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Title:
CONTROL SYSTEM AND METHOD FOR NEGATIVE DAMPING COMPENSATION IN MAGNETIC LEVITATION
Document Type and Number:
WIPO Patent Application WO/2008/075269
Kind Code:
A2
Abstract:
A magnetic levitation control system (100) includes a sensor (106) configured to measure a gap between an electro magnet and a stage and generate a gap measurement signal. A gap filter (112) is configured to receive the gap measurement signal and provide a phase lead signal which estimates and accounts for delays between the gap measurement signal and a compensation action. An estimation block (114) is configured to receive the phase lead signal and provide the compensation action in accordance with the phase lead signal such that negative stiffness and negative damping effects are compensated for in the control system.

Inventors:
BAKKER, Arjan F. (Groenewoudseweg 1, BA Eindhoven, NL-5621, NL)
Application Number:
IB2007/055085
Publication Date:
June 26, 2008
Filing Date:
December 13, 2007
Export Citation:
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Assignee:
KONINKLIJKE PHILIPS ELECTRONICS, N.V. (Groenewoudseweg 1, BA Eindhoven, NL-5621, NL)
U.S. PHILIPS CORPORATION (1251 Avenue of the Americas, New York, NY, 10020, US)
BAKKER, Arjan F. (Groenewoudseweg 1, BA Eindhoven, NL-5621, NL)
International Classes:
G05D3/12; G05B5/01; G05B6/02; F16C39/06
Attorney, Agent or Firm:
DAMEN, Daniel, M. (Philips Intellectual Property & Standards, High Tech Campus 44P.O. Box 220, AE Eindhoven, NL-5600, NL)
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Claims:

CLAIMS;

1. A magnetic levitation control system (100), comprising: a sensor (106) configured to measure a gap between an electro magnet and a stage and generate a gap measurement signal; a gap filter (112) configured to receive the gap measurement signal and provide a phase lead signal (113) which estimates and accounts for delays between the gap measurement signal and a compensation action; and an estimation block (114) configured to receive the phase lead signal and provide the compensation action in accordance with the phase lead signal such that negative stiffness and negative damping effects are compensated for in the control system.

2. The control system as recited in claim 1, further comprising a gap reconstruction module (110) configured to provide calibration between a measurement time and a filter time of the gap filter.

3. The control system as recited in claim 1, wherein the compensation action includes an output signal (Icaicuiated) modified in accordance with the phase lead signal.

4. The control system as recited in claim 3, further comprising an amplifier (118) configured to receive the output signal.

5. The control system as recited in claim 4, wherein the amplifier (118) reduces noise introduced by the gap filter (112).

6. The control system as recited in claim 1, wherein the gap filter (112) extrapolates a delay from previous gap measurement delays such that the phase lead signal is based on the extrapolated delay.

7. A magnetic levitation system (110), comprising: an electromagnetic core (102) configured to generate a magnetic field to levitate a stage (104) and maintain a gap between the stage and the electromagnetic core in

accordance with a realized current; a sensor (106) configured to measure the gap and generate a gap measurement signal; a gap filter (112) configured to receive the gap measurement signal and adjust the gap measurement signal to provide a phase lead signal (113) in accordance with past measurements to account for delays between the gap measurement signal and a compensation action; and an estimation block (114) configured to receive the phase lead signal and output a calculated current (Icaicuiated) which is amplified to provide the realized current such that negative stiffness and negative damping are compensated for in the control system.

8. The system as recited in claim 7, further comprising a gap reconstruction module (110) configured to provide calibration between a measurement time and a filter time of the gap filter.

9. The system as recited in claim 7, wherein the calculated current (Icaicuiated) is modified in accordance with the phase lead signal (113).

10. The system as recited in claim 7, wherein the amplifier (118) reduces noise introduced by the gap filter (112).

11. The system as recited in claim 7, wherein the gap filter (112) extrapolates a delay from previous gap measurement delays such that the phase lead signal is based on the extrapolated delay.

12. The system as recited in claim 7, wherein the electromagnetic core (102) and the stage (104) are employed in a vehicular system.

13. The system as recited in claim 7, wherein the electromagnetic core (102) and the stage (104) are employed in a semiconductor processing device.

14. The system as recited in claim 7, wherein the electromagnetic core (102)

and the stage (104) are employed in a contactless bearing system.

15. The system as recited in claim 7, wherein the electromagnetic core (102) and the stage (104) are employed in a medical imaging system.

16. A method for controlling a gap in a magnetic levitation system, comprising: generating (302) a magnetic field to levitate a stage and maintain a gap between the stage and an electromagnetic core in accordance with a realized current; measuring (304) the gap to output a gap measurement signal; filtering (308) the gap measurement signal to provide a phase lead signal in accordance with past measurements to account for delays between the gap measurement signal and a compensation action; and generating (312) the realized current in accordance with the phase lead signal to compensate for negative stiffness and negative damping in adjusting the gap.

17. The method as recited in claim 16, further comprising reconstructing (306) the gap measurement to permit estimation of the gap measurement without the need for collocated sensors.

18. The method as recited in claim 16, wherein generating (302) the realized current in accordance with the phase lead signal includes amplifying a calculated current output from an estimation block where the calculated current is based upon the phase lead signal.

19. The method as recited in claim 18, further comprising reducing (316) noise introduced by the filtering step.

20. The method as recited in claim 16, further comprising extrapolating

(310) a delay from previous gap measurements such that the phase lead signal is based on the extrapolated delay.

Description:

CONTROL SYSTEM AND METHOD FOR NEGATIVE DAMPING COMPENSATION IN MAGNETIC LEVITATION

This disclosure relates to magnetic levitation, and more particularly to a control system and method that compensates for negative damping.

Magnetic levitation (Maglev) refers to systems or devices that employ magnetic fields to repel an object or objects to counter balance other forces, such as gravity for example. Maglev may be employed in transportation systems where vehicles are levitated and propelled down a track. Other applications may include semiconductor processing (e.g., suspending a wafer supporting platen), contactless bearings (e.g., magnetically levitating shafts), or medical equipment (e.g., CT-scanners).

Stiffness is a material's capacity to push back when pushed, e.g., a spring resists compression. This behavior determines the material's strength and ability to dampen vibrations. This is positive stiffness. Some materials or systems have "negative stiffness": Their structure has been buckled or contorted in such a way that if pressure is applied, their stored energy only causes more compression in the same direction, e.g., a spring that, as you begin to press it, collapses on its own.

In magnetic levitation, a gap between a magnetic core and an object being levitated is said to inherently possess negative stiffness. Negative stiffness can be compensated for in magnetic levitation systems by a calibration formula that can counteract the negative stiffness effects. However, delays introduced between the time the gap is measured to the time the gap is compensated for often leads to negative damping. Negative damping cannot be accounted for in the same way as negative stiffness. Therefore, a need exits for compensating for negative damping in a magnetic levitation system. In accordance with present embodiments, a magnetic levitation control system includes a sensor configured to measure a gap between an electro magnet and a stage and generate a gap measurement signal. A gap filter is configured to receive the gap measurement signal and provide a phase lead signal which estimates and accounts for delays between the gap measurement signal and a compensation action. An estimation block is configured to receive the phase lead signal and provide the compensation action in accordance with the phase lead signal such that negative stiffness and negative damping effects are both compensated for in the control system.

A method for controlling a gap in a magnetic levitation system includes

generating a magnetic field to levitate a stage and maintain a gap between the stage and an electromagnetic core in accordance with a realized current, measuring the gap to output a gap measurement signal, filtering the gap measurement signal to provide a phase lead signal in accordance with past measurements to account for delays between the gap measurement signal and a compensation action, and generating the realized current in accordance with the phase lead signal to compensate for negative stiffness and negative damping in adjusting the gap.

In alternate embodiments, the gap measurement may be reconstructed to permit estimation of the gap measurement without the need for collocated sensors. The realized current is preferably generated in accordance with the phase lead signal, and a calculated current output from an estimation block is amplified where the calculated current is based upon the phase lead signal. The method may include reducing noise introduced by the filtering step. The method may further comprise extrapolating a delay from previous gap measurements such that the phase lead signal is based on the extrapolated delay. These and other objects, features and advantages of the present disclosure will become apparent from the following detailed description of illustrative embodiments thereof, which is to be read in connection with the accompanying drawings.

This disclosure will present in detail the following description of preferred embodiments with reference to the following figures wherein:

FIG. 1 is a block/flow diagram showing a magnetic levitation system in accordance with one embodiment;

FIG. 2 is a block/flow diagram showing a magnetic levitation system in accordance with a more detailed embodiment; FIG. 3 is a plot of magnitude (dB) or phase angle (degrees) versus frequency showing a reference signal and responses for different configurations of a gap filter in accordance with FIG. 2; and

FIG. 4 is a flow diagram showing an illustrative method for controlling a gap in a magnetic levitation system. The present disclosure describes a control system adapted for use in a magnetic levitation system, where negative damping is compensated for by anticipating delays between measurement and compensation for gap fluctuations between the levitated part or device (hereinafter referred to as a mechanical plant) and a magnetic core

hereinafter referred to as a plant core). In one particularly useful embodiment, delay is anticipated by a gap filter which provides a phase lead to counter act the negative damping experienced in the system. Instead of or in addition to the gap filter, past measurements or gap prediction criteria may be employed to more accurately predict the actual gap to assist in eliminating the negative damping. It should be understood that the present invention will be described in terms of a particular magnetic levitation system; however, the teachings of the present invention are much broader and are applicable to any magnetic levitation system that may have negative damping as a result of the delay introduced in a feedback loop. It should also be understood that the illustrative example circuitry may be adapted to include additional components or the components may be integrated on one or more integrated circuit chips. In addition, the components depicted may be implemented in software or on a stand-alone device or circuit. In particularly useful embodiments, the elements depicted in the FIGS, may be implemented in various combinations of hardware and software and provide functions which may be combined in a single element or multiple elements.

Referring now to the drawings in which like numerals represent the same or similar elements and initially to FIG. 1, a high-level diagram shows a magnetic levitation system 10 in accordance with one illustrative embodiment. An electromagnetic core 26 includes a winding or coil 24, which receives a corrected current to adjust the magnetic field generated by the core 26. The corrected current from an amplifier 22 functions as feedback to make adjustments to a gap 14 between the core 26 and a stage 12. The gap 14 is measured by a position sensor 16.

Stage 12 with magnetic levitation using the electromagnetic core (an E-core) 26 exhibits inherent negative stiffness. Calibration schemes may be employed to counter the negative stiffness effects, which may include the introduction of a predefined calibration delay. This may include providing a calibration constant or parameters in the governing equation that represents force, F, as function of effective gap, z, and current, I. More complicated schemes including rotations and gap imperfections may also be employed to compensate for this negative stiffness. If the negative stiffness is countered with a delay between measuring the position and using this position to generate a force, negative damping will be present.

For small disturbances in the gap, damping is equal to stiffness multiplied by the delay between measuring a position of the gap 14 and using this position for

compensation. Because the stiffness is negative, so is the damping. dF_

F, = F. - At dt dF dz

F t = F t _ l + — -At (1) dz dt

F t = F t _ l + [- 2^-At where F 1 is the nominal force on the object being levitated at time t, F t _ x is the nominal force at time t-1, F is the instantaneous force, At is the delay between the measurement and the compensation, z is the instantaneous gap, z (or — ) is the dt

( F\ ( F \ instantaneous change in gap, - 2 — is the stiffness, and - 2 — At is the damping

\ z ) \ z J coefficient for negative damping. Note that the damping is absolute and also depends on the nominal stiffness of the magnetic levitation, not on the compensated stiffness.

A simplified calibration formula shows that the stiffness is inherently negative:

I 2 I 2

F = Ci = Ci- with z = gap gap z (2) dF I 2 I 2 X F

— = -2Cι — = -2Cι — — = -2 — dz z z z z where Ci is a calibration constant, and I is the coil current. The stiffness will be negative as a result of the differentiation of the force function. Since the stiffness in equation (1) is negative the damping is also negative. In accordance with the present principles, embodiments described hereinafter will provide for compensation of this negative damping. A compensation module 18 is included to provide the compensation against not only negative stiffness but negative damping as well. After the gap measurement signal from the position sensor 16 is compensated, the compensated signal is used by a controller 20 to determine the output coil current. The coil current may be amplified by amplifier 22 before being used to energize the coil 24.

Referring to FIG. 2, a magnetic levitation system 100 is illustratively depicted to describe concepts in accordance with the present principles. Details of the individual block components making up the system architecture that are known to skilled artisans will only be described in details sufficient for an understanding of the present

invention. Portions or all of system 100 may be implemented on a central motion computer (e.g., a personal computer) or distributed on a stand-alone controller. System 100 includes a magnetic core or plant core 102. Plant core 102 may include an E-core or other magnetic device responsive to a feedback current e.g., having a winding (not shown) thereabout. The winding receives a current i R ea hz ed which includes an amplified and compensated signal as will be explained in greater detail below. Plant core 102 exerts a magnetic force F Rea i on mechanical plant device or devices (stage) 104. Mechanical plant 104 may include a vehicle, a platform, such as a platen for semiconductor processing, a rotating shaft, etc. A gap Gap R eai is maintained between the plant core 102 and the plant mechanics 104. The plant core 102 and the plant mechanics 104 comprise an inner loop 120 of system 100 which models the mechanical/physical aspects of the system 100. The inner loop 120 may include actual physical components or may include digitally modeled components. Gap Rea i may experience fluctuations, rotations or other deflections as a result of operating conditions. These fluctuations are determined and corrected or compensated in accordance with an outer loop 130.

Outer loop 130 includes at least one sensor 106. Sensor 106 may include an optical sensor, and inductive sensor, a mechanical sensor or any other device or software module to estimate the gap distance and fluctuations of the gap over time. Sensor 106 can be any device measuring position (e.g., an inductive sensor). Sensor 106 outputs a measured gap, GapMeasured- The gap measurement may be an analog or digital signal. If the signal is analog, it is preferably converted to a digital signal by an analog to digital converter 108. A gap reconstruction module 110 permits the sensors 106 not to have to be collocated with the gap of the E-core 102. Gap reconstruction 110 is optional when the gap measurement is collocated with the actual gap. If the sensor 106 is not collocated with the gap of the E-core 102, gap reconstruction 110 calculates the gap (Gap ca i cu i at ed) to account for the difference in location.

Any delay in the outer loop 130 (e.g., the calculating loop) with respect to the inner loop 120 (which is the real plant) causes negative damping. A gap filter 112 can be used to generate a phase lead signal 113 to compensate for the gap delay. In one application, the stage 104 is steered or otherwise experiences a change in direction, force or acceleration. At this moment, a gap estimate is needed from gap reconstruction 110 for gap filter 112. Gap filter 112 leads the Gap Ca icuiated signal to account for the delay between the measured gap (Gap MeaS ured) and the iReahzed signals. The gap filter 112 can be a simple lead

filter, a higher order filter (for better accuracy) or an estimation of the gap in the future based on several past measurements. Gap filter 112 may be implemented in such a way to compensate for all the delays encountered within the system.

The phase adjusted output from gap filter 112 is input to an estimation block 114 that predicts the output current Icaiuiated as a function of the force F applied to stage 104 and the gap (the phase lead signal 113) to provide the desired restoring force. Since the gap filter 112 is present, the delay is compensated for resulting in more accurate current estimation. This reduces or eliminates negative damping. Depending on the implementation, Icaiuiated may be digital and converted to an analog signal (voltage or current) by a digital to analog converter 116. An amplifier 118 may be employed to amplify or otherwise modify Icaiuiated to provide i R ea hz edto the core 102 in inner loop 120.

Any gap filter 112 which creates a phase lead will most likely amplify noise. However, this can be counteracted by an analog filter in the amplifier 118 using known methods. The result is that a phase lead is obtained and damping is compensated, without negative effects being visible in the gain. The system 100 may be employed in many different applications for example, in a vehicular system, in a semiconductor processing device, in a contactless bearing system, medical imaging devices, etc.

Referring to FIG. 3, the effect of various implementations of the gap filter can be seen in a Bode diagram. A baseline plot 202 shows magnitude and phase of the delay input to the estimation block without a gap filter. Plots 204, 206, and 208 show magnitude and phase of the delay input to the estimation block with different configurations of a gap filter, and increasingly show compensation for the phase delay. More compensation needs a higher gain of the gap filter. Also, in this case, the remaining negative stiffness 211 is visible on the left hand side of the plots where the lines become horizontal. The magnitudes are relatively the same for all plots.

Referring to FIG. 4, a method for controlling a gap in a magnetic levitation system is illustratively described. In block 302, a magnetic field is generated to levitate a stage and maintain a gap between the stage and an electromagnetic core in accordance with a realized current. The electromagnetic core includes a coil winding that is energized by the realized current to make adjustments to the gap. In block 304, the gap is measured by one or more sensors to output a gap measurement signal. In block 306, the gap measurement may optionally be reconstructed if needed to permit calibration of the gap measurement between the measuring and filtering steps. This may be as a result of signal

conversion or other delays or changes to the gap measurement signal before the gap measurement signal is filtered by the gap filter.

In block 308, the gap measurement signal is filtered by a gap filter to provide a phase lead signal in accordance with delay, e.g., using past measurements/history. The filter is designed to account for delays between the gap measurement signal and a compensation action. The compensation action is preferably performed by an estimation block that calculates an output current for the winding current to adjust/maintain the gap. In block 310, an amount of delay (e.g., phase shift) is extrapolated from previous gap measurements such that the phase lead signal is based on the extrapolated delay. This may be implemented using a look-up table or other memory storage device to determine or extrapolate an action based on past history. Block 310 is optional.

In block 312, a correction current or calculated current (or voltage) is generated by the estimation block in accordance with the phase lead signal to compensate for negative stiffness and negative damping in adjusting the gap. (The current can be represented by a voltage). In block 314, the correction current is generated in accordance with the phase lead signal, which may include amplifying the calculated current output from an estimation block where the calculated current is based upon the phase lead signal. The amplifier is used to amplify the calculated (correction) current output and may reduce noise introduced by the filtering step in block 316. The realized current is output from the amplifier in block 318.

In interpreting the appended claims, it should be understood that: a) the word "comprising" does not exclude the presence of other elements or acts than those listed in a given claim; b) the word "a" or "an" preceding an element does not exclude the presence of a plurality of such elements; c) any reference signs in the claims do not limit their scope; d) several "means" may be represented by the same item or hardware or software implemented structure or function; and e) no specific sequence of acts is intended to be required unless specifically indicated.

Having described preferred embodiments for a control system and method for negative damping compensation in magnetic levitation (which are intended to be

illustrative and not limiting), it is noted that modifications and variations can be made by persons skilled in the art in light of the above teachings. It is therefore to be understood that changes may be made in the particular embodiments of the disclosure disclosed which are within the scope and spirit of the embodiments disclosed herein as outlined by the appended claims. Having thus described the details and particularity required by the patent laws, what is claimed and desired protected by Letters Patent is set forth in the appended claims.