The vibration generator, that is, the mechanical execution part of the electrodynamic shaker, is usually referred to as the vibration shaker. The “electrodynamic shaker” including the power amplifier and the controller should be called the electrodynamic vibration test system. The basic structure of the shaker is shown in Figure 1.1, which mainly consists of the following components:
1) Working table and moving coil combination
The combination of the working table, moving coil frame, and moving coil as shown in Figure 1.1 is the active and the core component of the shaker. The working table is used to install the test product or fixture, which is the interface of the vibration output. The moving coil is wound with multiple groups of excitation coils on the metal frame. The power amplifier transmits an alternating current signal to the moving coil winding in the DC magnetic field gap. The magneto-electric induction effect forces the dynamic coil to produce reciprocating motion (vibration) driven by the alternating electromagnetic force.
The working table and moving coil assembly is an inertial mass (m) suspended on the support system (elastic coefficient k). Based on the requirements of the vibration test, the resonant frequency of this mass-spring system, that is, the “first-order resonant frequency of the empty table (no load on the table)” usually given in the technical indicators of the vibration table should be higher than the upper limit of the maximum operating frequency given by the vibration table. Therefore, the working table and moving coil assembly must be made of materials with a high specific stiffness (E/p) of elastic modulus E and density p (such as magnesium-aluminum alloy, beryllium, etc.).
2) Field coil and magnetic conductor
The field coil and magnetic conductor are composed of the excitation coil, magnetic steel body, pole core, pole plate, and short-circuit ring. When a DC current is passed through the excitation coil, a constant strong magnetic field is generated around the coil. This constant magnetic field flows along the magnetic circuit composed of the magnetic steel body, pole core, pole plate, and short-circuit ring to form a closed magnetic circuit. The driving coil (moving coil) with alternating current (vibration signal) interacts with the strong DC magnetic field in the air gap to generate alternating driving force to achieve “vibration” output.
The short-circuit ring in the figure is installed on the pole plate or pole core. It is a copper ring added to change the equivalent impedance of the magnetic circuit system. It plays a role in expanding the working frequency range, increasing the amplitude and reducing waveform distortion.
The excitation coil is a coil winding with a large current. It will emit a lot of “heat” during the electromagnetic conversion process. Therefore, good heat dissipation is one of the key technologies for winding the excitation coil and designing the vibration table. Moreover, during the operation of the vibration table, meeting the requirements of good cooling of the excitation coil and the moving coil is also a very important operating condition.
3) Shaker body, base and vibration reduction device
The shaker body actually refers to a collection of various components of the vibration table actuator part that is “stationary” except for the combination of moving parts such as the table top and the moving coil, including the mechanical structure suspension, support and guide devices such as the stand, ear shaft, support, vibration isolation spring, as well as the excitation coil and magnetic conductor that generate the magnetic circuit.