Introduction to the SPEEDBOX20
What the SPEEDBOX20 Can Do?
Key features of the SPEEDBOX20 include:
• High accuracy 200Hz speed output. This is the headline output of the unit. Inside the SPEEDBOX20 is a high precision dual axis silicon accelerometer. Readings from this unit are combined with the GPS speed output to give a 200Hz combined speed output that has both high accuracy and fast dynamic response with no interpolation.
• 20Hz GPS speed and position output. The raw GPS speed and position information is calculated and output at 20Hz.
• 2 Axis acceleration measurements. Raw longitudinal and lateral acceleration data is output at 200Hz. A 2g accelerometer is fitted as standard, which is suitable for most OEM testing and motorsport applications; a 6g option is also available.
• Wide range of output formats. Available output formats include CAN, RS232, analogue voltages and digital pulse. Custom output messages, such as NMEA over RS232, can be added on request.
• Configurable. The SPEEDBOX20 is supplied with a PC-based configuration utility that allows details of the outputs to be configured.
• Upgradeable firmware. The firmware of the SPEEDBOX20 is flash upgradeable through the serial port of a PC.
• Water resistant. The SPEEDBOX20 is housed within a machined aluminium billet casing that is water resistant to IP65 standard.
• Software and documentation. The SPEEDBOX20 is supplied with a comprehensive software and documentation CD.
What is the RTK option?
The SPEEDBOX-RTK is a factory-fit option that is available as an upgrade to the standard SPEEDBOX20. It combines all of the features of the standard SPEEDBOX20, with the following added extras:
• Yaw – this is defined as the rotation of the vehicle about its vertical axis
• Pitch – this is defined as the vehicle’s attitude being nose up or down
• Body slip angle – this is defined as the difference between the yaw and the heading
All of these RTK measurements are output at 20Hz.
Note: Whilst heading and gradient are output from the standard SPEEDBOX20, they are only available when the vehicle is moving and have no meaning when it is stationary. All of the core functions of the standard SPEEDBOX20 (including speed, acceleration, position and distance) are output with the same level of accuracy by the SPEEDBOX-RTK.
Figure 1: Difference between yaw and heading, pitch and gradient
For the angular yaw and pitch measurements, a second GPS receiver and antenna is used. A differential solution between the two GPS antennas is obtained using a technique called “Moving-base real time kinematic (MB-RTK) differencing”. This is an advanced processing technique that is fundamentally different from a normal GPS fix. It allows the position of the two antennas, relative to each other, to be determined extremely accurately (to within millimetres). Knowing the relative position of the second antenna to the first with such high precision allows yaw and pitch to be accurately calculated to within tenths of a degree. As yaw and pitch are “direct” measurements, they do not require the vehicle to be moving. The slip angle is calculated from the yaw and heading measurements once the vehicle starts to move.
The SPEEDBOX-RTK is a specialist measuring instrument. As such, there are a number of points that must be considered when using it:
• The processing techniques used require both antennas to obtain a full “carrier” GPS lock from a set of satellites common to both.
• A standard GPS solution requires four satellites. An RTK solution, however, requires a minimum of five satellites.
• As good GPS reception is so crucial in this application, the GPS antenna mounting instructions should be read and followed with special care by RTK users.
The limitations of the system must also be understood:
• A full carrier lock can often be lost during standard highway driving, especially in the presence of tall adjacent buildings, trees in leaf, bridges and other obstacles. Whereas the speed output is relatively robust to sub-optimal GPS reception, an RTK solution is not.
• A good clear view of the sky is recommended when yaw and pitch data from the RTK option is required. It is best suited to test and race tracks - where a clear view of the sky is largely guaranteed. Whilst the RTK is less suited for road testing or use in a built up, urban area, results may sometimes be obtained under these conditions, depending on the GPS reception available.
• When using the RTK option, it is possible to obtain an incorrect result under certain “unusual” conditions. Whilst this phenomenon is far more likely to happen when the vehicle is stationary and will only last for a short period of time, it must be borne in mind when interpreting the results.
• RTK works in a different way to a normal GPS fix, and its properties are somewhat different. A normal GPS lock will tend to “degrade” from good to poor, before losing a lock completely. An RTK solution behaves differently - it either has a very high accuracy lock, or no lock at all – there is very little in between.
All of the subsequent instructions for installing and operating the standard SPEEDBOX20 should be followed for the SPEEDBOX-RTK. Additional, RTK specific, notes are provided where required.