The benefits of AS/RS technology have been effectively realized in libraries. The amount of floor space required for books and periodicals has been reduced by ten-fold or more. The number of librarians required was reduced as well. Reshelving errors were eliminated. The location of each item while in the library is known with certainty. Despite these benefits, it is estimated that a few (less than 12) mini-load AS/RS systems have been installed in libraries.
This case problem involves determining the saturation point for a mini-load AS/RS system installed in a particular library. This is done be creating a graph of the cycle time for retrieving a book or periodical versus the arrival rate for such requests. The arrival rate resulting in the longest acceptable retrieval time is the saturation point. The smallest arrival rate of interest is 10 requests per hour. Assume that the arrival rate for retrievals is the same as the arrival rate for returns
The mini-load AS/RS system installed in one particular library has a capacity of 250,000 books and periodicals. There is a single aisle with identical racks on each side. The system is installed inside a secured vault for safety and security reasons.
Books and periodicals are stored in carriers that are 4 feet deep and 2 feet wide. Each carrier row is one of three heights: 10, 12, or 15 inches. Each item is stored in the shallowest carrier in which it can stand. Thus, vertical space is used most efficiently. Assume that the number of books and periodicals of each height is the same.
There are 36 carrier rows on each side of the single aisle. The height of the first row is 10 inches, the second 12 inches, the third 15 inches, the fourth 10 inches and so forth. There are 60 carriers in each row.
The S/R machine travels at a high rate of speed: 12.6 feet/second horizontally and 4.3 feet/second vertically. Assume that the S/R machine must travel either horizontally or vertical but not diagonally.
The process of retrieving a book or periodical is the following. A patron makes a request using the electronic library catalog system. The AS/RS fills one request at a time. The location of the item is completely random. The S/R machine moves from its idle location to the required carrier, extracts the carrier in 3 seconds, and places the carrier in the pick and delivery station. A librarian must remove the desired item from the carrier and record its status in the information system. This takes 7 seconds. The S/R machine remains idle at the pick and delivery station.
Next the librarian determines whether any item that needs to be returned to storage is of the same size as the carrier. If so, the item's new carrier location is recorded in the information system and the item placed in the carrier. Both steps combined take 7 seconds.
Assume the library is open 16 hours per day, 7 days per week.
Embellishment: The AS/RS system tests the carrier for weight restrictions. One in 100 tests fail. In this case, the librarian must remove the item as well as the newly entered location from the information system in 7 seconds. In either case, the S/R machine replaces the carrier and returns empty to its idle location.
Embellishment: Find the saturation point when the following procedure is used. The S/R machine does not replace a carrier that is at a pick and delivery station until the next retrieval request is made. At that time, a carrier is first stored and then the next carrier retrieved.
Embellishment: Limit the number of carriers stored at the pickup/dropoff station to a total of three. When the fourth carrier arrives, it is immediate returned to the same storage location by the AS/RS machine.
Case Problem Issues:
1. How should carriers be modeled?
2. How should the location of the carrier containing the book or periodical requested be determined?
3. How should S/R machine travel time be computed?
4. Specify the process for book and periodical returns.
5. What are good initial conditions for this simulation experiment?
6. What performance measures, other than cycle time, would be of interest?
7. What is the expected utilization of the SR machine?
8. How should verification and validation evidence be obtained?