R L Roebuck

Active Steering of a Tractor-semi-Trailer

This paper develops a path-following steering control strategy for an articulated heavy goods vehicle. The controller steers the axles of the semi-trailer so that its rear end follows the path of the fifth wheel coupling: for all paths and all speeds. This substantially improves low-speed manoeuvrability, off-tracking, and tyre scrubbing (wear). It also increases high-speed stability, reduces ‘rearward amplification’, and reduces the propensity to roll over in high-speed transient manoeuvres. The design of a novel experimental heavy goods vehicle with three independent hydraulically actuated steering axles is presented. The path-following controller is tested on the experimental vehicle, at low and high speeds. The field test results are compared with vehicle simulations and found to agree well. The benefits of this steering control approach are quantified. In a low-speed ‘roundabout’ manoeuvre, low-speed off-tracking was reduced by 73 per cent, from 4.25 m for a conventional vehicle to 1.15 m for the experimental vehicle; swept-path width was reduced by 2 m (28 per cent); peak scrubbing tyre forces were reduced by 83 per cent; and entry tail-swing was eliminated. In an 80 km/h lane-change manoeuvre, peak path error for the experimental vehicle was 33 per cent less than for the conventional vehicle, and rearward amplification of the trailer was 35 per cent less. Increasing the bandwidth of the steering actuators improved the high-speed dynamic performance of the vehicle, but at the expense of increased oil flow.

Factors influencing the energy consumption of road freight transport

Abstract Key factors that influence the energy consumption of heavy goods vehicles are investigated. These factors include engine efficiency, aerodynamic drag and rolling resistance, vehicle configuration (number of vehicle units), traffic congestion, speed, payload factors, and the use of regenerative braking. An accurate, validated model of the fuel consumption of a 38 tonne tractor-semitrailer vehicle is used as a basis to derive fuel consumption models of a number of other vehicle configurations. These models included a rigid four-axle truck with maximum gross vehicle mass (GVM) of 26 tonnes; a six-axle tractor semitrailer with GVM of 44 tonnes, with and without regenerative braking; a ‘B-double’ with GVM of 60 tonnes; and an ‘A-double’ with GVM of 82 tonnes. These vehicle models were driven over a simple hypothetical drive cycle with a fixed maximum speed and varying numbers of stops in a 10 km stretch of road. It is concluded that: (a) improving engine efficiency, unladen mass, rolling resistance, and aerodynamic drag can yield relatively small improvements in fuel consumption, compared with other factors; (b) larger vehicles are always significantly more energy-efficient than smaller ones when fully loaded; (c) transferring freight from articulated vehicles to smaller rigid vehicles for urban deliveries typically increases fuel consumption by approximately 35 per cent; (d) running vehicles partially loaded can increase the energy per unit freight task by up to 65 per cent; and (e) under urban start—stop conditions, the use of regenerative braking systems can reduce heavy vehicle fuel consumption by 25–35 per cent.

High-speed optimal steering of a tractor-semitrailer

A high-speed optimal trailer steering controller for a tractor–semitrailer is discussed. A linear model of a tractor–semitrailer with steered trailer axles is described, and an optimal trailer steering controller is introduced. A path-following controller is derived to minimise the path-tracking error in steady-state manoeuvres using active trailer steering. A roll stability controller is introduced by adding the lateral acceleration of trailer centre of gravity as another objective in the steering controller, so as to improve roll stability in transient manoeuvres. A strategy to switch between these two control modes is demonstrated. Simulation results show that the steering controller can ensure good path tracking of articulated vehicles in steady-state manoeuvres and improve roll stability significantly in transient manoeuvres, while maintaining the path-tracking deviation within an acceptable range. Tests with an experimental tractor–semitrailer equipped with a high-bandwidth active steering system validate the controller design and simulation results. The roll stability controller reduces the measured rearward amplification by 27%.

A systems approach to controlled heavy vehicle suspensions

This paper considers the design of a vehicle dynamics control system to control a tractor – semi-trailer vehicle fitted with active anti-roll hardware and semi-active ride suspension units. Key items of the system specification are discussed. The configuration of processors is considered, as are key issues of CPU requirements and communications. For reasons of reliability, safety, coding simplicity and modularity, a system of local controllers connected via a CAN bus to a global controller was selected. Details of this implementation, including safety precautions such as a vehicle dynamics watchdog, sensor consistency checks, localised processor watchdogs and a hardwired stop system are also discussed. Application to all areas of vehicle dynamics data collection and control is looked at.

Parameter measurement for heavy-vehicle fuel consumption modelling

A mathematical model is developed to predict the energy consumption of a heavy vehicle. It includes the important factors of heavy-vehicle energy consumption, namely engine and drivetrain performances, losses due to accessories, aerodynamic drag, rolling resistance, road gradients, and driver behaviour. Novel low-cost testing methods were developed to determine engine and drivetrain characteristics. A simple drive cycle was used to validate the model. The model is able to predict the fuel use for a 37 t tractor–semitrailer vehicle over a 4 km drive cycle within 1 per cent. This paper demonstrates that accurate and reliable vehicle benchmarking and model parameter measurement can be achieved without expensive equipment overheads, e.g. engine and chassis dynamometers.

Dynamic safety of active trailer steering systems

The dynamic safety of an active steering system for an articulated heavy goods vehicle is investigated. The vehicle is a tractor semi-trailer with two independently steerable axles on the trailer. Several different vehicle dynamics modelling approaches are used to investigate the aspects of the safety of the steering system. These include ‘back of envelope’ calculations, a single degree-of-freedom yaw model, a simplified yaw-plane model using Matlab SimMechanics, with realistic controller frequency response assumptions, and a complex multi-body model of the whole vehicle using TruckSim. Specific safety issues of concern associated with the primary active steering function are: (a) the necessary actuation bandwidth for stable response at high speeds, and (b) the performance implications of disturbance rejection requirements, e.g. side winds and split friction braking. It is found that vehicle tracking improves with increased bandwidth up to 8.3 Hz, but beyond this, performance is limited by other factors. Also, the steering system is able to reject off-tracking disturbances from side winds and split-friction braking, although the latter has a small effect. Additional ‘failsafe’ issues of concern are: (a) whether an independent centring system is necessary on each steerable axle or whether failure of an axle can be safely managed by steering the remaining axles in opposition, (b) the force levels needed in the automatic safety centring system, and (c) the maximum slew rate for centring the axles in an emergency. It is found that individual centring systems for each axle are necessary because axle ‘opposition’ is not a safe strategy for a trailer with two steered axles. The steering actuator is required to generate 32 kN during all modes of operation in order to maintain safety during the specified manoeuvre. A maximum steering slew rate of 11°/s is found to limit additional lateral acceleration to less than 0.2 g.

Implementation of active steering on longer combination vehicles for enhanced lateral performance

A steering-based controller for improving lateral performance of longer combination vehicles (LCVs) is proposed. The controller steers the axles of the towed units to regulate the time span between the driver steering and generation of tyre lateral forces at the towed units and consequently reduces the yaw rate rearward amplification (RWA) and offtracking. The open-loop effectiveness of the controller is evaluated with simulations and its closed loop or driver in the loop effectiveness is verified on a test track with a truck–dolly–semitrailer test vehicle in a series of single- and double-lane change manoeuvres. The developed controller reduces the yaw rate RWA and offtracking considerably without diminishing the manoeuvrability. Furthermore, as a byproduct, it decreases the lateral acceleration RWA moderately. The obtained safety improvements by the proposed controller can promote the use of LCVs in traffic which will result in the reduction of congestion problem as well as environmental and economic benefits.

Modelling rolling-lobe air springs

A semi-empirical mathematical model of a rolling-lobe air spring is proposed. It consists of an adiabatic gas volume, accounting for compression of air within the spring, a friction element accounting for friction between the rubber and the piston, and a velocity-dependent damping term. Tests were conducted on three different air springs in order to find parameters to characterise the model. The model provided accurate results for low and medium frequency testing. Above 30 Hz, discrepancies between the measured and modelled data were observed. These were due to excitation of resonances of the pressurised air springs rubber membrane.

Optimal control of a semi-active tri-axle lorry suspension

A vehicle model is developed that captures the dynamics of a semi-active tri-axle air-suspension system with load sharing between three axles. Linear control systems are designed for both ‘modified skyhook damping’ (MSD) and optimal full-state feedback (FSF) control. Non-linearities in the damping elements are modelled. The performance improvements of the control strategies are quantified by measuring the percentage reduction in dynamic body acceleration (BA) and dynamic tyre force (TF) compared to the optimal passive damping case. It is found that the FSF control offers the best theoretical performance when used with the non-linear damping element, achieving reductions relative to the optimum passive case of 38% in RMS BA and 20% in RMS dynamic TF. This optimum is found only when a gain amplification factor is added into the control loop.

Implementation of Trailer Steering Control on a Multi-Unit Vehicle at High Speeds

A high-speed path-following controller for long combination vehicles (LCVs) was designed and implemented on a test vehicle consisting of a rigid truck towing a dolly and a semitrailer. The vehicle was driven through a 3.5 m wide lane change maneuver at 80 km/h. The axles of the dolly and trailer were steered actively by electrically-controlled hydraulic actuators. Substantial performance benefits were recorded compared with the unsteered vehicle. For the best controller weightings, performance improvements relative to unsteered case were: lateral tracking error 75% reduction, rearward amplification (RA) of lateral acceleration 18% reduction, and RA of yaw rate 37% reduction. This represents a substantial improvement in stability margins. The system was found to work well in conjunction with the braking-based stability control system of the towing vehicle with no negative interaction effects being observed. In all cases, the stability control system and the steering system improved the yaw stability of the combination.