Material transport systems such as automatic storage/retrieval systems (AS/RS) are commonly used in business and industry. These transport systems can take on various forms, with one common approach employing a mobile machine such as an overhead hoist or crane which is supported by and displaced along one or more tracks. The mobile machine is powered and controlled by means of a connected trolley which is also attached to a support structure and moves with the machine in a direction parallel to the tracks supporting the machine. In the case of a crane, electric power and control inputs are provided to the mobile trolley via floor mounted rails, with an overhead rail provided to stabilize the machine. The crane typically operates in a narrow aisle which is very high, i.e., generally on the order of 90 feet in height.
The typical trolley support structure includes an elongated, generally closed housing which is open at the bottom. Disposed within the housing are thin, elongated copper conductor strips which carry electrical power and control signals. An upper portion of the trolley is disposed within the housing and includes conductive contact brushes which engage the conductor strips and transmit power and control signals. Suitable wire connections couple a lower portion of the trolley, which extends through the lower open portion of the support housing, to the mobile machine such as a hoist or crane. The mobile machine is also mechanically connected to the trolley so that the trolley moves with the machine as it is displaced along its support track.
Material transport systems employing multi-conductor strips in a generally closed housing as described above offer various advantages over more conventional single conductor strip systems. These advantages include reduced space requirements, isolation of the electrically active conductors from personnel, and more reliable electrical contact. The copper conductor strips employed in some of these prior transport systems have joints at close intervals. These joints cause excessive brush wear, arcing and poor electrical contact at high trolley speeds.
As this enclosed type of material transport system has gained wider acceptance, there has been an increasing demand for higher current capacities, increased transport speeds, and the incorporation of power and control conductors within the same compact housing. Reducing the size of the conductor housing makes it more difficult to employ large contact brushes on the trolley while maintaining acceptable separation between the various circuit components. In addition, the continuous copper conductor strips necessary to maintain good electrical contact at high trolley speeds are not perfectly flat over long distances. The typical continuous copper conductor has a slightly wavy outer surface over extended distances because of the thin profile of the conductor and the required loose fit of the conductor in the retaining slots within the housing. A contact brush having a long profile spanning the wavy irregularities in the surface of the conductor strip provides a reduced contact surface with the conductor strip, resulting in a reduction in current capacity.
In an effort to increase current capacity, some trolley systems employ conductor strips of increased thickness. Other approaches use multiple pick-up trolleys with their electrical leads connected in parallel. To achieve a continuous current of 100 amps, it is often necessary to connect four trolleys in parallel, or to connect as many as eight trolleys in parallel to provide 200 amps continuously which has become a common requirement. Connecting trolleys in this manner to increase current capacity is expensive, bulky, and inconvenient. In addition, the parallel connection of eight wires per conductor strip is subject to wiring error as well as overheating due to partial loss of contact.
The present invention addresses the aforementioned limitations of the prior art by providing a multi-conductor power bar system wherein conductor strips carrying input power and in some cases also control signals are disposed within an elongated, generally closed housing from which a trolley is suspended. The trolley employs multiple aligned conductor contact brushes for each power conductor strip and incorporates additional contact brushes for engaging the control signal conductor strips, where these are provided. The trolley includes various roller combinations for facilitating smooth transit of the trolley over joints in the housing at high speeds, while also preventing the trolley from lifting during acceleration and maintaining lateral alignment of the trolley within the housing.