April 4th, 2008 | Published in Google RechargeIT
One of the main purposes of our fleet of plug-in hybrids is to measure the performance of plug-ins compared to an unmodified hybrid car. We then post this information to our website, where anyone can see how our fleet is doing. As you can see, we collect quite a lot of information from our cars, and we actually collect more than we show to the public. Here's how we do it:
Inside the trunk of each of our instrumented cars sits a small, embedded Linux computer made by Soekris Engineering. They're called 'embedded', but they're really very powerful little devices: 256MB of SDRAM, 4GB of flash and a 266MHz, 486 like processor. In the past, we had much less than that as a desktop!
Whenever the car is running, or when it's plugged in and charging, the datalogger is switched on and powered by the car's auxiliary battery. Even plug-in hybrid cars have a standard 12V car battery in them (the 'aux battery'), although it's often smaller than in other cars. The aux battery is used to power the dashboard, locks, windows, etc. and is usually recharged by the gasoline engine when it's running. This battery is separate from the hybrid battery packs which drive the electric motor (known as the 'traction' battery).
To avoid draining the aux battery too much, we also installed a charger which charges the aux battery when the car is plugged in. In addition, the datalogger will switch itself off if the car is off and unplugged for five minutes or more.
Connected to the datalogger is an ammeter, a GPS receiver and an interface to the car's internal network (the CAN bus).
The ammeter measures the amount of power that the cars are drawing from the power grid when plugged in. (This includes the power that our aux charger is using too.) Of course, here at the Googleplex we already have a large solar installation which generates thousands of KWhs of power daily. In comparison, it only takes about 5 KWhs to completely charge one of our plug-in Priuses.
The GPS records the car's position and, more importantly for us, gives us an accurate timebase for our logs. Although the specific GPS receiver that we use is only good to half a second (plenty for us!), others can provide a signal which is accurate to fractions of a millisecond.
Most modern cars will have an internal network, called a CAN bus, which have very simple fully-connected, broadcast topologies. Over this network, many different devices in the car transmit their status and, by listening to these messages, we can find out nearly everything about the car. Some of the information which we don't process includes the angle of the steering wheel at all times, the amount that the brakes are pressed, and the current gear. So, you can see that the CAN bus is pretty comprehensive. Also connected to the CAN bus is our additional plug-in battery pack from Hymotion. That's how we communicate with the plug-in systems and monitor their state of charge.
At this point, we have a lot of data from the car, the plug-in battery, the ammeter, and the GPS pouring into the datalogger. In a future blog post, we'll talk about the internals of the datalogger and how the data ends up on the Google.org website.