SPEEEDBOX-INS Speed and distance accuracy
To validate the speed and distance of the SPEEDBOX-INS
This test measures the combined accuracy of the speed measurement and the pulse output. Measuring the speed accuracy of the SPEEDBOX-INS by comparing the results to those of a reference speed measurement technology was not possible, to do so would require a technology which has proven accuracy much higher than that claimed by the SPEEDBOX-INS, and nothing is available which can provide this. As such, validation of the speed has to be done indirectly. For the test, two reflective barriers were placed approximately 40 meters apart along a straight section of the test track. A high speed combined laser emitter/detector unit was mounted on the vehicle to shine out of the vehicle at 90° to the direction of travel, as illustrated below. As the beam hits each of the reflective barriers the laser sensor generates a pulse; producing a pulse at the start and end of the test distance.
Laser and reflective “optical” barrier installation
The test system was configured to count the number of pulses output by the SPEEDBOX-INS between the two reflective barriers.
The test consists of driving the test vehicle at a variety of speeds and whilst accelerating and decelerating past the barriers.
If the test vehicle follows an identical path between the barriers and no errors are present in the SPEEDBOX-INS output, the number of pulses counted between the barriers will be identical for every test.
If a scaling error exists on the SPEEDBOX-INS output, the error in the number of pulses counted will vary in proportion to average speed between the barriers.
If significant output latency exists in the SPEEDBOX-INS output, the number of pulses counted will rise or fall depending on the differences in speed between the entrance and exit barriers. The error from latency is approximately 0.44mm / ms latency / mph speed difference between start and end of test, neglecting 2nd order effects. A brake test from 60mph to 0mph with a latency of 10ms would give a distance error of approximately 0.26m
If the SPEEDBOX-INS was fundamentally inaccurate or inconsistent the pulse count would vary in other manners depending on the cause of the problem.
This type of test is exceptionally challenging for any speed measurement device – any deficiencies in the SPEEDBOX GPS, the GPS/accelerometer combining, or the output stages would be very clearly visible. In practice this test is even more demanding than a braking distance test and more clearly highlights any latency problems.
The test was performed 10 times, driving through the barriers at different speeds, braking, accelerating and keeping constant speeds. The results in the graph below show the difference in cm between the actual distance and the SPEEDBOX-INS measured distance against the average g-force for each run. As explained above these results validate the SPEEDBOX-INS speed and distance measurement accuracy.
Above: SPEEDBOX-INS speed and distance measurement deviation
Low Speed Tests
Low speed tests provide the severest challenge to the accuracy of non-contact speed measurement systems. Errors often show up as high noise when stationary. Many manufacturers remove this noise by implementing a crude zero-clamp on the output, so it is impossible to see any data below 0.5m/s, for example. This is a particular problem with all GPS only speed sensing systems as speed errors at rest are normally significantly higher than speed errors when moving (the reason for this is that noise on the speed output becomes a measurement error at speeds below the noise threshold, since a positive scalar speed value is always output).
The graph below shows a stationary speed output comparison between a competitor’s 100Hz GPS Only system, against the 200Hz SPEEDBOX-INS
Above: Stationary noise comparison between competitor’s GPS Only system and SPEEDBOX-INS
The SPEEDBOX-INS combines both GPS and inertial data to produce highly accurate speed measurements even at low speeds; this is achieved by combining the GPS data with the inertial data from the built in IMU.
There are three key advantages to combining GPS data and inertial data from the IMU:
- Faster update rates. GPS can be sampled at very high frequencies (there is no theoretical upper limit). However, the noise on the measurement increases along with the sample rate. In a perfect GPS environment up to 100Hz is possible, but at these high sample rates even driving past a tree can cause accuracy problems. In practical situations 20Hz is normally considered a good balance between update rate and low noise. In contrast, inertial data can be sampled at >1kHz without encountering noise inaccuracies.
- If the GPS signal is lost for a short period of time the measured accelerations are used to “fill in the gaps”. For example, driving under a bridge will stop a GPS receiver working for a few seconds, however no error would be seen on the SPEEDBOX-INS output as the speed data would be filled in by inertial data.
- Combining the accelerometer data and the GPS data also has the effect of reducing the noise on the GPS speed signal whilst testing in imperfect GPS conditions. These imperfect conditions could be the result of driving past trees or intermittent lock from some satellites becoming obscured by buildings etc.
GPS Stationary Positional Accuracy
The horizontal positional accuracy of the SPEEDBOX-INS is quite typical for a unit in its class, with accuracy to within 3m being obtained under most conditions. Position is output at 200Hz and uses the very latest processing techniques to eliminate jumps in position when the satellite constellation changes, whilst avoiding any lag in positional update when the unit moves. The example given shows the positional ‘walk’ whilst the unit was kept stationary for 30 minutes, using an antenna mounted with a clear, unobstructed view of the sky. In this example the positional accuracy is much better than the typical quoted 3m accuracy.
The vertical positional (altitude) accuracy is typically only half as accurate as the horizontal positional accuracy, this is entirely due to the geometry of the satellites visible from the antenna and affects all GPS systems. From any point on the surface of the Earth, there is a maximum spread of visible satellites of approximately 180° in the horizontal plane, but only 90° in the vertical plane.
Above: SPEEDBOX-INS stationary position trace for 30 minutes