Tuesday, 20 June 2006

Robot Assembly (3/4)


An assembly robot will never have the dexterity or intelligence of its human counterpart.  A manual worker is able to pick a part, in random orientation, from a storage bin and orientate it - ready for insertion.  The human senses of sight and touch are used for this purpose.  Whilst a manual worker can perform these tasks with comparative ease, a robot requires a large amount of computer processing power and many feedback devices to achieve any form of intelligence and, even then, the cycle time of the operation is so long as to make it uneconomic to use robots for bin picking.  Automatic feeders for robot assembly must, therefore, present parts to the workhead in a known orientation, or in a limited number of known orientations.  The attitude of the part on the feed track or pallet influences the number of robot degrees of freedom required.  More degrees of freedom are required for those parts which are inserted in a different attitude to which they are presented.  In the case of parts which cannot be presented in one known orientation, the final orientation must be carried out by a robot with extended capabilities.  This involves sensing and part manipulation, to achieve the required insertion orientation.

The robot work envelope poses limitations on the automatic feeder types that can be used in robot assembly.  Each part is presented to the workhhead at the end of a track or on a pallet.  The space occupied by the material of these devices must also be considered when determining the maximum number of parts that can be fed to any one robot.  Robots that can only access parts in the vertical axis must have parts arranged so that they can all be seen in plan view at the part presentation points.  Another problem arises when turret mounted grippers are used.  The grippers can occupy a large volume in space and this makes the avoidance of collision very important.  This situation can be investigated before the design of the robot assembly system is finalised.  The complete assembly process can be studied using computer simulation and there are many three-dimensional graphic simulation packages available that can identify if a collision is likely to occur.


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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