Technology drives performance of presence sensor range and accuracy

As sensors evolve, engineers can take advantage of new technology to improve sensor performance.
By Tony Udelhoven June 6, 2017

IO-Link is a standardized, point-to-point communication technology. This graph demonstrates where IO-Link products are used, in the level between conventional devices and networking products. Courtesy: TurckRange and accuracy are critical sensor functions that offer engineers and original equipment manufacturers (OEMs) a window into how processes are running at the device level.

This is especially true for presence sensors, which are used in automated environments. From detecting a product on a conveyor line, to monitoring liquid levels in containers, to ensuring components are in place, the reliable, exact detection of an object is vital to workflow.

By understanding the essentials of sensor range and accuracy, as well as how new technology affects these functions, engineers can choose the best solutions for their applications.

Essentials of sensor range and accuracy

Physics is key to presence sensing capabilities, and the rules of this science determine range and accuracy. Inductive proximity sensors and ultrasonic sensors are two common categories of presence sensors.

Inductive proximity sensors feature an internal coil technology that generates a radio frequency (RF) field to detect the presence of an object. For the best accuracy and precision, engineers should choose the smallest RF field required to detect the object.

This is because of repeatability and hysteresis. Repeatability is accuracy of the operating point over repeated operations, which is typically 2% or less of the sensing range. Hysteresis is the difference between when the sensor signals that an object is present as the target approaches the sensor face, and the point when the signal turns off as the target moves away. It is calculated as a percentage of movement in the sensing field, and is typically 5%.

For example, if an 8 mm sensor has a range of 3 mm, the repeatability would be 0.06 mm and the typical hysteresis would be .15 mm. A much larger 80 x 80 mm "hockey puck" style sensor with a 50 mm range would have repeatability of 1 mm and typical hysteresis of 2.5 mm. For applications that require very specific presence sensing, the 8 mm sensor would be more accurate because the on/off signal window is more precise.

For presence sensing at extended ranges, ultrasonic sensors are often a better fit. These sensors use sound waves to detect objects, by emitting a sonic pulse and then receiving the reflected signal.

Ultrasonic sensors can achieve reliable detection at ranges up to 6 m. The devices are also ideal for more complex presence sensing, such as irregularly-shaped or transparent targets, non-metallic objects, wide sensing areas and when dust or oil films are present.

Liquid level monitoring and glass detection are two application examples where ultrasonic sensors excel. Detection of clear objects such as glass can be challenging for vision-based systems, but these transparent materials still will reflect sound waves when sensors are properly installed. Liquids are exceptional at reflecting sound waves when the surface is clear, and ultrasonic sensors often are used to monitor liquid levels in containers.

Harsh environments also could substantially influence range and accuracy. Harsh environments can involve any number of environmental challenges, from aggressive chemicals to dust and other ingress. Material selection is critical to ensure a sensor is able to withstand these variables and reliably detect objects. Stainless steel is the best option when harsh chemicals are present. Brass is typically suitable for environments without chemicals. 

Manufacturers are incorporating IO-Link technology into existing sensors, such as Turck's factor 1 uprox3 sensor, to allow engineers to gather additional information from the factory floor. Courtesy: TurckHow new technology impacts sensor performance

Today’s presence sensors are experiencing performance improvements thanks to microprocessor and internal sensor technology advances. As new solutions come to market, engineers and OEMs are discovering how these innovations enhance the accuracy or extend the range of sensors in an operation.

One of these advances is IO-Link, which is a standardized, point-to-point communication technology designed to increase the data that can be gathered from a sensor and reported to a controller. It also has practical applications for data accuracy, especially in analog systems.

In traditional analog systems, a signal may be translated from digital to analog before ultimately being delivered to the programmable logic controller (PLC), where it is converted from analog back to digital. With each translation, data losses can occur. With IO-Link, however, sensor signals are transmitted digitally one time before being carried back to the IO-Link master and ultimately the PLC. Limiting the number of translations limits the opportunity for losses.

This technology also improves accuracy of sensing values because the digital resolution is fixed. An engineer can look at the binary digital signal, pick the points that represent the position, and make decisions based on the readings. An engineer doesn’t have to scale the analog signal over the desired measuring range any longer. The internal microprocessors are designed to enable more linearization to make digital signals even more accurate.

Beyond IO-Link, microprocessor technology is also having a fundamental impact on sensor design and capabilities. Companies can make smart sensors with diagnostic capabilities and linearize the internal signals to make a more accurate and repeatable sensor.

In the past, electronics took up extra space in order to be able to solder the leads to the printed circuit board. New flip chip designs have the solder connections on the underside of the package and can manage higher currents, and have more processing power and longer sensing ranges, in less physical space. This allows for more compact sensor sizes. Recent sensor releases feature designs up to 30% smaller than previous solutions, with up to 50% longer sensing ranges.

Sensor design will continue to evolve, presenting new opportunities for insights into process and workflow. By pairing knowledge of presence sensor essentials with technologies like IO-Link and increasingly powerful microprocessors, engineers can drive improved system performance and operations from the PLC to the device level.

Tony Udelhoven, vice president, sensors division, Turck. Edited by Chris Vavra, production editor, Control Engineering, CFE Media, cvavra@cfemedia.com.

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Key Concepts

  • Understanding sensor range and accuracy allows engineers to make the best choice for their application.
  • Engineers should pick the smallest radio frequency (RF) field required to detect the object because of repeatability and hysteresis.
  • Technology advances such as IO-link are improving sensor performance and are helping them be more accurate. 

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