SignalCalc Drop Test Demo Guide

SignalCalc Drop Test Demo Guide

Introduction Most protective packaging for electronic and other fragile products use cushion materials in the packaging that are designed to deform in response to forces experienced during impacts, drops and other impulsive loads during transportation. The deformation transforms the relatively high acceleration, short duration shock pulse experienced when two rigid surfaces collide (such as a product dropping on the floor) into a lower acceleration, longer duration event. For verification of package shock performance, most test procedures simply call out a series of freefall impacts (drop tests). Companies will often specify that the product be monitored during the impact to measure the acceleration transmitted through the cushion material. Part of that procedure requires determination of the critical velocity change of the product under test. The SignalCalc Drop Test application provides the user with the ability to capture and display the drop waveforms and perform "Fairing" (done by smoothing or low pass filtering) on the waveforms and to measure and report the peak acceleration and velocity change levels during package drops to better understand and meet the drop test requirements.

Typical Drop Test Machine

Running a Demo Hardware Set up and Launching the Application Begin by connecting OUT 1 of either the Quattro or Abacus platform to IN 1 and connect OUT 2 to IN 2. Launch the Drop Test software; run SignalCalc Drop Test 210 if connected to Quattro or SignalCalc Drop Test 710 if connected to Abacus. When the software launches, the following welcome screen appears:

The Drop Test application is equipped to simulate shock waveforms typically encountered in drop testing using the signal generators available in the hardware. To enable the use of the signal generators, check the Enable use of Signal Generators option which is accessed via the Test >> System Options menu. Selecting Test >> New launches a new (default) test. A Save As dialog prompts the user to save the test. Drop Test tests are saved in folders with a.drp extension.

Setting up the Test The Setup dialog can be accessed by clicking on the icon highlighted above, or, from the Setup menu in the main toolbar. There are 4 main tabs (Test, Inputs, Measurement Parameters and Tolerance Parameters) for setting up a Drop Test, with the optional 5 th tab (Outputs) available only when the signal generators are enabled.

The Test tab provides users with the ability to define a Test Title and Test Description, enable Run Notes, specify a Run Name and select the desired Run Increment protocol under the Run ID section, specify default engineering Units for acceleration, velocity and displacement and enter Comments for the test.

The Inputs tab has three sub-tabs: Front End, Info and Trigger. Activate channels 1 and 2 on the Front End tab. Set the Coupling to AC Diff or AC SE and set the mv/eu values for channels 1 and 2 to 100 and select g as the EU. The Info tab is where the user may specify point numbers and directions for each channel and Trigger tab is where the user selects which channel is to be the trigger channel along with the corresponding trigger level. Note that the default settings will work for the demo purposes.

The Measurement Parameters tab has several important set up areas: Measurement Parameters Select the analysis FSpan (Hz), Blocksize (number of samples per captured block of data) or Lines of resolution. Note: sample rate for the measurement = Fspan * 2.56. Trigger There is currently only one Trigger Type, which is Input. This means that one of the activated input channels is set up to the be the trigger channel. There is also a Captures per run setting; use Single for drop applications involving a single package drop, or Multiple for applications where repetitive impacts or drops are conducted for product qualification. ICP This is where the Check ICP function can be enabled. Uncheck the checkbox for demo purposes. SRS Parameters Available with the purchase of the Advanced Drop Test option are SRS Measurements. A summary of SRS Measurements is presented at the end of this document. SRS measurements are defined by a frequency range, bounded by Low Freq. (Hz) and High Freq. (Hz), the desired octave Resolution, a Ref. Freq. (Hz) which sets the center frequency on one of the digital proportional bandwidth filters that specified by the user and a Damping (%) value.

Fairing Parameters Captured drop waveforms in a drop test often have high frequency ringing and other mechanical and/or electrical noise superimposed on the primary response frequency. Unlike other shock test applications, Drop Test users generally do not wish to analyze this high frequency content in the signal and therefore it is standard practice to apply some form of fairing to the signal. Fairing is done by either Smoothing or Filtering the waveform. Smoothing is done by averaging consecutive samples, with the number of samples to be averaged over specified in the Smoothing Width column. Filtering is accomplished by low pass filtering the waveform; in the Low Pass Filter case, the user specifies the Cutoff Freq. (Hz) while in the Auto Filter case, the system automatically selects the cutoff frequency. Select Auto Filter for the choice of Fairing.

The Tolerance Parameters tab allows users to select the type of Tolerance Limits that will be applied to the measurements, the definition of the Tolerance reference and specification of User Limits for peak acceleration and velocity change levels. Tolerance Limits be imported from an external File, User defined or defined by a Standard. Note that this setting may also be set to None in which case no tolerances are applied. If limits are to be defined by a standard, the user then selects the standard applicable in the Tolerance dialog; two standards are currently offered: Milspec 810 and Def Stan 00-35. The user then chooses the waveform type that will be expected for the Drop Test from the three choices offered: Half Sine, Saw Tooth and Trapezoid. The user also enters the Expected Amplitude and Duration of the pulse. When the these parameters are entered the program calculates the expected velocity change and reports it in the Reference Velocity section. There are two criteria available for this calculation; the first is 10% Pk, which means that the program selects a pulse width corresponding to 10% of the peak expected amplitude, on either side of the peak and calculates the velocity change across this pulse duration. The second criterion calculates the velocity change as dictated by the selected Standard. Once the velocity change is calculated it may be transferred to the Results table in the run time environment using the Apply to Results button.

The user may also choose to enter the Target peak acceleration for each channel (as some channels in a real drop test may correspond to off axis measurements, etc.) Note that Results table also allows users during the live measurement to select Cursors as the velocity change calculation mode, which allows the user to manually position dual cursors across the captured pulse width for the velocity change to be calculate over.

User Tolerance Limit Definition Screen Saving of User Defined Tolerances

Tolerance Standards

Reference Waveforms

Inject different noise levels into the signals for channels 1 and 2. The Outputs tab is where the signal generators and activated. The signal generators automatically create simulated drop waveforms corresponding to the reference waveform type (Half Sine, Saw Tooth and Trapezoid) selected in the previous section. The user may specify parameters such as Delay, Noise and percent variation in pulse width and pulse amplitude injected into the signals. For demo purposes, it is recommended that user enter different values for Noise (%) for the different signal generators so as to display signals of varying form. Running a Measurement Once all the setup of Inputs, Measurement Parameters, Tolerance Parameters and Outputs is completed, the user can then exit the Setup dialog and proceed to the runtime screen.

The default runtime screen has various graphs for channel 1 displayed, the Control panel visible on the top-left portion of the screen, the SRS Parameters immediately below the Control panel and the Results and Fairing tables directly below the graphs. Before running a measurement, it is worth taking a few minutes to review the various signals available for display, which are accessed via the Display > New Graph menu. The various signals are described in the following section.

Signals available in Drop Test application Accx Measured Acceleration Waveform This is the last (transient) time-history of duration Tspan and resolution dt measured from channel x. Velx Calculated Velocity Waveform This is the result of integrating Accx. FAccx Faired Acceleration Waveform If the Fairing option is enabled, this is the faired acceleration time-history from channel x. Note: Fairing may be done by either smoothing the waveform (i.e., by averaging across consecutive samples) or by low pass filtering (which may be user defined or Auto). FVelx Faired Velocity Waveform This is the result of integrating FAccx. SAccx FFT of Accx This is the complex spectrum of Xx spanning Fspan with resolution df SVelx FFT of Velx This is the result of integrating SAccx. Refx Reference This is the expected shape and level of the captured Accx waveform and may be either defined by a standard (Mil 810, Def Stan, etc.) or user defined. TolUx Upper Tolerance This is the upper limit boundary for the measured waveform. TolLx Lower Tolerance This is the upper limit boundary for the measured waveform. SRSPx Positive SRS Spectrum This is the positive Shock Response Spectrum corresponding to Xx.

SRSNx Negative SRS Spectrum This is the negative Shock Response Spectrum corresponding to Xx. SRSMx Maximum SRS Spectrum This is the maximum Shock Response Spectrum corresponding to Xx. Note that the span of all Shock Response Spectra are determined by the Low Frequency and High Frequency settings while the resolution is determined by the Octaves setting and the placement of that resolution is determined by the Reference Frequency setting. RAccxyz Resultant Acceleration For accelerometers placed orthogonally (i.e., 90 to each other) at the same measurement location, the system provides a calculated resultant magnitude using the formula: Resultant Magnitude = 2 2 Mx + My + Mz 2 where, Mx, My and Mz are the magnitude of vibration in each of the three directions respectively. Note that the user must specify which channels make up a group of signals used in the calculation of the resultant. Also, the user may also calculate the resultant for just two channels (e.g., x and y). The Results and Fairing tables are combined in a tabbed dialog and each may be displayed separately as needed.

The Control panel offers three choices before a measurement has been started: Init This button initializes all displays and hardware settings and readies the system for a measurement. If ICP coupling is enabled, the various ICP sensors are powered and allowed time to settle. Start This button starts a measurement, which is triggered based on trigger parameters defined earlier. Monitor This button allows the start of free run (no trigger) measurements, typically used to verify that all accelerometers are working properly prior to a real drop test. The system will stream live time domain data through the displays.

The above screen shot shows the result of a Single drop waveform capture. When the capture is completed, the system waits until the user either Starts a new measurement, Ends the measurement or Recalculates various parameters in the current measurement (Note: SRS and/or Fairing parameters may be changed following a live capture, permitting recalculation of results).

The default layout shows the raw measured acceleration waveform captured, the faired acceleration waveform with the upper and lower tolerance limits, the calculated velocity waveform and the faired velocity waveform, all for channel 1. Note that the Results table shows all test parameters passing the user specified pass/fail criteria. Using the Display > New Graph function, the identical signals may be displayed for channel 2 as well. Display of Measurements for Channels 1 and 2

Measured and Faired Acceleration Waveforms for Channels 1 and 2 Tolerance limits may be moved around the captured waveform in any of the graphs (as allowed for by the standards) in order to enable the waveform to pass the limit test. Recall that both channels 1 and 2 had passed all test criteria. To illustrate the Move Limits feature, the user can click on the highlighted icon in Main Toolbar or use the Display > Move Limits menu to move the tolerance lines around the captured waveform

in the top-right graph (channel 1). Once the captured waveform intersects with the tolerance line, the Acceleration Tolerance test fails. Selecting Display > Reset Limits returns the limits to their original position. The Channel Status Dialog ICP check is initiated by clicking this button. The captured waveform may be inverted or flipped by clicking on the desired channel indicator box. This is provided for cases where the sensor orientation results in an inverted waveform relative to the defined tolerances.

Appendix A Important Drop Down Menu Items and Toolbars Test Menu Display Menu and Move/Reset Limits

View Menu and Controls Toolbar

Appendix B Shock Response Spectrum (SRS) Measurements Mechanical shock pulses are often analyzed in terms of shock response spectra. The shock response spectrum assumes that the shock pulse is applied as a common base input to an array of independent single-degree-of-freedom systems. The Shock Response Spectrum, or SRS, is used in modeling a mechanical component as a series of single degree of freedom (SDOF) spring-dashpot subsystems each with a constant damping ratio and varying natural frequency. Each spring-dashpot subsystem is considered a 2nd order linear system and is converted into the digital domain. The absolute maximum response of each spring-dashpot subsystem is returned as the SRS result for the corresponding natural frequency of the subsystem. A plot of the absolute maximum responses for all the natural frequencies is the Shock Response Spectrum. The shock response spectrum gives the peak response of each system with respect to the natural frequency. Damping is typically fixed at a constant value, such as 5%, which is equivalent to an amplification factor of Q=10. The shock response spectrum is particularly suited for analyzing pyrotechnic shock but it may also be used for evaluating classical pulses, such as a half-sine pulse. In the following example, consider a base input, half sine pulse of duration = 11 msec and peak acceleration of 50 g.

Total Captured Signal, Half Sine Response through 30 Hz Filter Response through 80 Hz Filter

Response through 140 Hz Filter SRS Spectrum showing peak response corresponding to each SDOF component Additional technical information is available in the Shock Response Spectrum (SRS) chapter of the SignalCalc Dynamic Signal Analyzer manual.