Improving our roads with sensors
07 October 2016
Roads are an extremely important part of our lives’ infrastructure. Without them we couldn’t easily travel from A to B, be that for work, pleasure or to keep food in our supermarkets and the wheels of industry turning.
The latest figures released in January 2016 showed that the number of cars alone on British roads had increased by 600,000 in the previous year. Designed, built and used for many purposes, roads need to be maintained until they reach a position when they have to be replaced.
Engineers are involved at every stage of a road’s life cycle design, construction and maintenance to those involved in looking at how vehicles and particularly tyres interface with the roads and cause their engineered characteristics to gradually degrade to an unacceptable condition.
This article looks at how, using pressure sensors, the engineers map the points of contact that tyres make with the road surface and illustrates some recent research relating to measuring this interface from the perspective of the surface.
Measuring the tyre/surface interface
The tyre/surface interface affects many road and runway properties from friction, noise, rolling resistance to how dynamic load is transferred down through the constructed layers. These interface properties are of interest to engineers and material specialists from the vehicle, tyre and road construction sectors.
Interface properties are important as they can provide vital information on not only the state of the road such as wear and tear, potholes, degeneration but also the characteristics of the road surfaces such as skid resistance, noise generation and rolling resistance. This information can be used, for example, as a way of predicting the likelihood of accidents due to speed or skidding in either wet or dry conditions.
So, how does a pneumatic tyre interact with a surface? The simplest way to look at this is to paint the tyre, load it onto cardboard and collect data such as gross contact area, groove area, contact length and width. The paint patch tends to be circular at higher tyre inflation pressures and lower load; and elliptical at lower tyre inflation pressures and higher load. Similar relationships between inflation pressure, load, contact length and width apply for different types of tyre. With regard to road design, this circular contact patch still forms the basis of how a tyre is assumed to interface with a road surface and transfer of load down through the different layers in the construction.
Although paint can show the contact patch of an under-inflated tyre to be different to an over-inflated tyre it cannot quantify the actual values and distribution of contact stress distributed throughout the patch.
Greater detail can be obtained using pressure mapping.
Figure 1 compares the contact patch for two tyres fitted to the same vehicle with the same loading and inflation pressures on a smooth surface. Although the contact areas are comparable, the distribution of vertical stress is significantly different. The semi-slick tyre has been designed to maximise grip in dry conditions, whilst the treaded tyre has been designed to optimise both dry and wet skid resistance.
The pressure mapping system used the Xsensor X3 IX500:128.128.10 supplied by UK measurement specialists Interface Force. This is a high pressure sensor with 16,384 sensing points and has been designed with a tough urethane cover that can withstand outdoor testing for tyre applications. The sensor is bendable and can conform to different surfaces. The IX500:128.128.10 provides a combination of higher resolution and faster data acquisition rates so that it can be used in low speed dynamic tyre testing.
Measuring grip and its accuracy
There are about 20 types of devices in use around Europe. Although they all measure skid resistance, the actual measurements can differ widely for the same road surface. The three most common devices used in the UK are the pendulum tester, GripTester and SCRIM. The pendulum tester is similar to a person sliding their foot along a surface to get a feel of its grip. The pendulum tester replaces the calibrated foot with a piece of spring-loaded standard rubber attached to the end of a pendulum arm. The GripTester is a longitudinal friction device in the form of a three wheel towed trailer that measures skid resistance by simulating the interaction of a fixed slip tyre with the road surface in a longitudinal direction. The SCRIM is a transverse friction measuring device that simulates interaction between a braked tyre and the road surface as an angled tyre turns into a corner.
Measuring contact pressure distributions on surface materials
Looking at the interface between the smooth GripTester friction measuring tyre and a typical English motorway surface material, a test was able to depict the distribution of contact stress corresponding to the road surface texture. This technique can be used to quantify the contact area for different types of surface materials, their contact pressure distributions and how these parameters change with time as the surface is trafficked.
Measuring tyre deformity in laboratory conditions
The example chosen relates to the laboratory method of measuring aggregate skid resistance. The test specimen was painted and then subjected to accelerated polishing involving interaction between the aggregate particles, solid tyre, abrasive and water. The test specimen was photographed and filters used to highlight where contact took place. Three dimensional modelling was used to determine the depth of tyre interaction into the test specimen surface texture. This example highlighted that only the top 1.17mm of the aggregate particle had been subjected to accelerated testing at this interface. This raises the question should use of this method continue as a method of aggregate selection?
It is extremely important to consider the role of all engineers when looking at the infrastructure, planning, maintenance and replacement of roads. As sensor systems advance and become more flexible, then more information can be gained from using mapping systems both in the laboratory and in ‘real-life’ situations. By sharing information and working together, engineers in all disciplines can help improve our road infrastructure and keep us safely moving.
Original academic paper was printed in Engineering Integrity Society house journal.
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