The Impact Of Contact Force On The Accuracy Of Hand-Arm Vibration Measurement

When measuring vibration on a hand, simultaneous measurement of contact force verifies whether the force magnitude is sufficiently rigid. The contact force also provides information on the operator’s work schedule and may help to instruct operators if they are using excessive or too little force when working with hand-held tools. Additionally, by knowing both the coupling force value and the vibration acceleration, it is possible to calculate the actual vibration energy dose that has been transferred to a hand.

Contact Force

The contact forces between the hand and the vibrating surface are: the push/pull force and the gripping force. The need for simultaneous assessment of contact forces and vibration magnitudes has been universally recognized and reflected in the ISO 15230 standard.

MEMS Accelerometers and Contact Force Sensors

In recent years accelerometers based on MEMS technology (Micro Electro-Mechanical Systems) became an alternative to piezoelectric sensors. MEMS transducers are widely used in micro-mechanical systems in the automotive, computer and audio-visual industries. Construction of MEMS consists in a moving mass of resistant boards placed on a mechanical suspension system frame of reference. As a result of movement (such as vibration), there is a change in the capacitance between the moving and the fixed plates (which form capacitors).

The advantage of MEMS is their dimensions that can vary from only a few microns to millimetres, which makes them a milestone in miniaturization. The list of the advantages of MEMS-based sensors is long and includes low cost, low power consumption, small size, resistance to mechanical shocks, full electromagnetic compatibility, and no DC-shift effect.

The introduction of MEMS accelerometers broke the barrier created by piezoelectric accelerometers in hand-arm vibration measurements. First of all, it reduced the cost of the complete system. Secondly, their small size allowed for being attached to human hands without any distraction to the performance of everyday activities even underneath anti-vibration gloves, therefore giving the true results of vibration exposure. Additionally, their size allowed for the installation of a contact force sensor next to the accelerometer, thus enabling measurement of the contact force simultaneously with tri-axial acceleration assessment. This constituted a strong basis for the creation of improved methods of hand-arm vibration assessment and new hand-arm vibration measurement standards.

Figure 1. Hand-arm vibration adapter with tri-axial MEMS accelerometer and contact force sensor installed

Use of hand-arm vibration adapters in accordance to ISO 5349

Before MEMS sensors were introduced, hand-arm vibration measurements were performed with piezo-electric sensors typically mounted on tools. The location of sensors was chosen not as recommended by the ISO—at the point of contact with the hand, but, to avoid damage, at the point most convenient and safe for the sensor itself. The introduction of MEMS hand-arm vibration adapters with force measurement capabilities solved that problem and enabled measurement exactly at the point of contact between the hand and the vibrating surface.

At the same time, the use of hand-arm vibration adapters raised a question about the accuracy of vibration measurements as a function of the contact force. To answer this question, 240 measurements were performed at the Polish National Research Institute at the Central Institute for Labour Protection. The impact of the coupling force on the vibration magnitudes has been assessed with Svantek’s SV106 human vibration meters and SV105AF hand-arm adapters (push force thresholds in tests were: 0 N, 20 N, 50 N, 100 N).

Hand-Arm Vibration and Contact Force Measurement

The study has been performed with two SV 106, SVANTEK’s human vibration level meters meet the ISO 8041:2005 requirements and are designed to perform measurements in accordance with ISO 5349-1 and ISO 5349-2 standards, with special SV 105AF hand-arm adapters mounted on the handles of vibration exciters.

During the measurement, the instrument was battery-powered. Two SV 105AF units, marked as A and B hand-arm adapters, have been attached with tape and beeswax to ensure the repeatability of measurements. One of the adapters (A) has been used for measurement with the operator, while the second one (B) is the reference. Both measurement points were located on the testing handle. During measurements, point A was located at the point of contact of the operator’s hand with the handle. Point B has been located below the zone of contact of the hand with the testing handle. As a reference, the accelerometer Brüel & Kjaer 4374, nr. NA2/11, and the PULSE Brüel & Kjaer 3560C, nr. NA2/84 analyzer have been used. The reference accelerometer has also been attached to the handle.

Measurement instrumentation

Instrumentation used to generate and control test signals:

vibration exciter, Ling Dynamic Systems V721, nr id. NA2/30/31,
vibration accelerometer, Brüel & Kjaer 4374, nr id. NA2/11,
amplifier NEXUS, Brüel & Kjaer 2692, nr id. NA2/62,
power amplifier, Ling Dynamic Systems PA 2000, Nr id. NA2/26,
signal generator Brüel & Kjaer 1054, nr id. NA2/25,
computer nr id. NA2/63,
analyser PULSE, Brüel & Kjaer 3560C, Nr id. NA2/84.

Instrumentation used for force measurements:

force meter, PI-W Movir MSZN-1, Nr id. NA2/32,
push force platform, CIOP PIB, Nr id. NA2/34,
testing handle, CIOP PIB, Nr id. NA2/33,
optic sensor, Sensor, CW 18/80, Nr id. NA2/43

Figure 2a. Hand-arm vibration adapter with tri-axial MEMS accelerometer and contact force sensor installed

Figure 2b. SV 106 Vibration Meters

Contact force measurement

During the experiment the push force has been measured with the use of special platform at the Polish National Research Institute at the Central Institute for Labour Protection, using PI-W Movir MSZN-1 meter and CW 18/80 sensor. Push force thresholds in tests were: 0 N, 20 N, 50 N, 100 N.

Figure 1. Example of contact forces measurement given by ISO 15230

The measurement goal and method

The goal of the experiment was to perform measurements of vibration acceleration on a testing handle with the use of SV 106 meters described in p. 2.1 in the presence of the different push forces applied by the operator and evaluate the effects of these forces on the vibration results.

The method of the study was to measure the weighted vibration acceleration in the direction parallel to vibration generated on a testing handle with the simultaneous use of two measurement sets described in p. 2.1. The vibration generated on a handle was compliant with the ISO 10819:2013 standard.

Adapter A was tested under defined push forces applied by < 10 different operators, whereas adapter B was used as the reference without the use of push forces.

Two testing signals were used in the study:

Vibration signal 1: a simulated signal of vibration accelerations generated by the vibration hammer,
Vibration signal 2: a simulated signal of vibration accelerations generated by the angle grinder

Measurements of hand-arm vibration were performed in accordance with ISO 5349-1:2001 Mechanical vibration — Measurement and evaluation of human exposure to hand-transmitted vibration — Part 1: General requirements and ISO 5349-2:2001 Mechanical vibration — Measurement and evaluation of human exposure to hand-transmitted vibration — Part 2: Practical guidance for measurement at the workplace.

Photo 3. Position of the operator on platform during the measurements.

Measurement results

The graph below presents the ratio of averaged results for 30 measurements for each contact force threshold at point A against the results at point B together with the standard deviation. The results from vibration signal 1 are marked in blue and vibration signal 2 in red.

Graph 1. The ratio of vibration values measured with the applied force against to the reference values with no force applied

The impact of contact force on measured vibration values.

On the basis of the conducted study, it has been defined that differences between measured vibration accelerations in points A and B for push forces in the range of 0–100 N do not exceed 1.2%, which means that the effect of the push force applied is irrelevant to the measured vibration values.

The minimum contact force threshold required for the accurate vibration acceleration measurements

Additional measurements with hand-arm adapters without the use of beeswax or mounting tape were performed to evaluate the effect of changes in the coupling of the adapter to the testing handle. It has been noted that for force thresholds below 20 N, it was necessary to ensure that the coupling between the adapter and testing handle was sufficient to prevent breaks in continuous contact during the measurements.

Conclusion

The conducted study proves that the effect of changes in the contact force thresholds applied by the operator is irrelevant to the measured vibration acceleration values. This assumption is valid for forces above the threshold of 20 N, below which it is necessary to ensure the correct coupling between the hand-arm adapter and the vibrating surface. Together with the force level drop below 20 N, the uncertainty related to the coupling increases rapidly. However, it is necessary to note that in practice, for tools generating high vibration amplitudes, the threshold of 20 N may not guarantee perfect coupling, therefore, higher threshold levels should be established.
At the time the ISO 5349 standard was introduced, it was practically impossible to perform force measurements together with tri-axial vibration measurements due to limitations in hardware.
At the moment, very small force transducers can be fitted right next to the MEMS-technology-based vibration accelerometer in a form of a hand-arm adapter, as specified by ISO 5349-2 and ISO 10819 standards. In comparison to the technique of mounting a sensor on a tool, the use of contact force allows defining the actual vibration exposure, whereas mounting on a tool bears the uncertainty of including into tests the periods when there was no contact with the operator’s hand.
As it has been proven, the use of hand-arm adapters provides the same accuracy of vibration amplitudes as in the case of standard piezoelectric vibration sensors mounted on a tool, but additionally offers the advantage of the best possible location of the measurement point—exactly at the point of transmission of the vibration signal to the hand of an operator and provides information on the actual exposure to the vibration.

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