Figure 1 presents the layout arrangements of the pressure sensing cells. The pressure sensing cells are located in the plantar anatomical partitions. The regional division method adopted is presented in the left top of Fig. 1. The heel can be further divided to anterior heel and posterior heel. The metatarsal region is divided further to three sub-areas as the first metatarsal, the second to the forth metatarsals and the fifth metatarsal by 17:22:12. The toes region is divided into two sub-areas as hallux and the lesser toes by 1:2 as shown in top left of Fig. 1. The spatial resolution of partition dividing is 0.25 mm. Figure 1 shows the layout arrangements of pressure sensing cells and the dark red spots represent sensors which will change size in the following study.

Pressure of each pressure sensing cell is calculated by summing the readout of the cells in the corresponding region of Tekscan 3000E. The pressure range of Tekscan 3000E is 0–2.1 kg/cm^{2} with a spatial resolution of 4 cells/cm^{2}. The thickness of Tekscan 3000E is about 0.1 mm and the weight is <10 g. Ten healthy young subjects (6 males and 4 females, age 25.9 ± 3.00 years, weight 61.0 ± 11.6 kg, height 170 ± 8.40 cm) were recruited for this study. All the subjects have no history of motion disorders, none of them suffered from any visible foot deformities. The subjects all gave their written informed consent in accordance with local Ethics Committee of Shanghai Jiao Tong University. Four walking modes: normal walking, walking with the ankle constrained, walking with knee constrained and walking with loads are investigated in this study. The joints are constrained with bandages and protective body gears that are fixed at certain positions to limit the movement range of the joints. Loads are added to subjects by wearing sandbags of 2 kg on the ankles. Subjects need to complete five times 10 step walking trials. Gait data of different configurations of plantar pressure sensing cells is collected in the laboratory and processed with MATLAB 2010b. The layout of Lin Shu’s insole is also investigated in this paper, three pressure sensing cells are placed in the metatarsal region and three pressure sensing cells are placed in the heel region [11]. COP coordinates are estimated through calculating the center of gravity [12] with Tekscan 3000E and each configuration of pressure sensing cells. The estimation errors are calculated by Eqs. (1) and (2).

$${\text{RMSE}}_{{{\text{xdirection}}}} = \frac{1}{4} \times \left( \begin{gathered} \left( {\frac{{\sum\nolimits_{1}^{{n1}} {\left( {COP\_X_{{mea}} \left( i \right) - COP\_X_{{ref}} \left( i \right)} \right)^{2} } }}{{n_{1} }}} \right)^{{1/2}} \hfill \\ + \left( {\frac{{\sum\nolimits_{1}^{{n2}} {\left( {COP\_X_{{mea}} \left( i \right) - COP\_X_{{ref}} \left( i \right)} \right)^{2} } }}{{n_{2} }}} \right)^{{1/2}} \hfill \\ + \left( {\frac{{\sum\nolimits_{1}^{{n3}} {\left( {COP\_X_{{mea}} \left( i \right) - COP\_X_{{ref}} \left( i \right)} \right)^{2} } }}{{n_{3} }}} \right)^{{1/2}} \hfill \\ + \left( {\frac{{\sum\nolimits_{1}^{{n4}} {\left( {COP\_X_{{mea}} \left( i \right) - COP\_X_{{ref}} \left( i \right)} \right)^{2} } }}{{n_{4} }}} \right)^{{1/2}} \hfill \\ \end{gathered} \right)$$

(1)

$${\text{RMSE}}_{{{\text{ydirection}}}} = \frac{1}{4} \times \left( \begin{gathered} \left( {\frac{{\sum\nolimits_{1}^{{n1}} {\left( {COP\_Y_{{mea}} \left( i \right) - COP\_Y_{{ref}} \left( i \right)} \right)^{2} } }}{{n_{1} }}} \right)^{{1/2}} \hfill \\ + \left( {\frac{{\sum\nolimits_{1}^{{n2}} {\left( {COP\_Y_{{mea}} \left( i \right) - COP\_Y_{{ref}} \left( i \right)} \right)^{2} } }}{{n_{2} }}} \right)^{{1/2}} \hfill \\ + \left( {\frac{{\sum\nolimits_{1}^{{n3}} {\left( {COP\_Y_{{mea}} \left( i \right) - COP\_Y_{{ref}} \left( i \right)} \right)^{2} } }}{{n_{3} }}} \right)^{{1/2}} \hfill \\ + \left( {\frac{{\sum\nolimits_{1}^{{n4}} {\left( {COP\_Y_{{mea}} \left( i \right) - COP\_Y_{{ref}} \left( i \right)} \right)^{2} } }}{{n_{4} }}} \right)^{{1/2}} \hfill \\ \end{gathered} \right)$$

(2)

where X and Y refer to the A/P and M/L direction, respectively; and refer to COP coordinate obtained with the configured sensing cell pattern; and refer to COP coordinate by Tekscan 3000E; n1, n2, n3 and n4 refer to the total number of samples of aforementioned walking modes, respectively. Gait data of four walking modes aforementioned is grouped together to calculate the total RMSE of COP coordinate estimations of the investigated configuration of plantar sensing cells, utilizing Tekscan 3000E’s result as the truth value.

Six sizes of pressure sensing cells are evaluated in this experiment by combining cells of Tekscan 3000E to obtain the desired size, as the red dots showed in Fig. 1. S1, S2, S3, S4 and S5 represents areas 1.5 cm × 0.5 cm, 1.5 cm × 1.0 cm, 1.5 cm × 1.5 cm, 2.0 cm × 2.0 cm, and 2.0 cm × 2.5 cm, respectively. Pressure sensing cells of fitting the foot contour (S6) are also investigated by combining cells of Tekscan 3000E in the corresponding area, and the division method is shown in the left top of Fig. 1.