Functional Safety Oriented Design of an Electro-Hydraulic Stationary Braking System

Stationary (parking) brake is a very important and safety critical function in many classes of machines. The new transmissions and the "by wire" systems increase the criticality of the role of stationary brake, as it is also an emergency (secondary) brake, and it's often used to hold the vehicle when the transmission is not locking the wheels. As an example, dual clutch and power-shift transmission gear systems, as well as hydrostatic transmissions under certain circumstances, are often unable to hold the vehicle stopped and this function is provided by the stationary brake. Due to the main need of having the brake actuated when vehicle is stopped, without any hydraulic and electric power, the brake con guration is normally a "negative" con guration, usually called "spring applied" because of the actuator con guration, but this con guration causes the brake actuation when de-energized, even in case of system failure. A thorough hazard analysis on fault cases, performed over the automatic stationary braking system of a big Agricultural machine, evidenced some fault cases, that could result in a unwanted actuation of the stationary brake, potentially causing serious damages. The paper deals with the electro-hydraulic braking system analysis and analyzes three different circuit architectures, that increase the Safety Performance Level of the solution, avoiding dangerous hazards and increasing the reliability of the entire system. The discussion will also be referred to Agricultural Tractors, due to the recent changes in homologation regulation within the European Union, introducing changes in brake and steer systems requirements.