Maintaining the extraction from the warehouse

1. Handling extraction from the warehouse (NZ)

1.1. Starting a extract operation

The entire conveyor including the transportation chain system is controlled by system NZ. When system NZ detects the possibility of extracting a car from any stack, an appropriate command is sent from the PLC to the PC on channel 2. Concurrently with this command the information of the availability of cars at the exit of each stack is given to the PC in the form of 5 words containing a ‘1’ (car available) or a ‘0’ (no car available). The reason for this is that a car could have been inserted into a stack at system PY but has not reached the exit of the stack physically. The PC at system NZ now performs the following operations:

 

1.1.1. Extraction based on priority

The operation personal in the control room was given the opportunity to define a number of 1 to 25 cars for each stack for priority extraction. This means that the cars of this stack will be extracted immediately regardless of their color code. If such a stack was found AND it is not blocked AND a car is available at the exit of the stack, the number of this stack is sent to the PLC by acknowledging the ‘request for extraction’ command previously given on channel 2.

1.1.2. Extraction based on color

If the check for priority extraction failed, a ‘search for the same color’ is performed. During each extract operation the color code of the extracted car is memorized. The next extract operation compares the color code of the previous operation with the color codes of the nearest-to-the-exit position of each stack. If the colors match AND that stack is not blocked AND a car is available at the exit of the stack, the number of this stack is sent to the PLC by acknowledging the ‘request for extraction’ command previously given on channel 2.
If there was no match in the color codes, the stack with the greatest contents is selected for extraction if there is a car available and the stack is not blocked.
Each color code is also combined with a counter, which will be incremented by the extract operation. When the counter has reached a limit that was defined through the MMI system the counter is reset to zero and the memorized color code of the last car is destroyed. This will cause the selection of a new color at the next extract operation.
Additionally, the software maintains a wrap-around pointer for the stacks. Lets assume that all five stacks are filled with car bodies of the same color code. Without this pointer the software would select stack number 1 until it is empty for consecutive extract operations, than stacks 2 through 5 in the same way.
With this feature, the wrap-around pointer is incremented on each extract operation. When stack five was processed, the pointer wraps around back to stack one.
The compare operation is started at that stack, where the wrap-around pointer is pointing to.

1.1.3. Extraction based on emergency

If none of the extract operations describe above have found a stack for extraction, a last attempt is made: using the wrap-around pointer the ‘availability values’ given by the PLC are used for extraction and one of the next stacks is selected. This situation may be found due to a damaged image of the warehouse. The image could show empty stacks while in reality there are cars in the stacks. Normally the warehouse would be blocked.