The Formula Student is a worldwide engineering competition for student teams designing and constructing a race car. Many teams are building electrical racecars powered by an accumulator. Before they can join racing competitions, they need to erase one danger: An unintended acceleration due to a pedal sensor error. Different from combustion racecars, electrical racecars only accelerate indirectly when the gas pedal is pushed, because a sensor needs to detect the movement and sends this information to the program which controlls the e-motors. This process can fail and e-motors fully powered cannot be stopped with hard braking. To solve this problem, E race cars need a so called “Brake System Plausibility Device” checking if the driver brakes simultaneously to engaded motors. For that, the power supply to the e-motors and the brake pressure need to be observed.
For this reason, we designed two PCBs which Aisler manufactured for us. I want to present some insights of this project here. We check the power delivery to the engines with a current sensor. The current sensor is not implemented within the current path, but measures the magnetic field caused by the current flow. The sensor is connected to our main pcb of our Brake System Plausibility Device. The other pcb is placed near to the brake padels and receives the brake pressure sensor values. This pcb drives a signal to the main pcb indicating that the brakes are engaged or not. To ensure a safe information transmission, we need to detect wheater the cable is broken or connected to power or ground. To ensure this, we transmit the information via a certain current level, drived by a current driver on the slave pcb. The main pcb now measures a voltage over a measurement resistor. A broken cable or short circuit to power/ground differs from the current driven by the slave pcb and can therefore be detected. The main pcb also controls a shut down relay which seperates the accumulator from the motors. If a brake implausibility or a failed communication is detected, the pcb opens this relay and the power delivery to the e motors is stopped.
