The Torsional Sensing Load Cell for
Occupant Positioning Sensing
Bob Bruns
bobbruns@pacbell.net
Introduction
Torsional sensing load cells can provide an accurate,
easy to implement and low cost solution to the
problem of occupant position sensing for air bag
deployment. Such load cells, placed on the corners of
the seat frame can provide accurate information
regarding not only the weight of the passenger, but by
calculation of the centroid, information of the position
of the passenger. This information can be used to
reduce the air bag deployment force for children,
small statured individuals and out of position
occupants. Their unique design means they can be
optimized for high output from normal forces, and yet
made strong in orthogonal directions to provide
maximum strength in the event of a crash. The
torsional sensing load cell is a proprietary
development of the GageTek Company.
It’s
predecessor, the helical load cell was the winner of the
ISWM Kardux Cup in 1998, recognized as the most
significant advancement in the weighing industry for
that year.
The Torsional Sensing Load Cell, theory
The equations of interest for the Torsional Sensing
load cell, TSLC, can be derived from the operation of
a spring. A spring converts the linear force to a
torsional moment in the wire. The general case of
loading results in three reactions, T, F and M. These
forces are shown in figure 1. The torsional force, T,
is the only force of interest in the Helical or Torsional
Figure 2. The Torsional Sensing Load Cell, TSLC
Sensing Load Cells. The torsion results in a strain
which is present on the surface of the wire and can be
measured by mounting shear pairs at diametrically
opposed positions on the spring at A and B.
Figure 3. The Slot Block TSLC
Figure 1. The Helical Load Cell with resultant
forces and moments
A derivative of the helix is the Torsional Sensing
Load Cell. This load cell is constructed by essentially
machining a spring out of steel. This configuration is
shown in figure 2. Instead of the helix, the sensing
area forms a circle with strain gauge pairs mounted in
diametrical opposition. A variation of this is the slot
block TSLC as shown in figure 3. In this design the
load cell has been simplified further by removing the
upper and lower loading surfaces and building a load
cell from a single block of steel with a slot cut into it.
In this case the strain gauges are mounted on a single
surface, in this case the gauges are placed on a single
surface of the load cell. This arrangement results in
Figure 4. Free body diagram of the TSLC
the easiest assembly and lowest possible cost.
The TSLC is insensitive to off axis loads because of
the manner in which the torsion propagates around the
length of the load cell. A free body diagram of the
TSLC is show in figure 4. A free body diagram of the
loading results in the following:
TA = (FLOAD )(a)
From this analysis is clear that the total torsion present
at the point of application of the strain gauges is equal
to the total force times the effective diameter. It is not
dependent on the placement of the load. The result is
the same for any positional placement of F within or
outside the diameter of the load cell.
The loading force F is measured by placing shear pairs
on the sensing element in a standard wheatstone
bridge configuration. This is shown in figures 5 and
6. In this case the element has been straightened for
purpose of illustration. Placed on the neutral axis if
the torsional element the gauge pairs reject parasitic
linear forces and moments resulting in an output
proportional to the total torsion, and so the total load
force.
The gauges are placed according to the following
rules:
•
Gauges may be placed on any point along the
circumference of the element, but should be a
mirror image across the diameter or slot. Ie.
Top-top, side-side, or bottom-bottom.
•
Gauges should be placed so the presence of
other forces or moments are rejected by the
TB = (FLOAD )(b)
TTOTAL = (FLOAD )(a) + (FLOAD )(b)
TTOTAL = (FLOAD )(D)
Figure 6. Formation of wheatstone bridge for
configuration of Figure 5.
complementary pairs.
Figure 5. Positioning of strain gauges on the torsional
element
•
Gauges should be in diametrical opposition to
each other across the diameter or slot.
•
The structural cross section at the points of
applications of the gauges should be identical.
Configuring the TSLC for a seat
Fitting the Torsional Sensing Load Cell to a seat is a
simple matter. Generally speaking, seat mounting and
slider arrangements fall into two general
configurations, vertical rails and horizontal. In either
case the base or feet of the seat structure must be
isolated from the sliding rails that transport the seat
forward and back. This isolation is in the form of a
gap separating the two parts of the seat structure.
the seat needs to be 2mm narrower. Load cells can be
riveted or bolted into position.
Horizontal rails are handled in similar fashion. In this
case the isolation gap is created by adding 1 mm to the
height of the seat.
Figure 8. shows two typical
configuration for such a seat . In the first case the
sliding rail is place on top of the stamped foot. The
load cell fits recessed within the foot itself. Another
common configuration is when the horizontal sliding
rail connects directly to the chassis and the seat is
attached to the upper, sliding part of the rail assembly.
The TSLC configuration in this case is to create the 1
mm gap and place the load cells directly within the
seat as shown in the lower detail of figure 8. In this
case the load cell is again completely protected from
abuse as the complete assembly is within the seat
structure.
Figure 7. The TSLC configured for a seat with vertical
rails
Figure 7. shows such a configuration for a vertical rail
structure. In this case the normal attachment between
the feet that attach to the chassis and the sliding
mechanism of the seat is normally accomplished by
horizontal rivets connecting the stamped parts. An
isolation gap is created by moving each foot outward
and attaching the TSLC as shown over the ends of the
seat assembly. Note that in this configuration, as with
all others for this technology, the load cells in no way
interfere with the sliding mechanism of the seat. Both
motorized and manual sliding mechanisms can easily
be accommodated. A gap of 1 mm has proven to be
sufficient for this application. This means that the
base of the seat at the chassis need to be made 2mm
wider, or conversely, the attachments at the base of
Figure 8. The TSLC on two types of horizontal rails
In all mounting configurations it is desirable to have
the slot running axially from front to back of the
vehicle.
This way the beam sections can be
strengthened and optimized for front or rear collision.
Strength exceeding 2000 lbs per load cell or 8000 lb
per seat is easily achieved. During a side impact the
gap on one side of the seat collapses and the load cell
becomes in effect solid under the compressive load.
Very high loads can be resisted.
Load cells can be constructed to be mounted in other
directions. Strength is optimized by shaping of the
beam element and rear cross section of the cell to
accommodate different loading and impact scenarios.
shows, a simple I beam without the web can be
constructed simply by drilling a hole through the
torsional element, perpendicular to the direction of the
propagation of the torsion around the element. The
back, or solid end of the element can be lengthened to
provide strength in bending in the horizontal plane.
This lengthening does not affect the output of the cell.
In this way load cells have been constructed with
nominal outputs of 1 mV/V at 200 pound load and
still meet a crash requirement of 35mph or
approximately 8000 pounds force in the forward
direction of the seat without failure.
Construction of the TLSC
One of the fundamental principle of the TSLC is that
the output is dependent only on the torsional
compliance of the section which incorporates the
strain gauges. One of the most important secondary
characteristics
of
any
seat
based
Additional output can be obtained by using piezoresistive gage elements, such as those from B.F.
Goodrich Aerospace. Typically this type of gage has
a gage factor of 20, or ten times the output of a foil
gauge. This results in a proportionally higher output
obviating the need for a high gain differential
amplifier at the output of the bridge.
A typical construction of the TSLC for a horizontal
seat rail is shown under one proposed scenario. The
simple element is formed by machined, cast or
stamped element. Gages are applied to the upper
surface. An interconnection element is applied and
bonded to the interconnection points of the gauges.
This could contain an amplification ASIC. A molded
connector is attached and finally a protective
encapsulation layer is applied to protect the assembly.
The result is a rugged, low cost load cell ready for
mounting on the seat rails. Typical specifications for
such a load cell are linearity better that 0.1% full scale
and temperature coefficient <2% full scale over a
temperature range of –20 to 60 C. The output of a foil
based system is typically 1 mV/V, and a piezo
resistive system is 20 to 100 mV/V.
Calculation of Centroid
Figure 9. Construction of the TSLC
system is the ability to withstand collision. Another is
to provide sufficient electrical output to the signal
conditioner so that noise is minimized and expensive
cabling and amplification is not required.
These two requirements means that the load cell
should be constructed to have high strain and in
torsion, and low strain in tension, compression and
bending. A simple and common structural shape that
meets this requirement is an I-beam. As figure 9.
Since load cells are place at each corner of the seat,
calculation of the center of gravity is a as simple as
calculating the centroid of the forces at the corners of
the seat. Four forces, F1..F4 on the corners of the seat
are all at fixed and known distances from the center of
the seat. The centroid distance from the center of the
seat in the y direction, ly is the sum of these forces
times the distances divided by the total force.
Likewise the side position, lx can be calculated in
similar fashion.
This calculation provides both x and y position, and is
appropriate for both front and side air bags.
Some ancillary forces are present which cause an error
to be present in the weight measurement. These
forces are shown in figure 11. Generally these error
forces come from two sources, the force of the
occupant’s foot on the floor of the vehicle, shown as
FF, and the force FSB, the force on the seat back caused
by a rear seat passenger putting his hands on the back
of the front seat. In both of these cases, these forces
resolve into horizontal and vertical components, with
only the latter having an effect on the weight
measurement.
Costs
The cost of manufacture for this technology is
dependent largely on the method of construction of
the cell element and the type of strain gauges used. In
each costing exercise undertaken with this technology,
costs of implementation for each seat is less than $20
per seat, or $5 per load cell. Of this cost
approximately 60% is for load cell elements and the
strain gauges.
The other 40% is allocated to
encapsulation, packaging and connectors. Piezo
resistive stain gauges are more expensive than foil or
electro deposited gauges. These costs are offset by
the higher output and reduced requirement for low
noise instrumentation electronics for analog signal
processing.
Advantages/Disadvantages of the torsional load cell
technology
Advantages:
•
Extremely accurate weight information.
•
Calculation of position by simple centroid
algorithm.
•
Long lasting, will never wear out.
Figure 10, Calculation of the occupant position by
centroid
•
Accurate over temperature.
Seat belt tension causes another minor error to the
extent of the tension attached to points off the seat.
However, with this technology, tightening the seat belt
around a child seat mount does not cause an erroneous
signal as with pressure pads, because the weight at the
feet of the seat is unchanged.
•
Resistant to damage by puncture, foreign
objects or abuse.
•
The technology is protected by U.S. patents
5,714,695, 5,872,319, 5925,832 and patents
pending.
Disadvantages:
Figure 11. Ancillary error forces on floor and seat back
•
Vertical rails require moving mounting holes
1 mm outward
•
Horizontal rails require increasing the seat
height by 1 mm.
•
Affected by loading from rear seats when rear
passengers place hands on front seats.
Conclusion
The Torsional Sensing Load Cell is a viable solution
to the problem of detecting occupant weight and
position for passenger side air bag deployment. Such
load cells have been developed for seats with both
horizontal and vertical rail systems, with motorized
adjustment and manual position adjustment. The load
cells can be constructed to fit virtually any seat
construction with minimal impact to existing designs
and only slight movement of 1 mm in the seat foot
mount point or change in seat height. They are
rugged, durable and accurate and stable over
temperature. Output can be increased with the use of
semiconductor gauges, minimizing cost of processing
electronics. For more information the author can be
contacted at the above email address or at 916/9440970 or 916/801-7640.