Power up with hydraulics

Once a declining power source, new initiatives in hydraulics are fueling their revival. University of Minnesota Professor, Kim Stelson, outlines work done on campus test beds and by progressive machine builders.

Power Up With HydraulicsBy Kim A. Stelson, Director of Engineering Research for the Center for Compact and Efficient Fluid Power

FLUID POWER has unexcelled capabilities that make it the only viable choice for some industrial machine power applications. Compared to an electric motor, a hydraulic actuator has 12 times more torque for the same volume, 100 times more power for the same volume, 500 times more power for the same weight, and 50 times the bandwidth for the same power.

Consequently, fluid power is a growing industry (See Figure 1 below) with component sales of $12 billion for the U.S. and $33 billion worldwide. Systems sales are one to two orders of magnitude higher.

Despite its intrinsic advantages, hydraulic power has been losing market share to electric-powered devices for injection molding machines. Although hydraulic machines still dominate the larger sizes, electric machines are displacing hydraulic machines in smaller sizes, where the higher power and force capabilities of hydraulics aren’t as important. There are key factors contributing to this trend. Hydraulic machines can be noisy, inefficient, and more difficult to maintain.

So, is the trend toward electric machines inevitable? Maybe not. There is considerable work underway to improve hydraulic power’s efficiency enough that it reemerges as a preferred power source.

U.S. Fluid Power Component Sales
U.S. Census Bureau’s Industrial Report for Fluid Power is based on an annual survey of U.S. manufacturers. Results for 2005 include the National Fluid Power Association’s (NFPA) estimates for November and December. Estimated growth for 2006 is a composite based on several independent forecasts.
Source: U.S. Census Bureau (Click image to enlarge).

Big Energy Saving Opportunities
In proportional valve or servo valve control, the flow to a hydraulic cylinder or other actuator is modulated by opening or closing an orifice in the valve. The power lost in the valve, P, is the product of the pressure drop, ∆ p, and the flow, Q, or P= ∆ p*Q. When the valve is fully open,  ∆ p is very small and little power is lost in the valve. At the other extreme, when the valve is fully closed, Q is zero so no power is lost. It’s at intermediate values that the power loss is a maximum, so it’s important to avoid partial load for efficient operation.

Pulse-width modulation (PWM) is one control approach that can almost entirely eliminate throttling losses in valves. The idea of PWM control is to either have the valve fully open or fully closed, but never partially open. The valves are opened and closed at a fixed frequency, and the fraction of time that the valve is opened regulates the average flow. This approach is commonplace in electronic systems, where SCR-controlled PWM systems have entirely replaced rheostats in everyday devices such as light dimmers.

Migrating PWM control to hydraulic systems is a research challenge. A high switching frequency is desirable, so ripple in the flow is more easily smoothed out. However, rapidly opening and closing a valve is difficult, particularly for high flow rates. Also, the opening and closing times for the valve must be as short as possible because throttling losses occur during the transition interval. If the opening and closing times are too long, the energy savings are reduced significantly.

Hydraulic Injection Molding Machine

Energy savings of 80% could be realized with energy-saving hydraulics.

To understand the potential energy savings of efficient hydraulics, consider a 180-ton hydraulic injection molding machine (See Figure 2). Such a machine consumes 32.1 kW of power, including 3.2 kW for the electric heaters and 28.9 for the hydraulics. The vast majority of the power for the hydraulics, some 23.9 kW, is lost through valve throttling. If throttling losses could be eliminated by using PWM control, the required power could be reduced to 8.2 kW, producing a 74% savings. If 20% of machines in use adopted a throttle-less approach, the resulting savings would be approximately $150 million per year for U.S. users and manufacturers.

Though not a pure machine application, excavators also show the energy-saving potential of hydraulics. Research shows that, with conventional controls, only 60% of the input results in useful work, while 18% of the energy is lost in pump inefficiencies, 18% is lost in valve throttling, and 4% is lost in the pistons and lines. If these losses could be reduced by more-efficient pumps and PWM control, and if regeneration could be employed to capture energy in one phase of the work cycle and reused in another phase, an estimated 30% energy savings would be produced. If this approach were adopted for all earthmoving equipment, estimated annual energy savings would be $1.15 billion.

A Global Research Challenge
The history of fluid power research is shown in Figure 3. In the 1950s and 60s, the U.S. dominated research in fluid power. During this era, high-performance, electro-hydraulic, servo-system technology was developed and refined. In the 1970s, the U.S. began phasing out hydraulic research to pursue other priorities. At the same time, Europe became active in fluid-power research, and founded research centers in 1969-70 at the University of Bath in England, RWTH-Aachen in Germany, and Linköping University in Sweden. Today, there are more than 30 of these research institutes. Hydraulics research also expanded rapidly in Asia beginning in the 1980s, and continues to grow in Japan, Korea, China, and elsewhere.

Meanwhile, little happened in the U.S. until 2001, when the National Fluid Power Assn. (NFPA) began a research effort involving industry and academia, which revived U.S. fluid-power research.

History of Fluid Power Research
(Click image to enlarge)

Seeking Efficient Fluid Power
The latest initiative in U.S. fluid-power research comes from the National Science Foundation (NSF), which recently announced a $15-million, five-year contract to support the new Engineering Research Center (ERC) for Compact and Efficient Fluid Power. NSF funding will be augmented by $3 million from universities and $3 million from industry. This is a dramatic accomplishment for fluid power in the U.S. ERC will create new fluid-power technologies, radically transform fluid-power practices, reduce energy consumption, and spawn new industries.

The center is distributed among seven universities. The lead university for the ERC is the University of Minnesota. The other core universities are Georgia Institute of Technology, Purdue University, University of Illinois at Urbana-Champaign, and Vanderbilt University. Outreach universities are Milwaukee School of Engineering and North Carolina A&T State University. Other institutions collaborating in the ERC are the Science Museum of Minnesota, Project Lead the Way, and, of course, the National Fluid Power Assn.

Research at ERC
ERC’s interdisciplinary and varied research agenda has three main efforts. New control approaches and system configurations will be developed to replace current, inefficient valve-throttling approaches. These include high-performance pump control, regeneration, and on-off valve improvements. By increasing the efficiency of fluid power in existing and new applications, ERC will save billions of dollars in energy, mostly in petroleum, and pay for the center many times over. PWM control and biomimetic distributed pumping and control are expected to spawn new pump motors and actuators with improved efficiency. These will be enabled by actively controlled tribological surfaces. Biologically inspired coatings also will reduce drag.

By decreasing size and weight, fluid power systems can migrate from heavy equipment to human-scale assistive devices, again creating more new industries. Phase-change energy storage also will likely create more compact energy storage and sources. Chemofluidic-actuation and free-piston engine compressors will provide order-of-magnitude better energy and power density for self-powered and mobile devices, enabling many new applications. Composite and functionally graded materials and integrating components into unified systems will further minimize the weight and volume of fluid power systems.

Understandably, no one will use new fluid power unless it’s safe, quiet, clean, and easy to use. ERC also will address problems with noise, vibration, leakage, contamination, and awkward interfaces. This will lead to wider, more efficient and more satisfactory use of fluid power.

Besides gains in the injection-molding industry, ERC projects other new industries will be created where compact and efficient fluid power can be used for underwater exploration, rescue operations, remotely manipulating nuclear materials, bomb disposal, medical and rehabilitation applications, and wearable or compact tools for home and industrial use. Improved compactness will enable fluid power to perform other tasks that aren’t presently possible.

Improved efficiency also will reduce petroleum consumption and pollution. If new fluid-power technology could cause a 10% improvement in overall fuel consumption for transportation, $24 billion in crude oil would be saved each year. The superior power density of fluid power makes it ideal for regenerative braking with field tests, showing 25-35% fuel savings for trucks. ERC also will develop high-density accumulators, making regeneration feasible for passenger vehicles, and resulting in larger energy savings. Savings also can be achieved in the construction, mining, agricultural and industrial sectors.

New fluid-power approaches developed at ERC will be demonstrated on six test beds. These are the excavator, injection molding machine, small Urban Vehicle (sUV), compact rescue crawler, fluid power assisted hand-tools, and fluid-power-assisted orthoses and prostheses. The excavator will be located at Purdue; the injection molding machine, sUV and fluid-power-assisted hand-tools will be at Minnesota; the compact rescue crawler is a joint project between Georgia Tech and Vanderbilt; and the fluid power assisted orthoses and prostheses will be at Illinois.

Other education and outreach innovations at ERC include:

  • Development of benchmark fluid power labs augmented with take-home laboratory modules
  • Collaboration with the Science Museum of Minnesota to develop permanent and traveling exhibits, educational materials on fluid power and an extracurricular fluid power program for middle schools and high schools
  • Collaboration with Project Lead the Way to include fluid power in a high school technology curriculum
  • Creation of industrial internship and co-op programs for both undergraduate and graduate students
  • Enhancement of continuing education in fluid power for industry through hands-on short courses and distance education.

  About the Author
Kim StelsonKim Stelson is Director of the Engineering Research Center for Compact and Efficient Fluid Power, and Professor of Mechanical Engineering at the University of Minnesota. You can contact him at kstelson@me.umn.edu.
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