Classifications

• • G—PHYSICS
  • G05—CONTROLLING; REGULATING
  • G05B—CONTROL OR REGULATING SYSTEMS IN GENERAL; FUNCTIONAL ELEMENTS OF SUCH SYSTEMS; MONITORING OR TESTING ARRANGEMENTS FOR SUCH SYSTEMS OR ELEMENTS
  • G05B5/00—Anti-hunting arrangements
  • G05B5/01—Anti-hunting arrangements electric
• • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
  • F16—ENGINEERING ELEMENTS AND UNITS; GENERAL MEASURES FOR PRODUCING AND MAINTAINING EFFECTIVE FUNCTIONING OF MACHINES OR INSTALLATIONS; THERMAL INSULATION IN GENERAL
  • F16C—SHAFTS; FLEXIBLE SHAFTS; ELEMENTS OR CRANKSHAFT MECHANISMS; ROTARY BODIES OTHER THAN GEARING ELEMENTS; BEARINGS
  • F16C32/00—Bearings not otherwise provided for
  • F16C32/04—Bearings not otherwise provided for using magnetic or electric supporting means
  • F16C32/0406—Magnetic bearings
  • F16C32/044—Active magnetic bearings
  • F16C32/0444—Details of devices to control the actuation of the electromagnets
• • G—PHYSICS
  • G05—CONTROLLING; REGULATING
  • G05B—CONTROL OR REGULATING SYSTEMS IN GENERAL; FUNCTIONAL ELEMENTS OF SUCH SYSTEMS; MONITORING OR TESTING ARRANGEMENTS FOR SUCH SYSTEMS OR ELEMENTS
  • G05B6/00—Internal feedback arrangements for obtaining particular characteristics, e.g. proportional, integral or differential
  • G05B6/02—Internal feedback arrangements for obtaining particular characteristics, e.g. proportional, integral or differential electric
• • H—ELECTRICITY
  • H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
  • H02N—ELECTRIC MACHINES NOT OTHERWISE PROVIDED FOR
  • H02N15/00—Holding or levitation devices using magnetic attraction or repulsion, not otherwise provided for
• • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
  • F16—ENGINEERING ELEMENTS AND UNITS; GENERAL MEASURES FOR PRODUCING AND MAINTAINING EFFECTIVE FUNCTIONING OF MACHINES OR INSTALLATIONS; THERMAL INSULATION IN GENERAL
  • F16C—SHAFTS; FLEXIBLE SHAFTS; ELEMENTS OR CRANKSHAFT MECHANISMS; ROTARY BODIES OTHER THAN GEARING ELEMENTS; BEARINGS
  • F16C2326/00—Articles relating to transporting
  • F16C2326/10—Railway vehicles

Definitions

• This disclosure relates to magnetic levitation, and more particularly to a control system and method that compensates for negative damping.
• Magnetic levitation 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).
• semiconductor processing e.g., suspending a wafer supporting platen
• contactless bearings e.g., magnetically levitating shafts
• 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.
• 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.
• 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.
• 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).
• delay is anticipated by a gap filter which provides a phase lead to counter act the negative damping experienced in the system.
• past measurements or gap prediction criteria may be employed to more accurately predict the actual gap to assist in eliminating the negative damping.
• 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.
• the illustrative example circuitry may be adapted to include additional components or the components may be integrated on one or more integrated circuit chips.
• the components depicted may be implemented in software or on a stand-alone device or circuit.
• 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.
• 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.
• 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 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
• z is the instantaneous gap
• z (or — ) is the dt
• damping is absolute and also depends on the nominal stiffness of the magnetic levitation, not on the compensated stiffness.
• 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.
• 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.
• a gap filter 112 can be used to generate a phase lead signal 113 to compensate for the gap delay.
• 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.
• 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.
• 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.
• a method for controlling a gap in a magnetic levitation system is illustratively described.
• 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.
• the gap is measured by one or more sensors to output a gap measurement signal.
• 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.
• 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.
• an amount of delay e.g., phase shift
• a correction current or calculated current 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.
• 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.

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• Engineering & Computer Science (AREA)
• Physics & Mathematics (AREA)
• General Engineering & Computer Science (AREA)
• General Physics & Mathematics (AREA)
• Automation & Control Theory (AREA)
• Chemical & Material Sciences (AREA)
• Combustion & Propulsion (AREA)
• Electromagnetism (AREA)
• Mechanical Engineering (AREA)
• Control Of Vehicles With Linear Motors And Vehicles That Are Magnetically Levitated (AREA)
• Magnetic Bearings And Hydrostatic Bearings (AREA)
• Feedback Control In General (AREA)

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• 2007
  • 2007-12-13 JP JP2009542310A patent/JP2010514991A/ja active Pending
  • 2007-12-13 WO PCT/IB2007/055085 patent/WO2008075269A2/en active Application Filing
  • 2007-12-13 KR KR1020097012568A patent/KR20090091300A/ko not_active Application Discontinuation
  • 2007-12-13 CN CNA2007800472180A patent/CN101568892A/zh active Pending
  • 2007-12-13 EP EP07859380A patent/EP2095198A2/en not_active Withdrawn

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