Operating principle of inductive sensors
The electric current, which flows inside the coil, generates an oscillating electromagnetic field. When a metallic item (object) gets into the field, the induced eddy currents decrease the amplitude of the oscillation. When this oscillation becomes lower than a specific threshold, the sensor switches. The parting of the metallic item reestablishes energy to the electromagnetic field; consequently the amplitude of the field increases until , above a certain threshold, the sensor switches again, returning to the initial state. Only metal items can generate enough eddy currents to modify the magnetic field’s oscillation amplitude generated by the sensor. Therefore, an inductive sensor detects only metallic items without being influenced by the presence of other materials, both solid (wood, glass, plastic, etc.) and liquids (water, oils, etc).
Operating principle of capacitive sensors
The capacitive probe generates an electrostatic field. When an item gets close to the capacitive probe, the oscillator starts to oscillate (and the amplitude). The amplitude of oscillations increases as the target moves closer to the sensor. Above a certain amplitude, the detection circuit switches the sensor. When the item separates from the probe the amplitude of the oscillations decreases until the sensor reaches a specific, at which the sensor switches again to the initial conditions.
The standard target used to determine the range of an inductive sensor is represented by a square soft steel, 1 mm thick, and the side equal to:
- diameter of the active surface (if Ø > 3 x Sn)
- • 3 x Sn (if 3 x Sn> Ø)
For instance: a M8 diameter sensor with a rated operating distance Sn = 2 mm has a target standard of L = 8 mm; otherwise, a M30 sensor with a rated operating distance Sn = 15 mm has a target standard side of L = 45 mm. The composition of the target has a large effect on the effective sensing distance. Depending on the material used, the flow rate changes according to the correction factors of each material. Therefore:
sensing range = (rated operating distance) x (reduction factor)
|material target||correction factor|
|Aproximate values. The correction factor depends on the material and the characteristics of he coil used. For specific sensor, refer to the technical specifications of the product|
The difference between the output activation point and the point of de-energizing when the target is gets closer and separates is called hysteresis. The hysteresis is necessary to prevent the occurrence of rapid activation and deactivation of the output either when the sensor is exposed to vibrations or when the target is stationary at the nominal sensing distance (Sn).
Correction factors of the target (capacitive)
Capacitive sensors can detect virtually any type of material, but the effective detection distance depends, besides on the size of the target, on its dielectric constant: materials with high dielectric constants (metals, water, ...) are perceived at higher distance than materials with low dielectric distances (flour, oils, etc).
Below is attached a partial list of materials with their dielectric constants:
|dielectric constants of common industrial materials|
|Acetone 19,5||Freon R22 e 502 (liquid) 6,11||Polystyrene 3,0|
|Acrylic resin 2,7 - 4,5||Gasoline 2,2||Quartz glass 3,7|
|Air 1,0||Glass 3,7-10||Resin of urea 5-8|
|Alcohol 25,8||Glycerin 47||Resin, polyvinyl chloride 2,8-3,1|
|Ammonia 15-25||Limestone 1,2||Rubber 2,5-35|
|Aniline 6,9||Marble 8,0-8,5||Sale 6,0|
|Aqueous 50-80||Melamine resin 4,7-10,2||Sand 3-5|
|Ash turned 1,5-1,7||Mica 5,7-6,7||Shellac 2,5-4,7|
|Bakelite 3,6||Milk powder 3,5-4||Silicone paint 2,8-3,3|
|Benzene 2,3||Nitrobenzina 36||Soybean oil 2,9-3,5|
|Carbon dioxide 1,0||Nylon 4-5||Styrene resin 2,3-3,4|
|Carbon tetrachloride 2,2||Oil of turpentine 2,2||Sugar 3,0|
|Carton 2-5||Oiled paper 4,0||Sulfide 3,4|
|Celluloid 3,0||Paper 1,6-2,6||Teflon 2,0|
|Cement 4,0||Paraffin 1,9-2,5||Toluene 2,3|
|Cereals 3-5||Perspex 3,2-3,5||Transformer oil 2,2|
|China 4,4-7||Petroleum 2,0-2,2||Vaseline 2,2-2,9|
|Chlorine in solution 2,0||Phenolic resin 4-12||Water 80|
|China 4,4-7||Petroleum 2,0-2,2||Vaseline 2,2-2,9|
|Ebonite 2,7-2,9||Poliyacetato 3,6-3,7||Wood, dry 2-7|
|Epoxy 2,5-6||Polyamide 5,0||Wood, wet 10-30|
|Ethanol 24||Polyester resin 2,8-8,1|
|Ethylene glycol 38,7||Polyethylene 2,3|
|Flour 1,5-1,7||Polypropylene 2,0-2,3|
Shielded and non-shielded models
The proximity sensors differ between shielded and non-shielded.
The shielded models have the sensitive part completely shielded of the sensor body. The field generated by the sensor is only present on the active face and therefore only detects a front positioned target. These models can be mounted completely embedded in the metal body of the machine. The non-shielded models have the sensitive part jutting out from the sensor body. The detection range is also present laterally to the active face. Thus, having a greater extension makes the detection range greater than that of the shielded models. These models must be mounted jutting out from the metal body of the machine.
It shows the pulse maximum frequency at which a sensor can activate the output while the target enters and exits the sensing range. This value depends on the type of sensor, on the target size, on the target distance from the sensing surface and on the target speed. It shows the maximum number of operations per second.