15 October 2009—Mother Nature is a technological genius. The latest group of researchers to seek out her wisdom is a team from General Motors’ Propulsion Systems Research Laboratory, in Warren, Mich., and the University of Illinois at Chicago. This week, at the IEEE International Conference on Systems, Man, and Cybernetics (SMC 2009), held in San Antonio, Texas, they presented their new electronic strategy for controlling an automotive powertrain that’s based on the method the human circulatory system uses to regulate blood pressure.
”An automotive engine is basically an air pump, but its operation is very similar to the heart,” says Hossein Javaherian, a GM Technical Fellow who is part of the team. It made sense, he says, that studying the heart’s control mechanism would allow control-system designers to learn how to optimize system performance more easily. That’s especially important, because carmakers are introducing more complex subsystems in response to demand for more powerful, responsive, and fuel-efficient vehicles.
One of those more complex subsystems is the electronic throttle control, where mechanical links between the accelerator pedal and the throttle actuator are swapped out for wires and microchips. Devising algorithms for running these systems has become increasingly difficult for control-system designers.
Electronic throttle control ”has to be intelligent enough to manage emergency situations safely as driver control of the throttle is relinquished,” says Javaherian. ”This requires extra vigilance in control-system design in order to save lives and avoid legal entanglement while automatically optimizing vehicle performance.” In today’s designs, that’s done through redundancy—either by introducing additional electronic hardware or by maintaining conventional mechanical links between the pedal and throttle positions as a backup. Both add complexity.
A biological solution, GM engineers figured, might do the job more simply. The human circulatory system has several control schemes in place, one of which is a negative feedback system called the baroreflex. The instant that blood pressure rises above its normal level, sensors called baroreceptors, located in the aorta in the chest and in the carotid arteries in the neck, detect the stretching of the artery walls and relay signals to the brain stem. These signals trigger the activation or inhibition of opposing branches of the autonomic nervous system. The sympathetic branch is responsible for constricting the blood vessels, increasing the degree to which the heart contracts with each beat, and elevating the heart rate. If blood pressure goes up, sympathetic nerve activity is suppressed. At the same time, the parasympathetic branch, whose function is to slow the heart’s pace and contractility, kicks in, causing a rapid return to the mean arterial blood pressure. For a drop in blood pressure, the reverse would occur.
In GM’s electronic analogue, the researchers coupled a linear and a nonlinear controller that map to the sympathetic and parasympathetic nervous systems, respectively. Together, they do the job all control systems are meant to do: correct the error between a real-world result and a desired value through a series of computations and rapid adjustments.
”Modeling our control systems after baroreceptors has enabled us to devise simpler and more robust control algorithms for the regulation of major engine variables, such as engine torque and air-fuel ratio, with significant improvements in vehicle performance and tailpipe emissions,” says Javaherian.
He notes that in the absence of this discovery, the team would have used more common control architectures to achieve the same goal. But this biologically inspired control structure changed the game by allowing the researchers’ algorithms to control engine variables, even as the system learned to tune itself during a wide range of vehicle maneuvers. The alternative would have meant time-consuming tuning of multiple controllers derived from and specialized to a mathematical engine model. That kind of system requires more memory and doesn’t handle changes in the system characteristics nearly as well as the baroreceptor.
The GM engineers designed the controller to handle a car’s powertrain, which includes the engine, transmission, and the emissions after-treatment system. But Javaherian says the baroreflex control scheme could be adopted in other areas of vehicle control. For example, it could help maintain ”vehicle stability and robustness in the face of external disturbances, such as one or more tires hitting a patch of ice or one or more fuel injectors failing,” he says.
Another important benefit, Javaherian says, is the design’s role in diminishing vehicles’ impact on the environment. He notes that the regulation of air-to-fuel ratio using the new control architecture can effectively reduce the amount of smog-forming nitrogen oxide, poisonous carbon monoxide, and even greenhouse gases while improving fuel economy.