Analog compensation of inductive sensor

Abstract

A magnetic bearing assembly having a signal conditioning circuit that includes an offset generator for increasing a digitally controllable analog offset to compensate for displacement errors in an unbalanced inductive sensor.

Description

Technical Field

[0001] Aspects of the present disclosure relate to sensors and, more particularly, to inductive sensors for use with active magnetic bearing systems. Background Art

[0002] Active magnetic bearing systems use a set of actuators (i.e., magnetic bearings) driven by a control unit to suspend a rotatable member (e.g., a shaft or rotor). To ensure stable magnetic suspension, sensors provide continuous displacement measurements to the control unit. Sensor technologies include capacitive, optical, magnetic, and electromagnetic. In an inductive sensor (which is a type of electromagnetic sensor), an external oscillator drives a current flowing in an inductor coil. When the suspended member changes its position, the inductance changes accordingly. This change is converted into a voltage and used in the control loop of the magnetic suspension.

[0003] In theory, the output of the sensor should be zero when the suspended member is perfectly centered, that is, 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. Traditional signal conditioning and processing techniques used to address the zero displacement error problem are: hardware-implemented quadrature compensation, demodulation angle correction, and software-implemented digital offset compensation. Unfortunately, hardware modifications require manual tuning and cannot be performed remotely, and digital compensation reduces signal resolution because the full-scale signal range at the input of the analog-to-digital converter (ADC) needs to be reduced to accommodate the offset. Summary of the invention

[0004] Aspects of the present disclosure include processing signals output from an inductive sensor used to sense the position of a rotor, etc., in a magnetic bearing assembly. In this regard, a programmable direct current (DC) offset is added to the sensor output signal V SENSE , to ensure zero output at zero displacement. Aspects of the present disclosure also compensate for imbalances present in magnetic bearing inductive sensors due to manufacturing, material differences, 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 spaced apart from a suspended object. 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 drive circuits electrically connected to the inductive sensors. Each drive circuit is configured to provide an input signal to a corresponding one of the inductive sensors so that each of the inductive sensors generates an output signal representing the air gap. An interconnection electrically connecting the inductive sensors to each other combines the output signals from the sensors to output a displacement signal. When the suspended object is at a predetermined position, the output signals from the inductive sensors cancel each other at the interconnection so that the first and second air gaps are equal to each other. When the suspended object is at the predetermined position, a non-zero displacement signal at the interconnection represents a displacement error. The assembly also 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 the position of the suspended object relative to the predetermined position based on the compensated displacement signal having the analog offset.

[0006] On the other hand, a magnetic bearing assembly includes a plurality of active magnetic bearings configured to suspend a rotatable member rotatable about a central axis. The assembly includes a plurality of inductive sensors, each associated with a corresponding one of the magnetic bearings and positioned radially outward from the central axis and spaced apart from the rotatable member. The sensor is configured to sense an air gap between the inductive sensor and the rotatable member. The assembly also includes a plurality of drive circuits electrically connected to the inductive sensors. Each drive circuit is configured to provide an input signal to a corresponding one of the inductive sensors so that each of the inductive sensors generates an output signal representing the air gap. An interconnection electrically connecting the inductive sensors to each other combines the output signals from the sensors to output a displacement signal. When the rotatable member is centered on the central axis, the output signals from the inductive sensors cancel each other at the interconnection, and when the rotatable member is centered on the central axis, the non-zero displacement signal at the interconnection represents a displacement error. The assembly also 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 the radial position of the rotatable member relative to the central axis based on the displacement signal having the offset. A magnetic bearing controller of the assembly receives the position signal for controlling the active magnetic bearing to minimize the radial displacement of the rotatable member relative to the central axis.

[0007] In yet another aspect, a method for controlling a magnetic bearing assembly includes providing an input signal to a plurality of inductive sensors. Each inductive sensor is associated with a corresponding one of a plurality of active magnetic bearings, and the active magnetic bearing is configured to suspend a rotatable member that can rotate about a central axis. The sensor is positioned radially outward from the central axis and is spaced apart from the rotatable member. The method includes sensing an air gap between each inductive sensor and the rotatable member, and receiving an output signal representing the sensed air gap from each inductive sensor. The method also includes combining the output signals from each inductive sensor at an electrical interconnection to output a displacement signal. When the rotatable member is centered on the central axis, the output signals from the inductive sensors cancel each other at the interconnection, and when the rotatable member is centered on the central axis, the non-zero displacement signal at the interconnection represents a displacement error. The method also includes receiving the displacement signal and adding an analog offset thereto to compensate for the displacement error, generating a position signal representing the radial position of the rotatable member relative to the central axis according to the compensated displacement signal with the offset, and controlling the active magnetic bearing according to the position signal to minimize the radial displacement of the rotatable member relative to the central axis.

[0008] Other objects and features of the disclosure will be in part obvious and in part pointed out herein. BRIEF DESCRIPTION OF THE DRAWINGS

[0009] Figure 1 is a block diagram illustrating an active magnetic bearing system according to an embodiment.

[0010] Figure 2 A method for communicating with Figure 1 An offset compensation circuit is used in conjunction with an active magnetic bearing system.

[0011] Figure 3 An alternative sensor location is shown according to an embodiment.

[0012] Corresponding reference characters indicate corresponding parts throughout the several views. DETAILED DESCRIPTION

[0013] refer to Figure 1 , the active magnetic bearing system 100 levitates a rotatable member 102 and holds it in place by applying controlled electromagnetic forces in radial and axial directions on the member 102. The radial magnetic bearing assembly 104 includes a plurality of electromagnetic actuators. Figure 1 In the embodiment of the present invention, the actuator takes the form of a controllable electromagnet 106, typically made of a lamination stack with copper coils wound around north and south poles. The rotating member 102 is typically referred to as a rotor and the radial magnetic bearing assembly 104 is typically referred to as a stator.

[0014] The suspension controller 110 or magnetic bearing controller applies current to the coil of the electromagnet 106 through the power amplifier 112 to generate an attractive force on the laminated iron rotor component, thereby levitating the rotatable member 102 within the bearing assembly 104. Depending on the application, the air gap or magnetic air gap between the bearing assembly 104 and the rotatable member 102 is, for example, 0.5 to 1.0 mm. For this reason, there is no contact friction or component wear, so no lubrication is required.

[0015] To achieve stable suspension of rotatable member 102 in active magnetic bearing system 100, the current in electromagnet 106 is continuously adjusted to maintain rotatable member 102 in a desired position. The position is continuously measured using a plurality of position sensors 114, which communicate information to controller 110, which in turn adjusts the current (e.g., by applying a control voltage at the input of power amplifier 112).

[0016] Figure 1 Also shown is a position sensor 114 for measuring any radial displacement of the member 102 relative to the central axis 116 of the bearing assembly 104. As described above, when the suspended member 102 is perfectly centered, i.e., at zero displacement, the output of the sensor 114 should be zero. Because this is often not the case due to manufacturing and material imperfections, the controller 110 must account for this zero displacement error. The position signal conditioning circuit 120 provides a position signal to the controller 110 that represents the displacement of the rotatable member 102 relative to the central axis 116 for controlling the active bearing assembly 104 to suspend the rotatable member 102. Although not shown, those skilled in the art are familiar with the use of monitoring systems for the active magnetic bearing system 100. Such monitoring systems provide real-time information of shaft position, vibration, current (load), temperature, alarms, safety management, etc. The controller 110 controls the suspension of the rotatable member 102 in response to a number of parameters.

[0017] Aspects of the present disclosure compensate for zero displacement errors by adding a DC offset to an analog measurement circuit that returns the zero mechanical displacement signal output to zero. Figure 2 , the inductive sensor 114 is part of a sensor assembly 202, which includes first and second oscillator circuits 204, 206, respectively, which are electrically connected at an interconnect 208 through the inductive sensor 114 (shown as inductors L1 and L2, respectively). The first oscillator circuit 204 is electrically connected to L1 and is configured to provide a first input signal to the first sensor 114. The second oscillator circuit 206 is electrically connected to L2 and is configured to provide a second input signal to the second sensor 114. The oscillator circuits 204, 206 are arranged or configured so that the second input signal is approximately one hundred eighty degrees (180°) out of phase with the first input signal. To this end, the oscillator circuits 204, 206 are arranged or configured so that the second input signal is approximately one hundred eighty degrees (180°) out of phase with the first input signal. To this end, the oscillator circuits 204, 206 are electrically connected to the inductor L1 and L2, respectively, and are configured to provide a second input signal to the second sensor 114. Figure 2The sensor output signals of the two sensors 114 are also shown to be approximately one hundred eighty degrees (180°) out of phase with each other. Interconnect 208 is electrically connected to position signal conditioning circuit 120 for transmitting sensor output signals V SENSE or displacement signal. Figure 2 The interconnect 208 is preferably configured to combine the output signals from the two sensors 114 so that when a first air gap or separation distance between a first one of the two sensors 114 and the rotatable member 102 is equal to a second air gap or separation distance between a second one of the two sensors 114 and the rotatable member 102, their output signals cancel each other.

[0018] In one embodiment, Figure 2 The sensor assembly 202 includes oscillator circuits 204, 206 that drive the sensor 114 to generate a sensor output signal V at an interconnect 208. SENSE . Output signal V SENSE The position signal conditioning circuit 120 is fed into the position signal conditioning circuit 120. In the illustrated embodiment, the position signal conditioning circuit 120 includes an analog amplifier 212 for increasing or decreasing V SENSE The oscillator 204, 206 of each driving circuit provides a gain for modulating V SENSE After the amplifier 212, the position signal conditioning circuit 120 includes a demodulator 214 for demodulating V SENSE , generating a demodulated displacement signal. Figure 2 The position signal conditioning circuit 120 further conditions the demodulated displacement signal using a low pass filter 216 .

[0019] After demodulation and filtering, the 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 an analog offset before the analog-to-digital converter (ADC) 222 converts the compensated displacement signal into a digital position signal, which is then output to the suspension controller 110. In one embodiment, the offset generator 220 includes at least one of a digital-to-analog converter (DAC), a digital potentiometer, and an adjustable resistor. In other words, the variable offset is generated using a DAC, but it can also be generated in different ways, including using a digital potentiometer or manual resistor tuning. It is important to perform compensation in the analog circuit to maximize the sensor resolution read by the ADC 222. In an example operation, the inductive sensor 114 driven by the oscillator 204, 206 generates analog sensor data and provides it 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 an analog output before conversion to a digital signal maximizes the input range of ADC 222, which results in increased sensor resolution. In addition, DC offset compensation is easy to control and extremely accurate. The offset generator 220 (e.g., implemented by a DAC) is digitally controllable while providing an analog offset. This allows the circuit to be easily tuned to correct any zero shift error locally or remotely.

[0021] Figure 3 Examples of alternative placements of inductive sensors 114 are shown. As shown, sensors 114a, 114b include a double-ended radial sensor arrangement for measuring the position of the suspended rotatable member 102 relative to the axis 116. Sensors 114c, 114d include a double-ended axial sensor arrangement for measuring the position of the suspended rotatable member 102 relative to a different axis 316 (shown as perpendicular to the axis 116). Sensor 114e includes a single-ended sensor arrangement for measuring the position of the suspended rotatable member 102 relative to the axis 316 (or axis 116), and sensor 114f includes a reference sensor for measuring the position of the suspended rotatable member 102 relative to a fixed object 318. Those skilled in the art will recognize that aspects of the present disclosure can be applied to different physical arrangements, wherein the sensor 114 is spaced from the shaft 102 from an axis that is not necessarily the axis of rotation 116.

[0022] The following are examples of offset compensation according to aspects of the present disclosure. Those skilled in the art who are familiar with conventional quadrature compensation will recognize that the digital offset compensation described herein produces the same net result as quadrature compensation.

[0023] In this example, the inductive sensor output is demodulated in phase with the excitation voltage, an analog offset is added, and then the compensated sensor output is low-pass filtered and converted from analog to digital. The process of tuning the offset compensation involves: placing the rotor at zero position; reading the output voltage of the low-pass filter; and adding an offset that is equal and opposite to the low-pass filter output. If the sensor is balanced, no compensation is required and the offset compensation value is 0. The sensor output is 0V, indicating zero displacement.

[0024] On the other hand, unbalanced sensors require compensation to achieve a 0V sensor output at zero displacement. To compensate for an unbalanced sensor configuration, a DC offset is added to the low pass filter output. An unbalanced sensor can be simulated by raising or lowering the simulated sensor inductance value to see a comparative response to position changes of the unbalanced sensor. For comparison, the offsets for changing the inductance of L1 (top) or L2 (bottom) are shown in Tables I and II below.

[0025] Table I

[0026] 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

[0027] Table II

[0028] 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

[0029] Ideal offset compensation (no phase shift in the sensor output) is given by:

[0030] Take the ideal sensor output as S out ,

[0031] S out =Asin(ωt)

[0032] The demodulated output is D out ,

[0033] D out =Asin(ωt)*sin(ωt)

[0034]

[0035] Take the offset value as The sum of the offset value and the demodulated output is O out ,

[0036]

[0037] After low-pass filtering and substituting after,

[0038]

[0039] The non-ideal offset compensation (the sensor output has a phase shift) is determined by:

[0040] If the sensor is not ideal

[0041]

[0042] From the sum / difference identity

[0043] S out =A(αsin(ωt)+βcos(ωt))

[0044] After demodulation

[0045] D out =A(αsin 2 (ωt)+βcos(ωt)sin(ωt))

[0046]

[0047] Adding an offset value after

[0048]

[0049] After low-pass filtering and substituting after

[0050]

[0051] In operation, a processor, computer, and / or server may execute processor-executable instructions (e.g., software, firmware, and / or hardware), such as those described herein, to implement aspects of the present disclosure. The processor-executable instructions may be organized into one or more processor-executable components or modules on a tangible processor-readable storage medium.

[0052] Unless otherwise noted, the order of execution or implementation of the operations of the various aspects of the present disclosure shown and described herein is not important. That is, the operations can be performed in any order, unless otherwise noted, and embodiments may include more or less operations than the operations disclosed herein. For example, it is expected that it is within the scope of the present disclosure to perform or implement a particular operation before, simultaneously with, or after another operation.

[0053] When introducing elements of the embodiments, the articles "a," "an," "the," and "said" are intended to mean that there are one or more of the elements. The terms "comprising," "including," and "having" are intended to be inclusive and mean that there may be additional elements other than the listed elements.

[0054] All depicted components illustrated or described may not be required. In addition, some embodiments and examples may include additional components. The arrangement and type of components may be varied without departing from the spirit or scope of the claims set forth herein. In addition, different or fewer components may be provided, and components may be combined. Alternatively or in addition, a component may be implemented by multiple components.

[0055] The above description illustrates embodiments by way of example and not limitation. This description enables those skilled in the art to make and use various aspects of the present disclosure, and describes several embodiments, adaptations, variations, substitutions, and uses of various aspects of the present disclosure, including what are currently considered to be the best modes for performing various aspects of the present disclosure. In addition, it should be understood that the various aspects of the present disclosure are not limited in their application to the details of the construction and arrangement of the components set forth in the following description or shown in the accompanying drawings. Various aspects of the present disclosure can have other embodiments and can be practiced or implemented in various ways. In addition, it should be understood that the words and terms used herein are for descriptive purposes and should not be considered restrictive.

[0056] It is obvious that modifications and variations are possible without departing from the scope defined in the appended claims. As various changes may be made to the above structures and methods without departing from the scope of the present disclosure, all matter contained in the above description and shown in the accompanying drawings should be interpreted as illustrative rather than restrictive.

[0057] In view of the above, it will be seen that the several advantages of aspects of the present disclosure are achieved and other advantageous results attained.

[0058] The Abstract and Summary are provided to help the reader quickly determine 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. This Summary is provided to introduce some concepts that will be further described in the Detailed Description in a simplified form. This Summary is not intended to identify key features or essential features of the claimed subject matter, nor is it intended to be used to aid in determining the claimed subject matter.

Claims

1. A sensor assembly for use with a magnetic bearing system configured to levitate an object, the sensor assembly comprising: first and second inductive sensors spaced apart from the suspended object, the first inductive sensor configured to sense a first air gap and the second inductive sensor configured to sense a second air gap; a plurality of drive circuits electrically connected to the first and second inductive sensors, each drive circuit configured to provide an input signal to a respective one of the first and second inductive sensors so that the first inductive sensor generates an output signal representative of the first air gap and the second inductive sensor generates an output signal representative of the second air gap; an interconnection electrically connecting the first and second inductive sensors to each other and combining output signals therefrom to output a displacement signal, wherein when the suspended object is at the predetermined position, the output signals from the first and second inductive sensors cancel each other at the interconnection such that the first and second air gaps are equal to each other, and wherein when the suspended object is at the predetermined position, a non-zero displacement signal at the interconnection indicates a displacement error; as well as A signal conditioning circuit is electrically connected to the interconnect, the signal conditioning circuit receiving and responsive to the displacement signal and including an offset generator for adding an analog offset to compensate for the displacement error, the signal conditioning circuit being configured to generate a position signal representing the position of the suspended object relative to the predetermined position based on the compensated displacement signal having the analog offset.

2. The sensor assembly according to claim 1, wherein: The first air gap represents a distance from the first sensor to the suspended object, and the second air gap represents a distance from the second sensor to a fixed object.

3. The sensor assembly according to claim 1, wherein: Each drive circuit includes an oscillator.

4. The sensor assembly according to claim 3, wherein: The drive circuit includes a first drive circuit and a second drive circuit, and wherein an oscillator of the first drive circuit is 180° out of phase with an oscillator of the second drive circuit.

5. The sensor assembly according to claim 3, wherein: The oscillator of each driver circuit provides a suppressed carrier for modulating the displacement signal, and wherein the signal conditioning circuit includes a demodulator for demodulating the displacement signal before adding the analog offset.

6. The sensor assembly according to claim 1, wherein: The signal conditioning circuit includes an analog-to-digital converter (ADC) configured to convert the compensated displacement signal from analog to digital after the analog offset is added to the displacement signal, and wherein the position signal output from the signal conditioning circuit is a digital signal.

7. The sensor assembly according to claim 1, wherein: The offset generator includes at least one of a digital-to-analog converter (DAC), a digital potentiometer, and an adjustable resistor.

8. A magnetic bearing assembly comprising: a plurality of active magnetic bearings configured to levitate a rotatable member rotatable about a central axis; a plurality of inductive sensors, each associated with a respective one of the magnetic bearings, each inductive sensor positioned radially outward from the central axis and spaced from the rotatable member and configured to sense an air gap between the inductive sensor and the rotatable member; a plurality of drive circuits electrically connected to the inductive sensors, each drive circuit 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 an air gap; an interconnect that electrically connects the inductive sensors to each other and combines output signals therefrom to output a displacement signal, wherein the output signals from the inductive sensors cancel each other at the interconnect when the rotatable member is centered about the central axis, and wherein a non-zero displacement signal at the interconnect when the rotatable member is centered about the central axis indicates a displacement error; a signal conditioning circuit electrically connected to the interconnect, the signal conditioning circuit receiving and responsive to the displacement signal and including an offset generator for adding an analog offset to compensate for displacement errors, the signal conditioning circuit configured to generate a position signal representative of a radial position of the rotatable member relative to the central axis based on the displacement signal having the offset; as well as A magnetic bearing controller receives and is responsive to the position signal for controlling the active magnetic bearing to minimize radial displacement of the rotatable member relative to the central axis.

9. The magnetic bearing assembly according to claim 8, wherein: The displacement signal represents an amount of displacement of the rotatable member relative to the central axis.

10. The magnetic bearing assembly according to claim 8, wherein: Each drive circuit includes an oscillator.

11. The magnetic bearing assembly according to claim 10, wherein: The drive circuit includes a first drive circuit and a second drive circuit, and wherein an oscillator of the first drive circuit is 180° out of phase with an oscillator of the second drive circuit.

12. The magnetic bearing assembly according to claim 10, wherein: The oscillator of each driver circuit provides a suppressed carrier for modulating the displacement signal, and wherein the signal conditioning circuit includes a demodulator for demodulating the displacement signal before adding the analog offset.

13. The magnetic bearing assembly according to claim 8, wherein: The signal conditioning circuit includes an analog-to-digital converter (ADC) configured to convert the compensated displacement signal from analog to digital after the analog offset is added to the displacement signal, and wherein the position signal output from the signal conditioning circuit is a digital signal.

14. The magnetic bearing assembly according to claim 8, wherein: The offset generator includes at least one of a digital-to-analog converter (DAC), a digital potentiometer, and an adjustable resistor.

15. A method of controlling a magnetic bearing assembly, the magnetic bearing assembly comprising a plurality of active magnetic bearings, the active magnetic bearings being configured to levitate a rotatable member, the rotatable member being rotatable about a central axis, the method comprising: providing input signals to a plurality of inductive sensors, wherein each inductive sensor is associated with a respective one of the magnetic bearings and is positioned radially outward from the central axis and spaced apart from the rotatable member; sensing an air gap between each inductive sensor and the rotatable member; receiving an output signal from each inductive sensor representing a sensed air gap; combining output signals from each inductive sensor at an electrical interconnection to output a displacement signal, wherein the output signals from the inductive sensors cancel each other at the interconnection when the rotatable member is centered about the central axis, and wherein a non-zero displacement signal at the interconnection when the rotatable member is centered about the central axis indicates a displacement error; receiving a displacement signal and adding an analog offset thereto to compensate for the displacement error; generating a position signal indicative of a radial position of the rotatable member relative to the central axis based on the compensated displacement signal having an offset; as well as The active magnetic bearing is controlled based on the position signal to minimize radial displacement of the rotatable member relative to the central axis.

16. The method according to claim 15, wherein: The displacement signal represents an amount of displacement of the rotatable member relative to the central axis.

17. The method according to claim 15, wherein: Each drive circuit includes an oscillator.

18. The method according to claim 17, wherein: The drive circuit includes a first drive circuit and a second drive circuit, and wherein an oscillator of the first drive circuit is 180° out of phase with an oscillator of the second drive circuit.

19. The method of claim 15, further comprising providing a suppressed carrier for modulating the displacement signal, and demodulating 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 after adding the analog offset to the displacement signal, and wherein The position signal comprises a digital signal.

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