To Err Is Not Divine

Brian Schriver

The hope of every manufacturing management team is to deliver defect-free product. But if that isn't feasible, then the next goal is to minimize defects as much as possible. That's where error proofing comes into play. While it in no way accepts even a small number of defects, its two-pronged approach aims to either prevent them from ever happening or to catch them as soon as possible. And the primary weapons in error proofing's arsenal are sensors.

Stopping Defects
Error proofing is the implementation of mechanisms to prevent product defects. Based on the philosophy that even the smallest number of defects is unacceptable, error proofing maintains that the best way to eliminate defects is to prevent them from happening in the first place.

There are three keys to stopping the proliferation of defects:

  • Awareness, which involves training, audio-visual aids, and general assistance for personnel to prevent mistakes
  • Detection, which is promoted through the introduction of manual or automated inspection techniques to filter out defects
  • Prevention, which includes process improvements or automation to ensure no errors are (or can be) made

Error proofing drastically reduces defects by preventing errors and by catching those that have been made as soon as possible. Examples of these two processes can range from color-coding, which helps the system distinguish between visually similar components, to sensing that determines if an automotive suspension assembly is moved to the next operation before a bolt is placed.

The Sensor's Role
Sensors play a key role in error proofing, and while most are used as simple detection devices, a few are designed to support preventative measures. Sensors used to detect defects come in all shapes, sizes, and technologies—from standard inductive proximity and photoelectric sensors to vision sensors capable of complex image analysis and specialty sensors designed to support the prevention aspect of error-proofing applications.

Sensors are used primarily to determine the presence or positioning of a part or a feature on a part. Limit switches are cost-effective and easy to apply, but the fact that their operation requires contact with the part prohibits their use in many applications. Inductive sensors are generally the most economical noncontact solution, assuming the parts to be detected are metallic and the sensor can be mounted close enough to accommodate the relatively short sensing ranges (<40 mm) associated with this technology. They're also rugged, making them a good all-around error-proofing solution in harsh industrial environments.

For applications requiring greater sensing distances and/or detection of nonmetallic parts, photoelectric sensors are often the best—and simplest—solution. Infrared sensors in standard sensing modes (e.g., diffuse, retroreflective, and transmitted beam), as well as more specialized sensors in background/foreground suppression, wide-angle, and fixed-focus modes, address the lion's share of error-proofing applications. Laser-based models are also available for sensing extremely small parts and part features at a relatively greater distance.

Specialized photoelectric sensors designed with error-proofing processes in mind address those situations where standard sensing packages are not suitable or are difficult to apply. Photoelectric light arrays using 2D scanning technology can create a light screen to sense an object regardless of its orientation. This is ideal for the detection of parts being ejected from a machine and a host of other applications focused on early-detection error proofing.

The same scanning technology is also used in parts-verification arrays (PVA) that are specifically geared toward error prevention, notably in bin-picking applications. Spanning the sensing fields of multiple PVAs across assembly station bins and wiring them to a controller programmed with the necessary logic can achieve a virtually error-free bin-picking process. Integrated job lights on the sensors show the assembler the correct picking sequence. In the event the assembler attempts to pick an incorrect part, a warning light indicates the error. Additional fault enunciation can be achieved via controller logic in conjunction with a tower light or audible alarm.

Vision technology represents the high end of the sensing spectrum in error-proofing applications, ranging from standalone vision sensors to complete vision systems. Complete vision systems usually perform complex inspection applications; require the integration of camera, lighting, processor (usually a PC), and software; and typically involve a fair degree of customization and programming expertise. Because of this complexity, vision systems are overkill in many error-proofing applications and their use cost-prohibitive. As a result, vision sensors have emerged as the preferred technology for applications requiring economical yet advanced sensing of presence or absence, completeness, position, markings, labeling, packaging, and components. Vision sensors combine an imaging device, lighting, and the necessary logic into a single, self-contained unit that provides a simple pass/fail output. Using multiple methods of evaluation (e.g., pattern matching, contrast, and brightness) to detect or differentiate objects by means of previously defined optical characteristics, the pass/fail output is used to separate good parts from defective ones. Detection parameters are set up through simple configuration of pre-defined functions, as opposed to the detailed software programming associated with vision systems.

Sensors are an integral part of the quality-control process. There's no shortage of sensing technologies to effectively address all your error-proofing applications, whether it's simple presence sensing or complex inspection. A complete and successful error-proofing effort will most likely involve a mix of these sensing technologies, as well as audio-visual aids, awareness training for personnel, and some creative thinking in the development of new preventative processes. All are key to driving not only product quality but also customer satisfaction. Manufacturing errors become product defects only if they make it out the door—and into your customer's hands.