Simcenter Testing Solutions Human Body Vibration

2021-05-26T14:08:51.000-0400

Simcenter Testlab

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Humans interface with many different types of products that generate vibration. These products include hand tools, seats, construction equipment, and more.

Human body vibration measurements assess the comfort and exposure effects of interfacing with these types of products.

Broadly speaking, the vibration assessment fall into one of two categories:

Human comfort and quality perception
Health effects due to long term exposure

This article provides background, a guide to applicable standards, and instructions for using Simcenter Testlab for the proper measurement and assessment of human body vibration.

Index
1.   Introduction
2.   Human Body Vibration Standards
3.   The Weighting Curves
4.   Whole-Body Vibration ISO 2631
4.1 Crest Factor Determines Calculation Method
4.2 RMS Method
4.3 Running RMS Method (Maximum Transient Vibration Value, MTVV)
4.4 Vibration Dose Value (VDV Method)
5.   Whole-Body Vibration in Simcenter Testlab
5.1 Getting Started
5.2 Weighting Filters and Signature Acquisition
5.3 Crest Factor, MTVV, and Time Data Selection
5.4 Running RMS or VDV Calculations
5.5 Vector Sum and Data Block Processing
6.   Hand-Arm Vibration ISO 5349
7.   Hand-Arm Vibration in Simcenter Testlab
8.   Human Body Vibration Example in a Vehicle
9. Human Body Vibration and Hand-Arm Vibration in Simcenter Testlab Neo
10. Conclusion

1. Introduction

Humans can be adversely affected by vibration depending on the amplitude, excitation frequency, and exposure duration. The vibration effects can vary widely: hardly noticeable to annoying to creating health concerns. At high levels of vibration and exposure, health and safety concerns include fatigue, reduced sensitivity, abdominal or chest pain, lower back pains, or even a condition known as white finger syndrome (Figure 1).

Figure 1. Potential health and safety concerns related to whole-body and hand-arm vibration.

Based on these concerns, the European Union created directive 2002/44/EC defining standards for human body vibration. Under this directive, it is mandatory for certain products to be certified according to the International Standards Organization (ISO - www.iso.org).

These requirements classify specific products that must meet and/or report vibration performance based on specified metrics. Based on their classification, some products cannot be sold within Europe without being certified to the standards.

The main ISO standards under this directive are ISO 2631 and ISO 5349. These standards define how to execute the test procedure including: take proper measurements, mount instrumentation sensors, apply weighting filters, and assess the exposure. The basic procedure is outlined in Figure 2.

Figure 2: Human Body vibration process starts with accelerometer measurement (left), which is frequency weighted (middle), to get a final result (right).

The human body is more sensitive to some frequencies than others, and the weighting function takes this sensitivity into consideration. It is applied on the measured accelerometer data which results in the predicted human body response. After the data is analyzed, the standards outline corrective actions to be taken.

While meeting requirements is important, the vibration metrics outlined in the standards has led to engineering the performance of products beyond the prescribed limits. By reducing the vibration levels lower than required, companies can gain a competitive advantage over competitors and create brand identity.

2. Human Body Vibration Standards

Standards from the International Standard Organization (ISO) cover how to perform and analyze human body vibration.

For example, some ISO standards defined in European Directive 2002/33/EC cover instrumentation and product specific information:

ISO 8662: Specific product-type applications, such as ISO 8662-2 for chipping and riveting hammers, ISO 8662-3 for Rock Drills and Rotary Hammers, ISO 8662-4 for Grinders, ISO 8662-5 for Pavement Breakers and Construction Work Hammers, and ISO 8662-6 for Impact Drills.
ISO 8041 and IEC61260: Define regulations involving the basic minimum standards and qualifications for the instrumentation and measurement equipment itself and data acquisition hardware and software.

Other ISO standards for Human Body vibration cover the weighting curves, how measurements are performed, and how to analyze the vibration data:

ISO 2631: ISO 2631-1 outlines Specific Frequency Weighting Functions and sensitivities for how the human body and specific body parts react to vibration. Specific Whole-Body Vibration test procedures are outlined in ISO 2631-2 for Building Vibrations, ISO 2631-3 for Motion Sickness, ISO 2631-4 for Vibration in Transport, and ISO 2631-5 for Impulsive Vibrations.
ISO 2631 defines and requires the use of the Wc, Wd, We, Wf, Wj and Wk weighting curves. These curves are for whole-body vibration calculations.
ISO 5349: Defines Hand-Arm Vibration test procedures. (Defines and uses Wh weighting curve)

Note that Human Body Vibration consists of the combination of the Whole-Body and Hand-Arm Vibration standards. This can be a bit confusing, because the term "Whole-Body" might be thought to imply that the Hand-Arm is included. In fact, the Hand-Arm is considered separately than what is termed Whole Body. This distinction is shown in Figure 3.

Figure 3. The human body as a dynamic system of spring/mass/dampers. Each system has a different natural frequency.

The human body is a dynamic system. Each body part has dynamic characteristics which respond to specific frequencies. The body can be viewed as a set of spring/mass/damper systems each with a unique natural frequency. Weighting functions are defined in the standards for each key body part (hand, arm, chest, head, etc) that account for these dynamic characteristics of the body parts.

3. Weighting Curves

Accelerometers measure all frequencies equally well in their specified frequency range of operation. The human body, on the other hand, responds differently per frequency. Weighting curves, as defined in the standards, capture this difference. The Weighting curves are used to shape the vibration data gathered from an accelerometer to predict whole body and hand-arm response due to the vibration.

Weighting functions are applied to the raw time signals when performing all calculations outlined in ISO 2631 and ISO 5439. These applied weighting functions are different based on:

Exposure location on the human body (hand, head, etc)
Position/posture of the body based on the handling of the test object
Axis or direction of the vibration

The positions, locations, and directions are shown in Figure 4.

Figure 4: Axes directions and positions for measurements and weighting locations for performing Whole-Body vibration analysis (left) and Hand-Arm vibration (right).

A summary table of the weighting functions that are applied at these locations, postures, and directions is shown in Table 1.

Table 1: Summary table of Human Body Vibration weighting filters and when they should be applied.

A more detailed definition of each weighting curve and how and when it is applied per ISO 2631 and ISO 5349 is listed below:

Wk – Applied in vertical whole-body vibration weighting to the z axis in a seated, standing or recumbent position. (ISO 2631-1)
Wd – Applied in horizontal whole-body vibration analysis, x and/or y axis, seated, standing or recumbent position. (ISO 2631-1).
Wf – Applied in vertical whole-body vibration weighting in the z axis. Low frequency analysis for evaluating motion sickness in a seated or standing position (ISO 2631-1).
Wc – Used in whole body vibration analysis. Applied to x–axis measurement for a seated person. Measurement location is on the seatback. (ISO 2631-1)
We – Applied to rotational whole-body vibration analysis, x, y, and z directions, seated position. (ISO 2631-1)
Wj – Applied in vertical head vibration weighting curve. Applied to x axis in a recumbent position. (ISO 2631-1)
Wh – Hand-Arm Vibration analysis and data processing. Used for all directions – x, y, and z. (ISO 5349-1).

The Wk,Wd, and the Wf curves are plotted as a function of frequency in Figure 5.

Figure 5: The Whole-Body (Wk, Wd, and Wf) weighting curves from ISO 2931. See applicable standard for exact information.

The Wk, Wd, and Wf weighting curves are shown in Figure 6.

Figure 6: Seat back, seat back recumbent, and recumbent head weighting curves (Wc,We, and Wj). See applicable standard for exact information.

The Wh filter for hand-arm vibration is defined in ISO 5349-1 and is discussed further in the Hand-Arm Vibration Section of this article. The frequency response is shown in Figure 7.

Figure 7: Hand-Arm (Wh) Vibration Weighting Curve from ISO 5349. See applicable standard for exact information.

The weighting filters are applied to the measured vibration signals to predict the effects of the vibration on the human body.

The weighting functions are similar to the A-weighting curve used to mimic the human ear’s sensitivity to sound in microphone recordings. For more information on the human ear and A-weighting, see the knowledge article "What is A-weighting?".

4. Whole-Body Vibration ISO 2631

Human Body Vibration is equivalent to the sum of Hand-Arm Vibration (discussed in the next section) and Whole-Body Vibration. Even though the hand and arm are parts of the human body, from the point of view of the standards, Hand-Arm Vibration is evaluated separately than Whole-Body vibration. The Whole Body standards do not include Hand-Arm vibration.

A typical process in performing whole body vibration evaluation is outlined below:

Step 1: Determine the duration of exposure time to vibrations under normal operating conditions. Only consider operating exposure and scrutinize work patterns for normal activity. For example, how many tools are operated daily and for what amount of time?
Step 2: Determine the magnitude of vibration. The magnitude should closely match the actual vibration experienced by the operator. Directly measuring acceleration for at least 20 minutes is an approach recommended and further detailed in ISO standard 2631-1:1997 and EN 14253:2003.
Step 3: Calculate Daily Vibration Exposure.
Step 4: Comparison of the Daily Vibration Exposure (for 8 hour basis) values against the Exposure Action Values (EAVs) or Exposure Limit Values (ELVs) for Health or Comfort, as defined in Human Vibration Directive 2002/44/EC. These are shown in Figure 8.

Figure 8: Exposure Action Value and Exposure Limit Value for Whole Body Vibration.

Corrective actions are then taken based on the recommendations based on Figure 8.

There are different methods used for calculating the daily vibration exposure value. Outlined below is a description of these methods and some insight into when to select each one.

4.1 Crest Factor Determines the Method

The Crest Factor (CF) is equal to the measured vibration in Peak format divided by the measured vibration in Root Mean Square (RMS) format. This can be expressed as CF= Peak/RMS. It is used to determine the type of analysis to be performed. The higher the Crest Factor, the “peakier” the data as shown in Figure 9.

Figure 9: Two weighted accelerometer time histories. The red trace has a higher crest factor and more peaks than the green trace.

It is important to restate that the values above (Crest Factor, RMS, etc.) are not calculated from original time history acceleration signals, but instead from the weighted time history signals. To choose between which calculations to use, the Crest Factor (CF) from the frequency-weighted acceleration signals is calculated:

For a Crest Factor greater than 9, Vibration Dose Values (VDV) values should be used.
For a Crest Factor less than or equal to 9, the RMS (Root Mean Square) method should be used.

The VDV and RMS methods are described in the next sections.

4.2 RMS Method

The Root Mean Square (RMS) method is best used for steady-state or continuous vibration environments without any impulsive or transient vibration events. Measurements should be made in the applicable exposure locations (outlined in the standard) and in the appropriate accelerometer directions (X, Y, or Z) with triaxial accelerometers.

The results are taken as inputs to calculate aw. The term aw is the weighted RMS value of the acceleration. See Equation 1 below.

Equation 1: Root Mean Square (RMS)

Where:

aw(t) is the weighted acceleration (translational or rotational) as a function of time (time history), in meters per second squared (m/s2), or radians per second squared (rad/s2), respectively;
T is the duration of the measurement, in seconds.

When applying this equation to each measurement location, the appropriate weighting curve needs to be selected and applied along with the applicable k factor (a scaling factor), which is a value of 1.4 for the X and Y directions, and a value of 1.0 for the Z direction.

Shown below is an example of how to apply this method to each axis for calculating a maximum value for daily exposure using this method as shown in Equation 2.

Equation 2: K-factor and exposure adjustments for each axes.

Where:

awi is the measured acceleration in each axis (i can equal x, y, or z) with the appropriately applied weighting function;
Texp is the expected exposure time
T0 is the duration time (Typically 8 hours)

Unlike the calculation for Hand-Arm Vibration where all three axes are always combined to determine the exposure value, the maximum of these calculated values in X, Y, or Z direction is used when calculating Whole-Body Vibration.

Additionally, if there is more than one exposure point measured, these values are totaled across the measured locations and their given axes.

For a given location, the summation (av) is performed on all directions/axes as shown in Equation 3:

Equation 3: Summation of axes directions.

The k value for the x and y axes is 1.4, and the value for the z axis is 1.0.

If multiple accelerometer locations are measured, this calculation is performed for each individual location. Then each location is summed together to get the total vibration exposure.

4.3 The Running RMS Method (Maximum Transient Vibration Value, MTVV)

The Running RMS Method is best suited for mostly steady vibration environments but also where transient shocks or impulsive events may occasionally occur.

The equations for calculating the daily exposure value using the running RMS method are the same as the RMS method, but they are performed over a time segment of one second. The formula also involves the integration of the frequency-weighted acceleration time history. It is expressed as shown in Equation 4:

Equation 4: Running RMS.

Where:

aw(t) is the instantaneous frequency weighted acceleration
τ is the integration time for the running average (1 second)
t is the time (integration variable)
t0 is the time of observation (instantaneous time)

The maximum value over the Running RMS calculation time is referred to as the Maximum Transient Vibration Value, or MTVV:

If MTVV divided by RMS (e.g., MTVV/RMS) is greater than 1.5, the MTVV value should be considered in addition to the Running RMS calculation for the daily exposure value.
If the MTVV divided by the product of RMS times the measurement period (T) to the one fourth power (e.g., MTVV/RMS x T^1/4) is greater than 1.75, the Vibration Dose Value (VDV) should be considered alongside the Running RMS calculation.

For more information about RMS calculations, see the knowledge article "Root Mean Square (RMS) and Overall Level".

4.4 Vibration Dose Value

The Vibration Dose Value (VDV) is used if the measured vibration (or vibrations) consists primarily of transient events, impulses, or shocks.

Below are the equations for each measured axis when using the VDV method shown in Equation 5:

Equation 5: Vibration Dose Value (VDV).

Where:

aw(t) is the instantaneous frequency weighted acceleration;
T is the duration of the measurement.

Like the RMS method, when applying this equation to each measurement location the appropriate “k factor” needs to be applied (Equation 6):

Equation 6: K-factor and exposure adjustments for each axes.

The highest calculation across the three axes here determines the VDV value to be used for the daily vibration exposure value. Again, keep in mind that if there is more than one exposure point measured, these values are totaled across the measured locations in their given axes in order to determine an overall exposure value.

Another consideration to be made when calculating the daily exposure value for Whole-Body Vibration is that if no one axis stands out in determining the maximum exposure value, the vibration total for Whole-Body Vibration can be calculated using the below in Equation 7 per ISO 2631-1:

Equation 7: Summation of axes directions.

The k value for the x and y axes is 1.4, and the value for the z axis is 1.0.

5. Whole-Body Vibration in Simcenter Testlab

Simcenter Testlab can be used to calculate Whole-Body Vibration during live acquisition and post processing. The steps for doing so are outlined in Figure 10:

Figure 10: Process to calculate whole-body vibration in Simcenter Testlab.

These steps are explained in the next sections:

5.1 Getting Started

Siemens Simcenter Testlab makes performing these calculations possible using the Human Body Vibration Add-In shown in Figure 11.

Figure 11. Human Body Vibration Add-In.

The Human Body add-in also requires the Time Signal Calculator Add-In. If using Simcenter Testlab token licensing, the add-in uses 6 tokens.

More information about token licensing in the knowledge article: "Simcenter Testlab Tokens: What are they, and how do they work?".

5.2 Weighting Filters and Signature Acquisition

With these two features activated, the user can go to the ‘Channel Setup’ Worksheet within Simcenter Testlab Signature and access the ‘Virtual Channel’ setup.

Select the FILTER_ISO2631 method to be applied to the measurement channels in real time (Figure 12).

Figure 12: The human body vibration curves available in Virtual Channels of Simcenter Testlab Signature acquisition module.

Once the weighting has been applied to acquired vibration signals, the weighted time signals can be used to calculate the Crest Factor (and the MTVV).

5.3 Crest Factor, MTVV, and Time Data Selection

These calculations can be found in the ‘Online Processing’ worksheet under ‘Frame Statistics’. Here the ‘Crest Factor’ and ‘Maximum’ can be clicked on to calculate the MTVV (Figure 13).

Figure 13: Offline Processing of Crest Factor and Maximum (for MTVV) in the ‘Time Data Selection’ Worksheet using Simcenter Testlab.

See the knowledge article "Simcenter Testlab: Calculating Statistics from Time Histories" for instructions on calculating the Crest Factor and Maximum of the weighted time histories.

5.4 Running RMS or VDV Calculations

Once the Crest Factor has been calculated the appropriate Human Body vibration metric can be selected.

The RMS or VDV methods can be found under the ‘Level Calculation’ tab in the ‘Section Settings’ of Signature Throughput Processing. Signature Throughput Processing can be activated under "Tools -> Add-ins". A new worksheet called "Time Data Processing" will be added to the workflow.

Select the appropriate calculation (RMS, Running RMS, or VDV as shown in Figure 14) based on the Crest Factor values described previously.

Figure 14: Level Calculation in Simcenter Testlab Throughput Processing for RMS, Running RMS and VDV methods.

For more information on using Signature Throughput Processing, see the knowledge article "Simcenter Testlab Throughput Processing Tips".

5.5 Vector Sum and Data Block Processing

Calculating VDV or Running RMS results in a single block of data per channel. To calculate a vector sum of the different vibration axes, turn on "Data Block Processing" under "Tools -> Add-ins". Here formulas for the vector sum can be created with multiplication by the appropriate k-factors (Figure 15):

User-added image

Figure 15: The "Data Block Processing" add-in is used to calculate the vector sum of the Running RMS or VDV data.

Finally, these calculated values can be compared to the requirements outlined within the ISO 2631 and ISO 5349 standards and corrective action can be taken if required.

6. Hand-Arm Vibration ISO 5349

Hand-arm vibrations result from holding a vibrating product in the hands at the locations shown in Figure 16.

Figure 16: For Hand-Arm vibration, the co-ordinate system for measurements can be either biodynamic (based on the orientation of the hand) or basiccentric (based on the orientation of the tool).

Manufacturers of the following products are often required to perform hand-arm vibration testing:

Chainsaws
Concrete Breakers / Road Breakers
Cut-off Saws, such as for stone
Hammer Drills
Handheld Grinders
Impact Wrenches
Jigsaws
Needle Scalers
Pedestal Grinders
Polishers
Power Hammers or Power Chisels
Powered Lawn Mowers
Powered Sanders
Scrabblers
Brush Trimmers and Cutters

IS0 8662 determines the classification of the product, while ISO 5349 defines the procedure used to apply the Wh curve to calculate values.

For measurement, a triaxial accelerometer is mounted onto the tool at the normal operating hand positions. This could be a single hand position with one triaxial accelerometer, or at both hand positions with two triaxial accelerometers (the exact test procedure is defined in the ISO procedures). Acceleration is measured in the X, Y, and Z directions. An example of a hand tool to be measured for hand arm vibration is shown in Figure 17.

Figure 17: A typical two accelerometer setup of a hand tool for measuring Hand-Arm Vibration. The white arrows in the upper left picture point to the accelerometers.

A typical setup for measuring hand-arm vibration includes two accelerometers. The two triaxial accelerometers are mounted at both hand positions. The hand tool is suspended by bands to mimic free-free boundary conditions.

After measurements are acquired, the Wh weighting function is applied according to ISO 2631 – specifically cited in ISO 5439 for hand-arm vibration.

7. Hand-Arm Vibration in Simcenter Testlab

To perform Hand-Arm vibration analysis, the "Human Body Vibration" add-in needs to be turned on under "Tools -> Add-ins" in Simcenter Testlab.

Once activated, in Simcenter Testlab the hand-arm weighting function can be applied to the data either:

Live: During acquisition using a Virtual Channel in the Channel Setup workbook of Simcenter Testlab Signature
Postprocessing: Using existing time data files using the Time Signal Calculator Add-in.

With the Human Body Vibration add-in activated in Simcenter Testlab, the FILTER_ISO5349 weighting function shown in Figure 18 becomes available.

Figure 18: ISO 5349 weighting filter available to be applied to time data.

After the filter is applied, the Root Mean Square based on the vector sum of the three directions is calculated as shown in Equation 8:

Equation 7: Summation of axes directions for Hand-Arm vibration.

This vector sum can be easily calculated in the Simcenter Testlab software with the ‘VECTORSUM’ calculation available in Signature Throughput Processing, shown in Figure 19. More instructions in the knowledge article "Calculate a Vector Sum to Make Understanding Vibration Data Easy".

Figure 19: VECTORSUM calculation available in Simcenter Testlab

After calculating the RMS value of ahv using Signature Throughput Processing, the value can be compared to acceptable exposure values outlined in ISO 5439. See the knowledge article "Simcenter Testlab Throughput Processing Tips" for more instructions.

For example, ISO 5349 outlines two main defined levels for exposure:

The Exposure Action Value (EAV)
Exposure Limit Value (ELV)

These are shown in Figure 20.

Figure 20. Exposure Action Value (EAV) and Exposure Limit Value (ELV) – ISO 5439

For example, if the ahv exceeds EAV = 2.5 m/s^2, then action is taken to reduce vibration, perhaps to reduce the risk of health risk for long period exposures. If the ahv exceeds ELV = 5 m/s^2 then the power tools use immediately should be stopped due to dangerous conditions.

8. Human Body Vibration Example in a Vehicle

The overall Human Body Vibration is the sum of the calculated Hand-Arm and the calculated Whole-Body vibration if applicable. Looking at the vehicle example in Figure 21, both Whole-Body calculations for the seated body and Hand-Arm calculations for the steering wheel need to be performed.

Figure 21: Human Body Vibration calculation in a vehicle requires both Whole-Body Vibration calculations and Hand-Arm Calculations to be performed.

First, the appropriate weighting filters need to be applied to correct directions and corresponding measurement locations, shown in Figure 22.

Figure 22: Applying the correct weighting curves to the appropriate measurement locations.

Next, the Crest Factor needs to be calculated shown in Figure 23.

Figure 23: Crest Factor Calculated in Simcenter Testlab.

See the knowledge article "Simcenter Testlab: Calculating Statistics from Time Histories" for instructions on calculating the crest factor.

Finally, the using the Data Block Processing add-in the total Human Body Vibration can be calculated as shown in Figure 24.

Figure 24: Final calculations for Human Body Vibration performed in Simcenter Testlab.

The applicable k-factor can also be added to the equations as needed.

9. Human Body Vibration and Hand-Arm Vibration in Simcenter Testlab Neo

With Simcenter Testlab Neo 2306 and higher, human body vibration methods are available in the Simcenter Testlab Neo Process Designer as shown in Figure 25.

Figure 25: Human Body Vibration process in Simcenter Testlab Neo.

With the "Vibration Comfort Analysis" turned on, there are two processes available to analyze vibration data for human comfort as shown in Figure 26.

Figure 26: From the "Process" ribbon, there are two human vibration comfort analyses available: "Hand Arm Vibration Analysis" and "Human Body Vibration".

The methods in these processes can be selected and have their processing parameters adjusted.

More about using Simcenter Testlab Neo in this knowledge article: Simcenter Testlab Neo Introduction.

10. Conclusion

Human body vibration testing and calculations can vary based on the product, classification, and exposure as classified by the ISO standards outlined in the European Union directive 2002/44/EC. These test and analysis methods are important to know in order to validate products that are require certification. Additionally, these standards and testing methods can be used to enhance designs based on customer perception and competitive differentiation.

Questions? Email scott.beebe@siemens.com or contact the Siemens Support Center.

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Simcenter Testing Solutions Human Body Vibration

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