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The
ADvanced VehIcle SimulatOR (ADVISOR) |
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The quest to produce a hybrid electric vehicle (HEV) challenges the imaginations of automotive engineers around the world. Numerous configurations must be weeded out and prototypes are prohibitively expensive. These design problems seem tailor-made for tools such as MATLAB, Simulink, Stateflow, and Real-Time Workshop, a suite of design tools from The MathWorks, Natick, MA. Using these tools, auto engineers can swap engines and battery banks, try different chassis combinations, and make decisions that will guide future design efforts for cars that don't yet exist. Already, two major players in the national effort to build an HEV have created simulators based on The MathWorks products that have become mainstays in HEV design. The ADvanced VehIcle SimulatOR (ADVISOR), developed at The National Renewable Energy Laboratory (NREL) in Golden, CO, and the PNGV Analysis Toolkit from Southwest Research Institute (SwRI) in San Antonio, TX, both take advantage of Simulink's user-friendly, object-oriented environment for developing models, performing dynamic system simulations, and designing and testing new ideas. The tools have set the course for HEV development at national labs, universities, and among the Big Three auto makers. "This is the next step toward realizing the development of these vehicles," said Scott McBroom, Senior Research Engineer at SwRI. "We're convinced this is the way to go." The nation's effort to build practical and efficient hybrid electric cars by 2004 began in earnest with software developers involved in the Partnership for a New Generation of Vehicles (PNGV), a government initiative announced by President Clinton in 1993. With increasing U.S. dependence on foreign oil -- most of which is used to fuel America's cars -- and increased awareness of the perils of air pollution, alternate transportation had risen to national attention. Hybrid vehicles seem to offer the best hope of a near-term solution. Computer models and early prototypes indicate that HEVs will boost fuel economy and cut emissions, while still providing the safety, performance, and range that American drivers demand. Under the PNGV program, NREL entered into contracts with Ford, General Motors, and Chrysler to produce hybrid propulsion systems by 1998, first-generation prototypes by 2000, and market-ready HEVs by 2003. In addition to the industrial partnerships, NREL has sponsored HEV research at the nation's leading engineering schools. Engineers at the Big Three auto makers are pushing to meet PNGVs lofty goal of tripling conventional gas mileage to around 80 miles per gallon. Chrysler, which joined the initiative in 1996, is still in the modeling phase, while GM and Ford, which have been in the program since 1993, are testing their concepts on mule vehicles. More than 20 organizations currently use ADVISOR, and the tool is continuously fed up-to-date component test data through user and university validation efforts.
What's Under the Hood? Hybrid vehicles use an electric motor and a bank of high-voltage batteries in conjunction with a "heat" engine, usually a conventional internal combustion engine (ICE). The components are arranged either in parallel or series configurations. Computer modeling with ADVISOR has already shown some basic characteristics for each. In a parallel hybrid vehicle, both the electric motor and the ICE are connected mechanically to the drive wheels. The ICE provides supplementary power when the vehicle is accelerating or climbing steep grades. Parallel HEVs don't need generators and have lower mass and better highway efficiency. They have the added benefit of redundant drive power, meaning the car can still be driven if one of the power systems fails. Parallel vehicles require a complicated mechanical coupling and a multi-speed transmission. Because the ICE must perform over a range of speeds, tuning can be difficult and efficiency is diminished. In a series HEV, only the electric motor drives the wheels. The ICE runs an alternator, providing power to the electric motor and the battery bank. Series HEVs have simpler mechanics, allowing more freedom for component placement. Because the electric motor provides enough torque throughout the range of required speeds, a single-speed gearbox can take the place of the transmission. Full power is available at all times in a series HEV, and because the ICE runs only when the batteries need charging, it can be tuned to run efficiently at a single RPM. Because series HEVs lack the redundant drive systems of their parallel counterparts, they are extremely sensitive to battery and electric motor efficiency. Some vehicles may have trouble negotiating long, uphill grades. For the two basic HEV configurations, there are a number of control strategies not found in conventional vehicles. While the efficiencies of each component are important, it is the way they are interconnected that really determines the value of an HEV design. Enter ADVISOR, with its ability to model all of the HEV components, monitor their interaction and response, and output the performance data. "There are many ways to design these systems, and if you were to design and build each one individually and test it, it would require an enormous amount of time and money," said Dr. Larry Michaels, Application Engineering Manager at The MathWorks. "By modeling on computer, you can evaluate all of these configurations very quickly and very cost effectively." ADVISOR is termed "steady-state" and "backward-looking" because its simulations begin with desired outputs and work back toward component choices and configurations. "You're trying to get into the ballpark with ADVISOR," added Dr. Michaels. "Trying to find a configuration that looks promising and that will meet performance and fuel economy goals. The idea is to see if this configuration should be pursued further and then to build prototypes based on that idea. Many times the negative answers are more important than the positive answers. The computer model is a way to quickly and easily see if you are ever going to get what you want out of it. Sometimes, even under ideal conditions, it won't meet goals. That idea can then be discarded." According to NREL's Tony Markel, ADVISOR, with all of the components modeled, allows HEV designers to use a variety of standard and custom driving cycles like the Federal Urban Driving Schedule (FUDS) and the Federal Highway Driving Schedule (FHDS). With the power of Simulink, ADVISOR easily can handle battery state-of-charge corrections and vehicle soak periods. The simulator gives designers quick and accurate assessments of important vehicle performances such as fuel economy, emissions, acceleration, and grade sustainability. ADVISOR results can then be run through MATLAB to find equilibrium points or ideal operating conditions. ADVISOR's Graphical User Interface (GUI) is designed to facilitate entry and modification of elements such as drivetrain configuration, transmission, vehicle type, energy storage system, generator, driving cycle, control strategy, and scaling. The GUI corresponds to a top-level Simulink block diagram, which shows the model's data flow from trip, through road load, driveline, motor and energy storage, and on to numeric outputs for fuels used and tailpipe emissions. Looking down two layers into the "road load" subsystem shows the interaction of next required speed and previous speed through other subsystems representing roll, climb, aerodynamics, and acceleration to arrive at figures for torque and speed required at the virtual vehicle's "wheels." In 1996, NREL tested five vehicle configurations with ADVISOR in pursuit of PNGV's goal of 80 mpg. These included three lightweight (series, parallel, and ICE) and two conventional-weight (parallel and ICE) configurations. Simulink's block group modeling platform allowed designers to easily manipulate each component and find the strengths and weaknesses of each design. Using ADVISOR, NREL engineers found that parallel and series vehicles showed the same fuel economy sensitivity to most parameters, but series vehicles were three times more sensitive to battery and electric motor efficiency. Results also showed that to reach the 80 mpg goal, ICE efficiency would have to be raised to nearly 35 percent and vehicle weight would have to drop to 1000 kg, about half the weight of today's conventional vehicles. While the conventional-weight vehicles modeled did not make the 80 mpg goal, they did manage a 20-percent improvement in fuel economy over today's cars. According to Markel, ADVISOR has enabled NREL to specify 176 distinct vehicles for evaluation, verification, and data storage. The findings may drive research into battery and electric motor technology, and accelerate NREL's goal of having most metal auto components made of aluminum by 2015.
Last year, NREL used ADVISOR in a new test that allowed for different control strategies in the series and parallel HEVs, an analysis made feasible only with computer simulation. These tests showed that the best-designed parallel vehicles got 24 percent better gas mileage than conventional cars, surpassing its series counterpart by 4 percent. ADVISOR has been well-validated at NREL and at a number of universities that use the simulator in their HEV design programs. ADVISOR's findings generally have fallen within 2 to 4 percent of actual measurements on HEV prototypes. A validation study at Virginia Tech in 1997 found that "the validation process shows that ADVISOR has extensive values as a simulation tool for HEVs." Software engineers at SwRI began looking for a way to take HEV development beyond steady-state, backward-looking models like ADVISOR, and offer a dynamic environment for designing, testing, and proving their ideas without hardware prototypes. SwRI used Simulink to put together modular component libraries of engines, vehicles, mechanical couplings, driveline components, energy storage devices, and a number of other HEV essentials. At the direction of the United States Council for Automotive Research (USCAR) -- a legalized consortium of Ford, General Motors, and Chrysler -- SwRI used those libraries as the foundation for their PNGV Analysis Toolkit. Participants in the PNGV since its inception, engineers in SwRIs Engine and Vehicle Research Division, were familiar with most of the widely used HEV simulators, including Simple-V and ADVISOR. SwRI chose MATLAB and Simulink for their PNGV Analysis Toolkit because of programming efficiency, Simulink's self-documenting feature, and customer familiarity with the software. Both the PNGV Toolkit's GUIs and the block diagrams go beyond simple choices of components, and require detailed descriptions of the components' performance values, allowing the HEV's subsystems to be tested. According to Ashok Nedungadi of SwRI, despite the dense models, the use of MATLAB GUIs "allows a novice to use the tool." The PNGV Analysis Toolkit is being used to evaluate new drivetrain options, test and debug control strategies, size and specify sub-components, and evaluate software and hardware modifications. USCAR now owns the software. Its members, who are engineers at the Big Three auto makers, will now be using the PNGV Toolkit to develop HEVs that meet PNGV's goals. Michaels agreed that new, dynamic, forward-looking simulators offer the best chance for both fine-tuning HEV components and designing and testing control strategies between them. For more information, contact The MathWorks at 24 Prime Park Way, Natick, MA 01760; Tel: 508-647-7000; Fax: 508-647-7101; e-mail: info@mathworks.com For more information about NREL's Hybrid Vehicle Propulsion Program, go to www.hev.doe.gov.
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