Russell Davison

PARTS CLASSIFICATION FOR FEEDING

It’s important to be able to classify or describe the features of a part so that particular part shapes can be identified. Firstly, a part can be classified according to it's basic shape, i.e. rotational or non-rotational. Each rotational or non-rotational part has a certain aspect ratio that allows it to be classified as being a disc, short cylinder, long cylinder, flat, long or cubic. Secondly, the amount of symmetry that a part possesses can be quantified. The amount of symmetry is determined by defining how often an orientation is repeated when the part is rotated through three mutually perpendicular axes. Thirdly, the amount of symmetry that a part possesses can be identified. The asymmetrical feature or features are those that cause the part not to have symmetry about an axis or axes. Fourthly, the bulk properties of a part can be identified to estimate the loss in performance of those feeders which deliver parts from bulk random orientation. Properties such as overlapping, tangling, nesting or stickiness reduce the feed rate and may even prevent feeding, depending upon the magnitude of the adverse property. Lastly, the physical properties of a part can preclude it from being handled by certain automatic feeders. Other properties, such as abrasiveness or a delicate surface finish, may cause problems with different feeder designs.

PERFORMANCE OF FEEDING DEVICES

Each robot assembly handling device has its own performance characteristics. A given device is able to handle a limited number of parts within a certain size range and geometry class. The orientation efficiency of a feeder, for parts with no adverse physical properties, is unimportant for robot assembly because the relatively long cycle time means that the demand rate for parts is low. The orientation efficiency for automatic feeders which sort out parts with adverse physical properties from bulk random orientation can be extremely low or zero if the adverse physical property is severe. Parts with severe adverse physical properties cannot be sorted from bulk random orientation and other methods of handling must be chosen. A typical solution to this problem is to present the part on a horizontal pallet transfer system. These handling devices are loaded manually or, preferably at the point of manufacture, using pick and place devices.

ROBOT ASSEMBLY HANDLING CHARTS

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.

ROBOT ASSEMBLY HANDLING EXPERT SYSTEM

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.

PRODUCT AND SYSTEM DESIGN FOR ROBOT ASSEMBLY SOFTWARE

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.

DATA ACQUISITION FOR ROBOT ASSEMBLY SEMINAR

A series of product design for robot assembly seminars were held at a UK university. They were well attended and the object of these seminars was to encourage industrialists to analyse their products, using a product design for robot assembly computer software application. The results of these studies were then investigated by university staff so that handling, gripping and insertion requirements for robot assembly could be recommended. These seminars were funded by the ACME directorate as a means of forging closer links between universities and industry. The results of the studies also gave direction to future research work at the university in the field of robot assembly. Interested parties were given a copy of a computer software application. Industrial product data was stored in standard ASCII files and this was easily manipulated by staff at the university. Statistics were produced that indicated trends in parts geometry and the physical properties of parts. These statistics showed the relative importance of various pieces of assembly automation for a cross-section of industrial products and they gave indicators for future assembly hardware development.

FUTURE WORK

I strongly believe that industry will only demand products and services if there is a genuine need for either. For this reason, my direction is heavily influenced by the continuously changing needs of my clients. Product information (available to clients) and calculated information (demanded by clients) is monitored through my consultancy contracts. My approach to the presentation of parts for robot assembly is changed to best suit the needs of the majority of my current clients and future clients. The results of the previously mentioned data acquisition seminars influenced the range of handling devices included in the database. It was necessary to include other devices to cater for particular categories of parts, that were thought to exist in smaller numbers than in reality. The findings also affected the handling expert system format. The sequence of questions was altered so that the minimum amount of information was required for the majority of parts. Later, a consortium of six companies was being formed to interface the product design for robot assembly software with a conventional CAD system. The object of this work was to allow a product designer, using a CAD system, to have the benefit of product design for assembly running in the background, which only became active when adverse robotic assembly properties were evident.

CONCLUSIONS

The presentation of parts is a topic often neglected by those considering robot assembly and yet it accounts for the majority of the cost for an installation. It is important to be able to describe the features of a part by the use of a parts classification technique that is sufficiently comprehensive to fully describe the part, without involving undue effort, or understanding, from the user. Parts presentation devices for robot assembly should have a high general-purpose content and a low special-purpose content. The orientation of the part during insertion affects the choice of handling device and the number of robot degrees of freedom. The classification of a part for handling can be a tedious process and it is important to only define features that are relevant for the selection of handling devices. This is best achieved by using an expert system approach and decision trees. The complex process of handling device selection can be carried out by computer software applications, thus eliminating the need to manually carry out many iterative calculations. The types of handling devices which best suit the needs of industry can be chosen by asking current and potential industrial users to specify their particular handling requirements. Most of the information relating to the design features of products, for robot assembly, can be extracted from a CAD system database.

I originally presented this article, "Design for Robot Assembly", as a guest speaker at the UK's 2nd National Conference on Production Research ...

SUMMARY

The design of products and systems for robot assembly requires a new approach to that used for manual and automatic assembly. Robot assembly is only effective if the robot’s flexibility is used to best advantage. Additionally, peripheral devices supporting the robot must also be adaptable to handle a wide variety of products and product parts. This is achieved by using equipment that is not designed specifically to handle a particular type of part with minor modifications to tooling, or the use of a different software application, the robot assembly system can be quickly adapted to assemble a different product or product style. By this method, robot assembly can be economically justifiable in many situations where it would otherwise have been precluded.

This article discusses the development of robot assembly systems and describes how product design plays an important role in the design of the equipment.

INTRODUCTION

There are three categories of system used in product assembly. These are manual assembly, automatic assembly and robot assembly. Whilst assembly can be classified in this manner, it is not uncommon to find an assembly system consisting of two or all of these groups to form a hybrid system. Manual assembly systems account for the majority of applications. Automatic systems are used in situations where the demand is high and there is no, or limited, change in the product styles being assembled. Robots have yet to make a significant impact in the field of assembly. It’s difficult to technically justify the use of assembly robots as the operation time of programmable devices is longer than that of dedicated automatic equipment. The economic justification of assembly by robots is equally difficult due to the characteristically small batch sizes for which these systems are appropriate. This has the effect of increasing the handling and insertion costs of the product being assembled.

Manual assembly is still used for more than ninety per cent of all assembly tasks. This is because many products are required in low volumes and with a high degree of variety. Robot assembly could account for more than fifty per cent of all assembly tasks if it could be made to be economic for much smaller annual production volumes. This could be achieved by assembling more than one family of products on one system. For this approach to be effective, two major conditions must be met. The proportion of re-usable, or general-purpose, equipment must be high and the time taken to re-configure the system for the assembly of the next product must be low.

Using existing technology, the industrial applications where robots can readily be used have been filled. These applications include paint spraying, spot welding and materials handling. Only a very small proportion of existing robots are used for assembly.

Robot assembly system equipment is either general-purpose or special-purpose. A robot assembly system should have a high proportion of general-purpose equipment and a low proportion of special-purpose equipment. The cost of a system, with a high proportion of general-purpose equipment, can be amortised by all the products that are being assembled by the robot. This is important when trying to economically justify the use of robot assembly for products required in low volumes. Under these conditions, many products or product styles, each with a low annual volume, can be grouped together and assembled on a single robot assembly station to obtain a high system utilisation.

THE HANDLING OF PARTS FOR ROBOT ASSEMBLY

Handling device selection for a particular part depends on the size and geometry of a part, as well as the rate at which the part is required. Each handling device has its own performance characteristics. This means that it is suitable for dealing with a limited range of parts. Small to medium sized parts, with features that can be seen in silhouette, can be handled by the most common of devices, the vibratory bowl feeder. Parts with no useful features for orientation purposes, or parts with adverse physical properties, are expensive to feed automatically and require special automatic feeding devices. These types of parts need to be re-designed to reduce their cost for automatic feeding. There are many properties of a part that would prevent it from being handled by vibratory bowl feeders, such as flexibility and stickiness. Parts with adverse properties such as these, and larger parts, must be handled by other feeding devices like magazine systems or pallet transfer systems.

The multi-part linear vibratory linear feeder can deliver different parts to a robot assembly station. It consists of two straight and parallel vibratory orientating tracks on a common drive unit. The rejected parts fall into return tracks and are brought to the start of the orientating tracks by a reciprocating elevator. The tracks can be CNC machined from a database of designs that are identified by an automated handling code for a particular part. Only the orientating tracks are replaced to changeover this multi-part feeder to handle other part types. The vibratory drive unit and reciprocating elevator are completely re-usable and the cost of these devices is divided between the different part types. The orientating track for this multi-part feeder is straight and it is much less expensive to produce than the curved orientating track of a vibratory bowl feeder. Applications of this feeder are limited to parts which require orientating devices simple enough to be produced in one set-up on a horizontal machining centre.

Gravity feed track magazines are simply short lengths of track which are loaded manually on-line or off-line. During off-line loading, a full magazine is substituted for a magazine when it becomes empty. These magazines are specifically designed for the particular type of part type and cannot easily be re-used for different types of parts. Although most of the gravity feed track magazine is special-purpose, the cost of these devices is relatively low. They are useful far feeding large parts and they provide an economic alternative to palletisation. Parts that are to be handled by this type of device must be stackable, for vertical magazines, and not susceptible to damage when the part is slid into position by the pusher.

The pallet transfer system consists of a walking beam transfer device to load a paternoster, an unload paternoster, and pallets. Full pallets are elevated by the load paternoster and transferred to the robot working zone by the walking beam transfer device. Parts are picked from the pallet and the pallets are then indexed to present a new pallet of parts to the robot. Empty pallets are offloaded from the walking beam by an unload paternoster that produces a stack of empty pallets. Virtually all of the pallet transfer system is general-purpose, with only the vacuum-formed part retainers being specific to a particular component. Pallets are loaded by standard means. Filling of the pallets at the point of manufacture is a very economic way of loading parts, although the cycle time of most manufacturing operations makes it difficult to use this method of loading. Parts are positively held in position on the pallet by ensuring that they are sandwiched between the underside of one pallet and the top of the one beneath.