Motion synchronization of multi-cylinder electro-hydraulic lift systems
Abstract
In this dissertation, motion synchronization of multi-cylinder electro-hydraulic (EH) lift systems is investigated. Among the different methods of synchronizing the displacement of multiple cylinders in a hydraulic lift system with unknown but bounded load, EH control presents a set of attractive trade-offs. A general model for an n-cylinder EH lift is developed that includes the vertical displacement, pitch and row motion of the load. It can be seen that without external synchronization control, the lift is an unstable system where the difference in cylinder displacements will increase under unbalanced load, and will result in significant pitch and row motion that will topple the load. Two prototype EH lift systems, a two-post and a four-post lift, are constructed to evaluate different motion synchronization control strategies. Linear Quadratic Regulator (LQR) and Cross-Coupled Controller (CCC) approaches are applied using the linearlized lift model. Experimental results show noticeable improvement over existing mechanical constraint solution. Due to the inherent nonlinear dynamics and uncertainties associated with the hydraulic system, a nonlinear perturbation observer is proposed to estimate the perturbation that combines the effect of parametric uncertainties and external disturbances. A nonlinear force/pressure control algorithm is then designed to improve force/pressure tracking using the result of the perturbation observer. By exploiting the unique structure of the lift system, an inner and outer loop control is realized through a two-step back-stepping synthesis approach. The outer loop motion synchronization control is achieved using linear MIMO robust control techniques and the inner force control loop is achieved using the previously developed perturbation observer based nonlinear force control. The overall system stability is shown under the assumption of bounded lumped perturbation. The feasibility of the proposed two-loop control strategy is verified using both simulation and experimental results. Experimental results show that the proposed controller achieves steady-state synchronization down to the sensor resolution level as well as demonstrates improved transient performance compared to the linear designs. Valve dynamics are ignored in the current design. Experimental data show that sensor noise and valve dynamics (bandwidth) are the two most significant factors that limit the performance of the system.
Degree
Ph.D.
Advisors
Chiu, Purdue University.
Subject Area
Mechanical engineering|Electrical engineering
Off-Campus Purdue Users:
To access this dissertation, please log in to our
proxy server.