CN204064363U - A kind of sensor circuit for thermal type gas quality flow meter - Google Patents

Abstract

The utility model relates to a kind of sensor circuit for thermal type gas quality flow meter, comprises by speed probe platinum resistance thermometer sensor, R _w, temp probe platinum resistance thermometer sensor, R _c, to pop one's head in platinum resistance thermometer sensor, R with speed _wthe reference resistance R of series connection _aand with temp probe platinum resistance thermometer sensor, R _cthe reference resistance R of series connection _bmagnitude of voltage according to being carried in four described resistance exports corresponding four road voltage signals, described sensor circuit also comprises two constant current sources and an accurate reference voltage, two constant current sources are respectively speed probe branch road and temp probe branch road provides constant drive current, and described accurate reference voltage provides reference voltage for the constant current source of speed probe branch road.The utility model has fast response time, measures accurately, realizes the features such as more simple and not influenced by ambient temperature.

Description

A kind of sensor circuit for thermal type gas quality flow meter

Technical field

The utility model relates to a kind of thermal type gas quality flow meter based on galvanostatic method.

Background technology

For traditional thermal type gas quality flow meter, its measuring method is mainly divided into two classes: constant difference method and constant power process.

Its basic structure of thermal flowmeter as shown in Figure 1.

Sensor core part is popped one's head in by speed and temp probe forms, and utilizes the resistance value size of popping one's head in vary with temperature relation and in conjunction with heat transfer principle to realize the collection of flow signal.By passing to certain current flow heats to speed probe, make it to produce forced-convection heat transfer with the gas that flows through in pipeline, heat exchange amount is directly proportional to mass rate, thus realizes fluid-velocity survey; Temp probe then for measuring fluid temperature (F.T.), realizes temperature compensation.

The basic model of thermal type gas quality flow meter is as shown in formula (1):

$q_m={[\frac{1}{B_1}\left(\frac{I_w^2{R}_w}{T_w-T_c}\right)-\frac{A_1}{B_1}]}^2---\left(1\right)$

Wherein, q _mrepresent mass rate, I _wexpression speed probe heating current; R _wexpression speed probe resistance; T _wfor speed probe temperature; T _cfor fluid temperature (F.T.); A ₁, B ₁for constant.

According to formula (1), analyze further and can draw two kinds of measuring methods that thermal flowmeter is the most basic.Work as T _w-T _cduring=constant, mass rate q _mwith heating electric power there is one-to-one relationship, the relation between both over-fittings thus the method for measurement quality flow is called constant difference method.Equally, when mass rate q _mwith temperature difference T _w-T _cthere is one-to-one relationship, by the relation of matching between them thus the method for measurement quality flow is called constant power process.

Constant difference method system chart as shown in Figure 2.Mainly comprise five parts: bridge section, differential amplifying part, PI controller part, static power source are biased and adder slices, power amplifying part (Current amplifier part).Each partial circuit parameter all needs correct coupling, especially the optimum configurations of PI controller, needs a large amount of experience and experiment, even artificial debugging, therefore, and its circuit realiration and debugging more complicated.

In addition, constant temperature method must increase extra temperature-compensation circuit.And this is the difficult point of research at present, producer is not also had to solve very well.

Constant power process circuit realiration is more difficult, is seldom used in thermal flowmeter.Its adjustment process as shown in Figure 3.Along with fluctuations in discharge, the heat taken away also changes, resistance R _wchange, electric power also therefore change, so need to change heating current I _wbut, R _walso can become thereupon, and then enter a circulation.So, how to design efficient control circuit and control algolithm, make keeping constant, is a difficult problem.Especially, the thermal inertia of platinum resistance thermometer sensor, is very large, and this just requires good feedforward control, further increases design difficulty.

As seen from the above analysis, no matter be constant difference method or constant power process, all need to design efficient control algolithm and control circuit, to realize the temperature difference or power invariability, which increase the complicacy of hardware circuit and software systems, and make system responses slack-off.

Utility model content

For the problems referred to above, the purpose of this utility model is to provide a kind of sensor circuit being applicable to thermal mass flow meter, and the hardware circuit of mass flow measurement is realized more simple and not influenced by ambient temperature, it is more accurate to measure.

For a sensor circuit for thermal type gas quality flow meter, comprise by speed probe platinum resistance thermometer sensor, R _w, temp probe platinum resistance thermometer sensor, R _c, to pop one's head in platinum resistance thermometer sensor, R with speed _wthe reference resistance R of series connection _aand with temp probe platinum resistance thermometer sensor, R _cthe reference resistance R of series connection _bmagnitude of voltage according to being carried in four described resistance exports corresponding four road voltage signals, it is characterized in that, described sensor circuit also comprises two constant current sources and an accurate reference voltage, two constant current sources are respectively speed probe branch road and temp probe branch road provides constant drive current, and described accurate reference voltage provides reference voltage for the constant current source of speed probe branch road.

The utility model is owing to taking above technical scheme, and it has the following advantages: circuit realiration easily, not influenced by ambient temperature, it is more accurate to measure, thus makes flowmeter corresponding speed fast, can realize flow Measurement accuracy without the need to extra control circuit.

Accompanying drawing explanation

Fig. 1 is thermal flowmeter basic structure;

Fig. 2 is constant difference method system chart;

Fig. 3 is constant power process adjustment process schematic diagram;

Fig. 4 is q under constant flow method _m-x matched curve;

Fig. 5 is constant flow method thermal type gas quality flow meter block diagram;

Fig. 6 is sensor assembly.

Embodiment

Below in conjunction with drawings and Examples and application scenarios, the utility model is described in detail.

1) constant flow method thermal type gas quality flow meter theory deduction

Can see in the thermal flowmeter basic block diagram of Fig. 1, when pipeline passes through the gas of certain flow, thermal flowmeter speed probe and the gas generation heat interchange flow through, when thermal flowmeter reaches balance, the heating electric power that speed is popped one's head in equals the heat that unit interval inner fluid is taken away, according to thermal conduction study and the theoretical relational expression that can obtain below of electricity, that is:

$I_w^2{R}_w=\mathrm{hA}(T_w-T_c)---\left(2\right)$

hA＝hdπl (3)

Wherein, I _wexpression speed probe heating current; R _wexpression speed probe resistance; H is Forced Convection Heat Transfer coefficient; A is speed probe heat exchange surface area; D is probe diameter; L is probe length; T _wfor speed probe temperature; T _cfor temperature is popped one's head in the fluid temperature (F.T.) recorded.

By thermal conduction study and channel theory, again introduce Nu Saier (Nusselt) number and Reynolds number:

$\mathrm{Nu}=\frac{\mathrm{hd}}{\mathrm{\λ}}---\left(4\right)$

λ＝at+b (5)

Nu＝CRe ⁿ (6)

$\mathrm{Re}=\frac{{\mathrm{ud}}_1}{v}=\frac{{\mathrm{\ρud}}_1}{\mathrm{\η}}=\frac{4}{{\mathrm{\η\πd}}_1}{q}_m---\left(7\right)$

Wherein, Nu is Nu Saier (Nusselt) number; λ represents the heat transfer coefficient of air, it is the linear function of temperature t, a, b is fitting coefficient, by looking into about the data in the thermophysical property of dry air under different pressures in thermodynamics handbook, under can different temperatures being simulated, the heat transfer coefficient λ of air and the variation relation of temperature, adopt a fitting of a polynomial a, b two fitting coefficient values; Constant C, n are determined by experimental data, are the empirical value provided in thermodynamics handbook; ρ represents fluid density; U represents fluid velocity; d ₁represent pipe diameter; V represents kinematic viscosity; η represents kinetic viscosity; ρ u combines and represents mass velocity.

Simultaneous formula (2) ~ (7) can obtain:

${\left(q_m\right)}^n={\left(\frac{4}{{\mathrm{\η\πd}}_1}\right)}^{-n}\·\frac{1}{\mathrm{\πlC}}\·\frac{1}{\mathrm{\λ}}\·\frac{I_w^2{R}_w}{T_w-T_c}=k\·\frac{1}{\mathrm{at}+b}\·\frac{R_w}{T_w-T_c}=\mathrm{kx}---\left(8\right)$

Wherein, get $k={\left(\frac{4}{{\mathrm{\η\πd}}_1}\right)}^{-n}\·\frac{I_w^2}{\mathrm{\πlC}};x=\frac{1}{\mathrm{\λ}}\frac{R_w}{T_w-T_c}=\frac{1}{a(T_c-273.15)+b}\frac{R_w}{T_w-T_c}.$

As can be seen from formula (8), when with steady current I _wto speed probe R _wduring power supply, k is constant, mass rate q _mand there is one-to-one relationship between fit variations x.Adopt biquadratic curve fit quality flow q _mwith fit variations x relation, as shown in formula (9).

q _m＝k ₄x ⁴+k ₃x ³+k ₂x ²+k ₁x ¹+k ₀ (9)

Wherein, k ₀, k ₁, k ₂, k ₃, k ₄obtained by matching quartic polynomial.Doing by utilizing standard scale the calculated value that rating test obtains k under different flow and fit variations x, can k be obtained by matching ₀, k ₁, k ₂, k ₃, k ₄, namely establish mass rate q _mand the relational expression between fit variations x.

Namely may be used on actual flowmeter survey experiment according to the formula (9) that matching obtains, obtain mass rate q _m, the method is constant flow method.Its experimental verification curve as shown in Figure 4.

2) constant flow method thermal type gas quality flow meter circuit structure design

Constant flow method thermal type gas quality flow meter block diagram as shown in Figure 5.Mainly comprise following six modules: voltage source module 1, constant current source module 2, sensor assembly 3, Signal-regulated kinase 4, A/D sampling module 5, main control unit 6, human-machine interface module 7, protection circuit module 8.

System power supply comprises voltage source 1 and constant current source 2.Voltage source 1 comprises general purpose voltage source and accurate reference voltage.General purpose voltage source provides power supply for chips all in circuit; Accurate reference voltage is that 80mA constant current source and 24 high-precision A/D samplings provide reference voltage.Constant current source 2 comprises to be provided the 0.5mA constant current source of electric current for temp probe branch road and provides the 80mA constant current source of electric current for speed probe branch road.Wherein, 80mA constant current source measured precision is up to 0.025%, and electric current variance is 9.62E-04, achieves high precision and high stability design.

Sensor assembly 3 is very simple, as shown in Figure 6, only by speed probe platinum resistance thermometer sensor, R _w, temp probe platinum resistance thermometer sensor, R _c, and two High Precision Low Temperatures of connecting with it waft reference resistance R _a, R _bcomposition, exports four road voltage signals.Utilize the accurate reference resistance method of series connection, even if constant current source disturbs due to circuit or other faults cause electric current to occur fuctuation within a narrow range, or in circuit, have other interference to appear at I _won branch road, still do not affect the measurement of resistance value.

Sensor assembly 3 exports four road voltage signals, corresponding with it, Signal-regulated kinase 4 comprises four road modulate circuits, the four road signals meeting sampling request after conditioning are sent into high precision 24 A/D sampling modules 5 and carries out analog to digital conversion, then be sent to main control unit 6 by SPI communication and carry out the process of digital filtering scheduling algorithm, calculate relevant voltage value and fit variations x, finally calculate flow value according to formula (9).

Main control unit 6 selects MSP430 single-chip microcomputer, and its peripheral module comprises human-machine interface module 7 and protection circuit module 8.The former 7 comprise again on the spot with teletransmission two parts: comprise liquid crystal display on the spot and by one-key communication; Teletransmission comprises pulse output, RS485 communication etc.Protection circuit module 8 is mainly and extends out house dog and power-down protection circuit etc.

3) key signal obtains and gathers

This part composition graphs 5 carries out the explanation explanation of main signal collection.

U _w, U _a, U _c, U _b: in Figure 5, can find out, at sensor assembly by 80mA constant current source and 0.5mA constant current source respectively to speed probe platinum resistance thermometer sensor, R _wwith temp probe platinum resistance thermometer sensor, R _cheating, then through signal conditioning circuit, further across AD sampling and CPU process, can U be obtained _w, U _a, U _c, U _b.

Mass rate q is derived in constant flow method thermal type gas quality flow meter theory part _m, fit variations x and k computing formula, namely

${\left(q_m\right)}^n={\left(\frac{4}{{\mathrm{\η\πd}}_1}\right)}^{-n}\·\frac{1}{\mathrm{\πlC}}\·\frac{1}{\mathrm{\λ}}\·\frac{I_w^2{R}_w}{T_w-T_c}=k\·\frac{1}{\mathrm{at}+b}\·\frac{R_w}{T_w-T_c}=\mathrm{kx}$

$k={\left(\frac{4}{\mathrm{\η}{\mathrm{\πd}}_1}\right)}^{-n}\·\frac{I_w^2}{\mathrm{\πlC}}$

$x=\frac{1}{\mathrm{\λ}}\frac{R_w}{T_w-T_c}=\frac{1}{a(T_c-273.15)+b}\frac{R_w}{T_w-T_c}$

Below the parameter wherein related to is illustrated one by one:

C, n are determined by experimental data, adopt the empirical value provided in thermodynamics handbook;

η represents kinetic viscosity, can obtain through tabling look-up;

D ₁represent pipe diameter, for known;

L represents probe length, for;

λ represents the heat transfer coefficient of air, can obtain through tabling look-up;

I _w, I _cthe heating current of expression speed probe and temp probe, namely the constant current source of 80mA described in the utility model and 0.5mA constant current source, in the utility model, utilize U _a/ R _a, U _b/ R _baccurately calculate its size further;

R _w, R _cthe platinum resistance thermometer sensor, value of expression speed probe and temp probe, it passes through U _w/ I _w, U _c/ I _ccalculate.

T _wexpression speed probe temperature, the temperature of platinum resistance thermometer sensor, Pt100 of demarcating during by making probe and the relational expression of resistance value draw;

T _crepresent temp probe temperature, i.e. fluid temperature (F.T.), the temperature of platinum resistance thermometer sensor, Pt200 of demarcating during by making probe and the relational expression of resistance value draw;

A, b represent fitting coefficient, by looking in thermodynamics handbook about the data in the thermophysical property of dry air under different pressures, under can different temperatures being simulated, the heat transfer coefficient λ of air and the variation relation of temperature, adopt a fitting of a polynomial a, b two fitting coefficient values.

4) flow measurement concrete steps

(1) be circuit power supply by voltage source; Accurate reference voltage provides reference voltage for speed probe branch road 80mA constant current source, and simultaneously for picking rate is popped one's head in and 24 high-precision A/D sampling A/D chip of temp probe provides reference voltage;

(2) 80mA constant current source and 0.5mA constant current source is adopted to be respectively speed probe R _wbranch road and temp probe R _cbranch road provides constant drive current;

(3) when air in pipeline flow changes, flow through that heat that gas takes away is corresponding to change, thus impel speed to pop one's head in platinum resistance thermometer sensor, R _wresistance changes, and sensor output voltage signal changes;

(4) output voltage values of popping one's head in enters 24 high-precision A/D sampling module and carries out analog to digital conversion, output voltage after Signal-regulated kinase conditioning;

(5) nominal data: then transformation result is sent to single-chip microcomputer by SPI communication and carries out the process of digital filtering scheduling algorithm, calculate relevant voltage value and fit variations x, according to standard scale flow q _mquartic polynomial is used to simulate relational expression coefficient between the two with fit variations x;

Flow q _mand there is one-to-one relationship between fit variations x, the quartic polynomial matching by below: q _m=k ₄x ⁴+ k ₃x ³+ k ₂x ²+ k ₁x ¹+ k ₀.

Wherein: $x=\frac{1}{\mathrm{\λ}}\frac{R_w}{T_w-T_c}=\frac{1}{a(T_c-273.15)+b}\frac{R_w}{T_w-T_c},$ In existing explanation above;

(6) calculate flow value in previous step through MCU to be shown on the spot by liquid crystal and pulse signal teletransmission exports, for user's observed and recorded and output, can also to be communicated with RS485 by button and realize man-machine conversation;

(7) simultaneously, the protection circuit such as house dog and power down protection, makes calling program run well and data are not lost, and guarantee system is normally run in whole process.

Claims (2)

1., for a sensor circuit for thermal type gas quality flow meter, comprise by speed probe platinum resistance thermometer sensor, R _w, temp probe platinum resistance thermometer sensor, R _c, to pop one's head in platinum resistance thermometer sensor, R with speed _wthe reference resistance R of series connection _aand with temp probe platinum resistance thermometer sensor, R _cthe reference resistance R of series connection _bmagnitude of voltage according to being carried in four described resistance exports corresponding four road voltage signals, it is characterized in that, described sensor circuit also comprises two constant current sources and an accurate reference voltage, two constant current sources are respectively speed probe branch road and temp probe branch road provides constant drive current, and described accurate reference voltage provides reference voltage for the constant current source of speed probe branch road.

2. sensor circuit according to claim 1, it is characterized in that, two described constant current sources are respectively 80mA constant current source and 0.5mA constant current source, and the former provides constant drive current for speed probe branch road, and the latter provides constant drive current for temp probe branch road.