The first edition of this book was intended to provide knowledge of hydraulic components, their operating characteristics and available circuit arrangements to assist in the design of hydraulic systems for a range of applications. For some circuits it is important to predict the dynamic performance of the overall system particularly where closed loop control is being employed.
For this purpose and to provide a general understanding of how parameters vary transiently analytical methods are developed that provide techniques for studying the dynamics of systems. In this second edition these features have been retained and expanded upon so as to evaluate the performance of some hydrostatic transmission examples. In the period since the first edition there has been a considerable expansion in the use of digital computers for control purposes which has been accompanied by developments in the instrumentation field and use of electromechanical valves.
This technology often involves closed loop control methods for example, the control of motor speed in a hydrostatic transmission. This raises issues of dynamic performance and stability, areas that are expanded in this second edition. It is useful to have an understanding of the mechanical losses in pumps and motors and a section has been added to model frictional processes in a typical hydrostatic motor which shows the effects of friction on torque and mechanical efficiency.
Independently operated valves for pumps and motors are not a new technological development but their use is becoming more apparent because of the options that are available for displacement control. The advantages of this technology are that conventional distributor valves and displacement change mechanisms are avoided which eliminates their frictional and volumetric losses and consequently results in increased effi-iency particularly at reduced displacement.
The design aspects of servo systems to provide this important feature are discussed. There has been a considerable development of industrial standards particularly in the fields of the determination of dangerous failure possibilities and system safety. These standards are referred to where applicable and discussed in the final chapter on systems management. Relevant standards are listed in the Appendix.
The introduction of electronic control into fluid power has created scope for its use in a wide range of machine applications particularly when operation by computers or programmable logic controllers (PLCs) is required. Electronic devices have improved the accuracy of control using closed loop control techniques in many applications that have traditionally been served by hydromechanical open loop systems.
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