Modal and Deformation Control
Controlling deformation of a test structure is the most effective way to replicate the stresses it will see when in use. This can also be used to stiffen the shaker table itself, and keep it rigid past its resonances.
The 800 Series Controller numerically breaks down the test structure into controllable degrees of freedom, including any modes that can be controlled by the actuators and measured by the control sensors.
Degree of Freedom View
The 800 Series Controller numerically breaks down the test structure into controllable degrees of freedom, including rigid body DoFs and mode shapes.
- In a 6DoF shaker table setup, these correspond to the 6 rigid body translations/rotations and the mode shapes of the table/structure.
- In a modal excitation configuration, these DoFs would correspond directly to the structure’s modes
Defining Which DoFs are Controlled
Not all degrees of freedom should be controlled at all frequencies, and the 800 series controller has various methods to avoid over driving your shakers to control DoFs that shouldn’t be controlled.
- The controller divides forces between the actuators evenly if more actuators than DoFs.
- Controlled DoFs can switch with frequency – In a modal test, different mode shapes can be controlled at different frequencies.
- Different #DoFs can be controlled at different frequencies within the same test.
- On a shaker table, this allows control of the translations/rotations at low frequencies and also controlling mode shapes at higher frequencies.
Controlling Out Modes of a 6DoF Shaker Table
The Team Tensor 6DoF table has 12 electrodynamic actuators (4 in each axis) for full 6DoF control. At low frequencies, the 800 Series controller uses the additional actuators by distributing the force equally; at high frequencies, it can use them to counteract table resonances and keep the table rigid to a higher frequency.
The first mode shape of the Tensor table is shown to the right.
The Tensor’s actuators can be actively controlled to fight the mode shapes of the table and keep the table rigid. The 800 Series controller can be instructed to do this by either defining the mode shape(s) explicitly and nulling them; or by defining a profile which implicitly requires modal deformation to be eliminated.
With control accelerometers at the corner of the table, the Tensor was run without controlling any modes (only controlling rigid body translations/rotations); and the results are shown on the top right plot. The first and second resonances are clearly seen in the 800-2000Hz range.