Stretchable Strain Sensor With Anisotropy and Application for Joint Angle Measurement

Flexible and stretchable strain sensors are expected to significantly contribute to new technologies and be applied in various applications, including human interfaces, smart clothes, and robotic skin. Hence, we have proposed a flexible film-shaped strain sensor that is composed of three elastomer films and two carbon-nanotube electrodes. It is thin, lightweight, and has low elasticity. Strain measurements taken by the sensor are based on changes in capacitance, which is proportional to the square of the area of the sensor’s sensing part. Consequently, the capacitance of the strain sensor does not show the direction of the strain. As some applications require the measurement of strain in a fixed direction, this paper proposes an anisotropic strain sensor. This anisotropic strain sensor decreases the effect of the strain in the fixed direction to less than 10%. The model of the anisotropic strain sensor and experimental results show that its capacitance is proportional to the square of the length of the sensing part in the stretch direction. To check the anisotropic strain sensor’s practicality, it is applied to estimate the angle of a wrist joint. The results show that the anisotropy decreases the root-mean-square error of the estimated angle to less than 3°.