“Inductive proximity sensors are non-contact proximity devices that set up a radio frequency field with an oscillator and a coil. The presence of an object alters this field and the sensor is able to detect this alteration.”1
These sensors are basically employed as proximity, position and displacement indicators. They can be installed in variety of applications such as:
- Valve positioning
- Measurement of liquid level in case of metals or other electro-conductive fluids
- Process and machine control
- Security purposes
The operating principle of inductive proximity sensors is very much identical to that of metal detectors.
One major drawback faced by inductive sensors is that they are capable of working with only metallic targets.
Two most commonly used inductive level sensor designs are shown in the figure below:
- Variable-inductance level transducer: In this type of transducer design, the tube comprising liquid is surrounded by a coil. As and when the level of liquid in the tank changes, the inductance of the coil also gets affected. This inductance change can be ultimately used to detect the liquid level.
- Transformer-type level transducer: This type of sensor design employs a twin-limbed core made up of iron which acts as a transformer. One limb of the core is surrounded by a coil which forms the primary winding of the transformer whereas the other part of the core is surrounded by the liquid in a tank whose level is to be measured. This liquid container forms a single turn of the secondary winding of the transformer. The electrical resistance of this turn tends to decrease with the increase in level of the fluid in the tube and vice versa. Owing to this behavior, the variations in the liquid level can be easily determined by assessing the power utilized at the primary winding.
Major specifications of an inductive proximity sensor that need consideration are mentioned below:
- Operating distance: It is the value of distance at which the switching mechanism operates.
- Repeatability: It is the rated distance within which the switching action occurs continuously. This parameter basically represents precision measurement.
- Field adjustability: The sensors having this capability are very adaptable and can be regulated while in operation.
- Minimum target distance: The minimum needed target size varies according to the sensor’s expertise.
Apart from the above mentioned parameters, one must also take into account factors like atmospheric conditions, power supply needs and material dimensions while specifying inductive level sensors.