Robot Parts (1/4)

I originally published this article under the title, “The Presentation of Parts for Robot Assembly” in the book “Advances in Manufacturing Technology”, Kogan Page, London, ISBN 1.85091.3951 ...

The presentation of parts for robot assembly involves the selection of the correct parts handling devices and it influences the robot degrees of freedom required. The design of appropriate feeders is discussed, with an emphasis on their flexibility.  A classification system is described that allows parts to be categorised by their design features and physical properties. The performance of an automatic parts feeder is shown to depend upon the design of the part that is being handled. A selection procedure is described that enables the correct handling device and robot configuration to be chosen for a particular application. An expert system is shown to be the best method of acquiring design information about the handle-ability of a part.  A software package that simplifies the selection of parts feeders and robot configurations is described. The importance of knowledge transfer between industrialists and researchers, in defining relevant handling devices, is discussed. The development of an enhanced CAD system is the subject of a further publication.

INTRODUCTION

The presentation of parts to a robot presents some of the most difficult problems in robot assembly. Single cell robot assembly systems may assemble a complete product consisting of several parts. These parts have to be presented to the robot at the correct rate and in a known orientation, or a limited number of known orientations. The rate of supply of parts to the robot cell is seldom a problem because cycle times are usually long. The orientation of the part, at the exit of the parts feeding device, is critical because this influences many other factors.  The orientation of a particular design of part at the feeder exit can be predicted using knowledge of handling device design. Parts are classified according to size, geometry, etc. so that feeding device performance can be qualified. Using a standard parts coding system, feeder performance can be matched with that required for a particular design of part. The orientation of the part, at the exit of the automatic feeder, can be predicted and the need for extra robot degrees of freedom can be determined. The presentation of parts for robot assembly is a complex problem and it’s best carried out using a software application.

PARTS PRESENTATION TECHNIQUES

A multitude of automatic feeders are available to handle a wide variety of parts. However, only a small proportion of these automatic feeders are economically viable for robot assembly.  For robot assembly, an automatic parts feeder must have a high general-purpose content and a low special-purpose content, so that the flexibility of the robot is not compromised by the inflexibility of its feeders. The vibratory linear feeder has a low cost special-purpose feed track that is mounted on a general-purpose drive unit and frame.  The device is very flexible because changeover is effected by removing the current feed track and replacing it with a feed track for the next part. The vibratory bowl feeder consists of medium cost special-purpose tooling that is mounted around the periphery of a general-purpose bowl. The feeder is generally inflexible and the time associated with part changeover makes it unsuitable for many applications with small batch sizes. The horizontal pallet transfer system has low cost special-purpose pallets that move into, and out of, the work zone by a general-purpose transfer system. Flexibility is achieved by using different pallet configurations or by simply changing the pallet contents. The 'Hitachi' type feeder works on a similar principle to the vibratory bowl feeder, with the special-purpose tooling being replaced by a vision system. Within certain geometrical and size limitations, this device is highly flexible; using a vision system to identify part orientations. The programmable belt feeder uses special-purpose pushers and gates, activated by a vision system or sensors, mounted above a general-purpose belt.  Product changeover is achieved by using a different vision system computer program or by replacing the pushers and gates.

Robot Parts (2/4)

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 Parts (3/4)

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.

Robot Parts (4/4)

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.