Selecting the Right Sensors for Industrial Applications

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There is a very wide variety of industrial sensors available in the market, and selecting the right sensor for an application can be challenging.

Industrial sensors cover almost all the sensor categories including proximity, position, velocity, level, temperature, force and pressure. These find application in extremely diverse areas including industrial process control, energy, aviation, safety and security, automobiles, healthcare and building automation to name a few.

Sometimes sensors form the core elements of products and solutions. These are an important part of the products that define quality, efficiency and safety of the intended application. From bare semiconductor chips and transmitters to complex systems, sensor solutions and technologies exist across the entire value chain. This article covers characteristics of sensors used in industrial process control and monitoring, including wireless sensors, followed by introduction to some leading manufacturers of industrial sensors.

Industrial process control

Industrial process control involves monitoring and control of machinery, systems and processes across a large number of industries including chemicals, pharmaceuticals, biotechnology, energy, water, oil and gas, plastic, paper, and food and beverages. The modern industrial processing and manufacturing systems are highly automated, ensuring that raw materials and energy are consumed in an efficient manner. There are many parameters to be monitored and controlled including pressure, temperature, level, flow rate, humidity, dust, corrosives, explosives, liquids and gases.

Industrial process control and automation (Image courtesy: https://www.halvorsen.blog)
Fig. 1: Industrial process control and automation (Image courtesy: https://www.halvorsen.blog)
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Sensors are the first element in a process control and measurement system. Precise control and accurate displacement/position measurement in extremely small scales such as nanometres and picometres has become increasingly significant during the last few years.

The main performance criteria for industrial sensors are sensitivity, resolution, compactness, long-term stability, thermal drift and power efficiency.

Some of the sensors used in industrial process control

Proximity sensors. These can be mechanical, optical, inductive and capacitive. They are widely used in industrial automation like conveyor lines for counting and jam detection, machine tools for safety interlock and sequencing. They are also used in detecting the presence or absence of objects.

Mechanical sensors are basically mechanical switches for on/off operations.

Optical sensors could be light sources like light-emitting diodes (LEDs) and phototransistors. These sensors are essentially non-contact type with no moving parts. They are small, fast-switching, and insensitive to vibration and shock. However, they require alignment, can be blinded by ambient light conditions, and may require clean, dust- and water-free environment.

Inductive and capacitive proximity sensors generally have a short range but are very robust and reliable. Inductive sensors use magnetic properties to detect the presence of metal objects. Capacitive sensors are normally used to detect the capacitance caused by non-metallic objects.

Capacitive and inductive proximity sensors (Image courtesy: https://cdn.automationdirect.com)
Fig. 2: Capacitive and inductive proximity sensors (Image courtesy: https://cdn.automationdirect.com)

Position sensors

These include encoders and linear variable differential transformers (LVDTs). Encoders are digital sensors commonly used to provide position feedback for actuators. These consist of a glass or plastic disk that rotates between a light source (LED) and a pair of photo-detectors. Plastic disk is encoded with alternate light and dark sectors, so pulses are produced as the disk rotates.

LVDTs consist of a magnetic core that moves in a cylinder. These are commonly used for position feedback in servomechanisms, automated measurement, and many other industrial and scientific applications.

Force sensors

Force and pressure sensors usually act as transducers; these generate signals as a function of the pressure being imposed on them. Force-sensing resistors are two-pin force sensors whose resistance changes when a force, pressure or mechanical stress is applied on the sensor surface. FN3050 sensor from TE Connectivity is a rugged force load cell, highly suited for process industry and test bench applications.

Vibration/acceleration sensors

Ceramic piezoelectric sensors or accelerometers are most commonly used to detect vibration. Triaxial accelerometers are used in mobile systems, cars, turbines and aircrafts. These provide vibration information and position data. Inertial measurement units measure linear and angular motion usually with gyroscopes and accelerometers. These are widely used in aircraft and missile navigation and guidance.

Level sensors

These sensors are very common in industrial process control. Selection of a suitable level sensor depends on its size and geometry. Industrial process control includes hydrostatic and optical level sensors, ranging from simple limit-value detection to precision continuous level sensing. Hydrostatic sensors can be installed as submersible sensors for positioning in the fluid or with a screw thread for attachment to the exterior tank wall.

Industrial wireless sensors

Most traditional sensors usually require a wire or cable to connect with the external instruments. The cable can be copper wire, twisted pair or fibre optic. However, transmission line cables cause signal and power losses. Also, wired network installation is cumbersome and takes much time. Wireless sensors offer flexibility of installation, resulting in improved process monitoring and control and also reduced installation and maintenance costs.

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