RU2430342C1 - Semiconductor pressure gage with frequency output signal - Google Patents

Abstract

FIELD: physics. ^ SUBSTANCE: pressure gage comprises transducer consisting of housing, membrane with meander-shaped strain gages that make branches of strain bridge, frequency converter comprising comparator and integrator arranged on operating amplifier with first capacitor in negative feedback circuit. Inverting input of operating amplifier is connected via second capacitor with first apex of strain bridge feed diagonal and, via first resistor of integrator, with one of apices of strain bridge measuring diagonal. Another apex is connected to non-inverting input of operating amplifier of integrator and to comparator second input. Besides, proposed pressure gage comprises additionally second resistor of integrator and second strain bridge with its feed diagonal apex connected to comparator output and second apex connected with bus "earth". Branches of second strain bridge also represent meander and are fitted in identical branches of first strain bridge. ^ EFFECT: higher accuracy and reliability. ^ 6 dwg

Description

The invention relates to measuring equipment and can be used to measure pressure in measurement systems, control and management.

A known construction of a semiconductor [1, 2] strain gauge absolute pressure sensor. The sensing element consists of a crystal, which is electrostatically connected to a glass washer in a vacuum, and a vacuum cavity is located inside the sensitive element between the crystal and the glass washer, which provides absolute pressure measurements. The crystal is made in the form of a square 4 × 4 mm (Fig. 1), with a central thin part providing pressure measurement, and is a silicon single crystal of the (100) plane. On the working part of the crystal, the strain gages 1-4 are formed by diffusion, combined into a bridge measuring circuit.

The disadvantages of the known sensor design are the relatively low sensitivity due to the limited supply voltage of the strain gage associated with the permissible dissipation power of the strain gages at a constant supply voltage, and the conversion error, depending on the instability of the power source of the bridge measuring circuit of the sensor. The conversion error from the instability of the power source is due to the fact that the voltage from the output of the measuring diagonal of the strain gage (units of millivolts) is directly proportional to the supply voltage of the strain gage.

The closest in technical essence to the proposed solution is a converter [3] of the strain gage unbalance signal to a frequency, the functional diagram of which is shown in FIG. 2, containing a strain gage bridge 5, an integrator 6 based on an operational amplifier 7 with capacitive negative feedback 8, a comparison device on the base of the operational amplifier 9 and the metering capacitor 10. The output frequency of this converter is determined by the formula

where ε _R is the relative change in the resistance of the strain bridge from the effects of the measured pressure; R ₁₁ is the resistance of the integrator, which includes the output resistance of the strain gauge bridge and the resistance of the cable line; C ₁₀ - dosing capacity.

As can be seen from formula (1), the frequency of the output signal of the converter is determined by the resistance 11 of the integrator 6 (R ₁₁ ), which includes the output resistance of the strain gage and the resistance of the cable line, the capacitance of capacitor C ₁₀ and the relative change in the resistance of the strain gage ε _R from the influence of the measured pressure, but independent of supply voltage.

A disadvantage of the known design is the low accuracy due to insufficient sensitivity. In addition, this design has low reliability, so, if one strain gauge fails, the sensor becomes inoperative.

The technical result of the invention is to increase accuracy by increasing sensitivity while maintaining the independence of the output signal parameters from the voltage supply of the strain gage. In addition, the technical result is to increase reliability due to the presence of two strain gauge bridges located in zones of identical deformations and operating temperatures. In case of failure of the strain gages of one of the strain gages, the sensor does not completely lose its operability and, after additional calibration, can perform its functions.

This is achieved by the fact that in a semiconductor pressure sensor with a frequency output signal, comprising a sensor consisting of a housing, an elastic element installed therein in the form of a membrane, square-shaped strain gauges formed on it, forming tensor bridge arms, a frequency converter of the signal from the strain gauge output, comprising a comparator and integrator made on the operational amplifier with the first capacitor in the negative feedback circuit, the output of which is connected to the first input of the comparator, the inverting input op the integrator’s amplifier through a second capacitor is connected to the first vertex of the strain gage power supply diagonal and through the first integrator resistor from one of the vertices of the strain gage measuring diagonal, and its other vertex is connected to the non-inverting input of the integrator’s operational amplifier and to the second input of the comparator, while the second vertex of the strain gage supply diagonal connected to the ground bus, a second integrator resistor and a second strain gage are introduced, the first vertex of the power diagonal of which is connected to the comparator output RA, and the second - with the tire "earth". The second integrator resistor is connected between one of the vertices of the measuring diagonal of the second tensor bridge and the inverting input of the integrator operational amplifier, and the other vertex of the measuring diagonal of this tensi bridge is connected to the non-inverting input of the integrator operational amplifier, while the shoulders of the second tensor bridge are also made in the form of a meander and placed so that are “inserted” into the shoulders of the first tensor bridge identical to them.

Figure 3 presents the elastic element of a semiconductor pressure sensor with a frequency output signal in the form of a membrane formed on it strain gauges in the form of a meander, forming the shoulders of the strain gages. Two tensor bridges are formed on this membrane. The first includes strain gauges R ₁₂ -R ₁₅ , the second - strain gauges R ₁₆ -R ₁₉ .

Figure 4 presents a functional diagram of the inventive pressure sensor. The circuit consists of strain gauges R ₁₂ -R ₁₉ combined in two strain gages and a frequency signal converter from the outputs of the strain gages.

Figure 5 presents the functional diagram of the inventive pressure sensor, assembled in the MicroCap program, as well as timing diagrams of the frequency converter signals.

A semiconductor pressure sensor with a frequency output signal operates as follows.

, где ε=ΔR/R - относительное изменение сопротивления тензомоста, и отрицательным "скачком" через дозирующий конденсатор C₂₁, равнымIn the steady state of the device, the output of the comparator 20 is followed by bipolar pulses of amplitude ± U ₀ . Let at the time t ₁ there was a change in the polarity of the output voltage from -U ₀ to + U ₀ . In this case, the voltage at the output of the integrator is due to a positive "jump" in voltage from one of the vertices of the measuring diagonals of the tensile bridge

, where ε = ΔR / R is the relative change in the resistance of the strain gage, and a negative "jump" through the metering capacitor C ₂₁ equal to

.

.

Given the initial conditions, we have

where C ₂₂ is the value of the capacity of the integrator.

.Under the influence of the strain gage unbalance equal to εU ₀ , the voltage at the output of the integrator will increase to a positive threshold level of the comparison unit 20, equal to

.

At the moment of equal response threshold and voltage at the output of the integrator based on the operational amplifier 23, the polarity of the output voltage will again change.

In this case, the voltage at the output of the integrator will be equal to

where τ _{and 1} = R ₂₄ C ₂₂ and τ _{and 2} = R ₂₅ C ₂₂ are the time constants of the integrator.

For the moment of equal voltage at the output of the integrator and the threshold level of the control system, the expression is valid provided that ε ₁ = ε ₂ = ε and R ₂₄ = R ₂₅ :

where T is the period of the output voltage, ε ₁ = ε ₂ = ε is the relative change in the resistance of the shoulders of the first and second tensor bridges.

From expression (4) we determine the frequency of the output signal

A comparison of expressions (5) and (1) shows that the sensitivity of the conversion of the signal from the pressure sensor due to the introduction of the second tensor bridge and the second integrator resistor is doubled. In this case, the output characteristics of the bridge measuring circuits of the sensor are averaged (the influence of the technological spread of the strain gauge parameters decreases, the non-linearity, temperature error, and so on are averaged). By increasing the sensitivity and averaging the characteristics of the bridge measuring circuits, the accuracy of the sensor increases while maintaining the independence of the output signal parameters from the voltage supply of the strain gage. In addition, reliability is improved due to the presence of two strain gauge bridges located in zones of identical deformations and operating temperatures. The same deformations and operating temperatures of both strain gages are ensured due to the fact that the shoulders of the second tensor bridge are also made in the form of a meander and placed so that they are "inserted" into the shoulders of the first tensor bridge identical to them.

In the event of failure of the strain gages of one of the strain gages, the sensor does not completely lose its working capacity and, after additional calibration, can perform its main function - to measure pressure.

The proposed design retains the independence of the output signal parameters from the voltage supply of the tensor bridge, since the conversion function (5), as for the prototype, does not include the supply voltage. This is due to the presence of negative feedback from the frequency converter, as a result of which, when the amplitude of the output signal of the converter changes, the voltage at the output of the integrator simultaneously changes in the form of a voltage jump proportional to the ratio of capacitances C ₂₁ / C ₂₂ and the output voltage from the measuring diagonals of the strain gages, with The values in the conversion function remain unchanged and do not affect the output frequency of the converter.

Experimental studies have confirmed the advantages of the proposed semiconductor pressure sensor with a frequency output signal compared to the prototype.

Figure 6 presents a graph of the dependence of the output frequency on the relative imbalance of the strain gage (TM) for circuits with one and two strain gages, built on theoretical and experimental data.

Thus, due to the distinguishing features of the invention, the accuracy and reliability of the sensor is increased.

Information sources

1. Mokrov EA, Barinov I.N., Tsibizov P.N. Semiconductor piezosensitive elements of microelectronic pressure sensors. // Publishing house of Penza State University - Penza, 2009 - 104 p.

2. Vaganov V.I. Integral strain transducers // Energoatomizdat - Moscow, 1983 - 136 p.

3. A.S. USSR No. 828406. The converter of the unbalance signal of the strain gage to frequency / Gromkov N.V., Mikhotin V.D., Shakhov E.K., Shlyandin V.M. // BI No. 17 dated 05/05/1981

Claims (1)

A pressure sensor with a frequency output signal comprising a sensor, an elastic element installed therein in the form of a membrane, square-shaped strain gauges formed on it, forming the shoulders of the strain gage, a frequency converter of the output signal from the strain gage, containing a comparator and integrator made on the operating room an amplifier with a first capacitor in the negative feedback circuit, the output of which is connected to the first input of the comparator, inverting the input of the operational amplifier of the integrator through the second The first capacitor is connected to the first vertex of the strain gage power supply diagonal and, through the first integrator resistor, to one of the vertices of the measuring diagonal of the strain gage, and its other vertex is connected to the non-inverting input of the integrator’s operational amplifier and to the second input of the comparator, while the second vertex of the strain gage supply diagonal is connected to the bus "Ground", characterized in that a second integrator resistor and a second strain gage are introduced, the first vertex of the power diagonal of which is connected to the output of the comparator, and the second to the bus ", the second integrator resistor is connected between one of the vertices of the measuring diagonal of the second tensor bridge and the inverting input of the integrator’s operational amplifier, and the other vertex of the measuring diagonal of this tensor bridge is connected to the non-inverting input of the integrator’s operational amplifier, while the shoulders of the second tensor bridge are also in the form of a meander and placed , which are “inserted” into the shoulders of the first tensor bridge identical to them.

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