Multi-loop with Phase Control

March 29, 2022

Multi-loop phase control synchronizes two or more shakers moving on the same axis while maintaining phase through level changes. Engineers can perform a multi-loop phase vibration test with one or multiple controllers, so long as they use the same test profile.

Multi-loop phase is ideal for oversized test items, as multiple shakers distribute the load throughout the structure. Shakers can be configured in several ways for a multi-loop phase test.

For example, dual shakers are a type of multi-loop phase control. Two shakers are in a push-push configuration if they face the same direction. When facing one another, they are in a push-pull configuration.

Fixture Design

Multi-loop phase is also an appropriate choice when mounting the DUT on a single shaker would be unrealistic. Some test items have multiple mounting points in the field, and a multi-exciter/single-axis (MESA) test would be more reflective of that environment.

dual-loop control shaker setup

In the article “Multi-Axis Vibration Testing of an Aerodynamically Excited Structure,” Chris Roberts and D. Ewins note that the test environment for items like rockets can vary significantly from the service environment, especially at the item’s extremities.

“The main causes for this mismatch can largely be attributed to the test setup itself, as fixture designs rarely replicate the in-service boundary conditions realistically and the excitation path in the test can also be very different from that in service.”

In other cases, a system may require a large shaker to meet the force requirements, lowering the usable frequency range. Using two shakers increases the force rating while maintaining the frequency range.

Forced Synchronization

In general, rigid coupling does not work well for multi-loop phase control. If one of the shakers moves out of phase, then the potential for damage increases. Instead, a dynamic coupling of the shakers is ideal, where both shakers monitor the control and adjust the drive when necessary.

In the article “Multi-Shaker Control: A Review of the Evolving State-of-the-Art,” Dr. Marcos A. Underwood, Tony Keller, and Russ Ayres state that the proper control method, “…has been found and field proven … in maintaining limit control of one or more limit channels while maintaining proper amplitude, phase, and coherence control and without violating any important structural constraints.”