RU2147119C1 - Pressure transducer - Google Patents

Abstract

FIELD: instrumentation, measurement technology for pressure measurement in gaseous, liquid and loose media. SUBSTANCE: pressure transducer has restoring member anchored on bearing member, three capacitive displacement transducers as minimum and power supply source. Capacitive displacement transducers can be connected to power supply source via switch individually or to inputs of differential amplifiers in pairs. Outputs of differential amplifiers are connected to analog-to-digital converters. Outputs of analog-to-digital converters are connected to computing-controlling device whose controlling input is connected to switch. EFFECT: enhanced precision of proposed pressure transducer during its long operation without any periodic calibration and provision for its matching with present-day information processing systems. 4 cl, 5 dwg

Description

 The invention relates to measuring equipment, in particular to devices for measuring pressure in gaseous, liquid and granular media.

 The development of science and technology makes more and more stringent requirements for pressure sensors in terms of improving accuracy over a wide range of uses and adverse operating conditions, while at the same time being cheap, the ability to integrate into microelectronic circuits, and into computer processing circuits.

 A promising direction in achieving the above requirements is the development and manufacture of pressure sensors from materials based on silicon using silicon microelectronics technology.

 A study of pressure sensors from leading companies in the USA, Japan, Germany, and the United Kingdom shows that the most promising pressure sensors are solid-state semiconductor sensors made in terms of a single-silicon silicon sensing element using group microelectronics technology. But they do not provide the required accuracy characteristics without periodic calibrations, which is not always acceptable.

 Known pressure sensors with sensitive elements made of monocrystalline silicon, made by the group technology of microelectronics and using a tensoresistive or piezoelectric effect to convert pressure into an output electrical signal (see, for example, US Pat. No. 4,314,226, NKI 338/4, 1982; EPO N 0109992, MKI G 01 L 9/06, 1984; AS USSR 1673894, MKI G 01 L 9/04).

 The strain gauge and piezoelectric transducers have significant drawbacks that limit the accuracy of pressure sensors associated with the dependence of the output signal on temperature, surface and thermal noise, low slope and sensitivity of the output signal.

 A known pressure sensor containing an elastic element mounted on a support element, capacitive displacement sensors of the elastic element, power source (see US Pat. RF N 1652839, IPC G 01 L 9/12, 1989).

 In the specified pressure sensor used capacitive displacement sensors. Along with their known advantage - ease of manufacture, they have several disadvantages, namely the presence of errors from the instability of the electrical parameters of the power source, as well as from the measurement of the electrical properties (dielectric constant) of the interelectrode medium due to the burning of materials, changes in temperature and humidity. These factors limit the accuracy of capacitive displacement sensors of the elastic element and, therefore, the accuracy of pressure sensors.

 The purpose and technical result of the invention is to increase the accuracy of the pressure sensor for a long service life without periodic calibrations, ensuring that it can be coupled with modern information processing systems.

 This goal is achieved by the fact that in the pressure sensor containing an elastic element mounted on the supporting element, capacitive displacement sensors of the elastic element, a power source, at least three capacitive displacement sensors are made, an additional switching device, differential amplifiers, analog-to-digital converters, and computing -controlling device, while capacitive displacement sensors through the switching device can be connected to a power source or in pairs with differential inputs of amplifiers, the outputs of which are connected to the corresponding analog-to-digital converters, the outputs of the analog-to-digital converters are connected to a computing and control device, the control output of which is connected to a switching device.

Besides:
- the elastic element is made with jumpers between its working part and the places of attachment to the support element, the jumpers have low rigidity in the direction orthogonal to the direction of the working movement of the elastic element;
- a temperature sensor is introduced into the pressure sensor, the output of which is connected to a computing and control device;
- the elastic and supporting elements are made of silicon-based materials;
- a pulse voltage source can be used as a power source.

 The proposed device allows to minimize the above errors of capacitive displacement sensors and thereby improve the accuracy of the pressure sensor.

 The introduction of an elastic bridge between the working part of the membrane (elastic element) and the places of its attachment to the support element eliminates the temperature deformation of the membrane due to the difference in the linear thermal expansions of the membrane and the support element and thereby reduces the temperature dependence of the pressure sensor readings by two orders of magnitude.

 Using a temperature sensor mounted on the support element allows you to enter a temperature correction in the pressure sensor readings and minimize its temperature error.

 Using a pulse voltage source simplifies the design and circuitry of the sensor.

The invention is illustrated in FIG. 1-4, where:
figa - the mechanical part of the pressure sensor in the context;
figb is a top view of the support element;
figure 2 - illustration of the connections of capacitive sensors to a power source and to differential amplifiers;
figure 3 is a structural diagram of a pressure sensor;
FIG. 4 is a structural diagram of a pressure sensor with a switching power supply.

In FIG. 1-4 is indicated:
1 - pressure sensor;
2 - an elastic element (membrane);
3 - supporting element;
4 - capacitive displacement sensors (C1, C2, C3);
5 - power source;
6 - switching device;
7 - differential amplifier (DU);
8 - analog-to-digital Converter (ADC);
9 - computing control device (VVU);
10 - jumper of the elastic element;
11 - temperature sensor;
12 - emphasis of the elastic element.

 The mechanical part of the pressure sensor (Fig. 1) consists of a pressure-sensitive elastic element (membrane) 2, mounted on the supporting element 3. The membrane is made of single-crystal silicon by liquid anisotropic etching. The supporting element 3 is made of glass brand OF7. The membrane is mounted on the support element by glass soldering.

 Special elastic jumpers 10 — decoupling between the working part of the membrane and the places of its attachment to the supporting element — are etched into the membrane. The jumpers have low rigidity in the direction orthogonal to the direction of the working movement of the membrane. This ensures a decrease in membrane deformation when the temperature of the device changes, as well as from relaxation of internal stresses in the attachment points during long-term operation.

 On the support element and the membrane, electrodes of capacitive displacement sensors 4 (C1, C2, C3) are made by metal (aluminum or gold) deposition. In the case of connecting and switching capacitive sensors according to the circuit shown in FIG. 4, metal electrodes are performed only on the support element, and the membrane is used as a common movable electrode for all displacement sensors (see Fig. 1).

 Stops 12 are made on the membrane, which protect against electrode shorting at pressures exceeding the maximum measured pressure.

 The electronic means of the pressure sensor are a switching device 6, differential amplifiers 7, analog-to-digital converters 8, computing and control device 9.

 Capacitive displacement sensors 4 are connected to the signal inputs of the switching device 6, the signal outputs of which are paired with the inputs of the differential amplifiers 7. The outputs of the remote control 7 through the ADC 8 are connected to the computing and control device 9. A power supply 5 is also connected to the switching device 6, and the control output VVU 9 is connected to the control input of the switching device. The temperature sensor 11 mounted on the support element is connected to the input of the VVU 9.

 The switching device can be performed on electronic switches HS-303RH / 883S, differential amplifiers - on operational amplifiers HS-3516RH, and ADCs - on integrated circuits FPGA HS-XC3020MS.

 The HS-80C86RH microprocessor can be used as a computing and control device, and a forward-biased p-n zener diode junction (for example, 2C147A) combined with an ADC can be used as a temperature sensor.

 The proposed pressure sensor operates as follows.

Capacitive position sensors are essentially capacitors C1, C2, C3 with corresponding gaps H1, H2, H3, linearly dependent on pressure P:
H1 = H1 _o -k1P; H2 = H2 _o -k2P; H3 = H3 _o -k3P,
where k1, k2, k3 are the proportionality coefficients characterizing the elastic compliance of the membrane.

According to the control signal from the VVU 9, the switching device 6 makes a serial connection of the displacement sensors 4 and connecting them to the power source 5 (current source). The capacitors are simultaneously charged, while the series connection ensures that the charges Q are equal on all capacitors, regardless of the parameters of the power source:
Q _C1 = Q _C2 = Q _C3 = Q = ∫ Idt,
where I is the charge current.

According to the next control signal from the VVU 9, the switching device 6 disconnects the displacement sensors 4 from the power source and connects them to differential amplifiers 7 in pairs (see Fig. 2b). In this case, a voltage proportional to the charge Q and inversely proportional to the capacitance of the sensor is maintained at the displacement sensors:
U _C1 = Q / C1; U _C2 = Q / C2; U _C3 = Q / C3.

Differential amplifiers 7 generate signals proportional to the voltage differences of the displacement sensors:
W1 = U _C2 - U _C1 = IT (1 / C2-1 / C1) = IT / E _o ES1 • [(H2 _o -k2P) S1 / S2- (H1 _o -k1P)];
W2 = U _C3 -U _C1 = IT / E _o ES1 • [(H3 _o - k3P) S1 / S3- (H1 _o -k1P)];
where E _o is the electric constant, E is the dielectric constant, S1, S2, S3 are the areas of the position sensor electrodes, T is the charge time.

Сигналы с ДУ 7, пропорциональные W1 и W2, через АЦП 8 поступают на ВВУ 9, где осуществляется вычисление действующего давления P по формуле (1). Коэффициенты пропорциональности k1, k2, k3 определяются экспериментально при тарировке датчика давления.The expressions W1 and W2 contain a number of parameters: I, T, E - characterized by a certain instability of quantities. However, the ratio W1 / W2 = a, while maintaining information on the pressure P, is free from the instabilities of the mentioned quantities. Using this ratio, you can determine the pressure value by the following formula:

The signals from the remote control 7, proportional to W1 and W2, are fed through ADC 8 to the VVU 9, where the effective pressure P is calculated by the formula (1). The proportionality coefficients k1, k2, k3 are determined experimentally when calibrating the pressure sensor.

The ratios S _i / S _j and ki / kj included in formula (1) are practically independent of the ambient temperature. Its influence on the value of the measured pressure will be provided through the values of the gaps H1 _o , H2 _o , H3 _o and the coefficient k1. To account for this factor, the calibration of the pressure sensor is carried out at several temperature values. From the measured data determine the temperature correction. According to the signal from the temperature sensor 11, the VVU 9 corrects the value of the output signal about the effective pressure by the value of the temperature correction.

When implementing the pressure sensor according to the circuit shown in FIG. 4, a pulsed power supply is used, and resistors R are used in the switching device to determine the required charge current I. The pulse duration of the power supply is chosen less than the time constant of the RC circuits formed by the resistors R and capacitances C1, C2, C3 of the displacement sensors. In this case, the voltage sensors will accumulate on the displacement sensors at the end of the pulse of the power source:
U _C1 = It / C1; U _C2 = It / C2; U _C3 = It / C3,
where t is the pulse duration of the power source. The operation of the switching device is synchronized with the pulses of the power source. Otherwise, the principle of operation of the pressure sensor according to the circuit in FIG. 4 is similar to that previously described.

 In the claimed pressure sensor can be used more than three displacement sensors while maintaining the principle of their use. In this case, it is possible to average the output value and thereby further increase the accuracy of the pressure sensor.

 At the enterprise NPK "Vector" made prototypes of the proposed pressure sensor with the implementation in them of technical solutions presented in this application. Experimental verification on prototypes confirmed the high technical efficiency of the proposed device.

Claims (5)

 1. A pressure sensor containing an elastic element mounted on a support element, capacitive displacement sensors of the elastic element and a power source, characterized in that it has at least three capacitive displacement sensors, an additional switching device, differential amplifiers, analog-to-digital converters and computing and control device, while capacitive displacement sensors through the switching device can be connected to a power source or in pairs with inputs of differential devices divisors, whose outputs are connected to corresponding analog-to-digital converters, the outputs of analog-to-digital converters - a computationally-control device, control output of which is connected with the switching device.  2. The pressure sensor according to claim 1, characterized in that the elastic element is made with jumpers between its working part and the attachment points to the support element, the jumpers have low rigidity in the direction orthogonal to the direction of the working movement of the elastic element.  3. The pressure sensor according to claim 1, characterized in that a temperature sensor is inserted into it, the output of which is connected to a computing and control device.  4. The pressure sensor according to claim 1 or 2, characterized in that the elastic and supporting elements are made of silicon-based materials.  5. The pressure sensor according to claim 1, characterized in that a pulse voltage source is used as a power source.

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