A Renaissance in Fluid Power

The prospect of a brighter future for fluid power technology may rest on new industry-driven research initiatives and educational activities. Formally organized through a National Science Foundation grant in 2006, a coalition of top industrial companies and researchers is mobilizing to put new ideas into old technology and has aggressive plans to transform the fluid power industry in the future.

“Fluid power has the highest power density of any technology, so it has capabilities to do things you can't do with electric motors. That's the promise,” says Dr. Kim Stelson, director of the Center for Compact and Efficient Fluid Power, the Engineering Research Center with headquarters at the University of Minnesota that spans seven major universities and is funded by the National Science Foundation (NSF). “We will believe this effort will lead to a renaissance in fluid power.”

Increasing the overall efficiency of fluid power is the most immediate goal, followed by a push to develop hydraulic hybrid trucks, buses and passenger cars. But longer term research also focuses on compact solutions to create fluid power source solutions for humanoid robots, biomedical devices and a new generation ofhand tools.

Stelson says the Engineering Research Center (ERC) is the best kind of support you can get from the NSF in engineering, because it is a large scale effort in a particular area. What is unusual is this initiative emerged from industry rather than the universities. Usually universities say, “we want to develop an engineering research center” and go out to seek support for the idea. Instead, industry came forward and said, “we need more fluid power research, can you guys help us?”

“Centers are also usually on absolutely new technology, nanotechnology or biomedical engineering,” Stelson says. “This is an old technology where we are trying to put new ideas into it and help to transform the industry.”

The Engineering Research Center, founded in June 2006 and now with more than 50 member companies, has developed an overall strategic plan, created five test beds to support key initiatives and created auxiliary projects that feed into these test beds.

The lead university is the University of Minnesota and core universities are University of Illinois at Urbana-Champaign, Purdue University, Vanderbilt University and Georgia Tech University. Outreach universities are North Carolina A&T University and the Milwaukee School of Engineering.

At the University of Minnesota, a test bed is being developed for hydraulic hybrid vehicles and there is current work on a hydraulic hybrid passenger car. At Purdue, work on an excavator will demonstrate near-term efficiency improvements.

Three test beds are focused on the longer term initiatives. Georgia Tech and Vanderbilt are working on a joint roving robot project. llinois has a fluid power orthosis and fluid power hand tools are being developed at Minnesota. Projects at a variety of universities feed into these test beds. The Milwaukee School of Engineering has a specialty in desktop manufacturing so they can make one-of-a-kind objects. The compact power source required for fluid power hand tools is being developed at Vanderbilt.

“We recognized in our strategy three areas in great need of improvement,” says Stelson, “the efficiency of fluid power, smaller and lighter, more compact solutions and overall effectiveness including human factors and noise issues.”

Focus on Efficiency

A primary area to explore is to dramatically improve the energy efficiency of heavy equipment used in manufacturing, aerospace, construction, agriculture and mining. If the overall efficiency of fluid power is improved by 10 percent, Stelson says the U.S. would save $7 billion in imported crude oil per year assuming oil is $50 per barrel.

If you look at the energy usage of a machine like an excavator or bulldozer, the hydraulic pumps and motors are inefficient and the throttling valves used for control are inherently energy-wasting devices. “Control is achieved by throwing away the energy you don't use,” he says.

The group has two initiatives in this area. One is PWM or high-speed on/off valves. The goal is to use high-speed switching valves to modulate the flow and not waste as much energy. A second approach is a hydrostatic solution for motion control using variable displacement pumps and motors. This could create a much more lossless position control system for each of the axes of motion.

Hydraulic Hybrids

A second strategic initiative is the opportunity to migrate fluid power into transportation. Currently, hydraulic hybrid vehicles, mainly heavy trucks and buses, are coming onto the market. But the group says there are reasons to believe this technology is superior and will offer benefits compared to electric hybrids particularly for heavier applications. At the same time, research is focusing on making the hydraulic hybrid passenger car a reality. “You cannot do that now with off-the-shelf technology, but we are trying to develop technology to make that possible,” Stelson says.

If you look at the energy density of an accumulator versus a battery, comparing an electric hybrid with a hydraulic hybrid, the energy density in the accumulator is not as good as energy density in the battery. But with the hydraulic hybrid, the power density of hydraulic pumps and motors is superior to the power density of electric motors and generators.

Stelson says some people tout the fact the Toyota Prius has regenerative braking, but he notes it is only 50 percent efficient depending on the drive cycle. The reason is the motors and generators are undersized and cannot accept all of the braking power. On the bench, electric motors and generators are very efficient, well into the 90 percent range, but the system can't realize that potential within the regenerative braking scheme.

In contrast, hydraulic pumps and motors are typically 80 to 90 percent efficient. But it's not a problem sizing hydraulic pumps and motors because their weight will easily fit into a car and they can accept the full braking power which makes regenerative braking more efficient.

He adds that there are two other ways to save energy with a hydraulic hybrid vehicle. One way is to turn off the engine when you don't need it and, since you have energy stored on-board, power the vehicle without the engine running. Plus, the engine can be operated at its most efficient operating condition, either off or almost at full throttle which is its most efficient condition.

On a typical spark ignition system, the maximum energy efficiency might be 30 percent. Efficiency drops to around 15 percent if the system is running at partial throttle. When cruising at highway speeds, the engine is a partial throttle because the engine is sized for acceleration/towing/hill climbing and not for cruising. Consequently, you are down to about 15 percent energy efficiency during cruising.

Another big negative hydraulic hybrids can address is the cost. Electric hybrid battery packs and motors add $3,000 to $7,000 to the cost of a vehicle and the average life of a battery pack is 90,000 miles. “There is a huge premium and we believe the hydraulic hybrid can be much cheaper, more reliable and because of the increased power density, the cars will offer a performance advantage,” Stelson says. “Not all of this is worked out and there are real challenges on how to control these things. There are also concerns about noise, drivability and how to make the accumulator hold more energy.”

“But I am predicting that we will see hydraulic hybrid trucks and buses coming on the market in the next few years,” he adds. “That technology will be refined, improved and with some breakthroughs we believe we can migrate it to passenger cars which would have huge social impact.”

Future Technology Development

Over a longer time frame, the group's research goal is to downsize fluid power solutions and develop a portable, compact fluid power source. This technology could help in the development of mobile, untethered robots that could work for long periods of time without external power. It could provide fundamental technology that could be used in fluid power hand tools and offer improvements in biomedical devices, such as prostheses, which could be powered by fluid power and operated for long periods of time without external power.

One possible technical approach is chemofluidic actuation. Torpedoes are propelled using a mono propellant, which produces a gas when it is exposed to a catalyst to create a chemical reaction. Researchers at Vanderbilt University are working on a hot gas vane motor that uses chemofluidic actuation over a rotary power source. The power and energy density of the system is promising and superior to batteries and motors.

Another effort is a free piston engine compressor. Currently, the way to make a pneumatic power source from an IC engine is to have a separate engine and compressor, which is a heavy and bulky approach. The IC engine takes the energy from the fuel and turns a crank to create compressed air. It's inefficient and bulky. The free piston engine compressor eliminates the mechanical interface. This new approach goes directly from the spark ignition using a diaphragm and check value arrangement, directly to compressed air in a small package. They are also working on a free piston pump which uses a hydraulic power source, which is more challenging, but has the potential to be more compact and higher power.

Focus on Collaboration and Education

The National Fluid Power Assn. (NFPA) has played an important role in this industry-wide investment and collaboration effort. And while the Engineering Research Center has been a catalyst, the NFPA was pivotal in competing with other applicants before the National Science Foundation to win that grant.

“We received a lot of direct feedback initially and ongoing from NSF about how the relationship that exists between our research base and our industry is fairly unique in their experience and is a model they'd like to see in other engineering research centers in different disciplines,” says Eric Lanke, NFPA executive director.

The group has helped develop a strategy to create interest in fluid power research among college students, graduate students and faculty. They have partnered with the Science Museum of Minnesota to develop a hydraulic hybrid car exhibit that will be shown at the IFPE Show in April 2008. Project Lead the Way is a non-profit foundation in Upstate New York and work has been done to develop curriculum to introduce fluid power to middle and high school students.

“Video productions created in partnership with Twin Cities Public Television have been developed to educate young people interested in science and technology careers on three themes,” Lanke says. “The first is fluid power makes the modern life we all enjoy possible. The second is the research discovery and dramatic positive effects it will have on the future. The third theme is about demonstrating rewarding careers and an industry going through a renaissance as it develops these new technologies.”