Proximity Sensors: How to Choose, Use Them
Matching the correct sensor to the application saves a lot of cost and aggravation with easier setup, proper operation, and longer life. Recent advances in inductive, photoelectric, and laser area sensing are giving users even more choices and enabling better decisions. Sensor manufacturers continue to innovate during and beyond the global manufacturing economic slowdown. Link to related resources.
Mark T. Hoske, Control Engineering
Matching the correct sensor to the application saves a lot of cost and aggravation with easier setup, proper operation, and longer life. Recent advances in inductive, photoelectric, and laser area sensing are giving users even more choices and enabling better decisions.
Sensor manufacturers have continued to innovate during the global manufacturing economic slowdown, and following positive economic predictions, proximity sensor sales are expected to strongly rebound in 2010, according to ARC Advisory Group; year 2012 proximity sensor shipments, for example, will exceed those of 2008. Brand labeling, partnerships, and modular design are among strategies sensor manufacturers are using to hold down production costs, says Florian Güldner, analyst and principal author of ARC’s “Proximity Sensors Worldwide
Outlook.” Modular sensor designs build on a standard set of components and interfaces within the sensor, Güldner says, creating reusable subsystems that can be produced in large volumes and used across multiple product families. This enables suppliers to buy certain modular components built by third parties, keeping end user costs stable while enabling innovation.
Choosing among inductive, photoelectric, and laser sensors requires knowledge about application requirements and sensor capabilities. Marcel Ulrich, product manager for Pepperl+Fuchs, explains the strengths and weaknesses of each type:
Inductive sensors detect changes in an electromagnetic field, so the target must be must be metal. Sensing ranges are short, typically within 2 in., and depending upon the physical sensor size, range often may be less than
Photoelectric sensors, as the name implies, project a beam of light and measure reflection to detect objects. Targets can be virtually any material. Distances range from thousandths of an inch for small fiber-optic models to several hundred feet for powerful through-beam types. Mechanical shock, dust, dirt, liquids and other contaminates can hinder optical performance. Variable target colors can cause difficulties for standard diffuse mode (direct detection) photoeyes. A myriad of specialty infrared and visible red photoeye options (fixed-focus, background suppression and fiber optic) can mimic laser precision at a lower price, says Ulrich.
Laser: Laser-based products, measuring time of flight, offer very precise optical detection, even at long distances. Although laser diodes have become more cost effective in recent years, laser-based photoeyes are the most expensive option of these three sensing technologies. Laser diode drawbacks, Ulrich says, include higher cost than other sensing technologies, temperature instability, limited life span, and eye safety concerns. (P+F has an “eye-safe” Class 1 infrared laser for sensing with a built-in Class 2 visible red laser for alignment.)
To help determine the best technology for the application, sensor suppliers offer online and in-person resources, as well as printed literature. Eric Simmons, product specialist for Sick, says Sick application engineers ask questions about application details such as object detecting, ambient conditions, mounting considerations, and other specifics. They may choose a sensor based on the initial description or request a target sample and drawing describing the application. “After receiving the sample and application details, the application engineer will be able to replicate the application at Sick and provide the recommended sensor type,” says Simmons. Sick, and others, also offers tools such as a sensor selection guide.
Schneider Electric Sensor Competency Center, created in 2005, incorporates sensors, technologies, and experts from the company’s Hyde Park and Telemecanique business units. SCC, located in Dayton, OH, says it provides “a single, integrated resource for all its customers’ sensor-related issues.” The SCC website has products, tools, literature, distributors, cross-references with other manufacturers’ sensors, newsletter, surveys, and videos, among other support and services. SCC’s online sensor selection tool allows searching by ultrasonic, photoelectric, inductive, and capacitive sensors, and by model number or partial model num
ber. This is useful when a part number is worn or documentation is unavailable. A “favorites” option allows comparison among various models.
For inductive sensors, technology advances have eased installation and improved durability, says Cory Nichols, sensors product manager from Eaton Corp. Recent improvements in Eaton cube and pancake style sensors include:
Auto-configuration : Some sensors can detect on power-up and intelligently determine if they were wired for NPN (ground switching) or PNP (positive switching) and adjust mode automatically;
Embedded intelligence : An onboard micro processor can provide potential for custom logic and allow factory customization in range and sensitivity, extending the range of sensor applications, including those with high electrical noise;
Rugged design : Vibration and impact absorbing potting compound can be used inside the sensor, making it more durable in harsh environments, increasing temperature range for use in heavy-duty outdoor applications, in vehicles, construction equipment, industrial wrappers, machine tools, and automated assembly lines; and
Flexibility : Complementary outputs may be available, such as normally open or normally closed.
Nichols says complementary outputs and auto-configuration allow end-users, OEMs, or system integrators to purchase one unit that can resolve a wider array of challenging sensor applications, instead of stocking multiple sensors.
Karen Keller, strategic marketing manager for Turck, says inductive sensor designs also can help electromagnetic compatibility (EMC) and avoid electromagnetic interference (EMI). Factor 1 sensors use separate, independent sender and receiver coils, so that ferrous and nonferrous metals have the same affect and rated operating distances are equal, she says. By eliminating a ferrite core, Keller says, factor 1 sensors operate at a higher switching frequency and are immune to EMI from electric welding equipment, lifts, and electronic furnaces. Turck uprox+ products, including Q10S, are advanced factor 1 inductive sensors.
Intelligent photoelectric sensing
Embedded intelligence in sensors can overcome challenges related to changing targets and ambient lighting. The Sick W12-3 photoelectric sensor, Simmons says, uses Sick’s third generation OES3 application-specific integrated circuit (ASIC) to help resolve four key challenges of background and foreground suppression.
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Black/white shift : Black on a target absorbs much more light than white. When a target changes color (either on the same package or after a recipe change), sensors may no longer detect the target, which would cause the sensor’s output to turn off and then back on. This must be handled by software, or with readjustment of the sensor.
Stray reflections beyond a sensor’s range may reflect back and flood its receiving elements. This could be caused by someone walking past with a reflective vest, a window being opened beyond the sensing range, a shiny object moving beyond the set point, and similar environmental changes.
High frequency lighting : New fluorescent lighting saves energy, but these high frequency lights can wreak havoc on photoelectric sensors, producing “chatter.” Embedded intelligence can fix that.
Another sensor’s light source : Cross talking occurs when two sensors are pointed at one another. Sensors are modulated to a unique frequency but may have instances when they are nearly in phase or a pulse is in the same modulation as its own light source. This can fool some sensors into thinking they see targets within the sensing range.
Clear, reflective object detection
A fifth challenge for photoelectric sensors is clear object detection, suggested Dennis Smith, technical marketing engineer at Banner Engineering.
“Detecting clear objects is a difficult sensing challenge in many real-world situations,” said Smith. The Banner Engineering World-Beam QS30 Clear Object Sensor controls how the emitted light striking the sensor reflector prevents false light signals from reaching the photodetector, Smith says. Design of the sensor allows its “microcontroller to detect small changes in the light level, so even clear objects that alter the light level only slightly will activate the sensor,” he says, making it “a very sensitive and highly reliable clear object detector.”
Another example is the new Allen-Bradley VisiSight line of general purpose photoelectric sensors from Rockwell Automation comes in a sealed, compact, cavity-free housing that minimizes the collection of dust and debris and allows easy sensor cleanup, company says. Various models target various applications: Diffuse models with an 800-millimeter sensing range provide adjustable sensitivity. Polarized retro reflective models with 3.5-meter sensing range come in adjustable or fixed sensitivity versions. Transmitted beam models provide 10-meter sensing distance, and infrared LED source models provide crosstalk immunity. A red light helps with alignment during setup and maintenance; a stability indicator flashes if the signal level is too close to the detection threshold. A patented ASIC provides linear sensitivity adjustment and noise immunity.
Mark T. Hoske , Control Engineering editor in chief, can be reached at MHoske@cfemedia.com .
Washdown photoelectric sensors handle high temperatures, chemical sprays in filling machines - ONLINE extra - photo gallery, more application details.
More sensor resources online
Applications using many sensors may benefit from using IEEE1451.4 Transducer Electronic Data Sheet (TEDS) to organize information about sensor manufacturer, model, and calibration, even with non-TEDS-enabled sensors, using a “Virtual TEDS” approach. Honeywell and National Instruments are among TEDS proponents. NI has more TEDS information . See more from Honeywell on TEDS .
Balluff BOS 50K photoelectric sensor has an extended range and enhanced background suppression. Learn more about this technology.
Carlo Gavazzi long range diffuse photoelectric sensor sees black up to 2 m, white and gray up to 2.5 m.
Honeywell SMART position sensor video . SMART stands for Superior Measurement Accurate Reliable Thinking.
Omron Industrial Automation E2EC - 2-wire & 3-wire dc subminiature proximity sensors have inline amplifiers for greater mounting flexibility.
Siemens’ new photoelectric proximity switches have special capabilities for handling delicate tasks-see video clips.
Sick laser measurement system sensors can be used for outdoor anti-collision and indoor profiling applications.
More sensors are on Control Engineering ’s industrial sensor channel .