Simulation Study of a Fail-Safe Steer-by-Wire for Heavy Earth Moving Machinery

Abstract Hydrostatic steering systems are commonly used in articulated heavy earth moving machinery (HEMM). In an earlier study, an energy efficient Electro-Hydrostatic Steering System (EHSS) was proposed, which works on Steer-by-Wire (SbW) principle. In the present article, the functional safety for Safety Related Parts of Control System (SRP/CS) for the EHSS is analysed and requirements are assessed. The analysis suggests that the novel EHSS is safe for critical control application like steering. A simulation analysis of the proposed EHSS is carried out on a ‘Y’ cycle operation of a wheel loader to analyse the effect of a major hazardous event on steering operation, and the ability of the proposed system to act in order to reduce the risk. It has been found that the proposed EHSS can overcome the most common mode of failure in primary steering. The primary steering is failed at two different points of operation and the secondary or emergency steering is activated to continue the steering operation as intended. The activation of emergency steering takes about 200ms in the case when primary steering fails while steering command is active, whereas 440ms when the primary steering fails during no steering command. The delay is mainly attributed to the diagnosis of failure in primary steering. In both the cases, it does not result in any hazardous movement of the steering actuators. The study demonstrates that with proper control, and even low diagnostic coverage, the proposed EHSS can meet the functional safety requirements.