Computer Control Systems in Hybrids

Today’s zippy computer processing speeds have helped hybrid cars become a reality. Hybrid computer control systems are extremely complicated. Without recent advances in processing speed and power, the invention of graphical software environments, and real-time operating systems, it would be difficult if not impossible for engineers to develop these complex hybrid computer systems. Also, the use of a high-speed "communication buses" on the vehicle, known as CAN (Controller Area Network), has played a huge role. These communication buses allow microprocessors (also known as an embedded controller) to talk to one another.

Two Powertrains Under Reins

What kind of communication are we talking about? Who is really talking and who is listening? Let’s take anti-lock brake systems (ABS), found on many cars, not just hybrids, as an example of this process. A brake controller for ABS can communicate information about wheel rotation and speed to the engine controller and transmission controller. The engine controller can determine how much acceleration (the amount the accelerator pedal is pressed) or how much torque the driver is asking for from the engine. The torque then can be communicated to the transmission to select the proper gear. If the brake controller indicates that there is a wheel spinning or slipping, this information can then be communicated back to the engine via the CAN bus to reduce torque and thus control the spinning or slipping wheel. We call this "traction control."

The hybrid system is a much more complicated control system than ABS. The hybrid powertrain is essentially two powertrains — a gas engine powertrain and an electric motor powertrain. The hybrid controller coordinates the two. Picture a horse and buggy hitched up to two animals, a horse and a donkey. The driver is trying to control both animals. The driver is the hybrid controller and the horse and donkey are the engine and electric machine working together. A tug on the reigns here, a little pat on the rump there, and the cart keeps moving down the road.

Systems Controlling Multiple Systems

With the hybrid controller, the communications occur between multiple layers of control and communications systems: electric motor controller, engine controller, battery management system, brake system controller, transmission controller, electrical grid controller, and some systems have 42 volts components as well. The control system integrates many of the functions that used to be only one controller’s responsibility. A good example of this complex coordination is regenerative braking. Part of the braking is shared between the standard foundation brake system—traditional friction pads, rotors, and drums—and the braking recovery system that sends electrical energy to the rechargeable batteries. Each component system knows what the other is doing and is adjusting its own status based on information provided by the other. In terms of regenerative braking, the net result is the car’s ability to stop in a normal controlled manner while recharging its batteries without having to plug in. Give thanks to high-speed computer processing for making it all possible.

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