Changing mobility and speed: vehicle dynamics measurement technology – its origins and ongoing development

Winterthur, December 2020 – Progress and mobility result in greater speed and higher efficiency. Recent decades have seen enormous advances in the precise acquisition of vehicle dynamics measurands, and this technology will continue to be a key factor in vehicle optimization as time goes on. We take a look into the past and the future – based on the example of Correvit sensor technology from Kistler.

Movements of people and goods are the backbone of modern societies, and they play an essential part in cultural and economic relations. The enormous increase in speed and mobility brought about by railways and automobiles is a major driver of growth and prosperity. The critical goal for vehicle developers is greater efficiency: and that means higher speed and lower consumption of resources. But at the same time, the global increase in traffic volumes – especially in conurbations – suggests that stricter control or even automation of traffic is likely to yield efficiency gains as compared to the individual modes of transport that are prevalent today.

How are these developments impacting vehicle measurement technology? Using the Correvit family of sensors from Kistler as an example, this article describes past and future changes in the scope of application for multidimensional position and speed measurement. The origins of contact-free and slip-free measurements of vehicle speed in relation to the carriageway date back almost 30 years. The principle of correlation – an innovative development in its day – made it possible to perform previously unimaginable measurements, which have mainly been used for full-vehicle dynamics characterizations. Then as now, the focus is on exploring the physical limits for agility, performance and steering, as requirements for active vehicle safety have continued to increase.

Optimizing tire performance

As user experience was accumulated and systems became more reliable, tire testing began to develop as an application area. In addition to the use of measurement technology to determine vehicle trajectory and kinematics, the dynamic transmission function of individual wheels or tires is a focus of attention here. A sensor head can now be attached directly to the wheel rim of the test car to supply data that extend the previous (grip) limits, thus allowing higher speeds. Again, one of the critical factors here is direct, slip-free measurement of the movement between rim and carriageway.

Tire performance is a highly important factor – especially in motorsport. It's not unusual for grip in the relevant temperature window and incipient tire wear to decide the results of a race. Here too, precise characterization – in advance – of the kinematic system properties is essential, and this has now become an established feature of the racing calendar. The state-of-the-art also includes derivation of physical parameters for virtual tire models, based on test results. Aided by appropriate calibration measurements, high-performance simulation tools open up new possibilities for comparing chassis and tire setups, and for optimizing lap time and racing strategy. It has now become impossible to win a professional race without accurate knowledge of the vehicle's transverse speed (or side-slip angle) and of the slip on individual tires – and this is precisely the information that Correvit sensors such as the new SF-Motion can supply.

From test drivers to test passengers

Application examples have concentrated heavily on the driver's experience in the past. But now, an entirely new chapter is beginning as the result of rapid developments in the field of (semi-)automated and autonomous driving. Different criteria and vehicle parameters are gaining importance: ride comfort and stress levels are attracting more interest alongside agility and steering feel. Extensive studies of test subjects focusing on aspects such as kinetosis (motion or travel sickness) are replacing the conventional performance-based approach that includes questionnaires for test drivers. There is evidently a general shift of emphasis away from drivers to passengers, all the more so as each group can have completely different subjective perceptions of the same objective event (such as a driving maneuver).

Developers are again turning their attention to objective measurements of dynamic driving characteristics, and functional safeguarding of advanced driver assistance systems (ADAS). Knowledge of the vehicle's current position, or the position it will assume in the immediate future, provides the most important basis for driver decisions that are increasingly supported by mechanical algorithms. Suitable sensors and computing systems must reliably map what the human eye perceives, and what the brain processes in real time during driving. Measurement errors or deviations can lead to fatal consequences for the vehicle occupants and others in the immediate vicinity.

Alongside accuracy, trust in vehicle dynamics measurement data is emerging as a new factor that also affects the choice of reference. Satellite- or horizon-based measuring systems need precise cartographic documentation of the driving environment, or they require an additional fixed reference. On the other hand, slip-free measurement relative to the road surface, as implemented with Correvit sensors, provides a direct reference to what is known as the 'ground truth'. The ideal would be to use a combination of both measurement principles so as to improve accuracy as well as the basis of trust. This goal is achieved by the S-Motion family of sensors from Kistler, which already implement the advantages of this kind of 'sensor fusion' in order to achieve substantially improved signal quality.

Taking to the air – but doing it safely

Going beyond road-based speed measurement, the next challenge involves a leap into the third dimension: robotized air taxis and autonomous drones could do much to alleviate the increasing shortage of space and reduce traffic jams that currently stretch for kilometers. For these applications, precise control based on exact spatial coordinates is essential – yet again throwing up entirely new questions for measurement technology. Nevertheless, these means of transport can also be tracked with the multi-sensor approach selected by Kistler. But for airborne vehicles of these types, the overall economical/ecological balance is still a major obstacle that must be overcome before such innovations can advance beyond the concept or prototype phases. Until the necessary conditions are met, Correvit-based speed measurement continues to be the standard reference for all vehicle tests.