MATERIAL-HANDLING SYSTEMS:AUTOMATED SYSTEMS

AUTOMATED SYSTEMS

Material handling is often a physically demanding, repetitive task. Although very few handling sys- tems are completely automated, mechanization and automation play a significant role in designing and operating effective and efficient handling systems. Some of the better-known examples of auto- mated material-handling systems include automated storage / retrieval (AS / R) systems, carousels / rotary racks, automated guided vehicle (AGV) systems, and robotic systems.

AS / R systems have been in use since the 1960s, although recent systems are much smaller in size. Each aisle in an AS / R system consists of a storage rack on either side and a S / R machine. (In some systems the S / R machine may not be aisle-captive.) The S / R machine is supported by a guide rail both at the top and bottom; this allows concurrent travel in the horizontal and vertical directions. Load pick-up and deposit is performed via a shuttle mechanism that allows the S / R machine to serve either side of the aisle without turning. Due to the shuttle mechanism, however, the rack used in AS / R systems is a specially-designed rack that is different from a standard single-deep selective (pallet) rack.

AS / R systems are capital-intensive systems. However, they offer a number of advantages, such as low labor and energy costs, high land / space utilization, high reliability / accuracy, and high throughput rates. The expected travel time equations presented earlier for single command (SC) and dual command (DC) cycles under concurrent horizontal and vertical travel [see Eqs. (1) and (2)] apply to AS / R systems as well.

There are many papers in the technical literature for the reader who wishes to obtain more detailed information on AS / R systems. In addition to results such as the expected value and / or distribution of SC and DC travel times (e.g., Chang et al. 1995; Foley and Frazelle 1991; Graves et al. 1977; Hausman et al. 1976; Kouvelis and Papanicolaou 1995; Sarker and Babu 1995), various papers have investigated operational issues such as S / R dwell point strategies, storage-retrieval sequencing, and storage methods (e.g., Chang and Egbelu 1997a, b; Egbelu and Wu 1993; Hwang and Lim 1993; Lee and Schaefer 1996; Peters et al. 1996), twin-shuttle S / Rs (Keserla and Peters 1994; Meller and Mungwattana 1997; Sarker et al. 1994), and storage-retrieval matching or AS / RS control strategies / design models (Han et al. 1987; Elsayed and Lee 1996; Lee 1997; Linn and Wysk 1990; Rosenblatt et al. 1993; Seidmann 1988; Wang and Yih 1997).

Carousels have been used in many storage / retrieval systems involving small to medium-sized parts. A carousel is basically a group of carriers that are suspended via trolleys from an overhead, closed-loop track. (Heavier loads may utilize floor-supported carriers.) Each carrier typically contains a set of shelves to store trays or tote boxes. Using a drive mechanism similar to a trolley conveyor, the carriers can be moved clockwise or counterclockwise around the track. Most readers would recognize a simple version of the carousel used at the dry cleaner, where garments are brought to the operator for retrieval.

Although most carousel applications in industry use human operators to store and retrieve the parts, fully automated systems have also been installed by replacing the human operator with auto- matic insert / extract devices to remove or insert the trays / tote boxes automatically. Also, depending on the plane of rotation, carousels are classified as horizontal carousels or vertical carousels.

When there are multiple items to be retrieved, the carousel must be turned and stopped several times, once for each item, assuming that the items are located on different carriers. Suppose each carrier in a horizontal carousel contains only one shelf, that is, the carousel is only one-level high. Further suppose several one-level-high carousels are stacked vertically. The result is a ‘‘carousel’’ where each level is powered and operated independently. Such a system is known as a rotary rack, which generally yields higher throughput and can support multiple extract / insert devices at the same time.

Another type of automated material-handling system that has been used successfully in manufac- turing and warehousing is the automated guided vehicle (AGV) system. An AGV is basically a fully automated cart that can pick up, route, and deliver unit loads from one point to another (within a network of pick-up / deposit (P / D) points) with no human intervention. The vehicle runs under the control of an on-board computer. (Actually, a vehicle may contain two or more microprocessors on board.) If centralized control is used, that is, if a central computer keeps track of all the vehicle movements and move requests in the system, it is quite common to have each AGV communicate

with the central computer via radio frequency (RF) communication. Although alternative guidance technologies exist, the most common is wire guidance, which consists of segments of wire buried in the floor. When energized, the wire generates a magnetic field that is sought by the vehicle.

Guidewire-free AGV systems (also known as self-guided vehicles, or SGVs) are also available from several vendors. SGV systems offer increased flexibility in those systems where the load routings and / or the P / D points change frequently. In a typical SGV system, the aisle structure is maintained by each vehicle as a road map. In moving a load, each vehicle follows the road map according to preprogrammed instructions. New road maps can be prepared off-line and downloaded to each vehicle

on an as-needed basis. Initially, the cost differential between AGV and SGV systems was significant. More recently, however, SGV systems have become cost-competitive with AGV systems.

Using the appropriate number of vehicles in an AGV or SGV system is very important. To obtain a quick estimate, one may use the model presented earlier for lift trucks operating under FCFS dispatching. However, due to possible congestion and blocking delays, and to capture more efficient, dynamic dispatching rules, many AGV / SGV systems today are designed via simulation. In fact, many simulation packages in the market have modules or logic built in to facilitate AGV simulation. In addition, the reader will find many papers on AGV systems in the technical literature. These papers address various issues, including system configuration, vehicle dispatching, guidepath design, and gridlock avoidance (see, e.g., Arifin and Egbelu 2000; Borenstein 2000; Bozer and Srinivasan 1991; Bozer and Yen 1996; Ganesharajah et al. 1998; Goetz and Egbelu 1990; Hwang and Kim 1998; Kim et al. 1997, 1999; Srinivasan et al. 1994; Yeh and Yeh 1998).

Robots have also played a significant role in material handling. Perhaps the most common robot used in material handling is the pick-and-place robot, which comes in a variety of configurations. Other robotic or robot-type systems used in material handling include automated item dispensing (similar to, or more advanced versions of, vending machines), palletizers / depalletizers, and gantry robots. Other robots, such as those used in welding and painting, are not considered material-handling robots. For further information on robots, the interested reader may refer to Nof (1999) or Tompkins et al. (1996, Chap. 6), among others.