Shear stress sensor sheet for monitoring foot-sole interaction

Problem definition

Sensors are all around us: they enable the interaction between ourselves and the environment or they deliver us detailed information about our surroundings. Many applications require sensors for recording shear stress, ranging from robot hands equipped with tactile sense in order to improve handling of objects, over prosthetic sockets to the foot-sole interaction detectors in sports and rehabilitation sciences. Especially when used in the medical sector, it is important that these shear stress sensors are thin, unobtrusive and easily embedded. In this masterthesis, we are going to develop a matrix of shear sensors which can be used for monitoring foot-sole interaction. Already available are sensors to measure the foot pressure distribution. However, the pathogenesis of an often occurring problem such as foot blisters is related to excessive frictional forces experienced on or under the foot. Therefore the goal is to realise a foot-sole interaction sensing sheet.

At our research group, Cmst (Elis Department), a 1-point shear stress sensor has already been developed. The sensor’s principle of operation is based on changing coupling of light between a light source and an adjacent photodiode. When both components, which are located on opposite sides of a transparent, deformable rubber layer, move laterally with respect to one another, a difference in photodetector current can be observed.

In order to map the shear interaction between a foot and a shoe sole, a matrix of several independent sensing points is necessary. 

Goal of this thesis

The goal of this project is to realise a 20cm x 30cm shear stress sensor sheet (corresponding with the size of a shoe) in a stretchable technology (available at Cmst), as schematically shown below. Two circuit layers will be designed and fabricated (one layer incorporating the photodiodes and another incorporating the LEDs) which will be laminated with a transparent deformable intermediate layer to obtain the shear sensor array. Since at least 1 circuit layer will consist of islands, interconnected using stretchable conductors, the different shear sensing points can move independently and a true sensor matrix is obtained.

Possible implementation of the shear sensor matrix

Apart from the physical realisation of the shear sensor itself, it is also needed to develop a low-power electronic circuit enabling the operation and the visualisation of the sensor. 
More specifically, the several subtasks in this master thesis are:
  • Design of the stretchable matrix:
    • Designing the shear sensor
      • Selecting appropriate PDs and LEDs
      • Optimising relative PD and LED positions on top and bottom layer
      • Organisation of various sensing points in a matrix
    • Calculating the optimal tracks connecting the different islands 
    • Fabricating the stretchable matrix at the Cmst clean rooms
      • Design of the electronic circuit
  • Driver electronics for the LEDs
  • Readout electronics for the PDs
  • Multiplexing electronics to readout sensors in a matrix configuration
    • Programming of a software application to visualize the applied shear stress. 
    • Demonstrator recording the shear stress between a foot and the sensing sheet.