RU2165602C2 - Semiconductor pressure transducer - Google Patents

Abstract

FIELD: measurement technology. SUBSTANCE: semiconductor pressure transducer is designed for use in primary pressure-to-electric signal converters. It has piezobridge formed on springy membrane of silicon sensitive element, arms of piezobridge being tied-up in common point connected to zero lead of power supply of transducer or to voltage wire of power supply transducer. Each arm has measurement output and power input, thermal corrector with two leads-out of reference voltages with different temperature gradients, differential amplifier of pressure signal. Controllable power supply source is built in each arm of piezobridge based on operational amplifier whose inverse input is connected to measurement output of arm of piezobridge, whose direct input is linked to one of leads-out of reference voltage of thermal corrector and whose output is connected to power input of arm of piezobridge and to one of inputs of differential amplifier of pressure signal. Thermal corrector of transducer includes operational amplifier with resistors and thermal bridge. One measurement lead-out of thermal bridge is connected via resistive divider to direct input of operational amplifier of transducer, its another lead-out is connected via resistor to inverse input of operational amplifier of thermal corrector. Resistive divider whose tap is first lead-out of thermal corrector is placed between inverse input and output of operational amplifier of thermal corrector for convenient adjustment. Lead-out of operational amplifier of thermal corrector is second lead-out of thermal corrector. EFFECT: simplified technical approach with preservation of independent multiplicative and additive components of error and enhanced linearity of output signal. 1 cl, 8 dwg

Description

 The invention relates to measuring equipment and can be used in primary converters of pressure into an electrical signal for automatic control systems, in information, control and other devices operating in a wide temperature range, in particular in electronic ignition systems of automobiles.

 Known semiconductor pressure sensor containing a piezoelectric bridge formed on a thin semiconductor membrane to which pressure is applied. Each bridge arm consists of two series-connected semiconductor strain gages, the shoulders are electrically connected at one end, each has a power input and a measurement terminal. The operational amplifier of the bridge power supply is connected by an inverse input to the point of connection of the shoulders of the strain bridge, its output is connected to the end of one of the shoulders. The measuring leads of the shoulders (the ends of the measuring diagonal) of the tensor bridge are connected respectively to the direct and inverse inputs of the second operational amplifier. The output of the second amplifier is connected to the power input of the other arm of the bridge. The voltage level, which is measured in accordance with the change in ambient temperature, is applied to the positive input of the first operational amplifier (UK patent N 2012967, MKI G 01 L 9/04, claimed 5.01.1979, date of initial priority 6.01.1978, published 1.08.1979 )

 Two types of temperature errors are present in the piezoelectric bridge signal. First, when the temperature of the crystal changes, the sensitivity of the strain gages changes, while the output signal changes in proportion to the change in sensitivity. This is the multiplicative component of the error; to compensate for it, one should change the supply current of the piezoelectric bridge inversely with the sensitivity change (which is done in the patent under consideration) or increase the amplifier gain accordingly. Secondly, the difference in the properties of piezoresistors, the presence of a balancing resistor, and temperature deformations of the membrane at a constant pressure lead to the appearance of an additive error that develops with a useful signal. To compensate, a value proportional to temperature should be subtracted from the piezoelectric bridge signal. In this patent, to compensate for this error, a resistive divider is used, the signal of which is fed to one of the inputs of the second operational amplifier.

 The disadvantage of this scheme is the incomplete use of the sensor supply voltage to supply the strain bridge, since an additional current-driving resistor is connected in series with the bridge. In addition, the circuit is inconvenient to configure; when compensating for additive error at elevated temperature, the initial setting at room temperature will be lost.

 The closest technical solution to independently compensate for the multiplicative and additive components of the error is a pressure sensor (Russian patent N 2086940 from 08/10/1997, MKI G 01 L 9/04, 19/04, priority of the invention 10.08.1995) containing a piezoelectric bridge with controllable a power supply for the shoulders of the piezoelectric bridge, a differential pressure signal amplifier with a separate bias input, and a temperature corrector consisting of a thermal bridge and two operational amplifiers that provide reference voltages to compensate for the additive and multiplicative stavlyayuschih error. However, the sensor is complex circuitry and contains 7 operational amplifiers. In addition, the power supply of the piezoelectric bridge arms used in this circuit leads to some nonlinearity of the output signal; it generates a quadratic error, which in real circuits is about 0.5% of the full scale.

 The aim of the invention is to simplify the circuitry while maintaining the possibility of separate, independent compensation of the multiplicative and additive components of the error, and increasing the linearity of the output signal.

 The analogue and prototype of the pressure sensor include a measuring piezoelectric bridge, a power supply for the shoulders of the piezoelectric bridge, a temperature corrector, and a differential pressure signal amplifier.

 The measuring piezoelectric bridge consists of strain gauges located on a silicon membrane and passive resistors. The membrane is formed on a silicon crystal by selective etching and perceives the measured pressure. The main function of this element is to isolate a signal proportional to the pressure acting on the membrane. The essential features of the piezoelectric bridge are the characteristics of the resistors that make it up: a passive resistor whose resistance does not depend on the stresses in the membrane; active or strain gauge, the resistance of which increases or decreases with increasing pressure on the membrane of the piezoelectric bridge. An important way to connect these resistors and the presence of external terminals: for supplying each of the shoulders of the piezoelectric bridge, the common point of the shoulders of the piezoelectric bridge and measuring output signal of each arm of the piezoelectric bridge.

 Another element of the sensor is the power supply of the piezoelectric bridge arms, the main function of which is the supply of piezoelectric strain gauges to a given current. Since the sensitivity of the strain gages decreases with increasing temperature, the current or supply voltage of the shoulders of the piezoelectric bridge is often made dependent on temperature. Typically, both shoulders of the piezoelectric bridge are connected to a single power source for the shoulders of the piezoelectric bridge. The number of power sources is significant: one for the entire piezoelectric bridge or for each arm of the piezoelectric bridge has its own source. In addition, it is essential to connect the terminals of the operational amplifier, on which the power supply of the piezoelectric bridge arms is usually performed.

 The temperature corrector of the sensor generates a temperature-dependent voltage with the required temperature gradient. Essential is the totality of external conclusions. A temperature corrector suitable for use in a patented sensor must have at least four outputs: Vcc power input; zero power - 0, the output of the first reference voltage Vdl and the output of the second reference voltage Vd2. The convenience of adjusting the sensor, as will be shown later, is determined by the possibility of independently adjusting the half-sum and half-difference of these gradients, which is also an essential sign. A thermocorrector consisting of a thermal bridge and two operational amplifiers issuing reference voltages to compensate for the additive and multiplicative error components is suitable for the patented circuit (Russian patent N 2086940 from 08/10/1997, priority of the invention 10.08.1995). The values of the temperature gradients of the output voltages of this thermocorrector can be adjusted independently, which means that their half-sum and half-difference can be independently controlled. However, the corrector is structurally complex, and the simpler options for constructing a thermal corrector that satisfy the above essential features are given below.

 The differential amplifier of the sensor pressure signal amplifies the piezo bridge signal to the desired level. Sometimes, to compensate for temperature errors, individual resistors of this amplifier are performed with temperature coefficients of resistance (TCS) significantly different from other ones. The prototype stipulates the use of a differential amplifier with a separate bias input. For use in a patented sensor, a differential amplifier is suitable without a separate bias input, i.e. having two signal inputs and an output, these outputs are available on any differential amplifier.

 The essence of the patented pressure sensor is the use of separate controlled power sources for the arms of the piezoelectric bridge for each of the two arms of the piezoelectric bridge. Each power supply for the shoulders of the piezoelectric bridge is built on an operational amplifier. Moreover, the temperature-dependent reference voltage from the temperature corrector is supplied to the direct input of the operational amplifier of the piezoelectric bridge power supply. The inverse input of the operational amplifier of the piezoelectric bridge arm power source is connected to the measuring output of the piezoelectric bridge arm, and the output is connected to the piezoelectric bridge arm power input and one of the inputs of the measuring differential signal amplifier. The convenience of adjusting the sensor is determined by the ability to independently adjust the half-sum and half-difference of the temperature gradients of the reference voltages of the thermal corrector; the implementation option for such a thermal corrector is specified in dependent clause 2 of the formula.

 In FIG. 1-8 are diagrams illustrating the invention. In FIG. 1, 2 and 3 show various options for constructing piezoelectric bridges; in FIG. 4, 5, 6 and 7 - options for constructing a thermal corrector; in FIG. 8 is a pressure sensor diagram illustrating the feasibility of a technical implementation of the invention.

 The piezoelectric bridge shown in FIG. 1, on the membrane contains 4 active strain gages pr1-pr4 in the zones of action of maximum compressive and tensile stresses. Here, each arm of the measuring piezoelectric bridge consists of two strain gauges increasing (pr2, pr3) and decreasing (pr1, pr4) their resistance when pressure is applied to the membrane. Both arms are united by a common point 0, and the strain gages pr1 and pr2 with different signs of resistance increment converge with increasing pressure in it. Each arm has a measuring output from the other terminal of the resistors combined at a common point (D1 and D2, respectively) and the power input of the piezoelectric bridge arm (Vp1 and Vp2).

 The piezoelectric bridge shown in FIG. 2, also contains 4 strain gages on the membrane. Each arm of the piezoelectric bridge consists of two strain gauges increasing (pr2, pr3) and decreasing (pr1, pr4) their resistance when pressure is applied to the membrane, and two passive resistors R1 and R2, the resistance of which does not depend on pressure. They are designed to balance the piezoelectric bridge to obtain the desired output characteristics and can be located both on the silicon crystal and outside it. A passive resistor in one of the shoulders may be absent. Here, both arms are united by a common point 0, and the strain gages pr1 and pr2 converge with different signs of resistance increment with increasing pressure. Each arm has a measuring output from the other terminal of the resistors combined at a common point (D1 and D2, respectively) and the power input of the piezoelectric bridge arm (Vp1 and Vp2).

 The piezoelectric bridge shown in FIG. 3, on the membrane contains 2 active strain gages pr1 and pr2 in the zones of action of maximum compressive and tensile stresses. Here, each arm of the measuring piezoelectric bridge consists of a strain gauge increasing (pr2) or decreasing (pr1) its resistance when pressure is applied to the membrane, and a passive resistor (R1 or R2), the resistance of which does not depend on pressure. Both arms are united by a common point 0, and the strain gages pr1 and pr2 with different signs of resistance increment converge with increasing pressure in it. Each arm has a measuring output from the other terminal of the resistors combined at a common point (D1 and D2, respectively) and the power input of the piezoelectric bridge arm (Vp1 and Vp2).

In FIG. 4 shows a temperature corrector containing a thermal bridge, consisting of four resistors, at least one of which has a significantly different one from other TCS. Such a corrector is able to perform its functions, i.e. give out two temperature-dependent voltages Vdl and Vd2 with the possibility of adjusting their half-sum and half-difference. However, it does require laser fitting of thermistors. The addition of series-parallel connected passive resistors, the resistance of which can be increased by laser fitting to the shoulders of the thermal bridge, can facilitate the adjustment, however, for the generation of the really required gradients of about + 2 · 10 ^-3 per degree, thermistors with a significant and linear in the working area of the TCS will be required, which is technologically may be difficult to do.

 In FIG. Figure 5 shows a temperature corrector containing a thermal bridge, the direct input of a differential amplifier is connected to the output of the measuring diagonal with a large temperature gradient, and the inverse input of the differential amplifier is connected to the output with a lower temperature gradient. The differential amplifier is built according to the well-known scheme on the operational amplifier DA1 with a resistive divider R1R2 in the direct input circuit, a resistor R4 in the inverse input circuit and resistor feedback. Here, an important sign for us is that not one resistor, as usual, is included in the feedback, but a resistive divider on the series-connected resistors R5 and R6. One output of the temperature corrector (Vd2) is taken from the amplifier output, the other (Vd1) from the tap of the feedback divider R5 and R6.

здесь Vt1, Vt2 - напряжения на измерительных выводах первого и второго плеча термомоста термокорректора. Балансировку можно проводить либо лазерной пригонкой резисторов термомоста, либо резисторов R1 или R2.Such a circuit is convenient in tuning to the required output voltage gradients. The temperature corrector can be balanced at the initial temperature t ₀ . This is the temperature at which the sensor is initially set up, it is from it that the temperature deviation t is counted. The balancing condition according to the notation of FIG. 5, 6 and 7

here Vt1, Vt2 are the voltages at the measuring terminals of the first and second arm of the thermal bridge of the thermal corrector. Balancing can be carried out either by laser fitting of the resistors of the thermal bridge, or resistors R1 or R2.

среднее значение (полусумма) температурного градиента обоих выходов термокорректора; δ_α - отклонение градиента первого и второго выходов от среднего значения

Для сбалансированного термокорректора значения полусуммы и полуразности температурных градиентов составляет

здесь α_m1 - относительный температурный градиент на измерительном выводе первого плеча термомоста, δ_m = α_m1 -α_m2 - разность относительных градиентов на измерительных выводах плеч термомоста.The reference voltages supplied to the direct inputs of the power sources of the arm of the piezoelectric bridge must have a certain temperature gradient α _i to compensate for temperature errors (Vd1 for arm 1 and Vd2 for arm 2)
Vd1 = V ₀ (1 + α ₁ t) = V ₀ (1 + α · t) -V ₀ δ _α · t,
Vd2 = V ₀ (1 + α ₂ t) = V ₀ (1 + α · t) + V ₀ δ _α · t,
here α ₁ , α ₂ are the temperature gradients of the output voltages of the thermal corrector; t is the deviation of the temperature from the original t ₀ , V ₀ is the value of the output voltage of the thermal corrector at the initial temperature t ₀ ,

the average value (half-sum) of the temperature gradient of both outputs of the thermal corrector; δ _α is the deviation of the gradient of the first and second outputs from the average value

For a balanced thermal corrector, the half-sum and half-difference values of temperature gradients are

here α _m1 is the relative temperature gradient at the measuring terminal of the first arm of the thermal bridge, δ _m = α _m1 -α _m2 is the difference of the relative gradients at the measuring terminals of the thermal bridge arms.

It is further shown that the half-sum of temperature gradients α compensates for the multiplicative component of the temperature error of the sensor, and the half-difference δ _α compensates for the additive component. Actually, the value of α required to compensate for the multiplicative error is about 20% per 100 ^o C or + 2 · 10 ^-3 1 / ^o C, the typical value of _{α α} is two orders of magnitude smaller in absolute value. Based on this, the real ratio of resistors R5, R4 and R6 is: R5 / R4 / R6 = 10/1 / 0.1, i.e., the values of these resistors differ by an order of magnitude. Since R6 is two orders of magnitude smaller than R5, the half-sum value α is determined mainly by the resistor R5. The half-difference value is determined only by the value of R6, i.e. half-sum and half-difference of gradients can be independently controlled by resistors R5 and R6 and temperature correction of the sensor is carried out.

 The given circuit is very flexible in configuration and can generate the required half-sum and half-difference values of the output voltage gradients even with negative values of temperature gradients on the measurement terminals of the thermal bridge.

 The disadvantage of this circuit is the dependence of the output voltages Vdl, Vd2 on the supply voltage of the thermal bridge Vcc. In FIG. Figure 6 shows a temperature corrector containing a thermal bridge, diode circuits included in its shoulders (one diode can be switched on), the rest of the circuit is similar to the previous one. The diodes of the thermobridge arms are powered substantially (by an order of magnitude) with different currents, which gives rise to different temperature gradients of the voltage drop across them. The values of the output voltages Vdl, Vd2 very little depend on the supply voltage of the thermal bridge Vcc.

 The thermal bridge of the thermal corrector shown in FIG. 7, contains transistors in the arms, between the base, emitter and collector of which resistors are connected. A single transistor, turned on in a similar way, is widely used to power the piezoelectric bridge pressure sensors. The collector of each transistor is connected to a power supply through a resistor, and the resistances of these resistors are significantly (an order of magnitude) different, which generates a significantly different current in the arms and different temperature gradients of the voltage drop across the collectors. The values of the output voltages Vd1, Vd2 from the supply voltage of the thermal bridge Vcc here also depend very little.

 A diagram of the pressure sensor constituting the subject of the invention is shown in FIG. 8. The circuit consists of a piezo bridge PM according to the embodiment of FIG. 2 formed on the elastic membrane of a silicon sensing element to which a measured pressure is applied. For use in the invention, any of those shown in FIG. 1, 2 and 3. options. Here is a general description of the piezoelectric bridge. Each arm of the piezoelectric bridge includes at least one strain gage, the shoulders are united at a common point, and the strain gages pr1 and pr2 converge with different signs of the increment of resistance when the pressure changes. In FIG. 8, the common point of the shoulders of the piezoelectric bridge is connected to the sensor supply voltage 0, however, it can also be connected to the sensor supply voltage Vcc. Each arm has a measuring output (D1 and D2, respectively) from the other terminal of the strain gages combined at a common point and the power input of the piezoelectric bridge arm (Vp1 and Vp2, respectively).

 Another circuit element is a thermal corrector TC made according to the embodiment of FIG. 4. Other variants of the temperature corrector shown in FIG. 5, 6 and 7. The temperature corrector is placed next to the sensitive element or is formed directly on the crystal of the sensitive element and generates two temperature-dependent signals Vdl and Vd2 with the possibility of independent adjustment of their half-sum and half-difference.

 The essence of the patented pressure sensor is the use of two separate controlled power sources for the shoulders of the piezoelectric bridge. The controlled power sources of the piezoelectric bridge arms are made on an operational amplifier. The inverse input of the operational amplifier of the controlled power supply of the piezoelectric bridge arms (DA1 for arm 1 and DA2 for arm 2) is connected to the measuring output of each arm of the piezomost. The output of this amplifier is connected to the input point of the power supply Vpi of the piezoelectric bridge arm, and the reference voltage Vdi is supplied from the thermal corrector TC to the direct input. A controlled power supply for the piezomost shoulders on an operational amplifier with a similarly connected direct and inverse input is also used in the prototype (Russian patent N 2086940 from 08/10/1997, priority of the invention 10.08.1995), but it alone feeds the entire bridge, and not a separate bridge arm . In the patented scheme, there are two of them, one for each arm of the piezoelectric bridge, which is why a piezo bridge with separate power inputs for the shoulders is needed.

 With this inclusion, the controlled power sources of the piezo bridge arms DA1 and DA2 combine the functions of a power source and a signal amplifier of the measuring output of the arm. The voltage at its output in accordance with the notation in FIG. 8 will be equal to :.

здесь pr₀ - номинальное значение тензорезистора пьезомоста; Δpr - изменение сопротивления тензорезистора при приложении перепада давлений к мембране; ρ1,ρ2 - относительное значение резисторов R1 и R2,

относительное приращение сопротивления тензорезистора, зависящее от температуры,β₀ - чувствительность пьезорезистора, реально составляющая около 2% на полную шкалу давлений P, γ - коэффициент потери чувствительности, составляющий около 20% на 100 градусов приращения температуры.

here pr ₀ is the nominal value of the piezoelectric strain gauge; Δpr is the change in resistance of the strain gauge when applying a differential pressure to the membrane; ρ1, ρ2 - the relative value of the resistors R1 and R2,

the relative increment of the resistance of the strain gauge, depending on the temperature, β ₀ is the sensitivity of the piezoresistor, actually making up about 2% of the full pressure scale P, γ is the coefficient of sensitivity loss, which is about 20% per 100 degrees of temperature increment.

 The outputs of the controlled power sources of the piezoelectric bridge arms DA1 and DA2, in addition to the piezoelectric bridge voltage input, are also connected to the inputs of the differential signal amplifier of the sensor, constructed according to the well-known scheme on the operational amplifier DA3 with resistors R3, R4 and R5 at the inputs and resistor R6 in feedback. In order for the differential amplifier not to amplify the common-mode component of the input signals, the ratios of the resistors R3 / R5 and R4 / R6 must be equal. However, the lack of amplification of the common-mode component of the piezoelectric bridge signal for the pressure sensor is not a decisive condition and can be compensated during tuning, so resistor R5 may be completely absent (have infinitely large resistance). In this case, the resistor R3 compensates for the influence of the input currents of the operational amplifier and may also be absent when using an amplifier with small input currents (have an infinitely small resistance).

Формула (1) описывает выходной сигнал схемы и состоит из трех слагаемых. Первое слагаемое

описывает усиление сигнала давления δr. Этот сигнал сам зависит от температуры и имеет аддитивную составляющую погрешности Δ(t), пропорциональную отклонению температуры, и мультипликативную составляющую погрешности Δ(P·t), пропорциональную произведению температуры и давления.The differential amplifier amplifies the difference of the signals V _o1 and V _o2 and generates an output signal V _p

Formula (1) describes the output signal of the circuit and consists of three terms. First term

describes the amplification of the pressure signal δr. This signal itself depends on temperature and has an additive error component Δ (t) proportional to the temperature deviation and a multiplicative error component Δ (P · t) proportional to the product of temperature and pressure.

пропорционально отклонению температуры t и при соответствующих значениях δ_α/и α может компенсировать аддитивную составляющую погрешности усиленного сигнала давления Δ(t). Причем влияние δ_α здесь на несколько порядков больше, чем α., поскольку разность (ρ1-ρ2) близка к нулю.The second term of the formula (1)

in proportion to the temperature deviation t, and at the corresponding values of δ _{α /} and α can compensate for the additive error component of the amplified pressure signal Δ (t). Moreover, the influence of δ _α is several orders of magnitude greater than α., Since the difference (ρ1-ρ2) is close to zero.

пропорционально произведению отклонения температуры и давления и, при соответствующих значениях α и δ_α, может компенсировать мультипликативную составляющую погрешности усиленного сигнала давления Δ(P·t). Причем влияние α здесь на несколько порядков больше, чем δ_α., поскольку разность (ρ1-ρ2) близка к нулю.The third term of the formula (1)

proportional to the product of the temperature and pressure deviations and, at the corresponding values of α and δ _α , can compensate for the multiplicative component of the error of the amplified pressure signal Δ (P · t). Moreover, the influence of α is several orders of magnitude greater than δ _α ., Since the difference (ρ1-ρ2) is close to zero.

The multiplicative and additive components of the error of the signal of the sensing element are compensated by the selection of the half-sum of the gradients of the reference voltages of the thermal corrector α and the selection of the half-difference δ _α .
Due to the symmetry of the input part of the circuit, mutual compensation of the bias and temperature drift of the amplifiers DA1 and DA2 is provided (Fig. 8). In addition, the symmetrical inclusion of the shoulders of the piezoelectric bridge both along the supply circuits and along the measuring circuits minimizes quadratic errors in the processing of strain gauge signals.

 If the temperature-dependent elements of the thermal bridge perform p-n junctions, as shown in FIG. 6 and 7, all sensor elements can be placed on the chip of the sensitive element.

Claims (2)

 1. A semiconductor pressure sensor containing a piezoelectric bridge formed on the elastic membrane of a silicon sensor to which the measured pressure is applied, each arm of the piezoelectric bridge includes at least one strain gauge, the piezoelectric bridge arms are combined at a common point connected to zero sensor power or to the sensor supply voltage, moreover, strain gauges with different signs of increment of resistance converge with a change in pressure, each shoulder has a measuring output of the shoulder of the piezoelectric bridge and a power input of the shoulder of the piezoelectric bridge a, as well as a temperature corrector having two outputs of the reference voltages with different temperature gradients, as well as a differential amplifier of the pressure signal, characterized in that for each arm of the piezoelectric bridge is introduced by a controlled power source of the shoulders of the piezoelectric bridge, constructed according to the scheme containing the operational amplifier, the inverse input of which connected to the measuring output of the arm of the piezoelectric bridge, the direct input is connected to one of the terminals of the reference voltage of the thermal corrector, and the output is connected to the power input of the arm of the piezoelectric bridge and to one of the odov differential pressure signal amplifier.  2. The semiconductor pressure sensor according to claim 1, the thermal corrector of which comprises an operational amplifier of the thermal corrector, resistors and a thermobridge, moreover, one measuring output of the thermal bridge through a resistive divider is connected to the direct input of the operational amplifier of the thermal corrector, and the second through a resistor with an inverse input, characterized in that between the inverse input and the output of the thermal corrector amplifier, a resistive divider is included, the tap of which is the first output of the thermal corrector, and the output of the operational amplifier of the thermal corrector is the second output of the temperature corrector.

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