MechatronicsSensors

Inductive Proximity

Inductive Proximity Sensors

Function Description

The most important components of an inductive proximity sensor are an oscillator (LC resonant circuit), a demodulator rectifier, a bistable amplifier and an output stage.

1Oscillator6External voltage

2Demodulator7Internal constant voltage supply

3Triggering stage8Active zone (coil)

4Switching status display9Sensors output

5Output stage with protective circuit

Fig. 1Block circuit diagram of an inductive proximity sensor

The magnetic field which is directed towards the outside, is generated via a half-open ferrite core shell of an oscillator coil and additional screening. This creates a limited area across the active surface of the inductive proximity sensor, which is known as the active switching zone.

When a voltage is applied to the sensor, the oscillator starts and a defined quiescent current flows. If an electrically conductive object is introduced into the active switching zone, eddy currents are created, which draw energy from the oscillator. Oscillation is attenuated and this leads to a change in current consumption of the proximity sensor. The two statuses - oscillation attenuated or oscillation unattenuated - are electronically evaluated.

Fig. 2Method of operation of an inductive proximity sensor

Only electrically conductive materials can be detected by means of inductive proximity sensors.

Depending on switch type (normally open contact or normally closed contact), the final stage is switched through or inhibited if a metallic object is present in the active switching zone. The distance to the active area, where a signal change of the output signal occurs, is described as the switching distance. The important criteria for inductive proximity sensors is therefore the size of the coil incorporated in the switching head. The bigger the coil, the greater the active switching distance. Distances of up to 250mm can be achieved.

A standardised calibrating plate is used to determine the switching distance of inductive proximity sensors. Only in this way can useful comparison of switching distances of different inductive proximity sensors be made. The standard measuring plate is made of mild steel (Fe 360 according to Eurostandards 25 and 27 or ISO 630) and is 1 mm thick. It is square and the length of a side is equal to:

  • the diameter of the active surface of the sensor

or

  • three times the nominal switching distance

The higher of the two values is to be used as the lateral length of the standard calibrating plate. Using plates with larger areas does not lead to any significant changes in the switching distance measured. However, if smaller plates are used this leads to a reduction of the switching distance derived.

Also, the use of different materials leads to a reduction of the effective switching distance. The reduction factors for different materials are listed in the table below.

Material / Reduction factor
Mild steel / 1.0
Chrome nickel / 0.70 - 0.90
Brass / 0.35 - 0.50
Aluminium / 0.35 - 0.50
Copper / 0.25 - 0.40

Table 1Guide values for the reduction factor

The above table shows that the largest switching distances achieved are for magnetic materials. The switching distances achieved for non-magnetic materials (brass, aluminium, copper) are clearly smaller.

Technical Characteristics

The table below lists the key technical data relating to inductive proximity sensors. The figures listed in this table are typical examples and merely provide an overview.

Object Material / Metals
Operating voltage / typ. 10 V… 30 V
Nominal switching distance / typ. 0.8 … 10 mm
max. E.g. 250 mm
Max. switching current / 75 mA… 400 mA
Ambient operating temperature / -25C… +70C
Vibration / 10 … 50 Hz,
1 mm amplitude
Sensitivity to dirt / insensitive
Service life / very long
Switching frequency / typ. 10… 5000 Hz
max. 20 kHz
Design / cylindrical, block-shaped
Size (examples) / M8x1, M12X1, M18X1, M30X1,
4 mm …  30 mm,
25 mm x 40 mm x 80 mm
Protection class to IEC 529,
DIN 40 050 / up to IP 67

Table 2Technical data of DC inductive proximity sensors

Many of the inductive proximity sensors which are available on the market have the following built-in precautions to guarantee simple handling and safe operation:

  • Reverse polarity protection (against damage as a result of reversing connections)
  • Short circuit protection (against short circuiting of output against earth)
  • Protection against voltage peaks (against transient overvoltages)
  • Protection against wire breakage (The output is blocked if a supply line is disconnected)

Fig. 3Inductive proximity sensor in threaded design

Notes on application

If inductive proximity sensors are fitted in metal fixtures, care should be taken that the characteristics of the proximity sensor are not be altered. Differentiation should be made here between the two different types of proximity sensors, i.e. flush-fitting and non-flush fitting proximity sensors.

Fig. 4Flush fitting inductive proximity sensors

Where proximity sensors are to be flush-fitted in metal, they must be installed in such a way as to ensure that the electromagnetic field is directed from the active zone forwards. In this way, the characteristic of the proximity sensor cannot be influenced by the method of assembly. In the case of series assembly of proximity sensors, a minimum gap corresponding to their respective diameter must be provided. This is essential in order to prevent the proximity sensors from influencing one another. The free zone in front of the proximity sensor should be at least three time the nominal switching distance of the proximity sensor used. The free zone is the area between the proximity sensor and a background object.

The advantage of flush-fitting proximity sensors is that these are very easy to install and space saving. Their disadvantage compared to non-flush-fitting proximity sensors is that although the external diameter of the proximity sensor housing is identical, the switching distance is smaller.

Fig. 5Non flush fitting inductive proximity sensors

Recessed proximity sensors which are mounted in a material which influences their characteristics (metal) require a free zone which surrounds the entire active area. However , these proximity sensors can be embedded in plastics, wood or other non-metallic materials without the characteristics of the proximity sensor being affected. This type of sensor can often be recognised by the coil head protruding from the housing of the proximity sensor.

Examples of application

Fig. 6Sensing the piston rod on a pneumatic or hydraulic cylinder

Fig. 7Detection of metallic workpiece carriers on a band conveyor

Fig. 8Sensing a cam controller by means of inductive proximity sensors.

Fig. 9Measurement of speed and direction of rotation.

Fig. 10Two inductive proximity sensors check the end positions if a semi-rotary drive.

Fig. 11Detecting end position of a press ram

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