Draft – Mar v6

NHTSA crash wall load cells

• Load cells should be based on 123.5 mm sq faces to enable a 125mm crash wall array

• Load cells capable of measuring up to 3 axial forces X,Y & Z• Load cells capable of measuring 2 moment measurements My and Mz.• Moment measurement should be inside the surface face dimensions of

the impacted side.• Moment measurement would be used to generate point load positions of

struck impacts on any one load cell face.

Specification requirements

Draft – Mar v6

F(x)

Typical moment measurement for load cell

Moment

• For typical moment measurement you are normally measuring outside sensor envelope from a fixed point on the sensor.

• In this case many types of sensor configurations can be used , i.e. single column, 4 column etc.

Fixed loading plate

Draft – Mar v6

NHTSA requirement for moment measurement

F(x)

Moment • NHTSA’s requirement is for moment measurement effect inside the surface face generated by F(x).

• This requires moment measurement to be possible for any position of force on the load cell face

• In this case the sensor type must be able to provide a definitive datum for moment measurement on the sensor face.

• This makes certain configuration of sensor difficult to accurately use for this type of measurement.

Draft – Mar v6

Example of 4 Column Sensor design

F(x)

Moment

• Graphs provide examples of outputs from a 4 column load cell .

• It shows a good moment profile over the load cell face.

• Unfortunately it produces the same output over two sides of the load cell making it impossible to distinguish where the force was applied

0 24.7 49.4 74.1 98.8 123.501234567

150KN100KN50KNm

V o

utpu

t

mm distance over load cell face

Draft – Mar v6

5 Column Sensor design

F(x)

Moment

• Using the central column in a 5 column design enables the centre of the load cell to be the datum point.

• This enables a +ve to –ve design over the face – providing distinct output for each impact over the load cell face.

• The trend line for each force is within 3%

-56 -43 -31 -19 -6 6 19 31 43 56

-8

-6

-4

-2

0

2

4

6

iWall Load Cell - Moment

75kN

Linear (75kN)

150kN

Linear (150kN)

225kN

Linear (225kN)

300kN

Linear (300kN)

375kN

Linear (375kN)

400kN

Linear (400kN)mm distance over load cell surfaceO

utpu

t (m

V)

Draft – Mar v6

F(x)

F(z)

F(y)

M(x)

M(y)

M(z)

• Load cell can be produced with either single axis or tri-axial force measurement.

• Torque measurement can be easily added in all 3 directions, due to the 5 column design

FTSS typical axis of measurement for crash wall load cell

Draft – Mar v6

Load Cell Base

Impacted Face of the Load Cell

Central Column

Strain gauge mounted on the columns

FTSS Crash wall load cell

Draft – Mar v6

5 Column Sensor design

-5 -4 -3 -2 -1 0 1 2 3 4 5

-15000

-10000

-5000

0

5000

10000

15000f(x) = 3708.44698344371 x − 67.9682320418987R² = 0.97749409430157

My Moment

My MomentLinear (My Moment)

X Axis = load cell mV output

Y ax

is =

mea

sure

d m

omen

t

Draft – Mar v6

5 Column Sensor design Calibration using 41mm square loading

pattern

• The load cell surface was split up into a 25 position matrix

• The 41mm square loading pattern then had varying forces applied from 50 – 300KN

41mm sq loading pattern

Draft – Mar v6

5 Column Sensor design

• Tested over 41mm sq loading patterns• For tested load cell with 160 different force and position impacts

over the load cell face.• This system provides accurate pin pointing of where the force is

applied on the load cell face typically within 4mm of actual position

• The largest error is over the outer columns which presently give slightly low readings of up to 10mm.

• With the present 5 column design the maximum point load error could be 10mm, however we are looking at improving this further with in the mechanical design.

Typical Characteristics