Developing Detection

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. In fact, year 2012 proximity sensor shipments are set to 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 report.

Matching the correct sensor to the application saves a lot of cost and aggravation with easier setup, proper operation, and longer life. And recent advances in inductive, photoelectric, and laser area sensing are giving users even more choices and enabling better decisions.

For instance, 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 5 cm, and depending upon the physical sensor size, the range often may be less than 1.25 cm. Ulrich says inductive sensors are cost effective and robust (compared to photoelectric or mechanical types). They handle impact, mechanical shock, liquids, dirt, and dust without a sacrifice in performance. Shapes include tubular, cube, flat square and pancake.

Photoelectric sensors – project a beam of light and measure reflection to detect objects. Targets can be virtually any material. Distances range from thousandths of a centimeter for small fiber-optic models to more than 100 m for powerful throughbeam types. However, mechanical shock, dust, dirt, liquids and other contaminates can hinder optical performance, and 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 sensors – measure time of flight, offering 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.)

Selection advice

To help determine the best technology for the application, sensor suppliers offer online and inperson 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 (SCC), created in 2005, incorporates sensors, technologies, and experts from the company’s Hyde Park and Telemecanique business units. SCC, located in Dayton, Ohio, says it provides “a single, integrated resource for all its customers’ sensorrelated 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 number. This is useful when a part number is worn or documentation is unavailable. A “favorites” option allows comparison among various models.

Inductive advances

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.

• Flexibility: Complementary outputs may be available, such as normally open or normally closed.

Nichols says complementary outputs and autoconfiguration 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.

Photoelectric intelligence

Embedded intelligence in sensors can overcome challenges related to changing targets and ambient lighting. The Sick W12-3 photoelectric sensor, Eric Simmons says, uses Sick’s thirdgeneration OES3 application-specific integrated circuit (ASIC) to help resolve four key challenges of background and foreground suppression:

• 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, these 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 this problem.

• Another sensor: 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 challenges

A fifth challenge for photoelectric sensors is clear object detection, suggests Dennis Smith, technical marketing engineer at Banner Engineering. “Detecting clear objects is a difficult sensing challenge in many real-world situations,” he says.

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. 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, Smith says, making it “a very sensitive and highly reliable clear object detector.”

Meanwhile, there is the new Allen-Bradley VisiSight line of general purpose photoelectric sensors from Rockwell Automation. These come in a sealed, compact, cavity-free housing that minimizes the collection of dust and debris and allows easy sensor cleanup, according to Rockwell.

Various models in the VisiSight line 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 m 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.

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When Sensing Gets Tough

High temperatures and exposure to chemical sprays make liquid filling machines a harsh environment for proximity sensors.

Packaging machines endure high pressure chemical sprays and temperatures from 20 °C to 100 °C, which can eat through traditional sensors like a kid in a candy store. Elopak, a maker of carton-based packaging systems for non-carbonated liquid food and non-food products, endured problems associated with discrete sensors that were not rugged enough for its aseptic filling machines.

Seeking fewer sensor failures and better support, Michael Ballinger, Elopak manager of electrical engineering, and his team searched for a position sensor that better matched application requirements, including the ability to withstand high temperatures.

The Elopak filling machine is central to the carton production process, completing most logistics for carton production. The filling machine forms, fills, and seals a carton before being safely packed into one of Elopak’s material handling systems. A typical filling machine can process up to 12,000 cartons per hour. Because of the variety of beverages that run through a typical filling process, the machine is subjected to harsh washdown conditions to maintain sanitary conditions.

Intense environment

The process begins with the filling machine accepting a cardboard sheet, which can include two or more layers of various materials such as polyethylene, paperboard, aluminum and OVOH-barriers, depending on the type of liquid being filled. The machine forms the sheet into a specified carton size and fills the carton with a liquid food product, such as milk, juice, or smoothies. Finally, the carton is sealed and a closure cap is applied.

An aseptic filling machine is a tough sensing environment. Elopak’s model E-PL90HA filling machine enables customers to produce and fill cartons with acidic products that have a oneyear shelf life. Cleaning cycles, which run every eight hours and last three hours, expose the machine and its control devices to temperature fluctuations from 20 °C to 100 °C, high-pressure sprays, and chemical sprays of Oxonia Active.

During carton production, the machine and control devices are exposed to temperatures between 70 °C to 90 °C, and are subjected to a sterility chemical spray of 35 percent peroxide (H2O2).

While other machine parts were withstanding such exposure, position control sensors were found to be failing under the intense cleaning. Water and chemical sprays caused condensation to build up inside the sensors, causing short circuits.

In addition, water seeped inside cable connection seals and corroded the connection pins, causing failure. Large water droplets also formed on the sensor face, creating false signals or no signals.

Rugged response

Elopak found a solution in ifm efector’s OG Series photoelectric sensors for washdown conditions. These have stainless steel construction, high temperature compatibility, and Ecolab certification. The 18 mm diameter sensors also have an IP69K international protection rating, which ensures the sensor housing is dust-tight and can withstand high-pressure, hightemperature washdowns.

Multiple ifm fiber optic cables are placed above passing cartons to verify that the top of the carton is completely formed. The cables feature stainless steel sheaths to withstand high temperatures and wet environments in the Elopak Pure-Pak aseptic filling machine. A high-intensity visible-red light source burns through water droplets on the sensor face, providing a reliable signal and assisting with installation alignment.

Sensor specifications and rugged construction also avoid “false signals or no signals that would result in bad cartons being produced,” says Ballinger. Labor is saved because machines no longer require frequent sensor replacements; machine efficiency has increased with installation of the OG sensors; and productivity has increased. Ballinger also expects more cost savings in the long run.

Mark Hoske is editor-in-chief, Control Engineering