Multi-Exciter

MIL STD 810 G – Test Method 527 – Multi-Exciter

 

SCOPE

 

Purpose
Multi-exciter test methodology is performed to provide a degree of confidence that the materiel can structurally and functionally withstand a specified environment, e.g., stationary, non-stationary, or of a shock nature, that must be replicated on the test item in the laboratory with more than one motion degree-of-freedom consideration. The laboratory test environment may be derived from field measurements on materiel, or may be based on an analytically generated specification.
Application
  1. General. Use this method for all types of materiel except as noted in MIL-STD-810G, Part One, paragraph 1.3, and as stated in paragraph 1.3 below. For combined environment tests, conduct the test in accordance with the applicable test documentation. However, use this method for determination of dynamic test levels, durations, data reduction, and test procedure details.
  2. Purpose of test. The test procedures and guidance herein are adaptable to various test purposes including development, reliability, qualification, etc.
  3. Dynamics life cycle. Table 514.6-I provides an overview of various life cycle situations during which some form of vibration (stationary or nonstationary) may be encountered, along with the anticipated platform involved.
General Discussion
This test method should be used to establish a degree of confidence that the materiel can structurally and functionally withstand a specified dynamic environment that is defined in more than a single-degree-of-freedom (SDOF) motion; i.e., in multiple-degree-of-freedom (MDOF) motion. Specification of the environment may be through a detailed summary of measured field data related to the test materiel that entails more than one degree-of-freedom, or analytical generation of an environment that has been properly characterized in MDOF. In general specification of the environment will include several degrees of freedom in a materiel measurement point configuration, and testing of the materiel in the laboratory in a SDOF mode is considered inadequate to properly distribute vibration energy in the materiel in order to satisfy the specification. As a result of the increased complexity of application of MET over even multiple application of SDOF single-exciter testing (SET), an analyst after careful review of the available data and specification, will need to provide rationale for selection of the MET method. MIL-STD-810G, Methods 514.6, 516.6, 519.6 and 525 provide guidance in developing the rationale and requirement for MET.
Reasons for selection of MET over SET may include the following.
  • MET provides a distribution of vibration or shock energy to the materiel in more than one axis in a controlled manner without relying upon the dynamics of the materiel for such distribution.
  • MET may be selected when the physical configuration of the materiel is such that its slenderness ratio is high, and SET must rely upon the dynamics of the materiel to distribute energy.
  • For large and heavy test materiel, more than one exciter may be necessary to provide sufficient energy to the test item.
  • MET allows more degrees-of-freedom in accounting for both the impedance matches and the in service boundary conditions of the materiel.
Terminology
Several terms need to be carefully defined for contrasting MET with SET. The term “test configuration” used in this document will refer to the totality of description for laboratory testing including the sources of excitation, test item fixturing and orientation. In either testing configuration, distinction must be made between excitation measurement in a vector axis of excitation and measurement on the test item in either the vector axis of excitation or in another vector different from the vector axis of excitation. Generally, to avoid confusion in specification and reporting, the vector directions of excitation and measurement must be specified in terms of a single laboratory inertial frame of reference related to the test configuration. In addition it is helpful to specify the test item geometrical configuration along with the dynamic properties such as mass moments of inertia relative to the single laboratory frame of reference.
  1. Single-Degree-of-Freedom (SDOF) – motion defined by materiel movement along or about a single axis whose description requires only one coordinate to completely define the position of the item at any instant.
  2. Multi-Degree-of-Freedom (MDOF) – motion defined by test item movement along or about more than one axis whose description requires two or more coordinates to completely define the position of the item at any instant.
  3. Single-Axis (SA) – excitation or response measurement in a unique single vector direction (linear or rotational). For rotational axis, the vector direction is perpendicular to the plane of rotation of the exciter or test item. Figure 527-1 displays a single-axis input in the vertical direction to an extended structure.
  4. Multi-Axis (MA) – excitation or response measurement that requires more than one unique vector for description. Refer to Figures 527-2 and 527-3 for MA examples of both two-axis and three-axis inputs to a common structure.
  5. Single-Input/Single-Output (SISO) – refers to input of a single drive signal to an exciter system in a SDOF configuration and a single measured output from the fixture or test item in a SDOF configuration.
  6. Single-Input/Multiple-Output (SIMO) – refers to input of a single drive signal to an exciter system in a SDOF configuration, and multiple measured outputs from the fixture or test item in a MDOF configuration. In general, for specification purposes the dynamic behavior of the test item will not be assumed to contribute to the output DOF, i.e., measured rotation of an extended test item that is being excited in a cantilever mode will still basically be considered as a SET with linear acceleration characterizing the output.
  7. Multiple-Input/Single-Output (MISO) – refers to input of a multiple drive signals to an exciter system configuration in a MDOF configuration and a single measured output from the fixture or test item in a SDOF configuration. This terminology is most used in measurement data processing where the single output is a composite of measurements from multiple inputs.
  8. Multiple-Input/Multiple-Output (MIMO) – 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. 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.
  9. Single-Exciter/Single-Axis (SESA) – application of a single exciter providing dynamic input to the test item in a single vector direction.
  10. Multi-Exciter/Single-Axis (MESA) – application of multiple exciters providing dynamic input to the test item in a single vector direction. For example, extended materiel might require excitation at the forward and aft end in a single vector axis as illustrated in Figure 527-2. For the case in which the two exciters are driven to a common specification with respect to both phase and amplitude, the output may be described basically in the one axis of excitation. For the case in which the two exciters are driven to independent magnitude and/or phase specifications, the output may need to be described in terms of a forward axis and aft axis and, perhaps, a rotational axis about the test item’s center-of-gravity (CG).
  11. Multi-Exciter/Multi-Axis (MEMA) – Application of multiple exciters providing dynamic input to the test item in a way that requires more than a single vector for complete description of excitation and measurement. Figure 527-3 displays a three exciter three axis test. Three axes vertical, transverse, and longitudinal are required to describe the test. Note that many multi-axis test platform configurations have been built in recent years. Common 6 exciter examples are the hexapod (Stewart Platform), MAST, and Team Cube. There are also over-determined actuated systems consisting of more than 6 exciters. In each case, the dynamic properties vary between designs and must be considered in the design of a MET.
Limitations
This method addresses very general testing configurations that use multiple axes of applied excitation to materiel. Generally, field deployed materiel has boundary (or impedance) conditions that are impossible to replicate in laboratory testing. The overall goal of MET is to achieve a distribution of materiel excitation energy that approaches that appearing during in-service deployment, while minimizing the effect of field boundary conditions experienced in deployment. It is realized that fixturing design limitations and/or other physical constraints may limit application of in-service environment in the laboratory. It is also realized that in-service measurements may not be adequate to specify the laboratory test configuration. As always, engineering analysis and judgment will be required to ensure the test fidelity is sufficient to meet the test objectives.
The following limitations also apply:
  1. This method does not address aspects of vendor-supplied software control strategy for MET.
  2. This method does not address the processing of in-service measurements for specification for MET.
  3. This method does not address advantages or disadvantages of Procedure I and Procedure II MET as defined in paragraph 2.2. The state of the art in MET is not such that a comprehensive comparison can be made at this time.
  4. This method does not address optimization techniques of the laboratory test configuration relative to distribution of the excitation energy within the test item.
  5. This method does not address technical issues related to axes of excitation and materiel mass and product moments of inertia. Nor does it address the need for specialized software for optimizing the axes of excitation with respect to mass and products of inertia.
  6. This method generally does not provide specific test tolerance information that is highly dependent upon the (1) test objective, (2) test laboratory measurement configuration, and (3) vendor software control strategy.
  7. This method does not discuss in detail the potential for efficiencies and efficacies of MET over SET leaving this as a part of specification of MET peculiar to the in-service measured environment.
  8. This method does not discuss optimum in-service measurement configuration factors consistent with MET.
  9. This method assumes that excitation is provided mechanically through electro-dynamic or servo-hydraulic exciters, and does not consider combined acoustic (refer to Method 523) or pneumatic induced modes of excitation.

 

TEST PROCESS

 

Procedure
The following steps provide the basis for collecting the necessary information concerning the platform and test item under MET testing.
Procedure ITime Domain Reference Criteria
  • Step 1. Select the test conditions to be addressed and mount the test item on the vibration shaker. Select the accelerometers that will serve as control locations and associated analysis techniques that will be used as test metrics (refer to Method 525, Annex A, and Annexes A, B, and C of this Method). Placement and polarity of all accelerometers must match that of the reference signals (refer to Annex A). Clearly identify each axis of excitation and provide alignment procedures to ensure all measurements are made precisely along each excitation axis. Use all inherent information concerning the dynamic/geometric configuration of the test item, including specification of the center-of-gravity of the test item in three orthogonal axes, modal characteristics of the test fixturing, along with all pertinent mass moments of inertia.
  • Step 2. If required, perform an operational check of the test item at test conditions. If the test item operates satisfactorily, proceed to Step 3. If not, resolve the problems and repeat this step.
  • Step 3. Subject test item (or dynamic simulant) to system identification process that determines the initial exciter drive voltage signals by compensation. For the MDOF case, the initial signals sent to the exciters for compensation must be statistically independent, and which form vectors that are linearly independent with respect to the DOFs to be tested. If a dynamic simulant is used, replace the dynamic simulant after compensation with the test item.
  • Step 4. Subject the test item in its operational mode to the TWR compensated waveform. It is often desirable to make an initial run at less than full level to ensure proper dynamic response and validate instrumentation functionality.
  • Step 5. Record necessary data, including the control acceleration time traces that can be processed to demonstrate that satisfactory replication of the matrix of reference time trace signals has been obtained.
  • Step 6. Monitor vibration levels and, if applicable, test item performance continuously through the exposure. If levels shift or a failure occurs, shut down the test in accordance with the test interruption procedure (paragraph 4.3.2). Determine the reason for the shift and proceed in accordance with the test interruption recovery procedure (paragraph 4.3.2).
  • Step 7. Repeat Steps 4, 5, and 6 the number of replications called out in the test plan.
  • Step 8. Remove the test item from the fixture, perform an operational check, and inspect it, the mounting hardware, packaging, etc., for any signs of visual mechanical degradation that may have occurred during testing. See paragraph 5 for analysis of results.
 
Procedure IIFrequency Domain Reference Criteria
  • Step 1. Select the test conditions to be addressed and mount the test item on the vibration shaker. Select the accelerometers that will serve as control locations and associated analysis techniques that will be used as test metrics (refer to Annexes A, B, and D of this Method). Placement and polarity of all accelerometers must match that of the reference signals (refer to Annex A). Clearly identify each axis of excitation and provide alignment procedures to ensure all measurements are made precisely along each excitation axis. Use all inherent information concerning the dynamic/geometric configuration of the test item, including specification of the center-of-gravity of the test item in three orthogonal axes, modal characteristics of the test fixturing, along with all pertinent mass moments of inertia.
  • Step 2. If required, perform an operational check on the test item at test conditions. If the test item operates satisfactorily, proceed to Step 3. If not, resolve the problems and repeat this step.
  • Step 3. Subject the test item (or dynamically accurate surrogate if available) to a system identification process. For the MDOF case, the initial signals sent to the exciters must be statistically independent and which form vectors that are linearly independent with respect to the DOFs to be tested. If a dynamic simulant is used then replace the dynamic simulant after compensation with the test item.
  • Step 4. Subject the test item in its operational mode to the specification levels, monitoring both auto and cross-spectral density terms. It is almost always necessary to make an initial run at less than full level to ensure proper dynamic response, and to validate instrumentation functionality.
  • Step 5. Record necessary data, including the control acceleration auto and cross-spectral estimates that demonstrate satisfaction of the overall test objectives.
  • Step 6. Monitor vibration levels and, if applicable, test item performance continuously through the exposure. If levels shift or a failure occurs, determine the reason for the shift, and follow the test interruption procedure (paragraph 4.3.2).
  • Step 7. Repeat Steps 4, 5, and 6 the number of replications as called out in the test plan.
Step 8. Remove the test item from the fixture, perform an operational check, and inspect it, the mounting hardware, packaging, etc., for any signs of visual mechanical degradation that may have occurred during testing. See paragraph 5 for analysis of results.

 

NOTE: Tailoring is essential. Please, ask to your confidence laboratory for further details about tailoring of test methods.

 

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