06.10.2011
The Working Group "Speed Measuring Instruments" is equipped with several reference facilities which are used to test speed-monitoring devices within the scope of the type-approval certificate of these instruments. The objects to be tested have to comply with the maximum permissible errors in service laid down in the Verification Ordinance (i.e. ± 3 km/h in the case of measured values of up to 100 km/h, and ± 3 % of the measured value in the case of values exceeding 100 km/h) with a very high confidence level. A reference facility is already installed in the direct vicinity of PTB, along a road where the speed is limited to 70 km/h. Another one is located along a federal motorway, on a section with no speed limit.
Figure 1 shows these two points of measurement. The maximum speed at which the reference facilities can detect the speed of vehicles with sufficient precision is 300 km/h.
Fig. 1: Reference facilities (left: A road; right: federal motorway – the field that is highlighted in grey characterises the measuring range)
The reference facilities measure speed based on the principle of displacement/time measurement:
For both lanes, the reference facility relies on 4 piezoelectric pressure sensors each which are built-in the road at a right angle to the direction of travel at a distance of 6 m from each other. The arrangement of the pressure sensors is selected separately for each lane, it is therefore possible to perform independent speed measurements. Figure 2 describes the schematic arrangement of the reference facility (the pressure sensors are represented in a different colour depending on the lane they are assigned to: green for lane 1 and red for lane 2). When a vehicle passes the facility, the pressure sensors provide pulse-shaped signals. According to the principle of the displacement/time measurement, three individual speed values v1, v2 und v3 are calculated for each lane from the time intervals of each signal and from the (known) distance between the sensors. A reference value v is formed and used as a valid result only when the maximum deviation of the three individual values from the mean value does not exceed a pre-defined error limit.
Fig. 2: Schematic representation of the arrangement of the reference facilities
Now, we will describe in detail how comparison measurements are performed, taking the example of a hand-held laser measuring instrument as a test object (Figure 3). Each measured value of the test object (if necessary with data concerning the lane and the direction of travel, etc.) is transmitted via an interface (possibly wireless) to the reference facility immediately after the measurement has taken place; there, it is plotted with the corresponding speed value measured by PTB's reference facility.
In addition, the speed values of the test object and of PTB's reference facility are also inserted into a video recording for a full documentation of the measuring situation.
Fig. 3: Arrangement of the testing facility, taking the example of a handheld laser measuring instrument
The results of such a comparison measurement are plotted in the form of a so-called "histogram" (Figure 4). This is a graphic representation of a frequency distribution. It represents the size of the deviations of the speed value of the test object as a function of the speed values measured by the reference facility. In addition, the histogram also shows the legally prescribed maximum permissible errors in the form of red lines. In order to obtain the type approval for verification, the test object may by no means exceed the maximum permissible errors – not even in a single case. The example shows the result of a comparison measurement consisting of 1255 individual measurements in total. The results show that the test object complies with an extremely high confidence level legally with the prescribed maximum permissible errors.
Fig. 4: Histogram representation of a comparison measurement
Contact person:
Frank Märtens, Dept. 1.3, WG 1.31, e-mail: frank.maertens@ptb.de