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Directions in Fluid Power

Article-Directions in Fluid Power

Directions in Fluid Power

Compact and efficient fluid power is both the name and the overarching goal for the coalition setting the research focus for developments in fluid power technology. But clearly the quest for energy efficiency is a force fluid power will use to re-shape its future. A cadre of engineering research projects is focused on new technology solutions that will impact the efficiency of the components in the fluid power drive train.

"Of the center's four goals, the first is to improve the efficiency of fluid power in current applications and achieve more than 50 percent reductions in energy usage," says Kim Stelson, a professor at the University of Minnesota and director of the Center for Compact and Efficient Fluid Power.

"It's easy to say when there is a critical need for energy efficiency that we should have started on this a decade ago," says Ed Howe, president of Enfield Technologies and chairman of the industrial advisory board for the center. "Today is our decade ago, and a decade from now we'll be glad we got our act together and started going at energy independence a little harder. We need to be more efficient and it's what the National Science Foundation wanted placed as a central priority for the research center. When the NSF says compact and efficient, that is what they mean."

Digital Hydraulics

A series of current projects are targeting fundamental research and ideas for improving the energy efficiency of fluid power components such as valves, pumps and motors.

"In the efficiency area, everyone is interested in high-speed valves that can turn on and off very quickly and still have reasonably high flow rates with small pressure drops," says Stelson. "Obviously there are trade-offs between those three characteristics but the goal is very practical, high-speed valves."

Two promising research projects for new valve designs are ongoing. One is a pulse-width modulated rotary valve development led by Professor Perry Li at the University of Minnesota. The second is high-speed poppet valves being explored by a team at Purdue University led by John Lumkes.

"I think that the rotary technology is exciting because, if you have a spool valve and a spool moving back-and-forth at higher frequencies, more and more energy has to go into the acceleration and deceleration of the spool each time you reverse direction," says Stelson. When the valve is rotated, since the accel and decel are eliminated, switching can be accomplished at much faster speeds. As the valve spins, it has ridges in a triangular pattern on it which, as it is translated along its axis, regulates the fraction of the time the flow goes to port A and what fraction of time the flow goes to port B. Port A can be hooked to an application and Port B could be back to the tank to achieve the high-speed switching."

"What we are trying to do is replace throttling valves which waste a lot of energy," he says. "Hydraulics is like electrical engineering before the solid-state era where you might have used a rheostat as a light dimmer, which would regulate the light but waste energy as well by dumping energy into the rheostat as heat. That is essentially what a throttling valve does and we need to replace those devices. Solid-state electronics on a light dimmer is a rapid switching on/off device and we are trying to provide the analogous solution and major energy savings."

The goal for the poppet valve project at Purdue is to create optimized valve modeling and develop equation structures that focus on how fluidic, mechanical and electromagnetic mechanisms interact in high-speed on/off valves, along with verifying results experimentally with a highly coupled valve prototype.

Lumkes says a prototype poppet valve has been developed, which is a three-way, two-position poppet sealing valve that is pressure-balanced. The geometry of the prototype is adjustable to achieve desired performance specifications suitable for modeling and verification. Stelson says the poppet valves have a tapered cone design which seals very well, have lower leakage and may be a superior technology with its ability to go to higher pressures.

"Depending on the driving voltage, system response is anywhere from .6 to 2 milliseconds, depending on the flow rate and voltage we want," says Lumkes. "That is switching 40l per min at a 10 bar pressure drop. It is an order of magnitude higher than a typical valve operating at that speed. We can also scale the design, and by doubling the stroke length, double the flow area and still have a valve that is capable of 1.5 to 2 millisecond response."

Lumkes says the design can handle large flows and still provide fast response, but the prototype itself is adjustable and actually validates the model for more than one particular motion. "Response is non-linear, especially with electromagnets as a function of stroke, driving voltages, hysteresis and the Eddy currents achieved at those speeds. We vary those parameters in the valve and verify that the model captures all of the non-linearities correctly," he says.

While development of the high-speed switching valve is important, the bigger picture is an approach to designing, simulating and using the valves in systems. "Without doing the prototype, it is only a theory so we definitely want to verify performance and build valves, but our goal is to also develop a methodology and suite of tools for designing valves for all of these different systems," Lumkes says.

The valve work is part of a growing move toward digital hydraulics for high-speed applications where conventional valves are too slow and there are opportunities to reduce throttling losses and increase system efficiencies. High-speed valves could be important building blocks and part of enabling isolation valves, swashplate displacement control, virtually variable displacement pumps, active pump control and optimized power management on a system level. Possible automotive applications could include hydraulic-hybrid transmissions, active damping and suspensions, electronic stability control, anti-lock brakes, fuel injectors and camless engines.

Hydraulic-Hybrid Vehicles

The second major goal of the CCEFP, to migrate fluid power technology into the transportation sector with hydraulic-hybrid vehicles, is also making significant progress. A variety of research projects are ongoing, along with the desire to investigate the smaller, more power-dense accumulators passenger cars would require. But the technology is now also taking to the road, first with garbage trucks and, later in 2009, with heavier trucks and buses.

"The short term future is that within a year, we will see significant hydraulic-hybrid vehicles," says Stelson. "The garbage trucks are just coming to market now. PeterBilt is a partner with Eaton Corp., who is developing the hydraulic subsystem. Bosch Rexroth also has a significant effort in this area, along with Parker-Hannifin, and all three companies are working on heavy vehicle, hydraulic-hybrid technology."

Stelson says after the garbage trucks, delivery vans such as UPS and FedEx will be next. Another area near-term is city buses and he says there are actually hundreds of city buses using hydraulic-hybrid technology on the streets of Beijing.

Waste Management Inc. said late in 2008 it was field-testing the first prototype parallel hydraulic -hybrid truck to be deployed in a waste collection vehicle. Four parallel hydraulic-hybrid-diesel collection trucks have been incorporated into Waste Management's fleet and are being tested in Fort Worth.

Hybrid vehicles hold great promise for the waste industry because collection vehicles have many cycles of braking and acceleration along a given route. The four Peterbilt 320 vehicles in Fort Worth use a Hydraulic Launch Assist(TM) (HLA(R)) system developed by Eaton Corp. to capture and store energy during braking, which not only improves efficiency but also reduces wear on brake pads. The stored energy is then transferred to accelerate the vehicle to the next pickup location, reducing fuel consumption and wear on the engine.

Though hybrid technologies have been successfully deployed in automobiles and light trucks, Class 8 vocational vehicles, a category that includes waste trucks, pose additional challenges to hybrid engine design. Among the largest vehicles on the road, Class 8 vehicles require a robust drive train that can handle heavy loads, and have multiple systems for compaction and lifting that draw power from the engine, complicating hybrid design.

Significant work is also being done in the area of more compact fluid power solutions. By decreasing size and weight, the goal is to migrate fluid power systems from heavy equipment to human scale assistive devices and new concepts for underwater exploration, medical and rehabilitation applications and wearable or compact tools for home and industrial use.

"Many companies are taking fluid power technology and applying it in broader ways to a wider, more expansive scope of applications," says Howe. "The skillsets and concepts of fluid power used to move a hydraulic actuator in a backhoe, all the knowledge of valves and pumps that make the backhoe work, are also being applied in a wide variety of other applications. Sophisticated engineers are giving fluid power a second look."

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