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Motivation and objective In recent years, electromechanical brakes (EMBs) have drawn significant attention from both research communities and automotive brake suppliers alike, as they are deemed to be a promising replacement for hydraulic brakes in the automotive industry. Compared to conventional hydraulic brakes, EMBs offer the potential for component, size, and weight reduction of the overall braking system, as well as faster response times, which result in shorter stopping distances and increased safety. In the scientific literature, the control of EMBs has been commonly addressed through cascaded architectures with inner-loops for motor velocity and current/torque control, and outer-loops for clamping force control, thereby requiring an accurate measure of this variable. However, the use of load cells to measure the clamping force is too expensive to be installed on production vehicles; strain gauges need to be calibrated regularly, they have a high sensitivity to temperature variations, and their installation is complex and time-consuming. With this in mind, we have investigated an alternative cascaded architecture with an outer-loop for deceleration control using wheel-speed-sensor (WSS) feedback. The objective is to study the benefits and drawbacks of such an approach in terms of technical performance and feasibility. Methodology & results For this study, a test vehicle was equipped with an EMB prototype and instrumented in order to have access to the WSS output. Firstly, a specific algorithm to process the WSS signal was implemented and tested under different driving conditions. The results show that by directly processing the WSS signal it is possible to obtain an accurate and smooth measurement of the wheel deceleration and eliminate the delay that would be obtained if these signals were retrieved from the CAN bus. Once the measurement of the deceleration was available, the control algorithm for the EMB was tested on the vehicle, initially with the EMB actuating on a single wheel while the remaining three maintained the original hydraulic braking (for security concerns), and then with EMB actuation on the four wheels. The algorithm’s inner-loops perform motor velocity and current control using field-oriented control techniques, whereas the outer-loop uses the wheel deceleration measurement to generate a velocity-equivalent reference for the actuator. The deceleration reference is generated as a function of the driver’s input/pedal stroke. The results so far show that, using deceleration feedback from the WSS, it is possible to achieve response times similar to those from conventional hydraulic braking, without compromising comfort. Limitations & future work One of the limitations encountered in this study concerns the accuracy of the deceleration measurement at low vehicle speeds. To overcome this constraint the deceleration-based control is switched to a (motor) position-based control at speeds lower than a certain threshold. Moreover, the tests conducted during this study considered only the use of the EMB for service braking. The use of deceleration-based feedback was not addressed during emergency braking or for parking brake. Finally, during the initial stages of the study it was detected that the test vehicle has two different types of WSS at the front and rear axles (AK-protocol vs square-wave output), which made necessary to adapt some hardware and software components before being able to process the WSS signal, thus slightly increasing the complexity of the overall implementation.
Hitachi Astemo: Dr. Missie Aguado-Rojas, Dr. Abdessamed Ramdane, Antony Auguste