Observer-Based Robust Force Control of Endurance Test Equipment for Steering System

In the automotive industry, endurance testing of chassis and vehicle components is critically important, as it directly relates to safety. Among them, endurance test of the steering system replicate the rack load that occurs when the driver steers while the vehicle is stationary or in motion. Accordingly, high-capacity, high-precision, and highly durable endurance test equipment (ETE) has traditionally been used. However, such high-performance ETE tends to be expensive due to complex production processes and component costs, requires a long lead time, and is also difficult to repair in the event of a malfunction. In practice, tests involving relatively low frequency and load specifications with simpler waveforms are frequently conducted, making it unnecessary for all test conditions to be covered by high-spec equipment. To address these issues, this paper proposes a newly developed multi-axis endurance test equipment (MAETE), optimized in its mechanical design to be suitable for frequently required, simpler endurance tests, while reducing cost and facilitating production and maintenance. Moreover, a method is introduced to enhance the rack force implementation performance by applying a model-based robust controller and a pre-compensator to the proposed equipment. The robust control technique mitigates model uncertainties in the multi-axis system to ensure control performance and stability. In particular, the disturbance observer (DOB) used in the outer force control loop follows the structure of a Reaction Force Observer (RFOB), which enables accurate force tracking without direct force sensing. The pre-compensator applied to rack force implementation alleviates the computational load on the feedback controller and disturbance observer (DOB), thereby improving load reproduction accuracy. Experimental results verify that the proposed MAETE significantly reduces cost, lead time, and maintenance burden compared to high-spec ETE, while still fulfilling the requirements for simpler endurance tests. This improvement is primarily achieved by utilizing modified commercial steering actuators and minimizing the number of custom mechanical components.