By Jeff Himes, Product Specialist, Balluff, Inc., Florence, Ky.
Most inductive proximity sensors are intended for general applications in largely benign environments, but operation in extreme environments of temperature, shock and vibration, washdown, and welding call for specially designed sensors.
This SteelFace® sensor has a rugged stainless steel face that withstands repeated impacts.
Inductive proximity sensors come in a vast array of standard types to solve almost any general-purpose sensing application. But what is available for an application that has unique requirements, such as extreme heat or cold, repeated impact, high pressure wash-down, or a welding environment? What is available when the mounting area is confined and even the smallest “general purpose” sensors are too large, or the machine being monitored is a vital piece of equipment where unexpected down-time is unacceptable? All these are legitimate requirements in the industrial environment and specialized inductive proximity sensors exist to meet these unique needs.
Extended Temperature Range
The document IEC 60947-5-2 sets the standard temperature rating of an inductive sensor at “-25°C to +70°C.” Some applications such as continuous outdoor use or indoor freezers may require sensors to operate below -25°C. Applications close to molten steel or glass operations may require sensors that operate above +70°C. It is common for manufacturers to provide sensors that have extended low and high temperature ranges in the same sensor. Ratings such as -40°C to +85°C or -40°C to +100°C are typical. Some manufacturers concentrate on meeting extremely low temperatures (-60°C) or high temperatures (+250°C). Sensors designed for extremely low temperatures are built with special attention to differences in the coefficient of thermal expansion of the various materials that go into the product. Adjacent materials with incompatible rates of thermal expansion may lead to separation and component breakage. High temperature-rated sensors can handle temperatures above the maximum ratings of most solid-state components. The vital electronics are housed in a remote-mounted module where they can be protected from direct exposure to extreme temperatures; the basic copper-wound coil is the only component that actually experiences the high ambient heat. The construction of high-temperate sensors typically employs a stainless-steel housing and a cable jacket made from Teflon® (PTFE) or silicone to resist chemical breakdown at high temperatures.
A ceramic based coating called SlagMaster® is used to repel weld slag on the face of this WFI sensor.
It is estimated that almost 70% of all inductive proximity sensor failures are caused by direct impact to the sensor face. Standard sensors are not designed to handle impact; they are non-contact sensing devices. Nevertheless, manufacturers understand the problem is often unavoidable and are continually testing various materials in an effort to create a more robust sensor face. One of the more popular face materials used today is stainless steel. Because an inductive proximity sensor is designed to detect the presence of metal in front of the sensing coil, it seems counter-intuitive that metal-faced sensor is even possible. Through careful engineering, however, it is possible to tune an inductive sensor to ignore the metal face and respond only to external targets.
Many of these sensors have a housing that is gun-drilled from one end of a solid steel bar. Gun drilling produces a single-piece housing where the body and face are one integral unit without any seams or gaps. The housing materials vary, but most are made from 303 or 316 stainless steel. Some sensor models have a stainless steel face almost 0.75 mm thick, providing tremendous physical protection to the vital sensor coil. In repeated impact applications such as manual part loading and unloading, these metal-faced sensors can last ten times longer than general-purpose inductive sensors. Metal faced sensors are available with standard, double, and even triple sensing ranges. Some target-selective versions are also available, which are specially tuned to detect “ferrous only” or “non-ferrous only” target materials.
Many food and beverage applications require the processing areas to be washed down daily with high pressure water and cleaning solutions to maintain sanitary standards. Per DIN 40050-9, an IP69K rating is defined for high-pressure, high-temperature wash-down applications. These enclosures must be dust tight and must also be protected against directed high pressure water (1,500 psi) and steam cleaning (+80°C). Many inductive proximity sensors are available with an IP69K rating and certainly the right models to select for this type of application. It might be tempting to save money by installing a sensor with a standard IP67 or IP68 rating, but don’t do it. Although it may last a short time, it will soon need to be replaced. IP69K sensors are typically manufactured with stainless steel housings with integral cable or connectors, and are available with or without function LEDs. If a sensor with an integrated connector is chosen, a matching IP69K-rated mating cordset is necessary.
These SuperShorty® sensors are the shortest type of self-contained inductive sensors.
The automotive body and frame industry, the “white goods” appliance industry, and other metal-forming industries employ many welding operations to produce a final product. These weldments include frame structures, body panels, and underbody support components that must be welded to properly form the subassemblies. Automated or manual “weld cells” create very hostile sensor environments due to high ambient temperatures, weld slag generation, part-loading impact, and the presence of strong electromagnetic fields. Uniquely designed inductive proximity sensors are made specifically to survive in this harsh environment. They have WFI (weld field immune) electronics, incorporate slag-resistant body coatings (such as PTFE), and have high-temperature-rated face materials (PTFE or similar material). The faces on these sensors tend to take the brunt of the welding abuse; therefore, some manufacturers offer optional ceramic-based face coatings to protect and extend the life of the sensor face. Many WFI sensors on the market also offer multi-metal detection at the same sensing range. These sensors are commonly referred to as Factor 1 or F1 models, since they have no sensing range reduction factor for non-ferrous materials like aluminum. Stated another way, their sensing range reduction factor would be equal to 1.0 (no reduction in sensing distance). Standard sensing models typically experience a reduction in sensing range for non-ferrous target materials. This sensing reduction varies per non-ferrous material and can be as much as 60% for aluminum. In this case, the sensor would have a reduction factor equal to 0.4 for an aluminum target. Factor 1 sensors do not have this limitation.
Extremely Compact Physical Size
To reduce machinery weight and shipping costs and to cut costly factory floor space requirements, machine manufacturers strive to keep their equipment designs as compact as possible. These newer, smaller models are not expected to compromise on performance, in fact they are expected to outperform their larger predecessors. Additional automation accomplishes increased performance, and sensors play an integral role. Historically M8, M12, M18, and M30 tubular sensors have been the sensors of choice, but these are starting to be replaced with smaller 6.5 mm, M5, 4 mm, and 3 mm sensors. This new generation of mini-sensors is smaller in diameter, length, and weight – all of which are important. To accommodate some of the shorter sensor designs (less than 20 mm long), a separate amplifier module is often placed “in-line” with the sensor cable to allow the sensor head to be smaller. More recently, fully integrated electronics contained in ultra-short 6.5 mm and M8 tubular sizes have come on the market. Some of these models are available with integral cables having overall housing lengths of only 6 mm and 10 mm, and with M8 connectors having body lengths of only 18 mm.
Reliable diagnostics are extremely important for high-throughput machines. These machines depend on the ability to identify process quality issues in real-time. Examples of these types of machines can be found in the printing and paper converting industries. The sensors on these machines need to be highly reliable and have their proper functioning guaranteed. Some sensor manufacturers are now incorporating internal sensor diagnostics that generate dedicated status outputs for monitoring via the host control system. These diagnostics allow the controller to verify the sensor’s signal integrity and monitor the “health” of the sensor itself. Rather than experience an unforeseen sensor failure or improper signal output, the status of the sensor can be monitored to recognize the need to replace or readjust the sensor before it fails or fails to operate correctly. This diagnostic capability allows for planned maintenance activities rather than unplanned downtime. Diagnostic functionality is now available with certain inductive, capacitive, and photoelectric sensors.
Filed Under: Sensors (position + other), Sensors (proximity), Test + measurement • test equipment