Proximity sensors

Proximity sensors detect the presence or absence of objects using electromagnetic fields, light, and sound. There are many types, each suited to specific applications and environments.

Inductive sensors

These non-contact proximity sensors detect ferrous targets, ideally mild steel thicker than one millimeter. They consist of four major components: a ferrite core with coils, anoscillator, a Schmitt trigger, and an output amplifier. The oscillator creates a symmetrical, oscillating magnetic field that radiates from the ferrite core and coil array at the sensing face. When a ferrous target enters this magnetic field, small independent electrical currents called eddy currents are induced on the metal’s surface. This changes the reluctance (natural frequency) of the magnetic circuit, which in turn reduces the oscillation amplitude. As more metal enters the sensing field the oscillation amplitude shrinks, and eventually collapses. (This is the “Eddy Current Killed Oscillator” or ECKO principle.) The Schmitt trigger responds to these amplitude changes, and adjusts sensor output. When the target finally moves from the sensor’s range, the circuit begins to oscillate again, and the Schmitt trigger returns the sensor to its previous output.

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If the sensor has a normally openconfiguration, its output is an on signal when the target enters the sensing zone. With normally closed, its output is an offsignal with the target present. Output is then read by an external control unit (e.g. PLC, motion controller, smart drive) that converts the sensor on and off states into useable information. Inductive sensors are typically 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. Because of magnetic field limitations, inductive sensors have a relatively narrow sensing range — from fractions of millimeters to 60 mm on average — though longer-range specialty products are available.

To accommodate 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, by far the most popular, are available with diameters from 3 to 40 mm.

But what inductive sensors lack in range, they make up in environment adaptability and metal-sensing versatility. With no moving parts to wear, proper setup guarantees long life. Special designs with IP ratings of 67 and higher are capable of withstanding the buildup of contaminants such as cutting fluids, grease, and non-metallic dust, both in the air and on the sensor itself. It should be noted that metallic contaminants (e.g. filings from cutting applications) sometimes affect the sensor’s performance. Inductive sensor housing is typically nickel-plated brass, stainless steel, or PBT plastic.

Capacitive sensors

Capacitive proximity sensors can detect both metallic and non-metallic targets in powder, granulate, liquid, and solid form. This, along with their ability to sense through nonferrous materials, makes them ideal for sight glass monitoring, tank liquid level detection, and hopper powder level recognition.

In capacitive sensors, the two conduction plates (at different potentials) are housed in the sensing head and positioned to operate like an open capacitor. Air acts as an insulator; at rest there is little capacitance between the two plates. Like inductive sensors, these plates are linked to an oscillator, a Schmitt trigger, and an output amplifier. As a target enters the sensing zone the capacitance of the two plates increases, causing oscillator amplitude change, in turn changing the Schmitt trigger state, and creating an output signal. Note the difference between the inductive and capacitive sensors: inductive sensors oscillate until the target is present and capacitive sensors oscillate when the target is present.

Because capacitive sensing involves charging plates, it is somewhat slower than inductive sensing ... ranging from 10 to 50 Hz, with a sensing scope from 3 to 60 mm. Many housing styles are available; common diameters range from 12 to 60 mm in shielded and unshielded mounting versions. Housing (usually metal or PBT plastic) is rugged to allow mounting very close to the monitored process. If the sensor has normally-open and normally-closed options, it is said to have a complimentary output. Due to their ability to detect most types of materials, capacitive sensors must be kept away from non-target materials to avoid false triggering. For this reason, if the intended target contains a ferrous material, an inductive sensor is a more reliable option.

 

Photoelectric sensors

Photoelectric sensors are so versatile that they solve the bulk of problems put to industrial sensing. Because photoelectric technology has so rapidly advanced, they now commonly detect targets less than 1 mm in diameter, or from 60 m away. Classified by the method in which light is emitted and delivered to the receiver, many photoelectric configurations are available. However, all photoelectric sensors consist of a few 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 designed to amplify the receiver signal. The emitter, sometimes called the sender, transmits a beam of either visible or infrared light to 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. Either way, deciding on light-on or dark-on prior to purchasing is required unless the sensor is user adjustable. (In that case, output style can be specified during installation by flipping a switch or wiring the sensor accordingly.)

Through-beam

The most reliable photoelectric sensing is with through-beam sensors. Separated from the receiver by a separate housing, the emitter provides a constant beam of light; detection occurs when an object passing between the two breaks the beam. Despite its reliability, through-beam is the least popular photoelectric setup. The purchase, installation, and alignment

of the emitter and receiver in two opposing locations, which may be quite a distance apart, are costly and laborious. With newly developed designs, through-beam photoelectric se

nsors typically offer the longest sensing distance of photoelectric sensors — 25 m and over is now commonplace. New laser diode emitter models can transmit a well-collimated beam 60 m for increased accuracy and detection. At these distances, some through-beam laser sensors are capable of detecting an object the size of a fly; at close range, that becomes 0.01 mm. But while these laser sensors increase precision, response speed is the same as with non-laser sensors — typically around 500 Hz.

One ability unique to throughbeam photoelectric sensors is effective sensing in the presence of thick airborne contaminants. If pollutants build up directly on the emitter or receiver, there is a higher probability of false triggering. However, some manufacturers now incorporate alarm outputs into the sensor’s circuitry that monitor the amount of light hitting the receiver. If detected light decreases to a specified level without a target in place, the sensor sends a warning by means of a builtin LED or output wire.

Through-beam photoelectric sensors have commercial and industrial applications. At home, for example, they detect obstructions in the path of garage doors; the sensors have saved many a bicycle and car from being smashed. Objects on industrial conveyors, on the other hand, can be detected anywhere between the emitter and receiver, as long as there are gaps between the monitored objects, and sensor light does not “burn through” them. (Burnthrough might happen with thin or lightly colored objects that allow emitted light to pass through to the receiver.)

Retro-reflective

Retro-reflective sensors have the next longest photoelectric sensing distance, with some units capable of monitoring ranges up to 10 m. Operating similar to through-beam sensors without reaching the same sensing distances, output occurs when a constant beam is broken. But instead of separate housings for emitter and receiver, both are located in the same housing, facing the same direction. The emitter produces a laser, infrared, or visible light beam and projects it towards a specially designed reflector, which then deflects the beam back to the receiver. Detection occurs when the light path is broken or otherwise disturbed.

One reason for using a retro-reflective sensor over a through-beam sensor is for the convenience of one wiring location; the opposing side only requires reflector mounting. This results in big cost savings in both parts and time. However, very shiny or reflective objects like mirrors, cans, and plastic-wrapped juice boxes create a challenge for retro-reflective photoelectric sensors. These targets sometimes reflect enough light to trick the receiver into thinking the beam was not interrupted, causing erroneous outputs.