CN102095359A - Force balance type transmitter of strain type displacement detecting element - Google Patents

Abstract

The invention relates to a force balance type transmitter of a strain type displacement detecting element, belonging to the technical field of automatic detection. At present, during the detection process of flow rate and liquid level, the widely used displacement detecting element of the force balance type transmitter is inductance output, thus the circuit of the transmitter is more complex, and the adjustment is more troublesome. According to the invention, the strain type displacement detecting element is used for the force balance type transmitter, and the displacement detecting element is resistance output, the circuit of the transmitter is more simple, the adjustment is easier, the whole structure of the transmitter is a negative feedback closed-loop system, and is composed of a lever system, a displacement detecting amplifier of a forward channel and an electromagnetic feedback device of a feedback channel; and when the amplification factor of the displacement detecting amplifier of the forward channel is greater enough, the whole property of the transmitter depends on the lever system with more stable link property and the electromagnetic feedback device of the feedback channel, and has nothing relationship with the stability of the displacement detecting amplifier, thus the cost performance of the force balance type transmitter of the strain type displacement detecting element is relatively higher.

Description

The transmitter of the force balancing type of strain-type displacement detecting element

Technical field

The present invention relates to a kind of transmitter instrument in the Automatic Measurement Technique field.

Background technology

At present, in testing processes such as flow, liquid level, still in a large amount of uses, typical case's representative is DDZ-II type and DDZ-III type differential pressure transmitter according to the transmitter of force balance principle.In this class transmitter, when tested pressure reduction changes, the input force that is proportional to tested pressure reduction is on lever system and make it produce displacement, the displacement detecting amplifier of transmitter is corresponding normalized current signal with this displacement conversion, this current signal is both as the output signal of transmitter, flow through simultaneously the electro-magnetic feedback device that is fixed on the lever is proportional to this output current with generation feedback force again, so, corresponding variation has all taken place in feedback force and input power, and lever system is equilibrated on the new position once more.Obviously, because above-mentioned dynamic balance relation, the output current of transmitter is proportional to tested pressure reduction.What DDZ-II type differential pressure transmitter used is high-frequency displacement detection amplifier, and uses aluminium detection lug and plane magnetic test coil as the displacement detecting element; What DDZ-III type differential pressure transmitter used is low frequency displacement detecting amplifier, and uses differential transformer as the displacement detecting element.Because the output parameter of these two kinds of displacement detecting elements is inductance value, the displacement detecting amplifier must comprise self-maintained circuit, thereby the circuit of amplifier is complicated, and the adjustment of transmitter is also cumbersome, and the reliability of transmitter, repeatability are also relatively poor.

Summary of the invention

The output parameter of considering foil gauge is a resistance, and subsequent process circuit is simple relatively, and has advantages such as measuring accuracy height, broad quantum, adaptability are strong, but uses the higher relatively transmitter of the force balancing type of strain-type displacement detecting element composition price ratio.

Shown in Figure 1 in the technical solution adopted in the present invention such as the Figure of description: the transmitter of development is a kind of transmitter of the force balancing type of strain-type displacement detecting element, its one-piece construction is a negative feedback closed loop system, by lever system, the displacement detecting amplifier of forward path, the electro-magnetic feedback device of feedback channel is formed, the displacement detecting amplifier of forward path adopts strain-type displacement detecting amplifier, strain-type displacement detecting amplifier is by strain-type displacement detecting element, electric bridge, voltage amplifier, several parts of V/I change-over circuit are formed, strain-type displacement detecting element comprises flexible member and the foil gauge two parts that are fixed on the flexible member, foil gauge is connected to electric bridge, electric bridge is converted to the input of change in voltage as voltage amplifier with the resistance variations of foil gauge, the voltage amplifier output voltage is converted to dc current signal through the V/I change-over circuit, this dc current signal is the output current signal of strain-type displacement detecting amplifier, the output current one side of strain-type displacement detecting amplifier is as the output current signal of whole transmitter, this electric current flows through the electro-magnetic feedback device that is fixed on the lever simultaneously to produce electro-magnetic feedback power on the other hand, the electro-magnetic feedback power of measured input power and the output of electro-magnetic feedback device acts on the flexible member of strain-type displacement detecting element in the differential force that comprehensive relatively back on the lever system produces, this differential force makes flexible member produce deformation (displacement) and by the elastic force of flexible member institute balance, the deformation of flexible member makes fixed thereon foil gauge produce corresponding resistance variations again, the transmitter of the force balancing type of strain-type displacement detecting element also can not use lever system, and the electro-magnetic feedback power of measured input power and electro-magnetic feedback device output is acted directly on the comprehensive comparison of carrying out power on the flexible member of strain-type displacement detecting element.In Fig. 1, k4 is the stiffness coefficient of flexible member, k ₇Be the amplification coefficient of voltage amplifier, β is the feedback factor of electro-magnetic feedback device, k ₁, k ₂, k ₃, k ₅, k ₆, k ₈Scale-up factor for the corresponding link of closed-loop system.Computing method according to the closed-loop system transport function can get:

$I=\frac{k_1\&CenterDot;\frac{k_3}{k_4}\&CenterDot;k_5\&CenterDot;k_6\&CenterDot;{\mathrm{<figure-callout\; id="7"\; label="k"\; filenames="HDA0000039862510000021.png"\; state="\{\{state\}\}">k</figure-callout>}}_7\&CenterDot;{\mathrm{<figure-callout\; id="8"\; label="k"\; filenames="HDA0000039862510000021.png"\; state="\{\{state\}\}">k</figure-callout>}}_8}{1+k_2\&CenterDot;\frac{k_3}{k_4}\&CenterDot;k_5\&CenterDot;k_6\&CenterDot;{\mathrm{<figure-callout\; id="7"\; label="k"\; filenames="HDA0000039862510000021.png"\; state="\{\{state\}\}">k</figure-callout>}}_7\&CenterDot;{\mathrm{<figure-callout\; id="8"\; label="k"\; filenames="HDA0000039862510000021.png"\; state="\{\{state\}\}">k</figure-callout>}}_8\&CenterDot;\mathrm{\&beta;}}\&CenterDot;f_1$

Since in strain-type displacement detecting element, the stiffness coefficient k of flexible member ₄Should design smallerly, and voltage amplifier amplification coefficient k ₇Design greatlyyer again, promptly

$k_2\&CenterDot;\frac{k_3}{k_4}\&CenterDot;k_5\&CenterDot;k_6\&CenterDot;{\mathrm{<figure-callout\; id="7"\; label="k"\; filenames="HDA0000039862510000021.png"\; state="\{\{state\}\}">k</figure-callout>}}_7\&CenterDot;\mathrm{\&beta;}>>1$

Therefore

$I\&ap;\frac{k_1}{k_2\&CenterDot;\mathrm{\&beta;}}\&CenterDot;f_1$

K in the formula ₁, k ₂, β is the proportionality constant of characteristic than stable element, therefore, transmitter output signal I just with measured input power f ₁Proportional.As long as the enlargement factor of the strain-type displacement detecting amplifier of forward path is enough big, the transmitter characteristic just only and k ₁, k ₂, β is relevant, and is and irrelevant with the scale-up factor of each link of strain-type displacement detecting amplifier, promptly irrelevant with the stability of these link scale-up factors, so strain-type displacement detecting amplifier can use relatively cheap components and parts.

The invention has the beneficial effects as follows, because the output parameter of foil gauge is a resistance, subsequent process circuit is simple relatively, compare with the transmitter of the force balancing type of other types, the cost of the transmitter of the force balancing type of strain-type displacement detecting element is lower, adjustment is easier, and foil gauge has advantages such as measuring accuracy height, broad quantum, adaptability are strong, but therefore uses the higher relatively transmitter of the force balancing type of strain-type displacement detecting element composition price ratio.

Description of drawings

Fig. 1 is a functional-block diagram of the present invention.

Fig. 2 is the structure principle chart of the embodiment of the invention.

1. foil gauges among Fig. 2,2. electric bridge, 3. voltage amplifier, 4.V/I change-over circuit, 5. direct supply, 6. pull-up resistor, 7. detection semi-girder, 8. zeroing spring, 9. permanent-magnet steel, 10. feedback moving-coil, 11. carriages, 12. elasticity shaft-seal diaphragms, 13. lever, 14. packing washers, 15. front end end covers, 16. the resilient seal endless belt, 17. interludes, 18. rear end caps, 19. slide cartridge, 20. support cup, forward horizontal stand chambers 21., 22. elastic seal rings, 23. Venturi tube, 24. the back balance chamber, 25. equilibrium liquid loop buffer spaces, 26. transmission forks.

Fig. 3 is the strain-type displacement detecting amplifier circuit figure of the embodiment of the invention.

Fig. 4 is the electro-magnetic feedback apparatus structure sketch of the embodiment of the invention.

27. magnet steel covers among Fig. 4, at the bottom of 28. magnet steel, the 29. soft iron hearts, 30. magnetic shunt path screws, 31. copper rings.

Embodiment

In the embodiment of Fig. 2, the present invention uses as the supporting transmitter of another patent (dynamic throttling element flowmeter of middle part through hole).Though still utilize Throttle Principle in measuring, the dynamic throttling element flowmeter of middle part through hole uses Venturi tube, nozzle and orifice plate as dynamic throttling element, do not need connecting pipe, therefore be difficult for stopping up, and can reduce difficulty of construction.All add packing washer (14) between the interlude (17) of dynamic throttling element flowmeter of middle part through hole and front end end cover (15), the rear end cap (18), be sealedly and fixedly connected each other, constituted a watchcase that is used for the rigidity that is connected with the detected fluid pipeline.In order to reduce fluid resistance losses and prevent that dirt from assembling in pipeline, dynamic throttling element adopts Venturi tube (23), a resilient seal endless belt (16) respectively is installed before and after the Venturi tube, resilient seal endless belt one end and the Venturi tube of Venturi tube front end are sealedly and fixedly connected, the other end is fixedlyed connected with the inlet seal of watchcase, resilient seal endless belt one end and the Venturi tube of Venturi tube rear end are sealedly and fixedly connected, the other end is fixedlyed connected with the exit seal of watchcase, the flow channel of Venturi tube and two resilient seal endless belt leak free detected fluid of formation between watchcase inlet and outlet.Be fixedly mounted on the Venturi tube two slide cartridges (19) respectively be fixedly mounted on two support cup (20) that front and back ends covers and be slidingly matched, form two moving sets, make Venturi tube under the promotion of detected fluid, to move forward and backward along pipeline axial within the specific limits.Fixing or elastic seal ring of semifixed installation between inward flange on the watchcase interlude and the front flange on the Venturi tube makes when measuring in watchcase, forms two mutual disconnected balance cylinders outside the Venturi tube, fills with equilibrium liquid in the balance cylinder.During measurement, forward horizontal stand chamber (21) equilibrium liquid pressure is identical with the anterior resilient seal endless belt of Venturi tube detected fluid wall pressure, and back balance chamber (24) equilibrium liquid pressure is identical with Venturi tube rear elastic seal band detected fluid wall pressure.After taking such technical measures, because two inside and outside pressure at both sides of resilient seal endless belt are identical, not influenced by radial force, therefore less demanding to itself physical strength, can adopt the non-metallic elastic material manufacturing, in like manner, Venturi tube also can adopt thin-wall construction and make with nonmetallic materials.The elasticity shaft-seal diaphragm (12) that is installed on the watchcase interlude is the fulcrum of lever (13), the elasticity shaft-seal diaphragm also plays the effect of sealing simultaneously, make the indoor equilibrium liquid of back balance can not leak into outside the watchcase, equilibrium liquid can be the water of cleaning, also can be mineral oil.Because liquid is incompressible basically, therefore the volume in the equilibrium liquid loop buffer space (25) between resilient seal endless belt, slide cartridge and the support cup three can be very little, in design if assurance its greater than forward horizontal stand chamber or back balance chamber equilibrium liquid overall volume in following issuable maximum compressibility of detected fluid maximum pressure effect.A lever bottom fixed installation transmission fork (26), respectively there is an open slot at the two ends of fork, hard-wired two straight pins place in the open slot on the Venturi tube corresponding position, and open slot and straight pin are formed moving sets, the axially-movable of Venturi tube can be changed into the rotation of lever.Lever upper position place has one to detect semi-girder (7), stick on the foil gauge (1) that detects on the semi-girder and the deflection that semi-girder produces can be converted to the variation of foil gauge output resistance under the thrust of lever, foil gauge should stick on the positive and negative strain regions of semi-girder respectively and be connected to electric bridge (2) by lead, form differential doube bridge or differential full-bridge, so that improve the sensitivity of electric bridge, the nonlinearity erron that reduces electric bridge and the drift of dummy gauge temperature characterisitic.Bridge output voltage is less, needs with voltage amplifier (3) it to be amplified, and the voltage after the amplification is converted to the current signal of 4～20mA to improve the teletransmission ability of transmitter output signal by V/I change-over circuit (4).Transmitter is a two-wire system transformer, the transmitter power supply is provided by direct supply (5) at a distance, the dc current signal of transmitter output is except that teletransmission exports pull-up resistor (6) to, this electric current also flows through a feedback moving-coil (10) that is fixedly mounted on lever top, the feedback moving-coil places in the permanent-magnet steel (9), therefore the DC current of transmitter output is big more, and the feedback moving-coil is just big more to the electro-magnetic feedback power that lever top applies.When lever balance, effect electro-magnetic feedback power and the thrust that acts on the Venturi tube of fluid thereon is proportional, just the thrust that acts on the Venturi tube corresponding to the DC current and the fluid of the transmitter of electro-magnetic feedback power output is proportional, because fluid acts on the square proportional of thrust and tested volumetric flow rate on the Venturi tube, so the DC current exported of transmitter and the flow of detected fluid have definite corresponding relation.Compare with using orifice plate, the dynamic throttling element of middle part through hole use Venturi tube have resistance to flow little, be difficult for stopping up, advantage that measuring accuracy is high.On the one hand, less resistance to flow can reduce the drag losses of the process pipe that flow detection causes, and has energy-conservation benefit; On the other hand, because the thrust that fluid acts on the Venturi tube in measuring process is less, need the electro-magnetic feedback power of this thrust of balance also less, so transmitter adopt simple lever get final product, and the double lever and the vector mechanism of complexity needn't adopting in DDZ-III type differential pressure transmitter.Transmitter adopts simple simple lever can make easier the finishing of adjustment of balance suspender belt (not drawing among Fig. 2) on the lever, simultaneously, in view of the power that acts on the simple lever less, it is thinner that lever just can design, it is smaller that the area of elasticity shaft-seal diaphragm also just can design, and the static pressure additive error that it causes is also less.Strain-type displacement detecting amplifier is made up of strain-type displacement detecting element, electric bridge, voltage amplifier, several parts of V/I change-over circuit, and its circuit as shown in Figure 3.The flexible member of strain-type displacement detecting element is exactly the detection semi-girder among Fig. 2, foil gauge R ₂, R ₃Stick on the normal strain district of detecting semi-girder, foil gauge R ₁, R ₄Stick on the negative strain district of detecting semi-girder, then each foil gauge is wired to electric bridge.Because than higher, and less demanding to its stability that is to say transmitter of the force balancing type, requires to detect the stiffness coefficient k of semi-girder to the requirement of displacement detecting amplifier enlargement factor ₄Enough little, voltage amplifier amplification coefficient k ₇Enough big, can reduce simultaneously the stability requirement that detects semi-girder and voltage amplifier with the reduction cost.Therefore, detect semi-girder and can select the little metal material of temperature coefficient of the elastic modulus, but require the stiffness coefficient k of semi-girder ₄Enough little, promptly very little differential force Δ f just can make foil gauge produce enough Δ R, and then makes subsequent conditioning circuit produce enough electric current changes delta I, and obviously, differential force Δ f is with respect to measured input power f ₁With electro-magnetic feedback power f ₂More little, the measuring accuracy of transmitter is just high more; As a same reason, voltage amplifier is selected the lower instrument amplifier AD8221 of price for use, and AD8221 is 8 pin chips, is to have the enough big enlargement factor (can be by selecting a suitable R to its requirement _GResistance realize), promptly as long as at foil gauge when certain strain value that is subjected to (should guarantee of the ultimate strain of this value during design) less than this foil gauge, the output voltage U of AD8221 _IMaximal value+the 4V that can reach the requirement of follow-up V/I change-over circuit gets final product.The V/I change-over circuit is selected XTR115 type two-wire system current transducer for use, and XTR115 adopts SO-8 miniaturization encapsulation, and its pin 7 is a power end, the direct supply U in the external output current loop _s, in the actual measurement process, transmitter of the force balancing type is usually in measure field, and direct supply U _sWith pull-up resistor R _LIn pulpit, therefore use XTR115 to allow on-the-spot transmitter be easy to realize that safe spark is explosion-proof away from measure field.The pin one of XTR115 provides+supply voltage of 2.5V to strain gauge bridge, and pin 8 provides+supply voltage of 5V for instrument amplifier AD8221, and pin 3 is+2.5V reference voltage source output current I _REFWith+5V voltage stabilizer output current I _REGReturn terminal, can be used as the public ground of input circuit.C is a noise reduction electric capacity in the circuit, and VT is the external NPN power tube of XTR115, and model is 2N4922, and the emitter of 2N4922, base stage, collector connect the pin 5,6,7 of XTR115 respectively, it and the power consumption that can reduce chip after the inner output transistor of XTR115 is in parallel.When detecting the stressed deflection of semi-girder, the voltage signal of strain gauge bridge output at first zooms into the input voltage U of 0.8～4V through instrument amplifier _I, the input resistance R by 20k Ω again _IConvert the input current I of 40～200 μ A to _I, after XTR115 obtains the dc current signal I of 4～20mA after amplifying 100 times _O, by pin 4 outputs of XTR115.In this force balance type closed-loop system of transmitter, when the displacement detecting amplifier magnification ratio on the forward direction passage was enough big, on enough big levels of precision, the input-output characteristic of transmitter only depended on lever system and electro-magnetic feedback device.Therefore, transmitter makes measured flux when measurement lower limit by adjusting the zeroing spring (8) on the lever system, and the output current of transmitter is 4mA; And range to regulate mainly be by changing the position of electro-magnetic feedback device on lever, and change feedback moving-coil tap (coarse adjustment) and realize by the magnetic induction density of regulating in magnetic shunt path screw (30) the change magnet steel air gap (fine tuning).The electro-magnetic feedback device by permanent-magnet steel, magnet steel cover (27), magnet steel at the bottom of (28), the soft iron heart (29), magnetic shunt path screw, copper ring (31) form.The feedback moving-coil that is fixed on the lever is in the magnetic field of permanent-magnet steel, move left and right therein, form magnetic circuit at the bottom of permanent-magnet steel, the soft iron heart, magnet steel cover, the magnet steel, the soft iron heart makes and forms uniform radial magnetic field in the annulus gap, thereby make the direction of current that flows through circular feedback moving-coil always vertical with magnetic direction, when the transmitter output current flows through the feedback moving-coil, will produce one and the proportional electro-magnetic feedback power of output current.

The course of work of this embodiment: when the flow of the detected fluid that flows through Venturi tube increases, fluid just applies a thrust bigger during than former flow to Venturi tube, and make Venturi tube produce a displacement to the right along pipeline axial, this displacement makes the lever that is connected on the Venturi tube be rotated counterclockwise around fulcrum, the lever upper end promotes to detect semi-girder crooked (generation displacement) left, stick on the foil gauge resistance that detects the positive and negative strain regions of semi-girder respectively and produce differential change, electric bridge, voltage amplifier and V/I change-over circuit are that the output current corresponding with detecting the semi-girder displacement changes with this differential resistance change transitions, promptly make the DC current of transmitter output also increase along with the increase of displacement, when the DC current of this increase flows through the feedback moving-coil, the electro-magnetic feedback power that the feedback moving-coil acts on lever top also increases thereupon, when electro-magnetic feedback power increases to when balancing each other with fluid thrust, lever is balance again, by means of the working method of this lever force balance, one of transmitter output and detected fluid flow have the output signal of definite corresponding relation.Need to prove that when the enlargement factor of strain-type displacement detecting amplifier was enough big, the displacement that detects semi-girder was very little in fact.

Claims (2)

1. the transmitter of the force balancing type of a strain-type displacement detecting element, its one-piece construction is a negative feedback closed loop system, by lever system, the displacement detecting amplifier of forward path, the electro-magnetic feedback device of feedback channel is formed, it is characterized in that: the displacement detecting amplifier of forward path adopts strain-type displacement detecting amplifier, strain-type displacement detecting amplifier is by strain-type displacement detecting element, electric bridge, voltage amplifier, several parts of V/I change-over circuit are formed, strain-type displacement detecting element comprises flexible member and the foil gauge two parts that are fixed on the flexible member, foil gauge is connected to electric bridge, electric bridge is converted to the input of change in voltage as voltage amplifier with the resistance variations of foil gauge, the voltage amplifier output voltage is converted to dc current signal through the V/I change-over circuit, this dc current signal is the output current signal of strain-type displacement detecting amplifier, the output current one side of strain-type displacement detecting amplifier is as the output current signal of whole transmitter, this electric current flows through the electro-magnetic feedback device that is fixed on the lever simultaneously to produce electro-magnetic feedback power on the other hand, the electro-magnetic feedback power of measured input power and the output of electro-magnetic feedback device acts on the flexible member of strain-type displacement detecting element in the differential force that comprehensive relatively back on the lever system produces, this differential force makes flexible member produce deformation (displacement) and by the elastic force of flexible member institute balance, the deformation of flexible member makes fixed thereon foil gauge produce corresponding resistance variations again.

2. the transmitter of the force balancing type of strain-type displacement detecting element according to claim 1, it is characterized in that: the transmitter of the force balancing type of strain-type displacement detecting element also can not use lever system, and the electro-magnetic feedback power of measured input power and electro-magnetic feedback device output is acted directly on the comprehensive comparison of carrying out power on the flexible member of strain-type displacement detecting element.

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