Inductive Proximity Sensors

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Chapter 5
Proximity Sensor

5.1 Introduction

A proximity sensors is the sensor able to detect the presence of nearby objects without any physical contact.
A proximity sensors often emits an electromagnetic field or a beam of electromagnetic radiation, and looks for changes in the field or return signal.
The object being sensed is often referred to as the proximity sensor's target. Different proximity sensor target demand different sensor. For examples a capacitive or photoelectric sensors might be suitable for a plastic target an inductive proximity sensors always requires a metal target.
The maximum distance that this sensors can detect is defined "nominal range". Some sensor have adjustments of the nominal range or means to the report
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Proximity sensors are commonly used on smart phones to detect accidental touch screen taps when held to the ear during a call. They are also used in the machine vibration monitoring to measures the variation in distance between the shaft and its support bearing.
This is common in the large steam turbine, compressor, and motor that use sleeve type bearing.

5.2 Inductive Proximity Sensor
A proximity ( inductive ) sensors is an electronic proximity sensors which detects metallic object without touching them.
The sensor consist of the an induction loop. Electric current generate a magnetic field, which collapses generating a current that fall toward zero from its initial trans when the input electricity of the ceases.
The inductance of the loop changes according from the material inside it and since metals are much more effective inductors than the other material the presence of metal increases the current flowing through the loop. This change can be detected by the sensing circuitry which can signal to some other device whenever metal are
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• An oscillators.
• A signal level detectors.
• A solid-state output switching devices.
• An adjustment potentiometers.

Fig. 5.2 capacitive proximity sensor

Capacitive proximity sensor are similar in size, shapes, and concepts to inductive proximity sensor. However unlike the inductive sensors which use induced magnetic fields to sense object, capacitive proximity generate to an electrostatic field and reacts to changes in the capacitance caused when a target enters the electrostatic field.

When the target is from outside the electrostatic field, the oscillator is inactives. As the target approaches, a capacitive coupling develop between the target and the capacitive probes.

When the capacitance reaches specified threshold, the oscillator is activate, triggering to the output circuit to switch states between the ON and OFF.


Fig. 5.3 working of capacitive sensor The ability of the sensors to detect the target is determined by the target size, dielectric constants and distance from to the sensor. The larger the target size, the stronger the capacitive coupling between the probe and the target. Material with the higher dielectric constant is easier to detect than those with lower value. The shorter the distance between target and probe, the stronger the capacitive coupling of the between the probe and the

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