3D contact patch measurement inside rolling tyres

Highlights

• Dynamic contact patch measurement inside rolling tyres.

• Mechanical stabilising system for constant observation of contact area.

• Full 3D field stereo computer vision deformation measurement.

• Large deformed contact area is made available as a 3D point cloud.

• Has the potential to provide insight into never before measured tyre dynamics.

Abstract

This paper presents a novel method for capturing the 3D profile of the inside of a rolling off-road vehicle tyre at the tyre-road contact region. This method captures the contact region at all times as the vehicle negotiates obstacles. The system uses a pair of inexpensive digital cameras (capable of capturing up to 300 frames per second) and features a purely mechanical stabilisation system to ensure that the cameras capture the contact region at any wheel speed or vehicle acceleration.

The captured images are processed using 3D computer vision techniques using an open source computer vision library called OpenCV. Stereo image pairs are used to create clouds of 3D points showing the profile of the inside surface with good accuracy. Various obstacles were traversed with the deformed tyre profile being compared to the undeformed profile. The system improves on current measurement techniques used to measure the contact patch by capturing a large region of the contact patch, providing full 3D surface geometry, as well as remaining centred on the contact patch irrespective of wheel rotation. The system also enables other imaging techniques to be used such as digital image correlation to determine velocity profiles as well as strain measurements.

Introduction

The tyre contact patch is an important region in both tyre and terramechanics research. Tyre deformation captures the force input to the vehicle. Thus, a better measurement of the deformation of the contact patch will provide more information which can be used to improve tyre modelling, model parameterisation and validation. For tyre model parameterisation the tyre rolling radius is also required to estimate the tyre longitudinal slip. On hard, flat terrain the assumption of a constant tyre roll radius is often made. In Miller et al. (2001), the rolling radius is estimated using the wheel speed and GPS speed measurement. However, this technique can only be performed under the assumption of no slip and this assumption is only valid when the vehicle is not braking or accelerating. Thus, the rolling radius is assumed constant during the duration of a braking or accelerating test, however due to load transfer the rolling radius changes which can lead to incorrect slip measurements. The modelling of tyres over soft soil also requires the knowledge of the deformation of the soil which is generally inferred from measuring the soil before and after being driven over. Measurement of the soil deformation while the wheel is in contact with the soil can also lead to better tyre-terrain interaction and soil models. The soil deformation, during tyre contact, could be determined by measurements of the un-deformed soil and the tyre deformation.The size and shape of the deformed region is difficult to measure under dynamic conditions as the region is not directly observable from outside the tyre. Numerous attempts have been made to measure deformation from inside the tyre using a variety of different strategies, however these attempts have been limited to either measuring single points or very small areas.

The simplest method in use is through direct observation by allowing the tyre to roll over a sheet of glass and recording images of the resultant deformation (Potenger, 2006) which, while successful, is limited in terms of attainable speeds, surface roughness, friction coefficient and vehicle manoeuvring. Other researchers have attempted to solve these problems by implementing tyre sensor systems which record the deformation on the inner surface of the tyre. The technique also only measures the size of the contact patch on a flat surface and has limited use on real terrain. Positioning a sensor on the inside of the tyre allows direct measurement of tyre deformation over varied terrain and a wider range of vehicle manoeuvres. However most techniques implemented thus far have the limitation that the sensors measured a very small area or even a single point.

Several examples of these small area measurement systems are found in literature. Magori et al. (1998) developed an ultrasonic sensor for vertical deflection measurement. Pohl et al. (1999), used surface acoustic wave sensors that was capable of measuring lateral and longitudinal deflection of a single tread element. An optical sensor, utilising a Light Emitting Diode (LED) positioned on the tyre surface allowed measurement of vertical and lateral deflection (Tuononen, 2009). A flexible capacitive sensor was developed by Matsuzaki and Todoroki (2007) which measured the strain in a small area of the tyre carcass. Xiong and Tuononen (2015) created a system which uses a line scan laser profilometer inside the tire to capture the inner profile of sidewall and tread. This system was successful at accurately measuring the inner profile and aimed to use the tread deformation to measure rolling resistance. These systems all suffer from the drawback of only having a useful sample data once per revolution as the sensors are fixed to either the tyre carcass or the inside of the rim.

An alternative was developed by Sandu et al. (2012) who used multiple infrared emitting diodes positioned around the inner surface of the tyre to gain a more complete picture of the deformation when tested in a terramechanics rig. The first true attempt to utilise a camera as sensing equipment was implemented by Hiraoka et al. (2009) to measure in-plane strain and out of plane displacement in a tyre. This principle was slightly altered to improve results by Matsuzaki et al. (2010) to provide improved surface features for image correlation. Another application of cameras for tyre measurement is found in Green (2011) where a single camera, along with a random speckle pattern, is used to measure deformation on a larger area. These methods, while proving the viability of cameras as a measurement technique, fail to capture the contact patch continuously as the tyre rotates.

This paper describes the design, implementation, and testing of a system which allows the entire deformed area inside a tyre to be measured using a pair of inexpensive, off-the-shelf digital cameras. The cameras capture images which are processed with a dense stereo correspondence algorithm to produce three dimensional (3D) data points representing the inner surface of the tyre. The system includes a mechanism to ensure that the cameras observe the deformed area at all times, lights to illuminate the area and a data acquisition system to record the images.

The system is a proof of concept design which aims to investigate the viability of using stereo cameras inside a tyre as a measurement technique for dynamic deformation, as well as to develop the experience and algorithms required to extract useful data from the raw camera images. Additionally the study aims to produce data which is currently unavailable to researchers in the vehicle dynamics and terramechanics communities and discover paths for future research. The system can not only be used to determine the shape of the tyre deformation but also may enable the measurement of the longitudinal or lateral deformation during traction or turning manoeuvres and the contact velocity profile from within the tyre using various DIC algorithms.