Selecting proximity sensors for diverse applications

Proximity sensors provide high-quality performance for an evolving range of applications.

By Tony Udelhoven August 9, 2012

In today’s industrial environments, most processes rely on monitoring systems to ensure product quality and process efficiency. Obtaining reliable results depends on selecting the appropriate sensor technology for the application. To accommodate these demands, sensors must be durable, flexible, and reliable, regardless of the environment. Proper sensor selection requires careful consideration of the sensor’s capabilities, limitations, and suitability for the intended application. 

Because of their versatility and high level of functionality, proximity sensors cover a diverse range of sensing capabilities. Popular proximity sensor types include inductive, capacitive, and ultrasonic. Although other types, such as optical, radar, and vision sensors are also considered to be proximity sensors, only the inductive, capacitive, and ultrasonic types are included in this article. 

Because they typically perform extremely well in a variety of applications, proximity sensors are well suited for many industries such as food and beverage, chemical processing, oil and gas, pharmaceuticals, discrete manufacturing, and building and construction. Their functionality, flexibility, and reliability allow proximity sensors to meet the requirements of most applications. 

Proximity sensor technology

Proximity sensors are inherently noncontact detection devices. Unlike traditional sensors that require a mechanical switch, lever, or plunger to be pushed against the moving target to adequately register the presence of an object, proximity sensors accurately detect an object without requiring contact. This noncontact design also protects the integrity of the sensor and promotes product quality by not subjecting sensing equipment or materials to unnecessary interaction. 

Proximity sensors are designed to provide accurate and repeatable operation under high-speed conditions. Performing at speeds as high as 5,000 Hz, these sensors can easily accommodate the demands of many fast-paced industrial applications. Because their accuracy can be calculated to within 0.001 in., proximity sensors offer the precision necessary to maintain efficient, effective production. 

Today’s proximity sensors are durable and flexible. Sensors are typically available with multiple IP ratings, which range from relatively clean environments to those with washdown conditions or temperature extremes. With various sizes, housings, and mounting options available, proximity sensors offer both versatility and application flexibility.

Performance versatility, application diversity

Different types of proximity sensors can solve a variety of sensing challenges. Using the right sensor for the right application minimizes downtime, reduces maintenance requirements, and enhances production efficiency. Understanding the various operating principles, performance capabilities, and application suitability of each proximity sensor type enables manufacturers to integrate the appropriate sensors to the right applications. 

Inductive sensors: Inductive sensors detect the presence of metal objects—whether they are ferrous or nonferrous. They can be used to detect the presence or absence of parts, to count objects, or in positioning applications. Inductive sensors are often used instead of traditional limit switches because they can operate at higher speeds than mechanical switches (Figure 1). Inductive sensors are also more reliable because they are more robust.

Inductive sensors generate a high-frequency electromagnetic field. They are typically constructed using a coil and a ferrite core. When a target passes through the sensor’s magnetic field, the current induced on the target’s surface changes the characteristics of the oscillator that generates the field, causing it to lose energy. The sensor is designed to detect this energy loss as a transition, which in turn triggers a signal to actuate a solid-state output to either an “on” or “off” state. When the metal object exits the magnetic field, the oscillator regenerates, and the sensor returns to its normal state. 

The output stage of a proximity sensor can be either analog or digital. Analog versions can be voltage (typically 0-10 Vdc) or current (4-20 mA). They typically provide a linear signal to allow distance measurements of up to nearly 2 in. Digital outputs are designed to be used in dc-only circuits or in ac/dc circuits. Most versions are configured with normally-open outputs, but other versions can be normally closed, or can incorporate both a normally open and a normally closed output. The versions using the Namur output are intended for hazardous locations and must be used with the appropriate interface device for intrinsic safety ratings to apply. 

Some advanced inductive sensors use multiple coils that enable them to detect all metals at the same range without requiring adjustments. Instead of a single coil inducing and being affected by eddy currents on a target, these inductive sensors use separate, independent sender and receiver coils. By detecting both ferrous and nonferrous components, these sensors provide an overall longer operating range. 

Inductive sensors are well suited for detecting metallic objects in machinery and automation equipment. Some inductive sensors are inherently immune to magnetic field interaction, making them useful in applications where alternative technologies would fail due to interference with magnetic fields, such as welding, lifts, and electric arc furnaces. 

Capacitive sensors: Capacitive sensors can sense motion, chemical composition, fluid level and composition, and pressure. They can be found in many industries and applications. Capacitive sensors can sense through lower dielectric materials, such as plastic or glass, or detect higher dielectric materials, such as liquids, which allow them to identify the level of numerous materials directly through glass, plastic, and other container compositions (Figure 2). 

A capacitive sensor’s active element is formed by two metallic electrodes positioned to form the equivalent of an open capacitor. These electrodes are placed in the feedback loop of a high-frequency oscillator. When no target is present, the sensor’s capacitance is low and the oscillation amplitude is small. A target approaching the face of the sensor increases the capacitance, thereby increasing the amplitude of oscillation, which is then measured by an evaluating circuit that generates a signal to turn the output “on” or “off.” 

Generally, capacitive sensors have a wide sensitivity band that makes them capable of performing well in difficult applications, such as sensing very small metal parts through a tube. Capacitive sensors are also available in different housing types, with barrel, rectangular, and probe styles available, and constructed to withstand a variety of demanding environments reliably.

Ultrasonic sensors: Ultrasonic sensors are used to detect the presence of targets and to measure the distance to targets in many automated factories and process plants. As with capacitive sensors, this technology can be used to measure multiple variables, such as wind speed and direction, tank fullness, and speed through air or water, and can detect targets in a solid, liquid, granular, or powder state. 

These sensors use an operating principle similar to radar or sonar. By generating high-frequency sound waves, ultrasonic sensors analyze the echo that is received after being reflected off the target. Then the sensor calculates the time between sending the signal and receiving the echo to determine the distance to an object. 

The atmosphere surrounding the target does not affect the accuracy and performance of ultrasonic proximity sensors, which ensures that results will not be skewed by the presence of dust or moisture. This makes them a reliable solution for harsh and demanding conditions. Further, since ultrasonic sensors use sound rather than light for detection, they are well suited for applications requiring clear object detection and liquid level measurement. 

Proximity sensors in action

In the oil and gas industry, inductive sensors provide a durable, reliable solution for measuring variables on offshore oil rigs. In order to be a suitable solution for this type of platform, sensors must be able to endure harsh sea conditions, such as saltwater, which can be corrosive and damaging to equipment. Therefore, implementing ATEX-certified inductive sensors can reliably measure the final positions of pipe handlers and other moveable components on the rig without performance being impacted by the elements. Plus, inductive sensors are not affected by contamination, making them impervious to oil, grease, and dirt. 

Inductive sensors can also be used in beverage processing applications. In order to maintain consistently high product quality, production chains must run smoothly, without unnecessary halts that can cause lost revenue and potentially jeopardize the entire process. Using rugged inductive sensors that can perform position monitoring without requiring readjustments minimizes costly production downtime. Sensors capable of recessed mounting protect equipment integrity by protecting the sensor from mechanical damage during operation. 

Capacitive sensors are used to detect a wide variety of materials, including liquids of varying viscosities or solids such as powders, rocks, and metals. For example, capacitive sensors are often used to detect granular or powered materials such as plastic pellets in hoppers of injection-molding machines. Specifically designed, capacitive sensors can even withstand explosive environments, allowing them to be mounted on grain elevators to detect materials such as rice, barley malt, corn, and soybeans. 

Proximity sensors are also important outside the factory and can become an important component on the field. Stadiums that feature retractable roofs must have a dependable monitoring system in place (Figure 3). These roofing systems involve several motion-control applications to open and close the roof. Motors are also used in jack screws to open and close rail clamps that lock the roof in place. To ensure safety and reliability, proximity sensors can be placed at each end of the jack screw’s travel path to verify that the motion is complete and the clamp is holding. Proximity sensors can also be strategically placed on roof panels to monitor the primary motion of the system, signaling to PLCs when to slow down the motors upon reaching fully open and closed positions. 

The future of proximity sensing

The need to improve production and increase profitability will continue to be driving factors in most industries. As sensor technology evolves, many more applications will be discovered. With their durability, flexibility, adaptability, and reliability, proximity sensors will continue to provide high-quality performance for an evolving range of applications.

Tony Udelhoven is director of the Sensors Division, Turck USA.