Few systems have become so challenging to repair as passive restraints. Keeping up with changes is critical to keeping your customers -- and your technicians -- safe.
CONTRIBUTED BY I-CAR
When a vehicle arrives at a collision repair facility, the passive restraint system has often deployed. Some airbags may have deployed, but are there airbags that did not deploy? What caused each airbag to deploy? Did it deploy correctly? Are there any live charges still able to deploy? These are questions technicians must ask and determine the answers to
each and every time they start a new repair job. To successfully restore today’s passive restraint systems, technicians must understand how restraint systems may deploy and where information about their operation can be found.
From the introduction of passive restraint airbags, not all systems have operated identically. There is no standard requiring airbags to deploy following one set of input parameters. The same can be said for the system output. If the system is a dual-stage airbag, for example, it is not required that all dual-stage airbags deploy the same.
When the first airbags were developed, there were no seatbelt pre-tensioners. There was typically only a front airbag for the driver. Occasionally there may have been a passenger airbag available, but only as an option. These first airbags either deployed or they did not. They were designed to deploy when a specific deceleration and impact threshold was exceeded.
Passive restraint systems today are much more complex. Depending on the vehicle maker, airbag deployment criteria may include any of the following, or more, measured crash data:
- Vehicle speed
- Rate of vehicle deceleration
- Occupant seatbelt use
- Occupant size and seating location
- Angle of impact
- Severity of impact
Preventing Occupant Injury
Airbags were first designed to deploy at a predetermined rate regardless of the type of impact. When this occurred, there was a lot of concern due to small-framed adults and people positioned too close being injured by the generated forces of the deploying airbag. Some of the airbag-related injuries included some deaths to children and small adults.
To address this problem, airbag deployment rates were changed, multiple-stage airbags were added, the airbag deployment criterion was revised, and the ability to turn the passenger airbag off was added.
Several vehicle makers responded to the problem by lowering the airbag deployment force by 20 percent to 35 percent. These “depowered” airbags are common in many 1998 model vehicles. For technicians, depowering alone does not affect current repair procedures. Another safety feature limits airbag deployment during less severe crash scenarios if an occupant’s seatbelt is buckled.
One improvement being explored to prevent airbag injuries includes altering the way an airbag deploys, such as an airbag that expands in diameter before it expands outward toward the occupant (radial deployment). This is accomplished by directing the expanding gas to the outer fabric layer first. Radial deployment is designed to prevent injury to an occupant sitting too close to the airbag. It also helps prevent head and neck injuries from the airbag contacting under the occupant’s chin. The driver’s airbag used on the 2004 Jaguar XJ8 uses a radial deploying airbag.
Dual-stage airbags are a popular added feature. As early as 1994, BMW began using dual-stage airbags in its vehicles.
Dual-stage airbags contain a high- and low-power charge, and they are designed to deploy at different rates depending on the severity of the collision. If a collision is severe, both igniters may deploy simultaneously. If the collision is not considered to be severe, there may be a delayed deployment between the igniters, or only one igniter may deploy.
A problem faced by vehicle makers was in the design of pick-up trucks. Pick-up trucks either have no rear seat, or inadequate room on the rear seat for securing a child seat. This was one of the reasons airbag cutoff switches were installed. The airbag cutoff switch disables the passenger airbag when the passenger is not large enough to withstand the deployment forces.
Decision-Making Systems
As early as 1988 BMW began transforming from a “to-deploy-or-not-to-deploy” situation, to a smarter system that decides if the seatbelt pre-tensioner and/or airbag should be deployed. For BMW, this decision is based on whether the occupants have their seatbelts buckled or not.
An example of how this type of system functions for the driver is in the event of a non-severe impact, the driver airbag may not deploy if the driver is wearing their seatbelt. In a severe impact, regardless of seatbelt use, the airbag will typically deploy.
Now, add in different vehicle makers—and the passive restraint systems that use dual-stage airbags—and the deployment possibilities are many. If the driver is not wearing their seatbelt during a non-severe impact, and the airbag does deploy, it may only deploy the reduced force charge. As impact severity increases and seatbelt use changes, depending on the passive restraint deployment parameters, during a severe impact the airbag may deploy both inflator charges in a simultaneous or in a staggered sequence.
With a system capable of making decisions, the restraint system electronic control module (ECM) determines how many stages and in which sequence the airbags should be deployed. On these systems, during a collision, the restraints system ECM determines which seatbelts are in use, and how quickly the vehicle is decelerating. These inputs allow the ECM to determine if an airbag should be deployed, which airbag or airbags should be deployed, how many stages should deploy, and in which sequence (if the vehicle maker permits delayed or staggered deployment of their dual inflator system).
General Motors Corp. (GM) explains the function and operation of one vehicle equipped with dual-stage airbags of the system this way: “The inflator modules have two stages of deployment, which varies the amount of restraint to the occupant according to the collision severity. For moderate frontal collisions the inflator modules deploy at less than full deployment, which consists of stage one of the inflator module. For more severe frontal collisions, a full deployment is initiated which consists of stage one and stage two of the inflator module.”
In its description of the Second Stage Airbag Deployment Check statement, one vehicle maker states, “if stage one has deployed, the airbag will be exposed. It cannot be determined by appearance alone that the second stage has deployed. If the second stage has deployed, the deployment circuit will be shorted and a diagnostic trouble code (DTC) will be set in the restraints control module (RCM) memory.”
If a passive restraint system part does not deploy, always read the system description and operation for the circuit before condemning a part as faulty or when you are unsure of how the system is designed to function.
Identifying a Passenger
As early as 1994, BMW was using a passive restraint system that was able to identify if an occupant was in the passenger seat. This system is called occupant recognition (OR). If no occupant is in the front seat, the passenger airbag is not designed to deploy.
Mercedes-Benz first developed the ability to identify when specific child restraints are secured to the front passenger seat and disable the passenger airbag. Starting in 1998, Mercedes-Benz began using a system called Automatic Child Seat Recognition (ACSR). In conjunction with Britax of Europe, these special type child seats are equipped with two transponders in their BabySmart seats.
Along with the operation of systems that monitor seatbelt use for determining airbag deployment, OR and an occupant classification system (OCS) also determine airbag deployment by the following:
- weight of the occupant in the front passenger seat. This is determined by either a silicone-filled bladder or strain gauges.
- stress on the seatbelt.
- proximity of the passenger to the airbag. This is either determined with a seat position sensor or ultrasonic sensors.
As an example, DaimlerChrysler categorizes occupants in three groups depending on weight. When a weight is detected on the front passenger seat approximately equivalent to a:
- rear facing infant seat, less than 17 kg (37 lb.), the OCS is designed to prevent deployment of the passenger airbag.
- six-year-old child, 21.5 kg (47 lb.), one production model vehicle uses a “low risk deployment” feature. This means that if specific crash criteria occurs, the least forceful airbag charge may be activated for an occupant between 17.5 kg (38 lb.) and 45 kg (98 lb.).
- 5th percentile adult, more than 45 kg (99 lb.), the airbag may be allowed to function normally, depending on the crash data received.
Airbags Other Than Front Airbags
Depending on the vehicle maker, airbags may be located virtually anywhere in a vehicle cabin. Airbags may be located in the knee bolster, door panel, seatback assembly of the front and or rear seat, or there may be a curtain-type airbag that drops down from the headliner.
Knee airbags may be located on both the driver and passenger side of the vehicle. The function of a knee airbag is to apply pressure against the front seat occupant’s knees or shins during a frontal collision. This type of airbag is designed to reduce the opportunity for the occupant to submarine under the front dash and reduce injuries to the knees. Only a few vehicle makers use knee airbags.
Door panel and seat back assembly airbags typically are designed to deploy during a side impact. The deployment of these airbags helps to limit the force at which the occupant makes contact against the side of the vehicle. Even though the airbags are only inflated for a fraction of a second, there is still protection provided for the occupant. There are different situations and configuration as to how seat back airbags may be installed. A 2002 Buick Regal uses a seat back airbag on the driver seat only, a 2002 Dodge Neon comes equipped with both driver and passenger front seat back airbags, and the 2004 Audi A8 seat back airbags are available for both the front and rear seat passengers.
Like a seat back airbag, curtain airbags also provide protection for perceived side impacts and roll-overs. Depending on the vehicle maker, but typically at a roll-over angle of about 52 degrees, the curtain airbags may be deployed. There are basically two designs of curtain airbags. The first and most common type deploys down from the headliner and covers all the side windows and pillars.
Another type of curtain airbag is the Head Protection System (HPS) by BMW. This type of curtain airbag is different because instead of being a rectangular airbag attached to the roof and hidden by the headliner, it is a tubular-type airbag that connects to the A-pillar and attaches back near the C-pillar. This tubular airbag has a rubber liner that does not permit the quick release of the inflation gases and remains inflated until the warm gases inside the tube cool.
Future Technologies
Advances in passive restraint technology continue to occur. One of the latest is Honda’s use of a side airbag cutoff switch. This system is designed to identify if an adult or child is present in the front passenger seat, and if they are out of position for the seat back airbag to safely deploy. According to Honda, an out-of-position child is “if a child leans sideways and the child’s head is in the side airbag deployment path.” According to Honda, an out-of-position adult is “if a short adult leans sideways or a larger adult slouches and leans sideways into the side airbag deployment path.”
One vehicle maker that is constantly updating and looking into better ways to provide occupant protection is Volvo. Volvo has developed a Volvo Concept Safety Car. Some advancements for occupant protection and collision avoidance used on this vehicle are a collision avoidance system, four-point seatbelts, a see-through A-pillar, which provides an increased field of vision, a B-pillar contoured to the seat frame assists in reducing the blind spot for the driver and an external airbag between the hood and the windshield. The airbag, located between the hood and the windshield, is designed to limit injuries to a pedestrian’s head by striking the A-pillar of the vehicle. This airbag is activated by a sensor located at the front of the hood of the vehicle and remains inflated for a few seconds to extend the protective cushion.
Another passenger safety system that is being used in some Mercedes-Benz and Lexus vehicles is a pre-accident system called PRE-SAFE by Mercedes-Benz and Pre-Collision by Lexus.
Pre-accident activation between vehicle makes and models will be different, though the pre-accident function is typically activated when the vehicle computer detects a specific criteria that is a typical input before a collision commonly occurs. One of these inputs is the computer detecting a panic-type stopping situation, or if an unavoidable obstacle is detected and a collision is anticipated. When this occurs, slack is removed from the seatbelts, the front and rear seats adjust to a pre-determined safe occupant position that will position the occupant in an ideal position for airbag deployment, the sunroof may close if opened, and the roll bar may activate, if equipped. Seatbelt slack on these types of systems is removed by a reversible electric motor.
Knowledge is Key
All collision industry professionals have the possibility of being faced with any one of the passive restraint systems mentioned in this article. Without knowing how the systems are designed to operate and what, or who, was in each seat of the vehicle, it is impossible to know if a system was working as designed by the vehicle maker. This makes it especially important to have access to the service manual information from the vehicle maker.
The different deployment scenarios that can occur with today’s advanced passive restraint systems is one of the subjects included in a new I-CAR program, “Advanced Restraint Systems (RES02),” coming soon.
