Hydraulic and Electro Hydraulic Control Systems By R. B. Walters
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Hydraulic and Electro Hydraulic Control Systems By R. B. Walters


Force and motion control systems of varying degrees of sophistication have shaped the lives of all individuals living in industrialized countries all over the world, and together with communication technology are largely responsible for the high standard ofliving prevalent in many communities. The brains of the vast majority of current control systems are electronic, in the shape of computers, microprocessors or programmable logic controllers (PLC), the nerves are provided by sensors, mainly electromechanical transducers, and the muscle comprises the drive system, in most cases either electric, pneumatic or hydraulic.

The factors governing the choice of the most suitable drive are the nature of the application, the performance specification, size, weight, environmental and safety constraints, with higher power levels favouring hydraulic drives. Past experience, especially in the machine tool sector, has clearly shown that, in the face of competition from electric drives, it is difficult to make a convincing case for hydraulic drives at the bottom end of the power range, specifically at fractional horsepower level.

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A further, and frequently overriding factor in the choice of drive is the familiarity of the system designer with a particular discipline, which can inhibit the selection of the optimum and most cost-effective solution for a given application. One of the objectives of this book is to help the electrical engineer overcome his natural reluctance to apply any other than electric drives. Another difficulty often encountered among all types of engineers is the unwillingness or inability to tackle the dynamics of hydraulic control systems in view of their relative complexity as compared with electric drives. Owing to the compressibility of the working fluid and the non-linear characteristics of hydraulic control devices, dynamic system modelling involves the manipulation of non-linear, high order differential equations.

This fact can have a daunting effect on all but the more analytically inclined engineers, and has contributed to the wide gap that exists between the control engineer and the average hydraulic application engineer. It has often led to the oversimplification of hydraulic system identification, which has frequently necessitated costly re-design and even resulted in litigation. It is hoped that this book will help in bridging the gap between the academic and the application engineer and thereby make some contribution towards the wider application of hydraulic control systems.

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Partly due to its relative complexity, hydraulic control system analysis is an ideal hunting ground for the mathematically biased engineer. Several analytical methods have been developed over the years and every specialist in this field has his own preference. The approach adopted in this book is based on method (3), with an extension into the time domain, facilitating the modelling of system transient response to any given duty cycle.

This concept, which permits system optimization in the frequency domain, has been developed and successfully applied over more than 15 years, with close correlation between predicted and actual performance over a wide range of applications, which is, in the final analysis, the ultimate criterion of credibility. In that context it is of interest to note that whereas the manuscript for the original edition was laboriously handwritten and subsequently typed and all graphs were manually drawn, the text for the latest edition was typed directly into a word processor and all graphs were generated and plotted by means of a specially adapted computer simulation program. Thus time marches on!

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