Integrated Inertial Measurements

It is largely accepted that the most accurate and cost effective way to carry out vehicle dynamics testing and development is with the use of GPS equipment. In some situations, however, it pays to also use inertial measurements to either augment the available data, or to overcome problems encountered in certain environments – such as a test track that isn’t in an entirely open area, or one which suffers from an obstructed view to open skies in some areas.

VBox3i_IMU04_RLCAB120

We’ve been supplying IMU integration with the VBOX3i data logger for a few years, but now we’ve got a new Inertial Measurement Unit which has a much higher performance due to tighter GPS synchronisation, better calibration and a higher sample rate.

Integrated Speed Measurements

Firstly, the new IMU04 has improved levels of velocity smoothing to cope with interruptions to the GPS signal, thanks to a new real time Kalman Filter which blends the inertial data with that of the GPS system. Although most test tracks are located in open areas, bridges over the test track and overhanging trees can lead to signal degradation– this is where the integration with the IMU signals ensures that GPS dropouts are eliminated.

KFspeed
In this example you can see that the GPS velocity trace (blue) drops outs underneath a bridge. The IMU integrated data (red trace) gives a much cleaner and un-interrupted signal.

For instance, carrying out manoeuvres such as a coast-down test (the results of which are critical to a manufacturer from a homologation perspective, as they form a significant proportion of their calculations of fuel consumption data released to the public) can be completed on a course that takes the vehicle under a bridge, which would not have been possible using GPS alone. IMU integration ensures a consistent velocity log is maintained during the momentary loss of satellite signal lock.

Indeed, there are some facilities that are not ideal places in which to conduct other procedures such as high-dynamic brake stops – they may be tree-lined, but are still used because they’re conveniently located. Integrated Inertial and GPS measurements allow for this convenience to be fully exploited because it can provide the same level of accuracy normally only possible from testing in an entirely open arena.

KFGoogleEarth
Interrupted GPS signal. The blue trace is GPS Position only; the red trace is the corrected position data obtained via integration of the accelerometer data from the IMU and GPS signals.

This is an extract from an article originally featured in Automotive Testing Technology – for the full content click here.

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