BASIC DESCRIPTION OF A VIBRATION TEST SYSTEM
The UD Vibration Test System provides a means of testing any item that require environmental
vibration testing, such as sine, random, or shock. Such testing will expose any weakness in the
item’s design and will ensure that the item is reliable during actual field use.
The Vibration System usually consists of five separate parts: the Shaker, the Field Supply, the
Power Amplifier, the Controller, and a Cooling Unit such as a Blower. In some of the smalleR systems, the Power Amplifier and Field are contained within one console, leaving four separate parts.
SHAKER:
The Shaker provides a table to which the items to be tested are attached. The principal parts of the Shaker are shown in the illustration below
The field coils energize the Shaker body with a flux path as shown, creating a high magnetic flux density in the annular air gap. The field coils are connected to the DC field supply. The armature assembly, consisting of the table and driver coil, is suspended in the annular air gap of the shaker.
When an alternating current is passed through the driver coil, electrodynamic forces are generated on the driver coil wire causing the table to move up and down as indicated by the double-ended arrow shown in the illustration. The table movement is determined by the magnitude and frequency of the drive signal from the vibration controller. The table motion depends on several factors including, but not limited to, the mass of the table, load attached, and
table suspension stiffness.
System limitations are dictated by the maximum mechanical stroke of the shaker as well as the power amplifier voltage and current output capabilities. Typically amplifier and shaker systems performance is limited by the following factors. At low frequencies (5 to 15 Hz), the system is limited by the maximum stroke of the shaker. Next the limit is typically the velocity limit for the shaker (40 to 100 Hz). This is generally associated with the amplifier output voltage capabilities.
Finally the limit to the performance is the shaker force limit (100 to 2000 Hz). This is a limit of the amplifier output voltage and current. Shakers come with three basic base configurations.
First is the Low Profile, This type of base contains casters and tracks or air floats to allow the shaker to be moved from one spot to another. This type of configuration is commonly used with Environmental Chambers. The second type of base is the Pedestal Base. The shaker is mounted to a base that is made primarily for vertical vibration only.
The pedestal base allows the operator to rotate the shaker
90° for horizontal vibration of LIGHT loads or to attach to a separate slip table. In smaller shakers, the isolation air bags are attached to the base and in the larger shakers, the isolation air bags are attached to the trunnions.
The third type of base is the Slip Table Base. In some cases the shaker is incorporated with a slip table base to allow quick and simple connection to an oil film supported slip plate for horizontal testing. Horizontal operation with a slip plate allows testing in three axis and can permit testing of very heavy payloads which could not be tested vertically due to the mechanical limitations of some armature suspensions.
The shaker includes protective interlock features to prevent damage due to over stroke or over temperature. These items are described in detail in the “Control Panel, Front Panel Descriptions” in the Amplifier Section of the Manual. Also refer to the Shaker And Field Schematic drawing in this manual.
FIELD SUPPLY:
The field supply is the DC source that energizes field coils in the shaker creating a strong magnetic field where the armature coil is suspended. Some shakers are designed with two field coils (upper and lower) and some with one. The DC source is generated from a three phase transformer. The secondary leads of the transformer are connected to a three phase rectifier assembly that generates the DC voltage. The field supply is operated from the power amplifier. As the amplifier is sequenced for operation, the field supply is automatically turned on at the appropriate time. When the field is contained in the power amplifier, the operation is the same, only the location of the three phase transformer changes.
POWER AMPLIFIER:
The power amplifier input is connected to the controlling drive signal source and the output is connected to the driver coils of the shaker. The controller generates a low power, low voltage signal which is amplified by the electronic switching power amplifier. The amplifier has the necessary power to cause the AC currents to flow through the driver coil to generate rated force. The power amplifier is designed to provide the necessary power levels through the operating frequency range without introduction of significant distortion.
CONTROLLER:
The controller is the intelligence of the system. Accelerometers (transducers) detect the motion of the shaker table and send it to the charge amplifiers. The output of the charge amplifier is then connected to the controller. Modern controllers are printed circuit boards mounted in an electronic box which is connected to a computer via USB interface. The controller works to maintain a predetermined reference vibration level. A closed loop control is established to maintain this reference level as shown on right.
COOLING UNIT:
The cooling unit provides forced cooling to the shaker’s heat-generating components such as the field coils and armature coil. In most cases, a blower is attached to the shaker via ducting, pulling air from the room through the shaker and out through the blower. The air is then expelled out the building, in most cases. Majority of the heat generated by the shaker is removed by the cooling unit.
I will be writing about calibration and how Electrodynamic shakers work in the next write up. Remember, running a successful vibration test can be difficult but an experienced and specialized vibration testing lab will make it look easy.