Analog compensation of inductive sensors

Abstract

A set of magnetic bearings (104) having a signal conditioning circuit (120) which includes an offset generator (220) for adding a digitally controlled analog offset to compensate for displacement error in unbalanced inductive sensors (114).

Description

Title of the invention: Analog compensation of inductive sensors Technical field of the invention

[0001] Certain aspects of the present invention relate to sensors and, more particularly, to inductive sensors for use with an active magnetic bearing system. State of the prior art

[0002] An active magnetic bearing system uses a set of actuators (e.g., magnetic bearings) driven by a control unit to levitate a rotating element (e.g., a shaft or rotor). To ensure stable magnetic levitation, sensors provide continuous displacement measurements to the control unit. Sensor technologies include capacitive, optical, magnetic, and electromagnetic technologies. In an inductive sensor, which is a type of electromagnetic sensor, an external oscillator excites current flowing in an inductance coil. When the levitating element changes position, the inductance changes accordingly. This variation is converted into voltage and adopted in the control loop for magnetic levitation.

[0003] Theoretically, the output of the sensors should be zero when the levitating element is perfectly centered, i.e., at zero displacement. In practice, this is not the case due to manufacturing and material imperfections. The control unit must account for this error, sometimes called offset error or quadrature error or zero displacement error. Conventional signal conditioning and processing techniques used to address the zero displacement error problem are: quadrature compensation implemented in hardware, changing the demodulation angle, and digital offset compensation implemented in software.Unfortunately, hardware changes require manual adjustment and cannot be performed remotely, and digital compensation reduces signal resolution due to the need to accept a reduced full-scale signal range at the analog-to-digital converter (ADC) input to accommodate the offset. Summary of the Invention .

[0004] Aspects of the present invention include processing the signal output by an inductive sensor used to sense the position of a rotor or the like in a magnetic bearing assembly. In this regard, a programmable direct current (DC) offset is added to a VSSense sensor output signal to provide a zero output at zero displacement. Some aspects of the present invention compensate by in addition to imbalances present in magnetic bearing inductive sensors due to manufacturing, material variance, etc. and ensure high sensor resolution.

[0005] In one aspect, a sensor assembly for use with a magnetic bearing system includes first and second inductive sensors positioned remotely from a levitating body. The first inductive sensor is configured to sense a first air gap and the second inductive sensor is configured to sense a second air gap. The assembly also includes a plurality of excitation circuits electrically connected to the inductive sensors. Each of the excitation circuits is configured to provide an input signal to a respective one of the inductive sensors such that the inductive sensors each generate an output signal representing the air gap. An interconnect electrically connecting the inductive sensors to each other combines the output signals of the sensors to provide a displacement signal.The output signals of the inductive sensors cancel each other at the interconnect when the levitating body is positioned in a predetermined position such that the first and second air gaps are equal to each other. A non-zero displacement signal at the interconnect when the levitating body is in the predetermined position represents a displacement error. The assembly further includes a signal conditioning circuit electrically connected to the interconnect. The signal conditioning circuit receives the displacement signal and includes an offset generator for adding an analog offset to compensate for the displacement error. The signal conditioning circuit is configured to generate a position signal representing a position of the levitating body relative to the predetermined position based on the displacement signal compensated with the analog offset.

[0006] In another aspect, a magnetic bearing assembly includes a plurality of active magnetic bearings configured to levitate a rotating member, which is rotatable about a central axis. The assembly includes a plurality of inductive sensors each associated with a respective one of the magnetic bearings and positioned radially outward from the central axis and spaced apart from the rotating member. The sensors are configured to detect an air gap between the inductive sensor and the rotating member. The assembly also includes a plurality of excitation circuits electrically connected to the inductive sensors. Each of the excitation circuits is configured to provide an input signal to a respective one of the inductive sensors such that the inductive sensors each generate an output signal representative of the air gap.An interconnection electrically connecting the inductive sensors to each other combines the delivered signals. by the sensors to output a displacement signal. The output signals of the inductive sensors cancel each other at the interconnection when the rotating element is centered on the central axis and a non-zero displacement signal at the interconnection when the rotating element is centered on the central axis represents a displacement error. The assembly further includes a signal conditioning circuit electrically connected to the interconnection. The signal conditioning circuit receives the displacement signal and includes an offset generator for adding an analog offset to compensate for the displacement error. The signal conditioning circuit is configured to generate a position signal representing a radial position of the rotating element relative to the central axis based on the displacement signal with the offset.A magnetic bearing controller of the assembly receives the position signal to control the active magnetic bearings to minimize radial displacement of the rotating element from the central axis.

[0007] In yet another aspect, a method of controlling a magnetic bearing assembly includes providing input signals to a plurality of inductive sensors. Each of the inductive sensors is associated with a respective one of a plurality of active magnetic bearings configured to levitate a rotatable member, which is rotatable about a central axis. The sensors are positioned radially outward from the central axis and spaced apart from the rotatable member. The method includes detecting an air gap between each of the inductive sensors and the rotatable member and receiving an output signal representing the detected air gap from each of the inductive sensors. The method also includes combining the output signals from each of the inductive sensors at an electrical interconnect to provide a displacement signal.The output signals of the inductive sensors cancel each other at the interconnection when the rotating element is centered on the central axis and a non-zero displacement signal at the interconnection when the rotating element is centered on the central axis represents a displacement error. The method further includes receiving the displacement signal and adding an analog offset thereto to compensate for the displacement error, generating a position signal representing a radial position of the rotating element relative to the central axis based on the displacement signal compensated with the offset, and controlling the active magnetic bearings to minimize radial displacement of the rotating element from the central axis based on the position signal.

[0008] Other objects and features of the present invention will be partly apparent and partly indicated herein. Brief description of the figures

[0009] [Fig.l] is a block diagram illustrating an active magnetic bearing system according to one embodiment.

[0010] [Fig.2] illustrates an offset compensation circuit for use with the active magnetic bearing system of [Fig.l] according to one embodiment.

[0011] [Fig. 3] illustrates alternative sensor positions according to one embodiment.

[0012] Corresponding reference numerals indicate corresponding parts in the drawings. Detailed description of the invention

[0013] Referring to [Fig.l], an active magnetic bearing system 100 levitates a rotating element 102 and holds it in position by applying controlled electromagnetic forces to the element 102 in radial and axial directions. A radial magnetic bearing assembly 104 includes a plurality of electromagnetic actuators. In [Fig.l], the actuators are in the form of controllable electromagnets 106, generally consisting of a stack of laminations with copper coils wound around the north and south poles. The rotating element 102 is often referred to as the rotor and the radial magnetic bearing assembly 104 is often referred to as the stator.

[0014] A levitation controller 110, or magnetic bearing controller, applies current to the electromagnet coils 106 via power amplifiers 112 to produce attractive forces on rolled iron rotor portions so as to levitate the rotating element 102 within the bearing assembly 104. The clearance or magnetic air gap between the bearing assembly 104 and the rotating element 102 is, for example, 0.5 to 1.0 mm depending on the application. For this reason, there is no contact friction or wear of the components and, therefore, lubrication is not necessary.

[0015] To achieve stable levitation of the rotating element 102 in the active magnetic bearing system 100, the current in the electromagnets 106 is continuously adjusted to maintain the rotating element 102 in the desired position. This position is continuously measured using a plurality of position sensors 114, which transmit information to the controller 110, which in return adjusts the current (for example, by applying a control voltage to the inputs of the power amplifiers 112).

[0016] [Fig.l] further illustrates position sensors 114 for measuring any radial displacements of element 102 relative to a central axis 116 of bearing assembly 104. As described above, the output of sensors 114 should be zero when levitating element 102 is perfectly centered, i.e., at zero displacement. Since this is not typically the case due to manufacturing and material imperfections, the controller 110 must account for this zero displacement error. A position signal conditioning circuit 120 provides a position signal representing the displacement of the rotating element 102 relative to the central axis 116 to the controller 110 for use in controlling the active bearing assembly 104 to levitate the rotating element 102. Although not shown, those skilled in the art are familiar with the use of monitoring systems for active magnetic bearing systems 100. Such monitoring systems provide real-time information on shaft position, vibrations, currents (loads), temperatures, alarms, safety management, etc. The controller 110 responds to a number of parameters to control the levitation of the rotating element 102.

[0017] Some aspects of the present invention compensate for the zero displacement error by adding a DC offset to the analog measurement circuitry, thereby resetting the zero mechanical displacement signal output. Referring to [Fig. 2], inductive sensors 114 are part of a sensor assembly 202, which includes first and second oscillator circuits 204, 206, respectively, electrically connected by inductive sensors 114 (illustrated by inductors L1 and L2, respectively) at an interconnect 208. The first oscillator circuit 204 is electrically connected to L1 and is configured to provide a first input signal to a first of the sensors 114. The second oscillator circuit 206 is electrically connected to L2 and is configured to provide a second input signal to a second of the sensors 114.The oscillator circuits 204, 206 are arranged or configured such that the second input signal is approximately one hundred and eighty degrees (180°) out of phase with the first input signal. Because of this, the sensor output signals from the two sensors 114 illustrated in [Fig. 2] are also approximately one hundred and eighty degrees (180°) out of phase with each other. The interconnect 208 is electrically connected to the position signal conditioning circuit 120 to communicate a sensor output signal VSense, or displacement signal. The interconnect 208 of [Fig.2] is preferably configured to combine the output signals of the two sensors 114 such that their output signals cancel each other out when a first air gap or spacing distance between the first of the two sensors 114 and the rotatable member 102 is equal to a second air gap or spacing distance between the second of the two sensors 114 and the rotatable member 102. The displacement signal represents an amount of displacement of the rotatable member 102 from the central axis 116. .

[0018] In one embodiment, the sensor assembly 202 of [Fig. 2] includes oscillator circuits 204, 206 driving sensors 114 to produce the sensor output signal VSSense at the interconnect 208. The VSSense output signal is introduced into the position signal conditioning circuit 120. In the illustrated embodiment, the position signal conditioning circuit 120 includes an analog amplifier 212 to increase or decrease the gain of Vsense. The oscillators 204, 206 of each of the driver circuits provide a suppressed carrier to modulate VSSense. Following the amplifier 212, the position signal conditioning circuit 120 includes a demodulator 214 to demodulate Vsense, resulting in a demodulated displacement signal. The position signal conditioning circuit 120 of [Fig.2] further conditions the demodulated displacement signal using a low-pass filter 216.

[0019] After demodulation and filtering, an offset generator 220 of the position signal conditioning circuit 120 adds a variable DC output to the displacement signal to compensate for any observed zero displacement error. To maximize sensor resolution, the offset generator 220 adds the analog offset before an analog-to-digital converter (ADC) 222 converts the compensated displacement signal to a digital position signal, which is then output to the levitation controller 110. In one embodiment, the offset generator 220 includes at least a digital-to-analog converter (DAC), a digital potentiometer, and a tunable resistor. In other words, the variable offset is generated using a DAC, but it can also be generated using different methods, including using a digital potentiometer or manual resistor tuning.The offset generator 220 includes a digital-to-analog converter (DAC) or a digital potentiometer or a tunable resistor. It is important to compensate in the analog circuitry to maximize the resolution of the sensor read by the ADC 222. In one exemplary operation, inductive sensors 114, driven by oscillators 204, 206, generate and provide analog sensor data to the position signal conditioning circuit 120. In turn, the position signal conditioning circuit 120 converts and processes the analog sensor data into a digital position signal for use by the controller 110 to control the active magnetic bearing assembly 104.

[0020] Advantageously, adding the analog output before converting to a digital signal maximizes the input range of the ADC 222, resulting in increased sensor resolution. In addition, the DC offset compensation is easy to control and extremely accurate. The offset generator 220 (e.g., represented by a DAC) can be digitally controlled while providing an analog offset. This allows the circuit to be easily adjusted to locally or remotely correct any zero offset error.

[0021] [Fig. 3] illustrates examples of alternative placements of inductive sensors 114. As shown, sensors 114a, 114b comprise a double-ended radial sensor arrangement for measuring the position of the levitated rotatable member 102 relative to the axis 116. Sensors 114c, 114d comprise a double-ended axial sensor arrangement for measuring the position of the levitated rotatable member 102 relative to a different axis 116 (shown as being perpendicular to the axis 116). The sensor 114e comprises a sensor arrangement at one end for measuring the position of the levitating rotating element 102 relative to the axis 316 (or axis 116) and the sensor 114f comprises a reference sensor for measuring the position of the levitating rotating element 102 relative to a fixed body 318.The first air gap represents a distance from the first sensor to the levitating body and the second air gap represents a distance from the second sensor to a fixed body. Those skilled in the art will recognize that certain aspects of the present invention may be applied to different physical arrangements where the sensors 114 are spaced from the shaft 102 by an axis that is not necessarily the axis of rotation 116.

[0022] The following is an example of offset compensation according to certain aspects of the present invention. Those skilled in the art familiar with conventional quadrature compensation will recognize that the digital offset compensation described herein produces the same net result as quadrature compensation.

[0023] In the example, the inductive sensor output is demodulated in phase with the excitation voltage, the analog offset is added, and then the compensated sensor output is low-pass filtered and converted from analog to digital. The procedure for setting the offset compensation includes the following steps: placing the rotor at zero position; reading the low-pass filter output voltage; and adding equal and opposite compensation to the low-pass filter output. If the sensors are balanced, no compensation is necessary and the offset compensation value is 0. The sensor output is 0 V, which represents zero displacement.

[0024] In contrast, unbalanced sensors require compensation to achieve a 0 V sensor output at zero displacement. To compensate for an unbalanced sensor configuration, a DC offset is added to the low-pass filter output. Unbalanced sensors can be simulated by increasing or decreasing simulated sensor inductance values ​​to see what the comparative response to a position change is with unbalanced sensors. For comparison, the amount of offset resulting from changing the inductance of L1 (top) or L2 (bottom) is shown in Tables I and II below.

[0025] TABLE I: Test L1 (pH) L2 (pH) Offset Compensation 0 (Calibration) 1.2 1 -0.0665 1 2 1 -0.0956 2 4 1 -0.233 3 10 1 -1.10 4 50 1 -7.34 5 30 1 -5.10 6 1 10 1.64 7 1 30 7.71

[0026] TABLE II: Test L1 (pH) L2 (pH) Offset Compensation 0 (Calibration) 1 10 -0.00552 1 1 20 3.25 2 1 30 6.11 3 1 1 -1.70 4 10 1 -2.75 5 20 1 -4.88

[0027] The ideal offset compensation (no phase shift in the sensor output) is determined as follows:

[0028] Take the ideal sensor output as Sout

[0029] Smit = Asi,^}

[0030] The demodulated output is Dmit

[0031] Dout = Asin (œt) *sin(w / )

[0032]

[0033] Take the offset value as C

[0034] The sum of the offset value and the demodulated output is O„ut

[0035] a (>.2Ç21) +c

[0036] After low-pass filtering and replacement C

[0037] LPF0Ut-^ (A + B)

[0038] The non-ideal offset compensation (phase shift in the sensor output) is determined as follows:

[0039] If the sensor is not ideal

[0040] = Asiniaiï + <p)

[0041] From the sum / difference identity

[0042] = A ( asin ( œt ) + (kos ( œt ) )

[0043] After demodulation

[0044] Dout = a(asin2 (œt) + [kos (œt) sin(œ / ))

[0046] After adding the offset value C [°° 47 1 7^, = A a ( 4 -^) + ^ S in(2« ( ) + C

[0048] After low-pass filtering and replacement C = 4

[0049] LPFout = i(Aa + B)

[0050] In operation, processors, computers, and / or servers may execute processor-executable instructions (e.g., software, firmware, and / or hardware) such as those illustrated herein to implement certain aspects of the invention. The processor-executable instructions may be organized into one or more processor-executable components or modules on a tangible processor-readable storage medium.

[0051] The order of performing or carrying out operations in accordance with aspects of the present invention illustrated and described herein is not essential, unless otherwise indicated. That is, the operations may be performed in any order, unless otherwise indicated, and embodiments may include additional or fewer operations than those described herein. For example, it is contemplated that performing or carrying out a particular operation before, concurrently with, or after another operation is within the scope of the present invention.

[0052] When introducing elements of the embodiments, the articles "a," "an," "the," "the," "said," or "said" are intended to indicate that there is one or more of the elements. The terms "comprising," "comprising," and "having" are intended to be inclusive and indicate that there may be additional elements other than the elements listed.

[0053] All components illustrated or described may not be necessary. Furthermore, some implementations and embodiments may include additional components. Variations in the arrangement and type of components may be made without departing from the spirit or scope of the claims as set forth herein. Additional, different, or fewer components may be provided, and components may be combined. Alternatively, or in addition, a component may be implemented by more than one component.

[0054] The above description illustrates embodiments by way of example and not limitation. This description enables those skilled in the art to make and use aspects of the invention, and describes several embodiments, adaptations, variations, alternatives, and uses of aspects of the invention, including what is presently considered the best mode for carrying out aspects of the invention. Furthermore, it is to be understood that aspects of the invention are not limited in their application to the details of construction and arrangement of the components set forth in the following description or illustrated in the drawings. Aspects of the invention may be embodied in other ways and may be practiced or realized in various ways. Furthermore, it is to be understood that the phraseology and terminology used herein are for the purpose of description and should not be construed as limiting.

[0055] It goes without saying that modifications and variations are possible without departing from the scope defined in the appended claims. Various modifications may be made to the above constructions and methods without departing from the scope of the present invention, it is understood that all the elements contained in the above description and represented in the associated drawings will be interpreted as illustrative and not limiting.

[0056] In view of the foregoing, it will be seen that several advantages of aspects of the invention are obtained and that other advantageous results are achieved.

[0057] The abstract and summary are provided to assist the reader in quickly determining the nature of the technical disclosure. They are submitted with the understanding that they will not be used to interpret or limit the scope or meaning of the claims. The summary is provided to present a selection of concepts in simplified form that are described in more detail in the detailed description. The summary is not intended to identify key features or essential characteristics of the claimed subject matter, nor to serve as an aid in determining the claimed subject matter.

Claims

1. Claims A sensor assembly (202) for use with a magnetic bearing system (100), the magnetic bearing system configured to levitate a body (102), the sensor assembly comprising: first and second inductive sensors (114) spaced from the levitating body (102), the first inductive sensor configured to detect a first air gap and the second inductive sensor configured to detect a second air gap; a plurality of excitation circuits electrically connected to the first and second inductive sensors, each of the excitation circuits being configured to provide an input signal to a respective one of the first and second inductive sensors such that the first inductive sensor generates an output signal representing the first air gap and the second inductive sensor generates an output signal representing the second air gap; an interconnection (208) electrically connecting the first and second inductive sensors to each other and combining the output signals thereof to output a displacement signal, the output signals of the first and second inductive sensors canceling each other at the interconnection when the levitating body is positioned in a predetermined position such that the first and second air gaps are equal to each other, and a non-zero displacement signal at the interconnection when the levitating body is positioned in the predetermined position representing a displacement error;and a signal conditioning circuit (120) electrically connected to the interconnect, the signal conditioning circuit receiving and responding to the displacement signal and including an offset generator (220) for adding an analog offset to compensate for the displacement error, the signal conditioning circuit being configured to generate a position signal representing a position of the levitated body relative to the predetermined position based on the displacement signal compensated with the analog offset.;

2. The sensor assembly of claim 1, the first air gap representing a distance from the first sensor to the levitating body and the second air gap representing a distance from the second sensor to a fixed body.

3. A sensor assembly according to claim 1, each of the excitation circuits comprising an oscillator.

4. A sensor assembly according to claim 3, the excitation circuits comprising a first excitation circuit (204) and a second excitation circuit (206), and the oscillator of the first excitation circuit being 180° out of phase with the oscillator of the second excitation circuit.

5. A sensor assembly according to claim 3, the oscillator of each of the excitation circuits providing a suppressed carrier for modulating the displacement signal and the signal conditioning circuit (120) comprising a demodulator (214) for demodulating the displacement signal before the analog offset is added.

6. The sensor assembly of claim 1, the signal conditioning circuit comprising an analog-to-digital converter (ADC) configured to convert the compensated displacement signal from analog to digital, and the position signal output by the signal conditioning circuit being a digital signal.

7. A sensor assembly according to claim 1, the offset generator (220) comprising a digital-to-analog converter (DAC) and / or a digital potentiometer and / or a tunable resistor.

8. A magnetic bearing assembly (104) comprising: a plurality of active magnetic bearings configured to levitate a rotating member (102), the rotating member being rotatable about a central axis (116); a plurality of inductive sensors (114) each associated with a respective one of the magnetic bearings, each of the inductive sensors being positioned radially outward of the central axis and being spaced from the rotating member and configured to detect an air gap between the inductive sensor and the rotating member; a plurality of excitation circuits electrically connected to the inductive sensors, each of the excitation circuits being configured to provide an input signal to a respective one of the sensors

9.

10.

11.

12. inductive such that the inductive sensors each generate an output signal representing the air gap; an interconnect (208) electrically connecting the inductive sensors to each other and combining the output signals thereof to provide a displacement signal, the output signals of the inductive sensors canceling each other at the interconnect when the rotating member is centered on the central axis, and a non-zero displacement signal at the interconnect when the rotating member is centered on the central axis representing a displacement error; a signal conditioning circuit (120) electrically connected to the interconnect (208), the signal conditioning circuit receiving and responding to the displacement signal and including an offset generator (220) for adding an analog offset to compensate for the displacement error, the signal conditioning circuit being configured to generate a position signal representing a radial position of the rotatable member relative to the central axis (116) as a function of the displacement signal with the offset; and a magnetic bearing controller (110) receiving and responding to the position signal to control the active magnetic bearings to minimize radial displacement of the rotating member from the central axis. A magnetic bearing assembly according to claim 8, the displacement signal representing an amount of displacement of the rotating element from the central axis. A magnetic bearing assembly according to claim 8, each of the excitation circuits comprising an oscillator. The magnetic bearing assembly of claim 10, the excitation circuits comprising a first excitation circuit (204) and a second excitation circuit (206), and the oscillator of the first excitation circuit being 180° out of phase with the oscillator of the second excitation circuit. A magnetic bearing assembly according to claim 10, the oscillator of each of the excitation circuits providing a suppressed carrier for modulating the displacement signal and the signal conditioning circuit (120) comprising a demodulator (214) for demodulating the displacement signal before the analog offset is added.

13. The magnetic bearing assembly of claim 8, the signal conditioning circuit comprising an analog-to-digital converter (ADC) configured to convert the compensated displacement signal from analog to digital, and the position signal output by the signal conditioning circuit being a digital signal.

14. A magnetic bearing assembly according to claim 8, the offset generator (220) comprising a digital-to-analog converter (DAC) and / or a digital potentiometer and / or a tunable resistor.

15. A method of controlling a magnetic bearing assembly (104), the magnetic bearing assembly comprising a plurality of active magnetic bearings configured to levitate a rotating member (102), the rotating member being rotatable about a central axis (116), the method comprising: providing input signals to a plurality of inductive sensors (114), each of the inductive sensors being associated with a respective one of the magnetic bearings and being positioned radially outward of the central axis and being spaced apart from the rotating member; detecting an air gap between each of the inductive sensors and the rotating member; receiving an output signal representing the detected air gap from each of the inductive sensors;combining the output signals of each of the inductive sensors at an electrical interconnection (208) to provide a displacement signal, the output signals of the inductive sensors canceling each other at the interconnection when the rotating element is centered on the central axis, and a non-zero displacement signal at the interconnection when the rotating element is centered on the central axis representing a displacement error; receiving the displacement signal and adding an analog offset thereto to compensate for the displacement error; generating a position signal representing a radial position of the rotating element relative to the central axis as a function of the displacement signal compensated with the offset; and controlling the active magnetic bearings to minimize radial displacement of the rotating element from the central axis as a function of the position signal.

16. The method of claim 15, the displacement signal representing an amount of displacement of the rotatable member from the central axis (116).

17. The method of claim 15, each of the excitation circuits comprising an oscillator.

18. The method of claim 17, the excitation circuits comprising a first excitation circuit (204) and a second excitation circuit (206), and the oscillator of the first excitation circuit being 180° out of phase with the oscillator of the second excitation circuit.

19. The method of claim 17, further comprising providing a suppressed carrier by the oscillator of each of the excitation circuits to modulate the displacement signal and demodulate the displacement signal before adding the analog offset.

20. The method of claim 15, further comprising converting the compensated displacement signal from analog to digital, and the position signal comprising a digital signal.

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