Any manual assembly process is a source of potential errors and quality lapses. Even the most conscientious operator is prone to mistakes. And, when parts look similar, the wrong part can easily be selected.

Fortunately, error-proofing technologies are available to monitor correct parts selection. Some assemblers are beginning to see the light with sensor-based systems that indicate the correct part to be installed and warn if the wrong part is selected.

"We all commit errors and mistakes, regardless of how skilled or experienced we are," says Jamie Flinchbaugh, a partner in the Lean Learning Center (Novi, MI). "Mistakes are inevitable, because people are not perfect. What really matters is how much that mistake is going to cost the company. That depends on when it is caught."

According to Flinchbaugh, it’s important to identify the potential of an error occurring beforehand. "Errors are cheapest to address up front, early in the assembly process," he points out. "The cost of quality decreases the closer you get to the source of the problem."

Flinchbaugh suggests using error proofing, which is also known in lean manufacturing circles as poka-yoke, to prevent errors from occurring or immediately point out a defect as it occurs. If defects don’t get passed down an assembly line, throughput and quality improve.

"Rework costs are killers and the administrative oversight is difficult to manage," notes Hamilton Bitely, sales and marketing coordinator at SpeasTech Inc. (Little Rock, AR). "Rework costs have labor, materials and inspection cost associated with it. Improving the final product quality eliminates errors and mistakes, which a company has to charge to warranty cost. The worst business situation any company can become involved with is bad product delivered to the customer."

Bitely recommends using new technology, such as smart part bins and light-directed picking systems to guide operators and help them improve accuracy.

Lighting the Way

Pick-to-light is a generic term that describes a class of products that guide operators using light. The systems detect when an operator passes his hand into a bin. Sensors on the bin openings detect breaks in invisible infrared beams within a conical-shaped sensing area. Lights above or below each bin direct the operator to the correct part choice. The correct pick sequence is programmed into each unit. Selecting the wrong part in the wrong bin activates an alarm, such as a buzzing noise, which alerts the operator to go to the correct location instead.

"Pick-to-light systems provide a fast and accurate way to automatically verify a correct pick," says Jim McNicholas, manager of controls engineering and product development at White Systems Inc., a division of FKI Logistex (Kenilworth, NJ). The systems allow manufacturing engineers to control the selection of hundreds of different parts while minimizing the work of assemblers.

McNicholas says pick-to-light systems can be installed on any rack or shelving unit and come in a wide selection of hardware and software configurations. They can be used on static shelving or automated carousel systems.

Pick-to-light technology evolved from the warehousing and distribution industry, where it is widely used. The paperless, light-directed order fulfillment systems are used to direct order pickers to the right pick locations and show correct pick quantity.

The systems tell people how much of each product to pick, explains Bernie McCabe, director of sales and marketing at Professional Control Corp. (Germantown, WI). "It’s not unusual to find some large warehouses with 5,000 to 20,000 light modules in operation," he says. According to McCabe, Wal-Mart Stores Inc. (Bentonville, AR) is the largest user of pick-to-light systems in the world. But, large manufacturers, such as Caterpillar Inc. (Peoria, IL) and Harley-Davidson Motor Co. (Milwaukee) use the technology in their parts distribution centers.

Light-based picking applications are becoming more popular in production environments, especially large operations that can afford to invest in the technology. "We are beginning to see more and more interest from manufacturers," says McNicholas.

For instance, when General Motors Corp. (Detroit) opened its new state-of-the-art Lansing Grand River assembly plant earlier this year, workstations were equipped with pick-to-light systems. Operators are assisted in selecting the right parts by bins that illuminate when parts are selected.

"Error-proofing occurs when an operator reaches in a rack for a part, such as badging or cladding that could vary with different vehicles," says Bob Anderson, plant manager. "The operator’s hand breaks a beam of light and a computer determines if the right selection was made. If the wrong part is selected, an error message goes off."

But, smaller manufacturers have also been using the technology to reduce errors and improve productivity. Pick-to-light systems are especially popular with assembly plants that have adopted lean manufacturing techniques such as parts kitting.

"We’ve had good success with kit-picking and workcell applications where parts are stored in a common picking area," says McCabe. "As companies get lean, we are seeing more opportunities for pick-to-light systems."

Kitting is a lean manufacturing practice in which assemblers are supplied with kits—a box of parts, fittings and tools—for each task they do. The kits of parts are picked ahead of time and placed in front of assemblers. This eliminates waste by reducing nonvalue-added activity, such as time-consuming trips from one parts bin to another to get the necessary material.

Kitting parts and components used to assemble products has proven to be efficient because:

• It allows the number of assemblers on a line to change based on the quantity of products needed. By kitting parts, a manufacturer can add or subtract the number of assemblers on a daily basis.

• It allows manufacturers to balance the workload throughout an assembly line to reduce the workload on an assembler who is unfamiliar with a particular job.

• Kitting is a way to double-check parts usage.

• Because the parts are in kits that are made to order, line setup time is eliminated.

An Efficient Concept

Pick-to-light technology is not a new concept. Indeed, it has been around for more than 20 years. But, the level of sophistication has improved lately because of new sensor technology, which has eliminated problems associated with earlier push-button-based picking systems.

"Some older guided parts picking systems use a button rather than an infrared sensor," says SpeasTech’s Bitely. "With that type of system, once you pick a part, you hit a button. But, that requires the operator to make two movements instead of one." Sensor-based systems eliminate that wasted motion.

"The idea of pick-to-light was developed to maximize picker efficiency by reducing the overall tasks that the picker performs," explains Pat Bornhoeft, vice president of Electronic Design Inc. (Kiel, WI). "By simplifying picking procedures, picking speeds dramatically increase while picking errors are significantly reduced. The purpose for designing advanced technology is to reduce the problems caused by the monotony of a picker’s day."

Bornhoeft says pick-to-light systems "eliminate the time-consuming, error-riddled task of reading a paper list and finding the right location." By directing pickers to the right locations, the errors caused by misreading text or numbers on a list or stock location do not exist and the overall speed of picking increases.

"When picking from a paper list, you can expect to pick in the range of 100 to 150 line items per hour," explains Bornhoeft. "With the advent of pick to light, picking speeds have increased to at least twice the speed of paper list picking at 300 to 350 line items per hour. With recent advancements in pick-to-light technology, picking speeds exceeding 500 line items per hour are not uncommon. With speeds like that, payback on a system can easily be less than 18 months."

Bornhoeft claims the use of advanced infrared sensors has improved light-based picking applications. "Some distribution centers have achieved more than 1 million picks without errors," he notes. "Accuracy rates of 99.95 percent are common."

With sensor-based pick-to-light systems, Bornhoeft says operators don’t have to push buttons to verify that they’ve been in the correct parts bin. "The quantity displayed at the location is automatically cleared once the item is picked," he explains. "This eliminates the two major problems found with conventional push-button pick-to-light.

"The technology allows the pickers to simply pick items rather than pick and push a button or push a button and pick, which costs valuable seconds per item," adds Bornhoeft. "One to 2 seconds per pick time savings can easily add up to hours over a full day of picking. In some applications, it is possible to pick with both hands from two locations simultaneously."

Another advantage of sensor-based picking systems is an alarm that visually and audibly alerts the picker if he reaches into the wrong location, once again reducing the human factor from the manual picking process. The sensor also allows a picker to double-check a pick after it is cleared by simply putting his hand back into the location, causing the quantity to light up as long as his hand remains in the location.

Because this new technology uses solid-state components to verify a pick, maintenance over the long term is reduced, keeping downtime to a minimum, Bornhoeft says.

Numerous Benefits

Light-based part picking systems offer numerous advantages, such as less stress on operators because they don’t need to read a paper list. "The technology allows any person to walk in and become as efficient as possible with little training," says Bornhoeft.

In fact, SpeasTech’s Bitely claims "training can be a 5-minute process instead of several hours. People just follow the lights to stay on the right track."

Another big advantage of pick-to-light systems is a dramatic reduction in paper. "At the core of any pick-to-light system is its database," says Professional Control’s McCabe. The systems are based on a Windows platform, so they can be reconfigured to accommodate changing needs. McCabe also points out that the systems are scalable, making it easy to add light modules and parts bins.

"We load information directly to an Oracle database, so all picking sequences are generated from the bill of materials for a particular model every morning," says a manufacturing engineer at a large manufacturer of hydrostatic pumps that has been using light-based systems for several years. "It’s a great information system that reduces the amount of paper we use. There is a lot of variation in the products we assemble and we also float operators. Light-based part picking reduces the number of questions that an operator has."

Pick-to-light systems maximize throughput while keeping costly errors to a minimum, but they are not entirely error proof. "Many systems being sold are highly technical," warns the Lean Learning Center’s Flinchbaugh. "When introducing technology into the error-proofing process, there’s still a possibility that it will fail. What if something is out of sequence or someone puts the wrong part into a bin? Also, operators can simply ignore warning buzzers or lights.

"Manufacturing engineers must realize that people are still part of the system," adds Flinchbaugh. "There are always opportunities for making errors. You need to constantly make error-proofing part of your assembly process. Most companies do a little bit to boost efficiency and then put it out of mind." That’s something that even the most sophisticated technology can’t overcome.