ADELANTO, Calif. – The following is the second of three articles by: Fabio Squadrani, Senior Manager, Braking Systems of Applus IDIADA co-authored with Deaglán Ó Meachair, brake-engineering specialist of


This article suggests how targets could be set for a vehicle equipped with a redundant braking system. This study is mainly focused on understanding and validating how failures affect the general performance of the vehicle. It primarily shows how partial system failures (ESP, EPB, IEB) affect the general braking performance, mainly in terms of deceleration, stopping distance, front/rear modulation, on slope braking performance and finally time-to-lock.

The first part of this article included a general introduction about brake concepts. In this 2nd part, a series of redundancy concepts are introduced: this is functional to the definition of the so called “primary metrics” for redundant braking systems.

  1. Redundancy Levels for Service Brake

The brake system must consider backup solutions that ensure continued safe operation of the vehicle when the primary system is not available due to any type of failure. The redundancy proposal for the brake system presents the operating characteristics for each level of redundancy and reference values regarding the performance expected in each case; in turn, it details the functionalities that must be available in the event of a failure to ensure safety during braking.

2.1 Redundancy Concept

The concept of redundancy is presented divided by levels, showing in each case the basic characteristics of actuation.

The primary actuation considers the operation of the full system, in this case the primary unit can boost and provide modulation in both the front and rear axles, therefore; it is expected to achieve high deceleration values. The regenerative brake is possible and commanded by the primary unit.

The secondary actuation is considered in the event of failure of the primary unit, which results in partial system performance. The front boost and modulation are provided by the secondary unit but the only modulation available for the rear wheels could by EPB actuation. It could be expected the EPB modulation on the rear wheels to support front hydraulic modulation to reach similar deceleration levels than the primary actuation. The regenerative braking is not available for the target vehicle due to hydraulic modulation is only available in the non-driven wheels.

Master Cylinder actuation is the fall-back solution in the event of failure of the primary and secondary unit; the deceleration performance shall reach the Regulation criteria in this case.

2.2 Functions

The function list contains all the functions considered to be available in the primary unit and those which should be available in the secondary unit as well.

The functionalities within the secondary actuation are divided according to vehicle performance and safety considerations in three categories:

  • Category A: Functions must be continually available, including during handover to redundancy system.
  • Category B: Function must be available, but intermittent interruption is acceptable.
  • Category C: Function is optional and does not form part of core brake system execution.
  • As an example, a list of functions could be as follows:

2.3 Primary Metrics

The redundancy concept proposal together with the function list (category A functions) allows to develop a primary metrics list for the brake system depending on whether the primary or the secondary unit operates.

The values either for IEB or RCU are proposed by IDIADA based on what the system could be able to achieve. As a reference, the regulation values are shown for those metrics subject to it.

About Applus IDIADA

With more than 25 years’ experience and 2,450 engineers specializing in vehicle development, Applus IDIADA is a leading engineering company providing design, testing, engineering, and homologation services to the automotive industry worldwide.

Applus IDIADA is located in California and Michigan, with further presence in 25 other countries, mainly in Europe and Asia.

About BrakeBetter

Deaglán Ó Meachair established to improve the efficiency of the planet’s electric vehicle fleet, by offering the automotive industry the insight and support required to move from legacy systems and grasp the vast potential available through electrification.