JPH0120646Y2 - - Google Patents

Description

[Detailed description of the invention] The present invention relates to a circuit that detects a change in capacitance of a pair of variable capacitors whose capacitance changes differentially depending on a measured quantity, and is particularly suitable for use in a capacitance detection type vortex flow meter. The present invention relates to a capacitance change detection circuit.

In a capacitance detection type vortex flowmeter, a movable electrode is displaced by fluid vibration caused by vortex generation, and the difference in the capacitance of a pair of variable capacitors that changes differentially is detected to determine the vortex frequency and the fluid flow velocity or Measuring flow rate. In this case, the capacitances C _H and C _L of the pair of variable capacitors are given by the following equations, where ν is the vortex frequency.

C _H = C _HO + C _HM sin2πν C _L = C _LO + C _LM sin2πν Here, C _HO and C _LO are fixed capacitances of about 100 pF, and C _HM and C _LM are capacitance changes of about 0.001 pF. Very small. Moreover, there is usually a difference of about 10% between the fixed portions C _HO and C _LO , and even if the difference in capacity is determined, the fixed portion cannot be removed. The fixed portions C _HO and C _LO change due to external factors such as temperature, and the difference (C _HO − C _LO )
Since the capacitance also fluctuates, it becomes a large noise, making it impossible to accurately detect changes in capacitance.

The present invention uses an operational amplifier in which a non-inverting input terminal is connected to a reference point and a capacitor is connected to a return circuit, and a first rectangular wave voltage is applied to the inverting input terminal via the first capacitor. After applying a second rectangular wave voltage via a second capacitor and converting minute capacitance changes into voltage with good S/N ratio, the output of this operational amplifier is synchronously rectified so that the DC component of the output signal is zero. Since the amplitude of the second rectangular wave voltage is controlled so that This is what I did.

FIG. 1 is a connection diagram showing an embodiment of the circuit of the present invention. In the figure, C _H and C _L are a pair of variable capacitors whose capacitance changes differentially depending on the measured quantity.
OSC is an oscillator that oscillates at a frequency, IV ₁ and IV ₂ are each inverters, the oscillation output of the oscillator OSC is added to IV ₁ , and the output V ₁ of IV ₁ is added to IV ₂ . Therefore, rectangular wave voltages V ₁ and V ₂ are generated at the output terminals of IV ₁ and IV ₂ with a phase shift of 180° as shown in FIG. The amplitude Vs of the square wave voltage V ₁ is constant, but
The amplitude Vr of V ₂ is controlled by the integrator output, which will be described later.
OP is an operational amplifier with a capacitor in its return circuit.
C _P , and the inverting input terminal (-) has a square wave voltage.
V ₁ is applied through capacitor C _H and
V ₂ is added via _CL . Further, a shield S is provided on the line connecting the inverting input terminal (-) and the capacitors C _H , C _L , and C _P . Shield S is OP
It is connected to the reference point as well as the non-inverting input terminal (+) of . SD is a synchronous rectifier circuit that synchronously rectifies the OP output _V3 using oscillation output. The IC is an integrator that integrates the synchronous rectifier output Vo, provides the output as the power supply voltage of the inverter IV ₂ , and controls the amplitude of the rectangular wave voltage V ₂ .

In the present invention configured in this way, rectangular wave voltages V ₁ and V ₂ whose phases are shifted by 180 degrees are used, and V ₁
is given to C _H and V ₂ to C _L , so OP's output V ₃
As shown in FIG. 2, the amplitude alternately switches between Vs·C _H / _CP and Vr·C _L / _CP in synchronization with the rectangular wave voltages V ₁ and V ₂ . Therefore, if the synchronous rectifier circuit SD performs synchronous rectification using the oscillation output of the oscillator OSC, the output voltage Vo will be Vo=C _H Vs - C _L Vr/C _P (1). Since the integrator IC integrates the output Vo of the synchronous rectifier circuit SD and controls the amplitude Vr of the rectangular wave voltage _V2 so that the DC component of Vo becomes zero, the following relationship holds true.

C _HO /C _P Vs - C _LO /C _P Vr = 0 Therefore, the output Vo of the synchronous rectifier circuit SD is C _HO /
When C _LO is set as α, Vo=C _HM + αC _LM /C _P sin2πν·Vs, and the influence of the fixed capacitance C _HO and C _LO can be effectively removed, and the capacitance change can be detected with high accuracy.

Note that in the above example, an integrator IC is used to control the amplitude Vr of the rectangular wave voltage _V2 so that the DC component of the output of the synchronous rectifier circuit SD becomes zero, but it is also possible to control Vr using a deviation amplifier. You may also do so. In addition, in the above, the first,
Although the case where the inverters IV ₁ and IV ₂ are used as means for generating the second rectangular wave voltage has been exemplified, it may be configured using other logic circuits.

As explained above, the circuit of the present invention uses an operational amplifier whose non-inverting input terminal is connected to the reference point and a capacitor is connected to the return circuit, and the first rectangular wave voltage is applied to the inverting input terminal. A second square wave voltage is applied via a capacitor, and a second square wave voltage is applied to the second square wave voltage.
After converting the minute capacitance change into a voltage with a good S/N ratio, the output of the operational amplifier is synchronously rectified and a second rectangular waveform is applied so that the DC component of the output signal becomes zero. Since the amplitude of the voltage is controlled, the influence of the fixed capacitance of the pair of capacitors can be effectively removed, and changes in capacitance can be detected with high accuracy. Therefore, if the circuit of the present invention is applied to a capacitance detection type vortex flow meter, it will be extremely effective.

[Brief explanation of drawings]

FIG. 1 is a connection diagram showing an embodiment of the circuit of the present invention;
FIG. 2 is an explanatory diagram of the operation. C _H , C _L ……Pair of variable capacitors, OSC……
Oscillator, IV ₁ , IV ₂ ... Inverter, OP ... Operational amplifier, SD ... Synchronous rectifier circuit, IC ... Integrator.

Claims (1)

[Scope of utility model registration request] first and second capacitors whose capacitances differentially change depending on the measured quantity; an oscillator; means for generating a first rectangular wave voltage of constant amplitude in synchronization with the output of the oscillator; means for generating a second rectangular wave voltage that is synchronized with the output of the oscillator and has a phase difference of 180 degrees from the first rectangular wave voltage, and a capacitor in the feedback circuit between the inverting input terminal and the output terminal, and a non-inverting input terminal connected to a reference point; means for applying the first rectangular wave voltage to the inverting input terminal of the operational amplifier via the first capacitor; means for applying a wave voltage to an inverting input terminal of the operational amplifier via the second capacitor; a synchronous rectifier circuit for synchronously rectifying the output of the operational amplifier with the output of the oscillator; and a DC output of the output of the synchronous rectifier circuit. and means for controlling the amplitude of the second rectangular wave voltage so that the component becomes zero.