Machine Design

Sling it, don’t swing it

Moving large and heavy loads using overhead cranes is time consuming and risky. Cranes need experienced operators trained to safely handle heavy loads. Loads that swing out of control collide with buildings, equipment, and other moving loads, causing major damage and endangering workers. A new approach to crane control recently tested at the Technical University in Prague (CVUT) reduces the danger of swinging loads and significantly enhances safety.

Swinging is a major challenge when maneuvering loads by crane. Most of the swinging takes place during and after the move, though other variables such as wind may start loads swinging as cranes sit idle. Even hall cranes protected from the environment may swing their loads while at rest.

Proper control of the crane, coupled with the removal of disturbance variables, minimizes this motion. Swinging generated by the load’s motion as it is being moved needs other remedies beyond control and minimizing disturbances.

There are two basic methods for dealing with the problem of swinging loads: using closed-loop control with feedback, or moving loads in trajectories that minimize swinging. Feedback control takes care of disturbance variables, but it also adds costs for monitoring load position. Moving in trajectories that prevent loads from swinging of the load, however, does not require monitoring the load position.

CVUT recently tested this second method. An antiswing crane using “input shaping” was tested on a lab model of a crane using a B&R controller from B&R Automation, Rosell, Ga. Before building the model, researchers had to first develop a mathematical model of the crane and a control algorithm for that model

One advantage of the portal crane is that the two directions of movement — sideways and forward/ backward — are independent of each other. To define the movement of a crane on one axis, researchers need to know the position of the load and crane trolley, plus the length of the cable suspending the load.

The trolley’s path and speed determines its position, and cable length can be calculated by measuring the rotation of the reeling drum. Load position is not measured directly when using input shaping. But load position does play an important role in the model where load position is replaced by a relative coordinate representing the difference between the load position and trolley.

Input shaping manipulates the input signal during transport to generate an antiswing crane movement. Once the trolley reaches its travel speed, both the trolley and load move at the same speed. When the movement is complete, the load should be stationary. A corresponding mathematical model determines the required trajectory and speed of the trolley to produce the needed antiswing crane motion.

Antiswing control was programmed into the model using several programming languages such as Structured Text (ST) and ANSI C. Data exchanges take place using global process variables on the controller. The ST program monitors the state of the crane as it switches between initialization, antiswing control, and error states. It also processes I/O signals, controls the crane via the control panel, and evaluates disturbance signals.

The algorithm for swing suppression was modeled in Matlab and Simulink and put into an Automation Studio project using B&R Tools’ Automation Studio Target for Simulink. The Simulink and B&R software let many existing Simulink function blocks, such as transfer functions, signal filters, control blocks, and lookup tables, be dropped into the simulation.

The control system met all of the requirements for antiswing control of crane movements. The tested algorithm for antiswing control using a B&R X20 control system has proven to be wellsuited for industrial use.

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