Introduction to the BRAKEBOX
What the BRAKEBOX Can Do
The BRAKEBOX is a self-contained, exceptionally accurate braking performance testing system. It can be fitted in almost any vehicle in minutes, essentially requiring only a 12V DC power source and a suitable GPS antenna mounting location, and can be operated by either engineers or technicians with a minimum of training through a simple and intuitive menu system.
The BRAKEBOX measures time, speed, distance and acceleration during short duration tests, which will normally be braking tests. Tests can be configured to start either when decelerating though a set speed, or in response to a trigger input, which would typically be triggered either by a brake pedal sensor or possibly an external laser barrier. Tests can be configured to end either when a certain speed is reached, which may be zero, or in response to a trigger input, which would typically be a laser barrier.
The BRAKEBOX uses an extremely accurate GPS engine, described in more detail in the next section, and as a result, accuracies of about 2 to 3cm in the distance measurement for a braking test from 100mph would be typical. The BRAKEBOX is also suitable for use in coast-down tests. Due to the nature of its operation, it is less suitable for long time scale tests. Race Technology stock a range of alternative data loggers using the same core GPS technology that are suited to longer tests if required.
How the BRAKEBOX Works
The BRAKEBOX uses an extremely sophisticated method to measure speed that goes well beyond the capabilities of typical “consumer” GPS units. The data broadcast by the GPS satellites is carried by a much higher frequency “carrier” wave. The carrier wave cannot be used to help calculate a more accurate absolute position, since it is impossible to know how many frequency cycles there are between the user and the satellite. However, by looking at the differences in the carrier wave cycle phase between successive measurement samples, the unit can in effect reference its new position extremely accurately to its last position, and hence calculate speed to a much greater accuracy than it can calculate position. In the event that a carrier lock cannot be obtained, the unit can also calculate speed by measuring the “doppler” effect, whereby the frequency of the transmitted data appears to shift depending of the relative speed of the transmitter and receiver. This method is not quite as accurate as the carrier method.
These highly accurate speed measurements are calculated in X, Y and Z directions, allowing the BRAKEBOX to calculate accurate path distance, heading, and either 2d or 3d speed. The distances are calculated by integrating the speeds, which leads to the stipulation that testing duration must be fairly short, since eventually error will creep in to the integration. Over the timescale of typical braking tests the method used provides unparalleled accuracy.
At rest, it can be observed that the 2d or 3d speed has noise present typically to about 0.1 or 0.2mph. At rest this noise is a bias in one direction to the speed, since regardless of the direction of the X, Y and Z velocities used, the speed is always positive. As soon the unit is moving, the noise is no longer a bias, since it then oscillates about the correct speed. In practice, the bias at zero speed makes no measurable difference to the result, since at the end of the test the vehicle is moving so slowly that the distance increment is negligible compared to the distances covered at the start of the test. The unit has an intelligent detection of zero speed built in, to ensure that the noise does not prevent the test from ending properly.