Plug-and-Work Material Handling Systems

One disadvantage of automated material handling systems is their relative inflexibility: once racks are installed and conveyors are laid, making even minor changes to a system can be cumbersome and expensive. However, recent progress in the capabilities and cost of basic system components, such as controllers, drives, and sensors, has made possible a new class of material handling systems having a much higher degree of flexibility. We propose underlying design principles for such systems and describe several prototype “plug-and-work” systems, which feature decentralized control and ease of reconfiguration. 1 Current Challenges The companies that comprise the Material Handling Industry are conservative by nature, and for good reason: heavy loads must be handled without injury; large volumes of different items must be handled without damage or loss; and picking, packing and inventory tracking must be done without error. It comes as no surprise, then, that manufacturers and users tend to be careful when designing and implementing new systems. Although the industry has implemented hundreds of successful automated material handling systems in the past, there are signs that business is or will be getting more difficult. For instance, a survey in Germany showed that users of automated material handling systems have reduced their investments in automated systems or are planning to “de-automate” significantly (see Figure 1). Although this study is now somewhat dated, we believe its underlying message is still valid. The study contends that many users consider automated systems too inflexible to cope with rapidly changing demands in structure, volume, and processes of today’s material handling processes. But what is it, exactly, that makes a modern, automated material handling system inflexible? We believe the following logic is behind this judgment: (1) A system is inflexible because it is time consuming and expensive to change in response to new requirements; and (2) current systems are difficult to change because functionality is spread over several levels of information, electrical, and mechanical networks, all of which must be changed simultaneously to provide new functionality. Figure 1: Reduction in automation in several production areas, according to ISI [3]. Statistics indicate the percentage of respondents who were planning or who had already implemented a reduction in automation. 1.1 Current (inflexible) designs We illustrate the point with a small example: Suppose an automated pallet warehouse retrieves items with an AS/RS-machine and moves them by conveyor to a transfer-carriage, which moves the pallet to the assigned picking position. After picking, the pallet is returned to the rack. The picked parts then move to a sorting and consolidation area. Suppose the need for higher picking capacity makes necessary an additional picking location, and that (luckily) there is enough space to accommodate that position. Which tasks must be performed to make this simple extension? First, additional sensors must be installed at the new stopping position. They signal to the PLC when the transfer-carriage is positioned correctly at the handover position. Second, let us assume that, in keeping with current practice, the necessary drives, rollers, sensors and structure-elements are already combined into one module. This module must be installed at the additional position. Third, the wiring between the drives, sensors of the picking position, and the sensors of the transfer-carriage one end and the control cabinet on the other end have to be installed and connected. If necessary, new control boxes have to be added to provide the required input-ouput ports for the PLC. Fourth, the input and output ports must be connected to the logic of the PLC. Essentially, this requires adding another position in the scheduling logic; that is, adding the sensors of the new position in the “check for free location” logic by connecting the port numbers of the sensors with the logical view of the added position. When the new position is actuFigure 2: High-bay warehouse with picking positions (“new” indicates added picking position). ally used, the code for moving the transfer-carriage to the new position has to be updated to include the new position as well as for the return of the pallet. Finally, the user-interface of the PLC has to be updated with the status of the new position. In order to do all this, the company must employ or hire personnel able to do the electrical work, and to link the physical view (mechanical, electrical) with the software logic. This is very important because the function of the system is distributed over several elements which are connected by a central control logic, usually implemented in a PLC. Documentation brings together the several areas of expertise needed to perform the necessary tasks to change the mechanical design, the wiring, sensors, drives, and controlling logic. These activities take at least one day, depending on the amount of work necessary for wiring and coding, and they usually cannot be done by the system operators. Instead, contractors or qualified personnel have to be brought to the site, along with the necessary equipment and material. We hope the reader acknowledges that such effort for such a small system change is unappealing at best, and that it supports the assertion that many automated material handling systems are “inflexible.”