Sensor and sensibility

by Simon Hargreaves

Our bikes are being micro-managed, literally. Microchips are as much a part of 21st century riding as an engine and wheels.

We're familiar with solid-state electronics in biking. Ignition swapped points for transistors in the 1970s, then went digital in the 1990s. Now, the need for greater power, fuel efficiency and emissions control means modern engines only work with computer-controlled fuelling, ignition, combustion and exhaust. When we open the throttle we pull the strings, but the ECU calls the shots. To do it, the ECU needs an accurate view of the world. And it gets it through its sensors.

There are five basic groups of sensors on a modern bike. Inductive sensors are the oldest, and use either a gear or perforated wheel to induce a voltage in a sensor (a ferrous core wrapped in wire), or a variable magnetic field to generate a signal in a semiconductor (Hall effect). This makes them good at measuring the speed and/or position of spinning objects like cranks, cams, wheels etc. They were first used to replace contact points in ignition systems but are also used to measure the change in wheel acceleration for traction control and ABS systems.

Then there are potentiometers that, like a volume control, measure via a change in resistance. They're good at measuring rotating or sliding positions, such as throttle position sensors, fuel level and exhaust valve position.

Thermistors are sensors that vary electrical resistance according to temperature. With no moving parts they're durable, and you'll find them doing the dirty work measuring ambient, airbox, coolant and and oil temperature.

And there's the oxygen sensor, which measures the oxygen content of exhaust gas by detecting the electrical current created by oxygen molecules migrating across a ceramic core. Without it, the ECU would be unable to run a catalytic converter.

Finally there are piezoelectric and piezoresistive sensors, that either generate a signal or change resistance when compressed or moved. These are the most interesting, because it's these that are at the cutting edge of traction control and active suspension.

Piezos are an example of micro electro-mechanical systems, or MEMs. As the name suggests, a MEM is a silicon chip containing a miniature mechanised system; it doesn't just compute (although it can do). The system might be fluid, optical, chemical of material, but it has moving parts. Which is pretty staggering when we're talking at the scale of less than 100 microns, or 0.01 of a mm.

Piezo sensors include the manifold absolute pressure sensor, located in the inlet manifold, which lets the ECU calculate the mass flow rate of air entering an engine. Piezos also 'listen' for the vibration from detonation inside the combustion chamber. And perhaps the sexiest of the lot feature miniature 'arms' embedded in a silicon chip, and that flex in three dimensions according the relative motion of the bike. A fixed 'arm' measures the electrical differential as a result of the flexing, which gives the ECU information about the angular velocity of the bike in three dimensions. It knows how fast you're leaning, braking, accelerating etc.

So the next time you're out for ride, it's sobering to realise your bike is as in touch with the world as you are.