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Monthly Archives: August 2011

Eddy Current Transducer

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Before going into depth about this transducer, it is important that you know about the theory of eddy currents.

Eddy Currents

Eddy currents, also known as “Focault Currents”, are currents induced in a conductor due to the magnetic field produced by the active coil. The conductor is placed in a changing magnetic field and the current is produced according to the change of magnetic field with time. The amount of eddy current produced will be more if the field strength is greater. When there is high field strength, the conductivity of the metal conductor increases, causing faster reversals of the field and hence more flow of eddy currents. Eddy currents will be produced in both conditions where either the conductor moves through a magnetic field or a magnetic field changes around a stationary conductor.  Even a small amount of the current will be produced in cases where a small change in magnetic field intensity is experienced on a conductor.

Like other currents in a conductor, eddy currents can also generate heat, EMF, and all types of losses. Its biggest disadvantage can be seen in a transformer, where power loss due to this, affects the device’s efficiency. This can be reduced by reducing the area of the conductor, or by laminating it. Since the insulator in the lamination area stops the electrons from moving forward, they will not be able to flow on wide arcs. Thus, they accumulate at the insulated ends and resist further accumulation of charges. This, in turn will reduce the flow of eddy currents. The amount of currents produced can also be reduced by using conductors having less electrical conductivity.

Eddy Current Transducer

This type of transducer is comparatively low in the measurement field and depends mostly on the quality of a high alternating source which is fed to a set of coils. One coil is called the active coil and the other provides temperature compensation (Compensating coil) by being the adjacent arm of a bridge circuit. A conducting material is kept close to the active coil so as to make it influenced by its absence or presence, or, by being any closer or away. Magnetic flux is induced in the active coil and is passed through the conductor producing eddy currents. The density of this current will be maximum at the surface and will lessen as the depth increases. This penetration depth can be calculated using the equation given below.

δ=1/fπμσ
δ-Penetration Depth (m)
f-Frequency (Hz)
μ-Magnetci Permeability
σ-Electrical Conductivity (S/m)

 The circuit diagram of an eddy current transducer/sensor is shown below.

Eddy Current Transducer
                                                   Eddy Current Transducer

Working

The active coil is kept closer to the conducting material and both of them are placed inside a probe. The compensating coil is kept further away from the conducting material. The high frequency source acts as the bridge circuit and feeds the coil across the two capacitors. The amount of eddy current produced becomes more as the distance between the conducting material and the active coil becomes less. This causes a change in the impedance of the active coil and thus unbalances the bridge circuit. The bridge circuit produces an output proportional to the amount of closeness between the conducting material and the active coil. The output of the bridge circuit is given to a low pass filter (LPF) and then its dc output is calculated. The high frequency allows a thin target to be used and also with this, the frequency response becomes good up to a target frequency 1/10th the supply frequency.

It should be noted that the diameter of the conducting material should be larger or at least same as that of a probe. If not, the output is prone to reduce linearity. If shafts are used as conducting materials, they should have a bigger diameter so that their curved surfaces effectively behaves as flat surfaces.

Applications

Since it is a non-contact device, it is suitable for higher resolution measurement applications. The device is used for finding out the position of an object that is conductive in nature.

  • Position Measurement

Since the output of an eddy current transducer represents the size of the distance between the probe and the conductor, the device can be calibrated to measure the position or displacement of the target. Thus, it can be applicable in monitoring or sensing the precise location of an object such as a machine tool. It can also be used to locate the final position of precise equipments such as a disk drive.

  • Vibrating Motion Measurement

The device is also suitable for finding the alternate positions of a vibrating conductor. Since a contact device is impracticable for this application, a non-contact device such as eddy current transducer is highly recommended. Thus, it can be applicable in measuring the distance of a shaft from a reference point or the to-and-fro movement of vibrating instruments.

Advantages

  1. Measurement of distance can be carried out even in rough or mixed environments.
  2. Cost-effective.
  3. The device is insensitive to material in the gap between the probe and the conductor.
  4. The device is less expensive and has higher frequency response than a capacitive transducer.

Disadvantages

  1. The result will be precise only if the gap between the probe and the conductor is small.
  2. The device cannot be used for finding the position of non-conductive materials. Another way is to connect a thick conductor onto the non-conductive material.
  3. There always occurs a non-linear relationship between the distance and impedance of the active coil of the device. This problem can be overcome only by calibrating the device at fixed intervals.
  4. The device is highly temperature sensitive. This can be overcome by adding a suitable balance coil to the circuit.

Magnetostrictive Transducer

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Magnetostriction

Magnetostriction can be explained as the corresponding change in length per unit length produced as a result of magnetization. The material should be magnetostrictive in nature. This phenomenon is known as Magnetostrictive Effect.  The same effect can be reversed in the sense that, if an external force is applied on a magnetostrictive material, there will be a proportional change in the magnetic state of the material. This property was first discovered by James Prescott Joule by noticing the change in length of the material according to the change in magnetization. He called the phenomenon as Joule effect. The reverse process is called Villari Effect or Magnetostrictive effect. This effect explains the change in magnetization of a material due to the force applied. Joule effect is commonly applied in magnetostrictive actuators and Villari effect is applied in magnetostrictive sensors.

This process is highly applicable as a transducer as the magnetostriction property of a material does not degrade with time.

Magnetostriction Transducers

A magnetostriction transducer is a device that is used to convert mechanical energy into magnetic energy and vice versa. Such a device can be used as a sensor and also for actuation as the transducer characteristics is very high due to the bi-directional coupling between mechanical and magnetic states of the material.

This device can also be called as an electro-magneto mechanical device as the electrical conversion to its appropriate mechanical energy is done by the device itself. In other devices, this operation is carried out by passing a current into a wire conductor so as to produce a magnetic field or measuring current induced by a magnetic field to sense the magnetic field strength.

Working

The figure below describes the exact working of a magnetostrictive transducer. The different figures explain the amount of strain produced from null magnetization to full magnetization. The device is divided into discrete mechanical and magnetic attributes that are coupled in their effect on the magnetostrictive core strain and magnetic induction.

Magnetostrictive Transducers
                                                 Magnetostrictive Transducers

First, considering the case where no magnetic field is applied to the material. This is shown in fig.c. Thus, the change in length will also be null along with the magnetic induction produced. The amount of the magnetic field (H) is increased to its saturation limits (±Hsat). This causes an increase in the axial strain to “esat”. Also, there will be an increase in the value of the magnetization to the value +Bsat (fig.e) or decreases to –Bsat (fig.a). The maximum strain saturation and magnetic induction is obtained at the point when the value of Hs is at its maximum. At this point, even if we try to increase the value of field, it will not bring any change in the value of magnetization or field to the device. Thus, when the field value hits saturation, the values of strain and magnetic induction will increase moving from the center figure outward.

Let us consider another instance, where the value of Hs is kept fixed. At the same time, if we increase the amount of force on the magnetostrictive material, the compressive stress in the material will increase on to the opposite side along with a reduction in the values of axial strain and axial magnetization.

In fig.c, there are no flux lines present due to null magnetization. Fig.b and fig.d has magnetic flux lines in a much lesser magnitude, but according to the alignment of the magnetic domains in the magnetostrictive driver. Fig.a also has flux lines in the same design, but its flow will be in the opposite direction. Fig.f shows the flux lines according to the applied field Hs and the placing of the magnetic domains. These flux fields produced are measured using the principle of Hall Effect or by calculating the voltage produced in a conductor kept in right angle to the flux produced. This value will be proportional to the input strain or force.

Applications

The applications of this device can be divided into two modes. That is, one implying Joule Effect and the other are Villari Effect.

  • In the case where magnetic energy is converted to mechanical energy it can be used for producing force in the case of actuators and can be used for detecting magnetic field in the case of sensors.
  • If mechanical energy is converted to magnetic energy it can be used for detecting force or motion.
  • In early days, this device was used in applications like torque meters, sonar scanning devices, hydrophones, telephone receivers, and so on. Nowadays, with the invent of “giant” magnetostrictive alloys, it is being used in making devices like high force linear motors, positioners for adaptive optics, active vibration or noise control systems, medical and industrial ultrasonic, pumps, and so on. Ultrasonic magnetostrictive transducers have also been developed for making surgical tools, underwater sonar, and chemical and material processing.