Onboard software safety study: automobile parking brake

Since 2007, CLEARSY has performed safety analyses on automatic parking brakes for a major French automobile manufacturer.

On high range cars, comfort and driving aids in particular take the form of the elimination of the traditional handbrake in favour of an intelligent parking brake. A button has replaced the lever and new related functionalities have appeared: the handbrake blocks when the vehicle stops, start-up assistance on inclines, dynamic deceleration of the vehicle in emergency situations…

The intrinsic simplicity of the traditional handbrake (a lever pulling a cable that tightens the callipers on the back wheels) in fact conceals the redundancy of the braking system imposed by automotive standards. When this vital organ is replaced by a calculator, an advanced safety analysis must be performed on it to ensure that the electronics do not cause damage to the initial function (immobilizing the vehicle according at the driver’s discretion) and that they do not lead to unwanted behaviour (for example, a sudden release).

In concrete terms, CLEARSY’s analyses focused on the conditions for releasing the parking brake in order to ensure that nothing exists in the situation software that could lead to an unwanted release.

To this end, CLEARSY has developed a complete readback methodology to define the scope of this type of study:

• Analysis and hypotheses for the calculator hardware (design of the safety hardware, analysis of redundancies…)
• Analysis and hypotheses for the low level layers of the software (operating system, interrupt vector…
• Compilation of partial results from the various software modules involved in the release of the parking brake
• Structuring of the partial results to reach a reasoning that can be reproduced and therefore guarantee software safety (nominally, as well as in the presence of a defect in the various calculator entries).

This methodology has allowed CLEARSY to propose an advanced, original verification solution for all software and hardware products for which safety must be demonstrated.