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  PHYSIC TERM PAPER

ON

 THE TOPIC

ELECTRICAL AND MAGNETIC

SENSORS

  AND THEIR UTILITY  

Name: Obomanu Alex JReg: 11013065Roll no: A40

Session: A1101Course: B. Tech (Hons) - CSEProgram code:1252

Review:

Small Magnetic Sensors for Space

Applications: 

Abstract:  Small magnetic sensors are widely used

integrated in vehicles, mobile phones, medical devices,

etc for navigation, speed, position and angular sensing.

 These magnetic sensors are potential candidates for space

sector applications in which mass, volume and power

savings are important issues. This work covers the

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magnetic technologies available in the market place and

the steps towards their implementation in space

applications, the Actual trend of miniaturization the front-

end technologies and the convergence of the mature and

miniaturized magnetic sensor to the space sector through

the small satellite concept.

Keywords: Miniaturized magnetic sensors, space

magnetometers, AMR-Anisotropic Magneto Resistance,

magnetic COTS-Components Off-The-Shelf 

Introduction :  The magnetic sensors in most common use today are

variable reluctance coils and Hall Effect devices. The drawback of 

variable reluctance sensors is that they generate a signal proportional to

the magnetic field’s rate of change. Signal strength therefore decreases

with decreasing speed, and below a certain flux change rate, the signaldisappears into the noise. The excess output voltage of the coil at high-

frequency magnetic fields also causes problems for circuit designers. Hall

Effect devices generate a very small raw signal because of low field

sensitivities (0.5–5.0 mV/100 Oe applied field), and the device

 performance is strongly temperature dependent. These features mandate

signal conditioning, and enquire that a certain minimum field be available

for device operation.

That is based on two phenomena: The magneto resistive effect

and the piezoelectric effect. These sensors consume no electrical power,

and easily produce a raw electrical signal with a magnetic field sensitivity

>10 mV/Oe. They combine the advantages of Hall sensors’ miniature size

and the passive nature of variable reluctance coil devices.

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Potential magnetic sensors for spaceapplications:1. As happens in almost every technological niche on

ground

2. Magnetic sensors are useful for many applications in

the space sector

3. Though the most representative application is the in-

orbit Measurement of the

Magnetic field, there are some others as magnetic

encoders

4. Angular and Position sensors

5. And magnetometers or gradiometers for planetary

magnetometer. Since magnetic

Applications are so varied; the choice of magnetic

sensor can be a difficult task.

A graph the panorama of the different magnetic

sensors: the most representative technologies used

for magnetic sensing are represented as a function of their magnetic

characteristics: minimum detectable field and dynamical range. The

applications have been depicted in Ben diagrams intersecting the bars of 

the technologies which can be used for the particular application. This

work is focused on potential COTS and small sensors for space

measurements of the magnetic field or magnetic gradient. The magnetic

field in-orbit can be measured for geomagnetic measurement purposes, or 

also inversely, to determine the relative orientation of a spacecraft in the

geomagnetic field. This is the purpose of magnetic sensors in ACS – 

Attitude Control Systems. In some missions measurement of the gradient

of the field is also needed.

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Magnetic sensors for space missions. The

implementation of magnetic COTS in

spacecrafts: As it has been introduced, the history of 

the magnetic sensors for space starts with the Soviet

Sputnik-3. Sputnik-3 was a 1.3 ton satellite devoted to

researching the upper atmosphere and near space. The

onboard instrumentation contained the first vector

magnetometer: a three axis fluxgate magnetometer,

which however did not succeed in the determination of the

direction of the Earth magnetic field due to the

uncertainty in the attitude of the spacecraft. Trial fluxgate

magnetometers have been widely used for monitoring the

magnetic fields of the Earth and Moon (Luna 1, Luna 2,

Pioneer Venus, Mariner 2, VENERA 1, Explorer 12, Explorer

14, and Explorer 15). Explorer 33 already foresaw a boom

for the magnetometer to avoid in part the Contribution of 

the spacecraft to the total magnetic field in the orbit of the

Moon.

Adaptation of COTS for space applications: As

described in the section above, both the functionality and

integration of magnetic sensing technologies are

continuously evolving. Moreover, as electronic devices and

sensors are becoming

Essential parts of our daily lives and as users test them

exhaustively the technology undergoes a continuous on-

ground qualification process by which reliability is

increased tremendously

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These magnetometers are required to measure the vector and scalar 

magnetic fields of the Earth with resolutions typical of the degrees 40 to

60 of the harmonics expansion for the field, the variations due to theionospheres interaction and other perturbations. The sensor technology

  par Excellence for measuring the Earth magnetic field vector is the

fluxgate because it is the best trade-off between resolution, stability and

  power consumption, mass and volume, and also some scalar 

magnetometers have been used to measure the intensity of the field or to

complement the measurement obtained with fluxgates. Fluxgates are based on the change of magnetic reluctance of a ferromagnetic core when

it is driven by an ac saturating field in the presence of a magnetic field.

The driving field is provided with the so called primary coil and the

changes in the reluctance are measured by means of the secondary coil.

These sensors are able to measure magnetic fields ranging from the MT

to the tens of PT and in the range of frequencies from dc to the order of 

the operation frequency (tens of kHz).

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Magnetic Fields in Industry and Medicine: There are

many places in industry and in medicine in which magnetic fields

smaller than the earth's field are of interest. The source of these

fields can be magnetized objects, electrical currents, or the Earth's

magnetic field. The low-field aspect of these applications can be

due to the distance to a magnetic object or the size of the object

itself.

All magnetic sources produce a magnetic dipole field if the

observer is at a distance from the source. Dipole fields decrease

as the inverse cube of the distance from the source. The fields are

also proportional to the volume of the source and to the maximum

magnetization at the source. A magnetized cylinder whose

diameter and length are one-half those of a larger cylinder at any

distance will have a magnetic field one-eighth as strong as the field

from the larger cylinder. In addition, doubling the distance from a

magnetized cylinder will decrease the field to one-eighth the field

at the original position. Distance and miniaturization lead to low

fields.

Objects made of soft magnetic materials are easily magnetized by

relatively small magnetic fields. These objects can be as simple as

the small iron pipes used as surveying markers or as complex as

entire automobiles and trucks. In one application, the objective is

to locate a buried object from a distance; in the other, it's to detect

the presence or passage of a vehicle close by. In both cases, the

smaller the field detected, the more useful the sensor. Also, the

field detected must be separated from the Earth's magnetic field,

which may be stronger than the field of interest.

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Various methods are used to subtract the Earth's magnetic field. Because

this field is constant, it can be subtracted in applications in which the

sensor is stationary. In applications for which the field of interest is time

varying, the Earth's field can be subtracted or filtered out.

Types of sensors:

• Variable reluctance (V/R) sensors with zero cross detection

• Single-element Hall effect sensors with zero cross detection

• Zero speed, differential Hall effect sensors with offset level

detection

• Differential Hall effect sensors with dynamic peak detection.

Variable Reluctance:

A V/R sensor or "MAG pickup" is basically a small generator that

produces an analogue voltage proportional to the size and speed

of a ferromagnetic target passing in front of the sensor. The output

voltage has an inherent characteristic that is ideal for certain types

of timing applications. The V/R sensor consists of a coil, a pole

piece, and a magnet; its equivalent electrical schematic

The variable reluctance sensor has an equivalent electrical circuit

consisting of a voltage generator example, the coil inductance

LCOIL, and the resistance of the coil wire RCOIL. The output is an AC

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voltage with amplitude and frequency proportional to target speed.

In this application, the AC voltage is converted to a digital signal

using a zero cross detection circuit and passed to the controller's

microprocessor.

The open circuit voltage, example, is proportional to the number of 

turns of the coil and the rate of change of the magnetic field:

  Example = n • d Φ / dt

Where:

  n = number of turns in the coil

d Φ = rate of change in magnetic flux

dt = rate of change in time

The voltage waveforms generated when a target passes in front of 

the sensor. The first tooth in the target profile has a width

approximately that of the diameter of the sensor pole

Piece; the second tooth is much wider. The magnetic flux

increases as the target passes in front of the sensor and

decreases as the tooth passes by. Because the output voltage

example is proportional to the flux differential with respect to the

change in time, it will first go positive as dΦ increases and then

rapidly swing negative as the dΦ slope changes from positive to

negative. The output voltage will return to zero when dΦ returns

to zero.

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The changing field f caused by the moving target generates a

voltage example in the sensor coil that is proportional to the

magnetic field's rate of change. For a small tooth, the digital output

changes state when example crosses through zero and creates a

precise timing signal t1 coincident with the CENTERLINE of the

sensor pole piece; for a larger tooth, however, the zero cross point

can occur at any point between t2 and t3.

Speed Sensors: As compared to a variable reluctance coil sensor, the

PSSM sensor detects near-zero speed (0.1 Hz), is smaller, and costs about

the same. Com pared to Hall and magneto resistive devices, the PSSM

sensor has better field sensitivity, better temperature stability, and costs

less. Figure 2 shows the stability of a sensor output at a 40ºC–160ºC

temperature range without any compensation circuit. The sensor produces

a sine wave electrical signal in tens of MILLIVOLTS when a magnet

 periodically passes by. The detection distance can be >1 in. when NdFeB

magnet of 3 mm DIA. by 5 mm long is placed in the object of interest.

The target’s speed is measured by the frequency of the output signal,

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which is an inverse of a time period between the passing magnets. A

wheel mounted with equally spaced magnets can thus measure rotational

velocity. With a built-in electrical circuit, the sensor outputs a standard

 pulsed square wave signal. Such a sensor with a bias magnet mounted

next to it can detect the rotational speed of a ferromagnetic gear at a

standoff distance of a few MILLIMETERS depending on the size (pitch)

of the gear teeth.

Flow Sensors High sensitivity and low power consumption are the two

most important criteria for flow measurement applications. Variablereluctance coil sensors are often used because they satisfy these criteria.

The PSSM sensor is well suited for this application because it requires no

 power source and offers better sensitivity in a smaller size than other 

magnetic sensors. Furthermore, it works at low frequency for better 

resolution in low flow measurements. An optional microprocessor-based

flow meter converts directly from flow rotor speed to flow rate.

Figure 3. A calibration curve of rotor 

speed can be seen as a function of 

liquid flow in a sensor with 1/3 in.

 pipe and a four-blade rotor.

Figure 4. A PSSM sensor placed next

to an electric wire produces a root-

mean-square signal output measured

as a function of an AC electric current

at 50 Hz.

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Electrical Current Sensors:  A sensor with high linearity over a

large magnetic field range is ideal for electrical current

measurement. Hall and magneto resistive sensors, because of 

their high linearity, are popular for this application. However,

their characteristics of zero offset voltage and temperature-

dependent output make electrical circuit design a complicated

matter. The PSSM sensor has been demonstrated in a simple

design and small size for electrical current sensors, current

switches, and relays. The device does not have zero offset voltage

and requires no temperature compensation for general use in

electrical current sensors. It can also be designed to maintain

linearity in magnetic fields >1000 O e for measurement of large

currents. 

Piezoelectric sensor applications:

A piezoelectric sensor is a device that uses the piezoelectric

effect to measure pressure, acceleration, strain or force by

converting them to an electrical signal. Piezoelectric sensors have

proven to be versatile tools for the measurement of various

processes. They are used for quality assurance, processcontrol and for research and development in many different

industries.

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Principle of operation:

Depending on how a piezoelectric material is cut, three main

modes of operation can be distinguished: transverse,

longitudinal, and shear .

Transverse effect:

A force is applied along a neutral axis (y) and the charges are

generated along the (x) direction, perpendicular to the line of force.

The amount of charge depends on the geometrical dimensions of 

the respective piezoelectric element. When dimensions a, b,

c apply, C(  x) = d(  x y) F(  y) b / a,

Where (a) is the dimension in line with the neutral axis (b) is in line

with the charge generating axis and d is the corresponding

piezoelectric coefficient.

Longitudinal effect

The amount of charge produced is strictly proportional to the

applied force and is independent of size and shape of the

piezoelectric element. Using several elements that are

mechanically in series and electrically in parallel is the only

way to increase the charge output. The resulting charge is:

C(  x) = d (  xx) F(  x) n,

Where d (  xx) is the piezoelectric coefficient for a charge in x-

direction released by forces applied along x-direction

(in PC/N). F(  x) is the applied Force in x-direction [N] and n

corresponds to the number of stacked elements.

Shear effect

Again, the charges produced are strictly proportional to the

applied forces and are independent of the element’s size and

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shape. For n elements mechanically in series and electrically

in parallel the charge is

C(  x) = 2d  xx F  xn.

In contrast to the longitudinal and shear effects, thetransverse effect opens the possibility to fine-tunesensitivity on the force applied and the element dimension.

Sensor design: Metal disks with piezo material, used inbuzzers or as contact microphones.

Based on piezoelectric technology various physical quantities canbe measured; the most common are pressure and acceleration.

For pressure sensors, a thin membrane and a massive base is

used, ensuring that an applied pressure specifically loads the

elements in one direction. For accelerometers, a seismic mass is

attached to the crystal elements. When the accelerometer 

experiences a motion, the invariant seismic mass loads the

elements according to Newton’s second law of motion F = ma.

The main difference in the working principle between these two

cases is the way forces are applied to the sensing elements. In a

pressure sensor a thin membrane is used to transfer the force to

the elements, while in accelerometers the forces are applied by an

attached seismic mass.

Sensors often tend to be sensitive to more than one physical

quantity. Pressure sensors show false signal when they are

exposed to vibrations. Sophisticated pressure sensors therefore

use acceleration compensation elements in addition to the

pressure sensing elements. By carefully matching those elements,

the acceleration signal (released from the compensation element)

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is subtracted from the combined signal of pressure and

acceleration to derive the true pressure information.

Vibration sensors can also be used to harvest otherwise wasted

energy from mechanical vibrations. This is accomplished by using

piezoelectric materials to convert mechanical strain into usable

electrical energy.

Electrical properties:

Schematic symbol and electronic model of a piezoelectric

sensor 

A piezoelectric transducer has very high DC output impedance and

can be modelled as a proportional voltage source and filter 

network. The voltage V at the source is directly proportional to the

applied force, pressure, or strain. The output signal is then related

to this mechanical force as if it had passed through the equivalent

circuit.

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Frequency response of a piezoelectric sensor; output voltage vs.

applied force

A detailed model includes the effects of the sensor's mechanical

construction and other non-fidelities. The inductanceL

m is due tothe seismic mass and inertia of the sensor itself. C a is inversely

proportional to the mechanical elasticity of the

sensor. C 0 represents the static capacitance of the transducer,

resulting from an inertial mass of infinite size. R 1 is the

insulation leakage resistance of the transducer element. If the

sensor is connected to a load resistance, this also acts in parallel

with the insulation resistance, both increasing the high-pass cut-off 

frequency.

Sensing materials:

Two main groups of materials are used for piezoelectric sensors:

piezoelectric ceramics and single crystal materials. The ceramic

materials (such as PZT ceramic) have a piezoelectric constant /sensitivity that is roughly two orders of magnitude higher than

those of single crystal materials and can be produced by

inexpensive sintering processes. The piezoeffect in piezoceramics

is "trained", so unfortunately their high sensitivity degrades over 

time. The degradation is highly correlated with temperature. The

less sensitive crystal materials (gallium phosphate, quartz, andtourmaline) have a much higher – when carefully handled, almost

infinite – long term stability.

 Applications:

Piezoelectric disk used as a guitar pickup

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Piezoelectric sensors have proven to be versatile tools for the

measurement of various processes. They are used for assurance,

process and for research and development in many different

industries. Although the piezoelectric effect was discovered

by Curie in 1880, it was only in the 1950s that the piezoelectric

effect started to be used for industrial sensing applications. Since

then, this measuring principle has been increasingly used and can

be regarded as a mature technology with an outstanding inherent

reliability. It has been successfully used in various applications,

such as in medical, aerospace, nuclear instrumentation, and as a

pressure sensor in the touch pads of mobile phones. In

the automotive industry, piezoelectric elements are used to

monitor combustion when developing internal combustion engines.

The sensors are either directly mounted into additional holes into

the cylinder head or the spark/glow plug is equipped with a built in

miniature piezoelectric sensor.

Summary:

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PSSM sensor technology shows great promise as a

replacement for other magnetic sensor technology. It will 

certainly be explored and developed for many 

applications such as information storage, magnetic 

recording read heads, magnetic imaging for non

destructive inspection and biomedical procedures,

detection of magnetic anomalies, surveillance, and 

mineral prospecting. 

Acknowledgements:

I wish to acknowledge the Technology for the confirmation of the

study of data related to the application of magnetic sensors in space

missions

Reference and Note:

1. Lenz J.; Edelstein, A.S. Magnetic Sensors and their applications.

 IEEE Sens. J.

2. Acuña, M.H. Space-based magnetometers. Rev. Sci. Instrum.

2002, 73, 3717-3736.

3. Keller, H.U.; OSIRIS Team. Osiris: The scientific

camera system onboard Rosetta.  Space Sci.

4. Watzin, J. A GSFC perspective on the execution of 

faster, better, cheaper. In  IEEE Proc

5. "Piezoelectric sensors". Piezocryst website