The quantitative measurement of articular motion is an important indicator of its functional state and for clinical and pathology diagnoses, and as an indicator of neurodegenerative disorders, which assists in the verification of joint and muscular limitation [1
], and to determine the effectiveness of rehabilitation exercises [2
]. Joint angle evaluation techniques can also be applied to improve sports performance [3
] and provide feedback information for prostheses control [4
]. The elbow joint motion has an important role in daily activities, and its amplitude can determine limitations, impairments, and can aid in medical diagnostics. The elbow articular motion can be analyzed during flexion–extension movement [5
]. This measurement can be assessed through a variety of methods, such as goniometers, video-based techniques, inertial measurement units, and encoders, among others [4
]. The most common is by using a goniometer, which can be made up of different types of sensors.
Currently, the main techniques employed in goniometers for this purpose are mechanical or electromechanical [6
], which are normally implemented with a strain gauge and resistive potentiometers. The major disadvantages in using strain gauge sensors are their inaccuracy, while the potentiometer generated discomfort in some users, which can limit the natural movement of the member [7
]. Another type of sensor used for this application is the inertial measurement unit (IMU), composed of gyroscopes, accelerometers and magnetometers. Although they are compact and lightweight, they present a high sensitivity with magnetic field interferences and can present high errors on the angle measurement [8
]. A video tracking technique is also implemented, but is very expensive and time consuming [4
Polymer optical fiber (POF) sensors are replacing conventional sensors due to several advantages, they are compact, lightweight, flexible, low cost, immune to electromagnetic interference [10
], in addition to presenting high stability, resistance to impacts and high strain limits, which enable the fiber to bend in angles with great amplitude [11
]. These advantages allow POF sensors to measure the magnitude of the range of movement of human joints, which has been applied in knee angle measurement [12
] and for spinal posture monitoring [13
]. Nowadays, several operating techniques have been proposed for POF sensors, among them, the intensity variation is the most common to measure joint angles [14
The principle of intensity variation for angle estimation consists in the attenuation of the optical power in proportion to the curvature angle of the fiber [15
], this attenuation can be measured with photodiodes. However, this attenuation is very low, so it is necessary to perform a lateral section, creating a sensitive zone, to improve the sensitivity of the fiber. The use of this technique allows the sensitivity control through the length and depth of the sensitive zone, as well as the roughness of its surface area [16
]. The sensitive zone is on the convex side of the fiber, thus, with its curvature there is a reduction of reflections on the concave side, so that the rays escape, causing a signal attenuation, which increases with the curvature rise [17
]. With different bending, it is possible to change the angle between the sensitive zone and the incident ray, changing the photodiode reading.
Some studies apply POF sensors to measure joint angles, such as Reference [18
], in which the POF sensor was applied for gait analysis and compared with the video tracking method in four participants, to show the repeatability of the sensor. This study applied the sensor in the elbow joint for only one participant during the throwing movement of an object, which demonstrates the applicability of a POF sensor to measure the angle of this articulation. Gait was also analyzed in References [18
], however, they did not compare this method with any other method. The studies presented had some limitations, such as not showing the repeatability of the sensor to measure the elbow joint angle in more than one participant and not performing a comparison with another method of joint measurement. Therefore, the aim of this study is to characterize and to implement a new portable, low-cost and wearable system to measure angles, based on a POF curvature sensor. In addition, we will present the system performance in tests with different angular velocities and its application for the measurement of elbow angles. We then compare the results with a consolidated resistive goniometer.
2. Experimental Analysis
A POF sensor is comprised of two modules: The emitter and the receiver of light. The emitter is a light emitting diode (LED) IF-E96 (Industrial Fiber Optics, Tempe, AZ, USA), that has a wavelength of 660 nm and is supplied by a 9-V battery. A resistance of 330 Ω was utilized in the circuit with the intent to limit the current in the LED. The receiver module is responsible for capturing the light from the fiber and it is composed of a photodiode IF-D91 (Industrial Fiber Optics, Tempe, AZ, USA) with a transimpedance amplifier with an adjustable gain. In all the experiments we used a multimode POF FB140-10 (Industrial Fiber Optics, Tempe, AZ, USA) with 160 mm, which is composed of three layers: Jacket, cladding and core. The jacket and cladding provide protection, while the core conducts the optical signals and is made of Polymethyl Methacrylate (PMMA), with 980 µm in diameter. An Arduino, at a sampling frequency of 10 Hz, made the acquisition of the signal response.
The sensitive zone was created by polishing the material with sandpaper (400 grit size) connected to a drill, to ensure a smooth and continuous surface, since the sensitive zone parameters of section length, depth, and surface roughness provided different sensitivities and can influence sensor response. The value of the sensitive zone length and depth was fixed at 14 mm and 0.6 mm, respectively [16
]. To obtain the desired section, the POF was positioned in a fixed support, which limits the drill to the desired length and depth specifications. The value of the sensitive zone depth was chosen because the fiber is very fragile in higher depths sections [15
] and this is the lowest limit in a sensitive zone depth, since the POF has a polyethylene jacket and to reach the core the depth must be at least 0.6 mm. Once the sensitive zone is defined, the sensor maintains repeatability of the performance in the same velocity. Figure 1
presents the parameters of the POF and the sensitive zone.
In order to show the applicability and robustness of the POF’s sensor, two different setups were made: Test bench and joint angle measurement, respectively. In both tests the POF sensor was compared and correlated with a resistive goniometer, the most common and marketable electrogoniometer.
2.1. Test Bench
The experiment test bench was made by positioning the POF sensor in a prototype, as shown in Figure 2
, which has a servomotor with position control, responsible for dynamic bending. The POF response was compared with the potentiometer response.
This test was performed in two ways: Quasi-static and dynamic. The tests were made with the aim of verifying the behavior and the performance of both sensors. The response curve of the POF sensor and the potentiometer were verified when they were bent. In the quasi-static test, the servomotor moved on sequential 10° steps in the 0–90° range and the results of the POF sensor and potentiometer were recorded. With these results a model to convert the sensors response (volts) in degrees was created. The dynamic test was made through bending the sensors in different angular velocities. The velocities used were 18, 36, 50, 80, 100, 140 and 200°/s and five repetitions were realized with 0–90° range, as this was applied to the conversion model obtained in the quasi-static test.
2.2. Joint Angle Measurement
To show the applicability of the POF sensor in joint angle measurement an experiment with 10 healthy participants (with ages between 19 and 31) was carried out. This study was approved by the National Committee of Ethics in Research, with a Certificate of Presentation for Ethical Appreciation, number 318,960.
The sensor was embedded in a 3D printed structure and set up at the elbow joint of the participants, through elastic bands, in such a way that the sensitive zone was positioned exactly in the joint area, to give better ergonomics, as shown in Figure 3
. For the experiment, the participants were instructed to realize the elbow flexion and extension movement five times, in an angular velocity at which they felt comfortable. The data, in degrees, from the potentiometer and the POF sensor were recorded at the same time during the movement.