They feed information to your ECU to control electronic riding aids... but how do they work?
Motorcycle electronics guru Bob Gray has 16 years’ experience as a data and enginemanagement technician and BSB crew chief.
Inertial measurement unit (IMU) is a term that often crops up on new motorcycles. IMUS are designed to provide a constant stream of measurements to the ECU, which it can then process to keep track of the bike’s attitude. The ECU uses this information to modify its control strategies – like traction control and anti-wheelie – which is where we benefit.
IMUS typically use two types of sensor; accelerometers and gyros. Accelerometers measure acceleration, while gyros measure angular velocity. To capture every way a chassis can move, there are typically three accelerometers inside an IMU (measuring longitudinal, lateral and vertical acceleration) and three gyros (measuring roll, pitch and yaw). These are referred to as six-axis IMUS, although some only use two gyros (roll and pitch), in which case they are five-axis IMUS.
Inertial measurements don’t describe the chassis’ attitude, only its movements. For example, the pitch gyro of a bike riding along a level road will have the same reading as a bike doing a perfect balance-point wheelie
(0°/s). That’s because the gyro measures movement, not angle. To calculate angle, the gyro measurements must be integrated with respect to time. Rewind a few seconds and you might find the pitch gyro on the wheelie bike had an average reading of 90°/s for 0.5 seconds, from which the ECU can work out the bike is currently doing a wheelie at 45°.
In terms of riding your bike, though, three things matter. First, the accuracy of the IMU measurements. Second, the quality of the filter used in the ECU to turn them into usable values like lean angle and wheelie angle. And third, the quality of the control strategy. If the quality of any one of those things is below par, you’re going to curse at some point because there’s either intervention when you don’t want it; or worse still, there’s no intervention when you need it. So, IMUS are great, but only if they’re part of a good system.
‘Accelerometers measure acceleration while gyros measure angular velocity’
az= This measures any sideways drift from the front or rear wheel
ax= This measures the lean angle of the bike
ay= This measures the acceleration and braking of the bike
Suzuki’s new GSX-R1000R uses a six-axis IMU to collect data