest Fluid Power Laboratory Power Engineering Faculty 313 Independentei Bucharest 060042 Romania Phone 4021 316 9643 Fax 4021 316 9645 Corresponding author Nicolae VASILIU E mail vasiliu fluid power pub ro The paper contains a report on some theoretical and experimental researches aiming to create high accuracy electro hydraulic digital servomechanisms used in high tech applications as actuators load simulators fly control systems dynamic test machines etc Computational methods control software design problems and experimental 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 maximum flexibility of the testing programs By using one derivative feed forward path from the system input and one derivative path from the system output the authors developed a hardware and software configuration that eliminates steady steady error for step ramp and parabolic input The Keythley ADwin PRO DSP high performance industrial computer and ADBasic programming language were u2. best due to inconsistencies in the system timer However Windows offers comfortable user interfaces multitasking functionality and great possibilities for network functionality In order to take advantages of a Windows environment and run fast and stable real time processes it is necessary to use the ADwin family of products ADwin real time systems are complete process controllers with analog and digital I O a local CPU and local memory Vasiliu and Vasiliu 2004 2005 ADwin systems use DSP s which guarantee response times of as little as 0 5 microseconds to an interrupt while maintaining complete software stability even in a Windows environment Since the local processor handles the process control and or data acquisition the PC processor is free to run a user interface program for example a man machine 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 programs runs on the processor of the ADwin board up to 10 processes can run simultaneously on one CPU with priorities assigned where required Processes can interact exchanging parameters and data Thus each process has its own independent timing but it is also possible to exchange parameters and data with other running processes Finally it is possible to develop complex applications with processes which interact more or less This is the case of multiple missiles launcher aerospac
3. d correction 05 0 0 05 Fig 10 Typical experimental sine input response Ts Fig 11 Numerical simulation by AMESim of sine input response Signal generator P compensator Speed correction 20 0 Nicolae VASILIU Constantin CALINOIU Daniela VASILIU Dragos ION GUTA 8 2 75 Generator ULV No correction Speed corection 2 45 1 r l Fig 12 Typical experimental improved step input response CONCLUSION The additional feedbacks are effective and easy to implement by the aid of modern industrial real time computers based on the digital signal processing This new control strategy was successfully implemented in the new generation of speed governors for hydro power units Vasiliu Calinou and Vasiliu 2006 10 11 12 13 REFERENCES CATANA I CALINOIU C AND VASILIU N 1999 High speed electrohydraulic servosystems Scientific Bulletin of the Politechnical University of Timisoara Tome 44 LEBRUN M CLAUDE R 1997 How to create Good Models without Writing a Single Line of Code Fifth Scandinavian International Conference on Fluid Power Link ping LEBRUN M 2004 EHA s Model Reduction Using Activity Indexes Recent Advances in Aerospace Hydraulics INSA Toulouse MARE J CH 1994 Dynam
4. e gunnery ground defenses and other special applications 4 EXPERIMENTAL TEST The experimental researches were carried out on a full scale general purpose load simulator and on a test bench designed and built in the Fluid Power Laboratory from the University Politehnica of Bucharest The test bench diagram is presented in figure 8 The digital compensators included in the overall loops are proportional or PI ones but any other type like fuzzy can be used A Stanford signal generator supplies the input signal of the system The use of an analog proportional compensator only offers a good dynamics but leads to a steady state error which strongly depends on the actuator load This fact was systematic checked by numerical simulation and extensive experiments A very simple control system cannot solve the problem of the accuracy even for low dynamic requirements The main characteristics of the test bench are the following industrial process computer type ADwin Pro KEITHLEY servo solenoid valves with integrated amplifier OBE BOSCH analog speed position servo controller AVPC BOSCH high speed actuator or heavy hydraulic cylinder inductive position transducers PENNY amp GILLES industrial computer IPC connected by an Ethernet interface with an industrial process computer The pressure supply is a constant one The control system diagram is presented in figure 8 The dual control system structure designed for high speed applications is present
5. ed by the aid of some derivative functional correction figure 3 which can be easy implemented by the aid of a high speed IPC Catana Vasiliu and Calinoiu 1999 8 In8 1 Fig 3 Servomechanism with derivative corrections This new configuration is described by the equation _ KimKp Kin T5 Y 8 K my Fos ToS K Pip 8 Kim KRK 3 The system follow error expressed as voltages becomes efs 5 5 Yp s K Z s 14 Hence aid Irs 1 K gt Rake ly bb 3 KK myFo s 15 TS 1 Kin jT a9 8 KintK pK For a P compensator H pals a Kp 16 and if we apply the condition Ty ia a7 the imput ramp steady state error becomes null for y t t The parabolic input signal Nicolae VASILIU Constantin CALINOIU Daniela VASILIU Dragos ION GUTA 4 yQ 50 18 is followed with the steady state error by 19 2K K Kp By the notation ay 14 Ky Ty K Kila 20 a PI compensator Hels Kelt 21 leads to the following equation Tns 4y8 T SY 8 KK my T SFy S T T s 1 at K mTa2 yrs T K mK RK T s K e s 22 KK m If a 0 the ramp input signal has no steady state error and the servomechanism rejects the force disturbances At the same time the system stability depends on the condition T m T gt 1 K mTa2 23 which has to be always checked during the design process A PID compen
6. ed in figure 9 Vasiliu Calinou and Vasiliu 2006 Nicolae VASILIU Constantin CALINOIU Daniela VASILIU Dragos ION GUTA 6 Fig 8 Test bench diagram SG signal generator SVEH high speed proportional valve SC analog servo controller IPC industrial process computer DAS data acquisition system i input signal e output signal PT position transducer ADWinPRO DSP industrial process computer ADwin Fig 9 Dual test bench diagram The theoretical results are found in good agreement with the experimental ones The improvement of the dynamic performance of the low speed servo systems for a sine input signal can be identified on the figures 10 and 11 The overall sine input amplitude was great enough to point out the improvement introduced by the control algorithm The sampling period of the real time control was 10 ms good enough for a maximum speed of about 45 mm s The tuning parameters of the digital error amplifier were correlated with the tuning parameters of the analog amplifiers included in the systems Some restriction was introduced by the numerical derivative corrections A good dynamics needs a close correlation of the Ty and Ta parameters by theory aid The step input response fig 12 also shows a good dynamics Improving the accuracy of the electro hydraulic servomechanisms by additional feedbacks Ul Generator No correction Spee
7. ics of the Electrohydraulic Rotary Servomechanisms Doctor Thesis I N S A Toulouse MARE J C CREGUT S 2001 Electro Hydraulic Force Generator for the Certification of a Thrust Vector Actuator Recent Advances in Aerospace Hydraulics INSA Toulouse VASILIU N VASILIU D 2004 Electro Hydraulic Servomechanisms with Two Stages DDV for Heavy Load Simulators Controlled by ADWIN Recent Advances in Aerospace Hydraulics INSA Toulouse VASILIU N VASILIU D 2005 Fluid Power Systems Vol I Technical Publishing House Bucharest VASILIU N CALINOIU C VASILIU D 2006 Modeling Simulation and Identification of the Electrohydraulic Speed Governors for Hydraulic Turbines by AMESIM 2006 European AMESim User Conference 4 Edition Strasburg VASILIU D 1997 Researches on the electrohydraulic servopump and servomotors Ph D Thesis University POLITEHNICA of Bucharest VASILIU N AND ANTONESCU L 1998 Hydrostatic Transmission for Aircraft Loaders AEROTEH Research Report VASILIU D CALINOIU C VASILIU N 2000 Governing Hydraulic Turbines By Servo Solenoid Valves Numerical Simulation And Test 2000 ESS CONFERENCE The Society for Computer Simulation Hamburg IMAGINE S A 2006 Advanced Modelling And Simulation Environment Release 4 3 0 User Manual Roanne Servo Solenoid Valves 1999 Technical Specification13 2 BOSCH Automation Technology Stuttgart Received February 16 2009
8. sator can lead to a better dynamic performance but the fine tuning is more delicate A simple numerical simulation can prove the above considerations For a typical servo system A 4 10 m Kq 0 013m As Ki 5 10 A V a 1 2 107 m Ns Kg 0 12 107 m Ns Zax 0 3m Ki My m f 120058 3 M 80 kg m The figures 4 7 show clear the advantages offered by the derivative corrections Yr V Y V t s t s Fig 4 Clasical response Fig 5 Clasical follow error 5 Improving the accuracy of the electro hydraulic servomechanisms by additional feedbacks Yv 4 Y V t s eroarea de urm rire t s Fig 6 Optimised response Fig 7 Optimised follow error 3 DIGITAL SYSTEM FEATURES The best solution for fast real time 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 times with predictable delays If the intelligence is not localized and dedicated but centered on a host PC platform all calculations are under the control of PC s operating system Windows and its available resources As a consequence there can be no guarantees for response times to either an external event interrupt or an internal timer interrupt Furthermore processes executing based on timer feedback will become erratic at
9. sed Also a configuration with PXI Controller from National Instruments and LabView development environment were used as a very flexible combination The experimental researches were carried out on a general purpose actuator designed and built in the Fluid Power Laboratory from the University Politehnica of Bucharest The same ideas were implemented in different kind of high performance applications as power units speed governors Key words Digital electro hydraulic control systems proportional valves control algorithms 1 INTRODUCTION A clasical electrohydraulic servomechanism figure 1 contins a position feedback and a P compensator only The dynamic performance is limited by the load mass mainly An efficient way of improving the overal damping is the use of an aditional force feedback supplied by a force transducer placed on the piston rod figure 2 The transient force feedback is more suitable for a general purpose servomechanism loaded by a random force Lebrun and Claude 1994 This hardware additional feedback can be replaced by a software one based on some 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 computation speed Mare 1994 i 1 HaKe GOH ss T K Fig l Basic electrohydraulic servomechanism Nicolae VASILIU Con
10. stantin CALINOIU Daniela VASILIU Dragos ION GUTA 2 Fig 2 AMESim simulation network with aditional force feedback in the loop 2 MATHEMATICAL MODELING The basic set of equations describing the dynamic behavior of an electrohydraulic servo system contains the following equations OK hal 1 dz V dP aP 2 Q m dt a m 4E dt 2 d z dz Bp ne a T 3 Pa P P 4 Simple transfer functions can be obtained by neglecting the fluid compresibility Z s Kn H m9 F 5s 4 gt Z s K mv H mv s F s s T 8 rs 1 6 and by accepting the practical condition f Kee p lt lt 1 7 Here MK Tn 2 8 is the equivalent time constant of the hydraulic motor including the overal pressure drop influence by the coefficient Ke a tKy 9 and 3 Improving the accuracy of the electro hydraulic servomechanisms by additional feedbacks Ka K K F et Ger 2 10 m m are the transfer factors on the two control channels System performance can be derived by the aid of this model A proportional compensator ensures a null steady state error E 0 for a step input signal y 1 t only The steady state error for a ramp input signal 1 Est E 11 depends on the overal speed gain K KRK K m 12 The high level applications as aerospace ones need the elimination of the steady state error both for step ramp and parabolic input signals This goal can be achiev
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