amp proposal

7/30/2019 AMP Proposal

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DEPARTMENT OF ELECTRICALENGINEERING


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Course: INDUSTRIAL PROJECT IVSubject Code: IPR413T (6 MONTHS)

IPR413R (RE-REGISTRATION)

INDUSTRIAL PROJECT PROPOSAL:

ACTIVEMAGNETICBEARINGSby


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Diakiese Ndudi Aubin209024489

A Project Proposal Submitted as a Partial Requirement for theBachelors Degree in Technology (BTech): Electrical Engineering

Control

In the Department of Electrical EngineeringFaculty of Engineering and the Built Environment,

Tshwane University of Technology

Supervisor: Professor Qi Guo-YuanCo-Supervisor:Mr Martial Tatchum

Starting Date:01/08/2013

STUDENTS DETAILS

STUDENT NUMBER: 209024489

TEL. NUMBER: 0762945738

FAX. NUMBER: N/A

CELL. NUMBER: 0762945738

E-MAIL:[email protected]


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PROPOSED TITLE OF PROJECT

ACTIVE MAGNETIC BEARING

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SIGNATURE DATE


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DETAILS OF INTERNAL PROJECT LEADER(Lecturer from TUT, if available)

SURNAME AND INITIALS: Qi G

INSTITUTE:University of Nankai (NKU) ,China

QUALIFICATION:PhD(Control Theory And Control Engineering)

(Nankai,China)

TELEPHONE NUMBER: +27123825385

FAX NUMBER: +27123825003

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SIGNATURE DATE


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DETAILS OF MODERATOR

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INSTITUTE: ..........................................................................................................

QUALIFICATION: ...................................................................................................

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SIGNATURE DATE


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DETAILS OF REPRESENTATIVE

SURNAME AND INITIALS: .................................................................................

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SIGNATURE DATE


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Contents

Table of Figures

1. ABSTRACTThe importance of Active Magnetic Bearings (AMB) is becoming more

significant these days in many industrial applications. The AMB system

achieves levitation (free suspension of a moving part in air without any

physical contact), and therefore eliminates any friction and need for

lubrication of the machine. The structure consists of a ferromagnetic rotor

maintained in free space by controlled electromagnetic forces. By doing so,

rotation speeds are much higher; losses and maintenance cost in the system

are significantly reduced.

Although Passive Magnetic Bearings (PMB) is simpler than the AMB in

terms of components (no controller), two main drawbacks are noticed on

that type of Bearing:

Firstly, it is impossible to achieve a six degree of freedom support due

to the restriction of the Earnshaws theorem.


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Secondly, the characteristics of the PMB, compared to the AMB, cannot

be changed easily during the operation. Reason why this project will be

entirely focused on the AMB design and control.

To achieve this purpose, the Active Magnetic Bearings (AMB) consists of

an electromagnet assembly, a set of power amplifiers which supplycurrent to the electromagnets, a controller, and gap(position) sensors

which provide the feedback required to control the position of the rotorwithin the gap. When the process starts, the power amplifier suppliesequal bias current to two pairs of electromagnets on opposite sides of a

rotor. The currents supplied enable the electromagnets to exhibit theirmagnetic forces. As the rotor deviates from its centre position; the

controller will offset the bias currents in order to balance the

perturbations caused by the deviation.

FIGURE : GENERAL DESCRIPTION OF AN ACTIVE MAGNETIC BEARING SYSTEM

2. BACKGROUND ON THE PROJECTNowadays, the use of energy in industrial systems is taken with much care.

The modern trend is to reduce the loss as much as possible in order to

optimize the utilization [2], to reduce the cost and therefore increase the

performance of the systems. Traditional rotary systems were proved to be

very lossy due to the friction[1] caused by the mechanical contact between

the rotor(moving part ) and the electromagnets(stationary part).With the

use of an AMB, frictionless structures are built, allowing most of the energy

delivered to be used efficiently. A magnetic rotor is suspended by an

electromagnet. In order to get an active control of the rotor, its position is

measured by a position sensor. The position signal is then treated by a

controller, which gives a current set point. This signal is then amplified by

the power amplifier, in order to get the necessary actuator current. The

actuator current is the output from a power amplifier, a Pulse Width

Modulated (PWM) signal to be distributed to the actuating electromagnets

for a rectifying force on the position of the levitated ferromagnetic object in

free space.The concept of AMB brings a revolution in aerospace industry

(turbo machinery [7], vacuum technology, machining, and airplane

transportation [2]).

Advantages [9]

Increased energy production

As there is no friction, the operation of classical rotor bearings is

maximized.
http://en.wikipedia.org/wiki/Electromagnethttp://en.wikipedia.org/wiki/Controller_(control_theory)http://en.wikipedia.org/wiki/Electromagnethttp://en.wikipedia.org/wiki/Controller_(control_theory)


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Reduced losses experienced when transferring energy from one

system to another one.

High energy and fuel consumption

Saving of fuel resources

No pollution and Fire Risk due to lubrication

High circumferential speeds

easily adjustable bearing characteristics

low maintenance cost

Enhanced ability to work at very high temperatures

active vibration control and passing of critical speeds

balancing and unbalance compensation

3. PROBLEM STATEMENTThe magnetic bearings are restricted by the Earnshaws theorem which

clearly states that a collection of point charges cannot be maintained in a

stable stationary equilibrium configuration solely by the electrostatic

interaction of the charges. Instabilities and non linearities are present

because the force exhibited is a function changing with current and varyingair gap as the rotor moves. Therefore, a controller needs to be designed. For

contactless levitation, the AMB rotor requires position control in 3

independent directions(X, Y, Z).However, this control is complicated by the

mutual coupling of the 3 displacements [1], which implies design of MIMO

controller. MIMO controller implementation is very complex, and results to a

more expensive system. Instead, a decoupled control using the SISO

approach is much simpler to be improved and modified. The latter approach

is only applicable where the rotor has small deviations from its nominal

position.

Prerequisite: Modelling

Since the Magnetic Bearing is a multivariable and non linear system, we

need to linearize it and adapt it as a SISO system for easy control.

Objective


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As per request of the supervisor, the project aim is to build a digital

hardware platform able to control and stabilize ferromagnetic object in free

space, using two approaches:

The classical method and model free control

Model free control of affine chaotic systems

Sub problem 1

Design of the AMB controller using the classical method and model free

control.

Sub problem 2

Design of the same controller using the model free control of affine chaotic

systems.

Sub problem 3

Simulation using MATLAB.

Data acquisition and instrument control using LABVIEW

4. PRELIMINARY LITERATURE SURVEYMagnetic Bearings are used in various industrial applications such as

electrical power generation, petroleum refinement, machine tool operation

and natural gas handling, compressors, turbines, pumps. Magnetic bearings

are used in watt-hour meters for home power consumption. Other

applications include flywheel energy storage, artificial hearts.

Two magnetic Bearing technologies are actually used, namely:

4.1 Permanent Magnetic Bearing

This method makes use of a permanent magnet to achieve the levitation of

the rotor .No control is needed and therefore use of sensors and control

circuit is eliminated. The system is simple, but the greatest limitation is the

Earnshaws theory [3] which clearly states that group of point charges

cannot be maintained in a stable stationary equilibrium configuration solely

by the electrostatic interaction of the charges. This means that a rotor

frequently charged will be very difficult to be stabilized in free space. Thesuccess of this approach is restricted to applications where we have a small

load[4] to handle. Many researches undertaken around the world have

stated that the Permanent Magnetic bearing is physically unstable as they

are passive, and therefore a controlled active system would be more

adapted to provide better response in terms of stiffness and damping [4].

One successful experiment of this type of bearing was achieved by the NASA

Laboratory [4] for spacecraft applications in January 2008.


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4.2 Active Magnetic Bearings

While the previous approach is based on a permanent magnet, the active

magnetic bearing consists of electromagnets of which the magnetic force is

varied by the current drawn to it. Sensors, Control circuitry and power

amplifier are added to provide the necessary adjustment in case of a

deviation of the rotor from its equilibrium position. A PID controller will beused for this project. The controller will calculate the error value as the

difference between measured variable and the desired specification point to

minimize that error by adjusting the control inputs. Based on its three terms,

the PID controller can be evaluated in terms of time:(Proportional) is

implemented for the present error, I(Integral)for all past errors up to present

and D(Derivative) for predicting future errors on the systems.

A digitally controlled system makes the process much easier, especially when

it comes to modify the functionality. Analogue control systems were

restricted in that sense that any change in the system requires a hardware

change, while a digital one is software wise. A digital platform is very flexible

to adapt various types of controllers by simply changing the software.

Unfortunately, magnetic force(between the electromagnet and the rotor) is a

function changing with current and varying air gap as the rotor moves ,which

causes non linearities[5]. Non linearities are also due to the flattening of

the magnetization curve due to saturation of the core material[6], the

hysteresis of the core material depending on frequency and amplitude and

the limitations of the current and slope due to power amplifiers[6,8]. Asthe Active Magnetic bearings have high nonlinearity and instability, design of

the controller is of a great concern. Many approaches have been introduced

to control magnetic bearing system and therefore to compensate for

magnetic nonlinearities and maximize the performance of magnetic bearing

control system.

4.2.1Mathematical and Mechanical Approaches

The big challenge in that matter is to find the right force to be exerted on

the rotor .We need to estimate a simplified formula for the energy stored in

the air gap between the ferromagnetic rotor and the electromagnet core.

From there, we can derive a formula for the magnetic force.

Energy Stored in the gap

Core

Number of


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Rotor

FIGURE : ENERGY STORED IN GAP VOLUME BETWEEN THE ROTOR AND THE CORE

From electrical basics, we know that the power is determined as:

Where W is the work accomplished during a certain amount oftime t. The power where V is the voltage and I the current .Incase of an inductor, the voltage across an inductor is

L is the inductance, so its a constant. Since and we have two pole faces

present then [1]

is the permeability of air

is the spacing gap

A is the sectional area of the gap

Nis the number of turns

Partial differentiation of work over length

Keeping the current flow constant from the above formula, we can derive the

force to be exerted by electromagnets.

As the force is the derivative of the work over displacement, the magnitude

of the force will be:

AirGap


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The change in the force can be calculated as:

Lets take then we have

Where and are respectively the force/current and the force/displacement

ratios.

4.2.2 Procedures to be followed in achieving the project

Its not enough to derive a formula for the force that needs to be exerted by

the magnets in order to achieve the levitation. Many considerations are to be

taken with care, such as:

Transfer functions designs

After having the formulas for the force, the next step is to split the system in terms

of block diagrams, representing each one a specific task in the whole process. The

transfer functions to be designed are:

a. Current and gap Control loops

Because a constant control is to be done based on the gap and the current

measurements, feedback loops are required. One feedback loop will be responsible

for the air gap distance measurement and the other one consists of providing theinput to the power amplifier to produce the necessary current for the adjustment.

The air gap feedback loop will be fed to the current feedback loop.

idesired

l

Controller

2Controller

1imeasure

Controller

2Set gap

Driving

Circuit

DrivingADC ADC

Bias current

Kl Ki

Gap

Sensor

Force to be applied

Dynamics

system

response


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FIGURE : GAP AND CURRENT FEEDBACK LOOPS

[1, 10]

The force to be applied follows the rules of a simple harmonic motion (SHM) and

Newtons second law.

b. Axial Coil transfer function

This is to find the transfer function between the current to be measured and the

FIGURE : ELECTROMAGNET REPRESENTED AS A RL CIRCUIT

Voltage across the electromagnet coil, based on the Kirchhoffs voltage law,

assuming a resistor in series with an inductor.

[1]

b. Controller design

The controller design is of a great importance in order to meet the specification of

the transient response, particularly in terms of settling time. For this project, we

will decide whether a PID or a PD controller will be used. Because the system is

limited to operate at certain frequencies, there is an operating range. A controller

will also be used to attenuate frequencies beyond the stable range.

d. Sensors transfer function

Transfer function is required to scale the sensor outputs to the exact value they

represent in real life.

Simulation of the entire program

After having all the transfer functions in place, the simulation will be follow to check

whether the combined system (all the transfer functions together) responds as

expected. From this point, if the performance is after review, the simulated system

will then be implemented.

5. PROJECT SCOPE , PLANNING & COST

5.1 Project Scope

TASKS START Duration(days)

Model & transfer functions design 1/8/2013 14Controller(compensator) design 15/08/201

314


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Stability Analysis(Root locus, frequencyResponse)

29/08/2013

10

Simulation 09/09/2013

22

Prototype Implementation 1/10/2013 14Review 15/10/201

325

Final Report Compilation 9/11/2013 7review and completion of the whole project 16/11/2013

10

5.2 Gantt Chart

5.3 Project Cost

Axial Position Sensor

Type: SKF CMSS665 eddy current position sensor

Number of sensors: 5

Unit price: 3700

Cost: 18500

Power Amplifiers

Number of channels: 10

Unit Price: 4000

Number: 1

Cost: 4000

PCI Card

Type: NI PCI-6013, Low cost 200 kS/s, 16-Bit, 16-AnalogInput Multifunction DAQ

Unit price: 5910

Number: 1

Cost: 5910

6. REFERENCES


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[1]Steyn N. 2006. Digital Control System for Active Magnetic

Bearings.Tshwane University of Technology, South Africa, August

2006, page 15

[2] L. Burdet2006. Active Magnetic Bearing Design and

Characterization for High Temperature Applications.Ecole

Polytechnique Federale de la Lausanne.Switzerland,September2006,page 7

[3] A.K. PilatActive Magnetic Suspension and Bearing.AGH

University of Science and Technology ,Poland, page 2

[4] W.Morales and R. Fusaro, A. kasak. Permanent Magnetic

Bearing for Spacecraft Applications.NASA Glenn research

Center ,Cleveland,Ohio pages 5-18

[5]Jawaid I. Inayat-Hussain.Nonlinear Dynamics of aFlexible Rotorin Active Magnetic Bearings ,2007 international Symposium on

Nonlinear Theory and its Applications,Monash

University,Malaysia,pages 1-2

[6] L.Pust and O.Szollos, Interaction of two strong nonlinearities.

[7]Jianhui Zhao,Mary E.F. Kasarda,Dewey Spangler,Robert

Prins,Daniel J.Inman.Active Magnetic Bearing System

Identification using current-position perturbation

[8] L.Pust.Weak And Strong Nonlinearities In Magnetic

Bearings,Insitute of Thermomechanics ASCRDolejskova 5 18200

Prague 8,Czech Republic,February 2004

[9]Rene Larsonneur.Design and Control of Active Magnetic

Bearing Systems for High Speed Rotation.Swiss federal Institute of

Technology,Zurich,Switzerland

[10] Chip Rinaldi Sabirin, Andreas Binder, Dumitru Daniel Popa,

Aurelian Crciunescu. Modeling and Digital Control of an Active

Magnetic Bearing System

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