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 modeshapes.  

  • In a 6DoF shaker table setup, these  correspond to the 6 rigid body translations/rotations and the modeshapes of the table/structure. 
  • In a modal excitation confituration, these DoFs would correspond directly to the structure’s modes
Kinematic Transformation
Tensor 12 Shaker Table

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 overdriving your shakers to control DoFs that shouldnt 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 modeshapes 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 modeshapes 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 modeshape of the Tensor table is shown to the right.

TensorMode1
Modal Control of Tensor

The Tensors actuators can be actively controlled to fight the modeshapes of the table and keep the table rigid.  The 800 Series controller can be instructed to do this by either defining the modeshape(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.

The tensor’s first modehsape was explicitly specified in the 800 Series Controller setup, and actively nulled.  The results are shown in the bottom right plot.  The first mode at 800Hz is actively nulled across the entire frequency band, and the table is now rigid past that first mode.