By Henry Menke

Industrial sensors have proven their effectiveness in basic automation tasks and are being used increasingly in error-proofing functions. Sensor-driven error proofing, often in concert with RFID, provides a simple and effective means of ensuring that a part is present and in the correct orientation or position. These sensors provide standardized outputs that are either discrete (yes-no) or analog (measurement or position). Integrating them into active error proofing schemes involves determining the level of error proofing required and applying the sensors for maximum effectiveness.

For sensors to work correctly, certain conditions have to be present: Parts need to be well fixtured; There needs to be a manageable number of inspection points per part; and the location of the detail on the part in question needs to be relatively constant.

Assembly and automation tasks use a wide range of sensing technologies to determine parts presence, feature detection/confirmation, hole presence/absence, and nesting validation. Discreet photoelectric sensors in thru-beam, retro-reflective, and diffuse reflective forms are all used to detect parts presence over a sensing distance greater than 30-40 mm and/or when non-metallic components must be detected.

When a simple, fixed yes-no response is not enough for successful assembly, an analog 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 to 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.

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 using either diffuse, or diffuse with background suppression techniques, or breaking a beam using through-beam or retro-reflective techniques.

Beam-type sensors can detect product details either based on shade differences, or position differences. Typical shade differences include of color shifts, surface finish, and polish levels. Position differences refer to changes in product position relative to the position of the sensor.

UV tracing is the most reliable method of error-proofing complex assembly tasks. There are two steps in the UV tracing process: The first is to apply a tracer material to the parts in question. The second is using a UV sensor to detect the tracer material.

The benefit of using UV wavelengths is that tracer materials are invisible to the human eye, inert, and do not impact product aesthetics. Many greases and lubricants inherently “glow” for UV sensors, so many engine test stand and powertrain manufacturers use UV sensors to detect leaks and overflow during lubrication tests.

The same error-proofing principles hold true for metal forming as they do for assembly and automation practices, with a few additional objectives. In conjunction with goals for stamping parts with zero defects, objectives are to protect dies from damage, prevent die lock up, and run production without interruption.

Discreet sensors are used to monitor stripper position, strip feed, pilot holes, and other features. They can also be used for slug-out and parts-out sensing in dies for error-proofing.

Short range analog sensors can measure bend angles, validate features, measure critical dimensions, and are often used to measure press parallelism on stand-alone error-proofing stations. Photoelectric sensors can be integrated to measure precise roll feed, parts out, and for slug-out detection.

RFID in error proofing
RFID in metal forming can, for example, assure all die segments are in place prior to stamping, and is used for die identification and tracking, which is important if a customer has hundreds of tools in house.

Sensors and RFID technology are also teaming up to minimize errors in the rework process. RFID tags located either on the assembly or the pallet, can control what should be and what has been done not only within each automation island, but between islands.

When a problem sub-assembly reaches the rework area, the RFID tag informs the rework PLC to program sensors to ensure that repair tools go to the correct place in the correct sequence, so that the correct repair procedure is the only possible action that can be taken. In this case, a human operator becomes simply an actuator, guided by software informed by the RFID tag and controlled by the sensors.

Flexible manufacturing requires the ability to make various product versions on the same manufacturing line. Exact versions being manufactured must be known, because different product versions have unique features to error proof. An RFID system that stores build data on a small data carrier affixed permanently to the build pallet is the most effective method for accomplishing this.

Sensor-based RFID systems have proven especially useful in machining operations where data is included on pallets that move in and out of machining stations. Before assembly begins, the data carrier is loaded with build information to instruct all downstream processes. 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 communication between the control system and data carriers.

When using a decentralized approach, the RFID system must support both read and write functions, handling data through standardized interfaces such as DeviceNet and Profibus. Before assembly begins, build information is written into the data carrier. The assembly system reads the build information at each station to determine what assembly and error proofing operation is required. In addition, 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. An 8-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.

See the entire article on Control Engineering website