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Understanding the more unusual sensors

10 March 2020

When sensors (photoelectric, inductive, pressure, temperature etc) were in their infancy, they were difficult to configure, and users were relatively inexperienced.

Over the following decades sensor manufacturers have had to meet the demands of industry and make sensors far more complex, in order to detect smaller differences, while also making them much easier to set up and use. To achieve this, the sensor gathers a large amount of data and is packed with intelligence to process this data and output a simple on/off signal or analogue value.

Although the built-in sensor intelligence means that most sensors will work to some degree in most applications, ensuring that they work reliably requires knowledge of each sensor’s strengths. Whilst these features are known for commonly used sensors, some of the more specialised sensors haven’t had as much general use in industry, so where they are best applied isn’t as widely known.

Specialised sensors include, Ultrasonic, Laser, Fibre-optic, Radar, Guided Microwave, Measuring sensors with I/O Link.

Ultrasonic

Ultrasonic sensors use a high frequency sound wave and time of flight to determine the distance from the sensor to the object. As the target must reflect the soundwave back to the sensor, a hard target will be better than a soft sound-absorbing target. E.g. A still liquid is a good target whereas foam on top of liquid is a bad target. Wave guides or stilling tubes can be used to eliminate the problem of foam on a liquid. The size and shape of the target also affects its performance; a flat target perpendicular to the sensor is good, but when the angle of the target varies from perpendicular, the sound wave gets increasingly reflected away from the sensor, making a hard flat target at 30? from perpendicular difficult to detect.

Ultrasonic sensors tend to have a wide and long detection area. A typical sensor could detect a 500mm flat plate out to 8m distance. At 4m distance the plate could still be detected if it were 500mm to the side of the centre line of the sensor, and a 25mm round rod could be detected 800mm from the centre line.

Ultrasonic sensors are good distance measuring sensors, especially if they incorporate temperature compensation, as the speed of sound changes with temperature. As the speed of sound also changes with pressure avoid using ultrasonic sensors where the pressure changes. Ultrasonic waveguides often have a narrow slit in them to avoid pressure changes.

An ultrasonic sensor relies on a moving transducer to create and receive a sound wave in order to detect an object. Anything that can interfere with this transducer movement or the sound wave will affect the sensor performance. For example, product labels, grease or a build-up of snow will interfere with the transducer movement, high winds and torrential rain can distort the sound wave.

Laser

Laser sensors use a single frequency and phase of light. This creates a narrow intense beam of light that does not spread out and disperse as most light does. This makes them ideal for detecting small objects at relatively large distances, or the absence of small features, like a thread missing from a hole. Laser sensors can also measure tiny height differences such as thousandths of a millimetre at short distances of a few hundred millimetres, or still detect a difference of less than 10mm at a range of 250m. The small diameter, intense light spot makes them easy to align and relatively immune to target colour. Mirrored surfaces can be difficult to detect, if they reflect the light away from the sensor, not back to it. 

In order for laser sensors to be eye safe they have very little light energy. This energy is concentrated in a very small area, so anything which disperses that energy will affect the performance, i.e. smoke, steam, rain or dirty lenses. As Laser beams are so narrow, vibration can severely affect the sensor by moving the detection spot off of its target. At 1m range a 0.5º angular vibration will move the spot 9mm

Fibre-optic

Fibre-optic sensors used to be the saviour of people wanting to detect something in a small space, now miniaturisation has produced some very small sensors. Fibre-optic sensors are still the go-to option for very small spaces. Fibres can be routed into the heart of a machine with the sensor mounted on the outside where it is easily accessible. Various fibres can withstand hostile environments, such as high vibration, extreme heat, wet, corrosive, explosive and electrically noisy environments. The disadvantages of Fibre-optic sensing are that you have to buy a fibre and a sensor, and that they have relatively short range.

Radar

Radar sensors use a frequency modulated continuous wave in the GHz frequency band to detect both stationary and moving targets. Their field of view can typically vary from 10? to 90? and ranges from 3.5m to 100m are available. The target must contain metal, water or a similar high-dielectric material.

Radar sensors are particularly suited to detecting stationary or moving vehicles in rain, snow, high and low temperatures and wind. They can also be used to detect a poor target in front of a good reference target. Radar sensors can be hidden behind a plastic facia panel or in a plastic box with negligible effect in performance.

Guided Microwave

Guided Microwave is used in liquid level measurement. An electromagnetic wave is emitted along the probe, when the wave reaches the medium, it is partially reflected due to the different level of permittivity in comparison to the surrounding air. The electromagnetic wave is recaptured by the sensor and the distance to the surface of the liquid can be calculated from the elapsed time. Foams on liquids can be reliably filtered out to give a true liquid level measurement.

If the Guided Microwave sensor is used in a metal container, its interior wall functions as an outer conductor for the electromagnetic wave. In non-metallic containers, a coaxial tube is used to replace the outer conductor. 

Guided microwave sensors are generally used in the following applications:

• Monitoring of coolants and lubricants in machine tools

• Monitoring of cleaning solutions and cleaning agents in mechanical engineering

• Filling systems in automotive production chains

• Level measurement in industrial processes subject to difficult environmental conditions

Measuring sensors with I/O Link

I/O Link has many benefits, such as automatic reprograming of replacement sensors and data collection. It also brings a whole new level of options to measurement sensors. Typically, measurement sensors used to fall into one of three categories: 1) Measurement with discrete switching points. 2) Measurement with analogue (0-10v or 0, 4-20mA) outputs. 3) Measurement with data output.

Measuring sensors with I/O Link now often incorporate categories 1 and 2 with data I/O. One sensor can now perform multiple tasks, discrete outputs for position limits or alarms, analogue outputs for indication, and data output for highest resolution control or analysis. 

I/O Link enables on-the-fly programming of set points, analogue windows etc. giving greater flexibility in changing applications. I/O Link also enables condition-based monitoring, facilitating local logic-based control and OEE (Overall Equipment Effectiveness)

Turck Banner have a wide range of sensors. No matter what kind of object or material has to be detected or inspected, or what-ever requirement has to be met, Turck and Banner have the right products for every application. 


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