Lecture - 4 - Inductance Transducers - German …eee.guc.edu.eg/Courses/Electronics/ELCT903...

1Dr.-Eng. Hisham El-SherifElectronics and Electrical Engineering Department

ELCT903, Sensor Technology

Will detect any conductive material

Quoted distances are for mild steel

Sensor performance can be affected by:

• Temperature

• Target material

• Target dimensions

Inductive Sensors



Operating Principle



-The sensor head is a ferrous core with a conductor coil winding around it.

- the oscillating supply circuit establishes an oscillating current, hence an oscillating electromagnetic field, around the sensor head.

-When metallic object enters the field of the sensor, it changes the electromagnetic field density around the sensor.

-The oscillating electromagnetic field induces eddy currents on the target metallic object.

- the eddy current losses draw energy from the supply circuit of the sensor, hence reducing the magnitude of the oscillations.

- the detection circuit measures the drop in the oscillation current magnitude and switches the out circuit transistor.

-The detection range is higher for ferrous metals than for nonferrous (aluminum, copper)



Inductance Transducers

Introductory Concept

Are used for proximity sensing when the presence or absent of an object must be detected with electronic non contact sensor

They are also used for motion position detection, motion control and process control applications

Inductance transducers are based on Faraday’s law

The induced voltage or electro motive force (emf) is equal to the rate at which the magnetic flux through the circuit changes.



The induced voltage equation

dtd

Nvφ=

Where N is the number of turns

BA=φ Where B is the magnetic field density

A is the area of the coil

Magnetic fluxφ

-If varying magnetic flux is applied to a coil.

-Then electromotive force appears at every turn of the coil.-

-If turn of the coil has the same cross section.

-Then the flux through each turn will be the same.



The voltage output can be changed by changing the flux enclosed by the circuit, this can be done by changing the amplitude of the magnetic field B or area of the coil A

dtBAd

Nv)(=

φψψφNwhere

dtd

dtdN

v === ,)(

Where N is the number of turns

is the total flux linkagesψ

1- The voltage generated is equal to the rate of change of flux linkages

2- The magnetic field B, produced by the current i in any circuit, is proportional

to the current and geometry of the coil



The total flux linkage of the circuit can be expressed in terms of a constant L, which is the inductance of the circuit

Inductance of the circuit is defined as the flux linkage per unit current

iN

iL

φψ ==

Flux is defined as

RNi=φ

Where

R is the reluctance of the flux path (reluctance is analogous to resistance in electrical circuits)



Self inductance of a coil L is expressed as

RN

RNi

iN

L2

==

where

N = number of turns

R = reluctance of the magnetic circuit

The reluctance is expressed as

Al

Rµ

=

Where

= effective permeability of the medium in and around the coil

= length of the coil in meters

= area of cross section of the coil, m2

µlA



GNlA

NL µµ 22 =��

��

�=

where

==lA

G Geometric factor

The equation shows that a change in self inductance of the coil can be caused by - a change in the number of turns

-The geometric configuration

- the permeability of the magnetic material

Any of these variables can be arranged to cause a change in displacement, there by causing a change in inductance as inductance transducers.



The inductance change can be caused by any of the following:-

- Variation in geometry of the coil (by change in the number of turns in the coil.

- Change in effective permeability of the medium in and around the coil.

- Change in the reluctance of the magnetic path or variation of the air gap.

- Change in the mutual inductance (by change in the coupling between coils 1 and 2 with aiding or opposing field).

The change in self inductance caused by the geometric configuration is the result of the coil arrangement. The displacement which is to be measured produces a change in the self inductance of the coil



Inductance transducers that are based on the principle of variation of mutual inductance use multiple coils.

The presence of an induced emf in a circuit due entirely to a change of current in another circuit is called mutual induction

Mutual Induction Transducers



Consider two coils 1 and 2 with turns N1 and N2.

The current i flowing in the coil 1 produces a flux

If R is the reluctance of the magnetic path,

then the induced emf in coil 2 due to current in coil 1 is

φ

( )dtRiN

dN

dtd

Ne��

��

�

==

11

222

φ

dtdi

RNN

e 1212 =

dtdi

Me 12 = where mutual inductance

RNN

M 21=

Theory of Operation



emf induced in coil 1 due to change in current in coil 2 is

dtdi

Me 21 = where mutual inductance

RNN

M 21=

the expression of mutual inductance is modified by the factor K , which presents the loss in flux linkages between two coils.

K is known as the coefficient of coupling between the two coils.

mutual inductance kRNN

M 21=We know that

RN

LR

NL

22

2

21

1 , ==

……………………………….1



from the above two equations

The mutual inductance 21LLkM =

Thus the mutual inductance between the two coils can be changed by variations in either of the self inductances or the coefficient of coupling

2

22

21

21 RNN

LL = …………………………………………2



Types of Inductance Sensors



-When a metal target enters the field, eddy currents circulate within the target.

-This causes a load on the sensor, decreasing the amplitude of the electromagnetic field.

-As the target approaches the sensor, the eddy currents increase, increasing the load on the oscillator and further decreasing the amplitude of the field

Digital Inductance Transducer



-The trigger circuit monitors the oscillator’s amplitude and at a predetermined level switches the output state of the sensor from its normal condition (ON or OFF).

-As the target moves away from the sensor, the oscillator’s amplitude increases.

-At a predetermined level the trigger switches the output state of the sensor back to its normal condition (ON or OFF).



-inductive sensors contain an oscillator circuit, with consists of a parallel resonance circuit with inductance and capacitance as well as an amplifier.

-the electromagnetic field is directed towards the outside, by means of a ferrite shell core of the inductance (coil).

-if an electrically conductive material is introduced into the electromagnetic stray field.

-according to the laws of inductance, eddy currents are created, which attenuate oscillation.

-depending on the conductivity, the dimensions and the distance of the approaching object, the oscillator can be attenuated to such an extent that the oscillation stops.

Attenuation of the oscillator is evaluated via subsequent electronic devices, and output signal is generated.



Output Configurations

-The load is connected between the output (A) and the negative side of the power supply (L-).

-The PNP transistor switches the load to the positive side of the power supply (L+)

-When the transistor switches ON, a complete path of current flow exists from L- through the load to L+



-The load is connected between the output (A) and the positive side of the power supply (L+).

-An NPN transistor switches the load to the negative side of the power supply (L-).



Complementary

-Transistor devices can also be complementary (four-wire).

-A complementary output is defined as having both normally open and normally closed contacts in the same sensor.



Series and Parallel Connections

-When sensors are connected in series all the sensors must be ON to turn ON the output.

-When sensors are connected in parallel either sensor will turn the output ON



Analog Inductive Sensor



-Inductive sensors Consists of an oscillator circuit made up of a parallel resonate circuit with coil (inductance and a capacitor (capacitance) as well as amplifier.

-The electromagnetic field is directed towards the out side by means of a ferrite shell core of the coil.

-If an electrically conductive material is introduced into the active zone of the stray field, eddy currents are induced into the material according to the laws of induction, which attenuate oscillation.

-Attenuation of the oscillator varies according to the conductivity, permeability dimensions and proximity of the object.

-Attenuation of the oscillator is evaluated via subsequent electronic stages.An output signal is generated which, within a defined measuring range, is proportional to the distance between the sensor and the material



Applications for Inductive Sensors

Drive shaft Run out/Motion

Drive shaft



-Inductive sensors are ideal for measuring rotating motion when the environment is dirty and accuracy less than a micron is not required.

-These measurements may be performed as part of a testing process or permanentlyinstalled in the final product.

-Examples

-are drive shafts or crankshafts running in an oil bath, or marine propeller drive shafts which may have water spraying or splashing in the sensing area, or power generator shafts which are monitored during operation to indicate bearing wear.

-The output of the sensor may be monitored by an operator or a computerized control system.



Valve Stroke, Piston Dynamics

Tests on internal combustion engines while they are running are ideal for inductivesensors.

Their immunity to oil allows them to be installed directly in an engine for test during operation.

One such test measures the position of the valve stem at its highest point.

The probe is installed in the engine and positioned to sense the position of the end of the valve stem at its highest point.



Inductive sensors are unaffected by oil and can be used to test parameters of internal combustion engine while they are in operation.



Calendar Roller GapRolling of sheets, coils, bars, and shapes is a process used to fabricate metals toproper thickness, shape, and texture.

This rolling process also controls thickness, perforation, and texture on paper products, rubber, plastics, drywall, engineered wood, and other materials.



-Inductive probes help control material thickness by monitoring the gap between calendar rollers.

-Depending upon material characteristics and process requirements, the material might be pre-heated or process working fluids could be present such as steam and solvents.

-The process requires precise control of roller gap to control quality and prevent rollercrash damage and premature wear.

-Inductive position sensors can measure the roller gap accurately in the hostile processenvironment while the material is running.

-This enables precision machine setup, real time roller gap control, and SPC data on the setup and running gap for all materialproduced.



Thread DetectionInductive sensors are a perfect technology for detecting the presence/absence or thequality of threads in tapped holes.

Inductive sensors can detect the presence or quality of threads in a tapped hole.



A traditional probe design in which the electromagnetic field radiates from the end of the probe can function as a thread sensor, but sensitivity is increased with specially designed thread detection probes.

These probes have internal coils turned 90°so that the field emanates from the side of the probe’s cylindrical body. This gives deeper penetration of the field into the tapped surface.

These are used in automated production lines to check presence/absence of thread to check for a broken tap

Sophisticated application uses the probes to monitor thread quality and depth to indicate the need to replace a worn tap.

Thread detection probes are also monitored during insertion to detect a broken tap still present in the hole. The automation system stops the insertion if a broken tap is detected, thereby preventing damage to the probe.



Stamping/Molding Die ProtectionWhen parts are formed through a stamping or molding process,

Inductive sensors monitor injection molds for proper closure.



Using inductive sensors during the forming process, real-time measurements can be made on part dimensions, die-half position/movement, mold closure, and mold core movement.

Inductive sensors are the only choice because a stamping process will produce significant shock, often in an oily environment; the molding process can impart high temperatures (200°C) and significant pressures (20,000 psi Pound square inch) on the sensor.

In addition, these systems can be used to measure critical machine setup dimensionssuch as die closure position and the gap between the mold and core to ensure propersetup.



Position and Velocity Sensor



-An inductive sensor and a gear set is often used as position andvelocity sensor for applications which require low resolution but very rugged position and speed.

- as each gear tooth passes by the proximity sensor the output of the sensor changes state between ON and OFF.

- this is basically is equivalent to a single channel encoder.

- this type of sensor is appropriate for one directional applications.

- the speed is determined by the frequency of pulses from the proximity sensor.

Lecture - 4 - Inductance Transducers - German …eee.guc.edu.eg/Courses/Electronics/ELCT903...

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