Textile Pressure Sensor Design for Sitting Posture Classification

Introduction to Posture Monitoring and Sensor Need

The relevance of analyzing sitting posture cannot be overstated in healthcare and biomedical realms. For this reason, there is a need to develop posture monitoring systems that are not only comfortable, but also economical and more efficient. Towards this end, Meyer, Arnrich, Schumm, and Troster propose a textile pressure sensor designed to both monitor and measure a person's body pressure distribution. The authors point out that "electrodes built with conductive textiles are arranged on both sides of a compressible spacer, forming a variable capacitor."

Over time, various techniques have been utilized in an attempt to gauge sitting posture. Some of these techniques include using radiographs for spinal curve examination, sitting state video analysis, and the utilization of specialized chair pressure sensors. All of these approaches have had one weakness or another — with the most prominent being prohibitive acquisition costs and complexity of use. The utilization of textiles in this context is therefore a welcome step in the right direction, particularly given that, as the authors point out, sensors of this kind are not only washable, but also lightweight and comfortable.

Textile Sensor Architecture and Design

The textile pressure sensor adopts a dynamic design made up of two parts: the textile sensor array and the measurement electronics. Textile wires are used to route electrical signals, which are linked and connected to devices adapted for communication and measurement. While the textile sensor itself is composed of three key components — a compressible spacer and two electrodes forming a variable capacitor — the measurement electronics make use of a dedicated converter. As the authors state, "the capacitance to digital converter AD7745 from Analog Devices is used to measure the capacitance of the sensor elements."

With regard to the textile sensor specifically, the induced pressure causes the thickness of the compressible spacer to vary. The two electrodes, as the authors explain, "are arranged on one side of the spacer as an array, while the other side consists of one common electrode, forming the capacitors between each electrode and the common electrode." This arrangement enables precise measurement of pressure distribution across the seating surface. More information on capacitive sensing technology provides useful background for understanding the operating principles of this design.

Sitting Posture Classification Experiment

In order to appraise and assess the sensor system's quality, the authors conducted a sitting posture classification experiment in which a total of 16 sitting postures were classified. Two sensor systems were used: the textile sensor system and a commercially available pressure mat placed below it for reference. The authors confirm that the textile sensor's pressure distribution is not affected by the placement of the reference sensor. They further note that "the two sensor systems have been synchronized so that only the frames of the reference system are used with a timely corresponding frame of the textile sensor system."

The chair used in the experiment was height-adjustable, ensuring that regardless of a subject's height, both feet could touch the ground while seated. The 16 postures evaluated were: seated upright (1), leaning right (2), left (3), forward (4), back (5), left leg crossed over the right (6), right over left (7), once seated upright and once leaning back (8) and (9), once while the knees are touching and once with the ankle rested on the leg (10)–(13), slouching (14), sitting on the leading edge (15), and slouched down (16). A total of nine subjects participated in the experiment.

The authors concluded that the recognition rate was 59% and 82% without and with the back sensor, respectively — figures that include hysteresis compensation. The reference system recorded a recognition rate of 56% and 84% without and with the back sensor, respectively. These results demonstrate that the textile sensor system performs comparably to the commercial reference system.