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This paper proposes a design approach for the network configuration of brake-by-wire (BBW) systems using the FlexRay communication protocol. Owing to the absence of mechanical or hydraulic back-ups, the BBW system needs to be highly reliable and fault-tolerant. The FlexRay network is shown to be very effective for such requirements of BBW systems by using hardware in-the-loop simulation (HILS), which allows developing and testing various algorithms and faithfully reproduces the actual system. The FlexRay protocols are designed using the FIBEX configuration tool appropriately for the control of BBW systems, and they are analyzed using the FlexRay communication monitoring tool. The results of HILS illustrate that the braking performance of a controller area network (CAN)-based network and that of a FlexRay-based network for BBW systems are very similar, however, the FlexRay-based network system is more reliable and ensures better fault diagnosis by monitoring more variables.

Currently major investments by Tier1 and vehicle manufacturers are made to implement and optimize safety critical automotive systems according to the ISO standard 26262 “Road vehicles functional safety”. The ISO 26262 standard describes methods to detect the safety critical faults of a system designed according to the rules of functional safety, but it does not describe how an actual implementation shall look like. Development of ISO 26262 standard compliant systems concentrates on optimizing and improving cost and performance in a competitive environment. More competitive and practical implementations use fewer additional hardware and software resources for safety control and error detection and have higher performance with less overhead. Microcontrollers already have implemented many safety related hardware functions, so called safety mechanisms to mitigate safety critical risks.

In this paper, we propose a hardware and a software design method considering functional safety for an electro-mechanical brake (EMB) control system which is used as a brake actuator in a brake-by-wire (BBW) system. A BBW system is usually composed of electro-mechanical calipers, a pedal simulator, and a control system. This simple by-wire structure eliminates the majority of bulky hydraulic brake devices such as boosters and master cylinders. The other benefit of a BBW system is its direct and independent response; this leads to enhanced controllability, thus resulting in not only improved basic braking performance but also considerably easier cooperative regenerative braking in hybrid, fuel-cell, and electric cars. The importance of a functional safety based approach to EMB electronic control unit (ECU) design has been emphasized because of its safety critical functions, which are executed with the aid of many electric actuators, sensors, and application software.

In this paper, for the fault-tolerant control of electro-mechanical brakes (EMBs), we continuously estimate several system variables and utilize these estimates to detect faulty sensor outputs. The clamping force is estimated by using the motor position and hysteresis phenomenon during clamping and releasing. Current sensor faults can be easily derived from the characteristics of a three-phase balanced circuit, and the motor position is estimated by applying different methods in low-speed and high-speed regions where a model-based approach is adopted. Using the proposed fault-tolerant logic, we can detect which sensor has a fault during driving or braking. Then, we use a bumpless transfer technique so that the control performance does not deteriorate when the estimated values are substituted for the measured values.