1、大学毕业设计（外文翻译）Electro-hydraulic proportional control of twin-cylinder hydraulic elevatorsAbstractThe large size of the cab of an electro-hydraulic elevator necessitates the arrangement of two cylinders located symmetrically on both sides of the cab. This paper reports the design of an electro hydrauli
2、c system which consists of three flow-control proportional valves. Speed regulation of the cab and synchronization control of the two cylinders are also presented. A pseudo-derivative feedback (PDF) controller is applied to obtain a velocity pattern of the cab that proves to be close to the given on
3、e. The non-synchronous error of the two cylinders is kept within 2mm with a constrained step proportional-derivative (PD) controller. A solenoid actuated non-return valve, i.e. a hydraulic lock, is also developed to prevent cab sinking and allow easy inverse-fluid flow. Keywords: Hydraulic elevator;
4、 Velocity tracking; Synchronization; Hydraulic lock1. IntroductionThe modern hydraulic elevator is currently an excellent and low-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 tra
5、vel distances. These include scenic elevators in superstores or historical buildings, stage elevators, ship elevators and elevators for the disabled, etc. In most cases, hydraulic elevators can be adapted to architectural design requirements without compromising energy saving and efficiency requirem
6、ents.In addition, the use of fire-resistant fluid makes the hydraulic elevator a suitable choice when elevators have to operate near hazards such as furnaces or open fires.Hydraulic drives are used preferably in elevators where large payloads need to be carried, such as for car elevators or marine e
7、levators. In heavy load cases, an elevator cab usually has directly acting or side-acting hydraulic cylinders. The direct-acting arrangement involves a deep pit, substantial risk of corrosion of the buried cylinders and the difficulty of replacing failed cylinder parts. Thus, in many situations the
8、side-acting hydraulic cylinder is preferred, despite the fact that it probably increases rail wear due to insufficient cab stiffness. In the extreme conditions, i.e. when large cab sizes and uneven payloads are involved, the cabs flexibility may even cause the guide shoes to stick to the rails, whic
9、h is very dangerous. Therefore, in such cases, a feasible solution is to arrange two directly acting cylinders symmetrically on each side of the cab, as shown in Fig. 1. It should be noted that smooth running cannot be ignored because people may be part of the payloads that accompany the freight. Th
10、e 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 generally used for elevator control with multiple hydraulic cylinde
11、rs, is schematically shown in Fig. 2. It is obvious from Fig. 2 that equal pressure-control is not suitable for a synchronized hydraulic elevator. When the payload is located on the right side of the cab, the left cylinder, with a lighter load, will move upward faster than the right one. The speed d
12、isparity between the two cylinders will not cease until the reaction forces actuated by the rails on both the lower left and upper right guide shoes attached on the cab are balanced by the hydraulic force difference. The non-synchronisation of the two cylinders can only be reduced by flow control, i
13、e. by ensuring that the fluid flows into the two cylinders per unit time are the same.This paper presents an electro-hydraulic system for the control of an elevator with twin cylinders that are located on each side of the elevator cab. The designed system consists of three flow-control proportional
14、 valves. A PDF controller is applied to velocity control whereas a constrained step PD controller guarantees the minimum non-synchronous error between the motion of two cylinders. The design of a newly developed solenoid-actuated non-return valve i.e. a hydraulic lock is also presented in this paper
15、 In this project, experiments are conducted with a normal size passenger cab instead of building a new large-size cab due to cost limitations. In order to achieve the flexible condition of a larger cab, the distance between the rails and their corresponding guide shoes in the side direction is exte
16、nded so that the cab has no constraints in this direction. Meanwhile, in the forward and backward direction, the cab is constrained by the rails just like a general passenger elevator. The synchronous motion control of the two cylinders in such an assembly is analogous to and even more difficult tha
17、n that of a larger cab with normal constraints.2. Electro-hydraulic control system design There are two different fluid power systems generally used in hydraulic elevators. the flow-restriction speed-regulation system and variable-delivery speed-regulation system. In the former system, the pump runs
18、 at a constant speed and the valve regulates the speed of the cylinder in both the upward and downward directions. In the latter case, the cab is operated by varying the speed of the pump, which is driven by a speed-controlled induction motor.The hydraulic system employed in this twin-cylinder eleva
19、tor works according to the flow-restricted speed regulation principle, in which the fluid flow into and out of the two cylinders is controlled by appropriate valve settings, with the output of the pump kept at a fixed level. In this system, there are three flow-control proportional valves -5-7 as sh
20、own in Fig. 3. Flow-control proportional valves act as throttle valves that restrict the fluid flow to a single direction. They can give a smooth stepless variation of flow control from near zero up to the valves maximum capacity. The flow rate through valve 5 remains almost invariable because a com
21、bination hydrostat maintains a constant level of pressure difference across the proportional valve, irrespective of system or load pressure changes. In the case of throttle valves, 6 and 7 in Fig. 3, their fluid flows will change with system or load pressure changes. Valve 5, here called velocity va
22、lve, controls the velocity of the elevator. The upward motion of the cab is driven by fixed-displacement piston pump 1. When motor 2 starts to work, the solenoid-actuated twin-position relief- valve 4 unloads the output from pump 1 to tank 20 and the opening of velocity valve 5 is kept at its maximu
23、m value. The solenoid of valve 4 is automatically energised, shifting the valve to its closed position and thus setting a relief pressure 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 val
24、ve 5, all the fluid flows into cylinders 12 and 13 and thus the cab velocity reaches its maximum value. The downward motion is caused by the dead load of the cab and its payloads. When the control panel receives a downward call, solenoid-actuated non-return valves 10 and 11 open and the cab velocity
25、 is controlled by valve 5. The larger the opening of valve 5, the higher the cab velocity. Velocity valve 5 directs pressurised fluid from the cylinders to tank 20 to lower the cab. Check valve 3 prevents pressurised fluid from driving the pump in its reverse working direction. Synchronous motion of
26、 cylinders 12 and 13 depends on the combined adjustment of the flow control valves 6 and 7. The steady-state flow through a throttle valve can be represented as where Q denotes the flow, Xv the spool displacement, P the pressure drop across the valve and K0 is a constant. If the pressure drop P rema
27、ins constant, Q is in direct proportion to Xv, which is in direct proportion to the electric current through the solenoid coil. The flow variations that are caused by the pressure drop variations can thus be compensated for by changing Xv. As mentioned above, fluid flows through the flow control pro
28、portional valves in only one direction. Valve groups 8 and 9, each of which consists of four check valves, are used to ensure that valves 6 and 7 work in their normal directions. Solenoid-actuated non-return valves 10 and 11 are specially designed to prevent the cab from sinking, which is normally c
29、aused by the leakage of the hydraulic components when the cab stops at a landing. The working principle of the solenoid-actuated non-return 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
30、 are energized will the cab move downward. In case of power breaks or other hydraulic element failures, emergency valve 14 lowers the cab at a lower speed.3. Electro-hydraulic proportional controlA suitable velocity curve, preset according to design specifications such as maximum acceleration, maxim
31、um rate of acceleration change and maximum running velocity, etc., is usually used to describe the running pattern of an elevator. If the cab velocity follows the given curve well, good riding comfort is assured. Open-loop control cannot achieve sufficient tracking accuracy because of variations in
32、payloads, fluid volume in cylinders and fluid viscosity. Therefore, speed feedback is needed to attenuate the influence of the various disturbances on the performance of an elevator. Furthermore, without closed-loop control, the non-synchronous motion of the two cylinders is inevitable due to the di
33、fferences in payload, friction and hydraulic flow 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 the whole system is shown in Fig. 4, which represents the elevator
34、 motion in upward direction. A similar block diagram can easily be deduced for downward motion. The cab velocity is measured by an encoder. The translational movement of the cab is transferred to rotation of the rotor of an encoder by a pulley. A two-element synchro-system is used to measure the rel
35、ative angles between the rotors of control transmitter CX and control transformer CT. Thus, the relative angle measured by the synchro-system is proportional to the height error between the two cylinders. As discussed above, the cab velocity is only determined by velocity valve 5 in Fig. 3, provided
36、 the synchronization valves 6 and 7 work in strict proportion to valve 5. In turn, under the same condition, the adjustment of valves 6 and 7 will not influence the cabs velocity. Hence, speed regulation and synchronization control can be realized separately, i.e. velocity controller 1 and synchroni
37、zation controller 2 can work independently. A pseudo-derivative feedback (PDF) controller, i.e. controller 1 as shown in Fig. 4 is applied to suppress the adverse effects of internal parameter changes such as fluid volume in cylinders and external disturbances such as payload and fluid-temperature v
38、ariations. As shown in Fig. 5, the PDF controller is easy to realize and insensitive to system-parameter changes and external disturbances . When m1(t) is small enough, the saturated non-linearity can be simplified as working in its linear segment, then the PDF controller parameters can easily be ob
39、tained.Suppose the system can be described byThen the three controller parameters are:where is 7.5167/ts, ts the settling time and kH the constant for adjusting the output amplitude of the controller. In situ tuning of controller parameters is required to ensure the optimal performance. Figs. 6 and
40、7 show the tracking performance of the cabs velocity following the given velocity curve with a full payload and with no payload, respectively. The difference between the desired velocity pattern and the actual velocity pattern is mainly due to the non-linear characteristics of the electro-hydraulic
41、proportional valve 5. However, the whole velocity pattern is very close to the designed pattern, and thus satisfactory riding comfort can still be guaranteed. A constrained step proportional-derivative (PD) controller, i.e. controller 2 in Fig. 2, is used to obtain synchronous motion of the two cyli
42、nders. The idea behind this PD controller is similar to the steering of a boat. When rowing a boat to keep it along a straight 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 rowers unavoidably delayed response,
43、the disparity between the boats real route and the given route cannot be kept small. An effective alternative method involves 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 approache
44、s an acceptable value. Cylinder 12 is taken as the reference cylinder, whose movement has to be followed by cylinder 13, say, the Fig.8. Non-synchronous height error curve under void payload. Fig.9. Non-synchronous height error curve under one ton unevenly placed payloadfollowing cylinder. The backl
45、ash of valves 6 and 7 is similar to the rowers delayed response to the boats route error. In each adjustment period of controller 2, its real output is only a fraction of the required value calculated by the PD controller. That is, the large error is reduced in each sampling period at a constrained
46、step until an acceptable height error is reached. This control scheme has proven to be effective in keeping the non-synchronous error within 2mm, as shown in Figs. 8 and 9. It should be noted that if the initial non-synchronous error during a sampling period is rather large, it would take some time
47、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-synchronous error will remain at small values throughout all the runs. To attain this goal, a sink-proof
48、device is needed since the different leakage rates of the two cylinders will directly increase the initial error of an elevator. 4. Conclusion An electro-hydraulic control system with three flow control proportional valves has been proposed for the control of elevator velocity and non-synchronous er
49、ror between the cylinders of a twin cylinder hydraulic elevator. A pseudo-derivative feedback control scheme has shown to be an appropriate technique to achieve a desired velocity pattern. Furthermore, this system 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 mm. A certain discrepancy between the desired pattern and the actual velocity pattern is due mainly to the hystere
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