- Minimizes Over-testing
- Reduces Risk of Damage to Critical Structures
- Measures Summed Forces
- Measures Force Differences (Moments)
- Simplifies and Expedites the Test Process
- Convenient and Easy to Implement
Optimizing 3 Component Force Sensor Installation for Satellite Force Limited Vibration TestingDue to the high cost, long development times, and uniqueness of sophisticated aerospace and other high-tech equipment, it has become imperative to implement
techniques that ensure the safety of such items during vibration qualification testing.
Conventional control using acceleration has been shown to cause significant over-testing that may result in damage to the unit under test (UUT). In
force limited vibration testing, the total input force to the UUT is measured and controlled, thereby limiting the “quasi-static” acceleration of the center-of-gravity and ensuring the integrity of the equipment.
The PCB® Force Limited Vibration Testing System meets most requirements for limiting the reaction force between the shaker and unit under test in random vibration testing. The use of piezoelectric, 3-component force sensors facilitates
easy and accurate measurement of the input force. This force relates directly, using Newton’s Second Law, F=ma, to the “quasi-static” acceleration of the structure’s center-of-gravity. Since design loads for aerospace equipment
are often given in terms of the “quasi-static” acceleration, the use of force sensors represents an ideal measurement approach for this application.
With force limited vibration testing, the force measurement signal is used
in the vibration shaker’s feedback control loop. This signal is compared to established force limits in the controller. At frequencies where the measured force exceeds the limit (at high Q resonances), the controller’s output signal (i.e.,
the input signal to the shaker) is “notched”. By reducing the input to the shaker at these frequencies, the reaction force between the test fixture and structure is maintained within specified limits.
Force sensors being used
with low outgassing accelerometers in thermal vacuum chambersAccelerometers, typically mounted on the test fixture, are also used in the control loop. They serve to limit the shaker’s excitation with respect to acceleration at frequencies other
than resonances for which force limiting prevails. In order to accomplish this, the shaker control system must have “extremal control” capability. Extremal control is the ability to establish feedback control with respect to the greater
of several inputs, either force or acceleration, as a function of frequency. It is also desirable to have “dual control”. Dual control is the ability to control both acceleration and force reference spectra.