Vehicle Crash Sensors: Remote Acceleration Sensor

Accidents involving automobiles are on the rise. Nevertheless, the rapid growth of technology and infrastructures has helped us take control of the situation to some extent.

A combination of active and passive safety sensors is generally required to significantly reduce road accidents. However, the surround sensor, like the video sensor, provides lots of information that can be used for developing current passive safety systems.

According to Koenning, M and Heger, T (2009), a typical passive safety sensor like acceleration sensor determines the extent of vehicle crash based on impact velocity, type of crash such as offset crash, full frontal crash, and mass of the crash participant.

The key role of the acceleration sensor as discussed by Pack, R et al (2005) in a research paper includes the following:

• Pedestrian protection – When an acceleration sensor is used in the bumper, it detects the collision with a pedestrian and activates the 'pop up' hood by a pyrotechnical activation thereby reducing the degree of freedom of acceleration of the vehicle, and protecting the pedestrian in case of an unavoidable accident.
• Occupant safety – Acceleration sensor generates an optimized firing choreography and activates the available restraint systems to reduce the injury risk of the occupant.

The following video by Gill Sensors demonstrates their acceleration pedal position sensor:

Acceleration Sensor by Gill Sensors.

Working of Remote Acceleration Sensors

Crash sensors available in today’s automobiles are more advanced. Some older sensors like simple pressure sensors are now replaced with highly advanced micro-machined accelerometers.

These sensors sense the deceleration taking placing during a vehicle crash or an accident. They also determine the position of the vehicle with respect to the road.

During a roll over, if the vehicle moves out of the horizontal plane, the sensor immediately sends the required information to the electronic control unit, which initiates air bag deployment. These sensors can be mounted in various parts of the vehicle, but sophisticated sensors in newer systems are usually placed within the electronic control unit or the airbag itself.

During deceleration, the sensor housing within the car stops at the same time as the car. However, the occupant inside the car continues to move at the speed of the car, which causes him to move forward and contact an electric switch to complete the circuit.

The electric impulse generated through the completion of the circuit is transmitted to the control unit or airbag. The control unit receives impulses from several acceleration sensors, determines which airbag to deploy, and sends back the impulse to the corresponding address for deployment. Each control unit has to receive two or more impulses from the sensors to deploy airbag.

When the control unit determines a crash, it sends an impulse to an inflator that ignites the sodium azide propellant inside the airbag. The ignition releases nitrogen that fills the airbag within few milliseconds before the occupant’s body is pushed forward. The nitrogen then starts escaping via the vent holes as the occupant falls on the bag thereby reducing the occupant’s momentum, and hence preventing the occupant’s impact.

Benefits of Remote Acceleration Sensors

Conventional acceleration sensors used in crash sensing technology usually evaluate data in one dimension, either lateral acceleration for side impact detection or longitudinal acceleration for front impact detection.

However, multi-axis sensors used in modern systems measure acceleration in more than one dimension at the same time. These types of sensors posses improved crash sensing performance required for rating tests of front pole impact and front small overlap impacts.

On the other hand, dual-axis sensors placed on the B-Pillar can provide enhanced sensing performance without the need for additional pressure or acceleration satellite sensor.

Traditional chassis-rail-mounted remote single-axis acceleration sensors may not rapidly detect pole impacts at the vehicle center under frontal accident conditions due to the relative lack of rigidity in the vehicle area. However, the front dual-axis acceleration sensor with Y-axis sensing used in the modern systems can sense the impact and lateral deformation of the vehicle more quickly. Moreover, it also posseses improved sensing performance to deploy side airbags in front oblique, resulting in improved occupant protection.

Rollover sensing is another potential benefit of multi-axis sensing systems. These systems measure the lateral and vertical acceleration to determine the rollover events for passenger cars, without the use of roll gyro. Thus, they offer potential cost savings.

Products – Latest Advancements

Some of the latest vehicle acceleration sensor systems available in the market include the following:

Central Acceleration Sensor

The central acceleration sensor is an automotive sensor technology introduced by Bosch. It is manufactured with the help of surface micro-mechanical technology. It can be integrated with the airbag control unit, and is capable of providing signals along the longitudinal and lateral axes of the vehicle. These signals in turn generate the airbag triggering decision and provide a plausibility signal.

Lateral Acceleration Sensor

The lateral acceleration sensor introduced by Volkswagon measures the lateral acceleration of the vehicle in order to determine the actual position of the vehicle. It can be located on the same housing as that of the rotation-rate sensor.

Low-g-Sensor

Bosch's low-g-sensor is designed for sensing the rollover of the vehicle. This sensor measures the acceleration of the vehicle along both the longitudinal and vertical directions. With the help of the angular rate signal, the rollover sensing algorithm of the sensor can detect the rollover event of the vehicle. It also transmits additional sensor signals with respect to the vehicle dynamics to enhance the rollover sensing application.

Sources and Further Reading

• Koenning, M and Heger, T, Usage of Surround Sensor Information for Passive Safety - Challenges and Chances, Transportation Research Board, 2009
• Pack, R et al, Pre-crash sensing countermeasures and benefits, Transportation Research Board, 2005.

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