Testing Electronic Stability Control With VBOX
With the recent introduction of mandatory fitment of ESC systems to passenger vehicles, manufacturers are faced with ever increasing investment in advanced testing equipment.
The documentation that describes test standardsFMVSS126 (US) and ECE R13H (EU) gives an indication of the type of equipment required to successfully undertake the necessary procedures. This includes such items as three axis accelerometer packs, laser or optical ride height sensors, and radar speed measurement systems, for which the current market solution is expensive.
The tests require that the steering response of the vehicle must be verified as falling within acceptable dynamic capabilities. This is done with a ‘Sine with Dwell’ manoeuvre whereby the
vehicle, travelling at 80kmh, must deviate from its original centreline path by at least 1.83 metres within 1.07 seconds of the initial steering input. The yaw rate ratios of the vehicle are also measured and compared at set time intervals, to prove that the ESC can control the heavy yaw movements without a spin being induced.
Measurement accuracy of this manoeuvre is critical, and accepted methods of capturing the data have employed not only the equipment already mentioned but also calculation based on double integration of the accelerometer data.
ESC testing being undertaken via the ‘sine with dwell’ test
The problem with double integration is that it is prone to error multiplication. If initial values aren’t extremely accurate the second order of calculations can render the data useless; consequently the accelerometer units themselves have to be of a very high specification (and therefore price) to ensure quality results.
To compensate for body roll angle the suggestion is made to use laser ride height sensors, which not only add to the overall cost but also to setup time. Finally, speed is supposedly best measured using radar which adds to the overall complexity of using several test components.
But there are alternatives. The VBOX GPS solution has major advantages before cost is even considered, simply due to the fact that less equipment is required and there is very much less to do in terms of installation. The 100Hz VBOX3i, coupled with an IMU, provides highly accurate speed, distance, heading, and yaw rate measurement at significantly lower cost than the combination of accelerometers, ride height sensors and radar. Additionally, the margin for error is lower when using VBOX as only a single integration is required to accurately measure lateral displacement, thanks to the very accurate speed and heading measurements provided.
Left: 100Hz VBOX3i and IMU provide a stable base for the ABD SR60 steering robot, Right: Complete technical setup, as fitted to a honda civic type-R
Taking readings from an antenna on the roof means that there is a need to compensate for vehicle roll – solved by mounting the VBOX inertial unit at the car’s centre of gravity and providing an automated calculation without the need for lots of post processing work with ride height data. This is where the Anthony Best Dynamics steering robot comes into the equation.
Racelogic have teamed up with Anthony Best Dynamics on several occasions and in this instance it is to provide a comprehensive solution for engineers to carry out these tests, which requires precise and repeatable control; the high torque and steering wheel speed required to perform the test means that a robot such as the ABD SR60 is essential. ABD take the VBOX CAN output for speed, heading, yaw and roll rate at 100Hz and feed it into the robot which conducts and guides the whole test. Firstly the car is ‘characterised’, by determining the necessary steering angle to achieve the required amount of lateral g (known as a ‘Slowly Increasing Steer’ procedure). The actual Sine with Dwell test is then performed with increasing yaw and displacement, on both right and left turns.
The ABD software combines the VBOX data and that from the robot, and produces an easily interpreted set of results that clearly show the level of displacement the car has achieved, at what steering angle, and over what period of time. This software also carries out calculations to compare yaw rate ratios at later phases of the Sine with Dwell test, which form the main body of evidence as to whether or not the ESC system passes, fails, or exceeds the regulation criteria. The combination of VBOX and ABD equipment ensures that overall test times, installation and post processing is reduced when compared to suggested setups.
The ABD/VBOX setup has been benchmarked against two other systems: a high expense accelerometer solution, and a Differential GPS Real Time Kinematic setup that gives 2cm positional accuracy.
The RTK DGPS solution is the ultimate in terms of measuring positional deviation, and under these particular test conditions where data is being used over very short time periods (1.07 seconds) the positional accuracy is a matter of millimetres. Consequentially the lateral displacement of the vehicle measured with the RTK DGPS data can be considered a reference, but a system like this is also very expensive, so not a suggested solution for every day ESC testing.
The benchmarking results prove that the VBOX GPS solution aligns with the reference measurements and also with that of the suggested equipment, and therefore meets the demands of the US and EU governing bodies. The difference is that with VBOX, the initial setup and outlay are much more palatable, no matter what size of organisation is using it.