Design method of robot kitting sytem for flexible assemble

Robotics and Autonomous Systems 8 (1991) 255-273 North-Holland

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Design method of robot kitting system for flexible assembly K i n y a T a m a k i a n d S h i m o n Y. N o f School of Industrial Engineering, Purdue University, West Lafayette, IN 47907, USA

Abstract Tamaki, K. and Nof, Sh.Y., Design method of robot kitting system for flexible assembly, Robotics and Autonomous Systems, 8 (1991) 255-273. This article describes the design method of robotic kitting systems, as an alternative parts entry process, which contribute to reduction of overall assembly costs. Utilizing robots rather than manual work to perform the parts kitting operation can, in certain cases, improve productivity, flexibility, and part flow control. The specific objectives of this article are to: (1) propose a design method for robotic kitting system, (2) develop design factors and criteria for planning robotic kitting system in accordance with individual steps of the design method, (3) describe the plan for robotic bin-picking workstation involved in a kitting system, and (4) explore robot hardware components (end-effector, manipulator, locomotion, and sensors) suitable for automated kitting systems, and (5) analyze the several configurations of robotic kitting systems using simulation tools.

Keywords: Robot kitting; Assembly automation; Design method of robotic kitting system; Design factors and criteria; Bin and kit.

1. Introduction

The method selected for the flow of parts into an automated assembly station can have a major influence on the resulting station cycle-times, total throughput, and overall productivity of the final assembly operation. This article describes the design method of robotic kitting systems, as an alternative parts entry process, which contribute to reduction of overall assembly costs. Setup cost constitutes a large proportion of the cost of production. Just-in-time (JIT) philosophy regards machine setup time as a major source of waste that can easily be reduced. A useful concept concerning setup time is the separation of internal setup from external setup. The object is to convert as much of the internal setup as possible to the external setup. Consequently, more attention must be paid to kitting and parts pre-

Correspondence to: K. Tamaki, System Science Institute, Waseda University, 3-4-1 Okubo, Shinjuku-ku, Tokyo, Japan.

sentation [12]. The process of manual kitting has already been applied for many years in a variety of industries. Based on a nationwide kitting sur-

Kinya Tamaki received his B.Sc. in Industrial and Management Engineering from the Institute of Musashi Technology, Japan, in 1981. He was granted his M.Sc. and Ph.D. in Industrial and Management Engineering in 1983 and 1989, respectively, from Waseda University, Japan. He was engaged in the WASCOR(WASeda C___QOnstruction Robot) Research Project from 1983-to 1989. The WASCOR research project for developing building construction robots was sponsored by the System Science Institute of Waseda University and 11 Japanese corporations. He is a Post Doctoral Researcher in the School of Industrial Engineering at Purdue University since 1989. His research interests include robot system design and its task description, design and evaluation of robot configurations, robot application (to manufacturing and construction fields), and research and development of production system design technology. Dr. Tamaki is a member of the Japan Industrial Management Association, the Robotics Society of Japan, the Japan Society of Mechanical Engineerings, and the Japan Society for Artificial Intelligence. He is the recipient of the Research Encouragement Prize (1987), Robotics Society of Japan, and of the Article Encouragement Prize (1989), Japan Industrial Management Association.

0921-8890/92/$05.00 © 1992 - Elsevier Science Publishers B.V. All rights reserved

K. Tamaki, Sh.Y. Nof

256 Table 1 Top ten design factors of national kitting system [21] 1. 2. 3. 4. 5. 6. 7. 8. 9. 10.

Reduces in-process inventory Improves control of parts flow Responds more effectively to product design changes Integrates storage and assembly Reduces direct labor expenses Able to handle small batch, high product mix Improves assembly efficiency Reduces cycle times at assembly stations Detects low quality parts and shortages prior to assembly Eliminates left over parts after assembly

vey of 15 industries including automotive, computers, electronics, and consumer products [21], the top ten decision factors to introduce kitting systems are illustrated in Table 1. The study of robotic kitting has shown that delivering all necessary parts for a single assembly in pre-oriented positions for easy grasping by an assembly robot in the form of a kit can provide the necessary line stock availability, line scheduling flexibility, and reduced station cycle time productivity for competitive results [22]. Also, the use of robots to prepare the kits themselves completes the automated link between part stores and final assembly. The critical design problem associated with designing robotic kitting systems concerns the process of selecting and integrating appropriate automated devices. These devices have to provide the most efficient and effective method for controlling flow and sequencing of multiple components between bins and kits. The specific objec-

Shimon Y. Nof is a Professor of In-

dustrial Engineering at Purdue University. His primary teaching and research interests are in design and control of intelligent production systems, industrial robotics and information systems. His current research projects involve interactive robotics and planners of cooperative work. He has also studied knowledge-based scheduling and manufacturing decision support systems. Dr. Nof is the editor of the Handbook of Industrial Robotics (Wiley 1985)/, Robotics and Material Flow (NorthHolland, 1986), consulting editor of Wiley's International Encyclopedia of Robotics (1988), and co-editor (with C.L. Moodie) of Advanced Information Technologies for Industrial Material Flow (Springer-Verlag, 1989).

tives of this article are to: (1) propose a design method for robotic kitting system (in Section 2); (2) develop design factors and criteria for planning robotic kitting system in accordance with individual steps of the design method (in Section 3); (3) describe the plan for robotic bin-picking workstation involved in a kitting system (in Section 4); (4) explore robot hardware components including end-effector, manipulator, sensors, and locomotion suitable for automated kitting systems (in Section 5); and (5) analyze the several configurations of robotic systems using simulation tools (in Section 6).

2. Overview of design method for robotic kitting system In this section, three issues are in relation to the design method for robotic kitting system. First, in terms of kitting as a parts entry technique, the technical terms of the bin and kit are defined. Then advantages as well as disadvantages associated with kitting techniques are listed. Second, major issues of kitting system design are discussed. Third, the design process itself is presented. The design factors and criteria are developed in accordance with each step of the proposed process. 2.1. Bin and kit parts entry technique Parts, components, and fasteners required in the final assembly operation can be transported using at least seven different methods as presented in Fig. 1. The container types presented can be classified in two primary categories: the bin and the kit. By definition, a bin represents a generic container of a single part type which is stored and transported as a single unit load. In conventional assembly operations, several bins are usually transported to a given assembly station in order to supply a bulk of components to the assembly operator to fill several orders. The concept of supplying a bin of identical parts to the assembly workstation is still highly successful for large batch production operations which do not require frequent change-overs of available line stock. Kitting, in contrast to bulk part handling, is defined as the grouping of various components,

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which make up a single assembly or subassembly, from several different bins into a standardized container (kit tray) for in-plant or inter-plant transfer between operations. The general flow of

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steps need to occur such as: (1) outside packaging materials need to be removed, (2) the lid or side panel of the shipping container needs to be removed, (3) inside packing materials which protect parts from tangling during shipment need to be removed, (4) parts need to be individually extracted, or in small batches for part feeders, registered for orientation and re-oriented for insertion. Each of these tasks could take a considerably long time if performed for each part and fastener at the final assembly station. The kitting process could be defined as a type of pre-assembly operation or stage where the tasks of acquiring, organizing, and pre-orientated parts can be consolidated in order to reduce the resultant cycle-time in the final assembly cell. This should usually increase throughput and improve overall assembly productivity. By consolidating these tasks in an off-line kitting facility, some advantages might be realized if the equipment used for part acquisition and registration could also be consolidated for use with multiple kit types. The consolidation of equipment in kitting station and the transportation of pre-oriented parts to final assembly could reduce or eliminate the need for on-line parts registration devices, thus justifying greater cost savings for the entire automation project. These benefits are just a few of the advantages that kitting parts off-line could provide in an automated assembly environment. Several other advantages as well as disadvantages associated with bin and kit parts presentation techniques are listed in Table 2.

2.2. Kitting design issues and scope of this research In many modern automated flexible assembly systems, the concept of kitting has been extended to include not only the component parts but the tools and fixtures. In terms of the component parts, Fig. 3 shows the alternative flow strategies of feeding and bin picking systems. The kitting flow strategies are closely integrated with the material-handling function. The kitting system must interface with the material distribution system as well as the warehouse and production systems [4]. Although a variety of issues has to be considered to build a kitting system, this article focuses primarily on the design of robotic kitting workstation for the component parts except the part feeding system.

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When designing kitting systems, these following issues become critical design factors which require careful analysis relative to the specific needs of assembly: the scheduling of kit orders, the part families involved in a kit, the location and configuration of kitting workstation, the recycling of kit containers, the logistics for bin and kit handling, the equipment selection, and the control and management of kitting information.

2.3. Design process for robotic kitting system In Fig. 4 the design process for planning the robot kitting system is proposed. The design factors and criteria which have to be considered when designing the kitting system are developed in accordance with individual steps of the proposed design process. The design factors are described in detail in Section 3. The design criteria are suggested in Table 3 as the goals to establish effective robotic kitting systems. In STEP 1 of the design process, a particular master production schedule, which specifies the independent demand for final-level product items, has to be developed. It might require greater production capacity than is available in the plant. Therefore, it is essential to know before releasing a production plan to the shop floor if it is a workable plan with regard to capacity.

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The production planning process results in a schedule for job release to the shop floor. Information about the product held in the bill of material file enables the production planner to transform job release information into a schedule of kit release. The information on each kit includes a 'pick list', that is, the total quantity of each component part, locations of these parts in the warehouse, the work center destination, and of course, the scheduled release date [12]. When inputting parts and bins to the kitting system in STEP 2, parts commonality and appropriate bin (and kit) container design have to be considered. Parts commonality enhances system flexibility and leads to a reduction in number of component parts. The bin and kit container design must be standardized in view of operationability of robotic kitting tasks. In terms of identifying the bin and kit tray, when bins are

stored in storage facilities such A S / R S , miniload, and carousel, each bin has a unique code number. The use of bar code labels on each bin (or kit) tray and the use of bar code readers at strategic points of entry can help to identify the bin (or kit) type and correctly route the bins (or kits) to the destination point. With regard to STEP 3, there are three primary locations of kitting station: (1) off-stores staging area, (2) workstation integrated with the main bin storage facility, and (3) in-transit staging area. Four examples of robotic kitting systems in the main bin storage facility and in-transit area are illustrated in Fig. 7. In this figure the kitting systems (a) to (c) are in the main storage facility, while system (d) is in the in-transit staging area. The logistics associated with robot kitting systems would involve the interconnection of several transport devices in order to: (1) carry part bins

Table 3 Design criteria in accordance with design process Scheduling of kits order

1. Flexibility in work scheduling: production change-over. 2. Ability of software and device to re-initialize in case of system failure.

Parts inputs to kitting system

1. High throughout of kit orders per hour. 2. Minimum of fixed work-in-progress (numbers of total kits in system - at workstations and idle queues). 3. Minimum average time in system (average time a kit order spends in total system including queue time). 4. High quality rate (percentage of correct kits in all orders). 5. Minimize any time delay between kitting task or indexing of parts bins. 6. Tight consolidation and easy control of part flow. 7. System modularity - a ability to function independently of human interface.

Kitting location and system location

1. Accessibility to parts and bins in storage facilities. 2. Configuration expandability for future production needs.

Bin transportation

1. Minimize or eliminate the bin-handling function. 2. Minimize transportation distance between storage facility and kitting workstation. 3. Minimize loading and unloading numbers.

Kit processing in robotic workstation

1. 2. 3. 4. 5. 6. 7.

Completed kit transportation

1. Appropriate load and unload station.

Recycling kit container

1. Maintenance of kit container.

High robot picking and part handling utilization. Robot's accessibility to bins and trays within the work envelope. Robot's ability to locate, grasp, and orient parts safely from bins. Utilize multipurpose gripper. Consider weight of parts and load stress on robot manipulator. Minimize depth of reach inside bin for robotic part acquisition. Minimize idle queue before inputting the workstation.

Robot kitting system

from storage (STEP 4), (2) transfer individual parts from bin(s) to kit tray(s) in the robotic workstation (STEP 5), (3) deliver completed kits to final assembly station (STEP 6), and (4) recycle back bin and empty kit containers (STEP 7). STEP 5 focuses on the design of the robotic kitting workstation, which is considered as a key item in the implementation of a robotic kitting system. The control of the robotic kitting workstation is discussed in Section 4.3. After the bins arrive into the robot kitting workstation, the robot

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begins to insert the correct quantity and types of parts into kit trays. In order to build the robot kitting workstation, bin arrival strategies and kit processing style have to be predetermined. The bin arrival strategies to the kitting workstation are classified as either: sequential or simultaneous bin arrival. In terms of the kit processing, it can be done either as single or as group kit processing. The hardware of a bin-picking robot includes end-effector, manipulator, vision and other sen-

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sors, and locomotion. The subject of designing hardware components of the bin-picking robot suitable for a kitting workstation is discussed in Section 5. In order to design the robot hardware to meet the variety of requirements in bin-picking operations, task description may be useful (see Fig. 5) [25]. A methodology for selecting robot manipulator configurations, following the data of the task description, has been developed by Tamaki [23]. As shown in Fig. 6 an application of the selection methodology to bin-picking robot manipulator configuration is described in detail in Section 5.2. Finally, the analysis and evaluation method of robotic kitting system configurations [22] are discussed in Section 6. As mentioned above, several examples of robotic kitting systems are illustrated in Fig. 7, based on the developed design factors and criteria according to the design process. The robot time and motion method, RTM [16], and the robot cell simulator SINDECS-R [17,18], are described as analysis tools that can be useful in modeling alternative robotic kitting system configurations for performance analysis and evaluation as shown in Fig. 8.

3. Design factors for planning robotic kitting systems The design factors for planning the robotic kitting systems are described in detail according to the design process in Fig. 4. These factors include: r a n d o m / b a t c h / o r d e r - m i x kit scheduling, bin and kit tray design suitable for robot bin-picking, kitting location and system size, sequential or simultaneous bin arrival, and single or group kit processing.

3.1. Random, batch, or order-mix kit scheduling When designing a kitting system the schedule of kit types required for assembly is a significant logistic issue. In general, there are three types of kit order schedules: random, batch, and order-mix combination. A random order schedule consists of many kit order types arriving in a random, first-in-first-out (FIFO), and 'fill-next-order', strategy which corresponds to the needs of a highly flexible just-in-time production facility. A batch order schedule refers to a kitting operation

which allows only a single kit type in the system over a finite time period and change-overs between kits are not allowed. The order-mix combination schedule utilizes both random and batch order processing capabilities, but would give a greater emphasis to batch orders. This type of scheduling strategy would continue to kit a large batch order until a single kit order with a higher priority enters the system. The issues determining the quantity of kits to be processed at one time refer primarily to the optimal batch size for the system and the robot's design capabilities. Some of the criteria include: (1) production scheduling ratios and sequence, (2) frequency of replenishing line stock buffers in final assembly, (3) the time delay for the kits to travel from the kitting station to final assembly, (4) the robot's accessibility to the trays, and (5) the effect that kit trays extracting from a rack or cart would have on the resultant cycle time, required staging space within the robot work envelop, and required special tooling devices.

3.2. Kit design suitable for robot bin-picking Kits themselves should be designed in order to satisfy three main functions: (1) the needs of assembly, (2) the distribution and handling requirements, and (3) conditions for long-term, extended use. The method used to present parts to an assembly operator can directly affect the efficiency and resultant cycle-time of the station. For manual assembly operations and for highly sensor-based robotic stations the use of a generalized and open bin-type kit container might be sufficient as a parts entry device. The use of a specialized fixture-type kit tray, however, may be preferable for most other, simpler robotic final assembly stations. The number of assemblies a kit can service is also an important issue. There exist four primary classifications for supplying the necessary parts: (1) one kit to service one assembly unit, (2) one kit to service several assembly units, (3) several kits to service one assembly unit, and (4) several kits to service several assembly units. The third and fourth alternatives generally represent inefficient replacements for the bin arrival systems. In the case where a large quantity of smaller parts such as fasteners are required, the second alternative may be beneficial; however, for most kit-

Robot kitting system

ting applications it is recommended to limit each kit to supply only the parts required for a single assembly unit. A specialized kit tray could be designed in one of two ways: (1) using small ridges between parts to eliminate part tangling during transport, or (2) using a fixtured tray to hold the individual parts in pre-oriented positions for easier part identification, location, and grasping tasks in final assembly. Investments in specialized kit trays should be carefuUy assessed due to the inevitable problem of changes in part design and assembly needs. One alternative kit design which provides for multiple applications combines the use of a standard tote base with a relatively inexpensive, specially designed insert tray on top. The top tray with part dividers designed specifically for the kit type could be easily interchaged based on changes in production ratios, or for new product introductions. The design of the bin and kit tray should be considered early in planning. Key considerations are [6]: (1) Design tray so that the same gripper that handles parts also can handle the tray. (2) Look for common features of parts. One design may accommodate several parts. (3) Be sure that the robot's gripper has enough clearance while picking and placing parts in the tray. (4) If parts are to be shipped to another department, determine if they have any special requirements. 3.3. Kitting location and system size

Regardless of the proximity of the main parts storage area in relation to the main assembly area, there exist three primary kitting station locations: (1) Off-stores staging area which would be located near and connected to a main storage facility, but not a direct function of its operation. (2) Kitting workstation integrated with a main bin storage facility, and (3) In-transit staging area which could be directly next to the assembly area or between production operations. Although most of the firms responding in the kitting survey [21] indicated that they primarily kit at an off-stores staging area, this type of

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configuration may not be an optimal design because of the excess transportation required to move the bins from the main storage location to the staging area and back. This greatly increases the time and cost of handling, increases the bin storage space required at the staging station, and provides little control of bin routing between parallel kitting operations. In the case of in-transit staging area, like the off-stores staging area, kitting system can also be limited with respect to volume and sequence of bins and kits which attempt to simultaneously enter the workstation. In some cases, the in-process staging area kitting operation could be useful for inserting specialized parts or subassemblies, which may have just arrived from manufacturing, into the kit tray in-route to the final assembly station. The kitting system which is designed to directly integrate the robot with the main bin storage facility for the bin-picking task could provide the most favorable configuration because it: (1) consolidates part flow and part inventories, (2) provides access to a larger part storage base, (3) reduces time delays associated with bin travel, and (4) reduces or eliminates the overall bin handling task. The size of the kitting systems is dependant upon: (1) the number of part types (bins) required for all kit types, (2) the throughput rate required to replenish kits to the final assembly operation, (3) the need for a synchronous robotic station to handle the variety of bin-picking tasks, (4) the consolidation of equipment for part flow control, and (5) the desired expandability for future operations. 3.4. Sequential or simultaneous bin arrival

When bins arrive sequentially at the kitting station they can be in a predetermined sequence and form an ordered queue. Each bin is indexed individually into the kitting workcell in order for parts to be extracted and inserted into the kits. Each bin can then be dispatched individually. In contrast, simultaneous bin arrival occurs when all the bins required for a kit or group of kits enter the workstation as a group, so that all of the necessary parts are available before the bin-picking task begins.

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Sequential bin arrival is used more efficiently for group kit processing while simultaneous bin arrival can be used effectively for either single or group kit processing. Due to the nature of the transportation equipment an ASR (Automated Storage and Retrieval) machine, a single carousel storage conveyor and a line conveyor are usually associated with sequential bin arrival. An AGV, a multiple-aisle miniload bin storage facility and multiple carousels are usually associated with the simultaneous bin arrival storage. The primary factors which determine the use of sequential or simultaneous bin entry methodologies include: (1) the number of bins required to fill a kit order, (2) the number of bins which are able to physically fit within the robotic work envelope at one time, and (3) the optimal sequence of bin picking operations to minimize gripper change-overs and kit tray indexing.

3. 5. Single or group kit processing When processing a single kit, the robot would (1) acquire an empty kit tray, (2) index to each bin sequentially, (3) transfer the necessary parts from the bin to the kit, and (4) dispatch the completed kit. When processing a group of kits, the robot would usually pick parts from the first indexed bin until all the kits in the batch are filled. Each bin will then be sequentially indexed until the entire batch of kits contains all of the necessary unit. The entire group of kits would then be dispatched as a complete unit. In the electronics industry, manually filled kits are usually placed onto mobile racks or carts [4], which can be transported by automated guided vehicles (AGVs) to the assembly center. Single kits are often transported using a type of belt conveyor.

4. Planning robotic kitting systems This section focuses on planning robotic kitting workstation as a main step of kitting system illustrated in Fig. 4. The following three topics will be discussed: justification of robotic kitting, the role of bin-picking robot, and control of robotic kitting workstation.

4.1. Justification of robotic kitting The benefits associated with consolidating the parts presentation tasks at a preassembly kitting station were stated in Table 2. There are several potential advantages of utilizing robotic technology to perform the kitting operation. The variety of trays, bins, parts and alternative insertion orientation within kit trays creates a highly complex set of instructions which, in the case of a large product mix variation, may be more applicable to programming than for a manual operator to maintain. This is particularly important for elimination of errors in kitting, which cause serious problems for throughout subsequent assembly operations. The same motivations that can justify robotic use in the final assembly process can be applied in the kitting operation, for example, improve accuracy, reduced downtime for breaks and holidays, reduced time to react to product changeovers, reduced labor and auxiliary production costs. In addition, the robotic operation can lead to improved material flow control through integration of all computer-controlled manufacturing and transfer subsystems. Another potential benefit is the systematic approach. By integrating robotic and computer-controlled equipment in the kitting station, it is possible, when needed, to bridge the automated storage facilities with the automated final assembly operation, through kitting, and provide for a totally automated, integrated, and self-controlled factory of the future.

4.2. Role of robot in kitting system The role of robot operations in a kitting system involves two basic operations: (1) quantity-picking operation: replenish the required quantity of a particular part to the bin tray, and (2) bin-picking operation: orient and insert parts from the bin to the kit tray. The use of bin-picking robots is still relatively new in comparison to other methods of part registration including part feeders, hoppers, vibratory bowl feeders and magazines. The latter devices usually have fixed mechanical designs which can only accommodate a single part type

Robot kittingsystem and need to be replenished manually with parts removed from bins. After parts are manufactured they are usually placed into bin containers or pallets for transportation and temporary storage. Due to widespread use of bin storage, there has been a considerable amount of work done on the development of robots which can acquire individual parts directly from bins for use in assembly [7,8,26,27]. The bin-picking operation, in general, requires the use of a vision sensor to locate and identify a component by its selected distinguishing features (edge, corner, hole . . . . etc.). Once the part's external edges are located, specialized grippers can be used to grasp and hold the part securely until the lifting and transport tasks have been completed. Due to the relatively large number and variety of parts which need to be handled through a kitting station, often at a consistently high throughput rate in order to keep up with final assembly production volumes, the use of a robot for picking parts directly from bins near the main bin storage facility, or at a staging area, could provide the overall most effective kitting system design. Section 4.3 will address the design and control of robotic kitting workstation based on the use of bin-picking robot. 4.3. Control of robotic kitting workstation The task associated with controlling the flow operations within the kitting cycle includes: (1) identify the kit tray upon entering the kitting workstation (if multiple types of kit trays are used, by programming bar code readings as inputs to the robot controller, a total distribution control network can be implemented); (2) begin accessing the current bin types; (3) after the bins arrive begin inserting the correct quantity and types of parts into the kit tray; (4) dispatch and route the kit to the current assembly station or kit storage location; (5) identify the kit tray upon entering the assembly station; and (6) dispatch and route the empty kit container back to the current kitting station if parallel non-competing kitting stations are used, or to the kit tray storage area. There exist several logistic issues which have an effect on kit flow control within the kitting

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cycle. These issues might include the determination of: (1) the quantity of different kit types required at each assembly station, (2) the minimum quantity of kits to be maintained in the on-line storage buffers, (3) the quantity of kits which must be consistently in-transit in order to replenish on-line buffer levels without delays, and (4) the most desirable location for the kitting operation in order to maintain consistent production levels and part flow control. A number of robotic kitting systems have been implemented in recent years (e.g., [4,5]). Computer-integrated robotic kitting systems, including the use of vision sensors and bar-code scanners could further provide a total control network to monitor the sequence of operations within an assembly workcell, including material handling, part transfer, quality inspection, and workload scheduling [12].

5. Bin-picking robot hardware design suitable for kitting system The design of robots to bin-picking operations has been considered as one of the most challenging in the area of building kitting system. In this section bin-picking robot hardware consisting of end-effector, manipulator, vision and other sensors, and locomotion is described. The appropriate design of robot hardware components has to be performed in order to meet the varied requirements in the kitting workstation. Therefore, task description in relation to the robot bin-picking operations is useful to design appropriate robot hardware components as well as peripheral equipment. Namely, the task description refers to specifying the requirements as capabilities required in the robot and peripheral equipment that are needed for kit processing workstation. Fig. 5 shows an example of a task description method which has been developed for the design of robot manipulator configuration. Based on the data of the task description, a methodology for selecting a suitable manipulator has been developed also as shown in Fig. 6. An application of the selection methodology to kitting manipulator configurations is discussed in detail in Section 5.2.

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5.1. End-effector in robotic kitting workstation It is necessary for a kitting robot to make provision for gripper changes for all bin-picking operations, in order to meet different picking requirement and to handle different-shaped parts. Of the various methods of gripping - mechanical, electromagnetic, vacuum gripper, etc. - each is generally only suitable for a small range of part types and sizes. Since gripper changes are nonproductive, they should be minimized. In principle, to minimize gripper changes, this can be achieved in one of several ways, for instance [13]:

(1) mount two grippers on robot manipulator and make use of the wrist-roll feature to select the appropriate one; (2) develop a 'universal programmable' gripper which can cope with a much wider range of part sizes and geometries; (3) design the parts of the product to minimize the number of gripper changes required for kitting them; (4) process the many kitting parts simultaneously by the use of the gantry type robot (see an example in Fig. 7, system (d)). This is perhaps the simplest way to reduce the impact of gripper changes, but its main disadvantages

Task Description for Designing Robot Hardware

Kitting Parts Information

* Type "Shape * Weight * Material Characteristics * Surface Conditions * Processing Precision

Robot Workstation Environment * Coordinates of Parts, Robot, and Peripheral Equipment * Workspace Conditions; Obstacle Existance * Clearance between Parts in Bin or Kit Tray

Kitting Task Process *Parts Initial Location (Position and Orientation) *Parts Final Location (Position and Orientation) *Sequence of Position and Orientation with regard to Part Movements

Allowable Workspace of Parts in Robot Workstation

Motion.Trajectory (MT) of Parts

Robot-Motion-Trajectory (RMT)

Robot-Workspace (RW)

Fig. 5. Task description for designingrobot hardware [25].

Robot kitting system

are the extra cost of duplicating all the fixturing required, and possibly space problems; and (5) mount several grippers on a turret. In the case of assembly operations, with six- or eight-station turret on which each gripper has limited versatility, the majority of products or sub-assemblies could be assembled without any change of gripper;

5.2. Manipulator in robotic kitting workstation Cartesian, cylindrical, and jointed-arm robots are commonly used in manufacturing. In robot bin-picking, spherical coordinate robots are used less, partly because they cannot reach into bins or reach all points of a container positioned below the robot. Nof and Robinson [11] and Nof [10] investigated robot motion economy principles for kitting and bin-picking for kitting and bin-picking.

267

They found that for a cylindrical robot structure, a horizontal bin or kit placement is generally preferred. Efficiency and flexibility are various primary factors to consider when designing the layout of the robotic kitting workstation and selecting the robot manipulator to perform the tasks. With regard to the design of manipulator configurations, task description and the design to satisfy the requirements specified by the description have to be implemented. The task description method that has been developed (Fig. 5) can apply its algorithm to the task description of bin-picking operations. The planner establishes task parameters in relation to kitting parts, workstation environment, and kitting task process, using a newly developed format sheet. The motion-trajectory (MT) in terms of part movements is described with homogeneous transformations which are composed of translation and rotation motions. The MT is trans-

Task Description Method of Robot Operations J Describe Information about Parts, Robot Workstation Environment, and Task Process

T I Specify Robot-Motion-trajectory (RMT) and Robot-Workspace (RW)

T

I

Database for Alternative Manipulator Configurations

FGeometric Analysis / a n d Classification IEach Alternative |Configuration

I_

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I

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Selection of Suitable Klttlng Robot Arm Configuration

t Selection of Suitable Kittlng Robot Wrist Configuration Determination of Link Lengths and Joint Movement Limits Involved in Selected Robot Manipulator Conflquratlon

i Kinematic Analysis of 5elected Klttlng Robot Manipulator Configuration

Rule-Base for Selecting Arm Configuration

T Rule-Base for Selecting Wrist Configuration

Feasible K l t t l n g Robot Manipulator

Fig. 6. Application of selection methodology to kitting robot manipulator configuration.

I

268

K. Tamaki, Sh. Y. Nor

formed to robot-motion-trajectory (RMT) expressed in robot coordinates. In other words, the R M T is the desired sequences of spatial points in kitting workstation that the manipulator has to move along from the initial location in a bin tray to the final location in a kit one. The robot workspace (WR) is determined to add the range of manipulator movement around the RMT. The R M T is an important input for designing the manipulator configuration consisting of appropriate combinations and sequence of joint types (revolution, pivot, and prismatic joint), while the RW is essential to set the link lengths and joint movement limits involved in a manipulator. In addition to the development of the task description method, a methodology for selecting a robot configuration, satisfying the requirements specified by the task description, has been developed [23]. Fig. 6 shows an application of this selection methodology to kitting manipulator configurations. The manipulator is viewed as a configuration connecting an arm and a wrist. To select a suitable arm configuration, arm database enumerating alternatives of arm configurations is scanned by a rule-base program that has to be developed, consisting of algorithms for selecting the appropriate arm. The particular arm configuration, capable of performing all translation motions in the RMT, is selected from these alternatives. Next, a wrist configuration is selected from the wrist data-base. This selection is based not only on rotation motions in the RMT, but also considering the required cancelling characteristic that depends on the already selected arm. The cancelling characteristic is a wrist ability for moving a part while keeping its posture, which cancels undesired changes of the part's orientation generated by motions of the arm. In terms of establishing the arm and wrist database [24], first, the possible types of arm and wrist joint combinations are exhaustively enumerated. Next, the duplicate or ineffective combinations among the enumerated alternatives are eliminated. All arm and wrist configurations included in the database have individually formulated equations with the rotational-transformation-tensor. Use of these equations helps to analyze and evaluate the geometric and kinematic characteristics of arm and wrist mechanisms. The arm configuration is analyzed and classified by

geometric shape of its workspace, while the wrist is done also in view of its geometric characteristics, such as orientation and canceling function. The programmability of the manipulator, as well as the configuration design, is what allows robots to handle a wide variety of parts and bins in many different motion-trajectories. Programming has to be extremely precise, and the system must be designed so that parts and trays are transferred accurately in and out of the robot's workstation. Because various motion-trajectories are required for the bin-picking operations of different types of parts, software probably will have to be customized. Some vendors provide generalized software. The user programs the product's size, gripping points, and the path the robot takes to and from the tray. 5.3. Vision and other sensors applications

Researchers have developed vision system to perform bin-picking tasks [2,3]. In practice, this approach has yet had only limited success due to the high level of computer processing required and the many task variables, such as ambient light levels, surface finish and obscured parts. Ultimately, these systems would enable the robot to pick components directly from the bin, and consequently, the local manual kitting station would become redundant. Sophisticated bin-picking experiments were performed at the University of Rhode Island [9]. The system consists of a six-axis robot manipulator, equipped sometimes with a two-finger gripper and sometimes with a flexible-hose-attached surface-adapting vacuum gripper, two cameras, an insertion tool and a regrasping workstation. The first camera is mounted on the manipulator to scan through a selected portion of the bin. The gripper then descends to pick up a part whose pose is subsequently identified by the second camera. Finally, with the aid of the insertion tool and the regrasping station, the part is manipulated to align with the predetermined configuration such that it can be transformed to the target site in a predetermined manner. The performance of acquisition procedure in this experiment is workpiece dependent. Based on over 200 trials, the ratio of successful acquisitions attained was 57% of the trials. An average cycle time of approximately 8 seconds was achieved.

269

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The first commercially available machine vision system with capabilities for bin-picking applications was the 'i-BOT-I' system, originally demonstrated in 1982 [14]. Two grippers are available, both parallel jaw type with visual and tactile sensing. The i-BaT uses gray scale image processing to provide specific part location/ orientation guidance to the robot controller. Part orientation is transmitted to the gripper which adjusts for the selected part. The intelligence (tactile and visual) in the gripper determines part location within the jaws and applies sufficient force to lift the part. Cycle time for part acquisition is less than 5 seconds, of which 2 seconds are robot transmit time. The second commercial bin-picking vision system was BinVision system, originally introduced in 1983. The BinVision system includes a robot equipped with a sophisticated vision subsystem which has the ability to locate and acquire randomly oriented, overlapping parts from a bin, conveyor, or other factory work area. Initial part location/orientation coordinates guide the robot to the part selected as easiest/quickest to acquire. Using closed-loop sensor feedback, and adjusting to subtle features using its integrated compliance capability, the robot applies sufficient, programmable force to lift and move the part. Typical application cycle time is 5 to 20

seconds, including image processing and robot transit time [14].

5.4. Robot locomotion required in automated kitting system In flexible machining systems, the requirements of small batch size and wide product variety often demand a tool variety that exceeds the capacity of machines' tool magazines. Robotic kilting can prepare kits of the tools required for the magazine modification. At IPA, Stuttgart, mobile robots (i.e., robots mounted on AGVs) select tools from a storage rack, transport them to the machine, and load the magazine [19].

6. Performance evaluation as part of the robotic kitting design

As mentioned previously, the design factors and criteria for planning robotic kitting systems have to be developed in accordance with each step involved in the design process as shown in Fig. 4. Based on the design factors and criteria, several configurations of robotic kilting systems can be recommended. Several such configurations were developed and quantitatively analyzed

270

K. Tamaki, Sh. Y. Nof

Table 4 SINDECS-R modeling definitions (b) Onboard robot-picking vehicle kitting systems configurations 1 robot which is m o u n t e d on an A S R machine and travels between bin locations 1 kit order platform on the A S R machine where the tray(s) are positioned during the kitting operation (station 11) 1 i n p u t / o u t p u t station (station 0) for tray storage/pickup and kit release from the system 9 bins (stations 1-9) of parts within the storage facility 1 temporary waiting station (station 10) for completed kit storage prior to being released from the system

by Sellers and Nof [22], using a robot cell simulator, SINDECS-R, and a robot motion time program, RTM.

6.1. Recommended configurations of robotic kitting systems Due to the diversity of storage, handling, robotic, and transport equipment available in the manufacturing market today, there exists an un-

limited variety of alternative robotic kitting configuration designs. Based on extensive research including a national kitting survey [21] and a qualitative analysis of over 20 different system configurations [20], it has been suggested that for most cases it is best to design the kitting facility as an integrated function of the main or auxiliary storage facility. For this reason, the following four different types of robotic kitting system, which also meet the design factors and criteria, were selected and analyzed in detail, and compared on the basis of their multi-criteria performance: (a) at main storage facility with a single aisle, end-of aisle robotic kitting system; (b) at main storage facility with a single aisle, on-board r o b o t / A S R (Automated Storage & Retrieval) system; (c) with a single bin carousel, single kit carousel, multiple kit processing robotic kitting system; and (d) bin and kit in-transit staging area with a gantry robot kitting system. These four families of robotic kitting configuration layouts are illustrated in Fig. 7. In the fol-

Table 5 R T M analysis for onboard robot-picking vehicle systems enter bin, aquire part, extract part (can vary in S-R for alt. parts) subtl 1 rl 5.0, 10.0 2 d2.0 3 rl 2.0, 3.0 4 grl 5 ml 2.0, 3.0 6 ml 5.0, 10.0 subt2 7 ml 5.0, 18.0 subt3 8 ml 5.0, 3.0 9 ml 5.0, 8.0 10 ml 2.0, 4.0 11 ml 1.0, 2.0 12 re 13 rl 1.0, 2.0 14 rl 2.0, 4.0 15 rl 5.0, 8.0 16 rl 5.0, 10.0

enter empty gripper into bin delay for vision system to find part estm. distance to closest part (varies) make contact with part; acquisition slowly extract part inside bin remove the part from the bin transport part to just outside center of kit lower gripper to insert into kit insert part into kit location assume approx, distance to part location = 4" slowly insert part into kit release part slowly move away from part raise arm to retract from kit carousel bring arm out of kit assume cycle starting position

RTM output values." Subtask 1 = Time to retrieve and acquire parts at bin (processing time at bin) is 8.5 seconds, which is 0.142 m i n u t e s / p a r t Subtask 2 = Time to transport part from bin to kit (time to and from station 11 in Travel Time Matrix) is 4.02 seconds, which is 0.067 m i n u t e s / t r i p Subtask 3 = Time to insert part into kit (processing time at station 11) is 16.7 seconds, which is 0.3 m i n u t e s / p a r t

Robot kitting system

271

27--

lowing section the evaluation results of the four configurations are summarized.

01-

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--

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IOI--

In order to analyze robotic kitting systems, the robot cell simulator SINDECS-R [17,18] was used to model and run the four configurations. With regard to SINDECS-R, a given system is defined solely in terms of robots, workstations, machines, and bins. There can be many different part types in the system, each with a different set of process steps or tasks to be completed at the various workstations, including quality control tasks. In this model, station 0 represents the tray input station, stations 1-9 represent the bins, station 10 is a dispatch queue station, and station 11 is the kit location station (see Table 4). So as to determine the process times at the kitting stations for the SINDECS-R input data, a robot time and motion method, R T M [16] was used to develop the individual task times associated with each of the configurations. An example R T M program, depicting the on-board r o b o t / A S R kitting system (b), is illustrated in Table 5. All of the configurations are modeled first with a waiting rule, and then without the waiting rule parameter. Specifically, the waiting rule means that each kit remains within the kitting area for a specified period of time before being released to assembly. For the models utilizing the waiting rule alternative, the kit dispatch delay is modeled as the final process operation on each kit at station 10 prior to release. The results of analyzing system (b) with respect to average production rate (orders/hour), average time in system ( m i n u t e s / o r d e r ) , and robot utilization (%) in system are tabulated in Table 6. As seen in this

t5 l-

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table, for a highly flexible operation, the input includes nine different types of kits representing single or group kit orders with small, medium, and large quantities of parts per kit type. In the simulation analysis four commonly used flow strategies are used: (1) FIFO - first-in-first-out which selects kit orders to enter the system on a random basis, but this order once established will remain consistent until processing is complete; (2) SPT - in which the kit orders are sequenced upon entry to the system from shortest order processing time in ascending order;

Table 6 Simulation results for onboard robot kitting systems (9 kit types) Performance

Wit waiting rule FIFO

Production rate (orders/h.) Time in system (min./order) Robot utilization (%)

1.111

SPT

Without waiting rule EDD

1.084

1.223

54.063

57.037

0.599

0.581

Order-mix

FIFO

SPT

1.122

2.500

1.416

49.686

53.433

23.947

0.661

0.603

0.603

EDD

Order-mix

5.212

2.644

44.143

11.677

22.710

0.586

0.802

0.610

272

K. Tamaki, Sh. Y. Nor

(3) EDD - in which each kit order has a user defined due date and orders are sequenced in the priority order of earliest due date first; and (4) Order Mix - in which a constant proportional mix of kit types is maintained in the system. The overall quantitative evaluation results of the four recommended robotic kitting systems are shown in Fig. 8. Although these results are for a specific case study and may not be a general results, this analysis illustrates an applicable evaluation that is part of the design method of the kitting system.

7. Conclusion The purpose of this article was to develop on effective design method of robotic kitting systems. The design factors and criteria required when planning a specific kitting system were established. In terms of bin-picking problems, the robot hardware components (end-effector, manipulator, sensors, and locomotion) which played an essential role in a kitting workstation were explored in order to design them suitable for the kitting system. Based on the developed design factors and criteria, four configurations of kitting system were listed. Then the four recommended configurations were evaluated by the use of quantitative simulation tools.

Acknowledgement This research was supported in part by a grant from Kajima Foundation in Japan, and by a CIDMAC, Purdue University grant. Previous contribution by C.J. Sellers to this research are acknowledged.

References [1] M. Adams, Automated kitting for assembly, Development in Assembly Automation, ISF (1988) 499-516. [2] T.S. Alubus and co-workers, Experiments in part acquisition using robot vision, Proceedings of the Autofact 11 4th Robots Conference (1979); see also SME Technical Paper MS 79-784.

[3] J. Birk, General methods to enable robots with vision to acquire orient and transport workpieces, Report No. N S F / S A 800334, PB81-148934 (1980). [4] S. Conrad and R. Pukanic, Process approach to planning a successful kitting system is outlined, Industrial Engineering (1986) 58-71. [5] F. Deaton, Automatic kitting, IBM Technical Disclosure Bulletin 25(11B) (1983) 6017-6018. [6] J.H. Fuchs, The Prentice Hall Illustrated Handbook of Advanced Manufacturing Methods (Prentice Hall, Englewood Cliffs, N J, 1988). [7] Intel Group, Brake drum bin picking at Intel, ISATA Automotive Technology and Automation Symposium, Oct. 6-10, Switzerland (1986) Vol. 2, 19-29. [8] L. Jacobson and H. Wechsler, Invariant image representation: A path toward solving the bin-picking problem, International Conference on Robotics, IEEE Computer Society (1984) 190-199. [9] R. Kelly, J. Birk, J. Dessimox, H. Martins and R. Tella, Acquiring connecting rod castings using a robot with vision and sensors, Proceedings of the 1st International Conference on Robotic lPtsion and Survey Controls, IFS (1981). [10] S.Y. Nof, Ergonomics of robots, in: R. Dorf, ed., International Encyclopedia of Robotics (Wiley, New York, 1988) Vol. 1,443-451. [11] S.Y. Nor and A.P. Robinson, Analysis of two robot motion economy principles, Israel Journal of Technology 23 (1987) 125-128. [12] P.D. O'Gorman and J. Browne, Kitting, in: R. Dorf, ed., International Encyclopedia of Robotics (Wiley, New York, 1988) Vol. 2, 753-761. [13] A.H. Reford and E. Lo, Robots in Assembly (Halsted Press, New York, 1986). [14] K.M. Richard, Industrial Robot Handbook (Van Nostrand Reinhold, New York, 1989) 405-414. [15] D. Richardson, Computer guided kitting makes us more productive, Modern Materials Handling 39 (12) (1984) 44-47. [16] A.P. Robinson, H. Lechtman and S.Y. Nof, RTM (Robot Time and Motion) User manual, Version 1.2, Research memorandum No. 84-12, School of Industrial Engineering, Purdue University, West Lafayette, IN (1984). [17] A.P. Robinson and S.Y. Nof, SINDECS-R User's manual, Research memorandum No. 84-10, School of Industrial Engineering, Purdue University, West Lafayette, IN (1984). [18] A.P. Robinson and S.Y. Nof, Flow control simulator for production systems with robots, Material Flow 3 (1) (1986) 113-120. [19] R.D. Schraft and coworkers, Mobile robots: Key-elements for integration of transport and handling functions, Proceedings of the 15th International Symposium on Industrial Robots (1985). [20] C.J. Sellers, Robotic kitting systems for flexible assembly, Unpublished M.S. thesis, School of Industrial Engineering, Purdue University, West Lafayette, IN (1987). [21] C.J. Sellers and S.Y. Nof, Part kitting in robotic facilities, in: S.Y. Nof, ed., Robotics and Material Flow (North-Holland, Amsterdam, 1986) 163-174. [22] C.J. Sellers and S.Y. Nof, Performance analysis of robotic

Robot kitting system kitting systems, Robotics and Computer-Integrated Manufacturing 6 (1) (1989) 15-24. [23] K. Tamaki, Facility planning - methodology for selecting robot manipulator configurations, Unpublished doctoral thesis, School of Industrial Engineering, Waseda University, Tokyo (1989) (in Japanese). [24] K. Tamaki, Y. Hasegawa and T. Ishidate, Fundamental research of joint configurations of industrial robot, Bulletin of Science and Engineering Research Laboratory, Waseda University, No. 119 (1987) (in Japanese). [25] K. Tamaki, Y. Hasegawa and T. Ishidate, Development of task description method for robotized large scale

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structure assembly system, Journal of Japan Industrial Management Association 39 (3) (1988) (in Japanese). R. Tella and J.R. Birk, General purpose hands for bin picking robots, IEEE Transaction on Systems, Man and Cybernetics SMC-12 (6) (1982). J. Turneey, T. Mudge and R. Volz, Solving the bin of parts problem, Vtsion 86 Conference, Society of Manufacturing Engineering, MI June 3-5 (1986) 4 (21)-4 (38). L. Waller and J. Shandle, Kitting saves money, but handle with care, Electronics, December, (1988) 37-38. O. Weight, MRP II unlocking America's productivity potential (Oliver Weight Ltd., Essex Junction, VT, 1981).