The Next Chapter for Hydraulics

Digital Displacement pumps bring hydraulics into the digital age by replacing analog mechanical actuation with digital solenoid valves and embedded software controls.

The Artemis demonstration excavator.
The Artemis demonstration excavator.

Since the invention of Digital Displacement® technology 20 years ago, Artemis Intelligent Power Ltd. has been developing and commercializing its use in hydraulics, working to increase the scale and scope of what it can deliver. “We’ve been increasing it from the few kilowatts of power of the early days up to the multi-megawatt level that we’ve now reached,” says Dr. Niall Caldwell, Managing Director of Artemis Intelligent Power Ltd.

The key premise behind Digital Displacement technology is the selective enabling of individual cylinders within a rotating hydraulic pump. Cylinders that are not required continue to idle. “It’s a fusion of mechanical mechanisms, which are fairly common in the hydraulic industry, with high-speed solenoid valves and a microcontroller which triggers those valves as the shaft rotates,” explains Caldwell. The goal is to provide controllable hydraulic power at a high level of efficiency.

While the technology has its roots in renewable energy, Caldwell says Digital Displacement technology offers the benefit of being inherently scalable. The company has demonstrated its use in a vehicle as small as a golf buggy, and spent the last five years—since being acquired by Mitsubishi Heavy Industries Ltd. in 2010—developing it for off-shore wind applications. There are currently two demonstration wind turbines, one onshore in Scotland, where the company is based, and one 20 km (12.4 miles) from Fukushima, Japan, floating in the sea. Both turbines feature a 7 megawatt (MW) hydrostatic transmission using Digital Displacement technology.

Caldwell says the company is now at a level where it’s ready to bring Digital Displacement technology to more mainstream use, such as in excavators and other heavy-duty mobile applications.

Digital from the ground up

According to Caldwell, what sets a Digital Displacement pump (DDP) technology apart is the fact that it is fundamentally digital. He says digital technology applied to hydraulics typically consists of a digital controller superimposed onto a fairly traditional pump design. The pump’s digital interface receives a signal from the vehicle CANbus or other digital command which is converted into an analog signal and “the same old fashioned analog control happens underneath the surface.”

The DDP is digital from the ground up, he says, because there is no analog conversion happening. “It’s a technology that could only work today because of advances in embedded control, timing and particularly the solenoid valves that we’ve developed.”

Featuring a radial piston configuration, the DDP uses a controller to fire the pump valves as the at just to the right time in the shaft rotation to ensure each cylinder produces the flow required of it. The controller determines how many of the valves and in what sequence they should be enabled in order to meet the instantaneous demands of the system.

A conventional analog variable displacement pump, on the other hand, would use a swashplate or bent-axis mechanism to change the length of stroke of each piston as the shaft rotates. The mechanism is typically hydraulically actuated, says Caldwell, and therefore susceptible to the associated delays, backlash and hysteresis of conventional hydraulic control. “By contrast, [with] our technology we do not change the stroke of our pistons,” he says.

All control functions are built into software instead of an analog controller, as would be the case with a traditional pump. This enables the DDP to be programmed to complete various types of tasks. “That gives us a lot of flexibility to configure it for different applications,” says Caldwell.

He says it is similar to tuning an engine electronic control unit (ECU); as the pump is turning, its internal variables can be accessed through the software and hundreds of parameters and functions can be controlled. “We can basically duplicate the control function of any conventional pump. We can also make it do things a normal pump can’t do.”

Control functions a DDP is capable of include torque limiting, pressure limiting, flow limiting, and various coordinated motions which would otherwise be impossible with a mechanical pump due to its inability to respond as quickly and precisely. “It’s a new chapter for hydraulics,” says Caldwell. “As we say, we’re leading hydraulics into the digital age. We’re taking a leap into completely digital pumps and realizing all of the benefits that this brings with it.”

One of the biggest benefits Caldwell says the DDP provides is a reduction in energy loss compared to typical pump designs. He explains that conventional variable stroke pumps when operating at low displacement tend to have a lot of fixed energy losses which are constant regardless of how much work they are doing. “That means that at part load they tend to have very poor efficiency,” he says. “While operating at full displacement, such a pump might reach 90% efficiency. However, when it’s at part displacement, which is probably where it spends most of the time, the efficiency will have dropped down to 70%.”

When completely idle, with the shaft turning but no pumping is taking place, the DDP wastes very little energy, typically about 1/10th of that of a swashplate pump. Testing conducted by Artemis on a conventional swashplate pump idling at 200 bar (2,900 psi) showed it lost about 6 kW (8 hp) of heat into its hydraulic oil. However, a DDP with similar 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 of the electrical energy of the industrial equipment.”     

By reducing energy losses, there is the potential to reduce or even eliminate the system oil coolers, bringing about even further efficiency improvements. In addition, as efficiency improves so does productivity because more work can be done with the same amount of engine power.

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. “This can make even more dramatic reductions to energy consumption.”

In a typical off-road vehicle, Caldwell says 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, by which it tends to get masked. With the DDP, users also get the advantage of using advanced controls to get diagnostic information and configure the pump in different ways.

Application demonstrations

In June of this year, Artemis announced the start of a project aimed at demonstrating the use of its DDP in an excavator. The company is working independently on the project and has no exclusivity to any OEM.

The company has purchased a mid-sized excavator and will install on it a tandem pump version of its E-dyn® 96 DDP technology which is used in stationary industrial machinery. Artemis is in the process of finishing baseline tests on the excavator. Once complete, it will swap out the machine’s tandem pump for its own DDP version and repeat the same tests.

Through the project Artemis aims to create a beacon for the off-road industry, showing it 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. “And so we’re coming along with something that’s radically different, a real step-change in terms of efficiency and also in controllability and smart feedback—our pump is a bit like a common-rail diesel engine in that it has a controller at the heart of it which knows and is tracking how the machine is working millisecond by millisecond.”

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 their own existing hydraulic pumps. This will allow them to take advantage of the Digital Displacement technology while still keeping their own distinct product identity.

Caldwells 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 it sees excavators as an application which will always require hydraulic actuation. “When you look at the function of an excavator with it’s very long travel hydraulic cylinders, very high power linear actuation requirements, it’s really one which is extremely difficult to do with anything other than hydraulic power.”

While electric machines have started making their way into some applications, particularly at lower power levels, Caldwell says hydraulics will continue to be “top dog” for applications requiring linear actuation. “There will always be hydraulics in excavators as far as I can see into the future,” he says.

“We have intentionally chosen the excavator for the market potential,” Caldwell continues. “But I think also it’s a heavy-duty, tough application. It’s one 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.”

Digital Displacement and E-dyn are registered trademarks of Artemis Intelligent Power Ltd. 

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