Background

Hand-held operating machinery expose workers to hand-transmitted mechanical vibration which can interfere with comfort, efficiency and workers’ long term health and quality of life. The use of the a vibration measurement technology aids the creation of best-practice preventive guidance by quantifying operator’s daily exposure at the workplace.

The evaluation of vibration exposure risks can be broken up into a number of distinct stages:

    • identify a series of discrete operations which make up the subject’s normal working pattern;
    • select the operations of interest to be monitored;
    • quantify the RMS acceleration value for each selected operation;
    • evaluate of the typical daily exposure duration for each operation identified;
    • calculating the 8-h energy-equivalent vibration total value (daily vibration exposure).1,2

The HAV-Sentry vibration measurement system is designed and manufactured in conformity to the industry standards related to the measurement and evaluation of human exposure to hand-transmitted vibration. These general requirements aim to provide practical guidelines of performing correct measurements and developing an effective strategy for hand-transmitted vibration measurement. Here we describe how our device fits the compliant picture and our technical road to contribute to the improvement of this established technique.

Sensor placement & mounting

The sensing unit of the Aegis is securely embedded in the textile glove at tool’s hand-driving point. This encloses the transducer which acquires and reports vibration magnitudes for the three directions of the biodynamic coordinate system defined in the section 4.2 of the BS EN ISO 5349-1:2001, on page 4.

We argue that tool-timing based on the idealised manufacturer-declared amplitude values defies the purpose of drawing a comprehensive picture of one’s daily vibration exposure. In addition, continuously relocating a fixed mounting accelerometer appears to be an elaborate and time-consuming task.

The HAV-Sentry system adopts the hand-held adaptor design detailed in the Annex D of the BS EN ISO 5349-2:2001, sitting comfortably between the vibrating surface and the palm and ensuring transmissibility of the actual vibration energy absorbed by the hand-arm system.

Weighting Factor

The frequency weighting Wh reflects the importance of different frequencies in causing injury to the hand. The range of application of the measured values to the prediction of vibration injury is restricted to the working frequency range covered by a frequencies from 4Hz to 2000Hz. Band-limiting high-pass and low-pass filters restrict the effect on the measured value of vibration frequencies outside this range where the frequency dependence is not yet standardised.

The Shannon-Nyquist theorem that specifies the sampling frequency of the accelerometer shall be at least twice the vibration frequency which the apparatus intends to acquire. Our accelerometer samples 4000 data points per second, which translate to band-limiting the frequency range between 0 and 2000Hz. The accumulation of vibration exposure, i.e. A(8), starts only when acceleration has passed 6Hz, preventing the user from faking vibration data through hand movements.

In the HAV-Sentry system, the frequency-weighting filtering is done in real-time on the Aegis device, as vibration data is acquired. The side plot shows the Wh factor generated by the embedded software has a 0.3% percentage error across the frequency range of interest, in line with filter response and tolerance limits detailed in section 5.5 of the BS EN ISO 8041-1:2017 referring to the accuracy at reference frequency under reference conditions.3

Daily Exposure Calculation

Vibration exposure measurement is performed continuously during normal working hours, from the time of user authentication via the ID Tag until the Aegis device is connected for data transfer, with no user input required in the process. The exposure value resulted from the utilisation of multiple tools in this interval is computed in real time by the method extracted from section 4.5 and 5 of the the BS EN ISO 5349-1:2001 on exposure characterisation through multi-axis vibration data acquisition.

For a batch of n vibration magnitude data points, the 3-axis root mean square acceleration arms is calculated for each of the 3 axes using equation (I) and stored in m/s2, along with the peak acceleration value.

The partial vibration exposure ahv, defined as the root-sum-of-squares of the three component values, is then computed using equation (II).

The 8-hours energy equivalent frequency-weighted vibration total value A(8), onto which the live exposure alerts are based, is determined every second using equation (III) and outputted in m/s2.

The standard evaluation of vibration exposure is solely based on vibration magnitude and exposure times. Factors unconsidered, but likely to influence the effects of human exposure to hand-transmitted vibration, are described in Annex D of the BS EN ISO 5349-1:2001, including but not limited to: gripping and feed forces applied by the operator, the posture of the hand and arm, the condition of the tool and the workpiece, etc. There is currently no guidance defined to evaluate additional factors. However, the standards recognizes that reporting of all relevant information is important for the development of improved methods for the assessment of vibration risk.

The HAV-Sentry system completes the compliant picture and contributes to a more comprehensive risk assessment by evaluating the real-life working scenario and providing a parallel stream of data with operator’s gripping strength, hand orientation and manually inputted anthropometric data.

Grip Strength

The HAV-Sentry system outputs average user gripping strength for every second. This data can be corelated to the task activity, contributing to the robustness of the best practice guides.

Vibration is absorbed by the human hand as a function of the grip force exercised on the tool handle. Higher grip forces generally increase the intensity of vibration experienced by the user, while also propagating the vibrational waves at larger distances within the human body.8 More specifically, the human biodynamic components representing hand dynamic characteristics such as: mass, stiffness and damping are directly proportional with how strong of a grip is maintained on the tool handle.10

While in contact with the vibrating machine handle, the non-linear coupled system of the human hand resonates at its natural frequencies.6 Resonance is the prime cause of Hand Arm Vibration Syndrome symptoms.10 Grip force & hand orientation could help determine the resonance points. Specific personalised recommendations can be given to workers to avoid unnecessary hazardous exposure due to tool resonance.

Hand Orientation

The orientation of the hand is an important parameter that need to be considered when formulating an accurate vibration exposure risk assessment. Here is why:

  • Symptoms of HAVS are accelerated if the hand-arm system is obliged to maintain a specific hand orientation.5
  • The hazardous grip method varies from individual to individual because every human hand is unique in geometry and density.8
  • Every human hand has corresponding vibrational mode shapes that manifest themselves at natural or resonant frequencies. Vibrational mode shapes are physical patterns which the hand forms when it vibrates at the natural or resonant frequency.7 The mode shapes of the hand can be activated if a particular gripping method or hand orientation is maintained during work.8

This information can be used to train operators to not only work safer, but also more efficient.

Anthropometric Data

The harshness of vibrational effect is indisputably correlated with anthropometrics.9 Detailed studies regarding the significance of individual body characteristics to HAV symptoms indicate that larger hand-arm sizes implicitly provide an increase of mass and results in an increase in magnitude of the biodynamic response.4 The palm soft tissue appears to absorb vibration more effectively and grant more effective isolation due to resembling a spring-damping mechanical system, thus, in this machine forced vibrational frequency range, hand size is inversely proportional to biodynamic response, and therefore larger hands are less vulnerable.10

Biodynamic components representing hand dynamic characteristics such as mass, stiffness and damping were found to be slightly higher for the female hand, therefore more prone to disorders in the sensorineural, vascular and musculoskeletal structures, even though vibrational energy transmitted to the hand was computed to be slightly lower for the female.10 This reflects the contribution of other relevant anthropometrics such as upper body strength and total body weight, which advantageously contribute to the appropriate absorption/assimilation of vibrational waves in a larger individual.8

The HAV-Sentry Dashboard hosts personalised profiles for each operator, where the anthropometric data is stored and can be updated.

[1] BS EN ISO 5349-1:2001 Mechanical Vibration. Measurement And Evaluation Of Human Exposure To Hand-Transmitted Vibration. General Requirements (2015) United Kingdom: BSI

[2] BS EN ISO 5349-2:2001+A1:2015 Mechanical Vibration. Measurement And Evaluation Of Human Exposure To Hand-Transmitted Vibration. General Requirements (2015) United Kingdom: BSI

[3] BS EN ISO 8041 Human Response To Vibration. Measuring Instrumentation (2005) United Kingdom: BSI

[4] Graham, B. (2000) Using An Accelerometer Sensor To Measure Human Hand Motion. Massachusetts: Massachusetts, Institute of Technology

[5] Griffin, J. (1996) Handbook of Human Vibration. 1st edn. London, Academic Press

[6] Harih, G., Nohara, R. and Tada, M. (2017) Finite Element Digital Human Hand Model-Case Study  Of Grasping A Cylindrical Handle. Journal Of Ergonomics 07 (02)

[7] Hartog, J. (2013) Mechanical Vibrations. Dover Publications

[8] Hayward, V., Visell, Y. and Shao, Y. (2016) Spatial Patterns of Cutaneous Vibration During Whole Hand Haptic Interactions. Proceedings of The National Academy of Sciences [online] 113 (15), 4188- 4193. available from http://www.pnas.org/content/pnas/113/15/4188.full.pdf

[9] Mansfield, N. (2005) Human Response To Vibration. 1st edn. Boca Raton, FL: CRC Press

[10] McDowell, T., Dong, R. and Welcome, D. (2005) Biodynamic Response At The Palm Of The Human Hand Subjected To A Random Vibration. Industrial Health 43 (1), 241-255

Augment your workforce!

Ready for the big step?

Not yet convinced?