Eddy current sensors detect the distance or the defects of metal objects without contact, dynamically with accuracy.

They are usually employed for measuring ferromagnetic and non-ferromagnetic substances. They are acceptable for applications in harsh industrial environments because of their exceptional tolerance, such as oil, dust, moisture, and field interference. Offered flexible and miniature versions, could also be used for measurement at a distance where the place is limited.

Principle of eddy current

Eddy current is electrical currents induced in metals by changing magnetic field. 

Eddy current flows in closed circular path within conductor, in plane vertical to the magnetic field.

Are eddy currents AC or DC?

The current always directed in one-way in case DC, where as it’s alternating one in case of AC. In DC, there is no oscillation whatsoever. Though in Eddy currents, however, have no fixed direction, are circulating in nature. The concept of AC and DC doesn’t apply to these currents, since there is no specific direction in which these currents flow.

The currents are not flown back to the source, since they’re electromagnetically induced. Hence the path between the source coil generally, and the load, which is the conducting surface, in this case, is incomplete. The currents don’t flow back to the source. The conducting surface, where actually eddy has been produced, dissipate their energy in the form of heat. And since the circuit is technically open, it can neither be called AC or DC.

Eddy current Vs induced current

An eddy current is an induced current but in metal or materials where you don’t want induced current such as the transformer’s core or mounting frame.

How is eddy current created in a conductor ?

When a conductor is placed in a time-varying magnetic flux, the change in flux due to varying magnetic fields induces small loops in the conductor, and current flows through these loops Faraday’s law. These currents are known as eddy currents.

How does an eddy current sensor work?

Eddy current sensor operating principle

Eddy current sensors utilize the principle of eddy current formation to determine the displacement. It forms when a changing magnetic field intersects a conductor. The relative movement causes a circulating stream of electrons, or electrons, in the conductor. These circulating eddies of electromagnets with magnetic fields that oppose the impact of applied magnetic fields. The more powerful the magnetic field, or higher the conductor’s conductivity, or even the higher the comparative speed of movement, the induced currents and larger the opposing area will be. Eddy current probes perceptions this creation of secondary areas to discover the space between the probe and target substance.

Applications of eddy current

There are various industrial applications in which eddy currents are exploited, without eddy it won’t function properly. Few examples are magnetic brake, electromagnetic damping based applications, induction heater, electrical power-meter, electromagnetic levitation, metal characterization, vibration and position measurements, structural testing, etc. Some of them have been explained in details as follows:

• Magnetic levitation and repulsive effects: Here eddy current based repulsion force is utilized in various applications. This is the basic criteria have been exploited during the application in magnetic levitation. This force can lift heavy objects against gravity such as train, monorail etc., these system also works friction freely.
• Induction furnace: eddy current might be utilized in melting of metals and for welding purpose, redesigning purposes, or for manufacture alloy. In an coil based heater, a high-freq AC is permissible to carry thru a coil which environs the respective metal to be molten.
• Magnetic braking in trains: Usually, trains are moving at excessive speeds, henceforth, the braking system of trains should be effective with smooth transition to free form jerking. Eddy current effect has introduced by strong electromagnet, situated directly above the rails, it activates eddy current in the rails to opposite direction of the train wheel rotation. It’s frictionless, so no mechanical linkage; henceforth, this brake operates on smooth transition without jerking effect, but only applicable to electrically powered train.
• Electromagnetic damping-based application: Few gauge or instrument i.e., galvanometer make use of the effect of eddy current. They non-magnetic fixed core by metallic material is utilized to generate eddy current coil oscillates, which in turn opposes the motion of the coil and takes it to rest by that opposing forces.
• In adjustable speed drive:  An eddy current coupled speed drive can be achieved variable speed as required for different industrial applications.
• Eddy current sensors measure vibration in steel galvanizing plants
• To measure the metal plate thickness of sheet metal , pipe or hollow-tubes also utilized eddy current sensor
• Cylinder position movement in an internal combustion engine has also eddy current sensor
• To Measure movement of hydraulic cylinders, eddy current sensor may be useful too.
• Employed in aero plane such as doorlocking switch and landing gearflap etc.

** Although eddy current is undesirable in some applications may create unwanted magnetic interference in the desired signal. Where we apply high field magnets, the analysis for error field created by eddy, need to be calculated and taken care for better accuracy.

Eddy Current Type Displacement Sensor

Principle of detection

High-frequency magnetic fields are utilized in the eddy current method. This high-freq. magnetic-field is created by flowing a high-freq currents within the coil placed inside the eddy current sensors called probes or sensor-head sometimes . Suppose a target (metal) introduced into this magnetic-field. In that case, electromagnetic induction causes magnetic flux to pass over the surface of that thing. Eddy current flows in the perpendicularly. This causes the impedance of the eddy current sensor to change. Hence, distance could be measured by this process.

The eddy current kind displacement detectors produce a pliable magnetic field by employing a high-frequency present into the detector head. When there’s a measurement item (metal) inside this magnetic field, then surplus current is generated around the magnetic flux, which passes through the object surface because of the electromagnetic induction result. This affects the impedance of the coil inside the detector head.

Since the space between the measurement item (alloy) along with the sensor tip gets smaller, the more considerable current is generated, and energy loss in the eddy current sensor tip rises. Because of this, once space is created nearer, the oscillation gets smaller. After space is higher, the oscillation gets greater. The detectors rectify the variants in the oscillation, which triggers an alteration of the DC voltage. But, linearity is fixed via linearization, and an outcome that’s proportional to space can be found.

The mutual interference impacts in that measurement sometimes.

Face to face mounting

Parallel mounting

There are several methods to interference in between; some of them are as follows:

• Eddy current sensors has to be installed with a spacing, so that no interference is instigated.
• Need to be installed, along with a different freq type.
• Need to be installed considering the interference prevention function.

To know more about Eddy current click Eddy Current Brake and Eddy Current Testing.

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Dr. Subrata Jana

I am Subrata, Ph.D. in Engineering, more specifically interested in Nuclear and Energy science related domains. I have multi-domain experience starting from Service Engineer for electronics drives and micro-controller to specialized R&D work. I have worked on various projects, including nuclear fission, fusion to solar photovoltaics, heater design, and other projects. I have a keen interest in the science domain, energy, electronics and instrumentation, and industrial automation, primarily because of the wide range of stimulating problems inherited to this field, and every day it’s changing with industrial demand. Our aim here is to exemplify these unconventional, complex science subjects in an easy and understandable to the point manner.