What is Vibration Control Testing?

Vibration control testing, also known as environmental simulation, allows engineering to validate the reliability of their products through controlled and consistent testing. These tests typically accelerate the durability validation process by producing equivalent lifetime contributions of vibration, but in less time. Companies that employ a vibration control program benefit from a positive return on investment (ROI) with reduced warranty, highly reliable products, and increased customer satisfaction.

Vibration control testing is the reproduction of equivalent vibration and/or shock environment experienced in the field or in a laboratory. This is typically, but not always, performed on an electrodynamic exciter also known as a shaker as shown in Figure 1. Vibration levels at key locations on the test object are controlled and monitored using a vibration control system.

Figure 1: Background – A shaker with test article mounted on an expander head, Foreground – A Simcenter Testlab vibration control system based on the SCADAS frontend

A physical test object is subjected to an equivalent amount of vibration that it would experience in the field in a laboratory setting where the test can be controlled as shown in Figure 2.

Figure 2: Field vibration (left) is reproduced on laboratory shaker vibration (right)

Vibration control tests are typically part of a larger environmental testing campaign to ensure a product will function properly in extreme environments. Besides vibration and shock, environmental tests also include:

• Humidity
• High and Low Temperatures
• Altitude
• Acoustics
• Solar Radiation
• Electro-magnetic Interference (EMI)
• Sand and Salt

Sometimes these tests are combined, for example, vibration testing may be combined with a climatic temperature chamber as shown in Figure 3.

Figure 3: Shaker table with environmental chamber for testing air bag sensors

Vibration control tests are used to reproduce events like aircraft take-off/landings, rocket launch, and transportation over rough terrain, etc. Additionally, vibration tests are used to screen for workmanship problems, catch premature failures, and improving analytical models.

Performing a vibration test in a laboratory setting has many advantages over field testing:

• Faster and Repeatable – by performing a test in a laboratory setting, the vibration can be reproduced in a faster and more consistent manner than field tests.
• More Information – lab tests are easier to instrument than field tests which can yield more information for design teams trying to optimize the life of the product.

To reproduce the vibration, a vibration shaker control system is used, which consists of specific parts to recreate the vibration environment.

Vibration Control System

The typical vibration control system consists of several different elements as shown in Figure 4.

Figure 4: Components of a Vibration Control Test System

Each element in the vibration control system has a specific purpose:

Vibration Controller

• PC with Simcenter Testlab Software – This is used to manage the test and determine the output needed to recreate the desired vibration levels. Note that Simcenter Testlab was formerly called LMS Test.Lab.
• Frontend (SCADAS) – An acquisition frontend is used to convert analog to digital signals and vice versa, the converted digital signals from accelerometers are measured and viewed on the PC, analog signal is outputted to amplifier to recreate vibration at specific levels.

Shaker System

• Amplifier – Gains the signal from the frontend and inputs into the shaker, depending on size of shaker, size of the test item and target vibration levels, a high level of voltage or current may be required.
• Shaker – Electrodynamic or hydraulic device that has a moving mass to recreate vibration. Shaker systems come in different configurations. For example, there are vertical and horizontal shaker tables to enable testing in different axes or directions of the product (Figure 5).

Figure 5: Horizontal vibration control shaker configuration (left) and vertical vibration control shaker configuration (right)

• Multiple Input and Multiple Output – Shaker tables to test multiple axes simultaneously, rather than one axis at a time. A product can to be tested in the vertical, lateral, and longitudinal directions simultaneously, instead of running three different tests, one test could be run in a third of the time (a 3x time savings).

• Shaker Force Rating and Limits – Typically shakers have a force rating based on the maximum test object mass, armature mass and required acceleration. Shakers also have maximum allowable displacements, velocity, and frequency. Based on the test article size, and desired vibration levels, a shaker with improper limits would not be able to reproduce the desired vibration levels.

• Shaker Fixtures – A shaker fixture rigidly mounts the test article to shaker, and holds test article in place. Often there is an expander head to enlarge the mounting surface of the shaker. Ideally, the first major fixture resonance should be above the maximum frequency being tested (Figure 7).

Figure 7: Left – Head expander for shaker, Right – Finite Element Analysis (FEA) showing first mode of vibration

Test Article and Transducers

Accelerometers

• Control – One or more accelerometers that are monitored and defined as control to recreate the required vibration levels. The sensitivity should be selected carefully based on the maximum vibration level expected. A low level vibration test might require a 100 mV/g accelerometer, while a high level vibration test might require a 10 mV/g accelerometer. Cable length to the measurement frontend should also be considered.

• Measurement Channels – Used to measure vibration on important parts of structure. Often called auxiliary channels.

Test Object

• Mounting – Test object should be mounted to shaker fixture to mirror how it is mounted in field.
• Axis/Direction – most times a test object is tested in one direction at a time, or simulated with multi-axis vibration.
• Test Item Positioning – Center of gravity of all masses should be placed over center of gravity of shaker system to avoid unwanted force moments.

These components are put together into a system as shown in Figure 8.

A selfcheck test is often performed prior to running a vibration control test to ensure that the complete system (frontends, amplifiers, transducers, shaker, test objects,…) are functioning and assembled correctly.

There are several different types of vibration modes that can be reproduced including sine, random, and shock as described in the next section.

Types of Vibration Control

The most common types of vibration reproduced by a shaker system is sine, random, and shock. In fact some believe random control testing and sine control testing make up over 70% of all the environmental simulation testing.

Random Control

In a random vibration test, a wide range of frequencies are excited and measured simultaneously as shown in Figure 9. The majority of vibration experienced by the test item in operational service is broadband in spectral content. That is, vibration is present at all frequencies over a relatively wide frequency range at varying intensities. Vibration amplitudes may vary randomly, periodically, or as a combination of mixed random and periodic.

Typically, a Power Spectral Density function is used as the target vibration, reference profile. Random vibration is often used for high number of cycle, low amplitude fatigue. Common test objects include small electronic components like electric circuit boards, avionic boxes, a complete missile and a full spacecraft.

More details about Random Control can be found here:

• Random-Control: Averages per Loop, Frequency Resolution, Weighting, and DOFs

Random with Kurtosis Control

Because not all vibration is Gaussian distributed random, time at peak vibrations can increased or decreased as shown in Figure 10. By controlling the kurtosis of the random signal, the probability distribution of vibration amplitudes are controlled.

The kurtosis statistic is used to measure the amount of peaks or “spikes” in the random vibration as shown in Figure 7. When kurtosis is equal to zero, there are less spikes and the random vibration is close to Gaussian random in distribution. The amount of spikes in kurtosis greater than 0 increases as the kurtosis number increases from zero.

Sine Control

Sine vibration is expressed as acceleration and a frequency. An environment dominated by sine vibration is characterized by a fundamental frequency and harmonics (multiples) of that fundamental. Often there will be more than one fundamental frequency. Each fundamental will generate harmonics.

The service vibration environment in some cases (low performance propeller aircraft and helicopters for example) contains excitation that is basically sinusoidal in nature, and with a very low broadband background. The excitation derives from engine rotational speeds, propeller and turbine blade passage frequencies, rotor blade passage, and their harmonics.

More details on Sine Control can be found here:

• Sine Control: Closed Loop Control Parameters
• Sine Control: Notching
• Sine Control: Estimation Methods

Sine Dwell

It is sometimes desirable to excite a structure at its resonant frequencies for an extended period of time to study the effects of fatigue on damping and possible resonant frequency shifts. Sine Dwell testing is commonly performed on aircraft engine blades, power generation turbines and vibration isolators.

Shock tests are performed to provide a degree of confidence that the unit under test can physically and functionally withstand transients encountered in handling, transportation, and service environments.

The procedures available for shock testing include:

• Functional Shock
• Material to be packaged
• Fragility
• Transit Drop
• Crash Hazard Shock Test
• Bench Handling
• Pendulum Impact
• Catapult Launch / Landing

Depending on the environment to be simulated a Classical Shock or Shock Response Spectrum (SRS) method will be selected. Typically performed on a shaker system Classical Pulses include Half Sine, Terminal Saw tooth, Square Wave and Trapezoidal.

It is also common to use a Shock Response Spectrum (SRS) as the target for a shock test. A Shock Response Spectrum (SRS) is a graphical representation of a shock, or any other transient acceleration input, in terms of how multiple Single Degree Of Freedom (SDOF) systems (like a mass on a spring) would respond to the transient input over a defined frequency bandwidth.

A typical step in the SRS based shock control process is a Shock Response Analysis. A wavelet decomposition is performed to produce an equivalent time history input that fits the shaker limits.

Mixed Modes – In some cases, the vibration environment is characterized by quasi-periodic excitation from reciprocating or rotating structures and mechanisms (e.g., rotor blades, propellers, pistons, gunfire). When this form of excitation predominates, source dwell vibration is appropriate. Source dwell is characterized by broadband random vibration, with higher level narrowband random, or sinusoidal vibration superimposed.

• Sine on Random – Some vibration environments, like that an engine produces, can have both a sinusoidal periodic component and a random component.

• Random on Random – Some products produce high bands of random vibration, like a bulldozer tracked vehicle (Figure 15) or the tractor that transport launch vehicles to the launch pad.

Time waveform testing consists of the replication of either measured or analytically specified time trace(s) in the laboratory with a single exciter in a single direction, and is performed to accurately preserve the spectral and temporal characteristics of the environment.

Until recently, the replication of time traces representing measured samples of field environments varying in time and even frequency, or a combination of both time/frequency variations, was not possible using commonly available exciter control system software. The advent of more powerful data processing hardware/software, and the implementation of advanced control strategies, has led to exciter control system hardware and software that permit convenient replication of extended time-varying test environments on a single exciter in a single direction in the laboratory. TWR test methodology strongly reflects the concept of “test tailoring.”

MIMO Control

Multiple Input and Multiple Output (MIMO) vibration refers to input of a multiple drive signals to an exciter system configuration in a MDOF configuration, and multiple measured outputs from the fixture or test item in a MDOF configuration as shown in Figure 17.

Until recently, the replication of time traces representing measured samples of field environments varying in time and even frequency, or a combination of both time/frequency variations, was not possible using commonly available exciter control system software. The advent of more powerful data processing hardware/software, and the implementation of advanced control strategies, has led to exciter control system hardware and software that permit convenient replication of extended time-varying test environments on a single exciter in a single direction in the laboratory. TWR test methodology strongly reflects the concept of “test tailoring.”

MIMO Control

Multiple Input and Multiple Output (MIMO) vibration refers to input of a multiple drive signals to an exciter system configuration in a MDOF configuration, and multiple measured outputs from the fixture or test item in a MDOF configuration as shown in Figure 17.

It is important to note that generally there is no one-to-one correspondence between inputs and outputs, and the number of inputs and number of outputs may be different. MIMO Control is utilized in two different applications:

• Multi-Exciter/Single-Axis (MESA) – the application of multiple exciters providing dynamic input to the test item in a single axis direction. For example, a long missile might require excitation at the forward and aft end in a single axis.
• Multi-Exciter/Multi-Axis (MEMA) – the application of multiple exciters providing dynamic input to the test item in a way that requires more than a single axis for excitation and measurement. Typically a MEMA requires three axes; vertical, transverse, and longitudinal to describe the test.

Acoustic Control

In acoustic control, a diffuse sound field is generated in a reverberation chamber. Normally wide band random excitation is provided and the reference spectrum is shaped. This test is applicable to material or structures that have to function or survive in an acoustic noise field such as aerospace vehicles, launch vehicles, power plants and other sources of high intensity acoustics.

Since this test provides an efficient means of inducing vibration above 100 Hz, the test may also be used to complement a mechanical vibration test, using acoustic energy to induce mechanical responses in internally mounted material.

Direct Field Acoustic Noise (DFAN) Testing

The Direct Field Acoustic Noise method, also named DFAN in the U.S., has been developed and is partly used today for qualification of satellites and components. The availability of commercial loudspeakers and amplifiers capable of generating the sound field required in a test has made the development of the direct field acoustic excitation method possible.

In a DFAN test, the specimen is placed in the middle of a loudspeaker circle and gets excited by a direct acoustic field. Modern loudspeakers and amplifiers deliver the required high decibels to obtain the target overall sound pressure level (OASPL). The vibration levels measured on the specimen during the DFAN test are comparable with those measured with reverberant field acoustic excitation.

See Knowledge Base article: Direct Field Acoustic Noise (DFAN) Testing for more details.

Applications

Vibration control testing is used by many different industries to qualify and improve the life of various manufactured products, including:

Military

Military equipment must survive intense environmental conditions while in service. Military standards are often used even in testing commercial products. The military environmental standard MIL-STD 810 was one of the first comprehensive vibration standards and is often referred to throughout industry.

It is not uncommon when testing military equipment that 100% testing is required for all units.

The latest revision of MIL-STD 810 can be found here.

Spacecraft

During launch and in space, satellites are subjected to tremendous amount of vibration and shock, which requires extensive testing and verification before launch.

Satellites are subject to several different types of tests during vibration qualification:

• Pyroshock – Rocket stage separation, solar panel deployment, antennas
• Acoustics – Launch simulation (reverberation and DFAN)

Transportation

In transit a product can undergo excessive vibration and shock. Everything including off the shelf commercial items should go through excessive survivability testing. This includes the testing of the article or object within the packaging.

Commercial Goods

To operate out of the box after transit and maintain long term reliability more and more manufactures utilize environmental stress screening to find workmanship problems and create premature failures prior to delivery.

Electronics

Commercial and noncommercial electronics alike must work on demand. Complicated circuit boards, avionic boxes, car entertainment systems, airbags, field communication systems, cell phones, computers, televisions ….. are all subjected to some form of vibration and shock prior to delivery.

Glossary of Standards for Environmental Testing

Defense and Space Standards:

• MIL-STD 810G – United States Department of Defense and details 28 testing methods covering a wide variety of environmental conditions such as rain, vibration, dust, humidity, extreme temperatures, shock and salt fog
• MIL-DTL-901E – Shipboard shock testing
• MIL-STD-331 – Simulated catapult launch/arrested landing
• MIL 167 – Military Standard, mech. vibrations of shipboard equipment
• NAVMAT P-9492 – Temperature Cycling and Random Vibration
• NASA-HDBK-7005, 2001 Dynamic Environmental Criteria, NASA Technical Handbook
• NASA-HDBK-7004, 2003 Force Limited Vibration Testing, NASA Technical Handbook

ASTM Standards

• ASTM D999 – Vibration testing of shipping containers
• ASTM D3580 – Vibration (vertical sinusoidal motion) test of products
• ASTM D4728 – Random Vibration testing of shipping containers

ISO Standards

• ISO 2247 – Vibration test at fixed low frequency
• ISO 8318 – Vibration tests using a sinusoidal variable frequency
• ISO 9022-10 – Combined sinusoidal Vibration, dry heat or cold
• ISO 9022-15 – Combined random vibration wide band: reproducibility medium, dry heat or cold
• ISO 9022-19 – Temperature cycles combined with sinusoidal or random vibration
• ISO 16750-3 – Automotive electronics

Other Standards

• IEC 60068-2-64 – Environmental Testing: Part II, methods, vibration, broad-band random
• DO-160 – Environmental Conditions and Test Procedures for Airborne Equipment
• EIA-RS-186 – Passive electronic component parts- Method 8- Vibration, High Frequency
• JIS C0040 – Environ. Tests Part II: Tests, Test Fc and Guidance: Vibration (sinusoidal)
• AC-156 – Earthquake
• 22-A11B-HAST
• RTCA DO-160 – Radio Technical Commission for Aeronautics (RTCA) publishes DO-160, Environmental Conditions and Test Procedures for Airborne Equipment
• ETS 300 019-2-0 – Telecom equipment transportation

This article is republished from the Simcenter Blog