Researchers in Europe are working to improve truck braking and looking at ways to achieve a quantum leap in brake performance. One of them presented their work to other truck and trailer engineers at the 14th Heavy Vehicle Transport Technology, held 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.
The first tests of the new system took place on the MIRA test track in the UK. In comparisons with an electronic braking system (EBS) the braking was much improved. On videos of the test, the trailers wheels can be seen stopping and starting as the EBS tries to reduce skidding, but on the new braking system the wheels can be seen to slow down until the trailer halts.
The new system uses bespoke high-bandwidth bi-stable valves, placed close to the brake chamber to minimise pneumatic delays, and sliding mode control. By being able to modulate quickly from some braking to no braking, the Slip Control (SC) system is able to more accurately control any slippage. This first test achieved a 16 per cent reduction in stopping distance, when compared to a combination fitted with EBS.
This is not the whole story – the new SC system could control ‘longitudinal’ force and slow the trailer down in a straight line. Unfortunately, because the wheels were constantly braking during this process, it minimised ‘lateral’ force. This is the force which keeps the tyres running in a straight line in the direction required.
As you increase the longitudinal slip, you lose lateral force, and if you are on a corner there is no sideways-acting force to keep the wheels running straight. The SC is a better braking system, but it reduces the ability to keep the truck in a straight line. It brakes better but when it loses lateral force the semi will tend to understeer off the road.
This lateral force problem does not effect EBS because every time it reduces the brake pressure to zero, the tyre is free running, lateral forces go up and keep the truck in a straight line.
The next stage of development needed the SC control to not only shorten stopping distance, but also improve directional performance. This meant the SC had to be enhanced by some form of controller to reinstate some lateral forces into the wheels and enable the truck to pull up in the correct line. The controller needed to ease off braking in order to retain enough lateral force at the wheels to keep the truck running true.
“Our controller design was a bit simplistic really,” says David. “If we reduce the slip a little bit, we lose a little bit of stopping force, but we get a substantially greater increase in lateral force. By sacrificing a little stopping force, we can gain a lot of lateral force.”
As the truck goes round a corner lateral forces are the same on all the wheels. When the brakes are applied the stopping forces on the wheels goes up but the lateral forces holding the truck in line reduce. The control actions of the system are based on the position the prime mover and trailer should be in, in terms of speed, articulation angle, steering wheel angle, and what the sensors on board observe to be the case. Inputs are selected and timed to return the truck to its ideal position.
“If at this point the trailer is understeering, we need to increase the lateral force,” says David. “So we break off the braking force on the steer axle allowing the side force to recover the line. If the prime mover is over-steering, we break off the brake force on the drive axle allowing the lateral force to recover.”
This principle can be applied to any axle where slippage sideways is detected. By monitoring the vehicle dynamics, the system can decide where the intervention is needed. Sensors on the truck and trailer feed back data to the controller. The driver’s steering input is also monitored.
The system then uses this data to work out how the vehicle actually is performing and what it should be doing at that speed with that steering angle. It then compares the two situations and creates, what David calls ‘slip attenuation vectors’, which are how much the slip should be adjusted on each axle to keep the truck in line while maximising braking force.
The system is being asked to brake hard, to which it has to add the attenuation factors to keep it on the road. This ‘Attenuated Slip Demand’ (ASD) decided where and when to reduce slip on each axle.
The system has to be very precise dealing with issues like two different possible causes of a large articulation angle, either trailer swing-out or jack-knife. Trailer swing-out is caused by reduced lateral tyre forces on the trailer so braking on the trailer backs off.
On the other hand, if the truck is jack-knifing, the trailer pushes the prime mover’s back end because of lower lateral tyre forces on the drive axle. Backing off on the trailer braking could be disastrous in this case. By introducing trailer side-slip data into the system, the ASD could differentiate between the two.