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 a previous study we addressed the control of EMBs using wheel deceleration feedback (as opposed to most approaches found in the scientific literature so far, based on clamping force or braking torque feedback). The control strategy uses a cascaded architecture, where the inner loops perform motor velocity and current/torque control and the outer loop performs wheel deceleration control using measurements obtained from the wheel-speed-sensor (WSS) signal. Vehicle tests showed the feasibility of the approach and that in general it is possible to achieve response times similar to those from conventional hydraulic braking without compromising comfort. Nevertheless, one of the limitations encountered in our previous study concerns the accuracy of the deceleration measurement at low vehicle speeds, as it made it necessary to switch from the deceleration-based control to a (motor) position-based control at speeds under a certain threshold. With this in mind, we now investigate the use of high-resolution WSS for the control of EMBs in the low-speed range. The objective is to study the benefits and drawbacks of such an approach in terms of technical performance and complexity of the signal processing algorithm. Methodology & results For this study, the front wheels of a test vehicle were equipped with two prototypes of high-resolution (Hi-Res) WSS, in addition to the sensors originally installed by the manufacturer. The prototype sensors were custom-made for our application by an external company. Both prototypes have a resolution higher than 48 pulses-per-revolution (which appears to be the standard in the automotive industry nowadays), and their operating principle is similar to that of commercial WSS. They consist of a rotating magnetic ring (encoder) attached to the wheel bearing, and a fixed pick-up sensor that detects variations in the magnetic field and delivers the output signal to be read by the ECU. The first Hi-Res WSS combines the vehicle’s original pick-up sensor (the same model) with a prototype magnetic ring that has a higher number of magnetic pole pairs. The second Hi-Res WSS combines the original magnetic ring with a prototype pick-up sensor that performs an interpolation of the internal signals to deliver a higher number of pulses per revolution. Tests were performed to compare the accuracy of the signals measured with the Hi-Res WSS against those obtained using the original sensor. In all cases the same signal processing algorithm was used in order to analyse as well the impact that the increase in number of output pulses might have on processing time. Some ideas for merging the information of conventional (low-resolution) WSS and the new Hi-Res WSS, thereby covering the entire speed range of the vehicle, are addressed as well. Limitations & future work The tests conducted during this study considered only the use of the Hi-Res WSS in open-loop (i.e. measurement-only) configuration. This is because the test vehicle in which the Hi-Res WSS have been installed is not (yet) equipped to perform closed-loop control tests of our prototype EMB. Work is ongoing to perform such tests.
Dr. Missie Aguado-Rojas, Advanced Engineer, Hitachi Astemo; Mr. Philippe Bourlon, Senior Advanced Engineering Mechanical Manager, Hitachi Astemo; Dr. Abdessamed Ramdane, Advanced Engineering Department Manager, Hitachi Astemo