How does an IMU (Lean Angle) sensor work?

An Inertial Measurement Unit (IMU) or lean angle sensor is used to measure the angular movement and acceleration of the motorcycle at around 100x per second. Its output data is a reference source for controlling lean angle cornering ABS, traction, electronic suspension, stability, and anti-wheelie control systems.

What does an IMU look like?

Below are photos of an IMU from Bosch. Bosch is currently one of the largest suppliers to the motorcycle industry, supplying the likes of KTM, BMW, Ducati, etc.

Bosch IMU
Bosch IMU sensor
Bosch IMU
Bosch IMU showing sensor ref angles

What’s inside an IMU?

Inside the IMU are Micro Electromechanical Systems, or MEMS. These are physical microscopic (1000th of a millimeter) components combining both gyros and accelerometers that can measure linear acceleration and changes in orientation in multiple axis.

An example of a MEM accelerometer is a tiny mass that sits on silicon springs called “seismic masses” suspended in a vacuum. Silicon springs are used, as they do not wear over time. As the mass moves, it produces changes in electrical capacitance or changes in electrical charge. These changes can be converted into precise measurements, as both the mass and distance moved are known.

One of the MEMS can be seen below against a human hair.

Human Hair on a IMU Chip
Human Hair on an IMU Chip

How does an IMU or lean angle sensor work?

The changes in electrical charge produced by the MEMs are fed into an onboard chip on the IMU, compared against each other, and also compared against wheel speed sensors. This data allows the bike ECU (onboard computer) to know the exact orientation, angle, speed, etc. of the motorcycle. This data can be used by the ECU to change braking pressure via the ABS, throttle position, engine torque, and ignition. All the parameters used to manage the stability control, lean angle ABS, anti-wheelie, etc.

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What does 5 or 6 AXIS mean?

When you look at the specifications of an IMU, they will describe if it’s a ‘6-axis’ or ‘5-axis’ IMU. This refers to the number of individual measurements available to the IMU.

5-axis = 3 accelerometers and 2 gyros
6-axis = 3 accelerometers and 3 gyros

If we take a 6-axis IMU, it can detect 3 linear (position) changes, left-right, up-down, and back-forward using the accelerometers. The gyros can detect 3 rotational (orientation) forces, side-to-side roll (lean), and the yaw when the bike changes direction.

 6-Axis IMU
6-axis IMU angles
 6-Axis IMU
6-axis IMU control

Putting it all together

As you can imagine, the IMU sends a huge amount of data to the bike ECU for processing. The ECU applies many algorithms to the IMU data, depending on the status of the bike and the reaction required.

Pinning the throttle fully open when the bike is doing 100MPH at a 50-degree lean angle in “Track” mode, requires a very different reaction to pinning the throttle at 50MPH and 20 degrees of lean angle in “Rain” mode. This different reaction is all made possible by the use of the IMU.

Thank you for the technology!