JP2648966B2 - Vacuum pressure gauge - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to a vacuum pressure gauge that measures the pressure of a gas at a particularly low pressure using a quartz oscillator.

(Technical Background of the Invention and its Problems) Conventionally, it has been known that the resonance frequency, crystal impedance, and the like of a crystal resonator in a resonance state change under the influence of the pressure of the gas in the atmosphere. Thus, a pressure gauge that measures the pressure of a gas is used.

For example, a gas pressure gauge disclosed in Japanese Patent Application Laid-Open No. 62-228126 has a configuration as shown in FIG. 6 and utilizes the fact that the crystal impedance (hereinafter referred to as CI) of a quartz oscillator changes with pressure. The pressure is measured.

That is, in FIG. 6, reference numeral 1 denotes a quartz oscillator placed in a gas atmosphere whose pressure is to be measured. As this crystal resonator, one having an appropriate vibration mode such as thickness-shear vibration and bending vibration can be used.

And 2 is a constant binary voltage with respect to the crystal unit 1;
For example, the switching unit selectively applies the positive and negative DC voltages ± ref having the same absolute value.

Reference numeral 3 denotes a current-voltage converter for converting an output current from the crystal unit 1 into a voltage.

Reference numeral 4 is a phase circuit, for example, a CR differentiating circuit for differentiating the output voltage of the current-voltage converter 3 and performing a phase so as to minimize the impedance of the crystal unit 1.
The output of the phase circuit 4 is supplied to the switching unit 2 via the comparator 5 to perform a switching operation.

Reference numeral 6 denotes an output unit for outputting the output voltage of the current-to-voltage converter 3 as the pressure of the gas in the atmosphere.
6a and an amplifier 6b for amplifying the rectified output.

That is, in the vacuum gauge having the configuration as shown in FIG. 6, the oscillation output of the quartz oscillator 1 is switched to the quartz oscillator 1 via the analog switch 2 which performs the switching operation.
A constant voltage ± ref is applied alternately, and the crystal unit 1 is driven at a constant voltage to resonate at a predetermined resonance frequency. Get
This is amplified as needed and displayed as, for example, pressure characteristics as shown in FIG.

However, in such a device, the CI value hardly changes particularly in a low pressure region of a high vacuum, so that it is difficult to obtain a sufficient resolution particularly in this region. Furthermore, since the quartz oscillator is driven at a constant voltage, there is a problem that a change in the CI value caused by a change in the drive current particularly in a region where the measurement pressure is low becomes a measurement error.

(Object of the Invention) The present invention has been made in view of the above circumstances, and can obtain a good resolution in a low-pressure vacuum region,
Further, it is an object of the present invention to provide a vacuum pressure gauge capable of measuring pressure accurately.

(Summary of the Invention) In the present invention, a quartz oscillator placed in a gas atmosphere whose pressure is to be measured is driven by a constant current and oscillated to obtain a signal proportional to the CI value. It is characterized in that after a voltage corresponding to the voltage is reduced, logarithmic conversion is performed and the result is output as a pressure signal.

(Embodiment) Hereinafter, an embodiment of the present invention will be described in detail with reference to the block diagram of the oscillation unit 10 shown in FIG. 1 and the entire block diagram shown in FIG.

That is, in the figure, reference numeral 11 denotes a quartz oscillator placed in an atmosphere of a gas whose pressure is to be measured. The crystal resonator 11 may be of a suitable vibration mode such as a thickness-slip resonator, a bending resonator, etc. In this embodiment, a tuning fork resonator is used.

Reference numeral 12 denotes an oscillation amplifier having one end of the crystal unit 11 connected to the input end. In this oscillation amplifier 12, a feedback resistor 13 is connected to an input / output terminal so as to obtain an appropriate amplification factor.

Then, the output of the oscillation amplifier 12 is given to a peak detector 13 to detect a peak value.
Is applied to the non-inverting input. The other input of the differential amplifier 14 is supplied with the reference voltage Vref.

The reference voltage Vref is set in consideration of a power supply voltage for driving the circuit element, an appropriate drive level for placing the crystal unit 11 on the resonance rod, a dynamic range of an output signal with respect to a measured pressure, and the like.

The output of the differential amplifier 14 is converted into a direct current through a low-pass filter 15, applied to one fixed input terminal of an analog switch 16, and a ground potential is applied to the other fixed input terminal of the analog switch 16.

The output of this analog switch 16 is connected to a capacitor 17
The signal is fed back to the other terminal of the crystal unit 11 via the.

Accordingly, the output voltage Vosc of the oscillation amplifier 1 is supplied to the peak value detector 13 to obtain the peak-to-peak value, and this value is controlled to be a constant voltage. Is a constant voltage.

Here, the output voltage Vosc, pp of the peak detector 13 is given by the following equation, where Rf is the feedback resistor 13 of the oscillation amplifier, and Vci is the output voltage of the low-pass filter 15.

Vosc, p−p = Vci · 4 / π · Rf / CI (1) When the half-wave peak voltage is detected for peak detection, the following equation is given.

1/2 · Vosc, p−p = Vref (2) The following equation (3) is derived from the equations (1) and (2).

Vci = π / 2 · CI / Rf · Vref (3) That is, since the output voltage Vci of the low-pass filter 15 is proportional to the CI value of the crystal resonator 11, the following equation (4) is given.

Vci = k1 · CI (4) where k1 is a proportional constant and the current Ixtl flowing through the crystal unit 11 is proportional to the value obtained by dividing the output voltage Vci of the low-pass filter 15 by the crystal impedance CI of the crystal unit 11. Therefore, the following equation (5) is given.

Ixtl = k2 · Vci / CI (5) where k2 is a proportional constant. By substituting the above equation (4) into the equation (5), the following equation (6) is obtained.

Ixtl = k2 · k1 (6) Therefore, the current flowing through the crystal resonator 11 is constant, that is, the crystal resonator 11 can be driven with a constant current.

Further, from the above equation (4), the output voltage Vci of the low-pass filter 15 becomes a signal substantially proportional to the CI of the crystal unit 11,
The relationship between the value and the pressure is as shown in FIG. 3, for example, when the pressure measurement target is nitrogen gas (N2).

That is, in FIG. 3, the horizontal axis is pressure (Torr) and the vertical axis is CI
(Kilo ohms), each of which is displayed on a logarithmic scale, and a sufficient resolution cannot be obtained in a low pressure vacuum region as it is.

For this reason, as shown in the block diagram of FIG. 2, the output signal Vci of the low-pass filter 15 is applied to the non-judgment input of the second differential amplifier 21, and a very low pressure, for example, a pressure of 10 ⁻⁵ Torr or less (at this time, The constant value Vp = 0 equal to the output signal Vci of the low-pass filter 15 is given to the inverting input of the second differential amplifier 21 to obtain the constant value Vp from the output signal Vci. = 0 minus the signal ΔVci,
This signal ΔVci is logarithmically amplified by the log amplifier 22 and the output signal V
A log is obtained, thereby improving the resolution, especially in the low pressure vacuum region.

The output Vlog of the log amplifier 22 has a pressure dependency as shown in FIG.

In other words, a quartz oscillator that is a sensor of this type of pressure gauge
A change in the CI value of 11 is at a certain pressure Ps, in this example about 2 Toor
In the pressure region lower than this, the so-called molecular flow region where the influence of the gas molecule flow is dominant,
In a pressure region higher than the pressure Ps, the influence of the viscosity of the gas becomes a dominant viscous flow region, and the principle of changing the CI value is different.

For this reason, as shown in FIG. 4, the CI value changes substantially linearly on the high pressure side and the low pressure side with respect to the pressure Ps, for example, about 2 Torr in this embodiment, but the change rates are different from each other. Will do.

For this reason, the output Vlog of the log amplifier 22 is applied to a linear amplifier 23 whose amplification factor changes to obtain a linear output with the pressure Ps as a boundary, and is amplified to obtain an output Vbend, which is further output. The signal is amplified by the amplifier 24 to obtain a substantially linear output Vout as shown in FIG.

With such a configuration, the crystal resonator 11 serving as a pressure sensor is driven by a constant current, so that the current flowing through the crystal resonator 11 in the resonance state is constant regardless of a change in the gas pressure of the atmosphere, and a stable resonance occurs. A state can be obtained, and a resonance state can be reliably obtained even under a high pressure at which the CI value becomes high.

Further, the CI value of the crystal unit 11 is affected not only by a change in the pressure of the atmosphere gas but also by a change in the drive current. However, in the above-described embodiment, an error due to the change in the drive current occurs due to the constant current drive. Thus, the pressure measurement accuracy can be improved without causing any problem, and when the CI value of the crystal unit 11 becomes low, an excessive current flows and the crystal unit 11 is not damaged or lost.

Since a signal having a CI value substantially proportional to a change in pressure can be obtained from the output of the low-pass filter 15, a constant value Vp = 0 is subtracted from this signal to perform logarithmic conversion. for example it is possible to obtain a resolution of about 1 × 10 ^-3 torr, yet measurable pressure range may be 1 × 10 ^-3 is torr~760Torr remarkably wideband single sensor.

Note that the present invention is not limited to the above-described embodiment. For example, when a change in CI value due to a change in temperature impairs measurement accuracy, the quartz oscillator, which is a sensor, is heated with a heater or the like and maintained at a constant temperature. You may make it.

(Effects of the Invention) As described in detail above, according to the present invention, the quartz resonator as a sensor can be reliably maintained in a resonance state, and there is no error due to a change in drive current, and even in a low pressure region. It is possible to provide a vacuum manometer capable of obtaining a resolution and capable of measuring pressure in a wide band.

[Brief description of the drawings]

1 to 5 are diagrams for explaining an embodiment of the present invention. FIG. 1 is a block diagram showing an oscillating unit, FIG. 2 is a block diagram showing an overall configuration, and FIG. FIG. 4 is a graph showing the pressure dependency of CI-CIP = 0 of a quartz oscillator, FIG. 5 is a graph showing the output of a linear amplifier, and FIG. 6 is a conventional pressure. FIG. 7 is a block diagram showing an example of a gauge, and FIG. 7 is a graph showing output characteristics of the pressure gauge shown in FIG. 10 Oscillator 11 Crystal oscillator 12 Oscillation amplifier 13 Peak value detection circuit 14 First differential amplifier 15 Low-pass filter 16 Analog switch 21 Second differential Amplifier 22: Log amplifier 23: Linear amplifier 24: Output amplifier

Claims (1)

(57) [Claims] A quartz oscillator placed in a gas atmosphere whose pressure is to be measured; an oscillation amplifier connected to one end of the quartz oscillator; a peak value detection circuit for obtaining a peak value of an output of the oscillation amplifier; A first differential amplifier that compares the output of the peak value detection circuit with a predetermined constant voltage and outputs the same; a low-pass filter that receives the output of the first differential amplifier; and a drive that is driven by the output of the oscillation amplifier An analog switch for switching the output of the low-pass filter and applying the output to the other end of the crystal unit; and an output voltage of the low-pass filter in a high vacuum state, the output of the low-pass filter being applied to one input. A second differential amplifier that is supplied with a predetermined constant voltage corresponding to the second differential amplifier and outputs the differential pressure; and that the output voltage of the second differential amplifier is logarithmically transformed. And a log amplifier that outputs the pressure signal as a pressure signal.