Building Dampers Question

[tommyedison]

I was looking for how Building Dampers work and I got this from howstuffworks.com

Building Dampers

Quote from the website

Passive -- This is an uncontrolled damper, which requires no input power to operate. They are simple and generally low in cost but unable to adapt to changing needs.

Active -- Active dampers are force generators that actively push on the structure to counteract a disturbance. They are fully controllable and require a great deal of power.

Semi-Active -- Combines features of passive and active damping. Rather than push on the structure they counteract motion with a controlled resistive force to reduce motion. They are fully controllable yet require little input power. Unlike active devices they do not have the potential to go out of control and destabilize the structure. MR fluid dampers are semi-active devices that change their damping level by varying the amount of current supplied to an internal electromagnet that controls the flow of MR fluid.

Inside the MR fluid damper, an electromagnetic coil is wrapped around three sections of the piston. Approximately 5 liters of MR fluid is used to fill the damper's main chamber. During an earthquake, sensors attached to the building will signal the computer to supply the dampers with an electrical charge. This electrical charge then magnetizes the coil, turning the MR fluid from a liquid to a near-solid. Now, the electromagnet will likely pulse as the vibrations ripple through the building. This vibration will cause the MR fluid to change from liquid to solid thousands of times per second, and may cause the temperature of the fluid to rise. A thermal expansion accumulator is fixed to the top of the damper housing to allow for the expansion of the fluid as it heats up. This accumulator prevents a dangerous rise in pressure as the fluid expands.

Will the semi-active damper move back and forth due to the contraction and expansion of the MR fluid?

How does the semi-active damper derive energy from the oscillations of the building as very little energy is required to operate it.

If somebody could point me out a more comprehensive source, I would be grateful.