Robust Control of Industrial Hydraulic Cylinder Drives - with Special Reference to Sliding Mode- & Finite-Time Control

Research output: PhD thesis

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In industry, performance requirements regarding machinery, applications etc., are constantly increasing, and with the development of reliable proportional flow control components to reasonable prices, the market is increasingly turning its attention toward controllable fluid power solutions. For series produced systems such as presses etc., dedicated controls are often developed. However, the great majority of the hydraulic systems developed, are produced in limited numbers for specialized applications, and here stand alone economically feasible digital controllers with ease-of-use interfaces are widely used. Such controllers typically provide the possibility to employ traditional linear controls such as PID schemes, and variants of this, with parameters tunable via graphical user interfaces. However, due to the intrinsic nonlinearities of hydraulic systems as well as the often limited knowledge of system parameters and control theory, hydraulic control systems are often implemented with poor results and with no indications on stability margins, robustness toward parameter perturbations, disturbances etc. Hence commissioning of such control systems is often an iterative and hence expensive process, making it difficult to comply with tight budgets and delivery deadlines.

The objective of this project is to overcome these issues, and develop controls aiming at robustness, consistent performance, simple parameter design and applicability under industrial conditions, i.e. with only pressure- piston- and spool position sensors, standard proportional valves- and control electronic hardware. The project is limited to consider only position control systems. To achieve the project objective, the possibility of online tracking of system parameters has been investigated, targeting compensations of nonlinearities and online controller adjustment. In regard to this, methods for compensation of the system gain have been developed - one based on the recursive least squares approach, and a model based type using a generalized system gain model and sensors.

In order to achieve consistent position control performance, simple parameter design and robustness in the presence of uncertain parameters- and disturbances, the field of sliding mode control has been investigated. Especially high order sliding mode control methods have been studied, due the intriguing possibility of maintaining the main properties of sliding mode control but with continuous control inputs. The applicability of second order modes has been investigated, and modifications of such controls have been developed based on homogeneity principles in order to provide more suitable controllers for hydraulic systems, than conventional second order sliding mode types. Also, an extension of the second order sliding algorithm known as the twisting algorithm has been developed, with compensation of local equilibria, and even an arbitrary order sliding mode design has been considered in a future perspective.

Experimental results reveal that the model based gain compensator may significantly improve performance of even control systems with linear controllers. The results of compensator-plus-control designs demonstrate improved tracking performance compared to common linear control methods based on best industrial practice. In particular homogeneous extensions / modifications of first- and second order sliding controls show to be especially suitable for hydraulic cylinder drives operating under industrial conditions. These controllers demonstrate superior performance compared with conventional methods, and may be commissioned with limited tuning effort. Combined with the proposed gain compensator, the resulting control structures are considered the main results in regard to the overall objective of the project.
Original languageEnglish
Print ISBNs978-87-92846-33-4
Publication statusPublished - 2014


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