Installing and Configuring an IMU for Motorcycle Applications

Setup

Firstly, it is important to align the IMU in the forward-facing direction of the bike motorcycle frame as accurately as possible as the channel “Heading_IMU” is dictated by the forward direction of the IMU. The VBOX IMU integration includes a process that will compensate for IMU mounting error and that compensated channel is called “Heading_IMU2”.

• In a single antenna setup, the VBOX will detect when the motorcycle is travelling in a straight line and compare the average Heading and Heading_IMU, then add the difference of these 2 channels to the Heading_IMU value to create Heading_IMU2.
• In a dual antenna setup, the VBOX uses Dual Antenna “True Heading” and compares this channel and Heading_IMU, adding the difference to the Heading_IMU value to create Heading_IMU2.

IMU integration will also need to be enabled within the IMU tab of VBOX Setup. Once enabled, there is an offset section, split into 2 options:

Relative to the antenna, the IMU is:

This allows you to input measurements from the antenna in a single antenna setup (or primary antenna A in a dual antenna setup) to the IMU. If using a roof mount, select the roof mount option and these options will no longer be available. Enabling "Roof-mounted IMU" simply fixes the X, Y and Z relationship between the antenna and the IMU.  

Relative to the front of the vehicle, the output reference point is:

This option allows you to enter the X, Y and Z offset values from a nominated reference point of the motorcycle to the IMU. This will then translate the data from the IMU location to another point on the motorcycle where all measurements will be made. These values include speed, heading and the position of the vehicle.

When “Roof Mount” is enabled, an automatic 1m Z offset is applied, translating the filtered speed down towards the centre of gravity of the motorcycle. This was created to lower the point of measurement to compensate for the lever arm effect. Though the translation function doesn’t explicitly mean “lever-arm correction”, the translated measurement point is typically used to compensate for the effects of “lever-arm”.

Note: In firmware version 3 of VBOX 3i, this auto application of 1m Z offset when roof mount mode is selected has been removed. It is now the user's sole responsibility to enter a required Z translation.

In firmware versions v2.8 and below, it is important to allow for the IMU to initialise for at least 30 seconds whilst stationary after the VBOX has acquired satellites. When initialisation is complete and you drive away, the Kalman filter will begin to make corrections based on the heading that was acquired when initialisation occurred, and the live heading measurement. If the motorcycle does not move away in a forward direction (IMU forward direction), then the Heading_IMU will receive an incorrect initial Heading value. If this value is greater than 30 degrees, then the Kalman filter cannot overcome this initial offset. If this occurs then the Heading IMU will remain incorrect, but the algorithm for Heading_IMU2 compensation still works and corrects the fault; this can explain instances where there is a difference between Heading_IMU and Heading_IMU2 data. 

Despite still showing the status as “Good” within VBOX Test Suite, this seeded Kalman Filter will cause the Heading_IMU measurement to be incorrect due to reasons outlined below:

• When the rider mounted the bike and moved the bike from a side stand angle to an upright bike position after the IMU and Kalman Filter had been initialised, then an angular change for the IMU can occur. If this angular change is more than 30 degrees, the Kalman Filter cannot overcome this initial offset, causing the Heading_IMU channel to be incorrect.
• Another reason could be the rider shadowing the GPS antenna, creating noisy GPS data that causes the Heading_IMU to initialise with an incorrect heading.
• Finally, if the vehicle does not move away in the forward direction of the Vehicle (IMU forward direction), then the Heading_IMU becomes seeded with an incorrect initial Heading value, causing the value to differ.

A feature was implemented in version 3 of the VBOX 3i firmware to assist motorcycle testers in cases of incorrect Kalman filter initialisation, where you can configure the IMU to “initialise only when moving”.

This feature allows the IMU to initialise when a certain speed is reached (18 km/h) which will aid the Kalman filter to initialise correctly in cases when it is difficult to remain stationary throughout the initialisation. It will also prevent early initialisation of the Kalman filter caused by erroneous movements of the antenna(s), such as when the rider shift their body position.

Once the configuration has been completed and the VBOX 3i is powered on, you will need to allow some time to acquire satellites. When the unit achieves satellite lock, you must drive in a straight line at a speed of more than 18 km/h. The IMU, and subsequently the Kalman filter, will begin the initialisation when the bike reaches the required speed and will begin the data generation for the respective channels.

Points to consider when installing an IMU on a motorbike When using a dual antenna VBOX 3i on a motorbike, you must bear in mind that the speed of the antenna is going to be much greater at the Primary Antenna (Antenna A) due to being a considerable distance from the motorbike’s centre of gravity. As a result, when the motorbike leans, the slip angle will fluctuate from the “True Heading” value to the “Course Over Ground” value. This is where we would recommend installing and utilising an IMU as it can be placed under the Primary Antenna (Antenna A) to translate the antenna point to a much lower centre of gravity through IMU integration and offsets. This means the user will achieve a more accurate slip angle as the speed at the Primary Antenna (Antenna A) is drastically reduced.  The Yaw rate that is derived from True heading (twin antenna heading) is NOT affected by motorcycle lean angle. However, the Yaw rate that is derived from True Heading is noisier than the Yaw rate measured from the IMU Yaw Gyro itself. The Yaw rate measured by the IMU is given in the coordinate frame of the IMU. Consequently, when a motorcycle leans, this measurement becomes inaccurate and requires angular compensation. Heading_IMU, when correctly initialized, can serve as the equivalent of True Heading from the Twin Antenna. Despite the bike leaning, the Heading_IMU channel calculates Yaw, Pitch, and Roll rate with the correct angular compensation. Thus, Heading_IMU indicates the direction in which the motorcycle body is pointing, irrespective of the lean angle. It is better to make a rate of change of Heading_IMU, to create a correct Yaw rate as inside the Kalman Filter, Heading_IMU is compensated for pitch and roll angle, so it would be best to make a rate of change of Heading_IMU, then you will get a YAW rate that is pitch and roll compensated and less noisy than the Yaw_Rate from twin antenna.