When the car gets a green light to go, it will accelerate to an appropriate speed. To determine the speed at which it is traveling, the car uses speed sensors similar to the sensor used to drive the car’s speedometer, but with a difference. The Google car’s sensors monitor the rotation of both rear wheels so that it can accurately measure the distance the car has travelled, and the speed at which it is traveling.

A modern speedometer doesn’t actually measure the rotation of the wheels. Instead, it measures the rotation of the output shaft of the gearbox which is connected to the wheels. In the original design patented by Otto Schulz in 1902, a small gear driven by either the output shaft of the gearbox, or one of the front wheels, drives a flexible rotating shaft (the speedometer cable) connected to the speedometer.

Inside the speedometer, a magnet connected to the cable rotates inside a small aluminium or copper cup. The cup is connected to the needle of the speedometer dial and there is a spring that pulls the needle back towards zero. As the magnet rotates, eddy currents are induced into the cup. The eddy currents set up their own magnetic field which, if there wasn’t a spring, would cause the cup and the needle to spin.

The spring, however, prevents the needle from spinning so that the needle comes to rest at the point where the two forces – the induced force in the cup and the force of the spring – balance each other out. The faster the magnet rotates, the more force is induced into the cup, and the further the needle is rotated.

All of this changed in the early 1990’s when the first all electronic speedometers began to appear. Electronic speedometers work a little differently to mechanical ones. To start with there is no rotating cable between the transmission and the speedometer. Instead, a sensor on the transmission output shaft sends electrical pulses to the speedometer which counts them.

So how does the sensor work?

There are a few options here. One possibility is to have a cam on the shaft that presses a micro-switch each time the shaft rotates. When the switch closes, it sends an electrical pulse to the speedometer. The disadvantage is that at high speeds, the switch will not have sufficient time to return to its resting position before the cam pushes it again, so it will be inaccurate. Mechanical switches are also prone to wearing out, which means that the switch would have to be replaced frequently.

To get around these problems, we could use an optical sensor to detect a reflective patch on the transmission output shaft as the shaft rotates. The sensor would be able to operate at high speeds and has no moving parts so it won’t wear out. The only problem is that car transmissions are notoriously dirty places. Dust and oil would gather on the sensor which would require constant cleaning. We need a sensor that is fast, has no moving parts and doesn’t mind at all if it gets caked in mud.

The sensor that is actually used is magnetic. A device known as a “hall effect sensor” is able to detect changes in the magnetic field. The sensor is placed near a small gear made of steel, sometimes with a magnet placed on the back of the sensor. As the gear rotates, the teeth of the gear move past the sensor and the magnetic field going through the sensor changes. The sensor detects these changes in the magnetic field and sends pulses to the speedometer.

Because mud and oil are non magnetic, the sensor doesn’t care at all if it gets dirty. The same type of sensor is also used in the anti-lock braking system to detect wheel rotation, and in the engine to detect the camshaft and crankshaft positions. The pulses are counted by the computer – more pulses per second means more speed.

The Google car doesn’t actually use the speedometer to measure speed. Instead, the rotation of both rear wheels is measured. This gives the system more information about how the car is actually moving. For example, as the car goes around a corner, the wheels rotate at different speeds. Having separate sensors on the wheels captures this extra information that the speedometer does not provide.

Putting it all together

So now our autonomous car has all the information it needs to navigate safely around our city. Its LIDAR can map the street layout including other cars, pedestrians, trees and the road. Its RADARs tell it how close it is to the cars in front of and behind it, its camera can see the traffic lights and it knows how fast it is going thanks to the sensors on its wheels. The GPS and mapping software in its processor unit allows it to know where it is in the world, where it is going and how it will get there.

The control software uses all of the data from the sensors and combines them using a set of rules that govern how the car should behave. The result is a system that is more alert, infinitely more patient, and not prone to fatigue like its human counterpart. The Google car is actually a better driver than any of us.

In 1973, the British science fiction writer and futurist Arthur C. Clarke postulated the third of what have come to be known as his three laws: “Any sufficiently advanced technology is indistinguishable from magic.”

To people living in the middle ages, the idea of carrying on a conversation with someone in another village via a small flat block of glass would have been deemed witchcraft. Even the moving pictures on its surface would have seemed like magic. Today we have mobile phones that allow us to talk to anyone anywhere in the world. To someone living in the early eighteenth century, the idea that people would be able to fly above the clouds from one city to another would have seemed preposterous, and yet at this very moment, around a million people are doing just that.

The idea of robots driving cars has been around since the mid twentieth century and yet right now, at the advent of the autonomous vehicle age, the prospect of self-driving cars seems more than just a little bit magical.