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SPEEDBOX / VehicleInstallation

Increased accuracy - Setting up Vehicle Installation using the SPEEDBOX Configuration Utility

A number of options are available regarding the installation of the system in the vehicle and the location of the desired output from the system. Setting these parameters correctly will ensure that the system can perform to its optimum accuracy, particularly during brake testing. The options available are summarised below.

  • A measurement offset can be applied to the output from both the SPEEDBOX and the SPEEDBOX-INS. This is used to move the output from the point of installation to the point of interest. For details click here.
  • For the SPEEDBOX-INS only, a user-defined lever arm can be used to separate the IMU from the GPS antenna. This may be used when it is wished to mount the IMU at a point of interest unsuitable for GPS reception, typically the vehicle center of gravity (CoG). For details click here.
  • For the SPEEDBOX only, the separation between the base unit and the antenna can be specified, resulting in increased accuracy during hard acceleration and braking. It is recommended that this measurement is always specified for the standard SPEEDBOX. For details click here.

Setting up the SPEEDBOX and SPEEDBOX-INS Measurement Offset

For both the SPEEDBOX and the SPEEDBOX-INS, a measurement offset can be set up, for example referencing the measurement to a specific point on the vehicle, a separate component such as a laser barrier used for triggering a test, or a reference system for direct comparison. Configuring this function correctly results in increased accuracy at the point of interest.

It is important to note that by using an output offset on the SPEEDBOX-INS, signals at the gyros picked up from vibration on the roof of the vehicle will give an increase in noise on the accelerometer readings. The longer the level arm and the higher the levels of vibration the larger this noise will be. To check if this noise is a real signal or due to the lever arm try with and without the offset and compare acceleration signals. If the noise is unacceptable then consider moving the sensor closer to the edge of the roof or mounting the INS unit inside the vehicle as described here

SPEEDBOX measurement output offset

SPEEDBOX-INS measurement output offset

Note: Measurements shown are only an example, actual values must be measured from your installation.

For Dual Antenna INS systems the offset measurement will be referenced from the rear "Base" antenna connected to Rf 1.

For both the SPEEDBOX lever arm and measurement offset, the locations are referenced to the SPEEDBOX unit. For the SPEEDBOX-INS measurement offset is taken from the INS antenna assembly to the offset point. Measure the distances below, ahead (and right for INS units), using negative values for opposite directions, and input these into the configuration software.

Setting up a User-configured Lever Arm for the SPEEDBOX-INS

By default, the SPEEDBOX-INS uses an IMU that is co-located with the GPS antenna within the INS antenna assembly, and hence the lever arm between the two is known and does not have to be entered by the user. In this case the "IMU to antenna offset" parameter must be left disabled for the SPEEDBOX-INS. This is the standard use-case for the SPEEDBOX-INS.

However, there may be some situations in which it is desired to mount the IMU for the SPEEDBOX-INS system within the vehicle, for instance to obtain acceleration measurements directly at the vehicle CoG with lower noise than if using an output offset with the IMU on the roof of the vehicle. This then necessitates the use of a separate GPS antenna on the roof. In this case, it is necessary for the user to specify the lever arm between the IMU and the GPS antenna.

The lever arm must be measured carefully, otherwise lever arm measurement inaccuracy may come to dominate the overall accuracy of the system. In particular, it should be noted that the lever arm is specified in the IMU co-ordinate frame. Thus it is generally necessary to mount the IMU perfectly flat and level in order to obtain accurate lever arm measurements. Since this is not always possible, particularly in pitch, the INS will report the pitch mounting angle of the IMU when the INS converges when a lever arm is configured. This can be used to determine if the lever arm configuration should be changed to account for the mounting angle.

Configuration of SPEEDBOX-INS user-defined IMU to GPS lever arm using the IMU to antenna offset option, corresponding to the physical situation shown below

Typical physical installation of separated INS IMU and GPS antenna

Setting up the SPEEDBOX Antenna Offset

For the standard (non-INS) SPEEDBOX, a lever arm can be set up between the antenna location and the SPEEDBOX base unit containing the accelerometers. The unit uses this information to model the dynamics of vehicle pitch to produce increased accuracy through braking tests that feature pitching movement either during or immediately prior to the test. This offset is optional, but is highly recommended for braking tests.

Please note that only the "ahead" and "below" measurements are used by the SPEEDBOX; the "right" measurement is ignored for non-INS systems.

Caution: Do not enable this parameter for the SPEEDBOX-INS unless deliberately splitting the IMU antenna and GPS antenna as described here.

Setting SPEEDBOX antenna offset using the IMU to antenna offset option

Note: Measurements shown are only an example, actual values must be measured from your installation.

Page last modified on September 07, 2023, at 12:31 PM