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This month's article has been written by Kim Hunter of Visual8 one of our most active US Certified SIMUL8 Solution and Training Providers. Their work includes combining simulation with scheduling algorithms and interactive graphics to produce powerful business planning solutions. Contact Kim at Kim.H@Visual8.com.

Simulating Gantry Crane Systems

A number of SIMUL8 users have asked us at Visual8 for technical support on modeling cranes. This is an interesting materials handling application that has challenged model builders since the birth of simulation. In fact, one of the very first uses for simulation - back in the olden days of Fortran and paper tapes - was modeling gantry crane operations in a steel mill.

The rules governing crane movements, the priority and sequence of moves, can seriously limit the output from the mill. Not having the luxury of visual output from their simulations these pioneers displayed the results of their simulations by physically moving a mechanical model of the gantry crane system over time.

Today, we are armed with sophisticated simulation tools like SIMUL8, which offer more powerful and immediate modeling capabilities. Let us show you a novel way to simulate a gantry crane using just a few SIMUL8 objects.

SIMUL8 queues or storage bins offer simulation builders a rich array of features to employ in their applications. In this simple single-gantry crane example we will use a storage bin to represent our gantry, let's call it sb_Gantry, and display it as a queue on the screen. The length of the gantry is set by its capacity and the size of the work items it contains. Thus a 50-foot gantry may have a capacity of 50 with work items of 1 foot - the size of the work items represent the distance the crane can travel at each incremental move.

We now fill our Gantry queue, on reset, with work items setting their image as spaces with one of the work items image set as that of a crane. Note the location of the crane in a global data item. Here we use a variable called gbl_Crane Position.

Now for the more interesting part - to simulate the movement of the crane we simply move work items in the Gantry queue. Moving the Crane work item forwards in the queue, to travel right, or backwards in the queue, to travel left, controls the direction of the crane. This assumes the queue is oriented from right to left in the simulation screen.

To manage the movement of the crane we created a separate work center called Crane Motor. To make this work center operate it is necessary to link it to a Storage Queue which has a start-up of one work item. The Crane Motor will then decide how long the crane movement should be using a Distribution called Crane Speed. Once the Crane Motor has completed one movement it will return to the storage queue. This will keep triggering the Crane Motor work center. Routing out from this work center is determined by the label Position, which will route either to the queue or the Load_Unload Work center depending on whether it has reached its destination or not. Now we will look at the logic that determines the cranes position and therefore its routing.

Recycling a work item between a queue and a work center in this way is a useful regulating technique that we use in many of our simulations.

As the crane moves incrementally over time we check to see if it's in the required destination position to either pick-up a load or unload the crane. Sophisticated users can adjust the crane travel time to reflect load conditions as well as crane acceleration and deceleration times.

Once the crane is in position we pass the control to a Load and Unload Work Center that will then simulate the loading or unloading of the Crane and the determine its next destination.

It's that easy. Now we leave you to think about the double-gantry crane problem!

More Information

If you would like some help on more complicated materials-handling systems feel free to give us a call and discuss it with one of our simulation consultants at Visual8 Corp. on our toll free line 1-800-463-4301.

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