Fig. 1: Design concept of unimodal tension, bend, twist, and shear sensors.
From: Flexible unimodal strain sensors for human motion detection and differentiation
a Top schematics show the applied forces and their directions for tensioning, bending, shearing, and twisting deformations in finite element simulations. The bottom panels show the piezoelectric response simulations of the uniaxially drawn piezoelectric PLLA films with different cutting angles under tensioning, bending, shearing, and twisting deformations. The first panel shows the piezoelectric PLLA film with a cutting angle of 45° (PLLA45) and its cutting schematic (first graph) with a cutting shape (black dash line) and a cutting angle (red dash line). The average responding voltage of all nodes of the PLLA film surface (Va) is shown in the middle. The same illustrative schemes are applied for PLLA-45, PLLA0, and PLLA90. b Left schematics show the cutting angles and cutting shapes of two PLLA parts from an original PLLA film for the unimodal sensors, and the right schematics show the corresponding parts after cutting out; c Top schematic shows the patterns of inner electrodes, and the front sides of two parts of each sensor are stuck together; the middle one shows the exposed area of each assembled sensor is coated with silver conductive paint as an outer electrode, except the lateral end of the sensor; the bottom one shows the inner electrodes are connected to a signal wire, and the outer electrode is connected to a metallic shield part (ground) of a coaxial cable. d Piezoelectric response simulations of two PLLA films composing each unimodal sensor under tensioning, bending, shearing, and twisting deformations to prove the unimodal sensors’ hypothesis. The first panel shows the piezoelectric response of two PLLA45s of the unimodal tension sensor under tensioning and bending. The generated voltages of the two PLLA45s are superimposed when tensioned, but they are offset under bending. The unimodal tension sensor under twisting and shearing is not simulated because the PLLA45 is insensitive to the two deformations, as demonstrated in Fig. 1a. The same illustrative schemes are applied for the unimodal bend, twist, and shear sensors.
