Saturday, 24 June 2006

Robot Parts (3/4)


The required attitude of a part, on insertion, influences the choice of handling device and it also affects the number of robot degrees of freedom required.  A particular feeding device, if it can handle the part under consideration, may be able to present a part in only one unique orientation or it may be able to present the part in a number of unique orientations. The orientation(s) of the part at the feeder exit are determined by considering the design of orientation tooling that is required.  For vision system controlled feeders, knowledge is required of whether or not the part's orientation can be deduced by the vision system. If the attitude of the part at the feeder exit is the same as that required for insertion then a minimum number of degrees of freedom are required from the robot arm.  If the attitude of the part at the feeder exit is different from that required for insertion then extra degrees of freedom are required. Parts which need to be re-orientated from the horizontal to vertical position require an extra roll or pitch axis and parts which are required to be turned end-to-end need an extra yaw axis. Additionally, certain parts may require that final orientation from the feeder is accomplished using a robot with limited sensory capability to define the orientation.  This is applicable to feeders which present the part in a limited number of known orientations. This knowledge can be collated to form a database from which it is possible to predict handling and dexterity requirements for the robot assembly system.  Various organisations have created database software applications for this design process.


It must be possible to describe a part being analysed so that the most appropriate feeding device can be selected.  A standard parts coding system is used to describe a part, as mentioned previously.  The sequence of questions which are asked to describe the part is very important. The response to certain questions may create a need for further questions to fully describe the part.  Alternatively, no further questions may be required. Additionally, a particular response to a question may dictate that only one handling device is appropriate, even before the part has been fully classified. Anybody using the 'selection of parts presentation device technique doesn’t want to be asked a lot of irrelevant questions and so a decision tree has to be developed to ask the minimum number of questions. Statements are presented in a structured format and these statements can be either true or untrue for a particular part. Branching forward only takes place when a particular statement is true, otherwise alternative questions are presented until a correct statement is chosen.  Questions are structured so that if a particular set of statements are untrue then the previous true response to a statement must have been incorrect and that statement is once again presented to the user. By this method, the minimum number of questions are needed to classify a part in terms of its handling suitability.


The presentation of parts for robot assembly is one section of a product and system design for robot assembly computer software application. It operates on eight screen pages. The first screen page allows the user to enter part numbers and descriptions to the application. The last three screen pages contain economic information and they provide the user with calculated information. The middle four screen pages are all concerned with defining the handling, and to some extent the insertion, requirements of the part under consideration. These four screen pages are displayed consecutively for each part and, when all the parts have been defined, the remaining three screen pages are displayed. In the handling section, the first screen page deals with adverse physical properties of the part. The second screen page deals with the geometrical symmetry features of the part. The third screen page deals with the geometrical asymmetry features of the part. The fourth screen page is used to define the insertion direction of the part and to determine if the part is potentially redundant.

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