ESP® was first developed by Bosch in 1995, and since 2014 every new car sold in Europe must have an ESP® system installed. This is due to the significant safety benefits such systems create. A study in Canada showed that ESP® equipped vehicles were involved in 41% fewer loss-of-control type collisions than similar vehicles without ESP®. This benefit increased to 51% fewer collisions of this type on icy, snow/slush covered roads.
ESP® is Bosch’s trademarked name for what is referred to generically as an Electronic Stability Control (ESC) system. Several other manufacturers have trademarked their own versions of these systems, so on a BMW you would find a Dynamic Stability Control (DSC) system. Honda produces their cars with Vehicle Stability Assist (VSA) and Porsche uses the term Porsche Stability Control (PSC). We will continue to refer to these systems as ESP® however, as 80% of vehicle manufacturers in Europe use the ESP® acronym.
Regardless of what it’s called, any new car will have a system with this function installed. This has been the case since 2014 in Europe, and 2011 in the US after a report found that making it a legal requirement would prevent 5,300- 10,300 fatalities and 168,000-252,000 injuries per year.
What does ESP® mean?
The term Electronic Stability Programme® refers to a system made up of several elements which work together to maintain the stability of a vehicle in motion and prevent skidding and rollover. Traction Control Systems (TCS) and Anti-lock Braking Systems (ABS) are important parts of an ESP®.
In any ESP® a microcomputer evaluates signals from an array of sensors, typically checking 25 times a second. The system determines whether the steering input (e.g., Where the driver is intending to steer the car) corresponds to the actual direction in which the vehicle is moving. Where the system detects that the vehicle is moving in a different direction to that intended, an ESP® will carry out a series of selective interventions to correct the course of the vehicle. These interventions can be through wheel specific brake applications, or through increased/decreased acceleration from the engine.
How does an ESP® work?
The key components of an ESP® are the ESP® Hydraulic Unit, wheel speed sensors, the steering wheel angle sensor, yaw-rate and lateral acceleration sensors, and the Engine Control Unit (ECU). These parts work together to detect the need for intervention, calculate the intervention required, and execute the intervention.
The steering-angle sensor calculates the position of the steering wheel by determining the steering angle. The control unit uses this input, along with the position of the brake pedal and accelerator pedal, to determine how the driver is intending the vehicle to move. This is termed the ‘desired state’ and is what the ESP® compares the actual movement of the vehicle against.
The yaw-rate sensor captures all movement of the vehicle around its vertical axis, and the information that it relays to the control unit is combined with that from the Lateral-acceleration sensor which measures the g-force of a turn. These inputs allow the control unit to determine how the vehicle is actually moving, or the ‘actual state’, and compare this to the desired state.
Wheel-speed sensors are passive or active (using either Inductive or Hall-effect sensors) which measure the direction of rotation and standstill of the wheels using magnetic fields. They send signals to the control unit which then computes the speed of rotation.
The difference between the desired and actual states, and the information from the wheel-speed sensors, allow the control unit to calculate exactly what braking/engine control interventions are required to bring the actual state in line with the desired state and avoid skidding or rollover.
Additional Benefits of Electronic Stability Programmes®
With the inclusion of ESP® loads of additional benefits are made possible. As an ESP® is able to control braking, hill-hold functions and hydraulic brake assist are made possible. The ESPs® yaw-rate sensors allow for rollover mitigation, and the wheel-speed sensors can detect the drag caused by a puncture or underinflated tyre, giving us the Tyre Pressure Monitoring System.
Typically, hill-hold control will keep the brakes applied for a couple of seconds after the driver releases the brake pedal. This enables the driver to switch between the brake and accelerator without use of the handbrake, and drive away comfortably from a hill start.
Hydraulic Brake Assist takes information from brake pedal position and the pressure gradient to determine whether the driver is braking strongly enough. This is critical as in emergency situations any hesitation in braking can lead to fatal consequences. Where the Hydraulic Brake Assist detects that the driver is not braking strongly enough, it increases the braking force to maximum.
Rollover is a particular problem for light vehicles with a higher centre of gravity, and the ESP® yaw-rate sensor can detect any potential danger from increased lateral-acceleration and intervene. The system will utilise the hydraulic unit to brake individual wheels, while reducing driving torque, to stabilise the vehicle and avoid rollover.
What are the drawbacks of ESP®?
Unsurprisingly, like other safety focussed features such as seatbelts and airbags, there aren’t really any significant drawbacks to ESP®. Certainly, from a safety perspective, they are plainly ‘a good thing’. Though they shouldn’t really be considered as performance enhancing, and their only limitation is on high performance driving.
The ONLY time you would ever want to limit ESP® or switch it off is when you were using your vehicle in a track setting, as it does restrict certain racing approaches to cornering. In Volkswagens when you ‘switch off’ the ESP® it doesn’t actually cease operating, but instead operates on a different set of parameters which allows for improved cornering performance in racing contexts. Certain other manufacturers have different driving mode settings, which amend the operations of the ESP® without requiring you to press the ESP® off switch.
One area where ESP® has created some complications is in collision investigations and crash reconstructions, as the interventions of the ESP® mean that traditional methods to reach conclusions about vehicle speeds and driver steering behaviour cannot be applied in the same way.