This invention relates to an article retrieval system and, more particularly, to an improved order picking process and apparatus which permits automated and efficient filling of a large quantity of orders, many of which contain multiple but different articles, by selecting the individual articles from an inventory which contains large number of articles including both high-demand articles as well as low and moderate demand articles. 
Numerous automated storage and retrieval systems (ASRS) have been developed, particularly for permitting storage and retrieval of containers of articles such as cases or cartons in warehousing arrangements.  Such ASRS typically involve storage racks wherein articles are disposed in horizontal rows and in vertical columns, with an automated stacker crane or the like being movable horizontally and vertically along the front of the storage rack and having an extendible article transfer mechanism associated therewith for permitting an individual article to be moved into a rack, or removed therefrom when retrieval is desired.  Such arrangements are also conventionally tied in with a central controller or computer which not only programs the movement of the stacker crane for storing or retrieving articles, but also inventories all of the stored articles within containers including the locations thereof.  Such storage/retrieval arrangements, however, are normally suitable solely for handling relatively large articles such as cartons or the like, and are limited with respect to the number of articles which can be handled within any defined time interval.  In particular, such arrangements are not suitable for mass handling of small articles, and particularly the storing and retrieval of small articles so as to assemble pluralities of such articles into pluralities of individual orders. 
To assist in automated order filling, various automated dispensing systems have been developed which permit storage of like articles in stacks, with the dispensing arrangement permitting dispensing of the lowermost article, such as on to a conveyor to permit collection thereon of a number of such articles which make up an order.  With such arrangements, the dispensers are designed for handling large quantities of like articles and hence are particularly suited for articles which are in high demand.  The structural complexity and overall cost of arrangements of this type, however, do not make dispensers of this type particularly suitable for the handling of articles and the filling of orders where the articles are in low demand, and particularly where there is an extremely large inventory of different low-demand articles from which individual articles must be selected to fill an order. 
Examples of dispensers of the aforementioned type, which are particularly suitable for dispensing a high volume of high-demand articles, are disclosed in U.S. Pat.  Nos.  4,000,821 and 5,271,703. 
Accordingly, while most order picking or fulfillment systems permit large quantities of specific articles or items, typically high-demand items, to be automatically picked or dispensed, nevertheless the low or moderate demand articles or items typically must be manually picked for subsequent consolidation with the high-demand articles which have been automatically picked or dispensed.  This thus results in the overall order fulfillment process being slower and less efficient than desired since the manual fulfillment phase of the order picking process thus controls the efficiency and speed with which the overall order-picking process can be carried out. 
It is thus an object of this invention to provide an automated storage and retrieval system for articles, particularly an order picking system which can be fully automated for permitting storage and retrieval of not only high-demand articles but also low-demand articles, whereby the improved arrangement is capable of overcoming the disadvantages which have been conventionally experienced with prior known systems. 
It is an object of this invention to provide an automated storage and retrieval system which is particularly suitable for handling an inventory of articles which includes an extremely large number of different articles, which inventory includes a small percentage of articles which normally represent the articles which are of high demand, with the remaining majority percentage of the inventoried articles representing those articles which are only of low or moderate demand, with the improved automated storage and retrieval system being capable of automatically retrieving not only high-demand articles but also the low and moderate demand articles so as to permit picking of the selected articles in accordance with an order so as to automatically combine the discrete picked articles which define the order. 
In the improved automated storage and retrieval system of this invention, the individual articles are typically stored in vertical stacks, each stack containing a plurality of identical articles, with the stacks being stored in cells which in turn are defined within movable containers or totes, each of which is subdivided into a rectangular array of upwardly opening cells, each capable of holding a stack of identical articles therein.  The totes are disposed for storage in racks which preferably include multiple vertically spaced shelves each capable of holding a large number of totes in side-by-side relation therealong, with the individual totes being movable into and out of the racks by automated storage and retrieval devices, such as stacker cranes or carousels of conventional construction.  The totes containing the high-demand articles will typically have identical articles defining the stack of each cell, whereas the totes containing moderate or low demand articles will have multiple stacks defined by different articles, whereby one or more cells contain a stack of one type article, and other cells contain stacks of different articles.  The ASRS of this invention includes an order picking system having at least two zones disposed in series along an order-collecting conveyor system.  One zone is for picking of low and moderate demand articles, and the other zone employs high-speed dispensers which support a plurality of upright stacks, each stack being of identical high-demand articles, for permitting dispensing of high-demand articles onto the order-collecting conveyor system.  A computerized controller assigns order-collecting areas or regions in spaced relation along the conveyor system so that individual picked articles within the two zones are deposited into a preassigned area to permit assembly of an order therein.  The order picking zone for the low and moderate demand articles typically involves several subzones arranged serially along the conveyor system, and each subzone includes a picker mechanism which includes a picking head which moves longitudinally through a predetermined distance along the conveyor, and transversely thereof.  A plurality of totes are supported on a platform adjacent and extending longitudinally along at least one side of the conveyor system, with the cells in each tote typically defining a plurality of stacks of different articles all having a preassigned coordinate (i.e., an X-Y coordinate) relative to the movement of the picking head.  The computerized controller causes the picking head to remove a desired article from the cell of the specific tote containing such article, with the picking head then moving transversely and longitudinally as necessary so as to thereafter transfer the article for deposit on the preassigned area of the conveyor system.  This process can be repeated several times within one subzone, and sequentially within the several serially-arranged subzones, to permit filling of a single order with the desired low and moderate demand articles which are deposited into the preassigned area on the conveyor system.  The partially assembled order thereafter moves under the dispensers in the next zone so as to permit addition thereto of any high-demand articles which are contained in the order. 
With the improved system of this invention, as summarized above, the totes which are disposed along the order-collecting conveyor system as associated with each subzone can be readily removed and interchanged by means of a storage/retrieval vehicle which effects transfer of totes between the storage racks and the picking areas (i.e. the picking interface).  Similar totes are disposed in series along the zone containing the article dispensers, and a replenishment mechanism which is movable transversely and horizontally along this latter zone is provided for removing a stack of articles from one of the replenishment totes and thereafter depositing the stack into an appropriate dispenser. 
Other objects and purposes of the invention will be apparent to persons familiar with systems of this general type upon reading the following specification and inspecting the accompanying drawings.