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The Volkswagen Golf VR6 has an engine power of 128kW channelled through the front wheels. It uses a traction control system where individual front wheels are braked. s ic m a n y D & y t afe S e l c i h e V n I ces Advan Traction Control Traction control was first used in heavy locomotives but is now applied to vehicles as diverse as heavy trucks and small front-wheel drive cars. Until recently, it was also used in Formula 1 racing as an aid to handling. By JULIAN EDGAR 4  Silicon Chip Why have traction control? The need for traction control is based on the idea that a driven wheel that is slipping excessive­ly is not providing the maximum possible power transfer to the road surface. However, completely preventing slip is not the aim; some slippage actually increases the tractive force obtainable. On dry road surfaces, the maximum accelerative force is available at slip rates of between 10% and 30%, while on loose sand and gravel the coefficient of accelerative force continues to increase with slip rate, with the Wheel speeds are sensed through the use of a toothed tone wheel and an inductive pick-up. The same sensors usually provide infor­mation for the anti-lock braking and the traction control sys­tems. maximum being achieved at a slip rate of more than 60%! Traction control systems usually work within the slip range of 2-20% and so will not provide adequate traction under all conditions. For this reason, most systems can be disabled with a dash-mounted switch. Wheel spin may occur on icy, muddy or gravel surfaces, where the coefficient of friction between the tyre and the road surface is low. It may be as a result of an increase in engine Electronic control of the throttle position is already carried out in some cars, using this geared motor. Integrating a traction control system which uses throttle control can therefore be carried out more easily in certain cars. torque being unable to be transmitted through the tyre to the road surface, as a result of too great a retardation through excessive engine braking, or as a result of large cornering and propulsive loads simultaneously being transferred to the wheels. Spinning drive wheels cause problems because they: (a) inhibit propulsion; (b) create handling instability because they can transmit little cornering force; and (c) lead to a high rate of wear on the tyres and drive mechanicals, especially when they pass onto a high friction surface and suddenly stop spinning. Control methods A number of approaches can be taken to limit wheel spin. The most obvious is that the engine torque output can be reduced by partially closing the throttle. This is easily done in cars using an electronical­ ly controlled throttle butterfly (“drive-by-wire”) but the reaction time using throttle control Fig.1: the layout of the Vehicle Dynamics Control system. In addition to the sensors required for ABS/ASR operation, sensors for vehicle yaw, lateral acceleration and steering angle are also used (photo: Bosch). March 1996  5 A combined anti-lock braking and traction control system for a commercial vehicle: (1) wheel speed sensors; (2) ABS/ASR elec­tronic control unit; (3) pressure control valve; and (4) solenoid valve. alone is slow. In diesel engines, the amount of injected fuel can be reduced to achieve torque reduction, while in turbocharged engines, boost pressure can be controlled. In petrol engines, the ignition system can be used to very quickly reduce the engine’s output. The spark advance angle can be altered or ignition pulses can be suppressed, causing a “miss”. However, an engine running with either excessive ignition retard or a deliberate misfire can produce excessive exhaust emissions and can have high exhaust gas temperatures. The latter is the case because the unburnt charge may ignite in the exhaust port! Simultaneous suppression of the fuel injector operation can be carried out to reduce these problems. The suppression of fuel injector signals will also cause a misfire and a consequent reduction in engine torque. Injector cutoff is often used on a rotating basis, 6  Silicon Chip with a cylinder shut off for a single cycle or “half” a cylinder shut off by the deactivation of a cylinder every other 720° cycle. This maintains engine smoothness and minimises crankshaft torsional stresses. The brakes can also be applied to the spinning wheel to slow it until its speed matches that of the non-driven wheels. By using this approach, the existing ABS (anti-lock braking system) hydraulic hardware can be utilised, with some hardware additions to cater for the extra traction control function. Truck traction control An example of a traction control system is the Bosch unit used on trucks and other heavy vehicles. It is integrated with the ABS system, making use of the ABS wheel speed sensors and hydraulic control unit. It uses a mix of engine intervention and brake application to control wheel spin. The Traction Control System (ASR in Bosch-speak) monitors the speed of the powered and unpowered wheels and recognises when a wheel is tending towards spinning. At this time, a dashboard light is illuminated, warning the driver of the presence of slippery conditions. The system controls the wheel speed of the powered wheels by two means: (1) Brake control – at speeds up to 30km/h, if a powered wheel is tending towards spinning it is braked and the speeds of the driven wheels synchronised. (2) Engine control – if both powered wheels are losing traction, the torque of the engine is reduced. At speeds above 30km/h, the spinning of either of the wheels is also prevented by a reduction in engine output. In addition to the braking and engine torque reduction approaches, trucks with air suspension on the leading or trailing axles can have the load on the powered axle briefly increased by up to 30%. This occurs when the traction control system relieves the non-powered Some Mercedes models use the sophisticated Vehicle Dynamics Con­trol, where any of the four individual wheels are braked to aid car stability during cornering. Sensors for yaw, steering angle and lateral acceleration are amongst those used. Above & right: the hydraulic control unit of a Bosch 2E ABS/ASR system. The electronic control unit (seen at right) uses hybrid circuits on a ceramic substrate and is combined with the hydraulic control unit. March 1996  7 Traction Control In Action WITHOUT TRACTION CONTROL WITH TRACTION CONTROL 1 4 2 5 3 6 This amazing sequence of photos showing the affect of the Vehicle Dynamics Control system, with the car cornering on a skid pan at high speed. Picture 1 shows the car understeering off line, mowing down the cones. By picture 2, the front outside tyre is giving off smoke as the car slides across the track in plough understeer. In Picture 3, it can be seen that the car is more than its own axle of its load by bleeding its air suspension bellows. Car traction control Powerful front wheel drive cars can have major wheel-spin problems, especially when accelerating from standstill. This is especially so because limited slip differentials are uncommon in FWD cars, because of the excessive torque reaction which would be felt through the steering wheel during differential lockup. The Volkswagen Golf VR6 uses a traction control system dubbed an Electronic 8  Silicon Chip width outside the appropriate cornering line. The righthand sequence (pictures 4-6) shows the same corner, same speed and same car – but with the VDC system operating. The amount of front wheel slip angle remains the same, as shown by the tyre smoke and amount of steering lock being used. But because the lefthand rear wheel is being braked, the car follows the chosen line. Differential Lock (EDS in German). The system uses only brake intervention to slow the spinning wheel. As with the truck system discussed above, EDS largely uses components already in place for ABS. The ECU continuously com­pares the speed of the front wheels, using appropriately placed sensors. If the difference in speed is greater than 110RPM, the slipping wheel is braked until it reaches approximately the speed of the non-slipping wheel. The system is activated until a road speed of 40km/h is reached, whereupon the effect of the system is gradually reduced. EDS also works in reverse gear, which may be desirable for those with very steep driveways! In very slippery conditions, the possibility exists that excessive brake temperatures may be realised – remember, this system doesn’t reduce the engine output. The electronic control unit continuously monitors the duration and frequency of EDS operation, with the probable temperature of the braking compon­ents calculated from these factors. When a preset level is reached, the EDS Fig.2: the VDC system controls understeer and oversteer by braking one of the wheels (photo: Bosch). system is disabled, although the ABS remains fully functioning. The system is very effective when one wheel is on a much more slippery surface than the other. In fact, with the left-hand wheels on dry tarmac and the right-hand wheels on a wet and icy road, a non-EDS Golf is able to transfer a drive force of 692 Newtons to the road, while an EDS-equipped car in identical circumstances can transfer 3112 Newtons – nearly 4.5 times as much. Interestingly, the VW EDS system can be retrofitted to recent ABS Volks­ wagens, with an additional valve block used on the hydraulic control unit and a new ECU used together with a redesigned wiring harness. Vehicle dynamics control This system, currently fitted to some Mercedes cars, is designed to prevent skidding during cornering. Unlike ABS and ASR, Vehicle Dynamics Control (VDC – I love all these acronyms!) can be activated even when the car is free-wheeling and when the driver is neither deliberately braking or accel- erating. Fig.1 shows a schematic of the system layout. While anti-lock brakes and traction control prevent longi­ tudinal wheel slippage, VDC attempts to prevent lateral slip, particularly when cornering. Both understeer (the front wheels laterally slipping and the nose of the car running wide) and oversteer (the rear wheels sliding sideways, with the tail of the car moving out of line) can be countered. If a car understeers when being cornered, the system cor­rects by braking the inner rear wheel, effectively rearwheel steering the car back into line. The controller can brake the chosen wheel almost to the point of locking and so the correcting effect can be very strong. Simultaneously with the braking of the single wheel, the speed of the car is slowed to a level appro­priate for the situation. This is achiev­ed by reducing the engine torque output by partially closing the throttle and/or by braking the other wheels. If oversteer is starting to occur, the system stabilises the car by braking the outer front wheel. Fig.2 shows the effect on vehicle stability of braking just one wheel. In addition to ABS and ASR components, the VDC requires sensors for yaw rate, lateral acceleration and steering angle. Furthermore, the controller needs information on whether the car is accelerating, free rolling or being braked. Longitudinal slip is derived from the wheel speed sensors, while a lateral accelerometer responds to the forces occurring in curves, with the analog sensor very sensitive in the range of ±1.4G. In addition, a yaw rate sensor is used to measure the speed at which the car rotates around its vertical axis. This device uses four pairs of piezo elements to excite a hollow steel cylin­der. The yaw is a measure of the shifting vibration nodes which occur within the cylinder. The ECU for the VDC system has a memory capacity of 48Kb – more than double that required for a combined ABS/ASR system. Next month we’ll discuss the traction control systems used on Formula SC 1 racing cars. March 1996  9