Green light stop, red light go
By Herbert van Even
With 600 volt accumulator pumping energy through the system, high voltage wires running through the cockpit and 4 motors guzzling 80 kW of power, a formula student car is a dangerous machine.
After testing, between events or during scrutineering, people will poke around in the fully operational DUT19. Because we don’t want to fry (friendly) scrutineers, the high voltage system has to be disconnected while this is done. For this cause the tractive system active light, or TSAL for short, shines proudly at the top of the main hoop. It’s green when there is no high voltage in the system but will violently start blinking red when the car is in ready to race mode.
Of course, the TSAL is just a stupid LED-strip, it can’t figure out the state of the car by itself. That is the task of the TSAL driver. This PCB measures three quantities to verify that you won’t be electrocuted when you decide to stab the car with a fork. It consists of a network of logic gates, hard wired to check measurements from the high voltage system. To ensure the TSAL driver speaks the truth, two mandatory measures are taken. Firstly, no software control is allowed. Since software is pure logic (although I start to doubt this when our software engineers are screaming at their screen), making the TSAL driver into a microcontroller with some peripherals is an evident choice, but because it isn’t stable enough, the FSG rules forbid it. The second directive says that the light must turn off when any error occurs during the transmission of signals. Image that 0V is the signal for a safe state, and the wire carrying the signal short to ground, then the TSAL will light up green, even if more than 100 amperes of current is rushing through the wires. Therefore, shorts, open circuits and discontinuities must be detected and signaled by turning of the TSAL.
As for this blog, it’s light goes out as well.