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RACELOGIC Support Centre

How to Mount the External IMU

Mounting

If you are using an external IMU04 in conjunction with VB3iS, it can be roof mounted, fixed rigidly to a vehicle through the fixing holes or secured using a telescoping mounting arm. It should be mounted in the direction of travel and level with the ground.

Roof-Mounting (Recommended)

The easiest method of mounting the IMU is by placing it directly on the vehicle roof, co-located with the GNSS antenna.
 

The IMU roof mount should be mounted on a braced, stable part of the roof, such as the area over the A or B Pillar.

Care should be taken to avoid placing the mount in a half braced position, where the front or rear is on a supported part of the roof, and the opposite end is on a flexible location. The potential pivot can induce a pitch rate oscillation that will influence the accuracy of the results.

Magnetic base (RLACS216)

A specially designed IMU Roof Mount (RLACS216) allows for an IMU04 (only) to be securely fastened within the machined enclosure while it's magnetic base ensures that it stays safely in place.

The antenna can then be placed directly above the IMU allowing for the data sources to be measured at the same point (if you are using a non-magnetic ground-plane antenna, the RLACS230 IMU04 Roof Mount should be used instead).


By co-locating the antenna and IMU on the vehicle’s roof the distance between the two does not have to be measured, as you do in a standard setup, leaving only the required translation to – typically – the vehicle’s centre of gravity. This method is easier and isn’t prone to human measurement error. Co-locating the antenna with the IMU also greatly improves the performance of the Kalman filter.

 

RLACS216.jpg
Suction mount (RLACS256)

When using a vehicle with a non-ferrous roof such as aluminium, carbon fibre or glass (sun roof), the magnetic based IMU Roof Mount will not hold and a suction mount is required along with the magnetic base.

With its three points of contact, the Vacuum Tripod Mount (RLACS256) ensures a very stable fixture and reduces IMU vibration that might be transmitted by the vehicle's roof.
 

The adjustable lever arms allow for the levelling of the IMU on a non-flat roof whilst extending the mounting options on the vehicle. For example, you can fix the mount partially or fully on a coupé's glass roof when trying to achieve a greater antenna separation in a dual antenna setup.

RLACS256.jpg

VB3iS IMU04 Mounting.png
 

Please contact vbox@racelogic.co.uk for more information, or to order an IMU Roof Mount.

Measuring Translation Points

When a roof mount is enabled using the VBOX Setup Softwarethe Front Panel of the unit or using the VBOX Manager, an automatic 1 m z offset is added, translating the filtered speed down in to the vehicle towards the centre of gravity. This can be changed by entyering translation values within the VBOX Setup Softwarethe Front Panel of the unit or using the VBOX Manager.

Customer required translation measurements should be made in the same plane as to which the IMU is mounted (i.e. perpendicular to the IMU base, not straight down to the ground).
 

g6846.png g6777.png

Fixed Mounting

It is also possible to fix the IMU firmly to the body of the vehicle, mid-way along the wheelbase. Try to position the unit as close as possible to the centre of the vehicle, making sure it is mounted in the direction of travel - as shown in the image below. It is also important to mount the sensor so that it is level with the ground.

For best results, mount the IMU and GPS antenna as close to each other as possible. For example: Bolt the IMU to the seat rails and place the GPS antenna on the roof directly above.


Note: The IMU and antenna should be mounted on the same rigid body, to provide a relative reference. So for a vehicle such as a truck cabin, both the IMU and antenna should be fitted to the cabin body (if cabin is air-sprung) or to the vehicle chassis.
 

Using a Mounting Arm

A flexible way to fix the IMU rigidly within the vehicle is by using the Racelogic Mounting Arm. The three-part telescopic handle is fully adjustable to any length between 70 and 150 cm to which another 20 cm can be added by extending a third section using the compression lever.

Both ends are fixed to an 8 cm x 13 cm plate which sits on a joint to accommodate for uneven surfaces. Pressed against the IMU on the floor and the vehicle's ceiling, the mounting prop ensures that the IMU is fixed tightly.
 

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Please contact vbox@racelogic.co.uk for more information or to order a mounting arm (RLACS212)

Measuring Antenna OffsetsIMU_Mount_3.png

When using either a fixed mounting point or the mounting arm, you must measure the relative position of the antenna in relation to the IMU to at least within +/- 5 cm. 

This will be the x,y,z offset values from the antenna to the IMU04.

These distances must then be entered into the VB3iS via inputting directly using the VBOX Setup Softwarethe Front Panel of the unit or using the VBOX Manager.

VBOX Setup Software

  1. Connect IMU04 to VB3iS, ensuring the unit is powered.
  2. Connect VB3iS to a computer using the supplied loom, connect an RLCAB001 cable to the 'CONFIG' lead and connect the serial plug to the computers serial port – this can be done via a serial > USB convertor if required.
  3. Open VBOX Setup and connect to VB3iS by selecting the correct COM Port.
  4. Select the 'IMU' menu and then the 'Settings' tab.
  5. Ensure IMU integration is 'Enabled', Location is set to 'External' and Roof mount is 'Disabled'.
  6. Enter the measured ahead/behind, right/left and above/below offset values within the 'Antenna to IMU offset' options.
    Note: The options will be labelled 'Antenna to reference offset' if being used in conjunction with a Roof mount.
     

VB3iS Antenna Offset VBS.png
 

  1. Select 'Write to unit' to save the settings.

Front Panel

  1. Connect IMU04 to VB3iS and then power the unit.
  2. Press the 'OK' button to enter the menu, navigate to 'IMU' using the 'Up' and 'Down' arrows and the press the 'OK' button.
VB3iS IMU Front Panel.png  
  1. Ensure IMU Integration is 'Enabled' and Location is set to 'External'.
VB3iS IMU Enabled.png VB3iS IMU External.png 
  1. Navigate to 'Ant to IMU Offset' using the 'Down' arrow, press the 'OK' button to enter the menu.
    Note: This menu is labelled 'Ant to Ref. Offset' if an External IMU is being used in conjunction with a Roof Mount.
VB3iS IMU External2.png VB3iS IMU Offset Front Panel2.png
 
  1. Use a combination of the 'Up' and 'Down' arrows and the 'OK' button to enter the measured ahead/behind, right/left and above/below offset values.
    Note: This menu is labelled 'The reference point is' if an External IMU is being used in conjunction with a Roof Mount.
VB3iS IMU Ext Offset Front Panel1.png VB3iS IMU Ext Offset Front Panel2.png
  1. Selecting the 'OK' button will move along digits and selecting past the final digit will store the value.
VB3iS IMU Offset Front Panel5.png  

Pitch/Roll Offset

If IMU04 is placed on a non-flat surface, the 'Pitch/Roll Offset' action within the Front Panel of the VB3iS unit or the VBOX Manager should be used to compensate the IMU attitude angle channels (pitch angle, roll angle).

This will zero the Pitch_imu and Roll_imu channels. Angle offsets calibration must be performed after the IMU Kalman Filter calibration has been completed, and the vehicle is static on a level surface.

  1. Press the 'OK' button on the front panel to enter the menu, navigate to 'IMU' using the 'Up' and 'Down' arrows and the press the 'OK' button.
VB3iS IMU Front Panel.png  
  1. Ensure IMU Integration is 'Enabled' and Location is set to 'External'.
VB3iS IMU Enabled.png VB3iS IMU External.png 
  1. Navigate to 'Pitch/Roll Offset' using the 'Down' arrow, press the 'OK' button to enter the menu.
VB3iS IMU External2.png VB3iS IMU Offset Front Panel6.png 
  1. Press the 'OK' button again to auto level the pitch and roll offset.
VB3iS IMU Offset Front Panel7.png  

Be aware that when mounting the IMU04 on an angle, the raw accelerometer and gyro data will be incorrect as the channels are not pitch compensated.