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FTSS Presents Flex-PLI-GTR (Flexible - Pedestrian Legform Impactor – Global Technical Regulation)
In the year 2000, the Japan Automobile Manufacturers Association, Inc (JAMA) together with
the Japan Automobile Research Institute (JARI) initiated the development of a pedestrian legform impactor to assess lower leg injury on pedestrians in car accidents. The legform at
that time used rigid long bones, so to improve biofidelity a flexible bone arrangement was considered.
In 2002, an initial design of Flex-PLI was made available, followed by the Flex-GT version in
2006. In September 2005, a Technical Evaluation Group (Flex TEG) was founded under the UN/ECE/WP29/GRSP/ informal group consisting of government and industrial parties to
evaluate the legform as a regulatory purpose test tool for Global Technical Regulation on Pedestrian Safety (PS-GTR). First Technology Safety Systems (FTSS) is part of this group as
the dummy manufacturer and was asked to review the GT design and manufacture the legform. The review kicked off in August of 2007 and highlighted a number of improvements;
the proposed Flex-PLI-GTR design was accepted in April 2008. The performance of the legform was to be unchanged to maintain biofidelity and to ensure existing test data remained
valid. Therefore, the existing overall design, size, mass and materials were maintained as much as possible.
The Flex-PLI-GTR legform represents a 50th percentile male leg designed for right side impact testing. It
simulates the flexible nature of the human bone and can assess pedestrian lower leg and knee injuries. During
a test, it is fired from a linear guided launcher onto the bumper of a static vehicle at a velocity of 40 km/hour.
The key improvements made in the latest version were: centralizing the knee ligament deflection sensors to
avoid impact direction sensitivity on curved car bumpers; balancing the cruciate ligament spring force in the
knee joint to prevent a twisting action between the two knee components; and the introduction of a full bridge
configuration to the multiple strain gages in the leg bones. This increased the voltage output by using both the
tensile and compressive strains of the bone and made the gages insensitive to elongation and temperature. In addition, handling was enhanced. Flex-PLI-GTR also has the capability of an onboard Data Acquisition
Systems (DAS). By using an onboard DAS, it helps in eliminating the usage of umbilical wires thus improving
free flight stability. It also helps in preventing wire damage which could cause data loss during a test. FTSS
also reviewed and updated the numerous quasi static calibration procedures for internal bones, thigh, knee
and lower leg assemblies. The dynamic calibration rig was also updated to provide more realistic loading and to improve reproducibility.
Instrumentation
The standard leg instrumentation has 12 channels; this includes three full bridge strain gages in the thigh and
four in the lower leg all measuring bone bending moments at spaced intervals along the bones. In the knee are
four string potentiometers measuring ligament elongation. An accelerometer is placed in the lower knee to
measure acceleration in the impact direction. Additional accelerometers and angular rate velocity sensors can be added to the leg but these are recommended for research only.
Leg Features
The leg bones consist of a segmented assembly mainly made from a high strength plastic with a fiber reinforced inner “bone”. Strain gages are bonded to the bones, and each gage channel set is calibrated
individually to establish gage sensitivity. Stainless steel wires limit bone bending at the injury threshold to
prevent bones being overstressed. Links are used to connect the segments to maintain an even spacing, and rubber buffers are used to prevent segment contact. Each leg sub-assembly is certified to biomechanical
corridors.
The knee consists of two sections: upper and lower. The knee joint is flexible by using springs and wires to
simulate ligaments. The springs are designed to meet the required ligament force and range of motion. The knee
is certified to biomechanical corridors. Onboard DAS is housed at either side of the knee upper section (six channels each side) and connector blocks are fitted in the lower knee to allow switching from onboard to
offboard DAS. Side covers are used to protect the wiring and electronics.
The flesh comprises of a combination of rubber and Neoprene foam sheets. The bulk of the rubber is in the
thigh to provide a humanlike flesh response and mass distribution.
Three Flex-PLI-GTR prototypes were manufactured and delivered to JARI in November 2008. One unit was
equipped with offboard DAS, one unit with onboard Messring M=BUS DAS and the third with onboard DTS SLICE DAS.
The M=BUS is an independent 6-channel logger 40x25x14 mm in size. The units can be daisy chained together via a single small coax cable ending in a terminator which checks system integrity and quality. Two units are
required on the Flex-PLI-GTR for standard instrumentation and packaged at either side of the upper knee. Each
logger is equipped with its own battery providing 17 seconds of record time (test duration time is around 1
second from firing). The system has a low friction disconnect on firing and on reconnection after the test data can be downloaded to a PC equipped with Messring’s Crash Soft 3 software for analysis. Time
synchronization of all channels is guaranteed over an integrated master and slave clock concept.
The DTS SLICE Nano data recorder is a modular system, where units are stacked together to provide the
required number of channels. The Base SLICE (31x26x6.5mm) contains the processor and memory while the Bridge SLICEs (31x26x5.5mm) are stacked on top providing channel functionality. Ten Bridge SLICEs can be
connected to one Base SLICE providing 30 channels. Due to the limited space on the Flex-PLI-GTR, two Base SLICEs were required; each stacked with two Bridge SLICEs handling 6 channels. For the prototype two super
capacitors were used to power the system after disconnect (to allow quick recharge,) but for future builds a
battery will be provided. The system is reconnected after firing and the test data is downloaded to the DTS software.
The initial results from the prototypes will be published at the upcoming Enhanced Safety of Vehicles (ESV) conference (www.esv2009.com).
FE Flex-PLI-GTR Consortium
Finite element models of the Flex-PLI-GTR are being developed in a consortium project sponsored by ten OEMs and FTSS. These OEMs collectively need models in all relevant explicit Finite Element Codes (LS-DYNA,
PAM-CRASH, RADIOSS and ABAQUS).
Alpha and beta releases are planned in May and December 2009, respectively.
The beta versions will be validated against an extensive experimental data set that has been defined by the
consortium members. The experimental database consist of approximately 30 material tests, fifteen component tests and up to 50 full leg tests.
Important parts of the validation are the full leg form load cases as illustrated in the photo (see above). In these
load cases, the leg form is kept in static position through a support connected at the top of the leg. An
impactor is hitting the leg at several locations in the knee region. As soon as the legform contacts the impactor,
the top connection is released and the leg is in free flight state. This test set-up offers inverse loading
conditions reflecting typical loading conditions as can be observed in vehicle testing but being very suitable for
model validation due to its simplicity. The inverse tests are being done at different speeds and for different
impact angles. Tests are repeated and even reproduced at different labs; Honda R&D Americas, Inc. and by BGS Böhme & Gehring GmbH at the Federal Highway Research Institute, Germany.
For detailed information, please contact your local distributor or visit www.ftss.com.
Typical impact with car bumper showing leg flexibility:
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