Fig 1.
Summary of methodology for investigations into sensor calibration, loading profile and sensor validation.
Fig 2.
[A] Cylindrical section of elastomer containing strain gauge and normal force sensor [B] Stress-strain curve of the elastomer under compression stress. Linear approximations for deformation were made for three regions of the curve (low, medium, and high stress magnitudes), sectioned by the compressive strains ε1 and ε2, with corresponding gradient E used for calibration. [C] Cylindrical section deformed by normal force only. [D] Cylindrical section deformed by both normal and shear forces.
Fig 3.
Shear stress system (SSS) sensor and placement in the sensing insole.
[A] Configuration of the strain rosette in the sensor with three strain gauges arranged at 0°– 45° - 90°, and the relationship between the local strain axes and the global applied shear direction axes (Medial-Lateral, ML, and Anterior-Posterior, AP). [B] Section view of the SSS sensor. [C] Top view of the SSS sensor and its dimensions. [D] Locations of SSS sensors in the sensing insole.
Fig 4.
Treadmill walking setup and data acquisition.
[A] Participant walking on a treadmill with the sensor insole system. The data acquisition system (DAQ) was attached to a belt, and each insole (left and right foot) has a separate but identical DAQ input. [B] Block diagram of the DAQ system, collecting data at 80 Hz.
Fig 5.
[A] Custom shear stress rig made of rigid 10 mm acrylic sheet plates which applied the force of the mechanical testing machine as a shear force onto the insole. The shear stress was calculated using the applied force and area of the custom indenter. The indenter’s compressive stiffness was 30.1 MPa, ~12 times stiffer than the silicone sensor of 2.5 MPa. [B] Custom normal stress calibration setup where the insole was placed on a compression platen.
Table 1.
Three conditions for mechanical bench top testing.
Table 2.
Participant visits and activity.
Fig 6.
Calibration investigation results.
Mean peak signal of shear stress (SSS sensor) total output (mV) from 10 cyclic triangular loading. [A]—Effect of area of loading on SSS sensor measured outputs. [B, C]- Effect of location of loading on SSS sensor output for medial and posterior respectively.
Fig 7.
Multiple linear regression analysis between the mean normal pressure profiles of participants’ walking data, with normal pressure profiles of an experimental benchtop mechanical test of indenters of different diameters.
[A] Experimental normal pressure profiles: (i) Indenter experimental setup, (ii) Normal pressure profile curves width increases with increasing indenter diameter, (iii) F-scan pressure result that shows the cross section used to obtain these values used in ii. [B] Participant pressure profiles: (i) In-shoe gait lab experimental setup (ii) An example of participant’s pressure profile over 20 gait cycles, showing the three normal pressure profiles of the foot at the calcaneus, first met head and hallux, (iii) F-scan pressure result that shows the cross section used to obtain the values. The image also shows the peaks for these three regions (Calcaneus peak CP, Hallux Peak HP and the metatarsal peaks MHP1 and MHP2). [C] Graphical representation of the regression analysis’ coefficient of determination (or R-squared) results. Larger circles indicate a higher R-squared value, and red circles indicate the maximum R-squared in the sensor group. R-squared values are shown above the circles, and maximum is indicated as red font.
Fig 8.
Shear sensor benchtop mechanical testing calibration and validation results, sampled for 2 seconds of the cyclic loading.
[A] Sensor calibration for both anterior-posterior (AP) and medial-lateral (ML) directions at a ‘medium’ level of posterior and medial shear loading of 1 Hz cyclic loading of up to 70 kPa shear stress, at a constant normal stress of 140 kPa. [B] Sensor validation test result at medium level of shear cyclic loading (up to 70 kPa), at a different loading frequency (~0.85 Hz) and different constant normal stress (125kPa). All results for the different configurations of loading are shown in Table 3.
Table 3.
Sensor error, repeatability and range for calibrated measurements and validation results in different loading conditions.
Table 4.
Peak plantar stresses for both participants and and repeatability investigations for both intra and inter walks.