By Meghna Deshraj

June 27, 2022

Magnetic Proximity Sensor
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A magnetic proximity sensor is a device that detects the presence of nearby ferrous material to activate its outputs. These sensors are often found in cars, washing machines, and garage doors, to name a few.

Most models of magnetic proximity sensors require the use of two coils for detecting objects. One ring is embedded in the sensor’s sensing area and generates an electric current induced by any magnetic field nearby. The other coil – which may also be inside the sensing area or be located remotely – receives these signals and converts them into a digital signal interpreted as binary values (1s or 0s). This data can then detect obstacles or send out an alarm if necessary.

The following are some of the most common magnetic proximity sensors on the market, along with their respective applications:

Axial Sensors. Axial sensors contain a primary coil placed near the object to be detected and a secondary coil located some distance away. It detects ferrous elements in a dedicated signal range by induction and outputs an electric signal when an object is in its sensing range. These sensors can also be used in open-loop systems; that is, systems based on “magnetic” instead of “mechanical” switches, which can be dangerous if they are exposed to moving parts (such as wind turbines).

These sensors are ideal for moving objects like cars and automatic doors, as they can detect the presence of ferrous material while being located at a distance. Axial sensors can also detect multiple objects within their range; that is, they can differentiate between different types of metal (steel, iron) and other materials.

Hall Sensors. Hall sensors contain two coils – one located inside the sensing area and the other in a remote position – arranged in parallel to allow for induction when there is a change in a magnetic field. This proximity sensor is commonly used when a system requires the detection of metallic material over a large area with a high level of sensitivity.

For example, this type of sensor controls the circuit of a washing machine. In the case of a garage door, the sensor can detect the presence of cars or other vehicles. There are several types and sizes of hall sensors available on the market. Still, they tend to be bulkier than some other sensors to minimize noise from environmental effects.

How do magnetic proximity sensors work?

A magnetic proximity sensor uses induction principles to detect nearby ferrous objects. The sensor is composed of a coil, which generates an electric current in response to the presence of ferrous metal nearby.

A magnetic field is generated when the sensor is placed near ferrous material. This field induces an electric current in the sensor’s secondary coil. This current then passes through the switch that determines the applied output. The higher the energy of this signal (the greater the amplitude), the more likely it is that the object detected is a valid target.

A magnetic proximity sensor’s sensing area is usually located inside and outside of its housing. For example, in a washing machine, one coil (which generates electricity) is situated in the washing drum while another – which detects metallic materials – can be positioned within a separate housing and be connected to control circuitry through an insulated cable.

The sensor’s sensing area can also be located externally, which should be fastened to the item to be detected. This is commonly seen in garage doors; the magnetic proximity sensor is placed outside and connected to a controller through an insulated cable that runs from the sensing area to the control circuitry.

The operation of magnetic proximity sensors is based on four basic principles: induction, signal propagation, amplification, and frequency shift. These principles are explained below:

1) Induction.

Electrical current is induced in any piece of ferrous material near a magnet when its magnetic fields interact with each other. A simple example of this can be seen when iron bars are placed near a magnet.

The induced current can be transformed into a voltage signal and amplified by an amplifier.

2) Signal Propagation.

Since magnetic fields are generated in concentric circles from a magnet, the signal generated by induction (induction current or electromagnetic wave) is also propagated in the same manner, that is, in concentric waves or rings.

3) Amplification.

The intensity of this current passing through the sensing coil depends on the number of loops where induced currents are induced and propagated, which means that more loops mean greater voltage.

4) Frequency Shift.

It is possible to change the frequency of this signal by varying the inductance of the coil. The most common way is to change the number of loops (more loops mean greater frequency). In this case, we are referring to an AC coil, which allows us to regulate the frequency to a greater extent by varying inductance in a range that allows for easy amplification and differentiation (6-15 kHz).

For a magnetic proximity sensor’s output to be interpreted and used as digital values, they must be conditioned first to remove unwanted noise and interference.

Where are magnetic proximity sensors used?

In an electrical circuit, magnetic proximity sensors are commonly used in industrial automation systems such as:

Magnetic proximity sensors have several benefits and drawbacks. This may be because of the sensor’s size or function (detecting a single object or multiple objects). The following are some of these benefits and drawbacks:

Many areas of use for magnetic proximity sensors include industrial automation, security, and robotics. A common example is magnetic proximity sensors for garage doors and automatic cars. Here the magnetic field produced by the sensor is interpreted to determine the presence of a vehicle approaching from outside the garage. In this example, the magnetic proximity sensors are placed outside the garage and connected to an amplifier through a cable.

About the author 

Meghna Deshraj

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