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 hydraulic 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 one. 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; 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 travel 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 requirements.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 elevators. 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 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, which 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. 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 generally used for elevator control with multiple hydraulic cylinders, 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 disparity 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.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 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. 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 extended 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 than 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 elevato