BERLINDespite the inherent benefits of autonomous vehicles, critics are concerned by a number of unresolved questions. For instance, will an automatic system remain reliable in the event of an emergency? And, what happens if the safety system breaks down due to a short circuit or other electrical fault?

To safeguard against total failure and a potentially dangerous situation, engineers at the Fraunhofer Institute for Reliability and Microintegration (IZM) have embarked on an R&D project dubbed HiBord. The goal is to develop an electronic disconnect device that is able to isolate any faults in vehicle electrical systems. The IZM engineers have already successfully tested a module in a BMW i3.

“In today’s electrical system architectures for automated vehicles, it is standard practice that areas affected by a fault are isolated by means of an overload protection system,” says Phillip Arnold, a research associate at Fraunhofer IZM. “This setup means that the affected component is shut down completely in the event of a fault.

“For automated and fully automated vehicles, such an approach is only viable provided there is redundancy for all components and the onboard electrical system,” notes Arnold. “That is, they are present in duplicate. This is expensive, increases weight and consumes space in onboard electrical systems.

In the HiBord project, Arnold and his IZM colleagues partnered with the Fraunhofer Institute for Integrated Systems and Device Technology to develop a disconnect device that shuts off faulty components in the electrical system while still safeguarding the supply of power to safety-critical components. This guarantees safe driving without the need to install a duplicate onboard electrical system.

“Although it sounds like an economy measure, this approach actually represents a significant improvement in terms of safety for autonomous driving,” claims Arnold. “In conventional systems, any undervoltage while on the road can trigger a sudden and uncontrolled failure of the entire onboard electronics, including braking and steering systems. This presents an unacceptable risk, particularly when traveling at high speeds.

“But, with our new module, part of the onboard electrical system continues to function as before, so that a fully automated vehicle would still have enough time to convey passengers to safety onto the emergency lane of a freeway or a parking lot,” says Arnold.

The Fraunhofer engineers used MOSFETs(field-effect transistors) to switch or block large electric currents. Equipped with 16 of these switches, the newly developed disconnect device is capable of switching up to 180 amperes of current.

“If this threshold value is exceeded—in the event of a short circuit, for example—the electrical switch opens and thereby shuts off the power,” explains Arnold. “Moreover, given that the MOSFET switches are capable of handling up to 300 amperes and therefore operate well below their maximum permissible load, they have a significantly longer lifetime than conventional solutions.

“In tests where [we] intentionally triggered short circuits, results showed that the module is capable of reliably isolating a current of up to 700 amperes without there being any propagation of the initial short circuit,” claims Arnold. “There are also clear advantages over conventional systems in terms of switching speeds.

“While a conventional fuse takes some 20 milliseconds to trip, the disconnect device detects a fault within 10 microseconds and only requires a further 300 microseconds before tripping,” Arnold points out. “This makes it more than 60 times faster than current fuse systems.”

“The module…is designed in such a way that it can, in principal, be used in any electric vehicle,” adds Arnold. “By protecting against a complete failure following sudden problems with the onboard electrical system, this new development marks a groundbreaking step toward safe and reliable autonomous driving.”