Assembling Products Correctly
It requires queuing and delivering the right stuff—at the right time
By Tom Rosenberg
Assembly Industry Manager
Manufacturing high-quality automotive parts means much more than building a quality product. The right product must be queued in the correct sequence and delivered at the right moment—time after time.
Two keys to making sure that the process meets this stringent production requirement are error proofing and data tracking.
There are two types of error-proofing techniques used in manufacturing: active and passive.
- Passive error proofing uses mechanical keying to make sure that a process can be performed correctly.
- Active error proofing uses sensing devices to verify that a process step was completed correctly, as well as tracking the overall process.
Passive error proofing provides a simple and effective mechanical means of ensuring that a part is present and in the correct orientation or position for processing.
Active error proofing addresses common production concerns:
- Incorrect part installation (wrong bolt size),
- Color mismatches (wrong trim piece),
- Missing parts (washer not installed),
- Insufficient fastening (torque not adequate),
- Insufficient lubrication (not enough grease),
- Product mislabeling,
- Products delivered out of sequence.
Industrial sensors, which have proven their effectiveness in basic automation tasks, are beginning to be used increasingly in the error-proofing function. Advanced sensor technologies are enabling them to compete successfully with traditional active error-proofing devices such as vision systems.
Sensors work well in the following situations:
The parts are well fixtured. The part is either contained in a fixture, or the operator can bring the part into an inspection station that can hold the required tolerances. There are a manageable number of inspection points per part, i.e. desired parameters can be verified by a reasonable number of sensors. Finally, the location of the detail on the part in question is relatively constant.
Sensors provide standardized outputs that are either discrete (yes-no) or analog (measurement or position). Integrating active error proofing involves determining the right use of sensors and the level of error proofing required.
Discrete sensors are simple and easy to integrate, while analog sensors are able to convey actual measurements or product position data. Outputs from either type of sensor can interface directly to existing control systems that handle the lockout, sequencing, or rework-diverter functions. Simple indicator lights, panel meters, or man/machine interfaces can also be used for evaluation.
Laser-based sensors offer a higher level of precision, ease of use, and cost effectiveness in error-proofing applications. Laser sensors detect product details by either using diffuse techniques or diffuse with background suppression techniques, or breaking a beam using through-beam or retroreflective techniques. Beam-break versions are reliable, accurate, and capable of long-range position detection.
Beam versions can detect product details either based on shade differences or position differences. Typical shade differences include detection of color shifts, surface finish, and polish levels. Position differences refer to changes in product position relative to the position of the sensor.
In application, easily seen laser spots assist operators by highlighting the specific product detail, and are especially useful for informing the operator about error locations after detection. The long-range capability of lasers allows them to be positioned around operators or moving tooling, enabling more points of detection in smaller areas. In addition, the precision available from laser sensors is often far in excess of what the machine fixturing can provide. The better the machine tolerance, the better the laser performs in that operation.
When a simple, fixed, yes-no response is not enough for successful assembly analog measurement, a sensor can provide the additional data essential for error proofing in flexible manufacturing environments. Analog sensors provide part-position information in the form of an analog signal that interfaces directly into the control system, allowing both actual measurements and continuously variable yes-no decisions. Some analog sensors offer one or more discrete outputs that also offer continuously variable yes-no decisions without impacting the control system.
Extreme accuracy for error proofing is seldom required in the actual manufacturing environment. The additional costs associated with extra microns of resolution are quickly lost in machine vibration, bearing wear, and fixture issues. Measurement sensors use technology that provides the resolution necessary for production applications, for example while fighting the negative effects of target color changes. For short to middle-range sensors, triangulation technology is used. For ranges up to 6 m, time-offlight technology is used.
Analog laser sensor solutions for complex applications and the laboratory can be custom designed.
It is critical for error-proofing sensors to be reliable and accurate. If an error-proofing solution fails to meet these criteria, operators or maintenance personnel will often bypass them. A bypassed error-proofing sensor creates an even greater chance for problems by creating a false sense of security in the operator. Also, reliability issues often occur when sensor alignment changes or dust and dirt form on a sensor's lens.
The self-contained through-beam sensor is designed for most errorproofing applications that are exposed to tampering or potential damage. The through-beam sensor comes in two styles: the C-frame and L-frame versions. Laser versions are available for parts that are extremely small with tight tolerances. The Class II laser is about 10x more accurate than the standard visible red versions.
Where the physical constraints of the C-frame sensor will not fit the application, the L-frame style is available. The L-frame works in applications where the C frame would be excessively large or two-axis movements might be blocked by one of the legs of the C-frame sensor.
Active error proofing doesn't always take place in a tight, well-defined area. In such a situation, some applications employ a vision system. In many cases, however, a simple area sensor would be sufficient to do the job. Examples where an area sensor would work include sorting parts based on height or width differences, detecting a part when exact position is not known, verifying when the operator has picked a part, or detecting parts accumulating on a feed track or delivery tube.
The area sensor works by creating a grid of light beams between an emitter and receiver. Emitter and receiver units come in heights of 100, 150, and 300 mm and can be separated to 2.1 m. The receiver has both a discrete and an analog output. The discrete output detects a part entering the active area. The analog output generates a 0-10V signal based on the position of the beams that are broken. This output has a resolution between 4 and 7 mm (depending on model), making area sensors well suited for sorting and verification applications.
UV tracing is the most reliable method of error-proofing complex assembly tasks—even better than a vision system. There are two steps in the UV tracing process. The first step is to apply the tracer material to the parts in question. The second step is using a UV sensor to detect the tracer material. When the sensor signals that the tracer is present, the part has been positively identified.
The advantages of UV sensors include reliability, accommodation of loosely fixtured parts, use of fiber-optic cabling for tight locations, simple teach-in controls, and compatibility with any control system.
Radio frequency identification (RFID) in error proofing gives manufacturers the flexibility to make various product versions on the same manufacturing system. To be able to do this, the exact version of the product being manufactured must be known, because different product variations have unique features to error proof. RFID systems store build data on a small data carrier that is permanently affixed to the build pallet.
Before assembly begins, the data carrier is loaded with the build information that will instruct all downstream processes as to the exact part version being manufactured. Correct assembly is verified by comparing the build information to what the error-proofing sensors detect. Build information can be kept decentralized on each build pallet, held centrally in the control system, or can act directly without intervention from the control system. These differences have a direct impact on the communication method required between the control system and the data carriers.
When using a decentralized approach, the ID system must support both read and write functions, handling data through standardized interfaces such as DeviceNet and Profibus. Before assembly begins, the build information is written into the data carrier. The assembly system reads the build information at each station to determine what assembly and errorproofing operation is required. Also, actual test results can be loaded into the data carrier for subsequent archiving.
When build information is maintained and referenced centrally in the control system, a simple and economical parallel, read-only interface can be used. This eight-bit interface connects directly to inputs on the control system, significantly reducing integration time. The control system establishes a virtual build sheet by equating the pallet number to a list of build sheets resident in the control system.
For highly specialized applications such as color matching and in-line vehicle sequence (ILVS) verification in the automotive industry, target applications use true color sensors. In this case, color sensors are tuned for the difficult darker shades typically found in automotive interiors. Totally self-contained, color sensors are able to learn three individual colors without the need for external lights or controllers. Setting the sensor is accomplished by teaching the intended color, and then assigning a tolerance level to that color setting. Two decision modes are also available to handle shiny or matte surfaces.
Parallel ID System Improves QC at Briggs & Stratton
The Briggs & Stratton Small Engine Facility in Polar Bluff, MO, builds 14,000 units a day of the Quantum four-cycle engine for power equipment such as self-propelled lawn mowers. The goal for Billy DePew, manufacturing engineering technician, was to improve traceability of completed engines coming off the line. This meant being able to trace a non-conforming part back to the pallet it was produced on, the date of manufacture, and the work shift, to check other engines of that batch to determine the exact corrective procedure.
A Balluff parallel passive inductive ID system was added to help identify specific engines within their production batch. With this system in place, engines that did not meet Briggs & Stratton's QA standards could be isolated by a time stamp (date/time of manufacture) that was applied to each block, providing traceability and accountability for work performed by Briggs' operators. The ID system helped Briggs and Stratton flag missteps before final engine assembly and reduce scrap, increasing throughput and the overall quality of its engine components.
A Balluff 60R read-only passive inductive identification system was selected to help improve quality tracking on the engine line. The BISC-60R-001-08P ID system installed at Briggs & Stratton consists of a parallel, self-contained read head, an eight-bit parallel (eight-byte addressable) device resembling a 30-mm inductive proximity sensor. This read head works with 1023-byte BISC-12805/L data carriers, one of each being mounted on the machining pallets. These data carriers are essentially EE-PROMS encapsulated in manufacturing-grade Duraplast material. They are capable of withstanding hostile manufacturing environments and rated from -20 to +70°C.
The 88 data carriers used in this application have a mounting hole in the center of the tag for pallet attachment. Data exchange between the carrier and the read head is noncontact and wear-free. The data and power for the data carrier are inductively coupled, and require no battery for operation or data retention, eliminating added cost and the risk of losing data. Precise alignment of the read head and data carrier is not necessary. The data carrier can be read on-the-fly while moving past the read head.
After the data carriers were mounted on the engine block pallets and the M30 read heads were mounted and aligned, the system was ready for integration into Brigg's control system. DePew integrated the Balluff ID system into his Mitsubishi PLC (Mitsubishi Electric Automation Inc., Vernon Hills, IL) and Telesis Technologies Inc. (Circleville, OH) workstation, which is equipped with an LCD screen used for visually checking data. All communications in the network are via RS-232.
Small engine blocks are installed on each of 88 production pallets moving on a conveyor through 54 machining stations. Cylinder bores are rough-cut, valve guide holes are bored, oil fill holes are cut, breather bores and cam bores are machined, and heads are tapped in sequence at the various stations. At the third machining station, the pallet number is automatically read off the data carrier on the pallet, and that number, plus the date, time, and crew/shift number, is coded by an operator into a pin stamper, which imprints the information on the corresponding engine block. These data are essential in pinpointing any out-of-spec components that must be removed from the system. Beyond that, the process provides greater traceability and quality control for the entire engine machining process.
This article was first published in the July 2006 edition of Manufacturing Engineering magazine.