How Traction Control Unit Work: A Complete Guide for Automotive Engineers
The steerability of a vehicle is not only lost when the wheels lock up on braking. The same effect arises if the wheels spin when driving off under severe acceleration.
Electronic traction control has been developed as a supplement to ABS. This control system prevents the wheels from spinning when moving off or when accelerating sharply while on the move. In this way, an individual wheel which is spinning is braked in a controlled manner. If both or all of the wheels are spinning, the engine control function reduces the drive torque. Traction control has become known as ASR or TCR.
Traction control is not normally available as an independent system, but in combination with ABS. This is because many of the components required are the same as for the ABS. Traction control requires only a change in logic control in the ECU. It also needs a few extra control elements, such as throttle control.
Traction control will occur to achieve the following:
(i) Maintain stability
(ii) Reduction of yawing moment reactions
(iii) Provide optimum propulsion at all speeds
(iv) Reduce driver workload
Advantages
(i) Improved tractive force
(ii) Better safety and stability on poor surface
(iii) Less driver stress
(iv) Longer tyre life
(v) No wheel spin on turning and cornering
Control function
An automatic control system can intervene in many cases more quickly and precisely than the driver of the vehicle. This allows stability to be maintained at times when the driver might not have been able to cope with the situation.
Throttle Control
This can be via an actuator, which can move the throttle cable, or if the vehicle employs a drive‑by‑wire accelerator, then control will be in conjunction with the engine management ECU. The throttle control will be independent of the driver’s throttle pedal position. This method alone is relatively slow to control engine torque.
Ignition Control
If ignition is retarded the engine torque can be reduced by up to 50% in a very short space of time. The timing is adjusted by a set ramp from the ignition map value.
Braking effect
If the spinning wheel is restricted by brake pressure, the reduction in torque at the affected wheel is very fast. Maximum brake pressure is not used to ensure passenger comfort is maintained.
System Operation
The description that follows is for a vehicle with an electronic drive‑by‑wire accelerator.
A simple sensor determines the position of the accelerator and takes into account other variables such as engine temperature and speed. For example, the throttle is set at the optimum position by a servo motor. When acceleration, the increase in engine torque leads to an increase in driving torque at the wheels. In order for optimum acceleration, the maximum possible driving torque must be transferred to the road. If driving torque exceeds that which can be transferred, then wheel slip will occur at least at one wheel. The result of this is that the vehicle becomes unstable.
When wheel spin is detected the throttle position and ignition timing are adjusted but the best results are gained when the brakes are applied to the spinning wheel. This not only prevents the wheels from spinning but acts also to provide a limited slip differential action.
This is particularly good when on a road with varying braking force coefficient. When the brakes are applied, a valve in the hydraulic modulator assembly moves over to allow traction control operation. This allows pressure from the pump to be applied to the brakes on the offending wheels. The valves in the same way as with ABS can provide pressure build‑up, pressure hold and pressure reduction. This all takes place without the driver touching the brake pedal.
