Absolute Length Sensor Based on Time of Flight in Stretchable Optical Fibers

Abstract

Soft and stretchable optical fibers are a promising all-polymer sensor element for extreme mechanical strains, pressures, bending, and other deformations. Compared to electronic sensors, the sensor elements are intrinsically stretchable and are not subject to electromagnetic interference. However, the typical approach of measuring deformation by tracking the transmitted light intensity is an integrating approach similar to resistance-based measurements; it does not provide information about the location or the nature of the deformation. This letter describes an alternate method for driving stretchable optical fibers: a consumer light detection and ranging chip connected to both ends of the fiber to measure its length via the time-of-flight (TOF) of a light pulse. This approach is a miniaturized, optics-based version of previous acoustic TOF and electronic time-domain reflectometry in robotics and human-interface devices. Such ranging systems collect spatial information using a small number of sensors. We found that the TOF approach can measure fiber lengths to within 1 mm on an overall length of ∼330 mm (>1% accuracy). Both the amplitude and TOF methods were sensitive to pressure that created microbending but in opposite directions. For bending radii greater than 7.5 mm, the TOF sensor was less sensitive to bending than the amplitude-based sensor. As a result, the TOF sensor accurately measured the perimeter of a shape with small-radius corners, while the amplitude-based sensor overreported the perimeter because of bending losses. Compared to earlier work, the TOF devices were less dependent on manufacturing variations that affected the signal level, such as fiber- source and fiber-detector alignment. We conclude that the amplitude method may be preferable for pure bending sensors, TOF sensors can distinguish pressure from stretching, and the TOF approach is notably better at measuring lengths over topography, such as in clothing-sizing and other wearable applications.