Electrodynamic Shakers

Labworks shakers utilize normal current/force motor principles to generate vibratory force. Electrodynamic force is inherently linear, and offers wider bandwidth with lower noise and harmonic distortion than mechanical or hydraulic based vibration generation. Most Labworks shakers are air cooled eliminating the requirement for oil or water used in conjunction with other types of shaker cooling.

Except its Inertial Shakers, Labworks electrodynamic shakers offer frequency response down to DC to insure good low frequency force capability. Upper frequency limits are controlled by the shaker armature's mechanical resonances and are extended by careful design to reach frequencies higher than most test specimen vibration test requirements.

Shaker General Description

Labworks General Purpose, Modal and Transducer Calibration shakers use a construction similar to common loudspeakers to convert electrical current flow into mechanical force over the widest frequency range with minimal spectral distortion of the input waveform. This moving "voice coil" configuration offers a large test article attachment surface with a lightweight moving mass.

The shaker's voice coil is attached to a suspended metalic support and test article attachment structure called the "armature". The shaker's armature is guided so that it is allowed to move relatively easily in the direction of the generated force and have the highest stiffness possible in all other directions. In this respect, these shakers are primarily unidirectional vibration devices. It is extremely important that the armature suspension be stiff in all transverse directions to minimize any lateral deflections caused by load attachment that could cause lateral armature coil deflection.

Labworks General Purpose, Modal and Transducer Calibration shakers utilize a "single-end" magnet structure configuration. This configuration offers several significant advantages over other types of magnet structures. Optimized, single-end shaker designs yield a larger armature coil diameter, giving these shakers a larger mounting surface, which is desirable for easy test article attachment. The single-end magnet structure also offers the easiest physical access for inspection and maintenance. No shaker body disassembly is required to service any dynamic component of the shaker.

Carbon fiber composite flexure components are used in the armature suspensions to maximize the available dynamic stroke while maintaining high lateral stiffness. Minimal use of rubber in the armature suspension reduces velocity related damping losses, therefore allowing higher velocity and better low frequency distortion characteristics.

Force Generation

Electrodynamic shakers are inherently force generators. Electrical current flowing in the armature coil interacts with the strong DC magnetic field of the shaker's magnet structure (body) to produce physical force. This force can be taken as being generated between the armature coil and the shaker's body. In this respect, since the armature is free to move relative to the body in the direction of the force, both the shaker's armature (and its attached test article) and the shaker's body are subjected to the generated force. If the armature coil current is varied, as in alternating vibration excitation, both the armature and the shaker body will be accelerated in response to this force and will each respond according to their inherent mass, with vibratory motion, each independent of the other less armature suspension stiffness considerations.

Shaker magnetic structures are designed to have extremely high DC magnetic fields concentrated in the internal area of the armature coil. Further, since high magnetic fields can be detrimental to some test article operation and test results, the magnet structure is usually designed to have a minimum of "stray" magnetic flux outside of the shaker body. This is especially significant in the area of test article attachment at the "top" of the armature. Exclusive use of high energy, centrally located magnets or field coils is extremely effective in both these areas.

Force generated by the interaction of the armature coil and the body DC field is proportional to the current flowing in the coil and the strength of the DC field. The generated force can be found from the following equation.

F=KBLI     where:

F=Armature coil force
K=.885 x10-7 (English units)
B=DC magnetic flux density
L=Length of armature coil conductor
I =Armature coil current

Displacement Limitations

Electrodynamic shaker armature displacement is limited only by the axial length of the armature coil and the physical limitations of the armature suspension system. Since most shakers are provided with an adequate axial coil length to maintain relatively linear force generation at low frequencies, the primary limitation is that of physical interference of the suspension components. Since shakers have an available operating displacement window, it is most common to rate and discuss vibration test displacement in peak to peak terms. For this reason, most engineering equations of motion involving vibration testing will use displacement in peak to peak units(sometimes called "double amplitude displacement").

The rated displacement of electrodynamic shakers is usually the maximum relative displacement available between the armature and the shaker body/suspension. When considering the suitability of a shaker for a given test, it is important to consider the various factors that may reduce the available test article absolute displacement.

Since the same force that is applied to the armature coil is also applied to the shaker body, the shaker body is also accelerated and has a displacement definable by the normal equations of motion. This body motion can have the exact opposite phase relative to the armature motion and therefore, must share the available relative (rated) shaker armature displacement with the armature and test article. In other words, the test article displacement added to the shaker body displacement must be less than the rated shaker displacement.

Another factor reducing the available displacement is the natural deflection of the armature suspension when a test article and fixture are placed on a shaker in the vertical shaker orientation. The weight of this added load offsets the armature downward and therefore reduces the available downward armature displacement. This reduction of the available stroke on one end of symmetrical alternating vibration reduces the allowable peak to peak displacement by double the amount of the deflection.

For most test articles, the shaker body weight is significantly heavier than the test article, fixture and armature and its displacement motion can be ignored. In that case, the required displacement equation found in the Systems Engineering section applies:

Dreq=g/.0511 f2 + 2w/k.

A normal maximum unsupported load weight for a shaker in vertical orientation is that which will reduce the available test article absolute displacement to 1/2 the rated, neglecting shaker body motion. This corresponds to the weight that will depress the suspension by 1/4 of the rated displacement. Labworks shakers are all designed with unusually large relative displacements to better accommodate unsupported load vertical operation.

Velocity Limitations

Shaker velocity limitations stem primarily from internal inductive heating of conductive armature components and damping loss heating of over-damped suspension components. Labworks shakers are designed with low stray magnetic fields which reduces the inductive heating of their armature components. Minimal suspension damping is utilized and for most applications, Labworks shakers have no velocity limitation other than that imposed by the maximum acceleration and displacement specifications.

Shaker systems, however, can have velocity limitations due to back emf requirements on the system amplifier. Velocity limitations are rarely a concern with Labworks systems. Please call with your specifications if extremely high velocities are required.