A Look at Magnetic and Inductive Sensing: Sensor Principles and Types
Seeing use in a variety of industrial applications for position sensing and object detection, inductive sensors are based on the phenomenon of magnetism and its resultant magnetic fields.
See Our Previous Articles on Industrial Sensors:
- Introduction to Industrial Sensors
- Optical Sensors and Common Types
- Lasers for Sensing and Object Detecting
- Infrared Sensors for Automation
- Temperature Sensors and Their Uses
Inductive sensors are based on the phenomenon of magnetism and its resultant magnetic fields. Although magnetism has been known for centuries, it was only first studied by British philosopher William Gilbert at the end of the sixteenth century. He discovered strong similarities between magnetic and electric properties, but more importantly, he was the person who discovered that the Earth has two magnetic poles.
Figure 1. Planet Earth is a giant magnet with a north and south pole. Image used courtesy of Adobe Stock
As we now know, every object with magnetic properties has a north pole and a south pole, and opposite magnetic poles attract each other. Magnetic polarity is caused by the arrangement of atoms in that object.
Furthermore, like gravity, magnetism produces forces that are exerted around it. Gravitational fields are produced in relationship to the mass of an object. In contrast, magnetic fields are produced by the atomic forces that are within magnetic materials as well as electric current. Magnetic fields also have magnitude and direction.
Magnetic fields are also produced by moving electric charges. This phenomenon was first discovered by physicist Hans Oersted in 1820. He observed that compasses worked differently when in the vicinity of electric currents.
Solenoids and toroids are devices capable of producing magnetic fields from an electric current. A solenoid consists of a coil with many wire turns, a metal housing, and a plunger. The magnetic field activates the plunger as an electric current runs through the coil. Toroids work based on the same principle as solenoids but have a circular shape. As a result, they are practical in applications that require higher field induction.
Figure 2. An iron core inductive coil assembly (center) used to form a common motor starting contactor.
As a complementary process, just like electric charges can produce magnetic fields, magnetic fields can produce electric currents. However, for this to happen, magnetic fields must change; in other words, constant magnetic fields do not produce electricity. The resulting electric current is then generated by a phenomenon called induction.
Induction is based on the well-known Faraday’s law of induction. It states that an induced voltage is equal to the rate of change of the magnetic field. This principle is behind most inductive sensors used today.
Magnetic and Inductive Sensor Types
Below, we will review some of the most common types of magnetic and inductive sensors.
Figure 3. A safety-rated reed sensor for a door application. Image used courtesy of Eaton
Reed switches consist of two small and thin ferromagnetic reeds in a special encasing. Their purpose is to manage the flow of electricity by opening or closing an electric circuit based on an induced magnetic field.
Like electric sensors, reed switches can be built for Normally Open (NO) and Normally Closed (NC) configurations. An NO reed switch would close the electric circuit upon the presence of a magnetic field. This happens vice versa for an NC switch.
Reed switches have numerous applications in home appliances, cars, and the medical sector. They can also be found in many places in manufacturing environments. One of the industry's most common uses of reed switches is for position sensing. Door open/close sensors are one common application among them.
Figure 4. Hall sensors (with yellow wires) placed around the stator coils of a brushless DC motor. Image used courtesy of Dialog Semiconductor
The Hall effect was discovered by American physicist Edwin Hall in 1879, hence its name. It states that an electric field is produced transversally to an object carrying an electric current and placed in a magnetic field. It is also one of the applications of the well-known right-hand rule. The great advantage of Hall effect sensors is that they can detect magnetic fields even when no changes exist. In other words, they can also sense static magnetic fields.
Common industrial applications of Hall effect sensors include DC transformers and position sensing in DC motors. Any non-contact DC current meter (ammeter) uses the Hall effect for operation.
Direct Magnetic Field
These are the most simple types of inductive sensors. They consist of a metal coil through which a magnetic force is exerted. This magnetic force will induce a current onto the coil, proportional to the magnetic field force.
The galvanometer is one of the earliest examples of a magnetic coil sensor. The galvanometer is an instrument that helps measure small electric currents based on the movement of a needle produced by a magnetic field.
Simple galvanometers with small ranges are often to detect unbalanced current in Wheatstone Bridge circuits, while larger galvanometers can be suitable for higher current ranges in industrial environments.
Figure 5. A high-frequency proximity switch. Image used courtesy of Cablematic
High-frequency Oscillation Switch
These switches are a subset of the proximity sensor. These sensors operate by producing a high-frequency magnetic field. When a metallic object obstructs the sensor, the current is induced onto the object, altering the oscillation frequency. A sensing circuit then detects these changes. These are very common proximity switches.
In the next article in our series, we will take a closer look at pressure sensors and the types used in industrial automation.