A portable plantar pressure system: Specifications, design, and preliminary results

2.1Hardware and software architecture

The proposed system enables the measurement of plantar pressure distribution during gait. It consists of a three-axis accelerometer placed on insoles to detect heel-strike and estimate foot orientation. The portable system is presented in Fig. 1 and consists of: 1 – PJRC Teensy 3.6 microcontroller with 32-bit ARM Cortex-M4 microprocessor with a clock speed of 180 MHz; 2 – HC-11 radio modules (transmitter and receiver); 3 – battery; 4 – fixing band; 5 – pressure sensors; 6 – MPU-9150 inertial navigation module; 7 – FFC tape.

Figure 1.

The portable system. 1) Microcontroller, 2) radio modules, 3) battery, 4) fixing band, 5) pressure sensors, 6) inertial navigation module, 7) FFC tape.

The pressure insole is leather-based and consists of seven layers (Fig. 2), namely a leather insert, a wire leather insert, a protective film, a 0.5 mm copper sheet, an insulating protective film, a polymeric film, and a 0.5 mm copper sheet. The component of the sensor that directly responds to pressure is a thin polymer film (4) covered with: conductive layers (3) and (5), insulating layers (2) and (6), and protective layers (1) and (7).

Figure 2.

Design of the resistive sensor.

Due to the number of layers, the maximum thickness of the insole is approximately 6 mm. The gait monitoring system was developed in LabVIEW software (National Instruments, USA) (Fig. 3). The insole is divided into 16 areas: heel (areas 1–2), mid-foot (areas 3–8), metatarsal (areas 9–14), and toe (areas 15–16). These areas are sufficient to support most of the body weight. The insoles proposed by the authors are European size 38 to 45. Shu et al. [20] proved that the number of sensors necessary to cover most of the body weight changes is 15.

Figure 3.

Gait monitoring system.

The software process uses a low-pass Kalman filter. Communication between the measuring system and the computer is based on two HC-11 radio modules in a frequency range of 433.4–473 MHz, depending on the selected work channel. One HC-11 module is connected to the microcontroller and acts as a transmitter; the other is connected to the converter and acts as a receiver. The HC-11 radio module (receiver) uses the PL2303 system and communicates with the computer via a USB-UART converter. The device exhibits such a low power consumption that the energy from a small battery is sufficient for collecting and recording the required data. Figure 4 presents a block diagram of the portable measurement system.

Figure 4.

Block diagram of the portable measurement system.

2.2Gait data measurement

The participants were healthy volunteers without neurological conditions or a reduced ability to perform daily routines and were recruited among the personnel at Bialystok University of Technology, Poland. The study was conducted in accordance with the Declaration of Helsinki. Written informed consent was obtained from each participant. The subject’s body mass was measured using a scale with a resolution of 100 g. The subject’s height was measured with the use of a stadiometer. For measuring plantar pressure distribution, the healthy volunteers were instructed to walk a distance of approximately 60 meters at their habitual speed outside of a standard laboratory. Each measurement was repeated three times per foot. The plantar pressure under 16 anatomical masks was measured per each step and normalized to the subject’s body mass. All the subjects were provided with an identical kind of footwear to control for differences in personal footwear. The precision of measurement and the accuracy of the system were validated against a system available on the market (T&T Medilogic Medizintechnik GmbH, Germany) during walking in the same healthy volunteers. The data was sampled at 60 Hz.

2.3Statistical analysis

Repeated measures ANOVA was applied to evaluate the differences in plantar pressure parameters. The mean value of plantar pressure was calculated in all masks for a single step. The results were expressed as means ± standard deviation (SD). The accuracy of estimation of plantar pressure distribution was validated against the results obtained from the reference system (Medilogic). The difference between data was assessed using Mann-Whitney U test. Statistical tests were performed using Statistica 13.1 (StatSoft, Poland). The significance level was set at p< 0.05.

3.1Testing of the sensor

The proposed sensor (size 10 mm × 10 mm) for plantar pressure collection was calibrated and the following parameters: sensitivity, linearity error, and hysteresis were assessed (Fig. 5, Table 1). The SNR value for the proposed sensor is 28.27 dB.

Figure 5.

Sensor calibration. a) Calibration setup, b) hysteresis of the sensor.

Figure 6.

Estimation of foot orientation. a) Axis definition, b) gait event detection.

3.2Testing of the portable system

Twenty two healthy male volunteers (24.0 ± 3.3 years; height 1.73 ± 0.07 m; and Body Mass Index 24.6 ± 2.5 kg/m2) participated in this study. The authors proposed a system consisting of 16 pressure sensors and a three-axis accelerometer to detect heel-strike and estimate foot orientation (Fig. 6).

Figure 7.

The experimental setup with images of plantar pressure during gait. a) The experiment, b) plantar pressure distribution for a typical man during walking. t is the time elapsed from heel contact to toe-off. s1–s16 – sensors.

Heel strike was collected by sensors 15–16, and the orientation of the foot relative to the position was recorded as shown in Fig. 6b. The value of 0 means that the foot is on the ground. Gait data was collected simultaneously by the new portable system and the system available on the market (T&T < edilogic Medizintechnik GmbH, Germany) and compared in real-time. The experimental setup during gait and an example of plantar pressure data normalized to body mass for each sensor are presented in Fig. 7.

The values of plantar pressure distribution compared to the commercial Medilogic system are presented in Table 2. There are no significant differences between the results obtained from both systems, i.e. the portable system and Medilogic (p> 0.05).