液压英语论文2

1、*Corresponding author. Tel.:65-874-6889; fax:65-779-1459. E-mail address:mpemma .sg (M.A. Mannan.Control Engineering Practice 9(2001367373Electro-hydraulic proportional control of twin-cylinderhydraulic elevatorsKe Li , M.A. Mannan *, Mingqian Xu , Ziyuan XiaoDepartment of Mechanical and Prod

2、uction Engineering, National Uni v ersity of Singapore, Singapore 119260, SingaporeSchool of Mechanical Engineering, Tongji Uni v ersity, Shanghai, 200092, People s Republic of ChinaReceived 4July 2000; accepted 27October 2000AbstractThe large size of the cab of an electro-hydraulic elevator necessi

3、tates the arrangement of two cylinders located symmetrically on both sides of the cab. This paper reports the design of an electro-hydraulic system which consists of three #ow-control proportional valves. Speed regulation of the cab and synchronization control of the two cylinders are also presented

4、. A pseudo-derivative feedback (PDFcontroller is applied to obtain a velocity pattern of the cab that proves to be close to the given one. The non-synchronous error of the two cylinders is kept within $2mm with a constrained step proportional-derivative (PDcontroller. A solenoid-actuated non-return

5、valve, i.e. a hydraulic lock, is also developed to prevent cab sinking and allow easy inverse-#uid #ow. 2001Published by Elsevier Science td .Keywords:Hydraulic elevator; Velocity tracking; Synchronization; Hydraulic lock1. IntroductionThe modern hydraulic elevator is currently an excel-lent and low

6、-cost solution to the problem of vertical transportation in low-or mid-rise buildings, and in those applications requiring very large capacities, slow speeds and short travel distances (Edwards,1989. These include scenic elevators in superstores or historical buildings (Hayes,1999; Anon, 1989; Schne

7、ider, 1986, stage elev-ators (Anon,1974, ship elevators (Laurent,de Fays, &Dambrain, 1988; Brouet, 1998 and elevators for the disabled (Iwainsky,Lauermann, &Spanier, 1990, etc. In most cases, hydraulic elevators can be adapted to archi-tectural design requirements without compromising en-ergy saving

8、 and e $ciency requirements (Schneider,1987. In addition, the use of re-resistant #uids makes the hydraulic elevator a suitable choice when elevators have to operate near hazards such as furnaces or open res (Umesh,1981.Hydraulic drives are used preferably in elevators where large payloads need to b

9、e carried, such as for car elev-ators or marine elevators. In heavy load cases, anelevator cab usually has directly acting or side-acting hydraulic cylinders (Wemhoener,1971. The direct-acting arrangement involves a deep pit, substantial risk of corrosion of the buried cylinders and the di $culty of

10、 replacing failed cylinder parts. Thus, in many situations the side-acting hydraulic cylinder is preferred, despite the fact that it probably increases rail wear due to insu $cient cab sti ! ness. In the extreme conditions, i.e. when large cab sizes and uneven payloads are involved, the cab s #exibi

11、lity may even cause the guide shoes to stick to the rails, which is very dangerous. Therefore, in such cases, a feasible solution is to arrange two directly acting cylin-ders symmetrically on each side of the cab, as shown in Fig. 1. It should be noted that smooth running cannot be ignored because p

12、eople may be part of the payloads that accompany the freight. The major issue when designing a control system is to ensure the synchronous motion of the two cylinders.The error due to the non-synchronous motion of the two cylinders caused, by an uneven load under equal pressure-control, which is gen

13、erally used for elevator control with multiple hydraulic cylinders, is schemati-cally shown in Fig. 2. It is obvious from Fig. 2that equal pressure-control is not suitable for a synchronized hy-draulic elevator. When the payload is located on the right side of the cab, the left cylinder, with a ligh

14、ter load, will0967-0661/01/$-see front matter 2001Published by Elsevier Science Ltd. PII:S 0967-0661(01 00003-X Nomenclature d? diameter of the seat of pilot ball 12of thehydraulic lockd A e ! ective diameter of rod 5of the hydraulic lock e non-synchronous height error between the two cylindersr ref

15、erence input to valve 5r reference input to valve 6and 7v velocity of the cabD ? outer diameter of main spool 11of the hydraulic lockD diameter of the seat for main spool 11of the hydraulic lockF force acted on rod 5of the hydraulic lockF force acted on pilot ball 12of the hydraulic lock F C electro

16、-magnetic force F Q spring forceK #ow-pressure coe $cientP A pressure in cabin C of the hydraulic lock P ? pressure in cabin A of the hydraulic lock P pressure in cabin B of the hydraulic lock P pressure at port PA of the hydraulic lock Q #owX T the spool displacementPthe pressure drop across a thro

17、ttle valve Fig. 1. Side acting arrangement with two cylinders.Fig. 3. Hydraulic circuit diagram of the twin-cylinder elevator. The hydraulic system employed in this twin-cylinder elevator works according to the #ow-restricted speed-regulation principle, in which the #uid #ow into and out of the two

18、cylinders is controlled by appropriate valve settings, with the output of the pump kept at a xed level. In this system, there are three #ow-control proportional valves *57as shown in Fig. 3. Flow-control propor-tional valves act as throttle valves that restrict the #uid #ow to a single direction. Th

19、ey can give a smooth stepless variation of #ow control from near zero up to the valve s maximum capacity. The #ow rate through valve 5 remains almost invariable because a combination hy-drostat maintains a constant level of pressure di ! erence across the proportional valve, irrespective of system o

20、r load pressure changes. In the case of throttle valves, 6and 7in Fig. 3, their #uid #ows will change with system or load pressure changes.Valve 5, here called velocity valve, controls the velocity of the elevator. The upward motion of the cab is driven by xed-displacement piston pump 1. When motor

21、2 starts to work, the solenoid-actuated twin-position re-lief-valve 4unloads the output from pump 1to tank 20 and the opening of velocity valve 5is kept at its max-imum value. The solenoid of valve 4is automatically energised, shifting the valve to its closed position and thus setting a relief press

22、ure for the system. At this stage, the regulation of the cab velocity is achieved by adjusting the electric current through the coil of valve 5. At the closing of valve 5, all the #uid #ows into cylinders 12and 13and thus the cab velocity reaches its maximum value. The downward motion is caused by t

23、he dead load of the cab and its payloads. When the control panel receives a downward call, solenoid-actuated non-return valves 10 and 11open and the cab velocity is controlled by valve 5. The larger the opening of valve 5, the higher the cab velocity. Velocity valve 5directs pressurised #uid from th

24、e cylinders to tank 20to lower the cab. Check valve 3 prevents pressurised #uid from driving the pump in its reverse working direction.Synchronous motion of cylinders 12and 13depends on the combined adjustment of the #ow control valves 6and 7. The steady-state #ow through a throttle valve can be rep

25、resented asQ K X T ( P , (1 where Q denotes the #ow, X T the spool displacement, P the pressure drop across the valve and K is a constant. If the pressure drop P remains constant, Q is in direct proportion to X T , which is in direct proportion to the electric current through the solenoid coil. The

26、#ow vari-ations that are caused by the pressure drop variations can thus be compensated for by changing X T . As men-tioned above, #uid #ows through the #ow control pro-portional valves in only one direction. Valve groups 8and 9, each of which consists of four check valves, are used to ensure that v

27、alves 6and 7work in their normal directions.Solenoid-actuated non-return valves 10and 11are specially designed to prevent the cab from sinking, which is normally caused by the leakage of the hydraulic com-ponents when the cab stops at a landing. The working principle of the solenoid-actuated non-ret

28、urn valve will be further expanded later in this paper. They lock the cab when the pump stops and thus can be called hydraulic locks here. Only when their solenoids are energized will the cab move downward. In case of power breaks or other hydraulic element failures, emergency valve 14 lowers the ca

29、b at a lower speed.3. Electro-hydraulic proportional controlA suitable velocity curve, preset according to design speci cations such as maximum acceleration, maximum rate of acceleration change and maximum running velo-city, etc., is usually used to describe the running pattern of an elevator. If th

30、e cab velocity follows the given curve well, good riding comfort is assured. Open-loop control cannot achieve su $cient tracking accuracy because of variations in payloads, #uid volume in cylinders and #uid viscosity. Therefore, speed feedback is needed to attenu-ate the in #uence of the various dis

31、turbances on theK. Li et al. /Control Engineering Practice 9(2001367373369 Fig. 4. Block diagram of the elevator control system.Fig. 5. Block diagram of the PDF control system.performance of an elevator (Watanabeet al., 1994; Tomisawa et al., 1991.Furthermore, without closed-loop control, the non-sy

32、nchronous motion of the two cylinders is inevitable due to the di ! erences in payload, friction and hydraulic #ow resistance between the two cylinders.Consequently, two closed loops are required to attain speed regulation and synchronization control at the same time. The control block diagram of th

33、e whole system is shown in Fig. 4, which represents the elevator motion in upward direction. A similar block diagram can easily be deduced for downward motion. The cab velocity is mea-sured by an encoder. The translational movement of the cab is transferred to rotation of the rotor of an encoder by

34、a pulley. A two-element synchro-system is used to measure the relative angles between the rotors of control transmitter CX and control transformer CT. Thus, the relative angle measured by the synchro-system is propor-tional to the height error between the two cylinders. As discussed above, the cab v

35、elocity is only determined by velocity valve 5in Fig. 3, provided the synchroniza-tion valves 6and 7work in strict proportion to valve 5. In turn, under the same condition, the adjustment of valves 6and 7will not in #uence the cab s velocity. Hence, speed regulation and synchronization control can b

36、e realized separately, i.e. velocity controller 1and synchro-nization controller 2can work independently.A pseudo-derivative feedback (PDFcontroller, i.e. controller 1as shown in Fig. 4is applied to suppress the adverse e ! ects of internal parameter changes such as #uid volume in cylinders and exte

37、rnal disturbances such as payload and #uid-temperature variations. As shown in Fig. 5, the PDF controller is easy to realize and insensi-tive to system-parameter changes and external distur-bances (Phalen,1977. When m(t is small enough, thesaturated non-linearity can be simpli ed as working in its l

38、inear segment, then the PDF controller parameters can easily be obtained.Suppose the system can be described by G (s bs #a s #a. (2Then the three controller parameters are:K B 1b (3 H ! a , K N 1b(3 H ! a ,(3K G Hbk&, where H is 7.5167/t Q , t Q the settling time and k &theconstant for adjusting the

39、 output amplitude of the con-troller.In situ tuning of controller parameters is required to ensure the optimal performance. Figs. 6and 7show the tracking performance of the cab s velocity following the given velocity curve with a full payload and with no payload, respectively. The di ! erence betwee

40、n the desired velocity pattern and the actual velocity pattern is mainly due to the non-linear characteristics of the electro-hy-draulic proportional valve 5. However, the whole velo-city pattern is very close to the designed pattern, and thus satisfactory riding comfort can still be guaranteed.370K

41、. Li et al. /Control Engineering Practice 9(2001367373 Fig. 6. Velocity curve under void payload (dashed:desired, continuous: measured.Fig. 7. Velocity curve under full payload (dashed:desired, continuous: measured.Fig. 8. Flow chart of the constrained step PD controller.Fig. 9. Non-synchronous heig

42、ht error curve under void payload.A constrained step proportional-derivative (PDcon-troller, i.e. controller 2in Fig. 2, is used to obtain syn-chronous motion of the two cylinders. The idea behind this PD controller is similar to the steering of a boat. When rowing a boat to keep it along a straight

43、 line, the rower exerts force on oars each time according to how far and how fast the boat is getting away from the line. Because of the rower s unavoidably delayed response, the disparity between the boat s real route and the given route cannot be kept small. An e ! ective alternative method involv

44、es the rower applying a fraction of the estimated forces each time the oars are operated. The boat will thus approach the given route step by step till the route error approaches an acceptable value. This algorithm, when used in controlling the non-syn-chronous error of two cylinders, can be describ

45、ed by the #ow chart shown in Fig. 8.Cylinder 12is taken as the reference cylinder, whose movement has to be followed by cylinder 13, say, the following cylinder. The reference input to valves 6and 7, i.e. r, as shown in Fig. 4, is proportional to that ofvelocity valve 5, r. The two constants in Fig.

46、 8, n andu K?V, are set by experience gained, which is based on a large number of tests. The backlash of valves 6and 7issimilar to the rower s delayed response to the boat s route error. In each adjustment period of controller 2, its real output is only a fraction of the required value cal-culated b

47、y the PD controller. That is, the large error is reduced in each sampling period at a constrained step until an acceptable height error is reached. This control scheme has proven to be e ! ective in keeping the non-synchronous error within $2mm, as shown in Figs. 9and 10.It should be noted that if t

48、he initial non-synchronous error during a sampling period is rather large, it would take some time to reach an acceptable level of error. If the non-synchronous error at the end of one elevator run can be retained at the beginning error of the next run, this process can be avoided and the non-synchr

49、onous error will remain at small values throughout all the runs. To attain this goal, a sink-proof device is needed since theK. Li et al. /Control Engineering Practice 9(2001367373371372 K. Li et al. / Control Engineering Practice 9 (2001 367373 di!erent leakage rates of the two cylinders will direc

50、tly increase the initial error of an elevator. 4. Design of a solenoid-actuated non-return valve One of the major tasks of this device is to guarantee a low non-synchronous error between the cylinders by preventing the leakage from the individual cylinders. Cab sinking will also obstruct freight loa

51、ding or unloading and passenger access or exit. The two valves 10 and 11, hydraulic locks, act to prevent cab sinking. Without them, the leakage, which is due to the clearance between Fig. 10. Non-synchronous height error curve under one ton unevenly placed payload. the spool and housing of valves 5

52、7, would denitely result in the sinking of the cab. A hydraulic lock that is used in the project is a modied version of a commercial hydraulic-controlled non-return valve with the addition of solenoid actuation. In its unmodied form, the valve has three ports: input port PB, output port PA and hydra

53、ulic control port PT. When #uid #ows from port PB to port PA (forward #uid #ow, it works like a check valve. In turn, when the #uid #ow reverses (inverse #uid #ow, it provides perfect cone sealing, which is much more reliable than clearance sealing. Its cone spool cannot be moved until it is acted o

54、n by the pressure of #uid at port PT. It is not desirable to use hydraulic controlled non-return valves in this elevator system since the pump does not work when the cab moves downward, thus no pressurised #uid is available at port PT to open the hydraulic controlled non-return valves and lower the

55、cab. Such a problem will not be encountered if a solenoid can propel the cone spool of a hydraulic controlled non-return valve when #uid #ows from port PA to port PB. Meanwhile, the desired cone sealing is retained. The assembly diagram of a solenoid-actuated non-return valve or hydraulic lock as me

56、ntioned above is shown in Fig. 11. A hydraulic lock is mainly composed of solenoid 1, propelling rods 4 and 5, main spool 11, pilot balls 12 and 22, etc. Port PA is connected to a cylinder and port PB to Fig. 11. Solenoid-actuated non-return valve. K. Li et al. / Control Engineering Practice 9 (2001

57、 367373 373 the hydraulic system. When the elevator moves upward, #uid from the pump enters port PB, opens main spool 11, and goes into a cylinder via port PA. After the elevator arrives at the desired landing, the pump stops providing #uid and the #uid in port PA is checked by main spool 11, pilot

58、ball 12 and pilot ball 22. As a result, the cylinder is locked. When the elevator is called to move downward, solenoid 1 is energised and its acting rod 2 propels rod 5 toward the right side. As rod 5 is proceeding, the cone surface at its left end propels rod 4 downward and opens pilot ball 22, ena

59、bling #uid in port PA to #ow into closed cabin C via hole 15. At this time, the pressures in cabins C and A are the same as that in port PA, P . The force ? acting on rod 5 at its left end is F d(P !P , 4 A A (4 guarantees low non-synchronous error by applying a constrained step PD controller. The test results show that the non-synchronous error can be kept within $2 m