This work demonstrates the creation and characterization of novel, textile, vibration-sensing electronic yarns (E-yarns), which have been incorporated into a vibration-sensing glove to create a proof-of-concept device for the monitoring of hand transmitted vibrations (HTVs). HTV exposure is frequently encountered in a number of professions, such as in the construction industry, where workers can experience prolonged exposure to significant levels of oscillatory motion (vibration) while working with hand-operated power tools. Overexposure can result in a variety of musculoskeletal, neurological, and vascular disorders [1
] meaning that monitoring HTV exposure is important: Disorders include Hand Arm Vibration Syndrome (HAVS) and Carpal Tunnel Syndrome (CTS) [3
The relationship between the use of vibrating tools and the manifestation of HAVS has been well documented in the literature [4
]. Studies have shown that a decrease in the vibrotactile perception thresholds of the fingers have a direct linear correlation with vibration exposure [7
]. Beyond this, research has also shown that there is a latency where HAVS symptoms do not begin to manifest until around 2000 h of exposure, with some indication of symptoms evident in more than 50% of the exposed workers (in this case forestry workers) after 8000 h of exposure [9
]. Therefore, by monitoring HTV exposure, the risk of serious injury can be reduced.
It has also been suggested that CTS, caused by the compression of the median nerve in the wrist, may be a result of significant HTV exposure in some cases. As highlighted in a review by Bovenzi [6
] a selection of case studies showed a higher occurrence of CTS symptoms in occupations where vibrating tools were employed [10
A seminal article by Griffin [13
] highlighted the critical physical variables relevant to HTVs, these being: magnitude, frequency, duration, direction, area of contact, contact force, posture, and environment. Vibration analysis typically employs Fast-Fourier Transform (FFT) Vibration Spectrum Analysis (FFT VSA), as vibration induced injury is both frequency and amplitude dependent. HTV vibration levels are typically recorded as an acceleration value (in ms−2
). A suitable transducer must therefore be sensitive over the correct frequency range for HTV monitoring, and sensitive to a range of amplitudes suitable to its application, which have been outlined in relevant standards [14
]. Vibrations from power tools can occur in any axis of motion (depending on the tool) and therefore three-axis measurements are preferable.
The position of the vibration transducer is critical to the accuracy of HTV measurements as the maximum amplitude of the vibrations experienced by the power tool operator will likely be at the point of contact with the tool. This will normally be the palm of the hand, however the placement of a vibration transducer in this location may cause discomfort to the end user and impede their use of the tool. As such, when measurements are taken, these typically occur at the wrist [16
], between the fingers [17
] and on the machine itself [18
]. Additionally, there is a commercially available transducer and glove solution that can be used to measure the vibration at the palm [19
], however the transducer appears to be relatively large and heavy (the transducer and cable weigh 6 g [20
]) and may affect the comfort of the user.
In this work, an innovative electronic textile solution for the monitoring of vibration has been created which allows for vibration transducers to be integrated within a glove at the palm of the hand and on the index finger without affecting the comfort of the end user. Electronic textiles have seen increasing development in recent years for sensing applications [21
] leading to innovations including strain sensing [23
], gas sensing [24
], and pressure sensing [25
A vibration-sensing electronic textile has been achieved by embedding small-scale tri-axial accelerometers into the core of a textile yarn using electronic yarn (E-yarn) technology. This resulted in a yarn with the look and feel of a normal textile. E-yarn technology has also been used to create other sensing devices such as temperature sensing E-yarns [26
], acoustic sensing E-yarns [27
], and light sensing E-yarns [28
]. While this is not the first instance of accelerometers being used in electronic textiles [29
] to the knowledge of the authors other works have not explored these E-textiles for vibration monitoring.
The production process to create the E-yarn, and the insertion of the E-yarn within a textile, has the potential to significantly dampen the vibrations and affect the sensors response. The E-yarns are constructed by first soldering the accelerometer onto fine copper wires. While the accelerometer is a commercially available device this is not a normal soldering practice (as instead the component would be attached to a printed circuit board). Further, not all of the solder pads are attached, and capacitors for bandwidth selection have not been used (the latter was to minimize the quantity of components needed within the E-yarn). This would likely result in the commercial device not providing an expected signal output. A similar approach was taken when developing the acoustic sensing E-yarn [27
The soldered component is then encapsulated within a hard, resin, micro-pod to protect the mechanically fragile solder joints, help guard the component from external mechanical and chemical stress, and to attach supporting fibers. The inclusion of the resin micro-pod can have a significant effect on the response of an embedded sensor as has been observed for temperature sensing E-yarns [31
] and optical sensing E-yarns [28
], with the inclusion of the micro-pod influencing the nature of the measurands reaching the embedded component. While the micro-pod is hard, and good vibration transmission is likely, careful characterization of the embedded component is important to ensure that vibration transmission is achieved over all of the relevant frequencies, as resins are known to dampen the mechanical vibrations and that this dampening can be frequency dependent [32
The encapsulated component, wires, and fibers are finally covered with an outer fibrous covering to consolidate the structure, facilitate further processing, and provide a normal textile feel. The inclusion of these soft fibers has previously been shown to affect the response of other sensors [28
], and the use of textiles to dampen vibration is well reported in the literature [33
]. The level of vibration dampening will likely be due to the structure and stiffness of the surrounding textile. This work will explore two covering designs, a knit-braid (as used in the majority of E-yarns to date) and a braid. Embedding the completed yarns within a textile fabric will have further potential to reduce the vibration transmission, and this must also be understood.
As each stage in the E-yarn production process could potentially introduce vibration dampening elements, this work carefully characterized the vibration-sensing E-yarns at each step during the production processes, and when embedded within a textile, over a range of vibration frequencies and amplitudes relevant to HTV monitoring.
This work builds on previous studies into the development of vibration-sensing E-yarns. The initial proof-of-concept vibration-sensing E-yarn utilized a slightly different design to either configuration shown in this work, and while it was capable of measuring vibrations, full characterization over relevant frequencies and amplitudes was not presented [36
]. This article specifically expands a short proceeding previously presented by the authors [1
] to include further data and a more comprehensive analysis, with this article introducing further characterization work of the E-yarn (amplitude dependence at different frequencies for the soldered and encapsulated components), additional repeatability data, the characterization of a different design of E-yarn, and the characterization of the E-yarns embedded within fabrics. Expanded experimental details have also been provided.
Ultimately this innovation has allowed a proof-of-concept vibration-sensing glove to be developed which incorporated four vibration-sensing E-yarns allowing for 3-axis vibration measurements to be taken at either the palm or the end of the middle figure; however the technology would also allow further vibration-sensing E-yarns to be integrated into a glove. This has the potential to providing a powerful new tool for understanding HTV and a possible solution for HTV monitoring in the workplace.
This work presented a vibration-sensing E-yarn that would be suitable for monitoring HTVs. The vibration-sensing E-yarns were carefully characterized over a series of vibration amplitudes and frequencies that may be relevant to HTV monitoring. The development of this yarn allowed for a vibration-sensing glove to be engineered.
It was observed that the incorporation of small MEMS accelerometers within the structure of a textile yarn had little effect on its behavior, suggesting that the micro-pod encapsulation and braided structure did not significantly absorb vibrations. A linear relationship between the vibration-sensing E-yarn’s response and vibration amplitude at the different frequencies was confirmed. A linear relationship was also observed between the vibration-sensing E-yarn’s response and the frequency of the vibration. These relationships could be used to calibrate the response from the vibration-sensing E-yarn to determine vibration in a real-world scenario. Multiple yarns were compared showing that each behaved in a similar fashion.
This work also showed an alternative vibration-sensing E-yarn design that utilized a knit-braided outer sheath. This design introduced more experimental errors, and the more complex relationship between the sensor response and frequency that was observed would make extracting meaningful data in a real-world scenario difficult. It was believed that these differences were due to the looser structure of the knit-braid in this vibration-sensing E-yarn’s design, which allowed the vibration-sensing element to move within the structure of the outer yarn. This informed the textile structure and materials used for the final vibration-sensing glove. This also highlighted the importance of the E-yarn design for correct transmission of vibration to the embedded component.
Finally, a fabric sample with an embedded vibration-sensing E-yarn was characterized, showing that the fabric had no effect on the E-yarns response. This allowed for a vibration-sensing glove to be constructed, which incorporated four vibration-sensing E-yarns.
While this proof-of-concept is an important step toward developing a glove to monitor HTV, further work will be needed to mature this device into either a viable health monitoring or research tool. Initially, specific use cases should be identified, and the E-yarns should be validated for the specific frequencies and amplitudes output by these tools. Depending on the amplitudes experienced this may require the accelerometer used to be changed. A comprehensive user trail would then be necessary to validate the glove. It is proposed that during such a trial the contact force exerted between the users hand and tool would also be recorded as this is known to be an important factor in vibration transmittance [13