As far as the trucking industry is concerned, brakes are brakes – or are they? There is a choice – drums or discs, and then the size of the drum being used, but the actual technology being used has been the same for some time. EBS (electronic braking systems), ABS (antilock braking systems) and stability control have entered the fray in recent years, but only to improve control of existing brake systems.
Researchers in Europe are going out beyond these limitations and looking at ways to achieve a quantum leap in brake performance. Two presented their work to other truck and trailer engineers at the 14th Heavy Vehicle Transport Technology, held recently in Rotorua in New Zealand.
Optimised Braking and Turns, is how Professor David Cebon, from Cambridge University in the UK, characterises the work he has been doing with his colleague Dr Graeme Morrison, as well as in collaboration with brake manufacturer Haldex.
In a current EBS braking system, individual wheels are slowed down to zero and then allowed to speed up again, then slowed again, to avoid skidding. These oscillations in speed take place every second.
As David says in his introduction to the project, “Conventional electronic/anti-lock braking systems for trucks have limited bandwidth due to the compressibility of air, the large volumes required to fill the brake chambers and sluggish actuation of the ABS modulator valves. Large amplitude cycling of wheel slip therefore occurs between almost free-rolling and almost fully-locked, causing inefficient emergency stopping performance.”
The control systems keep slippage in control by dumping all of the pressure in the system to allow the tyre to roll and reapplies the pressure to get further braking. The brakes are in an on/off cycle over and over again, brake pressure goes up and down, the system is working to both slow the vehicle and avoid skidding.
“This uses a lot of air and doesn’t stop very well,” says David. “The result of this kind of behaviour is trucks typically have stopping distances which are 30 to 40 per cent longer than cars.
“We’ve developed a new type of braking system. Early on we looked at whether we could do slip-control braking. We looked at an EBS system on an icy road and compared stopping distance against air usage.”
The prototype pneumatic Slip Control (SC) system was developed and tested by the Cambridge Vehicle Dynamics Consortium (CVDC), led by David Cebon. The system uses bespoke high-bandwidth bi-stable valves, placed close to the brake chamber to minimise pneumatic delays, and sliding mode control.
“We decided to take the same valve and the same brake actuator, but we controlled it in a different way to control slippage. Normal brake actuators take 40 milliseconds to open and close and use a lot of air when stopping. We discovered, if we went to a 15 millisecond valve, then we could get a much better response, considerably reducing stopping distance and air consumption.”
The team went on down to a three millisecond valve and improved performance again. Going further, to one millisecond didn’t improve things. The target was set to developing a braking system using valves which took just three milliseconds to open and close, less than a tenth of the time taken by current valves.
“We wanted to get down to three milliseconds, then we hoped to get a 30 per cent reduction in stopping distance and a 70 per cent reduction in air consumption,” says David. “That means we need a smaller air tank and use less power to fill it. We spent ten years trying to work it all out. After ten years and three PhD students we came up with this system with very fast bi-stable valves. There are two, one at the inlet, where we take air from the reservoir, and one on outlet through the exhaust valve.”