When electronic systems take control of an autonomous car, they need to perform perfectly every time. That’s prompting developers to build fault tolerance into systems.

The levels of redundancy will vary. If drivers are paying attention, they can act if a sensor fails. But when more autonomy leads to less driver interaction, engineers need to duplicate components to make systems fail-safe.

“The level of redundancy is coming out of the safety analysis and the safety goal,” said Steffen Linkenbach, Head of Systems & Technology at Continental Automotive North America. “We will definitely need more redundancies in power supply, actuators, and controllers than we have today.”

Without drivers, every facet of safety must be examined. Systems must constantly run diagnostics while they’re also performing their central role. If there are problems, the system must act to make sure accidents don’t happen.

“It’s not just redundant sensors, you need redundant power systems, communications, control units, and braking units,” said Erik Coelingh, Senior Technical Leader at Volvo Car Corp. “If something goes wrong, the car must detect that and put the vehicle into a safe state. It doesn’t have to continue with autonomous driving; it just needs to alert the driver and put the vehicle in the driver’s control.”

Keeping drivers in the loop is the simplest way to avoid the high cost of backup hardware. Many observers feel that it will take quite some time for the industry to move from semi-autonomous systems that rely on drivers to fully autonomous cars that let drivers do other tasks or sleep. Cost will be a huge factor in this progression.

“By eliminating the possibility of manual backup, these systems become very complex and therefore expensive,” said Brian Daugherty, Visteon’s Associate Director of Advanced Development and Intellectual Property.

In semi-autonomous vehicles that rely partially on drivers, it may be a challenge to keep them from becoming distracted. Strategists are already examining ways to ensure that they remain focused on the road, not the infotainment system.

“There are different ways to keep drivers involved,” said Kay Stepper, Regional Business Manager for Automated Driving at Robert Bosch Chassis Systems Control Division. “You can monitor the driver’s facial expression or eyes using a camera or you can require the driver to touch the steering wheel for a couple seconds. Automated trains require manual input so the machines knows the driver is still engaged.”