Proximity Sensors

No matter what they are called—discrete sensors, position sensors, or proximity sensors—they are a mainstay in the factory automation arsenal. They serve in all areas of manufacturing including continuous processing, batch processing, utilities, and discrete products. Technology has changed the mechanical limit switch (rollers, buttons—often teamed up with the mechanically adj...


No matter what they are called—discrete sensors, position sensors, or proximity sensors—they are a mainstay in the factory automation arsenal. They serve in all areas of manufacturing including continuous processing, batch processing, utilities, and discrete products. Technology has changed the mechanical limit switch (rollers, buttons—often teamed up with the mechanically adjustable solid stop, just in case—into an intelligent, rugged, accurate device with the ability to sense a wide variety of objects that can be wired into a sensor network if needed. In short, flexibility has become key to proximity sensors’ adaptability in a control system.

In a recent on-line survey conducted by Control Engineering and Reed Research Group, control professionals from the magazine’s database were asked about their involvement in specifying, recommending, and/or buying proximity sensors for their industrial control systems. Of the total respondents, 64% used proximity sensors for in-plant requirements, 21% for OEM (resale) requirements, with the remaining 16% using them for both OEM and in-plant requirements.

discrete control

While proximity sensors aren't normally used for process variables, their presence in process industries is increasing.

Everywhere, in all applications

Proximity sensors are widely used in manufacturing disciplines of all types, as can be seen in the accompanying chart. Usage is not even skewed toward discrete products manufacturing as one might expect. Respondents claimed that 45% of their prox sensors were used in both continuous and batch processing operations. Discrete products manufacturing accounted for 28% of the total. Continuous processing rated 10%, utilities 5%, and batch processing only 3%. The other category, which includes such areas as scientific and research services, transportation services, government and military use, etc., accounted for 9%.

The graph labeled “specific industrial uses for proximity sensors” shows some changes in applications in the past five years. Although machinery applications remained at the top of the list, the second, third, and fifth place applications showed some significant changes. Packaging/palletizing applications increased in popularity (moving up from fifth to second place since 2002), raw materials processing applications fell two slots to fifth vacated by packaging/palletizing. Fill level—liquids moved down just slightly to third place. These shifts in usage may reflect technology improvements. For example, as proximity sensors of all types have became more intelligent and “network ready” clearly their adaptability to applications, such as very large packaging operations, has become both practical and much easier.

discrete control

Changing technology

Big changes in usage have also occurred since the previous survey done five years earlier. According to respondents, current figures for proximity sensor usage at their facilities show inductive proximity sensors at the top of the list. The biggest change over the past five years has been the lessening of dependence on the limit switch, once a mainstay of position sensor technology. In the case of this latest survey, the other category, which contains both limit and safety switches accounted for only 7% of the total current reported usage. In the 2002 survey these categories rated number one and two in current usage. Technologies have again promoted changes in industries relying on these devices. Survey respondents listed the following characteristics as important or very important when specifying, recommending, or buying proximity sensors. These include, from most to least mentioned:

  • Short circuit protection;

  • Availability of alignment/set-up aids;

  • Reverse-polarity protection;

  • Automatic sensitivity adjustment;

  • Corrosion resistance;

  • Wash-down protection;

  • Self-teach features;

  • Intrinsic safety;

  • Analog output; and

  • Weld field immunity.

This list makes it clear that feature availability has trumped familiarity, leaving older technologies behind. There is a warning that goes with this. Some verbatim survey responses recommended careful scrutiny of new technologies before ordering. “Get help from vendors if necessary,” “pay very close attention to spec details,” and “demo the sensor(s) in its intended application if possible” seemed to be the most frequent advice. Specifically, Carl J. Pawley, manager, HV systems, General Atomics, adds, “Test the sensor in actual application to find [its] shortcomings.”

Chad Vincent, principal quality engineer for Baxter Healthcare, keeps an eye toward what’s next. “Fully understand your application and future applications prior to purchasing a particular type of proximity sensor,” he writes.

Standardization of sensor type and subsequent inventory benefits were also on the minds of many respondents. Harold Goins, senior control engineer for Sanford Manufacturing, felt that standardization was very important in any choice, particularly with ac vs. dc, pnp (sourcing) vs. npn (sinking), and brand. Or as Robert Horon, manufacturing engineer for Cummins Filtration says, “Find a good, common brand, and use it as much as you can in all operations.”

Proximity sensor vendors, related products

Using a list, survey respondents identified proximity sensor vendors they purchased from in the past 12 months. In decending order, these include: Rockwell Automation (Allen-Bradley), Banner Engineering, Turck, Omron Electronics, Honeywell (Microswitch), ifm efector, Pepperl+Fuchs, Eaton Electrical (Cutler-Hammer), Keyence, Sick, Siemens, AutomationDirect, Balluff, Schneider Electric (Square D/Telemecanique), and Baumer Electric. “Other” was selected by 7% of respondents. Other vendors and distributors are listed at under proximity sensors. Find system integrators with related experience at . Product Research reports are available free at / .

Multi-pixel vision sensor

discrete control
The Allen-Bradley Bulletin 48MS MultiSight is an optical multi-pixel sensor intended for use in industrial automation with challenging pass/fail sensing applications, such as multiple inspections for positioning, marking, labeling, packaging, and detection of presence or absence and completeness. It has a pass/fail PNP output, and uses three methods of evaluation (pattern matching, contrast, and brightness) to detect or differentiate objects by means of previously defined optical characteristics, such as for separating “good” and “bad” parts. An economical alternative to high-end vision systems for quality assurance, MultiSight’s advantages include reduced inspection time and cost, flexibility and ease-of-use, and reduced installation time. PC and configuration software provides users with a simple setup; Ethernet connectivity, integrated lighting and compact housing ease integration into existing systems. Rockwell Automation

Threaded lens mount photoelectric sensor

World-Beam QS30 Series photoelectric sensors come with a universal housing that features a 30-mm threaded lens mount. The device also has easy-to-see LED status indicators as part of its unique housing design said to suit most mounting requirements. Available in opposed, high-power opposed, polarized and non-polarized retroflective, diffuse, laser, and fixed- and adjustable-field sensing modes, the sensors also feature a rugged, sealed housing with side mount capability on all models. They also offer high-power sensing with ranges up to 200 m. Models come with standard 10-30 V dc and bipolar NPN/PNP outputs or 24-250 or 12-250 V ac with e/m relay outputs. They include light- or dark-operate selectable or configurable modes depending on model. Attached cables are either 2- or 9-m long. Euro-style quick disconnects also are available. World-Beam sensors are rated IP67 or IP69K depending on model. Banner Engineering

Metal detection sensors

Uprox+ sensors are capable of precisely detecting materials such as iron steel, stainless steel, copper, aluminum, and brass at extended sensing distances without a reduction in the rated sensing distance of the sensor. They are said to offer unrivalled performance due to a newly patented multicoil system, which replaces the wound coil found in conventional ferrite core inductive sensors. This results in up to 250% greater sensing distances and the flexibility to incorporate this technology in a variety of unique housing designs. They also feature an integrated predamping protection function to reduce the metal free mounting area in applications. This allows traditionally flush mounted sensors to be recessed by half a turn for increased mechanical protection. All Uprox+ proximity sensors adhere to the present EN50082-2 standard, yet they also exceed the strict provisions required by EN61000-4-6. Turck

Stainless steel sensor thwarts harsh conditions

discrete control
The E2FM Series inductive proximity sensors feature stainless steel housings specifically to offer superior performance and corrosion resistance in harsh factory environments. Applications include robotic welding areas, machine tools, and production lines subject to frequent washdown with harsh detergents and aggressive atmospheres. The cost-effective E2FM is said to far surpass the working life of the traditional plastic or common metal sensor housing that requires frequent replacement or maintenance because of harsh environmental attacks or rough contact damage. The E2FM sensing face and housing are manufactured of up to 0.8-mm thick SUS 303 stainless steel that also protects the internal sensing electronics against the ingress of damaging liquids or contaminants that might occur if the sensing face is damaged. It is designed to ignore metal chips, welding spatter, and other surface depositions that can confuse traditional sensors. It also has a 20% longer sensing range (10 mm) than other metal-housed sensors, allowing it to be mounted further from potentially damaging production line activity. Available in 2-wire dc and 3-wire dc models, all sensors feature connectors to simplify installation and maintenance. Omron Electronics LLC

High performance proximity sensors

Honeywell’s rugged, reliable proximity sensors are designed for use in aerospace, ordnance, and marine harsh environments. Using a variety of technologies, including eddy current killed oscillator, and Hall-effect, these sensors offer a high level of sealing and EMC/EMI/RFI performance as well as high reliability. These one- or two-piece products are said to meet demanding requirements, such as extreme temperatures, vibration, shock, and EMI/EMP interference. They are available in a variety of housing materials and termination styles. The sensors meet the requirement of U.S. and European AOM industry, in particular MIL standards, RTCA/DO 160, and DEF. Honeywell Sensing and Control

Inductive proximity sensors for safety applications

discrete control
The ifm efector line of fail-safe inductive proximity sensors now is available for safety applications. The sensors can detect any standard metal target, such as stainless steel or mild steel, and does not rely on a coded magnet or keyed target that are typically required for most safety applications. They also offer a large misalignment tolerance that reduces accidental tripping. A non-contact design provides safe operation by using a window technology. Safe operation is enabled when a metal target enters the zone, or window, of the safety sensor. Any target outside the enable zone will result in the sensor failing to safe. Safety sensors connect to AS-i Safety at Work, and their developer’s safety evaluation units, and standard safety relays. The sensor line is rated up to Cat 4 according to EN954-1 and SiL3 according to IEC 61508. ifm efector

Proximity sensors for ac/dc, extended range

discrete control
The Rhino cube-style inductive proximity sensors are two wire units that are available with a 20 mm sensing range and are rated for 20-250 V ac/V dc operation. These sensors are suited for automotive manufacturing and any number of other applications requiring a 120 V ac power source. The devices are flush mountable and feature a coated die-cast housing as well as a durable thermoset plastic sensing face for long life and reliable operation. Additionally, these sensors feature a five-way quick pivot sensing face for operational and mounting flexibility, as well as two corner LEDs for high-visibility status indication. Sensor models NBB20-L3M-US-V93 and NBB20-L3M-US-V12 have short circuit and overload protected normally open outputs. They are offered with 1/2-in. or 7/8-in. style quick disconnects respectively. Pepperl+Fuchs

High range analog proximity sensors

AccuProx family of analog inductive proximity sensors is said to sense objects up to four times farther away than that of typical shielded or unshielded tubular analog sensors without compromising output accuracy. With long-range, tight repeat accuracy and linear outputs, they are said to be ideal for applications that require precise position sensing and measurement or detection of small variances, such as error-proofing, saw blade deflection, material blemish detection, absolute angle detection, and part positioning. Unlike a standard inductive sensor, which outputs an open or close signal based upon target presence or absence, an analog sensor outputs an electrical signal that varies proportionately to the position of a metal target within its sensing range. AccuProx sensors can also sense ferrous and non-ferrous metals and can be used to assure proper distance, size, and thickness for various types of measurements. AccuProx sensors are available with current and voltage outputs on the same sensor. Both shielded and unshielded models are available in 12, 18 and 30 mm tubular barrels with a variety of connection options. Eaton Electrical

Alternate-frequency EM series sensors for side-by-side Installation

Compact sensor heads from Keyence offer multiple frequencies to allow close mounting without mutual interference. With a sensor head diameter as small as 3 mm for the cylindrical type and M5 for the threaded type, the EM series sensor is an effective space-saver. The sensor head is half the length of conventional M8-sized self-contained proximity sensors, which enables installation in limited spaces. The output indicator is located on the amplifier housing installed in the cable, enabling sensor operation to be easily confirmed. A high-tensile copper alloy output gives 5 times greater flexibility than conventional sheathed cables. Keyence Corporation

Capacitive proximity sensor offers mounting optionsdiscrete control

CQ28 programmable capacitive proximity sensor features what is said to be a unique thin, sleek, feature-rich design. Its flat housing offers numerous mounting options, while maintaining room for the visibility of two LEDs. Teach-in functionality is conveniently available through the control wire or pushbutton. Due to its thin design, the CQ28 offers numerous mounting options, including clamp fitting, tie-wrap mounting, and it can be adapted to recessed installations. Two LEDs show the current switching state and the quality of detection. Both NPN and PNP switching output versions are available. The CQ28 can reliably detect liquids and bulk materials through nonmetallic walls in sensing distances of up to 10 mm. Applications include sensing fill level of all types of media (such as liquids, solids, powders) in tanks, containers, or pipe systems. The sensor has enhanced electromagnetic compatibility, which reliably prevents spurious switching caused by interfering surge voltages that can come from electrostatically charged granulate in bulk material silos, mobile telephones, radio devices, magnetic valves, and frequency converters. Sick Optical

Sensors for safety

discrete control
The Type 2 light curtains from Simatic Sensors perform monitoring functions at resolutions of 20, 30, 40, and 90 mm. They cover a protective field heights of 150-1,800 mm and have integral test mechanisms as well as startup/restart inhibit and contractor control, which removes the need for the previously commonplace external test monitoring and evaluation devices. This feature also eliminates the often expensive overhead wiring and additional cabinet space that they require. Typical applications include printing and woodworking machinery, paper processing, textile and packaging machinery, pick-and- place machines, and warehouse and conveyor systems. The devices comply with the safety standards IEC/EN 95-1 (Category 2), IEC/EN 61496-1 (Type 2), and EN 61508 (SIL 2) as well as risk assessment in accordance with pr EN ISO 13489. A self-diagnosis system with a 7-segment display simplifies startup and high-speed diagnostics on site. Siemens Automation and Drives

Inductive and capacitive proximity sensorsdiscrete control

AutomationDirect offers a wide variety of inductive and capacitive industrial proximity sensors. Various designs are available in sizes from 3-30 mm, sensor prices start at $16. All ac inductive sensors are available in fixed or quick-disconnect cable styles. Direct current models equipped with electrical protection against short circuit, polarity reversal, overvoltage, and inductive load. Most models are equipped with LED status indicators. Many models offer adjustable shielded and unshielded versions for extended sensing ranges. The 30 mm capacitive sensors have NPN and PNP 3-wire models with 2 m cables and are available with normally open or normally closed outputs. Rectangular, miniature, and stainless steel body styles also are available. . AutomationDirect

Economical long range sensors

discrete control
Steelface sensors combine long range with low cost. The initial 2X range offering includes PNP normally open versions in 12 and 18 mm sizes. Both sensors are available with an optional Teflon coating that withstands slag buildup in welding applications. Steelface sensors feature one-piece gun-drilled 316 stainless steel housings that protect against impact damage and boast sensing ranges of 4 and 7.2 mm. With this extended range it is possible to move the sensors farther away from the target, lessening the chance of impact with the object being sensed. Usable in demanding environments, the M12 version exhibits no loss of range when flush mounted, giving them more flush mounted range than their developer’s more expensive 3X versions. Long range characteristics, combined with Teflon coatings, give them long term survivability in tough weld cell applications. Intended applications include sensing jobs in machining, metal forming, welding, food, and food preparation. Balluff Inc.

Ruggedized inductive cylindrical sensors

discrete control
Osiprox inductive cylindrical sensors were developed to exceed the demanding sanitary requirements of food and beverage processing and pharmaceutical applications. They are said to meet the demands of the aggressive environments involved in these industries, such as extreme temperatures and humidity, resistance to food and beverage products and vigorous equipment washdowns using chemical agents with IP67, IP68, and IP69 ratings equivalent with industry standards. The sensors are manufactured with FDA approved materials, which include cables, casing, and front face. Schneider Electric

Compact ultrasonic sensors go the distance

Series 20 Miniature ultrasonic sensors now include seven devices capable of long range measurement, up to 1,000 mm. When mounted up to 1 m from the target, these compact sensors will deliver highly precise non-contact proximity, position, and distance sensing on sound reflective substrates including clear, glossy, and opaque materials regardless of color. The sensors feature an integrated teach-in button that allows the user to customize settings, and a lock out feature that prevents unwanted output programming changes. A four-pin M8x1 plug connector provides easy wiring options. With an IP67 rated polyester housing measuring 20 x 42 x 15 mm, these compact sensors are said to be ideal for space restricted applications. All seven sensors are available with discrete (on/off) or analog outputs. They can be designed into heavy equipment, printing machines, and robotic components, and can monitor automated food and beverage, pharmaceutical, chemical processing, metalworking, textile, and packaging production lines. Baumer Ltd.

Author Information

Dick Johnson is a consulting editor with Control Engineering. Contact him at .

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