Sunday, 25 June 2006

Robot Parts (2/4)


It’s important to be able to classify or describe the features of a part so that particular part shapes can be identified.  Firstly, a part can be classified according to it's basic shape, i.e. rotational or non-rotational. Each rotational or non-rotational part has a certain aspect ratio that allows it to be classified as being a disc, short cylinder, long cylinder, flat, long or cubic. Secondly, the amount of symmetry that a part possesses can be quantified. The amount of symmetry is determined by defining how often an orientation is repeated when the part is rotated through three mutually perpendicular axes. Thirdly, the amount of symmetry that a part possesses can be identified. The asymmetrical feature or features are those that cause the part not to have symmetry about an axis or axes.  Fourthly, the bulk properties of a part can be identified to estimate the loss in performance of those feeders which deliver parts from bulk random orientation.  Properties such as overlapping, tangling, nesting or stickiness reduce the feed rate and may even prevent feeding, depending upon the magnitude of the adverse property.  Lastly, the physical properties of a part can preclude it from being handled by certain automatic feeders.  Other properties, such as abrasiveness or a delicate surface finish, may cause problems with different feeder designs.


Each robot assembly handling device has its own performance characteristics. A given device is able to handle a limited number of parts within a certain size range and geometry class.  The orientation efficiency of a feeder, for parts with no adverse physical properties, is unimportant for robot assembly because the relatively long cycle time means that the demand rate for parts is low. The orientation efficiency for automatic feeders which sort out parts with adverse physical properties from bulk random orientation can be extremely low or zero if the adverse physical property is severe. Parts with severe adverse physical properties cannot be sorted from bulk random orientation and other methods of handling must be chosen. A typical solution to this problem is to present the part on a horizontal pallet transfer system. These handling devices are loaded manually or, preferably at the point of manufacture, using pick and place devices.

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