HOW often have you heard a driving enthusiast say, “When I get into a performance vehicle, the first thing I do is switch off the electronic nanny”? I lost count more than a decade ago.
They don’t always realise that, apart from possibly endangering themselves and fellow road users, switching off the electronic stability control (ESC) deprives a driver of extra “skills” in the form of lightning reflexes and additional car control courtesy of a car’s electronic algorithms. Pressing that button also confounds ride-and-handling engineers that spend countless resources developing a system to keep you safe (and fast) on the road. In this article, we look at the theory and hardware used in modern vehicles to create ESC.
History
Compared with the automobile, ESC is a fairly recent technical development. BMW and Mercedes-Benz introduced traction control in 1987 that lacked the ability to control lateral dynamics.
Automotive supplier Bosch allied with BMW and Mercedes to develop the first production-ready ESC system in the mid-Nineties. BMW released its first system (DSC) in the 7 Series and Mercedes released its version co-developed with Bosch under the proprietary name electronic stability programme (ESP) in the S-Class.
Basic theory
The main function of an ESC system is to prevent a vehicle from losing lateral grip in a turn that can result in the driver losing control (spinning out). An ESC system is also capable of actively supporting the driver by enhancing directional stability during critical lateral dynamic situations like an emergency lane change to avoid an accident. The following scenarios are typical of a vehicle losing poise during cornering.
Understeer
When a vehicle is not following the driver’s intended line through a corner (see above) with the front end of the car pushing wide, it is called understeer. As this situation is the easiest handling characteristic for novice drivers to correct, most manufacturers build in a small amount of understeer in the car’s dynamics for safety reasons at the friction limit. The way that ESC mitigates understeer is to brake the inside rear wheel of the vehicle – creating a corrective torque moment round the vertical axis of the vehicle running through the centre of gravity (COG). This brings the front end of the car back on the intended path of travel.
Oversteer
When a vehicle is not following the driver’s intended line though a corner by rotating further round the vertical axis running through the COG of the car, it oversteers (see above). This handling characteristic is exploited by skilful individuals in a relatively new sport called drifting. The drift angle is controlled by balancing throttle application and steering input of specialised, rear-wheel-drive racing cars. On the road, oversteer can easily send a vehicle into a spin with catastrophic consequences. ESC avoids this situation by braking the outside front wheel of the car creating a corrective torque moment around the vertical axis running through the COG (see above). The reason for applying the torque moment on the front axle is that the rear has lost grip at this stage.
When ESC may safely be switched off
During extreme low-speed off-road driving, the ESC system may hamper traction if certain wheels are braked at the wrong time. Most four-wheel-drive vehicles with ESC have a special setting when low range is selected to alter the stability-control settings. Racing drivers piloting production vehicles around a racetrack may opt to switch off ESC as it allows them to steer a vehicle with the throttle as well as with the steering wheel. Vast amounts of talent and skill is needed to perfect the art of controlling a sliding car and even the best get it wrong at times.
Hardware:
ABS Module and Wheel-Speed Sensors
The functioning of the ABS module and wheel-speed sensors were covered in detail in the March issue of CAR. In short, the ABS control unit monitors individual wheel speeds and is able of supplying individual braking forces to each wheel for optimum braking performance as well as aiding vehicle stability as demanded by the ESC system.
Steering Angle Sensor
The intended steering angle of the driver is monitored by a sensor mounted on the steering column of the vehicle. Apart from the absolute steering angle, rate of turn is also measured.
Acceleratoin and YAW Sensors
The vehicle is fitted with an acceleration sensor to measure lateral acceleration of a vehicle when cornering. A yaw sensor measures the yaw angle (angle of rotation) of the vehicle round the vertical axis running through the COG.
Control Strategies
The ESC is a closed-loop system that measures the steering input of the driver and with vehicle speed known, calculates the expected lateral acceleration. The feedback loop consists of the measurements taken by the acceleration and yaw sensors. If the lateral acceleration measured is less than calculated, it may point to a vehicle that has exceeded the friction limit and which has started to slide. By monitoring the yaw angle of the vehicle, the ESC can decide if an understeer or oversteer situation needs to be corrected. Apart from braking individual wheels to regain dynamic composure, the engine control unit (ECU) may also cut the power delivery of the engine to aid the stabilisation process if needed. The modern ESC system acts so quickly and unobtrusively that the driver may be unaware of a possible life-saving intervention.
Many performance vehicles today offer a sport setting on the ESC system which increases the safety thresholds to allow some lateral slip before intervening. This enhances the driving experience for enthusiastic drivers by making it more exciting with the knowledge that there is still an active safety net. Remove that net, however, by deactivating the system completely, and the driver may soon find out that they are not as adept as previously thought with expensive repair bills, or worse, as a result.
Future
Future legislation will demand that ESC is standard on all new vehicles and may not give the driver the option of deactivating the system. One area where the system can “fail” is when a vehicle enters a bend too fast because the laws of physics still apply. Expect ESC systems of the future to be linked with GPS and to know the radii of all corners in advance. This would allow the ECU to limit the corner-entry speed of a vehicle (by reducing engine power or even autonomous braking) to avoid dynamic instability. Sceptics would say that it would remove the fun element of motoring but perhaps the best place for such fun is on a racetrack.
Traction-control system (TCS)
The main function is to limit wheel spin and maximise traction on the driven wheels, aiding stability. The tractive force available is the normal force times the friction coefficient between the driven wheels and road. The TCS control system can act in the following ways to stop/prevent wheel spin:
- Limiting the torque output of the engine by cutting fuel injections or retarding timing in spark-ignition engines (quick reaction). A slower intervention method in spark-ignition engines is by closing the throttle;
- Braking the spinning wheel(s) with the help of the ABS system;
- A combination of limiting the engine’s torque output and employing the clever braking functionality.
Read part 1 of Active Vehicle Safety, Modern ABS here.
