

THE INSERTION OF PARTS FOR ROBOT ASSEMBLY
An insertion operation is defined as being the action whereby a part is added to a work fixture, another part, or part-built assembly. This may involve a simple vertical downwards motion where the part is added to the part-built assembly, without being immediately secured. Alternatively, it may be a complex motion, such as that required for the application of an adhesive to a part. Each insertion process may require a different type of end effector and each process takes a certain amount of time to be executed. It’s possible to categorise each type of insertion process to define the type of end effector required and to estimate the time it would take to carry out the operation.
The end effectors may be accessed by the robot arm in many ways. The design of end effector, and the method of mounting it onto the arm, influences both the cycle time of the process and the cost associated with the insertion of a particular part. The simplest, yet most expensive and time consuming, method of accessing an end effector, is to use an
individual gripper, or tool, for each part or insertion process. The grippers and screwdrivers are stored in a rack within the work envelope of the robot arm. The relevant tool is picked from the rack, used for the insertion process, and then returned. The action of picking up the tool, and returning it, can often take longer than the insertion process itself.
Another method of inserting many different designs of parts is to use a multi-functional gripper. Only one gripper with a multitude of faces is used, for the internal or external gripping of parts. The time involved with gripper changing is eliminated, but the design of the gripper is complex and other tools cannot be mounted onto the same unit. Problems may also occur because only one set of jaws is being used for the insertion of many parts. The gripper designer has to ensure that the gripping force is sufficient to hold the part and yet not too excessive as to cause damage to the part. The varying force requirements can be met by additional gripper sensing. This, of course, increases the cost of this design of end effector.
The most efficient method of accessing a multitude of end effectors is to mount them onto an indexing turret. Between eight and twelve tools can be housed on one unit, depending on their size. Grippers, screwdrivers and othertools are mounted in a circle. This may be about a vertical, horizontal or inclined axis. The use of universal mounting plates, between the turret and the end effectors, allows interchange-ability of grippers and tools for product changeover. The time lost, due to gripper changing, is minimised because indexing of the turret occurs between movements to, and from, the parts feeders.
Most products, or sub-assemblies, have many possible sequences of assembly and it is important to recognise the most appropriate sequence, particularly in robot assembly. In all forms of line assembly, where moving work carriers are employed, it is good practice to secure parts as soon as possible because subsequent work carrier movements may cause a part to be displaced. This suggests certain precedences. If no movement of the part-built product occurs during assembly then the securing of parts is not important and a sequence of assembly can be chosen which involves a minimum number of gripper changes.
Consideration also has to be given to the appropriate action needed when a malfunction occurs. The decision to scrap, rectify or dismantle depends on the; value of the part-built assembly, frequency of the malfunction, labour cost and sequence of assembly. In single station robot assembly, an overriding consideration is the cost of gripper changing. The
optimal sequencing, linked with appropriate product design, can significantly reduce this cost. Computer software applications are available which, given the precedence constraints, identify the optimal sequence to minimise gripper changes. The cost of error recovery is important. The alternative actions need to be examined at each stage in the assembly build and the cost of these actions should be determined for all possible sequences. This activity is influenced by the chosen criteria of; minimum cost, maximum production or maximum profit.
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