HOW THE DIFFERENT SENSORS FUNCTION

Explore the diversity of sensor technology!

THROUGH BEAM PHOTO-ELECTRIC SENSOR

Light barriers are made up of a beam source (transmitter) and a sensor (receiver). In through beam photo-electric sensor systems, the transmitter and receiver are positioned opposite each other.

Beam sources used include 660 nm LEDs (visible red light) or 880–940 nm LEDs in the infrared range. Infrared light has the advantage of increased ranges for dark materials while also being invisible to the human eye. Red light versions have the benefit of simplifying sensor adjustment due to beam visibility. For applications requiring high precision (small component detection, repeatability), laser diodes are generally used. The receiver is normally a photodiode or phototransistor. Photoconductive cells are also used.

To achieve ambient light immunity in light barriers, the beams are modulated (especially for high range models) to differentiate them from the surrounding light.

RETRO-REFLECTIVE SENSORS

To save on an electrical supply at two locations, both transmitter and receiver can be integrated (in parallel but optically separate) within a single housing. The light beam is then reflected back from the opposing side.

DIFFUSE SENSORS

The light beam here is reflected back by the object itself located within the sensor range. The switching distance is determined by the reflective properties of the object's surface. The transmitter and receiver are positioned in parallel within a single housing.

LIGHT GRID

In addition to basic systems using just one light beam, there are also the light grid or light curtain systems which use multiple parallel or criss-cross beams. These systems can monitor large areas, e.g. access areas to a machine or an alarm-secured room. Light grids are much better than single light beams for opening elevator doors or door and gate systems.

 

 

FIBRE-OPTIC SENSORS

The optical and electronic components function separately and are connected via an optical waveguide. They are used, for example, where space is limited. Both through-beam and diffuse systems are available.

The functioning of capacitive proximity switches is based on changes in the electrical field around the sensor electrode (active zone). Capacitance is measured between the active electrode and the electrical earth potential. When a metallic or non-metallic object approaches the active zone, capacitance increases to affect amplitudes in an RC oscillator. This "switches" a downstream trigger stage and modifies its output status. Sensor sensitivity can be adjusted using a potentiometer, e.g. to set the required switching distance. The switching distance of a capacitive sensor can vary strongly and depends on the capacitivity, sensor diameter, material, and mass of the approaching object as well as on where the sensor is installed.

An inductive proximity switch (proximity sensor, position sensor) is an electro-sensitive sensor that reliably detects metallic (electrically conductive) objects.

It is comprised of three main elements: an oscillator, an evaluation unit, and an output function. The oscillator begins to vibrate when the inductive proximity switch is connected to a power supply. The electromagnetic field thus created is directed onto the active zone via a ferrite core which contains the inductor. An approaching object absorbs energy from the oscillating circuit resulting in a reduction in oscillating voltage. A switch-like binary signal ("object detected/not detected") is available at the output requiring no direct contact to the object being detected.

An inductive proximity switch uses a reduction factor which provides the switch-distance reduction for varying object materials. This is dependent both on the constructive properties of the proximity switch (e.g. housing material) and the object. For example, switching distance reduces by around 40% for an object made of brass. This can be problematic for certain applications and has therefore resulted in the development of the reduction-1-sensors, factor 1 (all-metal switch).

The functioning of this flow sensor is based on thermodynamic principles. A measurement probe is internally heated by several degrees Celsius above that of the flow material in which it is placed. When the medium flows, the heat generated in the probe dissipitates into the medium, i.e. the probe cools. The temperature set in the probe is measured and compared with the temperature of the medium. The temperature difference can then be used to determine the flow status for any medium.

The sensitivity of thermodynamic flow sensors depends on the thermal properties of a medium. The detection range for oil from a standard sensor is – due to oil's low thermal conductivity – three times greater than water, and for air, approximately thirty times greater. The technical specifications for a sensor are based on water when not otherwise indicated.

When electrically charged dust particles come into contact with a measurement probe (sensor rod), the electrical charge is transferred from the particles to the sensor rod (charged via static electricity). This tiny signal undergoes amplification within the electronics and produces a measurement signal that triggers a switch contact when exceeding a standard value.

Microwaves operate in electromagnetic waves from 1 to 300 GHz (wavelengths from 300 mm to 1 mm). Microwaves are electromagnetic waves and, like light, can be reflected and interrupted. They are reflected by metallic and electrical conductors and undergo only minimal absorption. Microwave barriers activate when the microwave between the transmitter and receiver is interrupted. Material-flow monitors activate, due to the Doppler effect, when a conveyor moves.

When container sides, housings or ducts are non-metallic, measuring can be performed externally. By using process adapters, microwave sensors can also be used for high temperatures.

Ultrasound refers to mechanical vibrations whose frequency exceeds the hearing capacity of the human ear (> 20 kHz). Ultrasound can be created and transmitted within solid, liquid or gaseous mediums. Sitron sensors function in gaseous mediums (air). They measure the time taken for the sound to go from the sensor to the object and back again. Fork sensors measure sound-wave amplitude. 

Through-beam mode is comprised of a transmitter and receiver. The receiver activates when the detection beam is interrupted.

Ultrasound sensors can be used irrespective of material, surface, colour and size. They detect transparent and glossy objects (including moving objects) and can be used in dust, dirt, fog and light. They do however have limitations for extremely hot or cold objects, foams or soft surfaces.

Contact

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D-30916 Isernhagen

Phone: +49 (0) 511 - 7 28 50 - 0
Fax: +49 (0) 511 -7 28 50 - 33
E-Mail: office(at)sitron.de

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