ME'scopeVES Application Note #27 - Impact Testing Using the Acquisition Window

App Note #27

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ME’scope Application Note #27 Impact Testing Using The Acquisition Window To follow the steps in this application note, your system must include the Visual ODS package and one or more VES-700 series options.

The Acquisition window only acquires time domain signals from the front end. All signal processing is done by the window.

THE ACQUISITION WINDOW Execute: File | New | Acquisition to open a new Acquisition Window. The locations of various control groups in the window are shown below.

Acquisition Window (left) Together with Structure and Data Block Windows Simplify Testing.

INTRODUCTION The VES-700 series Acquisition Options provide an Acquisition window within ME’scope that can directly control a broad range of multi-channel front end acquisition hardware systems, analyzers and recorders. The User Interface in the Acquisition window is the same, regardless of the acquisition hardware chosen.

The User Interface is designed specifically for structural testing. It consists of an Acquisition window connected to a Structure window, where the next measurement location is depicted, and a Data Block window in which measurements are accumulated. Animation of shapes from the measurements can take place even while measurements are being acquired.

Graphics Display, Spreadsheets and Control Tabs.

The front end measurement hardware is attached to the PC running ME’scope using the standard interface (USB, Firewire, Ethernet, ISA, etc) of that equipment. A single Acquisition window can be connected to many different types of acquisition hardware, but only to one front end at a time.

In this note, we will use a four-channel USB-interfaced analyzer for impact testing. This example will illustrate the advantages of graphical feedback and tight integration of the measurement and analysis processes. Previously acquired time domain data is provided in a Data Block file, so that you can simulate the test without using the actual hardware.

1.

Commands to Start, Stop and Scope the acquisition, and to Store each acquired measurement.

2.

Acquired Time Traces graphics area shows most recently acquired (raw) measurements. Full Format control and graphics area sizing are provided.

3.

Computed Traces graphics area shows results of signal processing on the acquired time Traces. Full Format control and graphics area sizing are provided.

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4.

Traces spreadsheet displays Trace properties from both graphics areas 2 & 3.

5.

Tabs provide control of acquisition and signal processing parameters.

6.

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Impact testing requires operator intervention with every measurement. Each impact must be free of overloads and “double-hits”.

Note: Double-hits are automatically detected and rejected when you apply a Force window to the impact hammer channel.

Channels spreadsheet displays properties of each input channel of the acquisition hardware.

IMPACT TESTING BASICS The most common Modal Test is a Single Reference roving impact test, using a roving impact hammer and a single reference accelerometer. This type of test is popular because it can be conducted with a minimum of equipment and test article preparation.

In a Single Reference roving impact test, you will mount one Reference accelerometer to the structure and then excite it at a multiplicity of Roving strike points and directions using a force-transduced hammer. The set of FRF measurements resulting from a Roving impact test will provide you with mode shapes defined at every Roving DOF (Point and Direction) at which you impacted the structure.

Double-hits occur when the hammer fails to rebound sufficiently after the impact. This causes ‘zeros’ in the spectrum of the impact force, as shown above. This contaminates the resulting FRF. For best results, the impact force spectrum should be nonzero over the measured frequency range. This is accomplished by selecting the stiffness of the hammer tip and the weight of the hammer to provide a spectrum with sufficient frequency content over the desired frequency range. When properly applied, the spectrum of the force should be a smooth curve that ‘droops’ about 10 dB across the frequency range of measurement, as shown above.

AUTO RANGING INPUT CHANNELS

The number of Reference DOFs at which accelerometers are mounted does not affect the number of DOFs in the resulting mode shapes. A Multiple Reference test, which uses multiple accelerometers, can influence the quality of your results, particularly if local, repeated or closelycoupled modes are encountered.

For more detailed information on the use of Multiple Reference testing, see application notes #5, #6, #14, #15 and #24.

Auto Ranging Channels during Scope Acquisition.

As shown above, the Input Range for all channels can be Auto Ranged by checking Auto Range on the Trigger tab, pressing the Scope button on the toolbar and tapping rapidly at the next Roving DOF on the test structure. This automatically maximizes the ranges of all active front end channels.

Good Impact (left) versus Double Hit (right).

During Acquisition, the Signal indicators in the Channels spreadsheet show the signal levels for every channel with each impact. Yellow indicates a peak value of less than 25% of the selected Input Range. Red indicates an overload, as shown below. Green indicates clean signals and undistorted measurements.

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App Note #27

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The Machine directions are: - axial (A) - horizontal (H) - vertical (V)

Note: The three Directions are always mutually perpendicular to one another for all axis types.

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A Roving DOF is always written as a Point number followed by a Direction. For example, 10X or -22Z or -33R.

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A negative sign means that the direction of positive motion is in the opposite direction from the axis direction.

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A Reference DOF is always written as a colon followed by a Point number and a Direction. For example, :3Z or :-17X.

Good Impact (left) and Overloaded Impact (right).

Executing: Acquire | Reject Impact (F11) removes the current impact from the spectrum average.

DATA ACQUISITION BOOKKEEPING A common source of error in an impact test is failure to record the excitation Point and Direction (the Roving DOF) at which each measurement is made. ME’scope provides an automated scheme using the test structure graphics for assistance.

Measurement Directions Each measurement Direction is defined at Point in a positive or negative direction. The measurement axes may be selected from Rectangular, Cylindrical, Spherical or Machine types.

Each transducer signal input to a channel of the front end hardware is identified as roving with a Roving DOF or fixed with a Reference DOF. The correct DOF for each channel must be entered in the Channels spreadsheet prior to acquisition. In addition, the Engineering Units and Transducer Sensitivity must be entered for each channel.

Measurement Sets

Typical Channels Spreadsheet Entries for an Impact Test.

Rectangular, Cylindrical and Spherical Directions. 

The Rectangular directions are X, Y and Z.

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The Cylindrical directions are: - radial (R) - tangential to the cylinder’s rounded surface (T) - along the cylinder’s axis (Z)

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The Spherical directions are: - radial (R) - perpendicular to the azimuth angle (T) - perpendicular to the angle of elevation (P)

Each time you acquire data, all of the simultaneously measured signals are assigned to a Measurement Set. The Measurement Set number is appended [between square brackets] to every channel DOF measured in the set.

The figure above illustrates a typical setup for a Single Reference roving impact test. Two signals are about to be acquired. These are designated as: 

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1Z – the Roving force (lbf) signal from the impact hammer.

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This displays Reference DOFs (with a red arrow) and Roving DOFs (with a blue arrow) for the current Measurement Set on the 3D model.

:1Z – the Reference accelerometer (g) attached at a fixed DOF (Point 1 in the Z direction) on the structure.

Note that this is a Driving Point measurement since both the force and acceleration have the same DOF. In a roving impact test, there will be one Measurement Set for every DOF impacted with the hammer.

Note: The Driving Point measurement does not need to be measured, but there are some good reasons to measure it first. Refer to application note #6 for details.

All functions calculated from the input signals are assigned DOFs based on the Channel DOFs. Each time you save measurements for a Measurement Set, the Acquisition window will increment to the next Measurement Set. In the following figure, each FRF and Coherence pair is identified by DOFs that include the Measurement Set number.

Displaying DOFs for the Current Measurement Set. When you select a previously defined Measurement Set or type a DOF into the Channels spreadsheet, Reference DOF and Roving DOF arrows are displayed on the 3D model for the corresponding points & directions. The Roving DOF (blue arrow) shows where the next hammer strike should be applied for the current Measurement Set.

Defining Measurement Set DOFs Graphically.

Trace DOFs with Measurement Set Numbers.

Graphical Definition of DOFs

PRE-DEFINING MEASUREMENT SETS The key to planning a test using Measurements Sets is to have a Structure model prepared before starting a test.

This 3D model can serve as a graphic guide to the testing sequence, thus avoiding any errors in making and labeling your measurements.

The DOFs for each Measurement Set can be defined graphically by clicking on Points and Measurement Directions on the 3D model.

Before this can be done, however, all of the test points must be numbered on the model, and the measurement axes oriented properly at each point.

Displaying Measurement Set DOFs

Execute: Measurement Set | Assign Channels. This initiates graphical selection of the Roving DOF for the current Measurement Set.

Execute: Measurement Set | Show Channel DOFs.

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1.

Click on the desired Roving DOF Point. The measurement axes will be displayed at that Point.

2.

Click on the end of one of the measurement axes displayed at the selected Point.

A Roving DOF (blue) arrow will be displayed at the selected Point & Direction and the Roving DOF will be displayed in the Channels spreadsheet.

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EXAMPLE ACQUISITION For this exercise, a Data Block file of previously acquired time histories will be used instead of actual front end hardware. The time histories from a series of 65 impact measurements are provided as one of the Project files.

This is accompanied by Acquisition, Structure and Shape files. To open this project: Execute: File | Project | Open. ď‚

Select Impact Acquisition.PRJ from the Examples\Modal Analysis subdirectory.

The I-Beam.STR and Impact FRF.ACQ windows will open.

Increment DOF Controls in the Channels Spreadsheet.

Incremental Definition of DOFs Execute: Display | Project Panel to close the Project Panel on the left of your screen. This will provide additional room for the two open windows.

The Channels spreadsheet contains fields (in the Measure tab) to enter the DOF (as a Point Number & Direction) for each Channel, as shown above. It also has a column of Increment DOF buttons that allow the Roving DOF to be automatically incremented with succeeding new Measurement Set. The Direction is incremented first and the Point Number is incremented after all directions have been incremented.

In test planning, these controls are used together with the Measurement Set controls on the Acquisition window toolbar. These controls are illustrated below.

Execute: Window | Arrange | For Animation to arrange the two windows side-by-side as shown above.

Measurement Set Controls on the Toolbar.

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Connect the Acquisition window to the I-Beam.STR structure window by selecting it in the drop down list on the toolbar.

Execute: Connect To | Front End and select Use BLK File from the selection list.

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In the open file dialog box, select I-Beam Time Histories.BLK from the Examples\Modal Analysis subdirectory, and click on OK.

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Setting up the Signal Processing On the Measurement tab: 1.

Click on the Frequency radio button.

2.

Check the FRF box, and (optionally) check the Coherence, Auto Spectrum and Cross Spectrum boxes.

3.

Select Linear Averaging and set the Number of Averages to 1.

The time domain Traces in the I-Beam Time Histories.BLK Data Block have one impact each. Therefore, spectrum averaging cannot be performed and Coherence is not meaningful.

On the Sampling tab: 65 measurements on the I-beam.

1.

Select the Number of Samples to be 2048.

In this following example, an I-beam was impacted at 65 different DOFs. Points 1 through 50 on the top and bottom plates were impacted in the (vertical) Z direction, and points 51 through 65 on the web were impacted in the (horizontal) Y direction, as shown above.

2.

Select the Maximum Frequency (Hz) to be 1280.

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without an external signal-conditioning box. Otherwise, turn it OFF.

On the Trigger tab: 1.

Click on the Trigger radio button.

2.

Select the Force (Channel 1) as the Trigger Source.

3.

Select + Slope for the trigger slope.

4.

Select the desired trigger Level as 1.0 percent of full-scale.

5.

Enter the number of Pre-Trigger Delay (20 Samples) to precede the trigger point.

6.

Check the Show box to mark the Force Trace with dashed lines at the trigger point.

Setting up the Input Channels

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Select a “starting point” voltage Input Range for each transducer. Note that most ICP transducers have a full-scale output of 5 Volts.

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In the MIMO column, set channel 1 to MIMO Input. Set channel 2 to MIMO Output.

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Select appropriate physical Units for the force and acceleration channels from the drop-down lists in these cells.

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Enter the Sensitivity constant for each transducer.

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Choose the Sensitivity Units from the drop-down list that opens in these fields.

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Select the appropriate time domain Window for each channel.

For impact testing, use the Rectangular window on all channels, or (optionally) use the Force window on the force channel and the Exponential window on the acceleration response channel. 

If the channel Window is either Force or Exponential, enter the appropriate Window Value.

For the Force window, this is the width of the desired window expressed as a number of time samples. For an Exponential window, this is the window amplitude value at the end the Trace. The exponential window always has a value of 1.0 at the beginning of the Trace.

On the Measure tab of the Channels spreadsheet: On the Setup tab in the Channels spreadsheet: 

Turn two Channels ON and the rest OFF.

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Select AC Coupling for use with piezoelectric force hammers and accelerometers.

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Turn Transducer Power ON if you are using an Integrated Circuit Piezoelectric (ICP) transducer

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Set the Point Number for both the Roving and Response channels to 1.

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Set the Point Direction for both the Roving and Response channels to Z.

The Point Number and Point Direction entries define the next pair of DOFs to be measured. 

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Set Increment Point to 1 for the Roving channel.

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MAKING MEASUREMENTS 

Set Increment Direction to Z for the Roving channel.

To acquire Measurement Set 1, Execute: Measurement Set | Select and enter 1.

Execute: Acquire | Start (F5) to initiate the acquisition.

Execute: Measurement Set | Number of Meas Set. Set the number of Measurement Sets to 65, and press OK. 65 Measurements Sets are created. 

Data will be read from the I-Beam Time Histories.BLK Data Block and processed in accordance with the Acquisition setup. The I-Beam.STR window will show where the measurement was made on the test structure. The Impact FRF.ACQ window will show the time waveforms “acquired” from the Data Block file in the upper graphics area, and the FRF in the lower area.

In the Increment DOF column, press the Apply button for the Roving (Force) channel.

Force channel DOFs 1Z to 65Z were automatically created for Measurement Sets 1 to 65. However, we want measurements in the Y direction (not Z) for Measurement Sets 51 to 65. To accomplish this: Execute: Measurement Set | Select (or use the or

buttons) to select Measurement Set 51.

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Change the Point Direction from Z to Y for Point 51.

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Change the Increment Direction from Z to Y.

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In the Increment DOF column, press the Apply button for the Roving (Force) channel.

Execute: Acquire | Save (F9) to save the measured FRF to a new Data Block file. The first time this command is executed, the New File dialog will open.

Force channel DOFs 51Y to 65Y should now defined for Measurement Sets 51 to 65.

With the 65 Measurement Sets defined and the tabs set up for signal processing, the Acquisition window can now be saved in a disk file. This file, together with the 3D model in the Structure file, provides a completely defines the roving hammer Impact test.

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Enter a name for the new Data Block file into which measurements will be accumulated, and press OK. The new Data Block will be displayed with the first (FRF) measurement in it.

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Minimize this Data Block window.

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Data Block for Accumulating FRFs. Note that the current Measurement Set number has advanced to 2 of 65 and that the blue (Channel 1) arrow has moved to Point Number 2 on the 3D model in the structure window.

Execute: Acquire | Start (F5) to initiate the next measurement.

Execute: Acquire | Save (F9) to save the next FRF to the accumulator Data Block.

Note that the minimized Data Block is displayed briefly as data is added to it. 

Continue to use the Start and Save Commands (or their equivalent F5 and F9 Function Keys) to complete all 65 FRF measurements.

Alternatively, you can use the F10 command.

Execute: Acquire | Save and Start (F10) to save the current measurement and initiate the next one.

When you have measured and saved all 65 Measurement Sets, you can display deflection shapes on the 3D model from the FRFs by executing the Draw | Animate command in the Structure window. 

Close the Impact Acquisition.ACQ window and curve fit the FRF measurements.

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Compare the resulting mode shapes with those of IBeam Modes.SHP.

Note: To compare mode shapes, your system must include the Visual Modal package or any package with option VES-400. For more information on curve fitting, see application note #13.

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Mode Shape of the I-beam. .