Monday, 26 June 2006

Robot Parts (1/4)

I originally published this article under the title, “The Presentation of Parts for Robot Assembly” in the book “Advances in Manufacturing Technology”, Kogan Page, London, ISBN 1.85091.3951 ...

The presentation of parts for robot assembly involves the selection of the correct parts handling devices and it influences the robot degrees of freedom required. The design of appropriate feeders is discussed, with an emphasis on their flexibility.  A classification system is described that allows parts to be categorised by their design features and physical properties. The performance of an automatic parts feeder is shown to depend upon the design of the part that is being handled. A selection procedure is described that enables the correct handling device and robot configuration to be chosen for a particular application. An expert system is shown to be the best method of acquiring design information about the handle-ability of a part.  A software package that simplifies the selection of parts feeders and robot configurations is described. The importance of knowledge transfer between industrialists and researchers, in defining relevant handling devices, is discussed. The development of an enhanced CAD system is the subject of a further publication.


The presentation of parts to a robot presents some of the most difficult problems in robot assembly. Single cell robot assembly systems may assemble a complete product consisting of several parts. These parts have to be presented to the robot at the correct rate and in a known orientation, or a limited number of known orientations. The rate of supply of parts to the robot cell is seldom a problem because cycle times are usually long. The orientation of the part, at the exit of the parts feeding device, is critical because this influences many other factors.  The orientation of a particular design of part at the feeder exit can be predicted using knowledge of handling device design. Parts are classified according to size, geometry, etc. so that feeding device performance can be qualified. Using a standard parts coding system, feeder performance can be matched with that required for a particular design of part. The orientation of the part, at the exit of the automatic feeder, can be predicted and the need for extra robot degrees of freedom can be determined. The presentation of parts for robot assembly is a complex problem and it’s best carried out using a software application.


A multitude of automatic feeders are available to handle a wide variety of parts. However, only a small proportion of these automatic feeders are economically viable for robot assembly.  For robot assembly, an automatic parts feeder must have a high general-purpose content and a low special-purpose content, so that the flexibility of the robot is not compromised by the inflexibility of its feeders. The vibratory linear feeder has a low cost special-purpose feed track that is mounted on a general-purpose drive unit and frame.  The device is very flexible because changeover is effected by removing the current feed track and replacing it with a feed track for the next part. The vibratory bowl feeder consists of medium cost special-purpose tooling that is mounted around the periphery of a general-purpose bowl. The feeder is generally inflexible and the time associated with part changeover makes it unsuitable for many applications with small batch sizes. The horizontal pallet transfer system has low cost special-purpose pallets that move into, and out of, the work zone by a general-purpose transfer system. Flexibility is achieved by using different pallet configurations or by simply changing the pallet contents. The 'Hitachi' type feeder works on a similar principle to the vibratory bowl feeder, with the special-purpose tooling being replaced by a vision system. Within certain geometrical and size limitations, this device is highly flexible; using a vision system to identify part orientations. The programmable belt feeder uses special-purpose pushers and gates, activated by a vision system or sensors, mounted above a general-purpose belt.  Product changeover is achieved by using a different vision system computer program or by replacing the pushers and gates.

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