Russell Davison

THE ORIENTATION OF PARTS FOR ROBOT ASSEMBLY

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.

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 other tools 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.

PRODUCT DESIGN FOR ROBOT ASSEMBLY

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.

CONCLUSIONS

The use of assembly 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.

I originally presented this article, "The Design of Hybrid Flexible Assembly Systems", as a guest speaker at the 6th International Conference on Assembly Automation ...

There is a requirement for a special kind of system to assemble products required in modest volumes with a degree of variety. A system which is as cost effective and efficient as hard automation, whilst providing the flexibility of manual assembly, is called a flexible assembly system. Within such a system, certain product parts may be required at a different rate to other parts. Some operations may require the flexibility and dexterity of a robot, or even manual labour. The resultant system would be a hybrid of many methods of assembly. This article recommends a technique to be used for the design of such a system, with the aid of a case study.

INTRODUCTION

The factory cost of a product is the addition of the manufacturing cost (e.g. casting, moulding, turning) and the assembly cost (e.g. manual, automatic, robotic). Industrial engineers continually seek new methods to reduce the factory cost of products. The current trend of exploiting cheap labour in developing nations, through “offshoring” creates a challenge for domestic manufacturers in the developed nations. Between 40 and 60 percent of the factory cost for many products is associated with the labour content. The majority of this cost is incurred during assembly. There are three reasons for this uneven split between labour costs in manufacturing and assembly.

(i) Manufacturing operations are usually done by, or with the aid of, a machine, i.e. turning, milling, drilling, etc. The manufacturing systems designer does not have the wide choice of the assembly systems designer because some degree of mechanisation must be used. It is then a logical extension to further automate the manufacturing process to reduce labour costs.

(ii) New processes have been developed which eliminate many manufacturing operations. Powder metallurgy is an example of such a process.

(iii) Most products are designed to be assembled manually. This often means that components are of such a design that they cannot be handled by automatic feeders. Additionally, many assembly insertion operations are too complex to be automated.

The assembly process is one of the last production processes to be successfully automated by the industrial engineer. However, as much of the factory cost of a product is incurred during assembly, it is this area where great productivity improvements can be made. The design of the assembly system should be undertaken with due consideration of the design of the manufacturing system and of the design of the product. The design of the assembly system, manufacturing system and product should be considered integrally. These three components, when combined, should create a product having the lowest factory cost at the desired level of quality. The design of a product and it’s associated production system is an iterative process, whereby product design features dictate the design of the production system and the capabilities of the production system determine the product design. The extent to which these actions can be carried out is only limited by the commitment of a manufacturer to a particular production system and product design.

A company may be already committed to a certain manufacturing system if there is prvious investment in capital equipment and tooling. Additionally, the external dimensions, performance or appearance of the product may be unchangeable. If a product is only part of a much larger assembly, the effect of changing a critical dimension may have expensive consequences for the rest of the much larger assembly. The performance of the re-designed product must be as good as, if not better than, the original design. The product may be one where visual appearance plays an important part in it’s acceptability in the market place. All of these factors place limitations on the engineer being able to specify the optimum product design and production system for that design.

It is easier to design the most economic assembly system for a product prior to commercial manufacture. In this case, there won’t be an inherited investment in manufacturing equipment or tooling, and the product design won’t have been finalised. If the product is well established, and has been produced for many years, the assembly systems engineer may be limited to a re-design of the assembly system alone. This is because a re-designed product may require expensive design modifications to the tooling used for the manufacture of the product parts. In these situations, a hybrid assembly system is required to meet the product requirements. A hybrid assembly system uses a mixture of methods during assembly of the product.

THE COMPONENTS OF A HYBRID FLEXIBLE ASSEMBLY SYSTEM

There are six methods of assembly and the simplest form is MANUAL ASSEMBLY. For high volume production, the operatives usually work on an assembly line. Other forms of manual assembly are a single worker assembling a complete product and groups of workers assembling a portion of the product.

For a more limited product range, a MANUAL ASSISTED method may be used, whereby workers are assisted by mechanical devices, such as automated parts feeders. The feeders present the parts to the worker in an ordered manner and the assembly time is reduced by eliminating the time taken to separate the parts from bulk random orientation. The reduction in assembly time is the basis for the economic justification of these devices.

The third form of assembly uses AUTOMATIC INDEXING assembly machines. These are rotary or in-line systems with a number of workstations. Automatic feeders supply components to workheads and they assemble the part to the fixture or part-built assembly. The workstations are ‘special-purpose’ and are dedicated to the assembly of only one product. Production volumes need to be high for the economic justification of these machines. Component quality must also be high to avoid excessive downtime caused by components jamming, etc.

The efficiency of an AUTOMATIC FREE-FLOW assembly machine is less dependent upon component quality. Transfer of work pieces between workstations is non-synchronous. There are small buffer stocks between each workstation and other workstations may operate whilst one is stopped due to a fault caused by, for example, a defective part.

The AUTOMATIC PROGRAMMABLE assembly machine has a non-synchronous transfer line and programmable workstations to assemble the parts, which are presented to the workheads by automatic feeders or, in the case of difficult components, part magazines may be used. The workheads execute one, or a number of, operation(s). Different computer programs, for each series of assembly processes, give the flexibility to assemble a variety of product styles on one assembly machine.

Robotic assembly is used for the assembly of products with large product variety, required in low volumes. Assembly operations are carried out by a robot which, itself, transfers the completed product onto the next operation.

THE DESIGN OF HYBRID FLEXIBLE ASSEMBLY SYSTEMS

The assembly process has two constituent parts and these are; the handling of components and the insertion of components. The design features of a part must be examined to decide if it can be automatically handled automatically or if it must be handled manually or placed in magazines. Similarly, the insertion process must be analysed to decide what type of workhead is required.

Various organisations have developed procedures that help the designer to estimate how easy it is to handle and orientate components by assigning a handling code to each part. The maximum feed rate and relative cost of the feeding method can then be estimated from this code. The parts which would require expensive automatic feeders or which could not be fed at the required feed rate can be identified. These parts must then be handled manually or in magazines/pallets. Additionally, certain parts cannot be handled automatically because they have other bad feeding qualities, e.g. they may be flexible or too light. The previously mentioned estimation systems also help the system designer to forecast the relative cost of the workhead required to insert a part into a part-built assembly. Those operations which require a complex path of insertion, or a large thrust, require more expensive workheads than for simpler operations. A list of parts (with their associated automated handling codes) and a list of operations (with their allocated automatic insertion codes) can be constructed from the preceding information.

If the product parts are listed in order of increasing handling difficulty levels then the most economical method of feeding a part to the workhead can be determined. Parts with low handling difficulty levels are fed by conventional vibratory feeders and, as the difficulty level increases, specially designed feeders/magazines/pallets/manual handling are used. The relationship between the handling difficulty level and the type of feeder to be used depends upon the required return on investment for the equipment.

The insertion operations can also be listed in order of insertion difficulty levels to determine the most economical method of insertion of a part into a part-built assembly. Greater difficulty levels can mean that the equipment is more expensive and, for assembly robots, more degrees of freedom are required for an insertion operation. If the difficulty level is too high then it’s necessary to employ manual workers for some operations.

When an assembly system is designed for a new product, the cost of parts handling and insertion can be reduced through re-design of the product. It’s usually not viable for an existing product to be re-designed, because of the tooling modification cost in the manufacture of the parts. Inevitably, therefore, the most economical method of assembly is limited to the existing product design, without design efficiency improvements.

The assembly handling and insertion codes determine which feeding method and insertion device are most appropriate for each part and operation. The part-built assembly has to be transported to each workstation between operations. This will either be synchronous or non-synchronous motion. Synchronous machines are generally less expensive than non-synchronous types, but they are limited by how many parts can be assembled on one machine. This is due to downtime and the space available.

It is desirable to construct a product from as many sub-assemblies as possible to achieve a high overall efficiency of the assembly system. These sub-assemblies should be common to all product styles, within the family of products. The variety can then be created in the final assembly of the product. If this approach is adopted then sub-assemblies will be required at a rate which is enough to justify the use of automatic indexing machines having dedicated workheads. The output from these machines can then be sent to the final assembly line via free transfer lines, to create a buffer stock of sub-assemblies. The buffer stock is necessary to minimise the effect of any indexing machine downtime.