Discussion: Is the future of fluid power digital?

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 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. 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. Other examples are ‘switched reactance hydraulics’ and ‘PWM electrohydraulic control systems’. 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. In that article he defined digital fluid power: ‘‘Digital Fluid Power means hydraulic and pneumatic systems having discrete valued component(s) actively controlling system output. Digital fluid power does not mean digital control of analogue components. Some borderline cases are the control of valve spool by using stepping motor or bang-bang positioning of actuators.’’. Linjama furthermore distinguishes between ‘‘parallel connected systems’’ and systems based on ‘‘switching technologies’’. Wang, et al. classify three different digital hydraulic systems.