A new groundbreaking technology could change the face of hydraulics – digital displacement technology (DDT). Developed by the UK technology development company, Artemis Intelligent Power, this award-winning technology is based on a simple fundamental idea: only use the cylinders that are required to meet the demand. This involves the selective enabling of individual cylinders within a rotating hydraulic pump. Cylinders that are not required continue to idle. By adopting the radial piston format and providing each piston with its own individually controlled solenoid valve, idle losses are minimised and high band-width control is available through switching on the cylinders on a cycle by cycle basis.
“It’s a fusion of mechanical mechanisms with high speed solenoid valves and a microcontroller which triggers those valves as the shaft rotates,” explains Dr Niall Caldwell, managing director of Artemis Intelligent Power. “The goal is to provide controllable hydraulic power at a high level of efficiency.”
At every scale, the inherent efficiency of DDT allows the boundaries of hydraulic machines to be expanded. This has been demonstrated by the application of a purely hydraulic drive to wind turbines with a power output of 7 MW. The combination of lightness and efficiency is also proving to be effective for improving the fuel efficiency of urban buses by nearly 30%. Other applications include regenerative braking in trains. The company’s big push is now in off-road applications such as front-end loaders.
An effective business model
Artemis works with leading global companies, including Sauer-Danfoss, Bosch Rexroth and Mitsubishi. Together they develop DD systems and power transmissions for a growing range of energy-saving applications.
Mitsubishi Heavy Industries (MHI) acquired Artemis in 2010 as part of its proposed R1,7 billion investment in the Scottish offshore renewables industry. The company was looking for a new technology to build wind turbines approaching the limits of mechanical gearboxes. The two companies jointly built a 2,4 MW transmission to retrofit into an existing turbine in Yokohama, Japan and a 7 MW wind turbine which is now installed at the Hunterston offshore wind turbine test facility in Scotland. A second 7 MW turbine, the world’s largest floating wind turbine, has been installed 20 km off the coast of Fukushima.
Numerous benefits
Electronic control makes DD machines more efficient when there is no load. Idling cylinders are not pressurised, so leakage flows are small and waste very little energy. Because they provide very fast, dynamic pump control, DDPs offer efficiency at a component level as well as a system level. The faster control response allows more precise and repeatable flow and pressure control for applications such as robotics, resulting in reduced overall system design costs. Other components can be simplified or removed from the system without sacrificing functionality. This is important in applications such as injection moulding machines, which operate with short bursts of activity followed by idle periods. Another advantage is that less waste heat is generated, sometimes eliminating the need for a cooling system.
When completely idle, the DDP wastes typically about a tenth of the energy of a swashplate pump. Testing conducted by Artemis on a conventional swashplate pump idling at 200 bar showed it lost about 6 kW of heat into its hydraulic oil. A DDP with similiar pumping capacity wasted around 600 W when off-loaded, which Caldwell says equates to it being 94-97% efficient at peak displacement. “The main message is a dramatic reduction in part-load losses, which means that over a real duty cycle, typically we’re talking about a quarter of the energy losses attributable to the pump compared to a conventional variable-displacement machine,” he says. “And that can be worth 10-15% of the fuel consumption of the vehicle or the electrical energy of industrial equipment.”
Caldwell says there is also a radical change in the way hydraulic circuits can be designed because the pump can have multiple independent outlets, with each providing a different flow and pressure. The pump can have a single outlet and do the job of a conventional pump or it can be configured with a different end-plate to create multiple independent pumps around a single shaft. “It means that you don’t have to have so much throttling of energy from one pressure to another, but rather you can directly generate whatever pressure is required by the load,” he explains.
In a typical off-road vehicle, a conventional pump may on average be 80% efficient. However, the system into which the pump is integrated may be closer to 60% efficient due to the design of other components such as the valves and the energy losses of the pump and actuator. “There’s a lot to play for in system efficiency if you can eliminate the energy losses associated with throttling valves,” he says. “That’s one of the benefits that comes down the line once you’ve taken full advantage of the DDP’s unique features.”
Additional benefits include a reduction in noise output. Instead of the typical high frequency whine of a swashplate pump, Caldwell says the noise emitted by the DDP is in the same frequency band as the noise of an engine.
Regenerative braking
The lower cost and ability to retrofit DD hydraulics into conventional systems is also behind a project to investigate regenerative braking in trains. Funded by Innovate UK, Artemis is working with the engineering consultancy, Ricardo and trainmaker, Bombardier on a hybrid rail vehicle. This project combines Artemis’ DD pump-motor transmission and Ricardo’s flywheel energy storage system. The goal is to improve the transmission system as well as to recover braking energy. Conventional diesel commuter trains dissipate about 35% of their overall mechanical engine energy as heat through braking, while the transmission can waste up to a further 30%. Hydraulic transmission and energy storage can tackle both losses. Other benefits could include reduced wear, as the flywheel takes much of the strain off the brakes. The hydraulic system is also good at returning stored energy and reducing the load on the diesel engine, so there could also be less pollution, noise and vibration as trains pull out of the station.
Revolution in off-road hydraulic machines
In 2016 Artemis announced a project aimed at demonstrating to major OEMs the potential of DDP to bring unprecedented levels of fuel efficiency and digital controllability in off-road hydraulic machines. The company purchased a 16 ton excavator and installed on it a tandem pump version of its E-dyn 96 DDP technology, which is used in stationary industrial machinery. Baseline tests on the excavator were successful and the machine’s tandem pump has been swapped for the DDP version, with fuel consumption expected to be reduced by 16% when the same tests are repeated.
Through this project, Artemis aims to create a beacon for the off-road industry, showing that hydraulic pumps have not reached the end of their evolution but that there’s another chapter yet to come. “The whole industry is converged on an axial piston design which hasn’t actually changed in principle much in 50 years,” Caldwell says. “We’re coming along with something that’s radically different, a real step change in terms of efficiency and also controllability and smart feedback.”
Upon completion of the demonstration project, Artemis will work together with interested OEMs to develop a DDP product which specifically meets their application needs. The company will supply the key enabling components of the DDP – the valves and the controllers – and the OEM can use its own existing hydraulic pumps.
Caldwell says Artemis chose to demonstrate the technology on an excavator because it is one of the top markets for high pressure hydraulic pumps worldwide, and he sees excavators as an application which will always require hydraulic actuation. Looking at the function of an excavator, with its very long travel hydraulic cylinders and very high power linear actuation requirements, it’s extremely difficult to use anything other than hydraulic power. While electric machines have started making their way into some applications, particularly at lower power levels, hydraulics will continue to be the choice for applications requiring linear actuation. “It’s a heavy-duty, tough application that is in some ways a touchstone for hydraulics and what hydraulics can do. We thought if we can make a hydraulic excavator work, which has such high demands in terms of controllability and power handling and reliability, then we can do anything.”
Many awards
The technology is still in the early phases of commercial application, but has received many accolades, the latest being the Royal Academy of Engineering MacRobert Award in 2015. This is the UK’s most prestigious award for technical innovation. Dame Sue Ion, chair of the MacRobert Award judging panel, said: “The company has achieved a technical advance of global importance. This is not simply evolutionary improvement but a complete step change. Another MacRobert judge, Nick Cooper FREng said the technology is an incredible invention and a brilliant piece of detailed engineering design. “Ideas that overturn established models can face an uphill struggle for acceptance. Nobody would have given DDT a second look, and the story of Artemis is about technical excellence, as well as faith, determination and belief in the invention”, he says.
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