At2 H 03 Proximity Generality

DEPT. of ELECTRICAL EQUIPMENT AND INSTALLATION TECHNOLOGY AT - A UT OMA TION T ECHNOL OGY PROXIMITY SENSORS == Generality == 1. Introduction

Solid-state electronic devices that are completely encapsulated (filled with epoxy resin) to protect against excessive vibration, liquids, chemicals and corrosive agents found in industrial environment.

Resin

Solid state device Use static technology or detection without physical contact

2. Applications



The typical electro-mechanical switch life cycle is not long enough



Very fast production rate (or for counting purposes), therefore fast switching frequency becomes necessary.



A fast electronic control system requires a bounce-free input signal.

Handout

AT2-222 – Sensors & Fiber Optic Technology

Chap. 3 – Proximity Sensors : Generality

AT2-222-H-03

Page 1

DEPT. of ELECTRICAL EQUIPMENT AND INSTALLATION TECHNOLOGY AT - A UT OMA TION T ECHNOL OGY



Solid state sensors are more efficient than mechanical contacts at handling very low load currents.



Due to their sealed construction, the presence of fluids, vibration, shock, dust, dirt and oil do not affect proximity sensors.



The object being detected is too small, too lightweight, or too soft to operate a limit switch.



Target must be sensed through objects such that plastic or glass.



Movement of the target is too precise for mechanical switches

Handout

AT2-222 – Sensors & Fiber Optic Technology

Chap. 3 – Proximity Sensors : Generality

AT2-222-H-03

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DEPT. of ELECTRICAL EQUIPMENT AND INSTALLATION TECHNOLOGY AT - A UT OMA TION T ECHNOL OGY 3. Advantages



Advantage No physical contact with the object



to detect 

Result No wear, possible to detect fragiles or freshly painted objects

High operating rates



Compatibility with electronic automated control system



High approach speeds



Fast response



Robustness, products fully



Excellent resistance to industrial

encapsulated in resin 

Solid state, no moving parts

environment 

Service life of sensor independant of the number of operating cycles.

4. Types & Symbol

There are two types: 

Inductive proximity sensors (detect metal only) Inductive sensor with NO contact



Capacitive proximity sensors (detect metal and nonmetal)

Capacitive sensor with NO contact

Handout

AT2-222 – Sensors & Fiber Optic Technology

Chap. 3 – Proximity Sensors : Generality

AT2-222-H-03

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DEPT. of ELECTRICAL EQUIPMENT AND INSTALLATION TECHNOLOGY AT - A UT OMA TION T ECHNOL OGY 5. Composition



Oscillator An electronic circuit for generating ac wave-forms and frequencies from a dc energy sources



Trigger Circuit (Schmit trigger) Detect oscillation (wave-forms) and change the output



Output Switching Device Normally consist of PNP/NPN (for dc) or triac (for ac), which gives the output signal

Proximity Inductive Output Switching Device

Trigger Circuit

Osc.

Output Switching Device

Trigger Circuit

Osc.

Coil

Proximity Capacitive

Handout

Plate

AT2-222 – Sensors & Fiber Optic Technology

Chap. 3 – Proximity Sensors : Generality

AT2-222-H-03

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DEPT. of ELECTRICAL EQUIPMENT AND INSTALLATION TECHNOLOGY AT - A UT OMA TION T ECHNOL OGY 6. Principles of Operation



Inductive Proximity Sensors 

When there is power supply. the oscillator operates to generate a highfrequency electromagnetic field through the coil.



When a metal object enters the high-frequency field, eddy currents are induced in the surface of the target. This results in a loss of energy in the oscillator circuit; consequently, this causes a smaller amplitude oscillation.



The trigger circuit recognizes a specific change in amplitude and generates a signal that will turn the solid-state output ON or OFF.



When the metal object leaves the sensing area, the oscillator regenerates, allowing the sensor to return to its normal state.



Capacitive Proximity Sensors 

The active face is formed by two metallic electrodes - like an “opened” capacitor. When there is power supply; the electrodes forms an electrostatic field.



As the target approaches the face of the sensor, it enters the electrostatic field. This causes an increase in the coupling capacitance, and the oscilator begins to oscillate.



The trigger circuit recognizes a specific change in amplitude and generates a signal that will turn the solid-state output ON or OFF.

Handout

AT2-222 – Sensors & Fiber Optic Technology

Chap. 3 – Proximity Sensors : Generality

AT2-222-H-03

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DEPT. of ELECTRICAL EQUIPMENT AND INSTALLATION TECHNOLOGY AT - A UT OMA TION T ECHNOL OGY

Inductive Proximity Sensor Output Stage (Normally Open)

Object Absent

Output Switching Device

Trigger Circuit

Osc.

Coil

Amplitude OFF

•

Object Approaching Sensor

Output Stage (Normally Open) ON

Output Switching Device

Trigger Circuit

Osc.

Amplitude

Coil

Oscillator Stalled

Object

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AT2-222 – Sensors & Fiber Optic Technology Chap. 3 – Proximity Sensors : Generality

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DEPT. of ELECTRICAL EQUIPMENT AND INSTALLATION TECHNOLOGY AT - A UT OMA TION T ECHNOL OGY

Capacitive Proximity Sensor

Object Absent

Output Switching Device

Output Stage (Normally Open)

Trigger Circuit

Osc.

Plate

Amplitude OFF Oscillator Stalled

Object Approaching Sensor Output Stage (Normally Open) ON Output Switching Device

Trigger Circuit

Osc.

Plate

Amplitude

Object

Handout

AT2-222 – Sensors & Fiber Optic Technology Chap. 3 – Proximity Sensors : Generality

AT2-222-H-03

Page 7

DEPT. of ELECTRICAL EQUIPMENT AND INSTALLATION TECHNOLOGY AT - A UT OMA TION T ECHNOL OGY

Handout

AT2-222 – Sensors & Fiber Optic Technology

Chap. 3 – Proximity Sensors : Generality

AT2-222-H-03

Page 8