Smart football footwear for advanced performance analysis and training purposes

摘要

Overview and Aims Kicking performance in soccer is a major skill that can strongly influence the success of a team. Existing methods for performance and activity monitoring of the actual foot to ball impact phase during different types of the kicking have their limitations. Based on the literature, the fundamental performance criteria when kicking a ball are kicking accuracy and characteristics of the foot to ball impact phase. The study aims to identify and characterise a piezoresistive, conductive polymer that can be used as a pressure sensor for measuring impact forces in an electrical format during a kicking action. The polymer will be developed into a high resolution pressure grid which will convert raw pressure data from the foot to ball impact phase into advanced kicking parameters. The advanced parameters studied here are the movement of the centre of pressure (COPx, COPy), COP velocity (v), normal force (FN), friction force (FF), impact duration time and peak forces location. The main goal product of the study is a smart football footwear for advanced kicking performance analysis and training purposes. Methods and Development In order to select the best piezoresistive material, material characterization tests were conducted for model function selection and electrical viscosity constant. Material characterization experiments included widely used tests such as: Creep, Stress Relaxation and Strain Rate Dependency as well as a new electrical methodology developed here - Electrical Creep and the discovery of two new electrical parameters: electrical viscosity constant (ɳ) and conductive stiffness. Next, the coefficients of determination during peak impact forces with calibration functions were measured. Development, calibration and validation of the sensor array system including its prime prototypes sensors and the smart footwear followed. The instrumentation, consisting of piezoresistive material used as a novel pressure array and a programmable microcontroller, measured the magnitude of the kick force and centre of pressure (COP) with respect to a soccer boot coordinate system. Results Initial experiments proved that an off-the-shelf conductive polyurethane has piezoresistive properties and can be used as a pressure sensor. Electrical viscosity model functions and constant (ɳ ) were determined for 10 conductive polymer specimens. ɳ ranged between 0.003-0.351 or 0.3%-35.1% and all specimens were found to follow a power law function. Electrical viscosity is a characteristic that represents the decrease of electrical change over time under constant mechanical load. Subsequently, peak impact forces were measured for the same 10 specimens and their calibration functions found. The most suitable specimen was found to be Rmat1 with a coefficient of determination of 0.989 and electrical viscosity constant of 18.6%. A unique sensor array system technique was then developed and patented. Three prototypes were created, using the Rmat1 specimen, to test the functionality and feasibility of using the system for different pressure mapping applications. Based on these prototypes, the smart kicking boot was developed. For system validation, the calculated system forces against the Kistler force plate data was FK; n = 58 with residual standard deviation σR= 125.6 N (r2 = 0.91252). σR is force dependent (σR= 0.0437 FK + 70.4), i.e. between 7.5% and 9% of FK at the range of 1-2kN. COP could not be validated due to a system limitation of the Kistler force plate in calculation of impact forces. The path of the COP between the boot and the ball for two curved kicks was plotted. Results showed the movement pattern and the location of the COP and exhibited a similar curve for both kicks. Additional advanced parameters against time of one kick were then calculated (COPx, COPy, COP velocity, normal force and friction force) and used to generate a colour coded 4D vector diagram. Discussion and Conclusions Pizoresistive sensors were identified as our preferred type for further investigation. After identification of a potential material (RmatFb), the off the shelf conductive polyurethane foam was electromechanically tested for application into a smart sensing system and proved possible for use as a piezoresistive pressure sensor. Next, we tested a second specimen (Rmat1x5 layers). Results showed that mechanical viscosity in Rmat1x5 follows a logarithmic law function whereas its electrical viscosity follows a power law function so only the power function viscosities were compared and showed a higher electrical viscosity value. The specimen was found to be more electrically viscous than mechanically. This new parameter, conductive stiffness, may become a gold standard benchmark for material characterisation in the future. We then studied the electrical viscosity constant of 10 conductive polymers. Whereas mechanical viscosity is a parameter that has been previously described in the professional literature, electrical viscosity is a new parameter described here. Based on electrical viscosity characteristics, Rmat2a was found to be the most suitable specimen for further development. Subsequently, the electro mechanical peak impact forces coefficients of determination and calibration functions were determined for the same 10 specimens. Rmat1 –vinyl was found to be the most suitable material for peak impact forces measurements. Once we developed the sensor array system and algorithms for pressure data, we designed three prototypes to trial the methodology using Rmat1. We successfully tested the system through different prototypes with differing feedback signals and proved the concept’s functionality. The final sensory system aimed to measure pressure distribution between the foot and ball and to calculate advanced parameters. The COP was tested for curve kicks and the COP data were displayed on a 4D colour-coded vector diagram model of a soccer boot. The COP data was constructed from four phases of the foot to ball impact. This data reveals new information about foot to ball dynamic parameters. A unique low cost instrumented system for soccer kicking in soccer was successfully incorporated into a soccer boot, calibrated, validated and tested during a full kicking motion. The smart soccer boot is useful for counting the number of kicks, assessing the magnitude of the kick force and displaying the COP. The sensor has high resolution, is thin and flexible, wearable and light weight. The results assist to illustrate the movement of the COP during the short impact phase between the foot and the ball.