EFFICIENT FLUID POWER CONTROL

Lead Research Organisation: University of Bath
Department Name: Mechanical Engineering

Abstract

The applications of hydraulics are diverse. Hydraulic actuation offers many benefits including compact and lightweight design due to high power density, fast response and good controllability. In most fluid power hydraulic systems, speed and force of the load are controlled using valves to throttle the flow and reduce the hydraulic pressure. This is a simple but extremely inefficient method as the excess energy is lost as heat, and it is common for more than 50% of the input power to be wasted in this way. An alternative method is to use a variable capacity hydraulic pump or motor. This is more efficient, but variable capacity pumps and motors are expensive.The proposed work investigates two methods of increasing the efficiency of hydraulic systems while maintaining good control of speed and force without the expense associated with variable capacity pumps. The first method is the Switched Reactance Hydraulic Transformer (SRHT), a novel device for controlling the flow and pressure of a hydraulic supply. The second method is the Electro-Hydrostatic Actuator (EHA). Both of these systems increase efficiency by removing the need for control valves. For both applications, active fluid-borne noise attenuation techniques may be necessary.Switched Reactance Hydraulic Transformer (SRHT):A new device for controlling the flow and pressure of a hydraulic supply is proposed. It consists of a high-speed switching valve and an 'inertance tube'. Acting as a transformer, the device is able to boost the pressure or flow. The device could be configured to provide the functionality of a variable capacity pump, a pressure relief valve, a pressure compensated flow control valve or a proportional valve. Each of these control modes can be achieved without an expensive variable capacity pump and without the inefficiency inherent in a control valve. Previous work highlighted problems of noise and parasitic power losses. If these problems can be overcome using more recent materials and techniques combined with careful design, it could provide a more cost-effective efficient alternative to pressure/flow control valves.Electro-hydrostatic Actuation (EHA):In EHAs, a variable speed electric motor drives a fixed displacement pump which delivers flow directly to a linear actuator. Moving from centralised power supplies to distributed multi-pump/actuator systems brings reductions in power levels for individual subsystems. Furthermore, valveless electro-hydrostatic actuation systems provide benefits of greater efficiencies compared to conventional valve-controlled hydraulic systems, further reducing the power requirements. EHA systems can suffer from noise problems because of the close coupling between pump and actuator, allowing direct transmission of pressure pulsation. The challenges are to achieve good dynamic performance while achieving higher efficiency, low noise and reduced system weight and size.Active Fluid Borne Noise Attenuation:Fluid-borne noise (FBN) is a major contributor to air-borne noise and vibration in hydraulic systems as well as leading to increased fatigue in system components. Although passive systems to reduce the noise have been shown to be effective, they require tuning to specific systems, their attenuation frequency range is limited and they may be bulky. Furthermore, attenuation devices based on expansion chambers, accumulators or hoses are likely to be unsuitable for EHA or SRHT systems as they add compliance to the system and would impair the dynamic response. Active devices, which add energy to the fluid to cancel out or destroy the pressure ripple to reduce noise levels, can be effective at a much wider range of frequencies and system designs without affecting the system's dynamic response. Both the SRHT device and EHA system may suffer from noise issues, and as such, will benefit from active noise attenuation.

Planned Impact

A direct beneficiary is the hydraulic system and component industry, which is worth about 1 billion to the UK economy, and growing at 10% a year according to latest available figures from CETOP, the European trade organisation. These systems and components are used in a diverse range of machines and industries (including automotive, aerospace, construction, marine, and defence), and so the customer base is immense. These developers and manufacturers will be able to produce more efficient machines, leading to other design improvements (such as smaller power supplies and cooling systems giving more compact designs). Thus these machines will be more competitive in many respects, not just in terms of meeting the demand for lower power consumption, and contribute to wealth creation. It is envisaged that this impact will be evident 5 years after the completion of the project and beyond. Reducing power consumption also contributes to the UK Government's commitment to cutting carbon dioxide emissions. As one example, according to recent Energy Technology Institute figures, nearly 10% of all ground-based vehicle emissions in the UK are from the off-road sector. This mainly constitutes vehicles for construction and agriculture which generally rely on hydraulic transmission and actuation, and to which this research will be directly applicable. Particularly as a result of working with industrial collaborators, the research staff will develop technical and transferrable skills which are valuable for both continued employment in academia as well as employment in industry. As described in the impact plan, there are five industrial collaborators spanning both manufacturers and users of fluid power systems and components. Industrial exploitation of results beyond these collaborators will be encouraged through dissemination and engagement activities including: presentation at industrial conferences, publishing articles in trade magazines and newsletters (BFPA and NFPA), through the Centre of PTMC's own website and newsletter, and holding an Industrial Workshop. The Centre's industry-funded staff (Technical Manager and Administrator) will assist with these activities.

Publications

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Cargo C (2014) Optimisation and control of a hydraulic power take-off unit for a wave energy converter in irregular waves in Proceedings of the Institution of Mechanical Engineers, Part A: Journal of Power and Energy

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De Negri V (2014) Behavioural prediction of hydraulic step-up switching converters in International Journal of Fluid Power

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De Negri V (2015) Modelling and analysis of hydraulic step-down switching converters in International Journal of Fluid Power

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Johnston N (2012) The transmission line method for modelling laminar flow of liquid in pipelines in Proceedings of the Institution of Mechanical Engineers, Part I: Journal of Systems and Control Engineering

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Johnston N (2013) An enhanced transmission line method for modelling laminar flow of liquid in pipelines in Proceedings of the Institution of Mechanical Engineers, Part I: Journal of Systems and Control Engineering

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Pan M (2013) Active control of pressure pulsation in a switched inertance hydraulic system in Proceedings of the Institution of Mechanical Engineers, Part I: Journal of Systems and Control Engineering

 
Description Hydraulic fluid power systems are widely used for motion control and offer many benefits including high power density, fast response and good controllability. However most hydraulic systems are very inefficient and this contributes significantly to global energy consumption and carbon emissions. This is because most systems use throttling valves to control the pressure and flow, turning the excess energy into waste heat.

Some novel techniques to enable control of hydraulic systems with high efficiency have been developed in this project. One is a "Switched Reactance Hydraulic Transformer" (SRHT). This is based on high speed switching valves to control the pressure and flow using pulse-width modulation. Two such valves were designed, built and tested in this project and were shown to be very effective. Some advanced and novel techniques were developed for analytical modelling of SRHT systems, measurement of unsteady flow with a very high bandwidth (several kHz), and real-time measurement of the speed of sound in the hydraulic fluid. These techniques can find application and value in diverse areas well beyond the initial scope of this project.

Other areas that have been developed include valve control of multiple loads using variable pressure, variable speed supply, focusing on a robot leg but applicable in diverse areas. This has been shown to be a practicable technique that provides greatly improved efficiency over conventional techniques.

Hydraulic fluid power is particularly prone to noise and vibration and this is particularly true for some efficient control techniques, for example SRHTs because of the rapid switching of pressure and flow. An active control technique has been developed and implemented and has been shown to provide excellent cancellation of pressure pulsations. This has great potential for providing practical noise control solutions without impairing the response or performance of the hydraulic system.
Exploitation Route The work has enhanced links with other Universities including Linz and Tampere, and may lead to collaboration, visits and exchanges. Hydraulics manufacturers including Parker and Rexroth are now working on the areas investigated in this project, and there may be scope for collaborative projects (possibly EU or Innovate UK funding). The findings are being incorporated into the University of Bath's fluid power CPD courses and disseminated into industry.
Sectors Aerospace, Defence and Marine,Agriculture, Food and Drink,Chemicals,Construction,Energy,Manufacturing, including Industrial Biotechology,Transport

 
Description The work has resulted in significant advances in understanding and development of efficient hydraulic systems. In the longer term it is expected to result in take-up of such technologies by industry, enhancing the UK's profile in hydraulic systems and components, resulting in significant energy savings, and enabling hydraulics to compete with other technologies such as electric actuation.
First Year Of Impact 2016
Sector Aerospace, Defence and Marine,Construction
Impact Types Economic