09-04-2011, 10:07 AM
Presented By
B.V.S.Phanindra
[attachment=11896]
Magnetic Bearings
INTRODUCTION:
Bearings
A bearing is a device to allow constrained relative motion between two or more parts, typically rotation or linear movement. Bearings may be classified broadly according to the motions they allow and according to their principle of operation as well as by the directions of applied loads they can handle.
Magnet
A magnet is a material or object that produces a magnetic field. This magnetic field is invisible but is responsible for the most notable property of a magnet: a force that pulls on other ferromagnetic materials like iron and attracts or repels other magnets.
Permanent Magnet
A permanent magnet is an object made from a material that is magnetized and creates its own persistent magnetic field.
Magnetic Bearing
A magnetic bearing is a bearing which supports a load using magnetic levitation. Magnetic bearings support moving machinery without physical contact, for example, they can levitate a rotating shaft and permit relative motion without friction or wear.
Principal of operation:
Magnetic levitation
Magnetic levitation, maglev, or magnetic suspension is a method by which an object is suspended with no support other than magnetic fields. Magnetic pressure is used to counteract the effects of the gravitational and any other accelerations.
DESCRIPTION:
It is difficult to build a magnetic bearing using permanent magnets due to the limitations described by Earnshaw’s theorem and techniques using diamagnetic materials are relatively undeveloped. As a result, most magnetic bearings require continuous power input and an active control system to hold the load stable. Many bearings can use permanent magnets to carry the static load, and then only use power when the levitated object deviates from its optimum position. Magnetic bearings also typically require some kind of back-up bearing in case of power or control system failure and during initial start-up conditions.
Two sorts of instabilities are very typically present with magnetic bearings. Firstly attractive magnets give an unstable static force, decreasing with greater distance, and increasing at close distances. Secondly since magnetism is a conservative force, in and of itself it gives little if any damping, and oscillations may cause loss of successful suspension if any driving forces are present, which they very typically are.
With the use of an induction-based levitation system present in maglev technologies such as the Inductrack system, magnetic bearings could do away with complex control systems by using Halbach Arrays and simple closed loop coils. These systems gain in simplicity, but are less advantageous when it comes to eddy current losses. For rotating systems it is possible to use homopolar magnet designs instead of multipole halbach structures, which reduces losses considerably. An example of this - that has solved the Earnshaws theorem - is the homopolar electro dynamic bearing invented by Dr Torbjorn Lembke.
Magnetic bearing system incorporates 3 technologies:
1) Bearing & sensors
2) The control system
3) Control algorithms
A control device for controlling displacement of a magnetically supported moving member according to a command, comprising:
1. A feedback circuit for detecting the displacement of the moving member to control the moving member to ensure stability and robustness of the magnetic support in response to the detected displacement, the feedback circuit comprising a closed loop composed of a displacement detector receptive of an output from a displacement sensor, an integral compensator coupled to the displacement sensor, a phase advancing compensator coupled to the integral compensator, and an electrical power amplifier coupled to the phase advancing compensator for effecting the magnetic support; and a feed forward circuit having an input terminal receptive of a command and an output terminal connected to the feedback circuit, and cooperative with the feedback circuit without disturbing the stability and robustness of the magnetic support for controlling the displacement of the moving member according to the command.
2. A control device according to claim 1; wherein the feed forward circuit has an output terminal connected to an input port of the electric power amplifier.
3. A control device according to claim 1; wherein the feed forward circuit comprises a low-pass filter connected to the input terminal, a high-pass filter connected to the input terminal, a compensative filter for effecting compensation of an output of the low-pass filter, a gain regulator for regulating a gain of an output of the high-pass filter, and a differential amplifier for differentially processing the outputs from the low-pass and high-pass filters to each other.
4. A control device according to claim 3; wherein the low-pass filter has a transfer function including a polynomial denominator preset according to desired response characteristic to the command, and the high-pass filter has another transfer function including another polynomial denominator having coefficients identical to those of the polynomial denominator of the transfer function of the low-pass filter.
5. A control system for controlling displacement of a movable member supported by magnetic support means, comprising: feedback control means for stabilizing the movable member in response to a displacement thereof caused by an undesired disturbance, the feedback control means comprising means for sensing a displacement of the movable member and for producing an output signal representative thereof, and compensating means receptive of the output signal for applying a compensating signal to the magnetic support means to stabilize the movable member; and feed forward control means responsive to an input command for controlling the displacement of the movable member without disturbing the stability thereof, the feed forward control means comprising input means receptive of the input command for producing a displacement signal corresponding thereto, the input means comprising a low-pass filter and a high-pass filter each receptive of the input command, and output means for combining the displacement signal with at least one of the output signal and the compensating signal of the feedback control means for receipt by the compensating means and the magnetic support means, respectively, the output means comprising means for combining an output signal from the low-pass filter with the output signal form the sensing means and for combining the difference between an output signal from the high-pass filter and the output signal from the low-pass filter with the compensating signal.
6. The control system according to claim 5, wherein the feedback control means comprises a closed loop including the sensing means and the compensating means.
7. The control system according to claim 5; wherein the input means comprises a compensative filter for effecting compensation of an output of the low-pass filter, a gain regulator for regulating a gain of an output of the high-pass filter, and a differential amplifier for differentially processing the outputs from the low-pass and high-pass filter with respect to each other.
8. The control system according to claim 7; wherein the low-pass filter has a transfer function including a polynomial denominator preset according to a desired response characteristic to the input command, and the high-pass filter has another transfer function including another polynomial denominator having coefficients identical to those of the polynomial denominator of the transfer function of the low-pass filter
B.V.S.Phanindra
[attachment=11896]
Magnetic Bearings
INTRODUCTION:
Bearings
A bearing is a device to allow constrained relative motion between two or more parts, typically rotation or linear movement. Bearings may be classified broadly according to the motions they allow and according to their principle of operation as well as by the directions of applied loads they can handle.
Magnet
A magnet is a material or object that produces a magnetic field. This magnetic field is invisible but is responsible for the most notable property of a magnet: a force that pulls on other ferromagnetic materials like iron and attracts or repels other magnets.
Permanent Magnet
A permanent magnet is an object made from a material that is magnetized and creates its own persistent magnetic field.
Magnetic Bearing
A magnetic bearing is a bearing which supports a load using magnetic levitation. Magnetic bearings support moving machinery without physical contact, for example, they can levitate a rotating shaft and permit relative motion without friction or wear.
Principal of operation:
Magnetic levitation
Magnetic levitation, maglev, or magnetic suspension is a method by which an object is suspended with no support other than magnetic fields. Magnetic pressure is used to counteract the effects of the gravitational and any other accelerations.
DESCRIPTION:
It is difficult to build a magnetic bearing using permanent magnets due to the limitations described by Earnshaw’s theorem and techniques using diamagnetic materials are relatively undeveloped. As a result, most magnetic bearings require continuous power input and an active control system to hold the load stable. Many bearings can use permanent magnets to carry the static load, and then only use power when the levitated object deviates from its optimum position. Magnetic bearings also typically require some kind of back-up bearing in case of power or control system failure and during initial start-up conditions.
Two sorts of instabilities are very typically present with magnetic bearings. Firstly attractive magnets give an unstable static force, decreasing with greater distance, and increasing at close distances. Secondly since magnetism is a conservative force, in and of itself it gives little if any damping, and oscillations may cause loss of successful suspension if any driving forces are present, which they very typically are.
With the use of an induction-based levitation system present in maglev technologies such as the Inductrack system, magnetic bearings could do away with complex control systems by using Halbach Arrays and simple closed loop coils. These systems gain in simplicity, but are less advantageous when it comes to eddy current losses. For rotating systems it is possible to use homopolar magnet designs instead of multipole halbach structures, which reduces losses considerably. An example of this - that has solved the Earnshaws theorem - is the homopolar electro dynamic bearing invented by Dr Torbjorn Lembke.
Magnetic bearing system incorporates 3 technologies:
1) Bearing & sensors
2) The control system
3) Control algorithms
A control device for controlling displacement of a magnetically supported moving member according to a command, comprising:
1. A feedback circuit for detecting the displacement of the moving member to control the moving member to ensure stability and robustness of the magnetic support in response to the detected displacement, the feedback circuit comprising a closed loop composed of a displacement detector receptive of an output from a displacement sensor, an integral compensator coupled to the displacement sensor, a phase advancing compensator coupled to the integral compensator, and an electrical power amplifier coupled to the phase advancing compensator for effecting the magnetic support; and a feed forward circuit having an input terminal receptive of a command and an output terminal connected to the feedback circuit, and cooperative with the feedback circuit without disturbing the stability and robustness of the magnetic support for controlling the displacement of the moving member according to the command.
2. A control device according to claim 1; wherein the feed forward circuit has an output terminal connected to an input port of the electric power amplifier.
3. A control device according to claim 1; wherein the feed forward circuit comprises a low-pass filter connected to the input terminal, a high-pass filter connected to the input terminal, a compensative filter for effecting compensation of an output of the low-pass filter, a gain regulator for regulating a gain of an output of the high-pass filter, and a differential amplifier for differentially processing the outputs from the low-pass and high-pass filters to each other.
4. A control device according to claim 3; wherein the low-pass filter has a transfer function including a polynomial denominator preset according to desired response characteristic to the command, and the high-pass filter has another transfer function including another polynomial denominator having coefficients identical to those of the polynomial denominator of the transfer function of the low-pass filter.
5. A control system for controlling displacement of a movable member supported by magnetic support means, comprising: feedback control means for stabilizing the movable member in response to a displacement thereof caused by an undesired disturbance, the feedback control means comprising means for sensing a displacement of the movable member and for producing an output signal representative thereof, and compensating means receptive of the output signal for applying a compensating signal to the magnetic support means to stabilize the movable member; and feed forward control means responsive to an input command for controlling the displacement of the movable member without disturbing the stability thereof, the feed forward control means comprising input means receptive of the input command for producing a displacement signal corresponding thereto, the input means comprising a low-pass filter and a high-pass filter each receptive of the input command, and output means for combining the displacement signal with at least one of the output signal and the compensating signal of the feedback control means for receipt by the compensating means and the magnetic support means, respectively, the output means comprising means for combining an output signal from the low-pass filter with the output signal form the sensing means and for combining the difference between an output signal from the high-pass filter and the output signal from the low-pass filter with the compensating signal.
6. The control system according to claim 5, wherein the feedback control means comprises a closed loop including the sensing means and the compensating means.
7. The control system according to claim 5; wherein the input means comprises a compensative filter for effecting compensation of an output of the low-pass filter, a gain regulator for regulating a gain of an output of the high-pass filter, and a differential amplifier for differentially processing the outputs from the low-pass and high-pass filter with respect to each other.
8. The control system according to claim 7; wherein the low-pass filter has a transfer function including a polynomial denominator preset according to a desired response characteristic to the input command, and the high-pass filter has another transfer function including another polynomial denominator having coefficients identical to those of the polynomial denominator of the transfer function of the low-pass filter
