Eddy-current damping

In this section, we will discuss vibration isolation systems based on suspension springs with eddy-current damping. To date, it is probably the most efficient vibration isolation system. The design and choice of springs are also discussed. An elementary theory of helical springs, sufficient for all the applications in STM and AFM, is presented in Appendix F. 

For eddy current damping typically conductive metal sheets are placed in close vicinity to magnets. When the sheets move or vibrate eddy currents... 

Simple mechanical isolation techniques include placing the SPM on an air-isolation table or a platform with low-frequency characteristics suspended by bungee cords. UHV-SPMs generally have a built-in eddy current damping/... 

First, the eddy current density is damped while penetrating into the conductor (penetration effect). Here the frequency dependence of the penetration depth implies that for deep lying cracks low frequencies must be used for obtaining a sufficient current density in the vicinity of the crack. Secondly, due to the induction law the induced current density at the surface jco is diminished when using lower frequencies. Therefore, in total, there is a certain excitation frequency which results in a maximum response field from the crack. 

The mechanism of suspension is related to the type of flow pattern obtained. Suspended types of flow are usually attributable to dispersion of the particles by the action of the turbulent eddies in the fluid. In turbulent flow, the vertical component of the eddy velocity will lie between one-seventh and one-fifth of the forward velocity of the fluid and, if this is more than the terminal falling velocity of the particles, they will tend to be supported in the fluid. In practice it is found that this mechanism is not as effective as might be thought because there is a tendency for the particles to damp out the eddy currents. 

When a conductor moves in a magnetic field, damping forces are generated by eddy currents induced in the conductor. The magnetic damper, with its reliability and thermal stability, has been utilized in various branches of engineering. A recent analysis of it is given by Nagaya and Kojima (1984). 

Damping torque loss DoLMotor = OLMotor- (by electric eddy current), depending on the rotational speed N, with... 

Although molecular diffusion itself is very slow, its effect is nearly always enhanced by turbulent eddies and convection currents. These provide almost perfect mixing in the bulk of each Hquid phase, but the effect is damped out in the vicinity of the interface. Thus the concentration profiles at each... 

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