15.03.2012
For PTB's reference facility for vehicle speeds, which is based on piezoelectric pressure sensors, the measurement uncertainty budget was verified by comparisons with a high-quality GPS inertial system.
The GPS inertial system RT 3100 (made by Oxford Technical Solutions Ltd.) is a combination of a GPS system with an inertial system and was mounted into a corresponding test vehicle. Figure 1 shows the set-up of the components of the RT 3100 unit. To obtain the complete GPS inertial system, this unit must only be complemented by a precision GPS receiver.
Figure 1: The RT3000 unit of Oxford Technical Solutions Ltd. (source: DTC – Dynamic Testing & Consulting).
From the signals of the acceleration- and yaw-rate sensors and from the information of the precision GPS receiver, special signal processors determine position-relevant and driving-dynamic measurands such as, for example, speeds, positions, accelerations, etc. in real-time. The sampling frequency of the measurement data thereby amounts to 100 Hz.
In order to measure, in comparison measurements, the velocity of the test vehicle (which is equipped with the GPS inertial system) exactly in the sensor field of PTB's reference facility, joint triggering was performed via the GPS coordinates. Further details of the setting up of the reference facility and of the triggering by means of GPS coordinates can be seen in Figure 2.
Figure 2: Set-up of PTB's reference facility with the GPS coordinates (falsified data) for the joint triggering with the GPS inertial system (Source of the picture: Google Earth).
Although the measurement principles were completely independent of each other, a comparison of the results of PTB's reference facility with those of the GPS inertial system showed a standard deviation of only 0.07 km/h (see Figure 3). This result impressively confirms the specified measurement uncertainty budget of the two facilities and shows that the reference facility is very well suited to carry out high-accuracy type-approval tests. The error limits laid down in the Verification Ordinance for test objects amount to 3 km/h and correspond to 40 times the standard deviation yielded within the scope of the comparison described above.
Figure 3: Results of the comparison measurements between the GPS inertial system and PTB's reference facility.
Contact person:
Frank Märtens, Dept. 1.3, WG 1.31, e-mail: frank.maertens@ptb.de