BSPD for Formula Student

Hello World! Welcome to our post about the making of the BSPD for our Phoenix, our 2021 Formula Student car. This car competed in FSA (Austria), FSN (Netherlands) and FSS (Spain) in the 2021 season.

The Formula Student has a rolebook that all the teams must respect in order to participate. This rules make the cars safe and similar to each other, and gives us the challenge of making the best out of the regulations.

The BSPD (Brake System Pausibility device) is a part of the shutdown system of the car, and it consists of a non-programmable circuit that prevents the driver from braking and accelerate at the same time, in order to prevent failures with stuck throttle pedals. The requirements are:

-Open by default

-Closed only when brake pressure <= 30 bar or throttle pedal position <= 25%

-If the opening condition persists for longer than 0.5s, the BSPD triggers and the shutdown circuit opens (the opening condition being brake pressure above 30 bars (Hard braking) and throttle position above 25%)

-After being triggered, the BSPD may become closed again either by power-cycling the LVMS or, if the opening condition is absent for longer than 10s, the BSPD resets itself

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Now, we continue with the proyect. In order to make the BSPD, we follow the digital path instead of the analog path, because we think is simpler and more reliable.

We will now briefly summarise how each of these parts works. The first stage is the sensor check, which includes checking the brake pressure and throttle position.

An LM393 comparator has been used for this purpose: if the voltage in the inverting terminal (- terminal from now on) is greater than the voltage in the non-inverting terminal (+ terminal), the output will be drawn to the voltage supply. If not, it is drawn from ground. it is supplied by 5V in one terminal, which is the voltage that most of the BSPD operates with (save for the relay).

To signal when the sensor input goes over a certain threshold, we can connect the + terminal to a voltage representing said threshold, and the sensor to the – terminal. This way, when the sensor signal overtakes our designated voltage value (the + terminal voltage), the comparator emits a “1” signal.

In respect of the opening condition of the circuit, with a truth table it becomes a simple issue:

We have defined the opening condition as 1 when present, and 0 otherwise. In this case we would need an AND gate.

For the part of the 0.5s delay, it is important to keep in mind that the BSPD must not be programmable. An analogue component that is well suited for things like delaying signals is the capacitor. Once the opening condition is triggered, a fixated current value will start to flow from the logic gate. With this, a capacitor and another voltage comparator we can design a simple schematic.

As the capacitor starts to charge, the voltage will raise, until the voltage of the + terminal overtakes the - terminal. The capacitance of the capacitor should be chosen so that this time is 0.5s.
In other words, the comparator is at 1 in its regular state. When the opening condition persists for 0.5s, the + terminal voltage becomes greater than the – terminal, and the comparator goes to 0.

Lastly, we would have the opening of the relay, since the shutdown circuit is routed through one in the BSPD. To actuate it without using any programmable devices, we can employ a transistor as an electronically actuated switch. The BSPD is a normally OPEN device, which is why before the comparator would normally be in 1. This way, if the components fail, the comparator changes to 0 and the shutdown circuit opens.

Thus, the final schematic would be as follows:

And these would be the gerber files of our design
We assembled them, and the BSPD worked well every time we needed. It passed the FSA and FSS regulations with ease.

The design was big, but functional. We are working now in the creation of a BSPD like this, but with SMD components.

Thanks to our sponsor JLCPCB for giving us the oportunity, and remember, here is your link:
https://jlcpcb.com/RAT

Dare to be MAD!