Brake Testing for the MOT (part III)
Note to MOT Testers: The following article appeared in November 2007 and information contained in it may not therefore be current - always check the MOT Manual if in doubt about any aspect of the MOT Test.
Future braking systems – and the MOT
Technological developments in the design of car and light commercial vehicles are accelerating faster than ever before.
The Passat has a complex reduction geared electric motor to operate the parking brake system.
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A key driver for this was the advent of the ECU, the vehicle’s own computerised system and designers are working ever harder to bring more systems under computerised control. For braking systems operated by hydraulic pressure, computerised control requires an electro-hydraulic link which is inevitably complex and expensive. So the next logical step is to develop braking systems using electrical servos – enabling direct computer interaction. Far fetched? It’s almost here…
A bit of history
Throughout the history of the motor car braking systems have varied widely in design. Early systems clamped the prop-shaft, others drew energy from the prop-shaft to provide power assistance. There were cable brakes, rod brakes and perhaps the best mechanical design, rod brakes operating on a wedge and roller principle.
Ultimately hydraulic disc brakes won the day, adapted from aircraft technology and first used by Jaguar in their racing ‘D’ types which swept the board at Le Mans – incorporated onto their production vehicles in 1958.
Then the Antilock Brake System (ABS) was introduced – for the first time a vehicle braking system had a degree of electrical control.
Concerned that ABS electrical faults could develop, VOSA (then the Vehicle Inspectorate) included ABS into the MOT by directing Testers to flag up a ‘reason for rejection’, should the ABS warning light sequence indicate a fault.
S-switch: Mercedes S type also has a combined electrical emergency and parking brake which differs from the Passat but their literature says that with the ESP switched off, and front wheels stationary, the rear wheels can be turned slowly in a “roller brake test stand”.
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Computer control
ABS enabled operation of the brakes on each wheel independently. Then separately, driven by the need for better engine emissions, the ECU arrived.
Suddenly electronic intervention into braking systems was achievable.
We now have electronic cruise control, ESP and other complex systems in the pipeline, which, depending on different driving situations, operate the vehicle’s brakes individually, often without the driver’s input or knowledge. The most recent of these systems – (generally operated via the vehicle ESP system) is the ‘disc wipe’ facility. In wet weather the system periodically slightly pressurises the brakes so the pads just touch the disc, generating enough heat, to evaporate surface water – reducing brake lag caused by the pads having to clear any water before they ‘bite’.
Yet none of these safety related systems are checked for the MOT.
The problem for VOSA and vehicle Testing authorities worldwide is that the only way these systems can be checked is by looking into the computer’s software programme – using a diagnostic tool. But with little or no commonality between vehicle makes regarding software, this is ‘mission impossible’. For emissions such commonality has been set into EU law by the EOBD system, but there’s no ‘standard’ for codes regarding brakes and other safety systems.
Pedal power – wet or wired?
Hydraulic brakes have traditionally had a major advantages over other systems. There’s little lag in the system, without complex mechanical linkages the operation is pretty much instantaneous between pedal pressure and activation.
Using incompressible fluid it’s easy to amplify the ‘gain’ in the system by the ratio between master and slave cylinder sizes, and power operation is easily achieved by a pressure activated power source – vacuum or compressed air for example.
But why not use electricity? Not so easy. The main obstacle is the high energy demand. Not only does the friction material need to be driven hard against the discs and/or drums, but may need to be held there for some time and under some pressure.
To achieve this electrical power from 12 volts incurs unacceptably high electric current levels – requiring heavy and expensive wiring and equipment. The alternative is to increase the voltage, (42 volts being suggested), reducing current demand.
High energy electrical condensers
Even then sudden “Oh my God!” snap emergency braking will probably need fast discharge electrical condensers to supplement normal ‘on line’ use. Perhaps this is now being seriously looked at; probably in the experimental stage. Nevertheless, it is unlikely that car makers or their component system suppliers like TRW (who often drive such developments), are addressing any effort at all as to how these systems will need to be ‘MOT Tested’, especially in the longer term when the vehicles become rather aged.
How, for example, are the electrical condensers to be checked? They can fail by leaking charge – emergency stops could be like Russian roulette! Historically, and surprisingly, there is still no legislative requirements on car makers as part of the ‘type approval’ regulations to ensure that these new sophisticated systems can be properly checked during an MOT.
A good example of this problem is the Volkswagen auxiliary braking system already in production.
Auxiliary brakes
Volkswagen’s sophisticated electro-mechanical system parking/emergency brake is driven by an electric motor with two stage gearing and a final lead screw to provide an overall reduction ratio of 150:1.
Firstly an electric motor drives a reduction gear via a toothed belt. The consequent 3:1 reduced drive then turns a geared swash plate which further reduces the ratio by 50:1. Finally a lead screw winds against the brake piston pushing the pad onto the disc.
The really interesting part comes when, should there be a situation where the parking brake is required as an emergency brake with the vehicle in motion. If the ‘parking brake’ button is pressed, the electronics detect that the wheels are rotating and rather than operate the parking brake, pressurises the hydraulic system to stop the vehicle.
The MOT Testing problem
The issue here is how can this be properly checked for the ‘MOT Test’? Is the problem electrical, mechanical or hydraulic? Is it with the parking brake only, or will the emergency system be affected?
The answer seems to be that the electro-mechanical parking brake cannot be properly checked at all. When the vehicle is moving it just won’t operate. The ECU thinks “the wheels are turning so this is an emergency, not parking”, and so pressurises the system to stop the vehicle – so that has to be checked with a decelerometer.
But with the vehicle stationary and the parking brake switched on, how to check it is holding the vehicle securely? After all a parking brake is an essential safety feature when vehicles are parked on a slope. Perhaps we’ll need artificial slopes in our Testing Stations!
The safety hazard
Braking systems with electronic intervention can be dangerous when it comes both Testing and servicing. On some modern cars the brakes activate if the ignition is switched on when the vehicle is stationary.
Injuries have resulted when a vehicle’s parking brake suddenly activates unexpectedly when being worked on with a technician’s hand in the mechanism – perhaps somebody switched on the ignition.
Should 42 volt high discharge electrical condensers be fitted in the future, they could be sitting there, fully charged with lots of electrical energy, poised and waiting for an unsuspecting technician or DIY motorist to make a mistake…
From hardware to software…
Since its inception in the early 1960s, the MOT Test has essential been an examination of the vehicle’s hardware. You check what you can see, and this will always be an important aspect of the MOT.
However, with modern cars becoming increasingly reliant on computerised systems, some sort of software check is bound to come sooner or later – and electronic interventions into the braking system seems to us to be a good place to start.
Aware of this, Martyn Pegg from VOSA’s Research and Development department recently instigated a conference at VOSA with car makers in attendance, to discuss the issues involved (see VOSA Matters).
The real problem is that European law does not compel car makers to ensure these ‘state-of-the-art’, software safety systems can be MOT Tested.
It seems that one set of Eurocrats set the ‘Type Approval’ regulations, and a different group decide how ‘MOT’ Testing should be done – and they don’t communicate with each other! It’s about time the Eurocrats in Brussels got their act together and realised that for road safety ‘MOT’ Testing is just as important as type approval.