ISO 5349: FITTING ACCELEROMETERS – VIBRATIONS MEASUREMENT SYSTEM HAND ARM

Summary:

A ISO 5349 defines the accelerometers must be rigidly mounted to effecting the measurements. The concept of rigidity derived from the definition function with one degree of freedom. Pursuant 5348 also recommends that the resonant frequency of the accelerometer shall be at least five times greater than the maximum frequency of interest which in the case of measuring the hand arm system is 1250 Prophet. Since the resonance frequency of the accelerometers typically used in these measurements much higher than this value, what is primarily concerned is the assembly of the resonance frequency.

In this article will be revised accelerometers mounting methods to perform these tests and the factors that must be taken into consideration in your selection.

Introduction

The title of the ISO 5349 "Mechanical vibration – Measurement and assessment of the exposure of individuals to the vibrations transmitted to the hand-arm system "is contradictory to what the standard is referred to in sensor assembly as" the accelerometers should be rigidly mounted on vibrating surface ". In fact being mounted accelerometers said form which are to be measured are the vibrations in the tool and not the vibrations transmitted to the hand arm system.

It is known that the assembly of the accelerometer's own hand and arm system influence the vibrations transmitted.

The influence of the frequency response of the accelerometer on the results of the measures

The accelerometers most often used in measuring vibrations in the hand-arm system are type piezoelectric. In them the generated electrical charge is proportional to the acceleration to which they are subject.

The concept of rigidity that comes from the standard definition function with one degree of freedom.

In a system of one degree of freedom is defined as the transmissibility function that shows the relation between the generated power and the transmitted power as a function of frequency, the following is graphically.

When Beta (natural frequency of the system with one degree of freedom divided by the frequency of the exciting force) is equal to a resonance phenomenon occurs, occurs and an amplification of the force transmitted.

In this role are defined three zones:

• Zone rigid behavior when Beta is much less than 1, wherein the transmitted forces are approximately equal to the forces generated;
• Flexible behavior, Beta is much higher when a, wherein the transmitted forces are smaller than the forces generated;
• Resonance zone, where beta is approximately equal to one and that the transmitted forces are greater than the forces generated. In this area the extent of Transferability varies proportionately as the damping.

It is often so accelerometers manufacturers showed the frequency response of an accelerometer in a manner similar to the function of one degree of freedom, As can be seen in the following figure for:

From the point of view of accuracy of the assay is intended when the accelerometer frequency response is to ensure that it works in the rigid zone of the transmissibility function, to ensure that the value measured by the sensor within the accelerometer is equal to the value at which the accelerometer assembly is subjected.

The rule states that the resonance frequency of the accelerometer and assembly should be at least five times the measurement range of the measurement in the case of hand arm system is 1250 Prophet. Since the resonance frequency of the accelerometers typically used in these measurements much higher than this value, what is primarily concerned is the assembly of the resonance frequency.

The influence of the frequency response of the accelerometer assembly

The ISO 5349 it is stated that "The mounting method should provide a linear frequency response over the frequency range to be measured, in other words, should not weaken or amplify and should not generate any resonances in this frequency range. "

It is known to influence the assembly of piezoelectric accelerometers in the frequency response of the measures. The assembly of the accelerometer generates a system with one degree of freedom with which it is associated to a natural frequency of the assembly often referred to as resonance frequency.

From the point of view of accuracy of the assay is intended where the assembly is to ensure that it works in the rigid area of ​​this function, to ensure that the value measured by the sensor inside the accelerometer is equal to the value at which the object is mounted is subject.

Just how the probe is brought into contact with the measuring point significantly influences the results of the measurements. As a general rule can be stated that, The more rigid is the binding of the probe to the measuring point, more rigor will that measure. So in the figure the best assembly presented below would be the Reference 1, com or accelerometer sixed by screwing Perne, it shows that the assembly of resonant frequency with a higher value.

According to the standard mounting resonant frequency would be higher than five times the measurement range 1250 Prophet, in other words, 6250Prophet.

This mounting resonance frequency value is only achieved with very rigid assembly, such as for example a flat planar surfaces in the magnetic base or mounting stud.

The weighting curves in frequency standard

A Norma ISO 5349-1 defines the following frequency weighting curve:

According to this curve is seen that the frequencies above 500 Hz are weighted at less than 4% frequencies between 8 and 16Hz.

This means that after applying the weighting, the percentage of the amount recorded for the measure, at frequencies above 500 Hz is less than the error associated with the measured frequency response of most accelerometers used for this type of measure (deviation from linearity below the 5%; Cross sensitivity less than 5%))

That is, the standard specific to a higher attenuation measurement error of accelerometers for frequencies higher than 500 Prophet.

It may be that the measurement error derived only from the inaccuracy of the accelerometer at frequencies between 8 e 16 Hz exceeds the weighted values ​​measured at frequencies exceeding 500 Prophet, especially when the importance of low frequency components, minors 100 Prophet, is much greater than the high frequency, above 500 Prophet, as is the case of most of the machines.

The location of the accelerometer assembly

The ambiguity of ISO 5349 arises especially na part 2, when referring to the location of the accelerometers. For one states that "making measurements directly on the palm are, normally, only possible using mounting adapters "and Annex D, gives as an example some adapters that clearly do not meet the required above with respect to the rigidity of the assembly at high frequencies, above 500 Prophet.

Being difficult to estimate, accurately, errors arising from the use of an assembly that does not reflect accurately the conditions of actual use, it can be stated, without incurring great risk, that the errors arising from a test assembly that is not true to the real conditions of use, are higher than those of assembling not be hard to vibratory phenomena with frequencies above 500 Prophet.

Thus only in cases in which the frequency spectrum of the vibrations of machines presents very dominant components above 500 Hz is revealed strictly required accelerometer mounting high rigidity. In other situations, that are most, should opt by the most realistic mounting possible and lose rigor in measurements at higher frequencies 500 Prophet.

Conclusion

In the measurements made in accordance with ISO 5349 only in cases in which the frequency spectrum of the vibrations of machines presents very dominant components above 500 Prophet, reveals strictly required accelerometer mounting high rigidity. In other situations, that are most, should opt by the most realistic mounting possible and lose rigor in measurements at higher frequencies 500 Prophet.

In a study, which use an installation that reflects in a realistic way, working conditions, the results reflect, with much greater accuracy, the level of vibration to which workers are exposed.

Below you can see a presentation on this topic.

References

1. Directive 2002/44 / EC
2. ISO 5349-1 and ISO 5349-2 from 2001 Mechanical Vibrations - Measurement and evaluation of the exposure of individuals to the vibrations transmitted by hand
3. Handbook of HUMAN VIBRATION, M.J.GRIFFIN, ACADEMIC PRESS, 1990
4. E.U. Guide to good practice on Hand-Arm Vibration – European Commission General Direction Employment and Social Affairs