Tuesday, October 26, 2021 · 16 min read
Car Seat Crash Test With a Catapult
The Dynamics Test Centre (Centre d’Essais Dynamiques - CED) is a public test laboratory open to any industrial or research organization. Among others, CED provides the test facilities for seat crash testing, including a powerful reverse pneumatic catapult. This is used by manufacturers in the automotive or aerospace industry to evaluate seat behaviour during an impact - to guarantee occupant safety in case of an emergency such as a crash.
The engineers wanted to view the test signals in real-time during the test. Now they use software and rugged data acquisition systems from Dewesoft for seat mechanism testing and crash test dummies calibration.
A seat crash test is almost like a rocket launch. A seat and a dummy are placed on the catapult test bench which is then to be released and accelerated - or decelerated - at specific speeds to simulate a frontal collision. Focused and organized technicians meticulously check the test specimen and set up - the test bench, the seat, the test dummy, each mount, cable, and sensor.
When everything is ready, everybody evacuates the test room. Behind a huge armoured glass window in the test control room, they wait. Then, the chief operator sounds a loud nerve-racking alarm. People now know that the test is imminent. And BAAANG !!! - you hear the air spring release and feel ground vibration - even in the isolated control room. The dummy takes the hit.
Days or weeks of preparation - and it all happens within less than a second! It is a one-shot measurement. And the seat with the dummy? Have you ever been on a roller coaster or experienced other sensational rides?
Now, imagine this banging into a wall. The test bench of CED is the most powerful in Europe. It is capable of projecting a 4 tons specimen at 90 kph along 1 to 2m which corresponds to the distance covered by a crash (vehicle deformation).
Seat crash testing
For cars, crash testing – or dynamic testing – is a mandatory regulated safety test to homologate vehicles. Automotive seats allow occupants to drive relaxed and safe. Nowadays, the seat can combine many mechatronic functions such as adjusting automation with an electric motor, heating, and even an active sound system or smart driver recognition system.
A seat combines hundreds of mechanical components. The most basic ones regarding safety are the anchorage mechanism - the inner track, the locking mechanism, and the bolts - and the seatback, not forgetting essential safety parts such as airbags and belts. Modern seat kinematics is complex. The seat mechanism components must be able to effortlessly recline, lift, adjust and swivel the seat, and then return it smoothly back to the driving position - see figure 1.
To implement a numerical model of resistance to vibrations and shocks, the CED R&D simulation department needs real road load data and accurate extreme mechanical stress in a real crash test situation. The science of materials and the study of surfaces - and contacts between them - are also applied to understand mechanical behavior and potential after-sales issues.
The dynamics test center - CED
The Dynamics Test Centre CED belongs to the Industrial Campus of Research and Innovation Applied to Materials (CIRIAM) – recently renamed Normand’Innov, and is supported by the regional council of Normandy in France. Based in the city of Caligny, CED includes test facilities such as huge six-axis vibration tables, and installations for climate durability, squeak & rattle testing, as well as the reverse pneumatic catapult.
The test center is equipped with several six-axis vibration tables for durability testing with a wide range of vibrations and loads. E.g., multi-axial simulation such as road, aviation, and seismic. See figure 2 for more details.
Those vibration tests can be combined with a removable climatic chamber for hot, cold, or humidity, or with a semi-anechoic chamber for squeak & rattle noise measurement, electric car development, acoustic comfort, and more. With the reverse pneumatic catapult, CED works for companies in the automotive and aerospace industry, like Airbus, Safran, Zodiac Aerospace.
The 61 hectares site in Caligny also comprises the FAURECIA seating mechanism production and a branch of ENSICAEN University. This worldwide R&D center for the design of seat mechanisms excels in internships and training in mechanical engineering and materials science.
Faurecia S.E. is a French global automotive supplier headquartered in Nanterre, in the western suburbs of Paris. It’s in the world top-10 of the largest international automotive parts manufacturers and at the absolute top for vehicle interiors and emission control technology. One in three cars worldwide is equipped somehow by Faurecia.
Reverse catapult testing
The powerful pneumatic reverse catapult at the CED test center is used for impact testing and real-time dynamics testing of automotive structures and equipment like dashboards, seats, or child seat restraints. The bench consists of a trolley that is projected by a hydro-pneumatic air spring to reproduce impact/shock situations on mechanical structures and equipment. It enables crash testing with 3.1 Mega Newtons, 90kph, 122 g, and a 3000 kg usable payload - see figures 3 and 4.
The device on which dynamic tests of seats and head restraints are conducted is a steel flatbed sled that runs on fixed rails. The sled is moved to simulate vehicle crash accelerations, re-creating the forces on occupants inside vehicles during real-world crashes. The changing acceleration or deceleration over the time duration of a crash is referred to as a crash pulse, and the key aspect of a sled is that it can be programmed to produce specific crash pulses.
To evaluate rear crash protection, vehicle seats are affixed to the sled. The sled is accelerated to simulate a stationary vehicle that's rear-ended by another vehicle of the same weight going 32 kph (20 mph). To accomplish this, compressed air is pumped into a special cylinder, thrusting a ram forward in a preprogrammed pattern of acceleration (crash pulse).
The catapult guarantees the repeatability of multiple deceleration pulses complying with the constructor’s specifications and meeting authority regulations and safety standards such as:
|ECE17||UN/ECE||Regulation No 17 of the Economic Commission for Europe of the United Nations (UN/ECE) — Uniform provisions concerning the approval of vehicles with regard to the seats, their anchorages, and any head restraints.|
|ECE94||UN/ECE||Regulation No 94 of the Economic Commission for Europe of the United Nations (UN/ECE) — Uniform provisions concerning the approval of vehicles with regard to the protection of the occupants in the event of a frontal collision.|
|ECER44||UN/ECE||Regulation No 44 of the Economic Commission for Europe of the United Nations (UN/ECE) — Uniform provisions concerning the approval of restraining devices for child occupants of power-driven vehicles (‘child restraint systems’)|
|Whiplash||Euro NCAP||Euro NCAP’s whiplash tests are designed to promote best-practice seat and head restraint design i.e. those designs which are known from accident data to provide the most effective protection in the real world.|
|Euro NCAP||Euro NCAP||The European New Car Assessment Programme is a European voluntary car safety performance assessment program. The safety rating is determined from a series of vehicle tests, designed and carried out by Euro NCAP.|
|US NCAP||National Highway Traffic Safety Administration (NHTSA)||A provision of the Safe, Accountable, Flexible, Efficient Transportation Equity Act: A Legacy for Users (SAFETEA-LU) requires new passenger vehicles to be labeled with safety rating information published by the National Highway Traffic Safety Administration under its New Car Assessment Program (NCAP).|
|FMVSS (202, 207, 208, 225)||Federal Motor Vehicle Safety Standards||Federal Motor Vehicle Safety Standards (FMVSS) are U.S. federal regulations specifying design, construction, performance, and durability requirements for motor vehicles and regulated Automobile safety-related components, systems, and design features.|
|AS8049||SAE Aerospace Standard (AS)||Performance Standard for Seats in Civil Rotorcraft, Transport Aircraft, and General Aviation Aircraft.|
The engineers of CED use a dedicated acquisition system and software on the catapult from KT Automotive GmbH, an offspring from Kayzer-Threde Gmbh, now part of the Kistler Group. However, with the standard crash test system, they were unable to view the test signals before, during, or immediately after the test. The test worked like a black box recording the data in standalone. To view the data and do their calculations the engineers had to go in post-analysis mode and export the data.
To develop the test bench and especially acquire the capability for strain gauge measurements requested by their clients, the engineers of CED were searching for DAQ system supplying:
6 channels strain gauge conditioner (full, semi, quarter bridge).
Compact & robust system to be embedded in the catapult trolley.
Shockproof from 25 to 60G (with a rate of around 50 tests/year).
Maximum sample rate 20kHz per channel.
Digital input to synchronize start acquisition with the catapult system.
Possibility to visualize measurements in real-time or to have test results status immediately at the end of the test. This was not possible with the existing system which works like a black box and needs a long post-treatment analysis.
The number of channels used depends on what needs to be analyzed - the structural mechanism of the seat, human behavior on the seat, or both.
The CED engineers chose the hardware solution proposed by Dewesoft - see figure 6:
KRYPTON 6xSTG: 6-channel universal strain gauges data acquisition system.
KRYPTON ONE 4xDI: 4-channel digital input module.
ECAT Power Junction: for injecting power supply.
Specific 50m flexible EtherCAT cable KAWEFLEX® dedicated for rolling trolley and repetitive mechanical movement.
Sample rate 20kHz per channel.
The Krypton One 4xDI receives a digital signal from the catapult trolley that triggers the measurement to synchronize the two data acquisition systems. As DewesoftX can start measurement on many conditions and with pre-trigger time, no data is lost and the results can be exported in multiple formats to be compared to any third-party data acquisition system.
Dewesoft hardware comes with the powerful data recording and analysis software - DewesoftX. A large choice of mathematical functions is available:
As it is easy to use, the engineers of CED can quickly create custom multi-physics displays with real-time calculations - see figures 9 and 10.
The crash test dummies
Crash testing involves a crash test dummy. Crash test dummies are designed to simulate the human response to impacts, accelerations, deflections, forces, and moments of inertia generated during a crash. They enable the study and development of crash-worthy structures and restraint systems.
Equipped with multiple built-in or mounted sensors they pick up and record a range of values during the test. The values that are recorded, along with other measurements made during the crash test, allow evaluating whether the test meets standards.
All details – the dummy’s clothing, the tension of the harness, the position of the chest clip, the tightness of the vehicle belt – are regulated by standards allowing for tests to be repeated and compared when done at different laboratory facilities.
The instrumented dummies at CED are made by the American manufacturer HUMANETICS, a global leader in the design, manufacture, and supply of crash test dummies faithfully modeling human beings. The company even supplies calibration equipment, crash sensors instrumentation, software modeling, and active safety testing equipment. And such dummies are not cheap toys. The price of one dummy is similar to that of a very nice family car.
In all physical crash tests, the dummies are used to measure the forces and assess likely injuries of vehicle drivers and passengers, adults or children. A range of destructive crash tests is conducted to simulate the most common types of crashes including frontal impact, side impact, run-off-road, and rear-end.
Per year, the CED test center makes around 600 to 800 crash tests - both frontal-reverse impact and side-impact crash tests - depending on the number of test specimens made. 20 to 30 measurement channels are needed for one dummy only. That is why additional channels such as Krypton are applied.
Krypton allows us to easily and quickly acquire up to six strain gauges on a test
says Baptiste Fleury, Crash Test Engineer at CED Normandy. “We use these gauges to measure the stresses at different places of the seats. For example, checking a potential weakness that has been identified in the calculation, dimensioning a component according to the measured stresses, or even providing data to the calculation on places that are difficult to model or simulate”.
Before a car seat hits the market, more crash tests may be performed. Manufacturers run crash test simulations throughout the design and development of a product. Early crash tests are done for research purposes to help determine what design areas need to be improved or clarify whether a potential product alteration will be beneficial or not. However, in some cases, the first crash tests are only done when the prototype seat is available.
Load cells, accelerometers, and displacement transducers are included in the dummies. They help engineers to evaluate what a real human body goes through in terms of shock and impact force. They are also used to evaluate the efficiency of the seat security systems such as:
Also, other transducers such as strain gauges are mounted on the seats. These allow the engineers to see how the materials used will behave in case of crash situations - if they are strong enough to avoid breaking, losing, or deforming.
Versatile DAQ modules
The new Dewesoft Krypton modules at CED are not just rugged but also versatile. In their metrology department - the “Dummies Clinic”, the engineers now also use these for other measurement tasks such as calibration of sensors and dummy health checks.
As crash test dummies simulate the human response to impacts, accelerations, deflections, forces, and moments of inertia generated during a crash - they take a heavy beating. Throughout their “lifetime” dummies are exposed to high shock levels and all the sensors inside - force, torque, displacement… must be checked regularly. This is done on specific test benches such as impactor test benches or tension-compression machines. The high accuracy and universal inputs of Krypton modules are well-suited for those calibration purposes.
The test procedures for dummies are regulated and described in test standards. The engineers either do a test on a dynamic pendulum impactor or an electromechanical test bench for traction and compression - see figures 13 and 14.
The dynamic pendulum impactor provides data on the behaviour of materials or components subjected to rapid loads. It is used to measure the response of an impact on the human Thorax - the chest region between the neck and the abdomen. When testing with the pendulum impactor, a known reference mass bumps into the dummy at a known velocity. The force/deflection response of the dummy’s sternum or chest bone is measured.
The electromechanical test bench is used to measure the sensor response curve. It applies a known static load or displacement and the deformation of the sensor itself is checked.
The entire data acquisition measurement chain can also be calibrated with an electrical reference generator signal. Every sensor has an electrical output that is scaled into a physical unit in the acquisition system. A calibration process is done to check if the data acquisition system itself measures the correct value. A calibrated signal generator is used as a known electrical voltage reference and the signal is sent to the inputs of the data acquisition system.
The Dynamics Test Centre CED in Normandy FRANCE has chosen rugged Dewesoft KRYPTON acquisition modules for their seat crash test catapult. With shockproof up to 100G, strain gauge, and universal STG amplifiers inside, KRYPTON units are suited for extreme vibration environment use and high-resolution measurement range.
Easy to use and versatile, we use the KRYPTON modules both for crash tests to study the dynamic behaviour of structures/materials and for high accuracy calibration operations
says the Crash Test Engineer Baptiste Fleury.
French television report on CED
Video report on CED test center
Crash test dummies at CED