weChook Racing: How we use data on circuit
Seeing as we’ve been espousing the virtues of data collection of late, I’ve decided I’d best write at least a bit about some of the things we do with our data.
I’m going to split this into two posts – the first covering how the driver uses the information during the race, and the second discussing what we do with the logged data in between sessions and race days, as well as what we’re hoping to achieve in the future.
From the Rockingham heat onwards, our cars were fitted with a screen that showed the driver a live readout of battery voltage, motor current and motor speed. Voltage isn’t much use on a moment to moment basis – it fluctuates with the current that is being drawn, meaning there’s no simple way to estimate the charge left in the battery
Motor RPM is also tricky to use – we know we need to keep the motor in its ‘happy range’ in order to keep power consumption low and stop the temperature from getting too high, but it’s difficult to plan a race using motor speed – without a good model of the motor, we don’t really know whether we need to hit 1750 rpm or 1800 rpm to make it to the end of the race!
That leaves battery current as the most useful resource to the driver. We’ve done plenty of testing, which shows that our best batteries have a capacity of roughly 25 Amp-hours, when discharged at a high current. For comparison, our worst batteries have a capacity of 22Ah, which can make a big difference when trying to reach the end of a race.
With 25Ah at our disposal, and an hours worth of racing to complete, the maths isn’t too hard; we need to hit an average of 25A over the course of the race to make sure we get to the end – simples!
With a single speed, relay controlled car, this is achieved through selecting the right gear ratio – get it wrong and you won’t reach the end of the race, or you will get there but at a slow pace. Choosing that gear ratio can be tricky – for me it came down to experience and voltage measurements. I’ll discuss more on this in the next post though!
Electric 2galoo had the luxury of a wide range of gear ratios, and a speed controller. By shifting up and down the gears and by varying the throttle input, Ian was able to target a constant rate of power consumption. At the International Final, this allowed us to control the rate that the battery went flat very nicely – gaining us a place on the last lap as the competition ran out of juice!
We’re currently developing a data logging product that will be available for sale to all greenpower competitors – read more about it here – http://wechook.com/?p=511 . If you’re interested in investing in one, get in touch with us on twitter (@Ramjet_gpt) or on the greenpower forum here: http://www.greenpower.co.uk/forum/discussion/3398/introducing-project-echook-nano