CN104848915A - Implementation method of capacitance liquid level sensor with high accuracy, self-adaptation performance and large measurement range - Google Patents

Abstract

The invention provides an implementation method of a capacitance liquid level sensor with high accuracy, self-adaptation performance and a large measurement range. Embedded hardware and software application system implementation serves as the basis, a multi-segment mechanical organization structure is adopted, a measurement processing algorithm model for rough judgment, medium, temperature and humidity change compensation and accurate calculation is established on the comparison calculation basis, the purpose of the 0.01mm liquid level measurement accuracy is achieved, self-adaptation of changes of measurement media, the environmental temperature and the environmental humidity is achieved, the measurement range can be several meters through the multi-segment mechanical organization structure, and the strong guarantee is provided for accurate detection of the oil level of oil tanks of automobiles/ships/aircrafts/agricultural implements/military movement carriers, the oil level of liquid fuel storage tanks and the water level of boilers.

Description

A kind of capacitive liquid level sensor implementation method of high-accuracy self-adaptation wide-measuring range

[technical field]

The present invention presents the capacitive liquid level sensor implementation method of a kind of high precision, self-adaptation, wide-measuring range, the liquid level measurement precision of 0.01mm can be reached, self-adaptation can be accomplished to the change of the change of measuring media and environment temperature, humidity, multistage mechanistic organization structure makes measurement range reach several meters, be mainly used in automobile, naval vessel, aircraft, agricultural machinery and implement, the fuel level in tank of military motion carrier and the accurate detection of boiler water level, liquid fuel tank oil level etc., provide strong guarantee for the liquid precises such as oil, water detect metering.

[background technology]

Liquid position change has linear approximate relationship with electric capacitance change, according to this principle, has occurred a series of capacitive liquid level sensor.Along with application popularization, the particularly Mobile solution such as automobile, agricultural machinery and implement, the problem that capacitive liquid level sensor exposes is increasing and serious:

● poor stability, measuring accuracy is low;

● the change of medium or slightly change, measuring accuracy degradation;

● temperature/humidity changes, particularly under low cold or high temperature, and measuring accuracy degradation;

● tracer liquid depth range is little.

Development Simple And Practical, high precision, by media variations-temperature variation-effects of humidity change, capacitive liquid level sensor compared with wide-measuring range, low cost, time and tide wait for no man, imperative.

[summary of the invention]

1 capacitance liquid level sensing principle demand

Capacitive liquid level sensor measuring principle is mainly based on following 2 points:

2.1 electric capacitys-Relationship between Liquid Level

As shown in Figure 1, battery lead plate right opposite amasss S and spacing l immobilizes for the simple structure block diagram of common capacitance liquid level sensor; Medium between plate is liquid medium ε below _b, be gas--air dielectric ε above _a; The wide w of battery lead plate, high H.

Simple plate armature, the electric capacity forming capacitor can be derived as follows:

$C=C_a+C_b={\mathrm{\ϵ}}_a\frac{S_a}{l}+{\mathrm{\ϵ}}_b\frac{S_b}{l}={\mathrm{\ϵ}}_a\frac{w*(H-h)}{l}+{\mathrm{\ϵ}}_b\frac{w*h}{l}=\frac{w}{l}{\mathrm{\ϵ}}_a*H+\frac{w}{l}({\mathrm{\ϵ}}_b-{\mathrm{\ϵ}}_a)*h$

More cylindrical sleeve barrel type capacitor in application, if external radius is respectively r and R in it, then the electric capacity of capacitor is:

$C=C_a+C_b={\mathrm{\ϵ}}_a\frac{2\mathrm{\π}(H-h)}{\ln \left(\frac{R}{r}\right)}+{\mathrm{\ϵ}}_b\frac{2\mathrm{\πh}}{\ln \left(\frac{R}{r}\right)}={\mathrm{\ϵ}}_a\frac{2\mathrm{\πH}}{\ln \left(\frac{R}{r}\right)}+\frac{2\mathrm{\π}}{\ln \left(\frac{R}{r}\right)}({\mathrm{\ϵ}}_b-{\mathrm{\ϵ}}_a)*h---\left(1\right)$

This shows: fix in sensor construction size, in media stabilize situation, electric capacity forms linear relationship with " liquid level ".For cylindrical sleeve barrel type capacitor, except height, what determine electric capacity is boss ratio, and is no longer directly relative area.

In practical work, medium is also unstable, and having with the change of humidity by temperature affects, and different liquids position height also has impact, and Here it is, and common liquid level sensor measures the basic reason place of being forbidden.

2.2 capacitance-resistance discharge and recharge computings

Electric capacity is directly measured, and relates to many factors and seriously floating, usually adopts the resistance R First-order Integral charge-discharge circuit that (Resistance)-electric capacity C (Capacitance) forms indirectly to carry out as shown in Figure 2.

RC circuit time constant: τ=R*C, charging, discharge time: t=t _r=t _f=ln10*R*C ≈ 2.3 τ.

The size of electric capacity, be embodied on duration of charging ability and discharge and recharge number of times, indirectly electric capacity can be obtained, so there is the capacitive transducer of two kinds of principle types: duration of charging ability CTC (Charge-TimingCapacitance) and relaxation oscillator RO (Relaxation Oscillator) by repetitive measurement duration of charging and the discharge and recharge number of times measured in certain hour.Obviously, the number of times measuring discharge and recharge more easily realizes and obtains higher measuring accuracy, and more capacitance sensing device adopts this principle, and electric capacity is larger, and the discharge and recharge time is longer, and the number of times of standard discharge and recharge follows the change of electric capacity to form linear relationship, the number of times of RC circuit discharge and recharge in certain hour follows the change of liquid position to constitute linear relationship; Can find out: the electric capacity that this sensor production is formed is not easily excessive, otherwise measuring accuracy will heavy discount greatly, usually in pf level, capacitance variations tens pf (only have electric capacity to manufacture little, could within a certain period of time, the discharge and recharge numeration that rapid formation is very large, improves measuring accuracy indirectly).

Definition does not add the air dielectric electric capacity before liquid for " basic electric capacity ", adds liquid to being full of the electric capacity of liquid for " change electric capacity "; " change electric capacity " on " basic electric capacity ", to the making of capacitive transducer with measure most important.If it is slightly large to manufacture electric capacity, ceramic disc capacitor can be increased to metering circuit, form " basic electric capacity ", thus more effectively carry out the measurement of measuring " change electric capacity ".

2 physical construction shape-designings make

According to principle and the demand thereof of capacitance liquid level sensing, the basic physical construction moulding of capacitive liquid level sensor:

● adopt cylindrical sleeve cartridge type electrode

Opposing plates formula capacitance electrode, cylindrical sleeve cartridge type capacitance electrode, without marginal effect of electric field, the radial external electromagnetic interference ability that suppresses is strong.

● all long combined type structure of multistage

Both greatly can improve measuring accuracy, and combined and enough measurement ranges can be obtained, the desirable 5 ~ 300mm of every segment length, hop count desirable 5 ~ 30.

3 measurement processing algorithm model structures

3.1 measurement processing method steps

● under normal condition, the measured value with measured value when no liquid and air and when being full of liquid--- set up comparison operation basis;

● according to each section of measured value and its air basic value and the relation being full of liquid base value, determine liquid-gas mixing section--- rough judgement;

● according to " mixing section liquid level ratio " and its " leading portion is full of the variable quantity of liquid value " revise each section " liquid-gas basic value "- medium, temperature, wet the change lining of degree is repaid;

● the accurate liquid position of mixing section is calculated according to the linear corresponding relation of measured value and liquid position--- accurate Calculation.

3.2 Survey control algorithm models

If under normal condition, each section of air value is s [i], full journey value is f [i]; Each segment length is H, and measured value is m [i].Capacitance measurement section is numbered from 0, from bottom to top.

Each section of capacitance change: c [i]=| m [i]-s [i] | (2)

The ratio of the relatively full section of mixing section liquid: $k_l=\frac{m\left[i\right]-s\left[i\right]}{f\left[i\right]-s\left[i\right]}---\left(4\right)$

The change of the full journey value of mixing leading portion: k _lc=f [i-1]-m [i-1] (5)

The change of mixing back segment air value: k _sc=m [i-1]-s [i-1] (6)

Mixing front-end temperature, humidity and media variations amount: k=k _lk _lc(7)

Especially, section headed by mixing section, post-calculation segments air themperature, humidity and media variations amount:

k＝(1-k _s)k _sc(8)

Basic value correction: $\left\{\begin{array}{c}s\left[i\right]+=k\\ f\left[i\right]+=k\end{array}\right.---\left(9\right)$

The liquid position of mixing section: $h\left[i\right]=H\frac{m\left[i\right]-s\left[i\right]}{f\left[i\right]-s\left[i\right]}---\left(10\right)$

Liquid position: $h={\mathrm{\Σ}}_0^{i-1}(H+1)+h\left[i\right]---\left(11\right)$

[accompanying drawing explanation]

Fig. 1 is " the simple structure block diagram of capacitive liquid level sensor ";

Fig. 2 is " RC charge-discharge circuit and time relationship schematic diagram thereof ";

Fig. 3 is " the embedded hardware system simple realization block diagram of described inventive method ";

Fig. 4 is " the embedded software application flows figure of described inventive method ".

[embodiment]

Invent described method, available microprocessor controller is that the embedded hardware and software application system simple and fast of core realizes.

1 embedded hardware System structure

Capacitive transducer electronic circuit forms primarily of RC charge-discharge circuit, analysis of accounts treatment circuit and data communication circuit.RC charge-discharge circuit, primarily of RC circuit, comparator circuit and on-off circuit composition, forms capacitance sensing module, has become peripheral hardware in conventional sheet that in individual devices or microprocessor controller, capacitance touch sensing is measured.Analysis of accounts treatment circuit can be completed by microprocessor controller and internal counter module thereof usually.Data communication circuit is usually local serial communication bus, and as RS485, CAN, industry ethernet etc., modern age, single-chip microcomputer was usually containing peripheral hardware in these communication module sheets.Therefore design of electronic circuits scheme mainly contains three kinds:

● there is communication function microprocessor controller+discrete capacitor metering circuit;

● there is the microprocessor controller of communication and capacitance sensing function;

● there is communication function microprocessor controller+capacitance sensing device.

Relatively these three kinds of schemes: the first scheme, impact is many with controllable factor, and poor stability, measuring accuracy not easily improve, and can not consider.First scheme, the capacitance sensing module that microprocessor controller is integrated, greatly, be easy to use, measuring accuracy is limited for basic electric capacity and change electric capacity.The third scheme, capacitance sensing is independently device, usually includes multiple adjustment means, and be called " capacitance sensing device ", measuring accuracy is high, but often basic electric capacity is little, and collocation low cost microprocessor controller just has outstanding advantage.Therefore latter two scheme is often used, especially the third.

Microprocessor controller, as the single-chip microcomputer of the main flow Cortex-M kernel by high performance-price ratio.

Fig. 3 gives the embedded hardware system simple realization block diagram of described inventive method.

2 embedded application softwares realize

The main contents of embedded software application programming are as follows:

Cycle period is sampled each measurement capacitance sections, and often wheel sampling terminates to produce and interrupts data receiver.

After system and capacitance sensing module thereof are initial, start that average value processing is done to sampled data at every turn when reaching filtering requirements data number, mixing section judge, real time temperature-humidity and media variations compensates, accurate liquid position calculates.

Asynchronous serial transmission UART (the Universal Asynchronous Receiver/Transmitter) receive interruption that corresponding RS485 communicates is set, according to external demand, complete the setting of sensor ID numbering, filter factor is arranged, basic operations value is arranged and computing liquid position data unofficial biography.

Basic operations value comprises air basic value and full journey basic value, adopts the inner integrated flash memory Flash of microprocessor controller to make non-volatile memory.

Mean filter, basic selection of times is 5, and corresponding filter factor is 0, and the corresponding actual filter times of other filter factor m is 5m.Obviously, filter factor is larger, and measuring accuracy is higher, but measurement data period of change increases, and measurement performance is blunt.

Fig. 4 gives the embedded software application flows figure of described inventive method.

Claims (3)

1. one kind " high precision, self-adaptation, wide-measuring range " capacitive liquid level sensor implementation method, it is characterized in that, have: " measurement processing algorithm model ", " multistage mechanistic organization structure " and " embedded hardware and software application system realizes ", the liquid level measurement precision that " measurement processing algorithm model " ensure that 0.01mm and the self-adaptation that measuring media change and environment temperature, humidity are changed, " multistage mechanistic organization structure " makes measurement range reach several meters long, and " embedded hardware and software application system " makes " measurement processing algorithm model " be achieved.

2. " high precision as claimed in claim 1, self-adaptation, wide-measuring range " capacitive liquid level sensor implementation method, it is characterized in that, " measurement processing algorithm model " has " judging roughly " of being based upon on " comparison operation basis ", " medium, temperature, the compensating for variations of humidity " and " accurate Calculation " mechanism, measured value when no liquid and air and measured value when being full of liquid are formed on " comparison operation basis ", according to each section of measured value and its air basic value be full of the relation determination liquid-gas mixing section of liquid base value i.e. " judging roughly ", each section of " liquid-gas basic value " i.e. " medium is revised according to " mixing section liquid level ratio " and its " leading portion is full of the variable quantity of liquid value ", temperature, the compensating for variations of humidity ", i.e. " accurate Calculation " linear corresponding relation according to measured value and liquid position calculates the accurate liquid position of mixing section liquid.

3. " high precision, self-adaptation, wide-measuring range " capacitive liquid level sensor implementation method as claimed in claim 1, it is characterized in that, " embedded hardware and software application system ", can with the capacitance sensing module in microprocessor controller and sheet thereof or outside sheet for hardware foundation, by embedded software application programming, realize " measurement processing algorithm model ".