WHAT IS PROXIMITY SENSORS , WORKING PRINCIPLE, AND TYPES

Proximity sensor

What is Proximity Sensor?

All types of sensors that make contactless detection are known as proximity sensors. But like limit switches you can detect them by physical contact with them. Proximity Sensors convert information on the movement or presence of an object into an electrical signal. There are three types of detection systems for making this conversion: firstly systems that use eddy currents generated in metal-sensitive objects by principle of electromagnetic induction, and secondly systems that detect changes in electrical potential when approaching a sensitive object and systems that use magnets and reed switches.

All sensors that provide non-contact detection of target objects that are close by or within the general vicinity of the sensor are known as proximity sensors, and generally classifies them as inductive, capacitive, ultrasonic, photoelectric, magnetic, etc. This Technical Explanation defines all inductive sensors that are used for detecting metallic objects, capacitive sensors that are used for detecting metallic or non-metallic objects, and sensors that utilize magnetic DC fields.

Classifications of Proximity Sensor:

Proximity sensors are divided into five main categories. They are

a. Inductive Proximity Sensor.

b. Capacitive Proximity Sensor.

c. Magnetic Proximity Sensor.

d. Photoelectric Proximity Sensor and

e. Ultrasonic Proximity Sensor.

a. Detection principle of Inductive Proximity Sensor:

Inductive Proximity Sensors detect magnetic loss due to eddy currents that are generated on a conductive surface by an external magnetic field. An AC magnetic field is generated on the detection coil, and changes in the impedance due to eddy currents generated on a metallic object are detected.

Other methods include Aluminium-detecting Sensors, which detect the phase component of the frequency, and All-metal Sensors, which use a working coil to detect only the changed component of the impedance. There are also Pulse-response Sensors, which generate an eddy current in pulses and detect the time change in the eddy current with the voltage induced in the coil.

In order for us to understand clearly:

The sensing material and the sensor form a transformer-like morphological structure.

Inductive proximity sensor- source Omron

The impedance changes can be viewed as changes in the resistance that is inserted in series with the sensing object. (This does not actually occur, but thinking of it this way makes it easier to understand qualitatively.)

b. Detection principle of Capacitive Proximity Sensor:

Capacitive proximity sensor- source Omron

Capacitive Proximity sensor Sensors detect changes in the capacitance between the sensing object and the Sensor. The amount of capacitance varies depending on the size and distance of the sensing object. An ordinary Capacitive Proximity Sensor is similar to a capacitor with two parallel plates, where the capacity of the two plates is detected. One of the plates is the object being measured (with an imaginary ground), and the other is the Sensor's sensing surface. The changes in the capacity generated between these two poles are detected.

The objects that can be detected depend on their dielectric constant, but they include resin and water in addition to metals.

c. Detection principle of Magnetic Proximity Sensor:

The reed end of the switch is operated by a magnet. When the reed switch is turned ON, the Sensor is turned ON.

Magnetic proximity sensor- source Omron

d. Detection principle of Photoelectric Proximity Sensor:

Photoelectric sensor- source Omron

Photoelectric Sensors detect the objects when changes in surface conditions and through a variety of optical properties. A Photoelectric Sensor consists primarily of an Emitter for emitting light and a collector for receiving light. When emitted light is interrupted or reflected by the sensing object, it changes the amount of light that arrives at the Receiver. The Receiver detects this change and converts it to an electrical signal. The light source for the majority of Photoelectric Sensors is infrared or visible light.

e. Detection principle of Ultrasonic Proximity Sensor:

Ultrasonic proximity sensor- source Omron

Ultrasonic proximity sensors are a common type of proximity sensor used in many manufacturing and automation applications. The basic detection principle of ultrasonic sensing is that the sensor sends out an ultrasonic pulse and receives a pulse back. Using the time difference between the sent and received signal, the distance to the object can be determined. A common design is to build both the transmitter and the receiver into the same housing, although they can also be housed in separate units like certain photoelectric sensors with separate emitters and collectors. Ultrasonic sensors work by using sound frequencies higher than the audible limit of human hearing (around 20 kHz), which is typically in the range of 25 to 50 kHz.

The method of setting the sensing distance:

The sensing distance is the distance from the sensing surface to the sensitive material position that allows for constant use, including the effects of temperature and voltage. It is about 70% to 80% of normal sensitivity. The proximity sensor output is ON when the sensing object is present in sensing distance, and the sensor output is OFF when the sensing object is away from the sensing distance. This can be easily understood by the image below.

A sensitive object used for measuring must be made of specific materials and have a specific shape and dimensions.

Sensing distance setting- source Omron

How to connect a Proximity Sensor in a Circuit:

How to connect a proximity sensor to the circuit by wiring is shown in the figure below. Depending on the transistor configuration and the presence of a sensing object nearby sensor outputs are considered as NC-normally closed or NO-normally open.

CConsider in PNP configuration when the output is low or off while the sensing object is absent, then we can consider the sensor as normally opened (NO). Similarly, if the PNP configuration output is low or off while the sensing object is present, then we can consider the sensor as normally closed (NC).

NPN and PNP wiring diagram

How to select a proximity sensor:

• Sensing object:

If the material you are going to sense is metal (iron, aluminium, brass, and copper) then you should choose inductive proximity sensor. Choose a capacitive proximity sensor if the material you are going to sense is metal, water, powder, and resins. Or if the object to be sensed is magnetic, the magnetic proximity sensor should be selected.

• Power supply:

Before choosing a proximity sensor we need to pay close attention to the power supply we are going to use. Proximity sensors are designed to fit many types of supply voltage AC, DC, AC / DC.

• Sensing distance:

The sensing distance must be selected by considering the effort of factors such as the temperature, the sensing object, surrounding objects, and the mounting distance between sensors. 

• Ambient environment:

Things to consider before choosing a sensor; Temperature or humidity, or existence of water, oils, chemicals, etc., also need to consider the degree of protection the ambient environment.

• Physical vibrations: 

To prevent sensors from vibrating loose, terminals should be solder instead of using terminal connectors. And the housing should be proper tighten. 

Features:

• Proximity Sensors detect an object without touching it, and they therefore do not cause abrasion or damage to the object. 
• No contacts are used for output, so the Sensor has a longer service life.
• Unlike optical detection methods, Proximity Sensors are suitable for use in locations where water or oil is used.
• Proximity Sensors provide high-speed response, compared with switches that require physical contact.
• Proximity Sensors can be used in a wide temperature range.
• Proximity Sensors are not affected by colors.
• Unlike switches, which rely on physical contact, Proximity Sensors are affected by ambient temperatures, surrounding objects, and other Sensors.
• Most of the proximity sensors are tow wire sensors. So easy to connect there is no too much complication in connection.