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BRAKEBOX / Accuracy

Overall system accuracy

The accuracy of the BRAKEBOX is typically 2cm. The following analysis explains the sources of these errors.

There are 3 factors that determine the accuracy of a brake measurement system:

  • Accuracy of the velocity measurements. The GPS system in the BRAKEBOX has a typical velocity bias error of about 0.01kph and a sample to sample noise level of about 0.1kph . This is currently as good as current GPS technology permits.
  • Latency of the speed measurements. The BRAKEBOX accounts for all speed latency effects and cancels them out to an accuracy of 6uS. Assuming that the vehicle is accelerating at 1g, this is equivalent maximum error of 0.0002kph.
  • Defining the exact start point of braking. The time that the braking measurement begins is to within 8uS. Assuming that the vehicle is accelerating at 1g, this is equivalent maximum error of 0.0003kph.

A typical error analysis for a typical 100kph to stop braking test assuming an average deceleration of 0.8g, lasting approximately 3.5 seconds is:

Error SourceCalculationError in braking distance
Error due to speed inaccuracies*0.01kph x 3.5s=9.7mm
Error due to speed latency25mS x 0.24m/s x 0.5=3mm
Error due to latency in defining braking point8uS x 27.8m/s=0.2mm
Total error bands 13mm
  • In practice since the noise on the speed measurements is Gaussian this will cancel out over a practical braking test lasting over a second and can therefore be ignored in the error analysis.
Page last modified on October 02, 2007, at 10:34 AM