SENSORS IN AUTOMATION

 SENSORS IN AUTOMATION

Sensors surround us all: in the consumer world of smart and mobile devices such as the capacitive or resistive based touchscreens on our smartphones and tablets or the motion accelerometers in gaming platforms; or in the home automation world with pressure and proximity sensors among many others in our dishwasher; and certainly they are a vital component in the fast developing future that is the ‘Internet of Things’, which is taking many of the headlines these days, and in the so-called Industry 4.0 drive to make manufacturing smarter. Sensors have also long been the virtual eyes, ears and nose and also the instruments of touch

in industrial automation and control machinery. They inform the system’s brain – usually the programmable logic controller (PLC) in an industrial context – with what it needs to know to make decisions. When integrated with PLCs and control products, sensors form a key part of the control and communication system and need to be a major consideration at the early stages of system architecture design.

 

Environment

In the industrial environment, sensors are used to measure an extremely wide range of environmental factors including temperature, motion, positioning, acceleration, weight, humidity, chemical composition, gases or pressure or liquid flow. Typical examples include ultrasonic liquid-level transmitters for continuous level measurement and alarm switching at a water treatment plant; or pressure, temperature and level sensors in the monitoring of hydraulic fluids and coolants in machine tool applications; or inductive proximity switches in automotive assembly lines. In fact today, there is virtually nothing that cannot be measured or sensed and used as an input to a system.

Sensors and transducers are designed for a wide range of environments, but equally important as acquiring the signal is conditioning and transmission to ensure it is safely conveyed to the central processing unit. For example, often a signal needs to travel over a distance and usually via cables.

To increase the signal-to-noise ratio, twisted-pair cable with shielding is typically used to cancel out induced noise. Depending on the surrounding environment and the distance of cable run, amplification of weak signals may be required, which can affect the type of signal chosen in the first instance. For example, 4-20mA is the de facto signal for long distances over hundreds of meters because it is immune to volt drop and induced noise. Millivolt and small-resistance signals such as those from thermocouples are generally weak signals and therefore need to be amplified prior to being sent over distance to controllers. Signal conditioners are therefore employed and fitted near to the sensor.

 

Position

One of the fastest growing areas in automation is machine vision technology, employing digital camera sensors and advanced image processing technology and widely used in many industries to automate the production, increase production speed and also improve quality. However, more traditional sensor types have also evolved rapidly. For example, one of the most important functions is position sensing and a vast range of linear and rotary position encoders is available. While some offer absolute encoding, retaining memory of the last position of the system following powerdown, the vast majority of applications can be served by incremental encoders. Encoder outputs need to be chosen carefully according to the electrical environment, application and external signal conditioning apparatus. Manufacturers such as Baumer therefore offer a range of configurations specifically designed to cope with these different requirements. Other major manufacturers in this area include Hengstler, Omron and SICK.

Proximity

Non-contact proximity sensing is another key area in automation and is used in a huge variety of positioning applications, ranging from hoisting in the factory environment to mobile lifting, automatic livestock feeding systems and automation in the food and beverage industry. The trend here is to provide improved performance using a traditional underlying technology with advances in specifications such as sensor speed, robustness and smaller size. One type of proximity sensor is the inductive proximity sensor, which uses magnetic induction to detect ferromagnetic metallic material. Suitable for use in dirty and wet areas, it is commonly used in traffic lights, car washes and many industrial automation processes, and while it is immune to dust, it is also limited to sensing distances of up to about 50mm. An example of this type of sensor is Schneider’s OsiSense XS8 range, which is designed for use in many industrial applications including packaging, materials handling and assembly. Key features of these sensors are that they can be installed quickly and easily on any machine or equipment due to the unique one-click mounting and the rotating detection head. They enable maintenance to be simplified due to the quick mounting and removal of LED status indicators that are clearly visible from any direction and from long distance.

A second type is capacitive proximity sensing, which detects objects that are conductive or simply have a dielectric different from that of air. This means the technology can detect both metallic and non-metallic material, which is a primary difference from induction-based proximity sensors. The technology is commonly used in applications to detect and measure position, humidity or fluid levels among others. Again, it is suitable for use in dirty and wet areas, and is immune to dust, but is limited to sensing distances of up to 40mm.

 

Light

Photoelectric sensors are also employed widely in industrial automation, using light to detect the distance, absence, or presence of an object by using a light transmitter, often infrared, and a photoelectric receiver. There are many types available that detect an object when the light beam is interrupted such as through-beam sensors, in which a receiver is located in line-of-sight of the transmitter and, or retro-reflective sensors, in which the transmitter and receiver are placed at the same location and use a reflector to bounce the light beam back from transmitter to receiver.

An example through-beam sensor is the E3JK series of photoelectric sensors from Omron, which also have a built-in power supply for greater system integration. This series of sensors have a high power and visible red LED for all models enabling easy alignment and long sensing distance. Another example of light-based devices, the DT50 and DS50 series of phototransistor distance sensors from SICK, uses either infrared or red light and provide a high degree of reliability and reliable measurement detection including in strong light conditions and on shiny targets. Typical application areas include steel and metal production, print and paper, automotive, warehousing and many more. And finally, when position and presence detection is required on a much smaller scale, fibre-optic and RGB colour sensors are becoming increasingly popular. In addition to small size and their ability to detect small object and movements, they are also resistant to electrical noise and can function at higher temperatures.

Finally, one of the fastest growing areas for light sensors is the dramatic increase in the use of visual information as a system input, particularly machine- vision technology, which today is being heavily used in manufacturing.

 

Level

Level sensors are another important kind of sensor for automation and control: essentially they detect the flow level of substances, usually liquids, slurries and powders. One example level sensor that uses ultrasonic technology is the MSP400 level transmitter from Mowbrey, which is designed to be mounted above a liquid and will measure the distance to the surface of the liquid. When programmed with details of the vessel, sump or open channel, the MSP400 calculates the level, content or flow and provides a 4-20mA signal proportional to the chosen variable. Two relays are provided for control functions and programming is achieved using integral push buttons or by remote communication using HART (Highway Addressable Remote Transducer Protocol).

 

Conclusion

These are just a few from hundreds of different sensor types available, and this is without even including pressure sensors, or RFID and barcode readers/scanners, which again come in a vast array of different types. Major manufacturers of proximity, light and pressure sensors and a huge range of accessories all available via RS include Telemecanique, Omron, Pepperl

+ Fuchs, Baumer, IFM Electronics, SICK Banner. In summary, the continuing development of sensor technology is vital to the flexibility and sophistication of both simple and complex control and automation systems in a multitude of industrial process and manufacturing industries.

For more information visit: www.rs-components.com/ sensors

 

Related Products

 

Mobrey MSP400 Ultrasonic Level Sensor

 

SICK Photoelectic Sensor DS50

 

 

Omron E3JK Sensor

 

Telemecanique Inductive Sensors