HYDRA multi-axis vibration test facility
Engineers developed the multi-axis vibration test facility HYDRA to complement the electrodynamic vibration test facilities. Compared to those facilities, it can:
- test much larger test specimens
- test much heavier test specimens
- test in any direction without changing the configuration
- test with larger displacements
- test at lower frequencies
Operational aspects
The six degrees-of-freedom (DOF) hydraulic shaker allows engineers to test specimens along both the vertical and the lateral axes with a single test set-up. They therefore no longer need to dismount, re-locate and re-instrument the payload for the different excitation directions. This reduces the effort involved in handling and in instrumentation, with consequent reductions in test duration and risk, and also provides flexibility in the sequencing of tests. In particular, engineers can perform x-, y-, and z-signature tests without particular effort before and after each single-axis test run.
The ’HYDRA overview’ diagram shows the aspects of the HYDRA configuration that are of most interest to the customer. The large octagonal test table has a span of 5.5 metres and is flush with the test floor. It facilitates the mounting of heavy and/or geometrically large specimens (for example, appendages such as solar arrays). Using this configuration, engineers can reduce the complexity and mass of test-specific mechanical support equipment and adapters.
The shaker is located adjacent to both the control room and the area for the customer’s satellite check-out equipment, which fosters good communication and working links between the user teams and the test operations staff. All shaker equipment and supplies are located in the basement of the building, mechanically isolated from the clean test area (class 100.000 Fed. Std. 209).
Cleanliness and safety
Detailed product assurance analyses during HYDRA’s design have led to a well thought out strategy for providing optimum protection for the test item against contamination and over-testing. The safety system is of the utmost importance, due to the presence of moving masses with high inertia and the large installed hydraulic power. The control system protects the specimen against all potential failures identified in various safety analyses, by initiating a ‘soft facility shutdown’.
The gap between the aluminium test table and the test floor is closed by a flexible seal (gap seal). This provides mechanical separation of the clean test area from the hydraulic equipment (actuators, bearings, valves, and so on) located below the table.
Dynamic performance and control
The test envelope that can be covered by HYDRA covers acceleration ranges from 20 milli-g to 3 g in the frequency band of 1-100 Hz. At frequencies above 20 Hertz, accelerations up to 5 g have been achieved.
Engineers designed and developed the vibration and control computer subsystem for HYDRA. It is based on online test value calculations for the control signal of the servo valves, taking the geometrical arrangement of the servo actuators on the test table into consideration. The processing power of vibration and control computer is provided by 36 digital signal processors (DSP), operating in parallel, which interface with the operator through several PCs.
To control the required excitation, 12 piloting sensors are mounted in four three-axial cubes close to the test specimen interface. Up to 16 additional notching sensors limit the excitations at certain locations on the test specimen to guarantee its safety.
Realistic flight-load simulation (transient testing)
In the future, HYDRA has the potential to perform tests with a much more realistic flight load representation than presently possible. Simulating the multidirectional transients at the interface between launcher and spacecraft would produce structural responses more realistic than the traditional sine or random tests.
HYDRA is unique in being able to perform mechanical vibration tests at frequencies between 1-100 Hz in all axes of motion (translation and rotation) simultaneously. This six degrees- of-freedom transient testing is expected to lead to significantly lower test loads than is presently the case. It can be implemented as soon as satellite programmes are ready to apply this new test method.
Technical data
| Seismic foundation | |
| Mass | 1 400 000 kg |
| Resonance frequency | less than 2 Hz |
| Test table | |
| Mass | 17 300 kg |
| Mass including table side universal joints and moving elements of actuator | about 22 500 kg |
| Flatness | 0.1 mm per m |
| interface points on a grid with a pitch of 320 mm | 229 x M20 |
| interface on a circle with a diameter of 2.74 m | 24 x M24 |
| Hydraulic supply | |
| Volume of oil in main tank: | 23.000 l |
| Flow rate of 5 high pressure pumps: | 5 x 350 litres per minute |
| Power consumption of five electric motors: | 5 x 200 kW |
| Volume of four piston accumulators: | 4 x 350 l |
| Volume of four nitrogen gas bottles: | 4 x 2000 l |
| Maximum noise level: | 95 decibels |
| Servo actuators | |
| Number of actuators | 8 |
| Maximum static force (at 305 bar system pressure): | 630 kiloNewtons |
| Working stroke: | ±70 mm |
| Maximum velocity: | 0.8 m/s |
| Maximum angular movement of cardanic bearings: | 5° in both axes |
| Electric equipment with PLC | |
| Apparent power: | 1.5 megaWatts |
| Effective power: | 1.2 megaWatts |
| Reactive power: | 0.3 megaWatts |
| Maximum size test | |
| Mass, including adapter: | 22 500 kg |
| Height above table surface: | about 12 m |
| Centre of gravity above table surface: | about 5 m |
| Test envelope | |
| Acceleration | 20 milli-g to 3 g |
| Frequency | 1 to 100 Hz |
| Sine sweep rate | 0.5 to 4 octave per minute |
For more information please contact Gaetan Piret, ESTEC Test Centre Manager.
Gaetan.Piret @ esa.int
