Special Issue on Digital Hydraulics

The wet and windy sources of renewable energy tend to have large forces and low velocities with widely variable and sometimes alternating energy flows. Furthermore productivity is better if the force presented to the input is able to respond to changes in magnitude, often in a complicated way. Finally it may not be economic to generate from the very largest power levels and so there has to be a graceful way to shed unwanted peak inputs. These characteristics are directly incompatible with the strict specifications for electricity networks, especially weak ones in remote places which are likely to be the first places where the renewable sources will be exploited. At the start of design work on the Edinburgh duck we tried to resolve these uncertainties with off-the-shelf highpressure oil pumps and motors. We found that power levels were not high enough, that efficiency was too low, that it was difficult to combine power from different sources and inconvenient to control machines by computer. This paper describes the design of a new type of hydraulic machine to overcome the difficulties. Variabledisplacement is achieved by enabling or disabling poppet valves rather than by the variation of machine geometry in the same way as switch-mode control in electronics. This can produce large reductions in part-load losses. The paper shows how machines can be used for wind, wave and tidal-stream generators and gives guidance on dimensions and weights. Torque ratings up to 10 8 Nm for slow machines and power ratings up to 10 MW for fast motors are possible.

Proceedings of the ICE - Engineering and Computational Mechanics, 2008

Hydraulic engineers and researchers deal with the scientific challenges presented by turbulent flow and its interactions with the surroundings. Turbulent flows are characterised by unpredictable behaviour, and, as yet, little systematic research has been conducted in natural systems. This paper discusses the implications of recent developments in affordable instrumentation, which was previously characterised by intrinsic weaknesses that adversely affected the quality of the signal outputs. A challenging application is the unsteady turbulence field in tidal bores. The interactions between open channel flows and movable boundaries and the atmosphere illustrate another aspect of our limited knowledge. Rapid siltation of reservoirs and air entrainment in turbulent freesurface flows are discussed. In both applications, hydraulic engineers require some broad-based expertise. In turn, the education of future hydraulic engineers is of vital importance.

Proceedings of the Institution of Mechanical Engineers, Part I: Journal of Systems and Control Engineering, 2012

Imagine the scenario in which an electric motor company came to the (correct) conclusion that the power density of hydraulic pumps and motors is much higher than that of electric generators and motors. Then imagine that the board of this company gave the (wrong) instruction to the R&D department to utilize the concept of hydrostatic machines to develop electric generators and pumps with the same power density as their hydraulic equivalents. The reasoning of the board parallels the reasoning of the Forward Look article 1 in which Rudolf Scheidl, Matti Linjama and Stefan Schmidt argue that the future of fluid power will become partly digital: ''The overwhelming success of digital concepts in information, communication and power electronics, as well as the analogy between electrical and fluid power systems, suggest that digital concepts should be beneficial in fluid power''. Or, to recast this statement: ''The overwhelming success of feathers for making birds fly, as well as the analogy between birds and fish, suggests that feathers should also be beneficial for fish''. In addition the authors emphasize that ''the positive connotations of the word 'Digital' are good selling points''. Do not be mistaken! I am a strong supporter of research in digital hydraulic systems, although I am not at all convinced that the future of fluid power will be digital. To quote Einstein: ''If at first, the idea is not absurd, then there is no hope for it.'' A lot of research in digital fluid power I consider to fall in the category 'absurd', in the sense of 'out of tune' or 'dissonant' with the ruling technology, and I am very curious to see what will be useful in the end. The idea to copy successful electrical concepts in the design of hydraulic systems and components is as old as the hydraulic industry itself. Examples are 'fluidics' or 'fluidic logics' in which a fluid flow is directed in order to perform analog or digital operations, similar to electronic devices. 2 The authors of the Forward Look article refer to another example, AC hydraulics, in which the principle of a three-phase a.c. network is converted to a three-phase alternating flow network. 3,4 Other examples are 'switched reactance hydraulics' 5 and 'PWM electrohydraulic control systems'. 6 Aside from some niche applications, none of these promising developments has been applied successfully in the market. In 2011, Linjama presented an overview of the stateof-the-art of digital fluid power. 7 In that article he

The hydraulic systems are mainly used for precise control of larger forces. The main applications of hydraulic system can be classified in five categories: 2.1 Industrial: Plastic processing machineries, steel making and primary metal extraction applications, automated production lines, machine tool industries, paper industries, loaders, crushes, textile machineries, R & D equipment and robotic systems etc.

In hydraulics, as with any technical topic, a full understanding cannot come without first becoming familiar with basic terminology and governing principles. The basic concepts discussed in the following pages lay the foundation for the more complex analyses presented in later chapters.

Software Quality is a term, which is commonly used in the wide Computer World. However, saying that the main goal for the developers and stakeholders is to achieve the software which is of high quality is too general. Measuring software quality is very complex, because it consists of evaluating software products, processes and resources, each of which is composite as well. The desired quality should be measured by considering diverse aspects of quality, which eventually will assemble to a complete picture of the quality developed. In our research we have concentrated on the software product evaluation, based on two developed systems. The product attributes such as size and complexity were first measured and calculated, to be able to compare the examined systems based on the obtained data.

Acta Geophysica