BRIDGE VOLTAGE INVERSION CIRCUIT FOR VACUUM GAUGE AND PRESSURE GAUGE SENSOR HAVING THE VOLTAGE INVERSION CIRCUIT

Abstract

The disclosed invention provides a bridge voltage inversion circuit for vacuum gauge and a pressure gauge sensor that includes the bridge voltage inversion circuit. The bridge voltage inversion circuit for a pressure gauge includes a reference capacitance, a sensor capacitance, and a transformer including a primary winding and a secondary winding that outputs a bridge voltage. The reference capacitor is connected to a first side of the secondary winding of the transformer, and the sensor capacitor is connected to a second side of the secondary winding of the transformer. The sensor capacitor senses and responds to a pressure, and a capacitance of the sensor capacitor is at a minimum when the pressure is at vacuum. The capacitance of the sensor capacitor at vacuum is less than a capacitance of the reference capacitor.

Claims

1. A bridge voltage inversion circuit for a pressure gauge, comprising: a transformer including a primary winding and a secondary winding that outputs a bridge voltage; a reference capacitor connected to a first side of the secondary winding of the transformer; and a sensor capacitor connected to a second side of the secondary winding of the transformer, wherein the sensor capacitor senses and responds to a pressure, a capacitance of the sensor capacitor is at a minimum when the pressure is at vacuum, and the reference capacitor and sensor capacitor are selected so that the capacitance of the sensor capacitor at vacuum is less than a capacitance of the reference capacitor, and wherein the bridge voltage is at a maximum amplitude when the pressure is at vacuum and a fold-over-pressure at which the bridge voltage is at the minimum amplitude is greater than a full-scale pressure.

2. The bridge voltage inversion circuit for a pressure gauge of claim 1 further comprising: a circuit that outputs a reference signal that drives the transformer; and a gain setting block for adjusting a signal gain before the transformer.

3. The bridge voltage inversion circuit for a pressure gauge of claim 2 further comprising an analog multiplexer that receives and multiplexes the reference signal and the bridge voltage signal and outputs the multiplexed signals to an analog-to-digital converter.

4. The bridge voltage inversion circuit for a pressure gauge of claim 3 further comprising a buffer that receives and buffers the bridge voltage.

5. The bridge voltage inversion circuit for a pressure gauge of claim 2 further comprising a power OPAMP driver that amplifies the reference signal, wherein the amplified reference signal is supplied to the primary winding of the transformer.

6. The bridge voltage inversion circuit for a pressure gauge of claim 1 further comprising an amplifier connected to the secondary winding of the transformer that receives and amplifies the bridge voltage signal.

7. The bridge voltage inversion circuit for a pressure gauge of claim 1 wherein the sensor capacitor is a diaphragm capacitor.

8. The bridge voltage inversion circuit for a pressure gauge of claim 1 wherein the capacitance of the reference capacitor is ten percent (10%) greater than the capacitance of the sensor capacitor at full scale.

9. A bridge voltage inversion circuit for a pressure gauge comprising: a transformer including a primary winding and a secondary winding that outputs a bridge voltage; a first sensor capacitor connected to a first side of the secondary winding of the transformer; and a second sensor capacitor connected to a second side of the secondary winding of the transformer, wherein the first and second sensor capacitors sense and respond to a pressure, a capacitance of the second sensor capacitor is at a minimum when the pressure is at vacuum, and the first sensor capacitor and the second sensor capacitor are selected so that the capacitance of the second sensor capacitor at vacuum is less than a capacitance of the first sensor capacitor at vacuum, and wherein the bridge voltage is at a maximum amplitude when the pressure is at vacuum and a fold-over pressure at which the bridge voltage is at the minimum amplitude is greater than a full-scale pressure.

10. The bridge voltage inversion circuit for a pressure gauge of claim 9 further comprising: a circuit that outputs a reference signal that drives the transformer; and a gain setting block for adjusting a signal gain before the transformer.

11. The bridge voltage inversion circuit for a pressure gauge of claim 9 further comprising a buffer that receives and buffers the bridge voltage.

12. The bridge voltage inversion circuit for a pressure gauge of claim 10 further comprising a power OPAMP driver that amplifies the reference signal, wherein the amplified reference signal is supplied to the primary winding of the transformer.

13. The bridge voltage inversion circuit for a pressure gauge of claim 9 further comprising an amplifier connected to the secondary winding of the transformer that receives and amplifies the bridge voltage signal.

14. The bridge voltage inversion circuit for a pressure gauge of claim 9 wherein the first sensor capacitor and the second sensor capacitor are diaphragm capacitors.

15. A pressure gauge sensor, comprising: a transformer including a primary winding and a secondary winding that outputs a bridge voltage; a first capacitor with one end at which pressure is applied and another end connected to a first side of the secondary winding of the transformer; and a second sensor capacitor with one end at which the pressure is applied and another end connected to a second side of the secondary winding of the transformer, wherein the first and second sensor capacitors sense and respond to a pressure, a capacitance of the second sensor capacitor is at a minimum when the pressure is at vacuum, and the first sensor capacitor and the second sensor capacitor are selected so that the capacitance of the second sensor capacitor at vacuum is less than a capacitance of the first sensor capacitor at vacuum, and wherein the bridge voltage is at a maximum amplitude when the pressure is at vacuum and a fold-over pressure at which the bridge voltage is at the minimum amplitude is greater than a full-scale pressure.

16. The pressure gauge sensor of claim 15 wherein the first sensor capacitor and the second sensor capacitor are diaphragm capacitors.

17. The pressure gauge sensor of claim 15 further comprising a power OPAMP driver that amplifies a reference signal, wherein the amplified reference signal is supplied to the primary winding of the transformer.

18. The pressure gauge sensor of claim 15 further comprising an amplifier connected to the secondary winding of the transformer that receives and amplifies the bridge voltage signal.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0020] The drawing figures depict one or more implementations in accord with the present concepts, by way of example only, not by way of limitations. In the figures, like reference numerals refer to the same or similar elements.

[0021] FIG. 1 is a circuit diagram of a prior art WYDE Gauge™ vacuum gauge analog front end circuit.

[0022] FIG. 2 is a chart illustrating the voltage amplitude to applied pressure relationship in circuit of FIG. 1.

[0023] FIG. 3 is a diagram illustrating a reference signal and a noisy signal from a prior art vacuum gauge.

[0024] FIG. 4 is a circuit diagram of an embodiment of a bridge voltage inversion circuit for use in a vacuum gauge front end circuit.

[0025] FIG. 5 is a diagram illustrating a reference signal and a clean signal from the embodiment of a bridge voltage inversion circuit shown in FIG. 4.

[0026] FIG. 6 is a diagram illustrating the voltage amplitude to applied pressure relationship of the embodiment of a bridge voltage inversion circuit shown in FIG. 4.

[0027] FIG. 7 is a circuit diagram of an embodiment of a bridge voltage inversion circuit for use with multi-electrode sensors.

[0028] FIGS. 8A and 8B show structures of an exemplary sensor employing the bridge voltage inversion circuit.

DETAILED DESCRIPTIONS

[0029] In this section, some embodiments of the invention will be described more fully with reference to the accompanying drawings, in which preferred embodiments of the invention are shown. This invention, however, may be embodied in many different forms and should not be construed as limited to the embodiments set forth herein. Rather, these embodiments are provided so that this disclosure will be thorough and complete, and will convey the scope of the invention to those skilled in the art. Like numbers refer to like elements throughout, and prime notation is used to indicate similar elements in alternative embodiments.

[0030] Embodiments of a bridge voltage inversion circuit for a vacuum pressure gauge/sensor are described herein. Embodiments of the bridge voltage inversion circuit provide the best signal quality at vacuum. As noted above, pressure readings are proportional to the signal amplitude of the bridge voltage; the mismatch of a reference capacitor current versus a sensor capacitor current provides the pressure reading. The reference capacitor is of fixed capacitance. When pressure changes, the sensor capacitor bridge voltage changes proportional to the change in pressure. At vacuum pressure, the amplitude of the bridge voltage is lost due to the presence of noise, and it is difficult to rely on the mismatch between the reference capacitor and the sensor capacitor. It is most desirable, however, to have the best signal at vacuum. Embodiments achieve this desire by inverting the signal so that the largest, rather than the smallest, capacitance difference (ΔC) is present at vacuum pressure.

[0031] Presently, there is an industry wide push towards lower full scale (FS) pressure gauges (e.g., ≤1 Torr). This undoubtedly presents a new level of challenge to sensor designers who are familiar with the inner workings of higher pressure gauges (e.g., ≥10 Torr). To answer this challenge, without changing the basic fundamental sensing electrical architecture, embodiments of a bridge voltage inversion circuit provide a sensor circuit with inverted amplitude bridge voltage (aka “INA”).

[0032] With reference now to FIG. 4 shown is an embodiment of a vacuum pressure gauge front-end 400 that includes a bridge circuit 408 that inverts the bridge voltage. In vacuum pressure gauge front end circuits, including prior art circuits such as shown in FIG. 1, sensor electronics consists of a transformer circuit design based on a coaxial cable primary and secondary winding. This transformer circuit design forms the so-called “bridge circuit” 408. The term “bridge” in this context means that there are inductance-capacitance resonance circuits on either side of the transformer 410. The transformer 410 includes a primary winding 421 and a secondary winding 422. The bridge voltage is output from the secondary winding. On one side of the bridge circuit 408, there is reference capacitor 412 to ground and one half the inductance from the secondary winding of the transformer 410. On the other side of the bridge circuit 408, there is the sensor capacitor 414 (e.g., a sensor capacitor for which capacitance increases as pressure increases) to ground and the other half of the inductance from the secondary winding 422 of the transformer 410. The reference capacitor 412 is connected to a first side of the secondary winding 422 of the transformer 410, and the sensor capacitor 414 is connected to a second side of the secondary winding 422 of the transformer 410. The sensor capacitor 414 may be a diaphragm capacitor, but can be any capacitor that changes capacitance based on pressure applied to the capacitor. The reference capacitor 412 is a fixed capacitor having a constant capacitance. If the resonance circuits from each side of the bridge circuit 408 are equal in inductance (L) and capacitance (C) values, then bridge voltage is zero. Whenever there is a mismatch in capacitance values, a sinewave with amplitude proportional to the capacitance mismatch amount would be generated.

[0033] In other words, the transduction between pressure and amplitude is generated through the order of pressure, sensor capacitance (C.sub.sensor), capacitance difference (ΔC), and bridge voltage. In prior art circuits, such as shown in FIG. 1, the capacitance values of the reference capacitor and sensor capacitor are chosen so that the bridge voltage (V.sub.amplitude) is proportional to pressure. For example, the capacitance of the reference capacitor may be C.sub.ref and the capacitance of the sensor capacitor at vacuum may be C.sub.sensor. As a result, as pressure increases, a positive ΔC=C.sub.ref−C.sub.sensor will result. At vacuum, C.sub.ref˜C.sub.sensor, the capacitance difference (ΔC) is the highest. The term “full scale pressure” here refers to the maximum pressure where the Gauge's accuracy specification is still guaranteed. For example, many gauges on the market today have full scale in the ranges of 10 Torr or 100 Torr.

[0034] With continuing reference to FIG. 4, in embodiments of vacuum pressure gauge front-end 400 with bridge circuit 408 that inverts the bridge voltage, capacitance values of the reference capacitor 412 and sensor capacitor 414 are chosen so that V.sub.amplitude is inversely proportional to the pressure. For example, the capacitance of the reference capacitor may be ten percent (10%) greater than the capacitance of the sensor capacitor at full scale. Alternatively, the capacitance (C.sub.ref) of the reference capacitor 412 may be fifty percent (50%) greater than the capacitance (C.sub.sensor) of the sensor capacitor 414 at vacuum. For example, the capacitance (C.sub.ref) of the reference capacitor 412 may be around 300 pF, and the capacitance (C.sub.sensor) of the sensor capacitor 414 at vacuum may be around 200 pF. The capacitance of the sensor capacitor 414 increases as the pressure at the sensor increases. Accordingly, in embodiments of vacuum pressure gauge front-end 400 with bridge circuit 408, the capacitance difference (ΔC) would be the most negative at vacuum, and the capacitance difference (ΔC) would approach zero (0) at full scale pressure.

[0035] A sine wave is used to drive embodiments of vacuum pressure gauge front-end 400. In embodiments, the signal spectral purity needs to be guaranteed in order for an on-board DSP engine to perform digital algorithms, for example, an algorithm to digitally filter out “common” noise between V.sub.ref and V.sub.sig. The sinewave signal, which is generated by a signal generator 402, is passed through a gain setting block 404 which may be used to adjust the signal gain. The signal then is passed through a power OpAmp Driver 406 to drive transformer 410. The power OPAMP driver 406 amplifies the signal, and the amplified signal is supplied to the primary winding 421 of the transformer 410. The signal is also passed to the output of the vacuum pressure gauge front-end 400 as V.sub.ref.

[0036] With continued reference to FIG. 4, the output of bridge circuit 408, i.e., the bridge voltage, may be amplified by an amplifier (a power OpAmp driver) 416 and alternatingly passed as V.sub.sig through an analog-to-digital convertor (ADC) 420 with V.sub.ref to the on-board DSP engine (not shown). An analog multiplexer 418 receives the reference sine wave and the bridge voltage signal, multiplexes the reference sine wave and the bridge voltage signal, and outputs the multiplexed signals to an analog-to-digital converter. In embodiments shown, winding of transformer 410 consists of a shielded cable (e.g., a coaxial cable), where the center core carries the bridge voltage signal V.sub.amplitude and the shield is driven by a unity gain buffer 417 with a replica of the center core signal. This is done to minimize the leakage current between the center conductor and the shield through the capacitances that created this current leakage path. In addition, the shield also prevents external EMI sources from interfering with the signal integrity at the winding center conductor.

[0037] In the case of prior art vacuum pressure gauge/sensor, with bridge circuit such as shown in FIG. 1, additional gain/phase adjust and OPAMP circuits are required for tuning the amplitude and phase of the bridge voltage. The need for tuning the amplitude and phase arise from the fact that at vacuum, where the bridge voltage amplitude is the smallest, additional signal gain is required for proper analog-to-digital (AD) conversion. Unfortunately, whenever the signal gain is increased significantly, there is always a risk that the phase relationship between V.sub.ref and V.sub.sig is altered. As the phase relationship between V.sub.ref and V.sub.sig become unpredictable, the digital algorithm between V.sub.ref and V.sub.sig cannot work properly. With the present embodiment of a bridge voltage inversion circuit shown in FIG. 4, these problems are avoided at the most critical pressure levels (i.e., at or near vacuum)-and the additional gain/phase adjust and variable gain circuits shown in FIG. 1 may be omitted.

[0038] With reference now to FIG. 5, shown are the sine waves of V.sub.ref (Reference Wave) and V.sub.sig (clean, large signal wave). The clean, large signal wave shown in FIG. 5 compares quite favorably to the noisy, small signal wave shown in FIG. 2 that is produced by the prior art bridge circuit shown in FIG. 1.

[0039] With reference now to FIG. 6, shown is a voltage-pressure graph showing voltage amplitude to applied pressure relationship of the embodiment of a bridge voltage inversion circuit shown in FIG. 4. Compared to voltage-pressure graph of FIG. 3, we see how bridge signal voltage V.sub.amplitude is at maximum (V.sub.max) at vacuum, slowly decreasing to a minimum (as pressure causes C.sub.sensor to increase until C.sub.ref is around C.sub.sensor which implies ΔC approaches 0). Further pressure increase as C.sub.sensor becomes greater than C.sub.ref causes ΔC to increase again until V.sub.max is approached before the sensor capacitor 414 shorts (the capacitor plates touch), as shown as P.sub.max in FIG. 6. As illustrated by FIG. 6, the bridge voltage is generated from an absolute value of the capacitance difference ΔC between C.sub.ref and C.sub.sensor. Consequently, for C.sub.ref and C.sub.sensor, ΔC will be C.sub.ref−C.sub.sensor at vacuum and will decrease to ˜0 as C.sub.sensor approaches C.sub.ref at P.sub._fo (fold-over pressure). When C.sub.sensor increases above C.sub.ref, ΔC will increase again as C.sub.sensor continues to increase as described herein.

[0040] As described herein, embodiments of the bridge voltage inversion circuit bring several added benefits beyond higher performance at or near vacuum pressure: [0041] No amplification needed.

[0042] An INA analog sensor circuit does not require additional amplification stages. This is a direct result of amplitude inversion on the bridge, i.e., voltage is lower when pressure increases. Since embodiments of the bridge voltage inversion circuit produce a higher voltage at vacuum, the SNR at vacuum is greatly enhanced. [0043] No need for gain and phase adjustment

[0044] The gain and phase adjustments are no longer needed here since the bridge voltage circuit signal does not need to be amplified when pressure is lowered and precise capacitance matching is not necessary either. [0045] Voltage vs. pressure's Linearity monotonicity is guaranteed.

[0046] Because there is no discrete gain stages along the signal path, the voltage-pressure curve can be one continuous curve across the entire full scale range of the gauge. This feature avoids the “stitching” of the voltage-pressure curve decade by decade, which greatly simplifies firmware design. [0047] SNR enhancement.

[0048] At low pressures, the signal to noise ratio is greatly enhanced as evident from the voltage-pressure plot shown in FIG. 5. As pressure steadily increases, the bridge voltage amplitude (V.sub.amplitude) decreases. Bridge voltage amplitude continues to decrease past a full scale mark P.sub._fs (full-scale pressure), eventually diminishing to zero when pressure reaches a “fold over” point P.sub._fo. Past the fold over point P.sub._fo, V.sub.amplitude increases again until it reaches V.sub.max. The diminished signal amplitude at higher pressures should not degrade the reading error percentage at higher pressures. [0049] Signal averaging

[0050] Due to enhanced signal-to-noise ratio (SNR) with V.sub.amplitude being at or near V.sub.max at low pressure levels, there is no need to unnecessarily increase the number of signal averaging in the digital domain. [0051] Reference Capacitor Choices

[0052] During the board assembly process, reference capacitor 412 needs to be matched to sensor capacitor 414 at P.sub._fo where zero amplitude pressure exceeds full scale. The exact location where P.sub._fo is in relation to P.sub._fs and can be flexible enough so that capacitors of certain fixed standard size can be picked with enough tolerances built in, as long as P.sub._fo is greater than P.sub._fs. [0053] Manufacturability

[0054] The easiness of using conventional capacitance sizes greatly enhances manufacturability of the sensors. There is no need for amplitude tuning circuitry for precision capacitance matching at vacuum pressure levels. [0055] Gain setting block and shortage protection

[0056] In the INA configuration of the bridge voltage inversion circuit 400, a gain setting block 404 is used to adjust the transformer 410 primary amplitudes. The purpose of this gain setting block 404 is to allow extra control of the bridge voltage amplitude in the event of pressure sensor short due to over pressure. Another benefit of using gain setting block 404 for shortage protection is to maintain the same amount of power delivered to the reference capacitor 412 in the event of a over pressure condition.

[0057] With reference now to FIG. 7, embodiments of bridge voltage inversion circuit 700 may be used with multi-electrode sensors. As shown in FIG. 7, bridge voltage inversion circuit 700 is used with two sensor capacitors 712 and 714 rather than a single sensor capacitor and a reference capacitor. The sinewave signal, which is generated by a signal generator 702, is passed through a gain setting block 704 which may be used to adjust the signal gain. The signal then is passed through a power OpAmp Driver 706 to drive transformer 710. The power OPAMP driver 706 amplifies the signal, and the amplified signal is supplied to the primary winding 721 of the transformer 710. This transformer 710 circuit design forms the bridge circuit 708. The transformer 710 includes a primary winding 721 and a secondary winding 722. A first sensor capacitor 712 and a second sensor capacitor 714 are part of the multi-electrode sensor and can both vary when pressure is changed. The first sensor capacitor 712 and the second capacity 714 may be diaphragm capacitors, but can be any capacitors that change capacitance based on pressure applied to the capacitors. The first sensor capacitor 712 is connected to a first side of the secondary winding 722 of the transformer 710, and the second sensor capacitor 714 is connected to a second side of the secondary winding 722 of the transformer 710. In the embodiments of the bridge voltage inversion circuit 700, the capacitance C.sub.sen1 of the first capacitor 712 at vacuum is greater than the capacitance C.sub.sen2 of the second capacitor 714 at vacuum. Bridge voltage inversion circuit 700 may be used, because the condition C.sub.sen1>C.sub.sen2 always holds true up to P.sub.max. A multi-electrode of the vacuum gauge sensor is a sensor construction by which external reference capacitors is not used. All sensor related capacitances are integrated in the sensor construction itself. The output of bridge circuit 708, i.e., the bridge voltage, may be amplified by an amplifier (a power OpAmp driver) 716 and alternatingly passed to a buffer 717 that receives and buffers the bridge voltage output from the transformer 710. The output bridge signal from the buffer 717 is passed to an analog-to-digital convertor (ADC) 720 and further to the on-board DSP engine (not shown).

[0058] With reference now to FIGS. 8A-8B, shown are structures of an exemplary sensor that employs the bridge voltage inversion circuit of the claimed invention. FIG. 8A sows a side view of the exemplary sensor, and FIG. 8B shows a front view of the sensor. The sensor 800 includes an inner capacitor 801 and an outer capacitor 802. Consequently, the inverted amplitude (NA) approach not only can be applied to the sensor capacitor/reference capacitor sensor construction, as shown in FIG. 4, the concept can be extended to sensors with integrated multiple electrodes, as shown in FIG. 7. More specifically, in the case of inner and outer electrode with concentric circle construction (other geometry variants possible), as shown in FIG. 8, the capacitance between the inner electrode 811 (sensor capacitor 801) and outer electrode 812 (sensor capacitor 802) with respect to a common ground plane often times cannot be perfectly matched during the manufacturing process. INA's way of intentional mismatch lends itself very well for this type of sensor electrode construction. In the exemplary sensor shown in FIG. 8, the inner capacitor 801 may correspond to the first capacitor 712 shown in FIG. 7, and the outer capacitor 802 may correspond to the second capacitor 714. The capacitance difference between the two sensor sides is a design choice based on many other factors, such as the tension of the diaphragm, spacing between diaphragm and electrode(s), sizes and shapes of each electrodes, etc. Therefore generally speaking, the larger the difference between the capacitances for both electrodes, the larger the signal amplitude we would likely have under vacuum conditions.

[0059] In conclusion, the inverted bridge voltage amplitude method of generating a signal amplitude inversely related to pressure is presented as part of WYDE Gauge™ platform. This method offers many advantages over the prior art in areas such as manufacturability, simplicity, and saving of circuit components while maintaining equal or better performance compared its predecessors. In addition, this method will prove to be invaluable for low pressure gauges at or below 1 Torr full scale.

[0060] The terms and descriptions used herein are set forth by way of illustration only and are not meant as limitations. Those skilled in the art will recognize that many variations are possible within the spirit and scope of the invention.