Proximity sensors detect the presence or lack of objects using electromagnetic fields, light, and sound. There are several types, each fitted to specific applications and environments.
These automation parts detect ferrous targets, ideally mild steel thicker than a single millimeter. They contain four major components: a ferrite core with coils, an oscillator, a Schmitt trigger, and an output amplifier. The oscillator creates a symmetrical, oscillating magnetic field that radiates from the ferrite core and coil array with the sensing face. Every time a ferrous target enters this magnetic field, small independent electrical currents called eddy currents are induced about the metal’s surface. This changes the reluctance (natural frequency) from the magnetic circuit, which actually cuts down on the oscillation amplitude. As more metal enters the sensing field the oscillation amplitude shrinks, and ultimately collapses. (This is basically the “Eddy Current Killed Oscillator” or ECKO principle.) The Schmitt trigger responds to these amplitude changes, and adjusts sensor output. As soon as the target finally moves from the sensor’s range, the circuit begins to oscillate again, and also the Schmitt trigger returns the sensor to its previous output.
In the event the sensor features a normally open configuration, its output is definitely an on signal once the target enters the sensing zone. With normally closed, its output is an off signal with all the target present. Output will be read by another control unit (e.g. PLC, motion controller, smart drive) that converts the sensor on / off states into useable information. Inductive sensors are usually rated by frequency, or on/off cycles per second. Their speeds range from 10 to 20 Hz in ac, or 500 Hz to 5 kHz in dc. As a result of magnetic field limitations, inductive sensors have a relatively narrow sensing range – from fractions of millimeters to 60 mm typically – though longer-range specialty merchandise is available.
To support close ranges in the tight confines of industrial machinery, geometric and mounting styles available include shielded (flush), unshielded (non-flush), tubular, and rectangular “flat-pack”. Tubular sensors, quite possibly the most popular, are available with diameters from 3 to 40 mm.
But what inductive sensors lack in range, they can make up in environment adaptability and metal-sensing versatility. Without any moving parts to utilize, proper setup guarantees extended life. Special designs with IP ratings of 67 and better are designed for withstanding the buildup of contaminants such as cutting fluids, grease, and non-metallic dust, in the atmosphere and also on the sensor itself. It should be noted that metallic contaminants (e.g. filings from cutting applications) sometimes impact the sensor’s performance. Inductive sensor housing is typically nickel-plated brass, stainless, or PBT plastic.
Capacitive proximity sensors can detect both metallic and non-metallic targets in powder, granulate, liquid, and solid form. This, together with their capability to sense through nonferrous materials, means they are ideal for sight glass monitoring, tank liquid level detection, and hopper powder level recognition.
In proximity sensor, both the conduction plates (at different potentials) are housed in the sensing head and positioned to operate as an open capacitor. Air acts being an insulator; at rest there is very little capacitance in between the two plates. Like inductive sensors, these plates are connected to an oscillator, a Schmitt trigger, plus an output amplifier. As a target enters the sensing zone the capacitance of these two plates increases, causing oscillator amplitude change, consequently changing the Schmitt trigger state, and creating an output signal. Note the main difference involving the inductive and capacitive sensors: inductive sensors oscillate before the target is present and capacitive sensors oscillate once the target is found.
Because capacitive sensing involves charging plates, it can be somewhat slower than inductive sensing … starting from 10 to 50 Hz, with a sensing scope from 3 to 60 mm. Many housing styles are offered; common diameters cover anything from 12 to 60 mm in shielded and unshielded mounting versions. Housing (usually metal or PBT plastic) is rugged to permit mounting very close to the monitored process. If the sensor has normally-open and normally-closed options, it is said to experience a complimentary output. Because of their capacity to detect most types of materials, capacitive sensors should be kept clear of non-target materials to avoid false triggering. That is why, when the intended target posesses a ferrous material, an inductive sensor is actually a more reliable option.
Photoelectric sensors are so versatile which they solve the majority of problems put to industrial sensing. Because photoelectric technologies have so rapidly advanced, they now commonly detect targets below 1 mm in diameter, or from 60 m away. Classified with the method where light is emitted and transported to the receiver, many photoelectric configurations are available. However, all photoelectric sensors consist of some of basic components: each has an emitter light source (Light Emitting Diode, laser diode), a photodiode or phototransistor receiver to detect emitted light, and supporting electronics made to amplify the receiver signal. The emitter, sometimes referred to as sender, transmits a beam of either visible or infrared light on the detecting receiver.
All photoelectric sensors operate under similar principles. Identifying their output is thus made easy; darkon and light-on classifications refer to light reception and sensor output activity. If output is produced when no light is received, the sensor is dark-on. Output from light received, and it’s light-on. In any event, deciding on light-on or dark-on ahead of purchasing is essential unless the sensor is user adjustable. (If so, output style might be specified during installation by flipping a switch or wiring the sensor accordingly.)
By far the most reliable photoelectric sensing is using through-beam sensors. Separated from the receiver with a separate housing, the emitter provides a constant beam of light; detection develops when a physical object passing between the two breaks the beam. Despite its reliability, through-beam will be the least popular photoelectric setup. The investment, installation, and alignment
of the emitter and receiver by two opposing locations, which might be a serious distance apart, are costly and laborious. With newly developed designs, through-beam photoelectric sensors typically provide the longest sensing distance of photoelectric sensors – 25 m as well as over is currently commonplace. New laser diode emitter models can transmit a highly-collimated beam 60 m for increased accuracy and detection. At these distances, some through-beam laser sensors are designed for detecting a physical object the dimensions of a fly; at close range, that becomes .01 mm. But while these laser sensors increase precision, response speed is equivalent to with non-laser sensors – typically around 500 Hz.
One ability unique to throughbeam photoelectric sensors works well sensing in the existence of thick airborne contaminants. If pollutants build-up entirely on the emitter or receiver, you will find a higher possibility of false triggering. However, some manufacturers now incorporate alarm outputs in the sensor’s circuitry that monitor the quantity of light hitting the receiver. If detected light decreases to some specified level without a target in position, the sensor sends a stern warning by means of a builtin LED or output wire.
Through-beam photoelectric sensors have commercial and industrial applications. In your own home, as an example, they detect obstructions within the path of garage doors; the sensors have saved many a bicycle and car from being smashed. Objects on industrial conveyors, on the flip side, can be detected anywhere between the emitter and receiver, provided that you can find gaps between the monitored objects, and sensor light is not going to “burn through” them. (Burnthrough might happen with thin or lightly colored objects that enable emitted light to pass through right through to the receiver.)
Retro-reflective sensors get the next longest photoelectric sensing distance, with a few units effective at monitoring ranges approximately 10 m. Operating comparable to through-beam sensors without reaching exactly the same sensing distances, output occurs when a constant beam is broken. But rather than separate housings for emitter and receiver, both of them are located in the same housing, facing the same direction. The emitter makes a laser, infrared, or visible light beam and projects it towards a specifically created reflector, which then deflects the beam back to the receiver. Detection occurs when the light path is broken or otherwise disturbed.
One reason behind employing a retro-reflective sensor across a through-beam sensor is perfect for the benefit of a single wiring location; the opposing side only requires reflector mounting. This contributes to big cost benefits both in parts and time. However, very shiny or reflective objects like mirrors, cans, and plastic-wrapped juice boxes develop a challenge for retro-reflective photoelectric sensors. These targets sometimes reflect enough light to trick the receiver into thinking the beam had not been interrupted, causing erroneous outputs.
Some manufacturers have addressed this challenge with polarization filtering, which allows detection of light only from engineered reflectors … instead of erroneous target reflections.
As in retro-reflective sensors, diffuse sensor emitters and receivers are found in the same housing. Nevertheless the target acts because the reflector, in order that detection is of light reflected away from the dist
urbance object. The emitter sends out a beam of light (most often a pulsed infrared, visible red, or laser) that diffuses in all directions, filling a detection area. The target then enters the region and deflects section of the beam to the receiver. Detection occurs and output is switched on or off (depending on if the sensor is light-on or dark-on) when sufficient light falls about the receiver.
Diffuse sensors can be found on public washroom sinks, where they control automatic faucets. Hands placed underneath the spray head behave as reflector, triggering (in this instance) the opening of a water valve. Because the target is the reflector, diffuse photoelectric sensors are usually at the mercy of target material and surface properties; a non-reflective target for example matte-black paper can have a significantly decreased sensing range as compared to a bright white target. But what seems a drawback ‘on the surface’ can actually be of use.
Because diffuse sensors are somewhat color dependent, certain versions are suitable for distinguishing dark and light targets in applications that require sorting or quality control by contrast. With merely the sensor itself to mount, diffuse sensor installation is normally simpler as compared to through-beam and retro-reflective types. Sensing distance deviation and false triggers brought on by reflective backgrounds generated the growth of diffuse sensors that focus; they “see” targets and ignore background.
There are 2 ways that this can be achieved; the first and most popular is via fixed-field technology. The emitter sends out a beam of light, as being a standard diffuse photoelectric sensor, however, for two receivers. One is centered on the required sensing sweet spot, and also the other about the long-range background. A comparator then determines whether or not the long-range receiver is detecting light of higher intensity than is being obtaining the focused receiver. If so, the output stays off. Only when focused receiver light intensity is higher will an output be produced.
The second focusing method takes it a step further, employing a wide range of receivers with an adjustable sensing distance. The unit uses a potentiometer to electrically adjust the sensing range. Such sensor
s operate best at their preset sweet spot. Enabling small part recognition, they also provide higher tolerances in target area cutoff specifications and improved colorsensing capabilities. However, target surface qualities, for example glossiness, can produce varied results. In addition, highly reflective objects outside the sensing area have a tendency to send enough light straight back to the receivers to have an output, especially when the receivers are electrically adjusted.
To combat these limitations, some sensor manufacturers developed a technology known as true background suppression by triangulation.
A genuine background suppression sensor emits a beam of light the same as a standard, fixed-field diffuse sensor. But rather than detecting light intensity, background suppression units rely completely on the angle from which the beam returns on the sensor.
To achieve this, background suppression sensors use two (or maybe more) fixed receivers accompanied by a focusing lens. The angle of received light is mechanically adjusted, allowing for a steep cutoff between target and background … sometimes no more than .1 mm. This is a more stable method when reflective backgrounds are present, or when target color variations are a challenge; reflectivity and color impact the intensity of reflected light, but not the angles of refraction employed by triangulation- based background suppression photoelectric sensors.
Ultrasonic proximity sensors are used in several automated production processes. They employ sound waves to detect objects, so color and transparency tend not to affect them (though extreme textures might). This may cause them ideal for many different applications, like the longrange detection of clear glass and plastic, distance measurement, continuous fluid and granulate level control, and paper, sheet metal, and wood stacking.
The most common configurations are the same like photoelectric sensing: through beam, retro-reflective, and diffuse versions. Ultrasonic diffuse fanuc pcb employ a sonic transducer, which emits a number of sonic pulses, then listens for their return in the reflecting target. When the reflected signal is received, dexqpky68 sensor signals an output into a control device. Sensing ranges extend to 2.5 m. Sensitivity, defined as the time window for listen cycles versus send or chirp cycles, can be adjusted through a teach-in button or potentiometer. While standard diffuse ultrasonic sensors give you a simple present/absent output, some produce analog signals, indicating distance with a 4 to 20 mA or to 10 Vdc variable output. This output could be transformed into useable distance information.
Ultrasonic retro-reflective sensors also detect objects within a specified sensing distance, but by measuring propagation time. The sensor emits a series of sonic pulses that bounce off fixed, opposing reflectors (any flat hard surface – some machinery, a board). The sound waves must go back to the sensor within a user-adjusted time interval; should they don’t, it is assumed an object is obstructing the sensing path and also the sensor signals an output accordingly. For the reason that sensor listens for modifications in propagation time in contrast to mere returned signals, it is great for the detection of sound-absorbent and deflecting materials for example cotton, foam, cloth, and foam rubber.
Just like through-beam photoelectric sensors, ultrasonic throughbeam sensors hold the emitter and receiver in separate housings. When an item disrupts the sonic beam, the receiver triggers an output. These sensors are fantastic for applications which need the detection of the continuous object, say for example a web of clear plastic. When the clear plastic breaks, the output of the sensor will trigger the attached PLC or load.