CSEM Scientific and Technical Report 2008

Cost-effective 2D Force Mapping for Medical and Sports Application
P. O. Gaggero, J. X. Brunner, S. H. Böhm
Two dimensional force mapping is interesting in many areas. Cost becomes a critical factor when building a sensor array with a large number of
sensing points or a large mapping area. The work described in this paper uses commercially available inexpensive bulk material in a novel way to
build a first demonstrator of a 2D force sensor. The built device implements a 8X8 sensing point matrix and has an image throughput of 50 images
per second.
The term 2D force mapping designates a device that
measures the spatial 2-dimentional repartition of the
mechanical forces applied on a given surface.
The physical principle
The sensing part is formed of a sandwich construction with
copper [1] -conductive foam [2] -copper as depicted in Figure 1.
By applying a force on the two copper plates the foam is
compressed and the effect of this mechanical deformation is
that the contact surface becomes larger, see Figure 1b).
Consequently, the resistance drops.
Figure 1 A simplified representation of the sensor work principle.
In case a) the measured resistance is bigger than in case b).
The measurement system
A 8x8 measurement point demonstrator has been built to
demonstrate feasibility.
Tow supporting layers are made of high density foam. Onto
the first layer, 8 copper strips of 1 cm width are glued, as
depicted in Figure 2, layer 1. Onto the second supporting layer,
90° rotated copper strips are glued (layer 3). Finally, the
conductive foam [3] is put in the middle (layer 2) and the entire
assembly is fixed by silicone based glue.
Figure 2: Layers of the sensing surface.
A simple electronic board scans the metallic strips for each
combination of line and column. The resistance at every
crossing of two copper strips is measured using a resistor
bridge circuit. Measurements are done using the incorporated
10 bits A/D converter of a microcontroller. The data are then
transmitted to a computer using USB. Data processing and
real time image display are done with custom software. For
74
a) b)
Layer 1
Layer 2
Layer 3
calibration purposes, a reference data set is subtracted from
the measured data set.
Results
The demonstrator was submitted to plausibility tests. Figure 3
depicts the footprint of a standing man without shoes. Figure 4
depicts a sitting man footprint. Systematic evaluation is
pending.
Discussion
It is possible to make a force sensitive pressure sensor with
8x8 measurement points out of inexpensive material.
Competing technologies are an array of individual sensors
whose main drawback is the quadratic increase of the needed
connection lines to each sensor.
Potential applications are foot pressure sensing in shoes,
smart bed sensors for sleep analysis, and carpets for activity
monitoring at the home of elderly people. There appear to be
no limitations regarding the eventual size of the sensor and
whole body mats seem possible. However, the sensing points
are electrically and mechanically coupled. Thus the
measurements are not completely independent from one point
to another. Foam patterning techniques could eventually be
used to minimize those effects.
Figure 3: Standing man on the force 2D mapping carpet.
Figure 4: Sitting man on the force 2D mapping carpet.
[1] 3M, 1181 scotch
[2] Schaumstoff-Technik-Nürnberg, E103/23, 10 mm
[3] Dow Corning, DC734RTV