Acceleration sensors in ETH electric racing cars

Acceleration sensors are used in the Formula Student racing cars to optimise grip. The aim is to achieve optimum driving characteristics by measuring and controlling the active damping.

Formula Student is the world's largest student engineering competition. More than 500 teams take on the challenge of building a fully functional racing car within one year. The regulations are kept very open and allow the young engineers to implement innovative concepts. The aim is to score as many points as possible in static and dynamic disciplines. On the one hand, it is important to build a fast, reliable car, and on the other hand, the implemented concepts have to be defended and also sold in front of experts from the industry and motorsport.

The Academic Motorsport Club Zurich builds a racing car for Formula Student every year. Almost 20 prospective mechanical and electrical engineers, students from ETH Zurich and the Lucerne University of Applied Sciences and Arts design, construct and manufacture a racing car. The result of the 2015 season is "flüela": an electrically powered racing vehicle with an acceleration from 0 to 100 km/h in under 2 seconds! (see picture) This is made possible by four self-developed wheel hub motors with 50 hp each, which allow individual control of all four wheels. The total vehicle weight is just 170 kg. A complete aerodynamic package is installed, which generates up to 2 kN downforce. In addition, countless carbon fibre reinforced plastics are used, including the one-piece monocoque, the rims and all parts of the aerodynamics. The chassis of "flüela" is equipped with semi-active dampers. These allow the damping curves and thus the dissipative forces to be adapted to the corresponding driving situation, thus always ensuring optimum road holding. With two overall victories at four events, the AMZ was able to secure first place on the world ranking list for another year.

In order to realise the full functionality of all actuators and their interaction, a multitude of sensors are built into the vehicle. These include an optical sensor that detects the current ground speed and allows precise measurement, despite slip on all wheels. In addition, an "Inertial Measurement Unit" is installed to determine the acceleration of the chassis as well as various safety systems such as countless temperature sensors to monitor the lithium-polymer cells, the power electronics and the motors.

The most important component of a vehicle are the tyres. They provide the only contact with the road and thus transmit all forces. From vehicle simulation and data analysis, it is known that tyre performance has the greatest sensitivity to lap time reduction. In order to understand the effects more precisely and to take measures, there is a great interest in the dynamics of the unsprung mass. For this reason, the AMZ is using wheel acceleration sensors from the company KELAG for the first time in the 2015 season. One such sensor is attached to each of the four wheel carriers (see picture). Electromagnetic compatibility is no easy task in an electric vehicle, where currents of several hundred amperes at 470 V flow. Thanks to the internal shielding and the strictly star-shaped cabling and earthing to the wiring harness, it is possible to work with excellent signal quality. KELAG's accelerometers measure wheel acceleration in the vertical direction with a bandwidth of -6 G to +6 G and were chosen in particular for their electrical and mechanical robustness, which enables reliable and precise signals even under difficult environmental conditions. The measured data is used to optimise the control of the semi-active dampers. This in turn results in increased road grip and thus shorter lap times.

A suspension simulation is used to develop the damper control. The vehicle is simplified with a quarter vehicle model, which represents the situation and interaction at a suspension and tyres. The aim of the simulation is to find the optimal damper curves so that the wheel load fluctuations on the tyres are minimised.

The system is excited with chassis movements and road irregularities measured and recorded on the real vehicle. The excitation profiles and frequencies influence how the unsprung and sprung masses react. In a car, one wants to keep the transmissibility of road unevenness to the chassis as low as possible in order to increase comfort. For a racing car, tyre performance is paramount. In order to achieve the lowest possible transmissibility to the wheel loads, the frequencies of the excitation are recorded with the sensors and the optimum damping ratios are determined by means of simulation.

In the 2016 season, the AMZ will again rely on the wheel acceleration sensor. The algorithm implemented in the damper control will be reset and the measured signal from the sensors will be used to react directly to bumps in the road. The new AMZ vehicle "gotthard" was already presented on 20 May on the occasion of the roll-out at Sauber F1 in Hinwil and will compete against international competitors at four events in Europe this summer.

Kelag contributed an acceleration sensor that measures the axis acceleration. The KAS903-12A was used for this purpose.

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