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FAQ >> Principle of the Crane Anti-Sway device

Principle of the Crane Anti-Sway device

Practical operation

To prevent violent operation, the variable speed drive have a progressive speed setting system which has the effect of optimising acceleration. These speed settings can be adjusted in terms of time and shape (linear, S curves etc.) to suit most applications. The diagram opposite represents the variable speed drive ramps with and without the Anti-Sway function. The ramp in the speed setting, linear in black, cannot be adjusted automatically and, because of this, the operator is left to control the swaying. The Anti-Sway function (red curve on the speed/ time graph), helps the operator to do this by ensuring that the acceleration or deceleration profile of each movement is adjusted in real time to the estimated position of the load. On this diagram we have only represented two profiles but, in practice, an infinite number of profiles is conceivable. On the right of the graph opposite, the red curve represents the speed profile as it is generated by the Anti-Sway function; the green curve the difference in angle between the load and the vertical. The position estimator carries out a sampling procedure every 40 ms (typical value) for each movement. You can see distinctly that the initial angle difference is rapidly controlled by the device as soon as the speed has stabilised. Each change in speed inevitably destroys this balance but the speed profile very rapidly controls this difference. At the end of the movement, an adjustable time out enables the load to be stopped and deposited under the best conditions.

The brain of the device

Without the corrective actions of an experienced operator, returning to a stable state of equilibrium may take several seconds. The swaying of the load only appears during the linear motion of the gantry crane (x movement) or when the trolley moves (y movement). Lifting the load following the z axis does not generate any swaying. The Anti-Sway principle is based on using a load position estimator and determines the difference between the perpendicular and the trolley. It will be noted that in this equation the length L incorporates the position of the centre of gravity. The Anti-Sway device uses the following information:
  • the translation speed of the two horizontal axes, i.e. the speed of the gantry crane (dx/dt) and that of the trolley (dy/dt),
  • the acceleration and deceleration speeds of these same axes (d²x/dt² and d²y/dt²),
  • the length of the cable,
  • the position of the centre of gravity.
The position of the centre of gravity is a variable figure that depends on the geometry of the load that the operator has to fill in. The swaying of the load is controlled by variable speed drive that adjust the translation and trolley movements by adapting the rate laws constantly. The useful information (speed, acceleration, if necessary length of the cable from a sensor connected to the motor) is directly accessible from the variable speed drive or using externally connected equipment (for example length of the cable by a cam box). From a configurable model the estimator calculates the expected swaying of the load by taking account of the information available in the controller, the length of the cable and the position of the centre of gravity.

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