With the growing acceptance of performance-based design (largely based on structural displacements), the need for accurate absolute displacement sensing has grown. Sohn, in Sensor Technologies for Civil Infrastructures, 2014 1.6.2 New sensors for displacement measurementsĬurrent transducers typically measure relative displacement over short distances (e.g., linear variable displacement transducers). GPS sensor systems can now provide accurate measurement of vehicle position and speed, and because of the small size and convenient packaging GPS technology is very suitable for vehicle testing and is becoming widely used. Commercial systems are available that are generally unaffected by the physical nature of the road surface, although wet road conditions can be challenging. Optical sensor systems are extensively used, for example based on an optical sensor reflecting off the road surface. It can be mounted on to the test vehicle on a suspension system to maintain ground contact at all times throughout the braking event and the distance is calculated from the rotation of the wheel. This is a separate lightweight wheel of known diameter but should not be confused with the term ‘fifth wheel’ applied to the coupling between the tractor and semi-trailer of an articulated commercial vehicle, see Chapter 4. A more accurate measure of vehicle stopping distance and also speed can be obtained from a ‘fifth wheel’. For long braking distances an accurate measurement of distance travelled can easily be achieved by marking the beginning and the end of the braking event by marks on the road, but this is seldom practical nowadays. for legislative testing, and if distance can be recorded on a time base, speed can also be determined. Such transducers need to be robust to heat, temperature and mechanical vibration because they have to be located close to the disc surface, and are more suited to laboratory dynamometer use rather than on-vehicle measurement.ĭistance travelled is an important parameter in determining braking system performance, e.g. those that utilise capacitative principles, are useful for this type of small displacement measurement. These are important parameters in brake-induced vibrations, particularly judder, as explained in Chapter 10, and non-contacting displacement transducers, e.g. the concentricity and ‘ovalising’ of brake drums and ‘coning’, ‘runout’’ and thickness variation of brake discs. Very small displacements need to be measured to assess circumferential variations in rotational precision, e.g. Linear variable displacement transducers (LVDT) are used to measure large displacements such as brake pedal travel or pneumatic actuator displacement, and an alternative is a ‘string potentiometer’ where a linear displacement is converted to a rotary displacement for measurement and can be easier to fit into confined spaces. Andrew Day, in Braking of Road Vehicles, 2014 Displacement and Distanceĭisplacement measurement is required to investigate and confirm system actuation.