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Ergonomics

Active car headrests – what are they and how do they work?

12 November 2022

Active headrests effectively protect the cervical region from injury, therefore they are increasingly used in modern cars. Depending on the design principle and mechanism of operation, they differ in the way they are activated. What are these differences and what is worth knowing about active headrests? 

Active headrest – what is it? 

Active headrests counteract injury to the cervical spine with built-in protective mechanisms. These systems are known as WHIPS (Whiplash Protection System) or AWS (Anti-Whiplash System). For a long time they were used only in the seats of higher-end cars, but due to their high effectiveness they can be found more and more often in middle-priced vehicles as well. These solutions are recommended by road safety and crash test organizations such as EuroNCAP, NHTSA and IIHS.

How does an active headrest work? 

Active headrests are activated during a rear-end collision. Their operation is automatic and involves "following" the occupant's head as quickly as possible to relieve pressure on the cervical region even before potential damage occurs, increasing the effectiveness of protection. SAAB engineers are considered among the pioneers in this field, having designed the popular SAHR system in 1997. Today, these solutions appear under different names and, depending on the manufacturer, differ in the way they are activated. Modern plastics that further improve safety performance, such as EPP foamed polypropylene, are used for headrests

How to set an active headrest? 

Although the purpose of the active front seat head restraints is to accommodate the movement of the passenger's and driver's heads in a crash, they also require adjustment. In most cases, you can adjust their height, as well as correct the tilt – either manually or with the push of a button. It is worth remembering that a headrest that is set too low may not protect against a sudden and strong bending of the head backward, which can result in serious injury. Adjusting the driver's seat and the correct position behind the wheel are prerequisites for safety regardless of whether the car is equipped with an active or passive head restraint.  

Types of active car headrests 

Head restraints, which become activated when a collision occurs, are divided into reactive and active. The main difference between them is the type of activation system. The former operate on a mechanical basis, based on a system of levers and springs located in the backrest. In active systems, on the other hand, actuators controlled by an electronic system play a leading role. The latter are considered faster and more precise. 

Reactive headrests for your car 

The first invented reactive head restraint system is believed to be the AHR, designed by Volvo engineers and used as early as 1987. Its main element is a lever located at the junction of the headrest and the seat. At the moment of impact, the head restraint tilts forward, supporting the passenger's head. In contrast, SAAB's pioneering SAHR uses a special plate built into the seat back. Under the pressure exerted by the passenger's body, the mechanism moves the head restraint upward and forward, preventing not only the head from moving backward, but also from hitting the head restraint too hard. IIHS research has shown that this solution reduces cervical spine injuries by as much as 43%. Other interesting systems include the self-inflating SIHR head restraints from Autoliv. In this case, air containers mounted in the backrests are used to protect the cervical region. Under pressure, it is pushed into pouches located in the headrests. In this way, the inflated headrest comes 5 cm closer to the passenger's occiput, thus providing head support. 

Active car headrests 

Active, electronically controlled systems include solutions where sensors are placed in the bumper or seat of the car. Such systems are used in Toyota cars, among others. The head restraint is activated when the sensors detect increased hip pressure on the driver's seat. Depending on the design of the head restraint, only the front half of the head restraint covered with soft foam can be pulled forward. Such a head restraint can be returned to its initial position. A more elaborate solution is PAHR, a system of active head restraints in BMW cars, among others. Its operation is based on collision sensors and computer-controlled pyrotechnic charges. When they explode, the fuses are released and the outer part of the head restraint is moved forward. 

Active and passive car headrest – what are the differences? 

Unlike active head restraints, which react to a collision situation and adjust to the movement of the passenger's body, passive head restraints only have a function of a static support for the head and act as an obstacle to prevent it from tilting backward. They can be an extension of the seat or be a separate component, just like an active head restraint. However, unlike the latter, they are generally connected rigidly to the backrest frame and do not have a forward and backward or angled adjustment mechanism. They can only be adjusted to the passenger's height by being raised or lowered. From the outside, it is sometimes difficult to distinguish between passive and active headrests, as they can look the same. The feature that visually distinguishes active from passive head restraints on some car models is their distinctive two-piece design.  

Also read: Safety systems in cars – Knauf Automotive 

Advantages of using EPP foam for headrest fillings 

In an era of innovation in automotive design, traditional foams are increasingly being replaced by modern plastics, such as EPP foamed polypropylene. The material is widely used today in the production of car headrests due to its excellent mechanical properties and minimal weight. Its ultralight, cellular structure filled with 95% air perfectly absorbs impacts without falling apart. The resilient, strong material deforms momentarily to return to its previous form when the pressure stops. This property has led it to find wide acceptance among manufacturers of car seats and other passive safety components. The simple, cost-effective compression molding method allows for the production of components of any shape, with grommets and extrusions for wiring, sensors or seat adjustment mechanisms. EPP plastic can also be easily combined with other materials used in car seat construction, in the form of metal reinforcements or soft top foams ensuring comfort. The technology provides great design flexibility, enabling the development of new design solutions. 

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