The Center for Compact and Efficient

Fluid Power


University of Minnesota
Minneapolis, Minnesota U.S.A.

Location Minneapolis, Minnesota, U.S.A. 
Responsible Leader: Professor Kim A. Stelson (Director), Ms. Stephanie Bettermann (Administrative Director)
Address Department of Mechanical Engineering, University of Minnesota, 111 Church Street S.E., Minneapolis, MN 55455 U.S.A.
Telephone number +1-612-624-4993
Fax number +1-612-626-7165
Email ccefp@umn.edu
Internet Site www.ccefp.org

From Editor

International Journal of Fluid Power would like to introduce the fluid power research and education centres with their expertise and particular interests in this column. Jumping from continent to continent we like to offer every research centre the opportunity to present itself.


FLUID POWER RESEARCH CENTRES WORLD-WIDE




 
1. Introduction



       
        The University of Minnesota has been active in industrial and National Science Foundation funded fluid power research since the late 1990s. Minnesota is the lead university in the Center for Compact and Efficient Fluid Power (CCEFP), a National Science Foundation Engineering Research Center (ERC) founded in 2006. CCEFP is funded at approximately $5.0 million per year and includes seven universities: Georgia Tech, Illinois Urbana-Champaign, Milwaukee School of Engineering, Minnesota, North Carolina A&T, Purdue and Vanderbilt. Research projects are organized in three thrusts (efficiency, compactness and effectiveness) that achieve the following societal benefits:  creation of a new fluid power technology that, with improved efficiency, will significantly reduce petroleum consumption, energy use and pollution; creation of a new fluid power technology that, with improved effectiveness, will make fluid power clean, quiet and safe for its millions of users; and creation of a new fluid power technology that, with improved compactness, will exploit its attributes in a new generation of devices and equipment. The CCEFP education and outreach program is designed to transfer this knowledge to diverse audiences: students of all ages, users of fluid power and the general public.


2. RESEARCH STRATEGY



All of the current University of Minnesota research fits within the CCEFP strategic framework. CCEFP research has four goals. The first goal is to dramatically improve the energy efficiency of fluid power in current applications; the second goal is to improve the efficiency of the transportation sector using fluid power by developing fuel efficient hydraulic hybrid technologies for small passenger vehicles; the third goal is to develop un-tethered portable human-scale fluid power devices; and the fourth goal is to make fluid power more acceptable and ubiquitous by making it quiet, clean, safe and easy to use.
The three-plane diagram was developed by the National Science Foundation as a mechanism to concisely show the interrelationship of the research programs within an ERC. The three-plane diagram for CCEFP is shown in Figure 1. The highest level of the diagram contains the integrated engineered systems or test beds. The middle level of the diagram contains the enabling technology to create the integrated systems. The lowest level of the diagram contains the fundamental research needed to create the enabling technology.  The diagram is organized into the three CCEFP research thrusts, efficiency, compactness and effectiveness.





Fig. 1:  Three-Plane Diagram

3. RESEARCH PROJECTS AND TEST BEDS



The University of Minnesota has conducted significant fluid power research over the last decade. Funding has come from the National Science Foundation, the National Fluid Power Association and industrial collaborators. A major step forward occurred in 1999 with the formation of the Fluid Power Consortium. The consortium promoted fluid power research and education at the University of Minnesota with financial assistance and advice from four local companies, Toro, MTS Systems, Eaton and Sauer-Danfoss. A brief description of some of the University of Minnesota Research projects follows.

3.1 Innovations in Fluid Power Components and Systems


Valve designs that utilize unstable flow forces To alleviate the bandwidth and flow rate limitations of single stage control valves, we developed a class of valves in which we consciously take advantage of fluid flow instability (that conventional valve designs try to avoid). Single stage control valves are inherently cheaper and more reliable than multistage valves. However, as frequency of operation and flow rates increase, single stage valves require large solenoids to overcome the large resistive flow forces. To extend the bandwidth and flow ratings without using very large and expensive solenoids, our research approach exploits both transient and steady fluid flow forces so as to improve the agility of the spool.

Smart sensing and smart sensors Another thrust of our research in fluid power system is the development of sensing and sensing methodologies. Recognizing that economical means of providing flow and pressure sensing is important for the commercialization of control concepts, we developed, with funding from the industry supported NFPA Cooperate Network of Research (CNR) program, a MEMS based integrated pressure, flow and temperature (PQT) sensor (Fig. 2) that is intended for integration into fluid power components (such as pumps). Flow measurement is based on pressure difference across a flow bend, thus minimizing energy loss.






Fig. 2:   MEMS Integrated PQT Sensor and a Dime


In additional to hardware sensors, we have also developed soft-sensing (i.e. observer) algorithms for determining, in real time, the valve spool displacement without using costly hardware sensors such as LVDTs. The soft-sensing scheme is unique in that it utilizes readily available solenoid voltage and current information, and the inductance characteristic of the solenoids in a non-linear observer to determine the spool displacement.

On/off valve based control of fluid power systems using self-spinning PWM valves The goal of this research is to develop energy efficient means of controlling fluid power systems that minimizes throttling loss. On/off valves can potentially be highly efficient because in both the fully on and fully off states, the throttling loss is small. By switching between these two states, and by varying the duty ratio of the on or off times, the mean flow or pressure can be modulated. We have studied, experimentally and analytically, the effect of variable design and operational parameters on system efficiency. The main obstacle to on/off valve based control is the lack of high speed on/off valves. They are necessary to ensure that the time spent during transitions between the two states (when throttling is significant) is minimized. Whereas a conventional valve with a poppet or spool element that travels linearly requires repeatedly accelerating and decelerating the valve spool (so that energy consumption is proportion to the 3rd power of the switching frequency), we are developing a pulse width modulated on/off valve that is based on unidirectional rotary motion. Thus repeated starting and stopping can be avoided and power is only needed to overcome friction (which is proportional to the 2nd power of the switching frequency). Moreover, the rotation scavenges energy from the fluid flow so that external energy is not required. This valve has been prototyped and is being integrated into a fixed displacement vane pump to achieve variable displacement function. Our long term goal is to develop methodologies for controlling pump/motors, transformers, hydrostats, and cylinder actuators using on/off valves. To date, a 3-way version of the rotary on/off valve that is integrated with a (40 lpm) fixed displacement pump to achieve variable displacement function has been prototyped and demonstrated. PWM frequency up to 90Hz, closed loop duty ratio modulation with 0-100% modulation time of less than 0.1sec have been achieved.  Further development will consider improved performance and configurations for regenerative applications.





Fig. 3:  Self-Spinning PWM Valve


Increasing energy storage density A significant deficiency of hydraulic systems is the lack of high energy density storage. Hydraulic energy storage is typically achieved in gas charged accumulators. Compared to electric batteries, their energy densities are two orders of magnitude lower. The goal of this high risk/high payoff project is to increase the energy density by an order of magnitude. If successful, it will enable energy regeneration in many mobile applications, such as hydraulic hybrid passenger vehicles. We are currently investigating an alternative storage approach that we called open accumulator that can achieve this goal.





Fig. 4:  Open Accumulator Architecture

Developing a hydraulic hybrid passenger vehicle A goal of the CCEFP is to utilize fluid power to enable significant fuel saving. One of the demonstration test beds is the hydraulic hybrid passenger vehicle, The target gas mileage of 100 mpg (2.35l/100km). The approach is to use a regenerative hydro-mechanical drive (HMT). The advantage of the HMT relative to either the parallel and series architecture is that it allows the bulk of the power to be transmitted efficiently through the mechanical transmission. The HMT architecture will enable optimal energy management, independent wheel torque control and regeneration.

Fig. 5:  Hydro-mechanical hybrid architecture

Fig. 6:  Hydraulic Hybrid Passenger Vehicle

Integrated algorithms for optimal energy use in mobile fluid power systems The goal of this project is to develop tools and methodologies for optimizing the power generation and power distribution in mobile fluid power systems.  With proper approaches, power consumption can be reduced while maintaining desired performance. Fundamental barriers to achieving optimal energy usage are (a) the understanding of when to use available power sources and (b) mitigating the smooth transition between the different modes of operation.  The foundation disciplines for achieving these goals are system dynamics, optimization, and feedback control.  The system dynamics aspects involve the creation of sufficiently detailed dynamic models that will serve the other two methodologies.  The optimization uses dynamic programming to create appropriate switching rules.  The feedback control maintains the stable transition between modes.

Hydrostatic dynamometer Traditionally automotive power train research and development have been conducted with electromagnetic dynamometers.  The ever increasing demand for reducing fuel consumption and emissions has driven the innovation of new technologies in engines, transmissions, and hybrid systems, which in turn requires significant flexibilities and transient capabilities of the dynamometer.  Given its superior power density, hydrostatic dynamometer is an ideal candidate for the next generation dynamometers.  This project consists of two research subjects.  One is to design and control a hydrostatic dynamometer (see Fig. 7) as a precise torque device that could supply or subtract torque in real-time under both steady state and transient operations.  The other is to develop a hybrid power train research platform where the hydrostatic dynamometer is used to mimic the drive train, vehicle load, and the alternative power source in coordination with the engine control system.  This platform will enable us to evaluate different hybrid architectures and control methodologies in terms of fuel economy and emissions without actually building the system.  

Fig. 7:  Hydrostatic Dynamometer


3.2 Passive fluid power machines that interact with humans


Passivity as applied to hydraulic systems – passified valves and passive teleoperation While the passivity concept is well accepted in the electromechanical domain, its application to hydraulically actuated systems is more recent and is pioneered by our group. A key obstacle to applying the passivity concept to hydraulic systems is that the inherent passivity properties of directional control valves (relative to electric motors) were unknown. Our research group performed the first passivity analysis on valves to show that they are in fact inherently non-passive and proposed passification methodologies, i.e. methods to modify the physical design or to control the valves so that they behave like passive two-port devices, for both single stage and multi-stage valves. The passification algorithm was later generalized using a bond graph representation. Since bond-graphs highlight the physical nature of the system, this new methodology greatly generalizes our previous results and enables a much broader class of mechatronic systems to be passified. The development of these "passive valves" has subsequently enabled the design of the first teleoperated control of hydraulic systems that possess the passivity, as experimentally demonstrated on the hydraulic backhoe setup in Fig. 8.

Fig. 8: Passive Hydraulic Teleoperation


Passive hydraulic human power amplifier A hydraulic human power amplifier with two assisted degrees of freedom was designed and built by a group of undergraduate students as their capstone project with funding from the NFPA Small Grant Program (Fig. 9). The objective for the machine is to amplify the applied human force in the assisted degrees of freedom so that the human can manipulate heavy objects while being physically connected to the task, similar to the concept of cobots. The paradigm is similar to teleoperation except that the master and slave systems are the same system. Because the machine must simultaneously interact with the human operator and the work environment, maintaining passivity (with a power scaling) is important for stable operation.

Fig. 9: Passive Hydraulic Human Power Amplifier

Our current research (within CCEFP) in the area of passive machines that interact of humans focuses on extending our passive teleoperation and passive human power amplification concepts to the pneumatic and chemo-fluidically actuated domains so that portable power sources for pneumatic and chemo-fluidic actuation (pressure bottles, portable compressors, or hydrogen peroxide fuel) can be passively controlled. If successful, truly portable, un-tethered, and powerful human assist devices can become a reality.  

Fig. 10: Passive Pneumatic Human Power Amplifier



Fluid power orthotics test bed The orthosis test bed is designed to demonstrate and integrate compact, efficient, and effective fluid power concepts in a challenging, un-tethered, human-scale device. Wearable orthotics to correct gait deficiencies present unusual challenges for fluid power systems as the packaging and the weight must be unnoticed by the user. Unlike prosthetics, where the inside of the limb can be used to place components, orthotics are all on the outside. The test bed is starting with planar ankle orthotics to correct foot drop, foot slap and joint weakness.  Figure 11 shows the prototype of a passive orthosis that captures energy from heel strike in a miniature accumulator that is used to drive a joint locking action that is timed to the gait cycle, all without electronics. Figure 12 shows the concept for an active orthosis that uses fluid power to generate torque to assist gait. 

                      

Fig. 11: Passive Orthosis                                                                                                                         Fig. 12: Active Orthosis





4. EDUCATION AND PRE-COLLEGE OUTREACH


The Center’s Education and Outreach program fills a long-recognized need. Despite fluid power’s ubiquitous presence as an industry enabler, hydraulics and pneumatics instruction is typically scant, particularly in engineering college curriculum. Some of the major education and outreach activities of the Center are described below.

4.1 Fluid power content in Project Lead The Way (PLTW) curricula

In partnership with PLTW and the National Fluid Power Association (NFPA), the CCEFP is working to enhance and expand fluid power content in several PLTW courses that are a part its middle and high school curricula. PLTW programs are now established in all 50 states and the District of Columbia, engaging 7,000 teachers and 5,000 counselors who work with 200,000 students.  PLTW’s fluid power course content is focused and enriched with the help of subject matter experts from industry (through the help of NFPA) and from the Center’s faculty and staff.



 Fig. 13:  Students and an instructor participating in a PLTW activity

4.2 Pneumatic training for FIRST Robotics teams

FIRST is an international robot competition for high school students. In 2008 there were 1,500 FIRST Robotics teams involving 37,000 high school students. Since inception, FIRST programs have impacted 156,000 students. In a pilot program for the 2008 competition, the Center developed a pneumatics workshop and field-tested it among several Minnesota- and Georgia-based FIRST teams. Next year, this workshop will be made available to other FIRST teams in other locations. The Center is connecting its diversity efforts to FIRST by sponsoring an, all Native American, FIRST Robotics team located in Cloquet, MN.



 Fig. 14: FIRST Robotics team members with an instructor. 

4.3 Delivering fluid power education through the core curriculum of mechanical engineering

Consensus reached at a recent NFPA Education/Industry Summit reaffirmed what has long been widely assumed: new departments and new four-year undergraduate degrees in fluid power are not realistic goals. But, inserting fluid power into the core mechanical engineering curriculum is. The CCEFP is working to develop curriculum material to insert into system dynamics courses and fluid mechanics courses, which are part of every mechanical engineering program in the world. Textbook material suitable for a typical system dynamics course is being under development and will be field test in the fall of 2008 at the seven CCEFP schools. With this start as a foundation, the Center’s long range goal is to reach all 283 ABET accredited mechanical engineering programs in the United States, followed by universities in other parts of the world.

4.4 Interactive exhibits

The Center is working with the Science Museum of Minnesota (SMM) to develop interactive exhibits on fluid power that engage the public. The Hydraulic Hybrid Vehicle Exhibit, developed by Center students working in collaboration with SMM professional exhibit designers, made a trip to the 2007 Minnesota State Fair where it was     seen by thousands of fair goers, and was featured in the CCEFP’s booth at the 2008 International Exposition for Power Transmission and its show partner, CONEXPO – CON/AGG. Together, these shows hosted more than 100,000 attendees. The exhibit’s permanent home is on the floor of the SMM. 



 Fig. 15: CCEFP’s exhibit at the Minnesota State Fair, 2007


5. INDUSTRIAL COLLABORATION AND TECHNOLOGY TRANSFER

Fifty-seven companies support the CCEFP with funding and in-kind donations. CCEFP annual industrial membership fees are around $650,000, and CCEFP companies have donated $250,000 worth of fluid power equipment to the Center. Twenty-two CCEFP member companies are represented on the Industrial Advisory Board (IAB). 






 

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