HARMONIC ELIMINATION CIRCUIT, POSITION DETECTION DEVICE, MAGNETIC BEARING DEVICE AND VACUUM PUMP

Abstract

Provided is a circuit that eliminates harmonics generated in a synchronous detection circuit to achieve low vibration and low noise, along with a position detection device, a magnetic bearing device, and a vacuum pump. An odd-order harmonic of a sensor carrier frequency can be eliminated from a displacement signal by setting a duty of a switch of the synchronous detection circuit to a specified value. Conditions for a pulse generation method are adjusted to generate a pulse at a phase angle of 180 degrees + 360 degrees x n. A duty of a pulse for a synchronous detection switch is set such that a positive-side area and a negative-side area of a harmonic waveform are equal to each other. Moreover, the pulse duty is adjusted to center the phase angle at which a sensor signal has a highest sensitivity.

Claims

1. A harmonic elimination circuit that eliminates configured to eliminate a harmonic signal from an ac waveform signal on which the harmonic signal is superimposed, wherein a duty of a pulse synchronized with the ac waveform signal is set such that a positive-side area and a negative-side area of the harmonic signal are equal to each other.

2. The harmonic elimination circuit according to claim 1, wherein the duty of the pulse is generated through switching to any of an output resulting from inverting amplification, an output resulting from non-inverting amplification, or a zero output.

3. The harmonic elimination circuit according to claim 1, wherein the pulse includes at least one pulse during a half period of the ac waveform signal, and the duty of the pulse is symmetrically generated with respect to a peak value of the ac waveform signal in a phase progression direction.

4. The harmonic elimination circuit according to claim 3, wherein, to eliminate a third-order harmonic, the pulse is generated from one pulse having a duty of 2Π/3 [rad] during the half period of the ac waveform signal.

5. The harmonic elimination circuit according to claim 3, wherein, to eliminate a fifth-order harmonic, the pulse is generated from one pulse having a duty of 4Π/5 [rad] during the half period of the ac waveform signal.

6. The harmonic elimination circuit according to claim 3, wherein, to eliminate a seventh-order harmonic, the pulse is generated from one pulse having a duty of 6Π/7 [rad] during the half period of the ac waveform signal.

7. The harmonic elimination circuit according to claim 3, wherein, to simultaneously eliminate a third-order harmonic and a fifth-order harmonic, the pulse is generated from three pulses each having a duty of 2Π/3 [rad] during the half period of the ac waveform signal.

8. The harmonic elimination circuit according to claim 3, wherein, to simultaneously eliminate a third-order harmonic and a seventh-order harmonic, the pulse is generated from three pulses each having a duty of 2Π/3 [rad] during the half period of the ac waveform signal.

9. The harmonic elimination circuit according to claim 3, wherein, to simultaneously eliminate a third-order harmonic, a fifth-order harmonic, and a seventh-order harmonic, the pulse is generated from seven pulses each having a duty of 2Π/3 [rad] during the half period of the ac waveform signal.

10. The harmonic elimination circuit according to claim 1, wherein, a duty that generates a sine wave PWM is used to generate the duty of the pulse.

11. The harmonic elimination circuit according to claim 2, wherein a switch that disconnects and connects power is controlled with timing of the zero output.

12. A position detection device comprising: a position detection unit that detects a position of an object; a carrier-wave-signal supply unit that supplies a carrier wave signal to the position detection unit; and a detection unit that detects a position signal resulting from the detection by the position detection unit through switching using a switch synchronized with the carrier wave signal, wherein the position signal is formed of an ac waveform signal on which a harmonic signal is superimposed, and a duty of a pulse that drives the switch is set such that a positive-side area and a negative-side area of the harmonic signal are equal to each other.

13. A magnetic bearing device comprising: a position detection unit that detects a position of an object in non-contact relation; a magnetic bearing unit that controls the position of the object by using an electromagnet; a carrier-wave-signal supply unit that supplies a carrier wave signal to the position detection unit; and a detection unit that detects a position signal resulting from the detection by the position detection unit through switching using a switch synchronized with the carrier wave signal, wherein the position signal is formed of an ac waveform signal on which a harmonic signal is superimposed, and a duty of a pulse that drives the switch is set such that a positive-side area and a negative-side area of the harmonic signal are equal to each other.

14. A vacuum pump comprising: a rotating body; a position detection unit that detects a position of the rotating body in non-contact relation; a magnetic bearing unit that controls the position of the rotating body by using an electromagnet; a carrier-wave-signal supply unit that supplies a carrier wave signal to the position detection unit; and a detection unit that detects a position signal resulting from the detection by the position detection unit through switching using a switch synchronized with the carrier wave signal, wherein the position signal is formed of an ac waveform signal on which a harmonic signal is superimposed, and a duty of a pulse that drives the switch is set such that a positive-side area and a negative-side area of the harmonic signal are equal to each other.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0058] FIG. 1 is a diagram of a position detection circuit as an example of the present disclosure.

[0059] FIG. 2 is a diagram illustrating a relationship between a fundamental wave and a third-order harmonic component.

[0060] FIG. 3 illustrates an example of typical set values of a pulse width for eliminating odd-order harmonics.

[0061] FIGS. 4A to 4H are waveform charts each illustrating a relationship between a fundamental wave and a pulse duty when a pulse is set in the example of the set values in FIG. 3.

[0062] FIGS. 5A to 5D are diagrams each illustrating an example of a waveform of a sensor dc signal and a degree of elimination of the harmonic.

[0063] FIG. 6 illustrates an example of respective pulse waveforms from individual switches including an inverting switch, a zero-output switch, and a non-inverting switch within one period (0 to 2Π).

[0064] FIG. 7 illustrates an even-order harmonic noise component of a carrier frequency.

[0065] FIG. 8 is a diagram illustrating a method of generating a duty of the pulse.

[0066] FIGS. 9A and 9B are diagrams each illustrating a method of removing high-frequency switching spike noise.

[0067] FIG. 10 is a longitudinal cross-sectional view of a turbo molecular pump.

[0068] FIG. 11 is a diagram of a conventional position detection circuit.

DETAILED DESCRIPTION

[0069] A description will be given below of an example of the present disclosure. FIG. 1 illustrates a diagram of a position detection circuit as the example of the present disclosure. Note that the same components as those in FIG. 11 are denoted by the same reference numerals, and a description thereof is omitted.

[0070] As can be seen from a comparison with FIG. 11 which is a diagram of a conventional position detection circuit, the position detection circuit in the example of the present disclosure is different from the conventional position detection circuit in that a zero switch 27 is provided to have one end connected to ground 25 and another end connected to the connection point 21 between the inverting switch 13 and the non-inverting switch 15. To the zero switch 27, an operation signal 29 is input.

[0071] Next, a description will be given of effects of the example of the present disclosure.

[0072] A synchronous detection switching operation is equivalent to multiplication between an input signal and a rectangular wave. Accordingly, in a synchronous detection output, not only a fundamental wave component, but also a result of multiplication between Fourier transform f(x) of the rectangular wave shown in Numerical Expression 1 and odd-order harmonic noise appears as a dc signal.

[00001]$\begin{array}{cc}\text{f}\left(x\right)=\frac{4}{\pi }\left\{sin\left(x\right)+\frac{1}{3}sin\left(3x\right)+\frac{1}{5}sin\left(5x\right)+\frac{1}{7}sin\left(7x\right)+.Math.\right\}& \text{­­­[Math. 1]}\end{array}$

[0073] For example, when an input signal is contaminated with third-order harmonic noise of a sensor carrier frequency as shown in Numerical Expression 2, an output signal is contaminated with a noise component as shown in Numerical Expression 3.

[00002]$\begin{array}{cc}\text{Vni}=Asin\left(3x\right)& \text{­­­[Math. 2]}\end{array}$

[00003]$\begin{array}{cc}\text{Vno}={\displaystyle {\int }_0^{\pi }\left(Vni\times f\left(x\right)\right)dx}\simeq {\displaystyle {\int }_0^{\pi }\left(Asin\left(3x\right)\times \frac{4}{3\pi }sin\left(3x\right)\right)dx=\frac{2A}{3}}& \text{­­­[Math. 3]}\end{array}$

[0074] This occurs since, as illustrated in a diagram representing a relationship between a fundamental wave and a third-order harmonic component in FIG. 2, the inverting switch 13 and the non-inverting switch 15 each for the synchronous detection illustrated in FIG. 11 are turned ON during a 180-degree period corresponding to a half wave of the fundamental wave, and consequently the third-order harmonic component is integrated over 180 degrees × 3 = 540 degrees, i.e., over 1.5 periods to result in generation of a dc component.

[0075] As a solution to this problem, in the present disclosure, a duty of a pulse for operating each of the synchronous detection switches, the number of the pulses, and the phase of the pulse are adjusted to prevent appearance of the harmonic component in the output.

[0076] For this purpose, an input to the synchronous detection switches is changed from a conventional non-inverted-signal and inverted-signal two mode operation as illustrated in FIG. 11 to a non-inverted signal, inverted signal, and zero output (no input) three mode operation as illustrated in FIG. 1. Then, each of the pulse duties is set to a specified value. The zero output may be formation of 0 volts with a circuit, but may also be processed with software on the assumption that a signal is not received as an input to a smoothing circuit 23 during a zero-output-mode period.

[0077] The odd-order harmonics of the sensor carrier frequency can be eliminated from a displacement signal after detection by setting the duty of each of the switches of the synchronous detection circuit to a specified value.

[0078] Conditions for a pulse generation method are basically adjusted such that pulses are generated at phase angles of 180 degrees + 360 degrees × n (n is a positive integer including 0). Then, the duty of the pulse from each of the synchronous detection switches is set such that a positive-side area and a negative-side area of a harmonic waveform are equal to each other.

[0079] In addition, the duty of the pulse is adjusted such that phase angles at which a sensor signal has a highest sensitivity are centered. In other words, when the sensor signal is a 360-degree sine wave, the duty of the pulse is preferably adjusted around the phase angles of 90 degrees and 270 degrees.

[0080] FIG. 3 illustrates an example of typical set values of a pulse width for eliminating the odd-order harmonics which are calculated on the basis of the conditions. FIG. 3 illustrates timings of turning ON/OFF the inverting switch 13 for an angle (0 to Π) of a half sine wave of a fundamental wave.

[0081] FIGS. 4A to 4H are waveform charts illustrating a relationship between the fundamental wave and the pulse duty when the pulse is set in the example of the set values in FIG. 3.

[0082] For example, when a third-order harmonic is to be eliminated, the duties in the individual modes are assumed to be such that, of phase angles of 0 to 360 degrees of a full wave of the carrier frequency, 0 to 30 degrees are for the zero output mode, 30 to 150 degrees are for the non-inverting mode, 150 to 210 degrees are for the zero output mode, 210 to 330 degrees are for the inverting mode, and 330 to 360 degrees are for the zero output mode.

[0083] Accordingly, 120 degrees (= 150 - 30 degrees) during a non-inverting operation is equivalent to 360 degrees (120 degrees x 3) in the third-order harmonic, and the third-order harmonic is sampled exactly for one period. Consequently, the third-order harmonic does not appear as positive or negative noise in the output of the synchronous detection circuit.

[0084] FIG. 4C illustrates an example in which, to eliminate such a third-order harmonic, as the switch for the synchronous detection, one pulse having a duty of 120 degrees is used.

[0085] The inverting switch 13 is turned OFF when an angle of the fundamental wave is between and including Π and 2Π.

[0086] Meanwhile, the non-inverting switch 15 is turned OFF when the angle of the fundamental wave is between and including 0 and Π. ON/OFF timing for the non-inverting switch 15 when the angle of the fundamental wave is between and including Π and 2Π is set to a value obtained by adding (Π) to the angle in the example of the set values illustrated in FIG. 3.

[0087] Note that the zero-output switch is turned ON while each of the non-inverting switch 15 and the inverting switch 13 is OFF.

[0088] As illustrated in the example of the set values in FIG. 3 and in the waveform charts in FIGS. 4A to 4H, by appropriately controlling timing for the switch for each of the modes and the number of pulses, it is possible not only to independently eliminate the third-order harmonic, the fifth-order harmonic, and the seventh-order harmonic, but also to simultaneously remove, e.g., the third- and fifth-order harmonics, the third- and seventh-order harmonics, and the third-, fifth-, and seventh-order harmonics.

[0089] Each of FIGS. 5A to 5D illustrates an example of a waveform of a dc signal at the connection point 21 for the synchronous detection pulse set as described above and additionally illustrates a result of calculating a degree to which a high-order harmonic is eliminated. In other words, each of FIGS. 5A to 5D illustrates an example of a fundamental wave to be originally detected, a waveform when a detection duty is changed for a waveform contaminated with harmonic noise, and a value of a component of a dc signal.

[0090] As illustrated in FIG. 5A, conventional control uses a monopulse having a duty of 180 degrees. At this time, a dc component of the fundamental wave is 0.636, while a dc component at the time of harmonic noise contamination is 0.699, and a 9.9% noise component is included. Meanwhile, as illustrated in FIG. 5B, to eliminate the third-order harmonic, the present example uses a monopulse having a duty of 120 degrees. As a result, the dc component of the fundamental wave is 0.550, while the dc component at the time of harmonic noise contamination is 0.537, and a 2.5% noise component is included. Thus, the harmonic noise can be reduced to about ¼ of that in the conventional example.

[0091] In FIG. 5C, to eliminate the third-order harmonic and the fifth-order harmonics, three pulses each having a duty of 120 degrees are used, as illustrated in the drawing. Pulse waveforms for the individual switches including the inverting switch, the zero-output switch, and the non-inverting switch in one period (0 to 2Π) at this time are illustrated in FIG. 6.

[0092] As a result, the dc component of the fundamental wave is 0.526, while the dc component at the time of harmonic noise contamination is 0.530, and a 0.7% noise component is included. Thus, the noise component can drastically be reduced to about 7% of that in the conventional example. Meanwhile, in FIG. 5D, to eliminate the third-order harmonic, the fifth-order harmonic, and the seventh-order harmonic, seven pulses each having a duty of 120 degrees are used, as illustrated in the drawing. As a result, the dc component of the fundamental wave is 0.510, while the dc component at the time of harmonic noise contamination is 0.510, and the noise component can be reduced to 0.0%.

[0093] Thus, it will be understood that, by appropriately setting the duty of the synchronous detection pulse, it is possible to efficiently eliminate the harmonic noise.

[0094] Note that, when the third-order harmonic is eliminated, odd-order harmonics related to the third-order harmonic are also simultaneously eliminated. In other words, 3rd-order x 3 = 9th-order harmonic, 3rd-order x 5 = 15th-order harmonic, ... and the like are simultaneously eliminated. Likewise, when the fifth-order harmonic is eliminated, odd-order harmonics related to the fifth-order harmonic are also simultaneously eliminated. Likewise, when the third-, fifth- and seventh-order harmonics are simultaneously eliminated, the third-, fifth-, seventh-, and ninth-order harmonics are eliminated, and it follows that all the single-digit harmonics can be eliminated.

[0095] Note that, as illustrated in FIG. 7, when, e.g., the third-order harmonic is to be eliminated, an even-order harmonic noise component of the carrier frequency has a reverse-polarity dc component resulting from integration of the inverted signal and the non-inverted signal with each other. Since a positive-side area and a negative-side area in a portion overlapping the pulse duty are equal to each other and are cancelled out, the even-order harmonic noise component does not appear in the output.

[0096] As a method of generating the pulse duty, when settings exactly as illustrated in the setting example in FIG. 3 are made using a microcomputer or the like, high-accuracy harmonic elimination as illustrated in FIGS. 5A to 5D is possible. However, the pulse duty can also be generated as follows, though the accuracy of harmonic elimination slightly decreases.

[0097] In other words, as the pulse duty, a duty that generates a sine wave PWM can be used. The duty that generates the sine wave PWM is obtained by dividing one period of a sine wave into a plurality of time periods and determining, for each of the time periods resulting from the division, a duty of a rectangular wave having an average amplitude substantially equal to an average amplitude of the sine wave. For example, as illustrated in FIG. 8, respective amplitudes of the sine wave from the oscillator and a triangular wave of a PWM frequency are compared to each other, and the switch is turned ON during a period during which the amplitude of the sine wave is larger. Thus, the pulse duty is generated by simple processing, and the harmonic noise can be eliminated.

[0098] As described above, by setting the duty of each of the switches in the synchronous detection circuit to the specified value, it is possible to eliminate, from the displacement signal after the detection, switching noise of odd-order harmonic components of the sensor carrier frequency with which the displacement sensor signal is contaminated. Therefore, it is possible to implement a low-vibration and low-noise magnetic bearing device with no need to add a high-cost component and without increasing cost.

[0099] Next, a description will be given of a method of eliminating high-frequency switching spike noise generated in the electromagnet power amplifiers or in the inverter for driving the motor 121.

[0100] In the turbo molecular pump 100, the electromagnet power amplifiers and the inverter for driving the motor 121 perform PWM control over power. At the moment when each of the switches is turned ON/OFF during a power switching operation, an abrupt voltage change occurs in the electromagnets and in the motor 121, and consequently extremely-high-frequency switching spike noise may be generated.

[0101] The spike noise has an extremely short duration time compared to one period of the displacement sensor and a high frequency. Accordingly, when the sensor signal is contaminated also with the spike noise, the spike noise may appear in a dc signal from the displacement sensor.

[0102] In the present example, there is the zero output (no input) mode in which the sensor signal is not transmitted to the output. Therefore, by controlling the power switch circuit such that power switches are turned ON/OFF during the zero output period, the spike noise is prevented from contaminating the sensor signal.

[0103] Specifically, power switching frequencies of the electromagnet power amplifiers and the inverter for driving the motor 121 are synchronized with an even order of the carrier frequency of the displacement sensor to effect ON/OFF timing for the power switches in the vicinity of 0 degrees and 180 degrees of a sine fundamental wave of the sensor.

[0104] Note that, when the power switching frequencies are synchronized with an odd order of the carrier frequency of the displacement sensor, the odd order of the carrier frequency is likely to appear in the sensor output, and therefore the synchronization of the power switching frequency with the odd order of the carrier frequency is preferably avoided.

[0105] Due to the PWM control, the ON/OFF timing for the power switches varies around a center value, but is overall concentrated on the vicinities of the 0 degrees and the 180 degrees of the sine fundamental wave.

[0106] FIG. 9A illustrates a fundamental wave to be originally detected, combined third-, fifth-, and seventh-order harmonic noise, and a waveform (full harmonic) contaminated with the spike noise in the conventional example. At this time, a pulse is a monopulse, and the detection duty is 180 degrees.

[0107] By contrast, FIG. 9B illustrates waveforms when, to eliminate the third-order harmonic, a monopulse is used, and the detection duty is changed to 120 degrees. In addition, an ordinate axis represents a value of a dc component. As can be seen from a comparison between FIGS. 9A and 9B, the spike noise conventionally observed in the vicinities of the 0 degree and the 180 degree has disappeared.

[0108] In other words, by concentrating the ON/OFF timing for each of the electromagnet power amplifier switches during normal operation or the like on the vicinities of 0 degrees and 180 degrees of the sensor carrier of the displacement sensor and setting the switch of the sensor in the vicinity thereof to the zero output, it is possible to reduce the possibility of contamination of the sensor signal with noise from the electromagnet power amplifiers or the like.

[0109] Note that, various modifications can be made in the present disclosure without departing from the spirit of the present disclosure. The example and the individual modifications each described above can variously be combined with each other.