Braking breakthrough

Written by: Tom Shelley | Published:

Advances in pneumatics are being applied to improve dramatically the efficiency of lorry braking. Tom Shelley reports



A new initiative to improve the control of the anti-lock braking process on heavy goods vehicles is expected to bring major reductions in both stopping distance and air usage.
Undertaken by one of the UK’s leading laboratories as part of an industry-sponsored research project, the move – which also involves using new faster-responding valves – forms part of a concentrated effort to progress the safe handling of articulated lorries. Significantly, the same methodology and hardware have the potential to improve control in a wide range of other pneumatic applications.
Professor David Cebon, professor of Mechanical Engineering at the University of Cambridge Department of Engineering, has for the last 14 years been running the Cambridge Vehicles Dynamics Consortium. This currently comprises ArvinMeritor, Camcon, Denby Transport, Firestone, Fluid Power Design, FM Engineering, Haldex, Mektronika Systems, Mira, Qinetiq, Shell UK, Tinsley Bridge and Volvo Trucks.
According to Cebon, a heavy goods vehicle’s anti-lock braking system works by periodically locking the wheels and releasing them.
“Cycling is expensive, in terms of air usage, and not very efficient, with long time delays,” he says. “And the algorithms are relatively crude. The time between lock steps is about a second.”
Maximum braking force occurs when the wheel just begins to slip. So, if the wheel brakes can be maintained at or near this position, braking will be maximised and air consumption minimised.
The problem is that the braking force versus wheel slip curve depends on weather, road surface, state of tyres and other variables. Therefore the system has to constantly monitor where this is, to the best of its ability; in order to establish this. That means knowing exactly what the speed of the vehicle is over the ground, as well as the rotation speeds of the wheels.
For experimental purposes, ground speed has been measured using an optical sensor, but Cebon points out that this would be of no use in a commercial system. The sensing system also needs to take into account the effects of Coriolis forces when making turns, as well as gravity on hills, which in theory requires the use of three accelerometers and three gyroscopes. In practice, the system would only have two low-cost inertial sensors.
“We could use £5,000 or £100 gyros,” he states. “We use £100 gyros.”
The package adopted claims an accuracy of 0.2m/s after 10s of braking.
On account of the costs involved in undertaking tests and development on a real truck, even on a rolling road, most of the control algorithm development has been undertaken on a HiL (Hardware in the Loop) test rig in the laboratory. This combines a real-world disk brake and air actuation system with a simulation of truck braking, running on a computer. The model of the braking truck was constructed using Simulink and the micro controller in charge of the brakes using dSpace.
The system takes in vehicle and wheel speed measurements, and continuously estimates optimal slip. Starting with a conventional ABS brake actuation system, the test rig was able to demonstrate that mean fully developed deceleration could be increased by 6% and air usage halved. However, a paper authored by Frank Kienhöfer, a research student at the time and now at the University of Witswaterand, states: “The transient performance of the slip controller is poor (rise time of 1s) as a result of the sluggish response of the ABS valves. This causes an approximate 10% increase in stopping distance.”
This led to the development of a new brake manifold with high bandwidth, binary-actuated valves developed with Camcon Technology. These valves flip between fully open and fully closed, with permanent magnets holding each valve in a given state. A short electrical pulse delivered to a solenoid momentarily weakens the magnetic field from the holding permanent magnet, allowing a compressed spring to snap the valve to the opposite state.
Two of Camcon’s valves have been tried: the ‘Vibrator’, originally revealed in Eureka’s February 2002 edition, which is able to flip from fully open to fully closed in 200 microseconds, but is limited to a 1.5mm diameter orifice; and the ‘Pushpull’, which takes 2ms, but has an allowable orifice diameter of 3.5mm.
In the prototype system, two Pushpull valves are used for the inlet, allowing rapid inflow of air when the brakes are applied, and another pair are used to discharge the brake chamber. Faster acting Vibrator valves are used at the inlet and outlet to make small adjustments to keep the braked wheel at the required slip point.
The Pushpull valves are controlled using a 20 Hz PWM (Pulse Width Modulated) signal, while the Vibrator valves are controlled by a 250 Hz PWM signal. The system requires that there be an inner control loop nested within the outer slip control algorithm loop to keep the brake chamber pressure at the brake demand pressure.
The bottom line is that the system has now reached the point where it can reduce stopping distance by 15% and air consumption by 40%. The latter is important to truck operators, because it reduces vehicle weight, which can then be replaced by revenue-earning cargo. The improved braking system also results in reduced tyre wear.
As a modification, Cebon describes it as fairly straightforward.
“You don’t have to replace the whole system,” he says.
As for seeing it in action on the road, there will be winter testing in 2008 in Sweden.
“We have a prototype device in the lab and are improving the algorithms and designing second generation hardware to go on our truck,” he adds.

Pointers

* System prevents wheels from locking up, instead holding them so that they only slip in such a way as to maximise braking force

* The system shows dramatic improvement in stopping distance, air consumption and tyre wear using conventional pneumatic valves. Results are even better with advanced valves

* Present state of development is a 15% reduction in stopping distance and a 40% reduction in air consumption


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