Types of sensors in automatic control (PT2): Proximity Sensors

Proximity sensors (or simply “PROX”) react when objects are near the sensor. In most cases, this distance is only a few millimeters

Reliable operation even in harsh environments (eg outdoor environment or grease environment). Proximity sensors usually detect the end position of the machine part and the output signal of the sensor triggers another function of the machine.
The main benefits of industrial proximity sensors are:
Operation / installation is simple and easy
Attractive prices (eg cheaper Photovoltaic Sensors)
Today, Proximity Sensors are present in many types of industries and applications. Some examples:
– Automobile manufacturing industry
– Machine tool industry
– Food processing industry
– Utility vehicles (for example, trucks, agricultural machines)
– Car washer

Types of Proximity Sensors

There are two types of industrial proximity sensors:
Inductive proximity sensor detects objects by creating an electromagnetic field. Of course, the device only detects metallic objects.
The capacitive proximity sensor detects objects by creating electrostatic field of electrostatic discharge. Therefore, this device can detect all kinds of things.
Although touch sensors only detect metal objects, they are much more common in the industry. These sensors are less susceptible to external interference such as EMC and – last but not least – these sensors are cheaper than capacitive sensors.
The next page will introduce you to some technical theories on the operation of the touch sensor.

How the sensor works
The touch probe sensor consists of a coil wound around a magnetic core at the touch. High frequency waves traveling through the wire core create an oscillating electric field around it. This electromagnetic field is controlled by an internal circuit.
When a metal object moves towards this field, it creates an electric current (eddy current) in the object.
These electric currents act as transformers, so that the energy in the coil detects less and fluctuates; The strength of the magnetic field decreases.
The monitoring circuit detects reduced oscillation and then changes the output. The object was discovered.
Because this principle of operation uses electromagnetic fields, the sensor senses superiority to the photovoltaic sensor for environmental resistance. For example, oil or dust usually does not affect the operation of the sensor.
 Sensor output
Today, most sensor sensors have either NPN or PNP logic transistor output characteristics (see right). These are also known as DC-3 wire.
In some cases of installation, people use Proximity Sensors with two connections (negative and positive). They are called DC-2 wires (see diagram below).

Openning contact / closing contact

The asynchronous proximity sensor (NO) and normally closed (NC) sensors describe the state of the sensor’s output signal after the detection or non-detection of the object.
Openning contact: High voltage signal, when detecting object; Low voltage signal when no object
Closing contact: High signal when no object; Low signal when detecting object.
The example on the left shows the DC-2 Proximity Sensors with normally open outputs (NO). Output works when the object moves near the sensor.
Move your mouse (= object) through the sensor to make the light bulb glow
Now, let’s look at an example of a similar illustration to the normally closed output (NC). The lamp shuts off as soon as the object (mouse) moves closer to the sensor.
Asymptotic proximity sensors with both NO and NC outputs are called opposite types.
Note: NO / NC type is used for both sensor and capacitive sensor. This picture shows the capacitance sensor.

Detection Distance – Standard Rate
Distance detection is an important technical parameter when designing Proximity Sensors in the machine.
There are three types of Proximity Sensors that detect short, medium and long detection distances.
Detection distance is stated in the specifications of the Proximity Sensors based on the sensor’s axial displacement criteria. This standard item is a 1 mm square steel sheet, the main component being iron (determined according to EN 60947-5-2).
Note: For objects moving toward the touch surface, the detection distance will be different!
Distance Detection Reduction Factor
Depending on the type of metal used, the detection range may be less than the detection distance. The following table provides the approximate proximity detection distance of a standard Proximity Sensor for various metal materials. Details about dependence on the types of metal are contained in the technical information of each sensor sensing document.
Note: Special touch sensors have a distance independent of the distance of the metal to be used. They are also called Proximity Sensors “Coefficient 1”.

Impact of Object Size
Detection distance is also influenced by the size of the object (the smaller the object will reduce the detection distance).
At the same time, the type and thickness of the coating also affect the detection distance.


Sensor delay describes the difference between the distance at which the sensor operates and the distance at which the sensor returns to its initial state.
Small latency allows to precisely position the object.
The value of the latency is usually in the range of 5-10%.

Response Frequency
According to EN60947-5-2, the response frequency determines the number of repetitions that can be output each second when the standard test object is sent to the sensor several times in advance.
See accompanying diagram for measuring method: Distance must be 50% of the detection distance; Pulse-pause ratio is defined as 1: 2 (see figure: M with 2M).
Note: If the pulse-pause ratio is different (eg, 1: 1), the maximum response frequency will be lower. The maximum response frequency also decreases if the distance is higher or lower than 50% of the detection distance.

Sensitive Touch ProtectedThe protected proximity sensor is composed of a shield around the magnetic core. This plate has the effect of leading the electromagnetic field to the front of the head.
The protected proximity sensor can be immersed on the metal surface, if space is tight. This also has the benefit of being able to protect the sensor mechanically.
However, the detection range is limited, but it can easily be attached to the surrounding metals without any effect.
Sensor Touch Unprotected
Unprotected sensor without protective layer around the magnetic core. The difference between protected and unprotected sensors can be easily observed.
This design provides greater detection distance than Proximity Sensors are protected. The unprotected sensor has a detection distance that is nearly double that of the protected type with the same diameter.

Uninsulated Proximity Sensors can not be mounted on an unprotected surface with metal surfaces. Therefore, the mechanical protection is lower. Because the magnetic field extends to the edge of the sensor, it can be influenced by the metals in the area. Proximity Sensors that are not protected are also more sensitive to mutual interference.

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