The Role of Sensors in Error Proofing

Summary

The significant benefits of error proofing both automated and manual assembly procedures are becoming increasingly evident in all industries. The objectives are both increased quality and increased productivity. Often increased productivity comes at the cost of quality. However, with new sensor technologies, assemblies can be validated and error-proofed automatically in-process rather than post-process. This saves steps in the manufacturing process, and eliminates significant costs. High-precision photoelectric sensors, dimensional gauging sensors, light screens, and camera-based technologies, as well as manual techniques can be applied to ensure critical product attributes. And these same techniques can also be used to measure and assess process variability over time.
 
Error proofing is quite different from post-assembly inspection that can result in potential rework or scrap of parts. The very essence of error proofing is designing the assembly process in a way that the error or failure cannot occur. It is both proactive and preventive. Here are six valuable functions that these sensors perform along with typical application examples.
 
Part Presence/Position.

Verifying that a critical part is in place and in the correct position before the next step in the process is a common requirement. Photoelectric sensors can easily achieve this objective with the latest models able to detect even the smallest and narrowest edges of parts. In the illustration, the two sensors can form a convergent point a mere one-hundredth of an inch to accurately detect the edges of semiconductor wafers. In this example the sensors not only verify the presence of the wafer, but also can detect if the wafer is skewed in the cassette.
 
Short Range Measurement.

With the advancement of sensing technology in the last several years, users are now able to integrate cost effective measuring capabilities into their processes with resolution as high as 0.0001 inch. Parts can be automatically inspected for critical attributes prior to the next manufacturing step. In the illustration, laser sensors are checking a cast wheel rotating in a fixture to assure that there are no voids or excessive run-out prior to machining.
 
Long Range Measurement.

New, long range sensors can look inside a machine or process where a shorter range sensor will not fit or survive, or is intrusive to the process. Ranges are now available for inspecting at any distance from one to 164 feet with repeatable accuracy of 0.04 to 0.12 inches. This capability opens up thousands of new inspection applications. The illustration shows a single sensor measuring the range of motion of an automotive seat back to verify it is able to adjust in three angles of recline, without regard to seat material, color or texture.
 
Light screen part profiling.

Many parts or assemblies cannot be inspected with traditional single-beam sensors. Oftentimes the user needs to determine if any one or more of several parts are in the proper position on an assembly. A light screen system can profile an entire part for missing or misplaced components. In the example, a light screen is inspecting the surface a cylinder head to make certain that all valves are seated in the head in an automated assembly process.
 
Image recognition.

Oftentimes an entire product area must be inspected because the flaw could be anywhere in the defined area. Image sensors are designed to easily solve these applications with a camera that counts pixels and then compares the count to a pre-determined reference count. In the application example, the sensor inspects a plastic steering column component for irregularities in a flange area, and rejects parts that register 80% or less white pixels, compared to a good part.
 
 Manual bin pick verification.

The accuracy of humans in sequential manual assembly is also a huge issue in error proofing. PLC-controlled "picking" systems use lights indicating which item to pick next, and a light screen in front of each bin to verify that the correct part has been taken. These systems increase quality percentages for assembly operations by reducing missed parts, and parts assembled in the wrong order. Along with error-proofing the assembly process, they also increase worker efficiency by verifying at all times where an assembler last left off during the assembly process, even after a break or work-stoppage. This totally visual communication system is also a worldwide solution that eliminates training obstacles such as language barriers, and technical ability. The illustration shows a warehouse application.

In today's marketplace, it is not good enough to simply make parts. Manufacturers and customers demand high quality that is measurable and verifiable, for each and every part or assembly. Error proofing reduces the time spent on quality inspections and rework, in addition to lowering the reject and scrap percentages. This adds up to reduced manufacturing costs and higher plant profitability. Manufacturers are now able to optimize their production speeds and produce quality products faster than ever before.