An Investigation of Anti-lock Braking Systems for Heavy Goods Vehicles

An articulated lorry was instrumented in order to measure its performance in straight-line braking. The trailer was fitted with two interchangeable tandem axle sub–chassis, one with an air suspension and the other with a steel monoleaf 4–spring suspension. The brakes were only applied to the trailer axles, which were fitted with anti–lock braking systems (ABS), with the brake torque controlled in response to anticpated locking of the leading axle of the tandem. The vehicle with the air suspension was observed to have significantly better braking performance than the steel suspension, and to generate smaller inter–axle load transfer and smaller vertical dynamic tyre forces. Computer models of the two suspensions were developed, including their brakes and anti– lock systems. The models were found to reproduce most of the important features of the experimental results. It was concluded that the poor braking performance of the steel 4– spring suspension was mainly due to interaction between the ABS and inter–axle load transfer effects. The effect of road roughness was investigated, and it was found that vehicle stopping distances can increase significantly with increasing road roughness. Two alternative anti–lock braking control strategies were simulated. It was found that independent sensing and actuation of the ABS system on each wheel greatly reduced the difference in stopping distances between the air and steel suspensions. A control strategy based on limiting wheel slip was least susceptible to the effects of road roughness. INTRODUCTION Over the last few years, there has been a trend towards use of air suspensions on heavy vehicles, replacing traditional steel leaf–spring suspensions. The Commission of the European Community has actively encouraged this trend in the belief that softer suspensions, such as air, produce less road wear [1]. The differences between these suspensions in braking has not been considered in detail. It has also become mandatory, since October 1989, to fit anti-lock braking systems (ABS) to all heavy goods vehicles in the UK, and this legislation is likely to be extended following the drafting of new legislation in the EEC [2]. Robinson [3] measured the stopping distances of heavy vehicles, with and without ABS, and showed that under certain conditions the action of the ABS can extend stopping distances. However, his overall conclusion was that these small increases in stopping distance are a small cost compared with the benefit of improved controllability, and the elimination of jack-knifing. Robinson also found that using independent ABS systems on each axle in multiple–axle suspensions always gave shorter stopping distances than controlling all of the brakes with the output of an ABS sensor on a single axle. A computer simulation study of an ABS system fitted to a ‘walking-beam’ suspension was performed by Fancher et al [4]. The researchers found that the ABS could cycle at 3Hz which would ‘tune-in’ to one of the pitch modes of vibration of the vehicle, causing large oscillations, and increasing stopping distances. The simulation agreed well with the experimental results. The main objectives of the study described in this paper were: (i) To measure and analyse the differences in straight-line braking performance of two conventional UK suspensions, one with steel springs and one with air springs, both fitted with ABS; (ii) To investigate the performance of various ABS control strategies: (a) a standard commercial system with wheel-speed sensors on a single axle; (b) the same system but with a sensor on each wheel; and (c) an unconventional system controlling the longitudinal slip of each wheel. EXPERIMENTAL PROGRAMME The experiments were conducted on a long, straight section of the Transport Research Laboratory (TRL) test track during a period of three weeks in August and September 1992 (see [5] for details). The test vehicle was a four axle articulated tractor/semi–trailer combination. The tractor had two axles with steel suspensions on each, and the trailer had a detachable subchassis to which instrumented tandem axle air and steel suspensions were fitted in turn. The vehicle was tested with each subchassis to assess its straight–line braking performance under a variety of different operating conditions. The instrumentation measured the vertical and longitudinal (braking) forces applied to the braked axles, as well as the longitudinal acceleration and speed of the vehicle. The trailer was a compartmentalised fuel-oil tanker, and the unladen gross weight of the combination was approximately 13 tonnes. For the laden tests, a gross weight of 31 tonnes was achieved by completely filling four of the seven compartments in the tanker with water. The trailer suspensions were: (i) a Crane Fruehauf air suspension with Rubery Owen–Rockwell axles, and (ii) a Crane Fruehauf monoleaf wide spread 4–spring suspension, with Crane Fruehauf axles. Both suspensions were tested with the same set of Dual 11R22.5 tyres, and both subchassis were equipped with identical ‘Skidchek MGX’ anti–lock braking systems manufactured by Grau Limited. The brakes on the two suspensions were made by different manufacturers, but all brakes were fitted with new Duron P2001 linings immediately before the tests. The ABS systems measured wheel rotation on the leading axle of the tandem suspensions, and controlled the brake torque applied to both axles with the same output air pressure signal. The pneumatic system of the vehicle was modified so that the trailer brakes could be applied independently of the tractor brakes, by a test engineer in the driver’s cab. The system was designed so that normal braking action by the driver would over-ride the test and cause all brakes to be applied if necessary. The brakes were only applied to the trailer axles, so as to emphasise any suspension differences that would otherwise be masked by the behaviour of the tractor. The instrumentation and experimental procedure are described in [6]. All of the braking tests presented in this paper were conducted from an initial vehicle speed of 18 m/s (40 mph). This speed was chosen so that the trailer brakes would dissipate approximately the same amount of energy as they would under normal operation, with all vehicle brakes operating, when stopping from 27 m/s (60 mph). (This assumes the trailer contributes