With the high voltages used in hybrids, automotive safety takes on new meaning. “One of the things that the hybrid vehicle brings to the auto industry is safety concerns,” said Dennis Stephens, principal staff engineer, Continental. “We have to think about the fact that now we have 300, 400, 500 volts floating around in a car, so there are a lot of redundant systems in place, both hardware, software, and interlocks that prevent the user from ever getting to those voltages.” Ensuring safety has several different aspects.

One of the initial safety concerns is the lithium-ion chemistry. “For the old nickel-metal hydride batteries we can look at stack of cells and sort of say are we balancing,” said Stephens. “For the new lithium batteries, we have to look at every cell. So if every cell is 3 V and you have to go up to 300, you have a lot of them in series that you have to constantly monitor.”

Constantly monitoring galvanic isolation is another safety-critical area. This requires sensing that the battery is isolated from the chassis. “There is some microprocessor brain power inside these batteries and battery management systems to constantly monitor that,” said Stephens. Contactors inside the batteries open up and turn off the battery if any fault protection flag goes high. This interlock capability prevents users from accessing the high-voltage terminals.

PHEVS have another problem that is different from existing hybrids. “Right now, a vehicle floats above the ground,” says Stephens. “When my car is plugged into the outlet in the garage, now I have mains voltage on my car.” The issues that this creates have to be worked out. Perhaps the bigger issue is the standards that need to be established that “aren't there yet” according to Stephens.

Model-based design (MDB) may provide answers to new components, safety and the need for overall development of a vehicle that is even more complex than one with just an internal combustion engine.

“For hybrid vehicles, because you are putting a lot of new components together, you really have to optimize your design on the top level, on the system level, and this is where modeling, simulation and model-based design can help engineers, ” said Wensi Jin, Automotive Industry marketing manager, The MathWorks. A tool recently introduced by The MathWorks, called Simscape language specifically simplifies the effort of engineers to implement newly developed components into existing models. The new IGBTs mentioned earlier provide a good example.

“If they are optimizing one component, they can take out the original block from a shipping product for an IGBT, describe their own IGBT in Simscape language and essentially this new block will go into the system level models,” said Jin. As shown in Figure 6, Simscape allows engineers to describe a component in language and generate a graphical model. Simscape is based on the widely used MATLAB language. The difference is instead of a data-flow-oriented programming environment, Simscape uses a network approach, an acausal modeling environment for modeling the physical network.

With model-based design and Simscape, designers can model on different levels and focus on those areas that specifically impact safety. In an accident situation, safety involves the disconnecting of high-voltage components. “Things that are very difficult to do in the real vehicle, you can do easily in the laboratory environment with models,” said Jin. “You can model the fault management system and be able to run these models in a modeling system environment.”

In addition to its use in the model-based design, The MathWorks Real-Time Workshop Embedded Coder product for MathWorks Release 2008a was recently certified by TÜV SÜD Automotive GmbH. The certification was for safety-related development according to IEC 61508. This could be a step toward a basis for standardization of at least one aspect of the high-voltage system. TÜV granted the certificate based upon a workflow for typical automotive applications that addresses “Application-Specific Verification and Validation of Models and Generated Code.”

Software optimized batteries are one thing but producing automotive-grade lithium-ion batteries is another. Improved batteries have been the weak link in hybrid and electric vehicles since the first EV hit the road in the early 20th century, but the final critical piece of the hybrid, PHEV and EV could be falling into place. Near the end of the third quarter of 2008, Continental AG opened a factory in Germany to manufacture lithium-ion batteries for hybrid vehicles. The battery will initially be used in Mercedes-Benz S 400 BlueHYBRID that will be in production in 2009. As shown in Figure 7, the battery is compact enough to mount under the hood of the vehicle.

Continental's battery has a volume of 13 liters, weighs 25 kilograms and allows electric motors in cars to supplement the output of combustion engines by up to 19 kilowatts of power. The battery system consists of the lithium-ion cells and the cell monitoring system, the battery management function, high-strength housing, cooling gel, a cooling plate, a coolant feed and the high-voltage connectors. Integrated electronic circuitry monitors the battery's overall health, temperature and energy capability as the system ages. If excessive temperatures occur, a safety interlock switches the battery off. A Cell Supervision Circuit (CSC) monitors the status and controls the interaction of the single cells. The CSC adjusts the charge condition to ensure all cells are loaded equally.

1. http://www.nri.co.jp/english/opinion/papers/2007/pdf/np2007114.pdf.

Randy Frank is president of Randy Frank & Associates Ltd., a technical marketing consulting firm based in Scottsdale, AZ. He is an SAE and IEEE Fellow and has been involved in automotive electronics for more than 25 years. He can be reached at r.frank@ieee.org.

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