Monday, 19 June 2006

Robot Assembly (4/4)


Three factors determine how easy it is to use an assembly robot for a product.  Each product part should be examined with respect to these three important qualities. In order of priority, they are the; necessity of the part to be separate from those which have already been assembled; ease with which the part can be handled, and the ease with which the part can be inserted.  By considering these factors in turn, the most economical design of product can be chosen for robot assembly.  A measure of the assemble-ability of the product is the 'design efficiency', and this is related to the above factors.

A part is considered to be necessarily separate from those previously assembled if one of four conditions apply to the part.  Otherwise, it can be eliminated.  Firstly, if the part or sub-assembly moves relative to its mating part during the normal function of the final assembly then it must be a separate part.  Secondly, if the part or sub-assembly must be of a different material than its mating part (eg. for insulation, vibration damping) then it must be a separate part.  Thirdly, if disassembly of the part or sub-assembly must be allowed for (e.g. servicing requirements, recycling) then it must be a separate part.  Finally, if the part or sub-assembly, when combined with it’s mating part, would prevent the assembly of other separate parts (except where the part's only function is to fasten) then it must be a separate part.

The majority of insertion processes take place along, or about, the vertical axis.  If the action of insertion for a part is not in the vertical axis then the process should be analysed to see if the more complex insertion path is really necessary.  If possible, it should be re-designed to take place in only one axis.  The vertical axis is always the preferred axis because the weight the part acts in this direction and assists, not hinders, the operation.  The robot cost is lower if insertion processes are kept simple.  This is because complex operations need more robot degrees of freedom and each degree of freedom requires an individual pneumatic, hydraulic or DC servo motor which increases the cost of the equipment.  Additionally, the potential profitability of the equipment is reduced because the cycle time of the operation will also be increased.


The use ofassembly robots will increase in the future if the ancillary equipment, i.e. end effectors and parts feeders, are as flexible as the robot.  The feeding devices should present the parts in a known orientation so that the dexterity required from the robot is low.  The cycle time of the operation would be lowered and, consequently, the assembly rate increased.  The flexibility of the feeders is ensured by using devices with a low special-purpose content.  An indexing turret, used for gripper mounting, minimizes the time lost due to gripper changing.  For any form of gripper mounting, the cycle time can be minimised by using a sequence of assembly which needs the least number of gripper changes.  Operator involvement can be minimised by developing strategies which allow the robot to recover from error situations, without the assistance of manual labour.  The cost of robot assembly can be minimised by designing the product for robot assembly.  This involves using the minimum number of parts and ensuring that the parts can be easily handled and inserted.

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