Vehicle platooning, where on highways a group of cars play “follow the leader” and pass steering, braking and acceleration control to the lead vehicle, is technology that’s now very close to reality.
The implementation of vehicle platooning can improve:
· fuel economy
· highway capacity
The first public highway tests of the Safe Road Trains for the Environment (SARTRE) platooning system on a motorway took place near Barcelona, Spain, in May 2012.
According to measurements and simulations carried out by the project partners, the estimated fuel consumption saving for high-speed highway operation of platoons is in the region of between 10 and 20 percent, depending on inter-vehicle spacing and geometry. The aerodynamic effects are such that even the lead vehicle will reduce its fuel consumption, albeit less so than the benefit derived by the following vehicles of the platoon.
Safety benefits would arise from the reduction in accidents caused by driver action and driver fatigue, and the effective utilisation of existing road capacity would also potentially result in reduced journey times. Safety is also considerably improved in congested traffic as the response time of the vehicles in the platoon is almost instantaneous.
In addition to making better use of the available road space, congestion would also be reduced through the avoidance of ‘ghost’ traffic jams where driver reaction time delays cause a ripple effect upon traffic flow. This is possible because the platoon control system is able to react much faster than human drivers would be able to; something that also potentially enhances safety for participating vehicles.
For users of the technology the practical attractions of a smoother, more predictable and lower-cost journey, and the opportunity to make use of additional free time, would be considerable.
A year into the work of the project, following extensive system design, simulation and testing, a single prototype following vehicle demonstration had been carried out.
Just under two years later, the now completed project – which was part-funded by the European Commission under the Framework 7 program – has delivered extensive trials of multiple vehicle platooning both on the test track as well as on the public highway.
The project was led by Ricardo UK Ltd, its partners included Applus+ IDIADA and Tecnalia of Spain, Institut für Kraftfahrzeuge (ika) of the RWTH Aachen University, Germany, and SP Technical Research Institute of Sweden, Volvo Car Corporation and Volvo Group of Sweden.
Trained driver in the lead vehicle
The concept of platooning, as envisaged by the SARTRE system, involves a convoy of vehicles where a trained professional driver in a lead vehicle guides a line of following vehicles. The system is designed to be able to accommodate a range of different vehicle types including cars, trucks and long-distance coaches, and is intended to be able to operate within a mixed traffic environment alongside conventional vehicles.
Drivers would be alerted via a human machine interface (HMI) to the presence and destination of nearby platoons and would be able to request access. Alternatively they would be able to pre-book a place via a web-based application, rather in the same way that airline or rail tickets can be purchased today.
Once the platoon has been joined and control has been handed over, the joining vehicle would be driven autonomously under the supervision and control of the lead vehicle driver.
Each car or truck within the platoon continuously and automatically measures distance, speed and direction and adjusts to the vehicle in front. All vehicles are totally independent and can leave the procession at any time by resuming control via their own HMI. But, once in the platoon, drivers can relax and do other things while the platoon proceeds towards its long-haul destination.
The SARTRE system, as developed and demonstrated by the project, involves the use of cameras and radar systems for relative position sensing, GPS for absolute positioning, and automotive standard 802.11p WiFi modules for inter-vehicle communications.
While the response to unexpected situations external to the platoon – for example speed changes due to congestion or lane change manoeuvres to avoid stranded vehicles or other obstructions – is automatically dealt with via the decisions of the lead driver, it was essential that the research team included procedures for problems within the platoon.
“The on-board systems of the SARTRE-equipped vehicles were developed according to standard approaches for fault-tolerant systems,” explains Ricardo chief engineer for SARTRE, Eric Chan.
“A system such as this requires a very high safety integrity level, which is reflected in the requirement for self-monitoring. If for any reason the system appears to be developing a fault or degraded condition, relevant safety margins are increased: for example, by automatically increasing the inter-vehicle distances and warning the participating drivers to be ready to resume control of their vehicles.
“In the event that communication cannot be re-established for any reason with the driver of a following vehicle, the lead driver would handle the situation appropriately, for example by simply bring the platoon to rest on the hard shoulder so that relevant action can be taken.”
In the first year of the program, SARTRE focused upon the concept development and feasibility. Issues investigated included usage cases, human factors and behaviours associated with platooning, core system parameters, and specification of prototype architecture and modes of application.
Before moving to the first vehicle tests early in the second year, the SARTRE project team carried out extensive simulator-based work so that human factors in the implementation of road train technology could be thoroughly investigated.
A sample group of men and women of varying ages and driving experience were tested using a simulator providing a 120-degree forward field of view via three LCD screens through which a total length of 18 km of virtual motorway could be driven. The simulator incorporated a steering wheel with force feedback, realistic manual/automatic transmission controls and a haptic seat installation; the combination provided a highly realistic virtual driving environment.
This simulator-based work has enabled the team to assess in detail the response of drivers both while participating in platoons and while driving independently in an environment in which platoons are operating.
Key findings from the human factors study included assessments of the acceptable distance between vehicles and the acceptable length of a platoon (by both platoon and other drivers). In this respect it appeared that the level of acceptable platoon length was in excess of the prototype system under development, with those taking part in the study considering platoons of up to fifteen cars to be acceptable, whereas the SARTRE project focused on a more modest platoon of no more than five.
In parallel with the simulator-based work, other members of the team spent much of the first year and a half of the project working on the development and testing of the on-board systems for the first SARTRE-equipped vehicles.
In addition to the optical and radar sensors incorporated in both lead and following vehicles, the necessary communications and control systems architecture needed to be defined, developed and tested.
To prepare for the first multi-vehicle tests, a Volvo-owned truck was equipped as a platoon lead vehicle, with an S60 sedan fitted as a participating vehicle to be driven autonomously as a following vehicle. The first live vehicle tests carried out in late December 2010 provided the first opportunity for testing outside the environment of the simulator.
“We were very pleased to see that the various systems already work so well together the first time,” said Eric Coelingh, engineering specialist at Volvo Cars. “After all, the systems come from seven SARTRE member companies in four countries.”
“The first tests were focused on the control and sensor systems, and we also spent some time evaluating the first iteration of the HMI,” adds Eric Chan.
“Issues such as longitudinal and lateral string stability were also assessed. The two-vehicle platoon was driven at up to 40 km/h with a gap size of 10 metres. Both ‘join’ and ‘leave’ manoeuvres were also tested. The initial program of testing was very successful and we gathered sufficient information to enable further development of the SARTRE system to continue.”
More vehicles, higher speeds
Over the remaining two years of the project, the team successfully increased vehicle speeds up to 90 km/h as well as closing the inter-vehicle distance to no more than four metres.
In addition, the SARTRE platoon demonstrations were extended up to a total of five vehicles, a level considered representative of a modest scale of implementation.
The first live tests of the SARTRE road train concept operating on a public highway in mixed traffic took place in May 2012 on a motorway close to Barcelona in Spain. For these tests, a platoon comprising a Volvo XC60, a Volvo V60 and a Volvo S60 plus one truck automatically driving in convoy behind a lead vehicle, was operated at typical motorway speeds.
“We covered 200 kilometres in one day and the test turned out well. We’re really delighted,” says Linda Wahlström, project manager for the SARTRE project at Volvo Car Corporation.
“During our trials on the test circuit we tried out gaps from five to fifteen metres. We’ve learnt a whole lot during this period. People think that autonomous driving is science fiction, but the fact is that the technology is already here. From the purely conceptual viewpoint, it works fine and road trains [platoons] will be around in one form or another in the future. We’ve focused really hard on changing as little as possible in existing systems. Everything should function without any infrastructure changes to the roads or expensive additional components in the cars.
“Apart from the software developed as part of the project, it is really only the wireless network installed between the cars that set them apart from other cars available in showrooms today.”