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08/02/2008
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Tom Shelley reports on the latest advances in all wheel steering for tractor semi-trailers and its likely consequences for vehicle design
By steering the wheels on each axle on a heavy goods vehicle and improving the control algorithms, it is possible to steer much more precisely, get long tractor trailer combinations round roundabouts without their either cutting in or swinging out and improve high-speed stability.
The technology can also be used to reduce the risk of rollovers, when heavy goods vehicles change lanes and keep vehicles on track if they hit icy conditions or strong side-winds .
As well as improving safety, the technology will allow goods vehicles in the UK to be made longer, with double trailer units, which are much more fuel efficient per tonne of cargo carried.
The technology comes from the same Cambridge Vehicles Dynamics Consortium responsible for the improved anti-lock braking technology described in the January edition of Eureka.
Professor David Cebon, professor of Mechanical Engineering at the University of Cambridge Department of Engineering and head of the project explained that the basic idea is to provide, "Electronic control of all the wheels so that the back end of the vehicle truly follows the front for all conditions".
Compared to other methods of vehicle steering, conventional or "Command steer", he said that his method, "Is a much more complicated mathematical relationship".
The wheels on each axle are steered by a high performance hydraulic cylinder. The actuators have been specially developed for the task, and are, "Not based on any commercial product although they do have commercial components". He also said that, "There are all sorts of safety issues" associated with possible hose failures or loss of hydraulic pressure, loss of electric power or computer control signals. In the event of any failure, the strategy is to turn the wheels back to their centrelines. Fluid power is supplied by a separate hydraulic system with accumulators that are maintained charged. Professor Cebon said that, "We have an interlock so you can't do anything until they are charged up".
Conventional articulated vehicle steering is by one of two methods. In most vehicles, the front wheels are turned, all the other axles are fixed - so they follow as best they can. A small number of vehicles use "Command steer", which involves turning the trailer wheels as well as front wheels in a fixed geometric relationship. Command steer systems are designed specifically to work well for low speed circular paths.
Conventional steering leads to the rear end of the trailer swinging in by as much as 5m on a typical corner. If one were to employ what is termed a "B double" trailer, the problem is even worse, and there is no way such a vehicle can get round a British roundabout. There is also a problem with tyre scrub and resulting severe tyre wear and road surface damage particularly when there are three axles on the semi trailer.
Commercially available command steer systems reduce these problems, but do not eliminate them. There is also a tendency for the tail of the trailer to swing out by nearly a metre at the start of the turn. And because dynamic forces are also involved, Command Steer systems have to be disabled at higher speeds - otherwise they can become unsafe.
The new system is much more sophisticated, with various sensors to detect what is actually happening. It works at any speed and on any path - not just circles. Part of the development programme is to improve truck handling generally, and prevent rollovers occurring as a result of sudden lane changes.
The bottom line for operators, is that it should be possible to make heavy goods vehicles longer, with double trailers instead of single. If this is done, a study entitled, "Economic Efficiency of Long Combination Transport Vehicles in Alberta", published in 2001 found that use of these vehicles results in a reduction in overall shipping costs of 29 per cent, a fuel saving per tonne of freight of 32 per cent, and a 40 per cent reduction in road wear. There would also be considerably fewer vehicles on the road, so traffic congestion would be reduced. Another possibility is for normal-length vehicles with trailer steering to be used in more constrained street layouts. This can potentially eliminate a lot of double-handling of freight that takes place in out-of-town freight depots. This will again substaintially reduce freight transport costs and fuel consumption.
The same technology is potentially also applicable to "Bendy buses" and other long and heavy vehicles.
Pointers
* By making each axle of a heavy goods vehicles steerable, and incorporating sophisticated sensing and control, it is possible to almost eliminate tyre scrub and cutting in and swinging out going round corners
* It then becomes practicable to make heavy goods vehicles much longer, and with two (or more trailers or semi trailers) as are used in Canada and Australia and get them to go round a British roundabout without getting stuck
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Author
Tom Shelley
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