Motion System Design

Ac servomotors hold sway in highrise buildings

If you’re up in a tall building on a windy day or right after a tremor, you know that — contrary to that Big-Band theme — it DOES mean a thing if it ain’t got that swing

Two active dampers reside in the penthouse 70 stories up in the MM21 Landmark Tower, an office and hotel complex in Yokohama, Japan. It is owned by Mitsubishi Estate Co. Ltd. (See illustration to the right.) Each damper consists essentially of a rope-suspended, 170-metric- ton mass driven by mutually perpendicular ball screws. (1 metric ton = 51,000 kg.) Vector-drive-powered ac servomotors run the screws. Mitsubishi Heavy Industries Ltd. (MHI) developed the pendulum-type, tuned active damper.

How it works

To counteract building sway effectively, the pendulum must have the same natural period as the building. The period of a simple short-stroke pendulum system relates directly to the square root of the active length of the pendulum; that is, to the square root of the distance from the pivot to the point where you can consider the mass concentrated. By directing the pendulum to operate in the plane in which the building sways, and at a length that produces the same natural period as that of the structure, forces from damping-mass acceleration and deceleration counteract structure sway.

Figure 1 shows a simple pendulum. In most practical high-rise structures, such a system also has a simple problem: too much length. To avoid that, MHI uses a multistage pendulum. It effectively folds the length by interposing shorter rope lengths that can pivot and lengths of frame that cannot. As Figure 1 shows, a 3-stage system reduces the needed length to one third that of a simple pendulum, thus keeping system headspace needs low. Now, if the mass can be driven in the pendulum system in such a way that its motion counteracts the forces of building sway, the amplitude of structural sway will be reduced.

Figure 2 shows such a theoretical system, with computer-controlled motion guided by sensors on the building and on the damper mass. A servomotor powering a ball screw delivers the linear motion.

Such a system is still not enough, however. A structure may sway in any direction. The pendulum system needs a means to direct its motion onto the plane of structure sway and to control the motion of the mass. It needs to be able cover building motion to and away from any point of the compass.

That’s what the system of Figure 3 does. It is an X-Y motion arrangement consisting of two pairs of linear drive and bearing systems. Two ball screws drive in the X direction at opposite sides of the framework; two, likewise in the Y direction. For each ball-screw drive, an ac servomotor powered by a vector-control adjustable- frequency drive runs a geared speed reducer with output shaft attached to its ball screw. A universal joint connects the underside of the damper mass to the X-Y system. The mass connects to the power-transmission system but does not ride on it; the effect is to minimize system size and frictional resistance.

For the 296-m-high steel and steel-reinforced concrete Landmark Tower, two dampers are on the first floor of a penthouse 282 m above ground. Each is 9 m wide and 4.9 m high, with maximum stroke of 1.7 m from neutral. Active rope length adjustment devices, Figure 3, permit period adjustment between 4.3 and 6 sec to allow accurate tuning.

Motors and drives

Motor output is 90 kW per direction. AC servomotor and vector drive systems like those in Figure 4 are a smart choice here because of good response characteristics such as usable speed range and acceleration rate, and especially because of the lack of brushes. Motor brush wear can be important on high duty-cycle applications with continual acceleration and deceleration.

In regular operation, a high-response system with accelerometers picks up any building motion. It issues computer-output driving signals to ac vector control inverters that run the servomotors, moving the damper mass in the correct opposing direction.

Several fail-safe functions can stop motor operation. For example, a sensor quickly detects any malfunction and prevents any motion that would accentuate rather than reduce sway. Any servo system failure brings motor-power cut-off, whereupon the system acts as a passive rather than an active damper. Abnormal damper displacement brings motor power cut-off and brake application.

Active damping future

Steady growth of high-rise building construction for hotels, apartments, offices, and other uses, could bring unpleasantries for more occupants because of sway from wind or quake. Such sway can sometimes even interfere with normal services and communications in the buildings. MHI sees a market here for its tuned active damper, building on experience in bridges, stacks, and towers. A recent installation is in the control tower at the Kansai New-International Airport. Reliance Electric, supplier of the servomotors and vector drives, also sees such servo-based stabilization systems continuing to grow.

MHI already has much experience in tuned-mass dampers (those without drive motors). It has more than 30 such units on stacks to prevent vortex-induced sway, and another 30 on pylon and girder bridges under construction or completed. You’ll also find them on observatories and communications towers, and MHI is working on higher-frequency, lower-amplitude machine foundation dampers.

MHI says that, on an installation like the Landmark Tower, the sensors can detect vibrations imperceptible to humans. When they do, the damper immediately goes into operation. It will also “...reduce effects of strong winds remarkably. Even a typhoon will pass almost unnoticed.” Moreover, “...the damper can absorb tuned mode vibration of the building incurred by most of the shocks from a minor earthquake. Of course, it cannot nullify the effects of a major earthquake, but it can bring residual vibrations under control rapidly.”

And the good performance and reliability characteristics of vector-driven ac servomotors will help building operators stay atop high-rise sway problems.

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