Brake Testing with VBOX
Vehicle and tyre manufacturers are constantly looking to reduce braking distances, speed up response times and improve stability.
Improvements in braking systems and tyre compounds are getting smaller each year, meaning it has become increasingly important to measure braking distances accurately.
Why use VBOX to perform Brake Tests?
VBOX is a powerful data acquisition system used by almost every vehicle and tyre manufacturers to measure the speed and position of a moving vehicle. It uses GPS signals to provide engineers with up to ±2cm positional accuracy, allowing them to plot even the smallest improvements in performance.
It has the ability to be connected to a brake trigger, which uses GPS time to monitor exactly when pressure is applied to the pedal, and allows for test data to be viewed quickly and easily using a laptop or tablet PC – reducing the time needed to monitor results.
GPS Position relies on precise measurements of the distance from the receiver to the satellite, and therefore suffers from numerous effects which can reduce the quality of the signal. These include atmospheric effects which delay the signals from the satellites and reflections from nearby objects such as buildings which introduce multipath, again adding to the length of the signal.
Luckily however, GPS Velocity can be measured using a different method which measures the change in signal from the satellite, or Doppler Effect. By measuring this change, the errors which normally affect GPS have very little influence over the quality of the signal, and the resultant velocity measurement is phenomenally accurate. Another great aspect of GPS is that all satellites have atomic clocks on-board, and by utilising this signal, the timing remains stable to within less than a millionth of a second.
Therefore, instead of using position to measure distance, the accurate Doppler derived velocity is integrated using the precise time signal to derive distance. The result of the combination of these two is an extraordinarily accurate distance measurement.
How is this verified?
There are a number of tests which we have carried out over the years to verify and improve our measurement algorithms for applications in vehicle testing. One such test is to place two reflective strips on the road at a known distance apart. Using a laser sensor connected to the trigger input of a VBOX, the vehicle is then driven between these two points a number of times, and the distances compared. In such tests, the VBOX 3i will always be within 3cm in 1000m, which is about the same as the measurement uncertainty due to the slight deviation of the vehicle during the driving.
Braking distance verification
A very common test in the development of braking systems and tyres is the braking distance test. As tyres and braking systems have improved over the years, the need for a more precise method of measuring braking distance has increased. This distance is tricky to measure as it requires capturing the exact point the brake pedal is triggered and relating this to the start of the braking distance. The vehicle is moving quite fast at this point, (~28 metres per second for 100km/h), so any error in this measurement can have a significant effect on the end result. The VBOX 3i overcomes any such concerns by using a technique borrowed from photogrammetry, where the input from the brake trigger is timed to an accuracy of 50 nanoseconds.
Using this method, a very precise trigger to stationary distance is measured. In the old days, they used to have a chalk gun attached to the brake pedal trigger, and then measure the distance from where the car came to a stop from where the chalk mark was on the road!
When position is an important factor, the VBOX 3i can utilise the growing number of Glonass satellites (Russian equivalent of GPS – currently at 21 satellites) to provide an RTK (Real Time Kinematic) solution which gives ±2cm positional accuracy.
As this tests starts when the pedal is pressed, it measures both the response of the braking system, as well as the performance of the tyre, brake pads, discs and other components.
These tests are useful for analyzing the performance of tyres due to the consistency of braking distances recorded.
This involves both sides of the vehicle running on dis-similar surfaces to determine how the braking system copes with different levels of traction. This tests also allows engineers to analyse the stability of the vehicle.
Brake and turn
This comprises of the vehicle performing a brake test during a turn (i.e. not in a straight line). This helps engineers to determine the stability of the vehicle during such a manoeuvre.
For any further information or some advice on which system will suit your application, please contact us