MIMO Vibration Control Overview

MIMO Testing has gained a huge momentum in the past decade with the development of multiple shaker table systems, the availability of Multiple Input Multiple Output (MIMO) controllers, and the readiness of the standards (e.g., Mil STD 810G method 527 and IEST DTE 022 working group recommendation,). The usage of multiple shaker testing systems includes military, defense and space organizations, with their use expanding to commercial and automotive industries as well.

In the real world, structural vibrations are excited from sources in all directions. To simulate a real-world vibration environment, the testing must be performed simultaneously in more than one direction. MIMO testing is necessary for many applications, such as large structure testing with a single shaker requiring extensive fixturing, large structure testing with a single shaker providing insufficient force, or tests requiring simultaneous multi-axis excitation (translation only, or with rotation). MIMO testing is recommended when SDOF testing is inadequate to properly distribute the vibration energy required to satisfy the specification.

MIMO testing with simultaneous multiple direction excitation decreases the overall testing time by eliminating the time required to change the fixing of the DUT to the table and change shaker orientations (e.g., from vertical to horizontal). In general, MIMO Testing provides a distribution of vibration energy to the test article in more than one axis in a controlled manner without relying upon the dynamics of the test article for such distribution. The physical configuration of the test article is such that its slenderness ratio is high, thus Single Exciter Testing must rely upon the dynamics of the test article to distribute energy. For large and heavy test articles, more than one exciter may be required to provide sufficient energy for the test item. MIMO Testing allows more degrees-of-freedom in accounting for both impedance matches and in-service boundary conditions of the test article.

The multi-shaker system ranges from Multiple Exciter Single Axis (MESA), to Multiple Exciter Multiple Axis (MEMA), with 2 to 6 shakers involved, or even with up to for single axis, three axis translational shaker table, 6 DOF Multi Axis Shaker Table (MAST) table, etc. 

The Spider MIMO Control System utilizes multiple shakers. Multiple control channels are individually assigned with a defined profile. The control process of MIMO Control is expanded into a Matrix fashion in contrast to the Scalar fashion of single shaker control.

For a shaker system with the number of drive X equal to m, and number of Control Y equal to n, it will follow the system equation,

{Y}nx1 = [H]nxm {x}mx1

The [H]nxm is the system transfer function matrix, which is typically evaluated during the pretest stage. {Y} is the linear spectrum vector of the responses (controls), and {X} is the linear spectrum vector of the drives.

The number of control channels can be the same as the number of drive channels, which is referred to as square control; or they can be different, which is called rectangular control. When the number of the control is larger than the number of drive (shaker), it is over-defined control. In the opposite situation, it is under-defined control. Square control and over-defined control are more commonly used than under-defined control.

MIMO Random Control, like MIMO Sine Control, can control phase between shakers and between axes. By maintaining a multi-dimensional system matrix, the Spider system can determine the contribution from each shaker to the overall response and properly differentiates for each shaker so that proper, accurate, safe control is assured. The complex issue of singularities is addressed with an elegant solution that permits intricate tests to be performed without having to resort to test segmentation as an attempt to avoid singularity.

In a Random test, MIMO produces true Random with one control per profile. The same quality of control offered by Single Shaker Random control is inherent in MIMO Random control. Adaptive control guarantees rapid equalization and accurate control when non-linear responses occurs. This also reduces the time required to achieve full level testing.