Electro-hydraulic proportional control of twin-cylinder hydraulic elevators

The 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 ±2 mm 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.