Automation Addresses Odd-form Assembly
December 31, 1969 |Estimated reading time: 16 minutes
It is no longer a question of whether capable handling equipment is available. Rather, are the technology and equipment properly matched to the task?
By Gregory W. Holcomb and Tawnya Henderson
While the words "odd-form" put fear into the hearts of many past manufacturers, today's assembly systems are technically advanced enough to handle this demand without difficulty. The challenges have been met; now, it is a matter of understanding the technologies and equipment so that the right solutions can be applied to meet individual needs and tasks.
The continued advancement of this technology not only provides the industry with the ability to successfully automate its manual assembly processes and improve profitability but also to facilitate a more globally competitive market, thus creating a demand for effective odd-form automation greater than ever before.
Key to the success of odd-form automation is component locating and handling options flexible and robust enough to handle various components typical of this assembly. Solutions must compensate for board variances, body-to-lead variations and lead-to-lead tolerance stack-ups to ensure accurate location and insertion. To determine which locating and handling options are best suited to automate a specific odd-form assembly, requirements must be reviewed carefully, and matched to the most suitable odd-form technology and equipment.
Odd-form RequirementsBy being specific on present and future needs and working with the supplier, the required accuracy, speed and flexibility can be determined. Then, pairing the requirements with the most suitable locating and handling strategies, reasonably assures that the optimal odd-form assembly equipment has been selected for the job. There are two information categories to be evaluated (Table 1):
Vacuum picking commonly is used for odd-form SMD handling and can be combined with the adaptive compliant technology to provide total part handling flexibility for through-hole and SMDs.
1. Application-specific information: What is the component mix to be assembled? Is the mix low (one to three parts), medium (four to five parts) or high (six parts or more)? Are similar component types being assembled or are there many different component styles that vary significantly in size and shape? What is the variation range in the body-to-lead and lead-to-lead part geometries? What percentage is surface mount vs. through-hole? What is the volume of odd-form components to be assembled (low, medium or high)? How many boards per panel and placements per board? How many of each part number is needed to place or insert? Are the parts "balanced," or are there many more of some types to be placed/inserted than others? Is lead cut/form or lead snap-in retention required? Is lead clinching after insertion required? And lastly, is pin-in-paste being used?
2. Production-specific information: How many shifts a day/week will be run? What are the throughput requirements? What packaging type will be used (what is available)? How often will a new board changeover be required? And, what are the standards for the number of systems supported per operator?
(Right) Adaptive compliant assembly head handles all odd-form parts without head or tool changes. It can be combined with simple vacuum picking to provide total component-handling flexibility.
It is very helpful to communicate these issues with the supplier. By providing information on the number of boards and the part volumes needed, the machine or machine combination necessary to achieve desired throughput can be determined. Similarly, by evaluating the component mix and its unique challenges, the best way to handle them will be apparent. The part mix, consistency and packaging also will establish flexibility requirements.
Also important in establishing the latter's needs is board changeover frequency. For example, one automotive manufacturer runs a line three shifts per day using the same or very similar boards. Hence, flexibility is not a priority. Rather, speed, accuracy and throughput are the main concerns. On the other hand, a contract manufacturer (CM) may change boards several times a week, thus requiring the greatest flexibility. Wherever flexibility needs may fall, the technology is available to handle them most efficiently as long as the correct parameters have been established.
Once application and production needs have been reviewed and evaluated, several viable options remain for locating (finding the part or leads), handling (picking the part) and implementing (properly placing or inserting the part on the board). The following is an overview of these options to better understand the methods and technologies available.
Odd-form Lead-locating TechnologiesDirect lead acquisition. This method is used for through-hole odd-form parts to locate components by their leads using dedicated grip fingers. This more dedicated approach is the fundamental method used on systems such as dual-inline packages (DIP) and radial- and axial-lead inserters. It is well suited for low-mix applications and components with poor lead integrity, i.e., those that require lead form or retention during insertion, such as splayed DIPs.
Table 1. An assembly technology/automation match-up.
On the down side, because of its dedicated aspect, each lead pattern requires a unique jaw set. Although lead gripping can, in some instances, eliminate concern for tilted body parts, tool changes still are required to compensate for lead pattern variations of different parts. Another problem with lead gripping is the large opening required to release the lead and clear the body. That this, in turn, requires a significant amount of space adjacent to the part being inserted indicates that lead gripping is not well suited for densely populated boards. To obviate this problem, a pusher mechanism can seat the part successfully after release, nevertheless adding complexity to the tooling.
Optical lead finding. Locating leads optically requires that both the X- and Y-locations be determined. There generally are two ways to accomplish this:
- Profile the leads With lead profiling, leads are scanned by vision or laser equipment in one view to determine their X-locations. The part then is passed through the optical scan a second time to determine the Y-locations. By mathematically combining the two scans, the pin locations for insertion can be determined. However, each time a part is passed through a scanning station, it is at the cost of cycle time. Additionally, because of a shadowing effect, it is impossible to "see" all the leads if inspection is part of the application (Figure 1). However, there is some inspection value with optical systems for bent leads: Because of the shadowing, missing leads will not be seen, and in most instances, the few leads that are bent can be detected tactically and searched or rejected at the board. In production, the actual time spent detecting bent leads using tactile sensing is insignificant compared to that required to inspect every lead of every part with optical systems.
- View the lead ends Optical lead finding also can be accomplished via lead-tip imaging using vision cameras. While the first optical method requires two scans of the part (X and Y), this method only requires one view the end view. By looking at the lead ends directly, X and Y can be determined with one image, thus saving precious production time. Most lost time is in the actual move through the camera since imaging itself typically is very fast.
While this method works with numerous parts, there are some limitations with the single-end view approach. For example, to reliably image leads, lighting and algorithms must be changed to deal with varying tip configurations and background colors, making this method better suited to low-mix applications. (Note the various surfaces illustrated in Figure 2.)
Lead registration in the feeder. This final locating technology physically finds through-hole part leads while the component is still in the feeder. Then the part is gripped by the body for placement/insertion. It is a simple approach that best stabilizes the part and provides constant control during handling and insertion. It also provides reliable lead location, robust picking and keeps the lead perpendicular to the board holes for true insertion (Figure 3).
The flexibility gained through body gripping allows for effective component assembly, ranging from geometrically stable parts (using a simple handling strategy) to those with greater geometric variances (using a more flexible, adaptive compliant handling strategy). When matched with the latter approach, picking a part by the body vs. the leads provides greater agility and the ability to handle component "brickwalling" and other common odd-form assembly challenges.
Odd-form Component Handling TechnologiesThe following handling technologies are available with many different heads or grippers (single, multi, compliant, etc.), as well as with automatic head, finger and other tool changers. For successful insertion/placement detection, tactile sensing must be included in all these technologies. Vision, when needed, also can be used with them.
Vacuum: Pneumatic vacuum picking handles parts by using a vacuum tip or quill, which when placed on a flat surface of a component, will grasp, move and place the part using suction and release. This technology most commonly is used for surface mount device (SMD) placement, which is more likely to have a flat surface for easier picking.
Figure 1. Shadowing makes it impossible to "see" all the leads when using optical lead profiling methods.
Vacuum technology can be very cost effective as an add-on if a surface mount machine is available and existing tools are used. It also is good for pocket-type tape formats, especially SMT packaging, the technology being limited, however, to parts with flat or smooth surfaces. Parts not easily vacuum picked can be adapted by adding a special flat surface piece to them, which is removed after part placement. Although this solves the problem, it obviously adds assembly and disassembly steps to the application, as well as additional cost. Tilted parts, such as taped components, typically are poor candidates for simple vacuum picking because the leads are left going into the board at an angle, a situation not conducive to robust insertion.
Figure 2. Lead tip variations. To reliably image leads, lighting and algorithms much be changed to deal with the varying configurations and background colors.
Simple pneumatic: This gripping technology uses air pressure to open and close gripper jaws, which typically are tooled with dedicated fingers. It is the simplest and least expensive handling technology, but limited because the gripper jaws, controlled by air pressure alone, are either completely open or closed. Although this is not a problem when picking a part from a feeder, placement on a densely populated board can pose a problem. Hence, simple pneumatic handling should be limited to low-density, low-mix placement.
Figure 3. Lead locating in the feeder keeps the leads perpendicular to the board holes when coupled with an adaptive compliant handling technology.
Servo-actuated technology: Operated by a small electric motor in the grip, this system handles parts with jaw movement of components to any pre-programmed point. Such movement makes it possible to handle different-sized parts on densely populated boards without fully opening the jaws after placing/inserting a part. However, with the advantage of the programmed movement comes the disadvantage of mechanical design complexity, as well as software programming requirements for each part and application.
Adaptive-grip technology (AGT): This technology handles parts with jaws operated by air pressure. It also features controlled jaw opening and closing via a specialized cylinder that permits automatic adaptation to any part size. Adaptive-grip technology provides a grip release that automatically opens from a part of any size or shape within a few thousandths of an inch. This is ideal for insertion/placement into densely populated boards and for easy accommodation with high-density and brickwall applications (Figure 4).
Figure 4. A comparison of handling technologies relative to board density requirements.
With AGT there is no need for electric motors and drives, and the technology does not require application-specific programming, which is the key operational difference between the adaptive-grip and servo-actuated methods.
Implementation StrategiesThe location and handling technologies outlined may be integrated into many different physical forms for execution by various suppliers. Some are combined with, or in addition to, others. And while some overlap others, the physical manifestation of these technologies generally falls into one of the implementation strategies outlined below. These strategies take the application and technology requirements defined to this point and match them with the physical performance levels necessary to handle the job:
Single-tool strategy uses a single dedicated tool for handling odd-form components. It also can be equipped with automatic head or finger changers and is adaptable for vision location when required. Handling with single-tool strategies includes four technologies: vacuum (for SMT or mixed-technology applications), simple pneumatic, servo-actuated and adaptive-grip technology.
A single-tool grip generally has the least weight so that it can move quickly. Although it is the least complex implementation choice, it also is the least flexible and can only be used for very low-mix applications. Automatic head or finger changers generally can improve flexibility, but only at the cost of increased cycle time. Single-grip tool strategies are best used for applications requiring low flexibility with only one to three different part styles to be assembled and with medium to high throughput.
Multi-tools strategy uses several tools on one head, turret or indexing wrist, which then assemble several different part types simultaneously. Com- ponent balance is a huge issue with multi-tool handling and must be evaluated carefully to make this a viable option. (Even systems equipped with automatic head and jaw changers are subject to this problem.) For example, if a five-up tool is selected and six parts are to be assembled, the gripper can only pick five; it then must return to get the single leftover part, causing speed loss over other more flexible methods. Additionally, depending on the part types, vision location may be required. Handling technologies used with multi-tools include a combination of the tools listed above.
Figure 5. Implementation of single-plane compliant tool. The tool is positioned over the part with the robot centerline aligned with the lead centerline (a), allowing for simple insertion (b).
However, if component assembly is balanced, this strategy can provide more speed and flexibility than a single-tool approach. Generally, it can handle up to six part types without changing heads or jaws. If more than six parts must be assembled, head or jaw changes likely will be required. Tool changing can be more complex since jaws and grip modules must be matched as well as the combinations of jaws and applications. For this reason, this implementation approach is best used for applications featuring a well-balanced, medium parts mix.
Compliant ToolsThis implementation strategy consists of two general approaches:
1. Single-plane compliant tools (two-dimensional (2-D)) can accommodate various through-hole and surface mount odd-form component styles and lead variations. It normally does not require vision location if the parts are geometrically consistent. The typical handling technologies used with single-plane compliant tools are servo-actuated and adaptive-grip.
While the single-plane compliant tool can accommodate various odd-form parts without the need for vision, it cannot compensate for tilted parts such as capacitors, single-inline packages (SIP), hybrid SIPs, inductors and many others. Therefore, this approach is best used for applications with lower parts mixes, which only require compensation for skewed or twisted parts such as connectors or other "precision" molded components not typically tilted.
Figure 6. The 2-D compliant handling approach, which cannot compensate for tilted parts.
As shown in Figure 5, the 2-D tool is positioned over the part with the robot centerline aligned with the lead centerline (5a). The body centerline is not a concern even though it is the body that will be gripped. This is because the compliance shifts to compensate for the X-Y translation in the body as the jaws grip the part. This maintains the lead centerline with the robot centerline, allowing for simple insertion (5b). However, with 2-D adaptive implementation, if the part body is tilted relative to the leads (as with many parts), the 2-D compliance cannot compensate for the tilt because it is a single-plane device only (Figure 6).
If a tilted part is picked using 2-D compliance, optical locating could be used. However, the leads would end up angled to the board at the point of insertion. This angled-lead condition makes for less reliable insertion because of pinch points and possible solder defects, especially with pin-in-paste (Figure 7). Note the void potential on the top left. The only way to guarantee perpendicular lead entry is to use three-dimensional (3-D) adaptive compliance.
Figure 7. An angled lead creates a less reliable insertion condition, especially with pin-in-paste applications.
2. Adaptive compliant tools (3-D) facilitates body gripping regardless of part geometry or condition. It does not require lead-find tools since lead location control is never lost. Using this 3-D adaptive approach, the assembly head can comply with all six degrees of freedom, thus creating a totally flexible handling strategy to accommodate virtually all odd-form through-hole and SMD types. It also can handle all part tolerance variations, including tilted, skewed and twisted parts. The handling technique typically used with this strategy is adaptive-grip.
Adaptive Compliant Technology (ACT)ACT combines the most flexible handling technology with the simplest mechanical design. Because it can handle odd-form through-hole and SMDs without head or finger changes, it is also best suited to applications requiring a medium- to high-mix of component types with varying tolerances or many board changeovers.
Figure 8. Implementation of 3-D adaptive compliant handling technology as applied to a tilted component. The tool is positioned over the part (a) and compensates for any tilt (b). The part is then "picked" (c), inserted (d) and released (e).
Figure 8 shows a component with a tilted body. The only way to handle this part is with 3-D adaptive compliance. The leads are registered in the feeder and the 3-D tool is positioned over the part (the "approach," 8a). As the adaptive grip closes on the tilted body, the 3-D adaptive unit then compensates for any tilt, twist or translation of the body relative to the leads ("adaptive compliance," 8b), which is performed without disturbing the leads. (Leads as small as 0.015" can be handled with this tool.) Once the grip closes, the compliance is locked, preserving the lead location. The part then is removed from the feeder with lead location locked in and ready for insertion (the "pick," 8c). The part can be seated directly into the board without requiring a pusher (the "insert," 8d). If any leads are bent, a search move is initiated. If insertion is unsuccessful, the part is rejected and a new part inserted. Finally, the jaws open only enough to release the part without disturbing adjacent components (the "release," 8e). An important attribute of this approach is that no tool changes are required for virtually any part, enhancing flexibility and speed.
A mix-technology implementation approach uses two or more of the appropriate handling technologies and strategies discussed above to handle both through-hole and multiple SMDs on the same system. Although these tools provide the necessary flexibility to handle mixed-technology components, the degree of flexibility and throughput achieved depends on the handling technology. While tool additions can mean more system complexity, this range of capabilities can compensate by providing many benefits.
ConclusionLooking for the most flexible or least costly odd-form assembly equipment is not recommended. With all the available viable options in technology, it is recommended instead to refine needs, match them to the appropriate technology and select the equipment that best meets the requirements of today and those well into the future. No matter the direction, the cost savings realized from improved quality, less rework and the speed that automation delivers will pay its way many times over.
GREGORY W. HOLCOMB and TAWNYA HENDERSON may be reached at CHAD Industries, 650 W. Freedom Ave., Orange, CA 92865; (714) 998-8399; Fax: (714) 998-5438; E-mail: gholcomb@chadnet.com and thenders@chadnet.com.