How the elevator of the automatic warehouse system works

26 November 2009

A lot of people have been asking how the elevator in the automatic warehouse system works. You must have noticed that the elevator movement, required for the forks to load and unload a box from the rack or the monorail, is done automatically. In any case I’d like to share with you how our internal controller performs this automated movement. Please note that this article only applies to versions 1.2.3 or above of ITS PLC Professional Edition.

Figure 1. Automatic warehouse elevator and forks.

The following grafcets illustrate the logic behind the automated elevator movement.

Figure 2. Grafcet G0 and grafcet G1.

Forks Left, Forks Center and Forks Right correspond to sensors 2, 3 and 4 respectively.

*Elevator Up, Elevator Down and Box at Elevator are internal sensors that indicate the actual position of the elevator and if there is any box loaded. You don’t have access to these sensors. What you should remind is that sensor 5 is ON during the load and unload actions, so you can use its transitions to sequence movements in your algorithm. Grafcet G0 is pretty self-explanatory and it shows how the elevator behaves if the forks are at left, center or right position and if it is loaded with a box or not. Grafcet G1 shows that if you remove a box during the Loaded and Ready to Unload steps, it will stop any action in Grafcet G0 and move the elevator to the Unloaded position (initial step of Grafcet G0).