A New Flexible Optical Fiber Goniometer for Dynamic Angular Measurements: Application to Human Joint Movement Monitoring

The electronic measurement of the angle between two planes is generally performed by using the so-called electrogoniometers. The major drawback in using such devices is the presence of a fixed hinge that imposes a fixed center of rotation. This can cause problems when measuring the bending angle in some joints, such as Cardan or human joints, which have a variable rotation center. Based on an optical fiber, a sensor measuring the relative angle in a rotating joint has been developed. This joint makes use of the intensity modulation of a laser beam propagating in a single-mode optical fiber, due to the changes of its polarization status originated by the rotation of contiguous portions of the fiber, where controlled birefringence has been induced by the joint rotation. A prototype of this sensor has been developed with a range of the relative angle of 90$^{circ}$ , a resolution of less than 0.01$^{circ}$, and a standard deviation of 0.1$^{circ}$. The main advantages of this innovative sensor are lightness, flexibility, high speed of reaction, and high accuracy. This paper describes the development of the proposed sensor, with particular reference to the applications of human joint movement monitoring. Additionally, the equipment implemented for the test is illustrated, and results from laboratory tests are reported and discussed.