Fully Automated Vibration Table System For Simplifies The Testing Process With An Intuitive Interface, Increasing Laboratory Productivity
Frequency Range
Electrodynamic shakers operate through a wide frequency range that is typically from 5 Hz to 3,000 Hz. Most test specifications in the automotive and transportation industry emphasize low frequencies (ie: below 1,000 Hz) while military vibration specifications normally call for testing out to 2,000Hz and electronics industry specs can go as high as 3,000 Hz.
Armature
A lightweight armature is favorable and will permit testing at higher g levels.
• The armature structure should provide a significant amount of stiffness.
• Material of construction is often magnesium or aluminum.
• Magnesium has a very high strength-to-weight ratio and provides superior damping, making it a favored material.
• Smaller lighter armatures may be appropriate for testing smaller products, while larger armatures can eliminate the need to use a head expander, reducing system mass and improving transmissibility.
Centering & Support
The armature needs to remain centered in its travel. Using an optical sensor to locate the armature and an automated pneumatic fill & drain system, the armature, bare table or loaded, will stay true to its course. Merely centering the armature at the beginning of a test is neither adequate nor safe. Advanced systems possess the ability to continuously center the armature while a test is in progress. Another feature of most shakers is a centerpole and bearing shaft that helps to keep the armature properly aligned during operation. It is good practice to load fixture and product weight overthe center of the armature to avoidoverturning moments. In the event the payload center of gravity cannot be located over the centerof the armature, additional guidance may need to be added to thesystem to prevent shaker damage.
Head Expanders & Plates
If large or multiple products extend too far beyond the edge of the armature, the product could be damaged or overtested. Using a head expander or head plate to increase the armature mounting area and properly tie the fixturing and products should alleviate this potential problem. Head expanders and plates should be designed for proper stiffness (ie: gusseted and welded) and should not be bolt-together structures since bolted joints reduce energy transmission. Favored materials of construction are magnesium or aluminum.
Fixturing
In addition to acting as a mounting interface between the shaker and the product to be tested, a vibration fixture needs to be rigid and lightweight. The vibration fixture should also transmit a uniform distribution of energy from the armature to the test item. In many cases, vibration is applied in three orthogonal axes. Specialized vibration fixture designs permit the fixture to be rotated for testing in the X, Y, and Z axis. It is common to find the weight of a fixture to be two to three times heavier than the products to be tested. Fixtures should mount easily to the armature and products should mount easily to the fixture. Quick changes from product to product and axis to axis help to maximize equipment utilization and improve lab productivity.
Slip tables
A sliptable assembly is used when vibration is required in the horizontal axis. For this purpose, the shaker is mounted in a trunnion which allows the shaker to be rotated 90°. A single piece table construction with a solid trunnion limits the relative body motion of the shaker and improves low frequency/high displacement performance of the overall system by solidly linking all of the reaction mass of the shaker to the slip table base. The solid trunnion and base configuration also limits the potential problems associated with misalignment which can plague a two piece system. Beyond the standard oil film systems, several options such as guideline tracking and hydrostatic bearings,are available to further control true and consistent horizontal vibration performance.
Cooling
Shakers consume significant electrical power which is converted to heat. Cooling of field coils and armature coils is mandatory in an electrodynamic vibration system. Shakers up to 15,000 lbf are typically air-cooled, while high force shakers (20,000 lbf and above) are liquid-cooled. Air-cooled shakers can be set up to exhaust warm air outside the facility during warm months.
Provisions can be made to damper the warm air back into the facility during colder months. The blower for an air-cooled shaker can be mounted outdoors or it can be placed in a sound deadening package and remain indoors.
Chamber Interface
When integrated with a chamber to perform combined environment tests, the shaker needs to be equipped with certain items. A thermal barrier, using sheets of G10 material and/or flexible silicon rubber will protect the shaker from being exposed to the temperature extremes of the test chamber. Properly designed, this flexible seal will also provide a protective barrier against moisture. Casters and track are one way to roll the shaker under a chamber. Another option is to utilize an air-glide system that floats the shaker on a cushion of compressed air.Applications where the shaker remains fixed in its position and the chamber rolls back and forth present another viable option.The ability to generate and run sophisticated combined environment tests from a single user interface can be a huge advantage.This simplifies data entry and synchronizes the individual pieces of equipment so they work in concert to carry out a fully integrated test.
Vibration Isolation
Electrodynamic shakers are capable of generating high forces over a wide range of frequencies. To limit the amount of unwanted vibration energy from being transmitted into the floor and throughout the facility, pneumatic mounts between the shaker and the floor are filled with compressed air to isolate the transmission of this energy. Shakers are relatively heavy pieces of equipment and, as such, are typically mounted on the ground floor of the facility
Noise Levels
An electrodynamic shaker running a full force random profile can be as loud as a jet engine. While performing a sine sweep to find a resonance point, the shaker can start out as a low-pitched hum and rise to an ear piercing scream. It is for these reasons that a sound enclosure should house the shaker system. It is a wise idea to place the control and amplifier console outside of the room for purposes of safety and comfort. A multi-pane window providing a view from the control room into the shaker room is advised for those situations where visual access to the product
under test is critical.
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