is now the technology used almost universally where accurate measurements of vehicle speed and distance travelled are required. GPS has largely replaced 5th wheel and optical technology over the past decade, offering higher accuracy, easier installation, lower latency and lower running costs compared with the legacy technologies.
What is unique about the SPEEDBOX2 product range is that it uses the most advanced GPS technology available and combines this information with accelerometer data using a Kalman filter. The result is a system which outperforms GPS-only sensors in every regard. Accuracy, latency and signal to noise ratio are all dramatically improved. Output accuracy is also maintained even during temporary GPS signal outages.
SPEEDBOX2 connectivity is highly flexible and includes four analogue outputs, four analogue inputs, CAN output, two RS232 outputs and a USB port for connection to a PC. A configurable pulse output is also included, which can be used to replicate the output of most 5th wheel sensors. The SPEEDBOX2 has an expansion port for an optional very high accuracy external inertial measurement unit (in addition to the internal accelerometers used in the SPEEDBOX2 base unit). The new SPEEDBOX2 also has new advanced very high speed triggering capabilities and uses high quality industrial connectors.
Software support for the SPEEDBOX2 is also greatly improved with the introduction of the new Performance Monitor software, which allows braking, acceleration and speed tests to be performed and analysed interactively in conjunction with a laptop computer.
The core GPS speed measurement technology of the SPEEDBOX2 is an evolution of that used in the original SPEEDBOX – the Race Technology PurePhase GPS system. The PurePhase system has been designed from the ground up for vehicle testing and is intensively optimised for this application. All ‘off the shelf’ commercial GPS systems focus on getting the best positional accuracy; in contrast the PurePhase system not only calculates high accuracy positional information, but also separately calculates velocities using advanced signal processing techniques.
The SPEEDBOX2 combines 20Hz GPS data with the accelerometer information to calculate speed and distance at a true 200Hz with no interpolation. This method of combining the data streams takes advantage of the best features of both measurement types – without any of the drawbacks of either. The accelerometer has lower noise, lower latency and higher bandwidth than the GPS receiver, and the GPS receiver has a much higher absolute accuracy than integrated acceleration alone. Therefore the SPEEDBOX provides higher accuracy and great signal robustness than either accelerometer or GPS-only sensors.
One very common question concerns the choice of 20Hz as the sampling rate for the GPS receiver - why not use a higher sampling rate, since faster is generally perceived as better? There are two very good reasons: Firstly, a typical road car has a resonance at around 3Hz, so at 20Hz the sampling rate is plenty high enough to ensure that no dynamics are missed. Secondly, whilst there is no fundamental limit to how fast GPS can be sampled, measurement noise increases linearly with sampling frequency so high speed sampling is undesirable. The noise of a 20Hz GPS receiver is typically one fifth that of a 100Hz receiver.
The resulting outputs from the SPEEDBOX2 offer class-leading accuracy. Distance measurements obtained by integrating the speed over a set course show excellent accuracy – typically a few centimetres over distances of many tens or even hundreds of meters. The accuracy of the distance measurement from a 100mph brake test to a full stop is typically in the range of 2-3cm.