RU1781637C - Measuring converter of differential capacitive pickup - Google Patents

Abstract

Usage: in measuring technique, in particular for measuring the parameters of electrical circuits. SUMMARY OF THE INVENTION: improving the accuracy of measurements by increasing the sensitivity of the device, as well as expanding the functionality due to obtaining a linear conversion function when working with any type of differential capacitive sensors. The measuring transducer comprises a control unit 1, a reference voltage source 14,

Description

The invention relates to a control and measurement technique, in particular to devices for measuring the parameters of electrical circuits.

Known devices for converting electrical capacitance to a constant voltage,

The disadvantage of the above devices is the non-linearity of the conversion function and the insufficiently high sensitivity when working with differential capacitive sensors.

Closest to the proposed technical essence is a device containing a reference voltage source, the output of which is connected to the first fixed contact of the first switch, the second fixed contact of the first switch is connected to the first fixed contact of the second switch and connected to the device common bus, mobile contacts the first and second switches, respectively, through the measured and reference capacitors are connected to the input of the amplifier, the output of which is through a synchronous detector dinene with the second fixed contact of the second switch, and the output of the synchronous detector is the output of the device, a rectangular voltage generator, the output of which is connected to the control inputs of the switches and synchronous detector

In order to increase the sensitivity and obtain a linear function of conversion into a device containing a reference voltage source, the output of which is connected to the first input of the switch, the first and second capacitors of the capacitive sensor, the second terminals of which are connected through the charge amplifier to the input of the synchronous detector, the first output of the first capacitive capacitor the sensor is connected to the output of the switch, the following are introduced: a control unit, a fetch-storage device and three keys, the output of the reference voltage source is connected to the first the output of the first key, the second input of the switch is connected to the first output of the second key

and connected to the output of the retrieval-storage device, which is the output of the device, the first output of the third key is connected to the common bus of the device, the second conclusions of the keys are connected through the second capacitor of the capacitive sensor to the input of the charge amplifier, the first output of the control unit is connected to the control input of the second key , the second output of the control unit is connected to the control input of the switch, the third output of the control unit is connected to the control input of the first key, the fourth and sixth outputs of the control unit are connected respectively etstvenno first and second control inputs of the synchronous detector, a fifth output of the control unit is connected to the third switch control input, a seventh output control unit is connected to the control input of the sample-storage device having an input connected to the output of the synchronous detector.

Significant differences of the claimed invention are the introduction of three control keys and a retrieval-storage device into the device, as well as new communications: the output of the reference voltage source is connected to the first output of the first key, the first output of the second key is connected to the output of the selector - storage and the second input of the switch, the output of the synchronous detector is connected to the information input of the sample-storage device, the first output of the control unit is connected to the control input of the second key, the third output of the control unit audio connected to the input of the first switch controlling the first and second control inputs of the synchronous detector are connected respectively to the fourth and sixth outputs of the control unit, the control zhod sampling-storage device is connected to the seventh output of the control unit.

In FIG. 1 shows a functional diagram of a measuring transducer of a differential capacitive sensor.

The device comprises a control unit (CU) 1, which includes a rectangular voltage generator 2, the output of which is connected to the input of the first integrating circuit 3. The output of the second integrating circuit 4 is connected to the input R of the counter with decoded outputs 5. The output of the first integrating circuit 3 is connected to the input C of the counter with decrypted outputs 5. The output 4 of the counter 5 is connected to the input of the integrating circuit 4 and connected to the input R of the T-trigger 6. The T input of the trigger b is connected to the output of the first integrating circuit 3. The output of the counter 5 is connected n with the input of the inverter 7, the first input of the logic element 2 OR 8 and is the first output of the control unit 1. The output of the inverter 7 is the second output of the control unit 1. The outputs 1 and 3 of the counter 5 are connected respectively to the first and second inputs of logic element 2 OR 9. Input inverter 10 is connected to the output of generator 2. The output of logic element 2 is connected to the first input of logic element 2 AND 11, the output of which is the fourth output of control unit 1. The output of trigger 6 is the fifth output of control unit 1. The output of logic element 2 is connected to the first input logical element 2, the output of which is the sixth output of the control unit 1. Output 21 of the counter 5 is connected to the second input of logic element 2 OR18, connected to the first input of logic element 2 and is the third output of control unit 1. The output of inverter 10 is connected to the second inputs of logic elements 2, 12 and 13, the Output of the element 2 is the seventh output of the control unit 1.

In addition, the device shown in FIG. 1, contains a reference voltage source 14, the output of which is connected to the first input of the switch 15 and connected to the first contact of the key 16. The second input of the switch 15 is connected to the first contact of the key 17. The first contact of the key 18 is connected to the device common bus. The output of the switch 15 is connected to the first terminal of the capacitor of the capacitive sensor 19, and the second terminals of the keys 16, 17 and 18 are connected to the first terminal of the capacitor of the capacitive sensor 20. The second terminals of the capacitors 19 and 20 are connected to each other and through a series-connected charge amplifier 21, a synchronous detector (SD) 25 and a sampling-storage device (SEC) 31 are connected to the second input of the switch 15. The first, third and fifth outputs of the control unit 1 are connected respectively to the control inputs of the keys 17, 16 and 18. The second output of the control unit 1 is connected with control input n reklyuchatel 15. Fourth m sixth ECU 1 outputs connected respectively to the control inputs DM 25. Seventh yield BU 1 is connected to the input

control of the I / O 31. The output of the I / O 31 is the output of the device.

The charge amplifier 21 comprises a capacitor 22 and a resistor 23, the first terminals of which are connected to the inverting input of the operational amplifier (op amp) 24, which is the input of the charge amplifier 21. The non-inverting input of the op amp 24 is connected to the device common bus. The second terminals of capacitor 22 and resistor 23 are connected to the output of op-amp 24, which is the output of charge amplifier 21.

The synchronous detector (LED) 25 contains: a capacitor 26, the first output of which is the information input of the LED 25. The second output of the capacitor 26 is connected via a key 27 to the inverting input of the op-amp 30, which is connected to the first output of the capacitor 29. The non-inverting input of the op-amp 30 is connected to a common to the device bus and through a key 28 is connected to the second output of the capacitor 26. The second output of the capacitor 26 is connected to the output of the op-amp 30 and is the output of the LED 25. The control inputs of the keys 28 and 27 are the first and second control inputs of the LED 25, respectively.

The sampling-storage device (SEC) 31 contains a key 32, the first output of which is the information input of the SEC 31, The second output of the key 32 is connected to the non-inverting input of the op-amp 33 and connected to the device common bus through a capacitor 34. The output of the op-amp 33, which is the output of the I / O 31, is connected to the non-inverting input of this amplifier. The control input of key 32 is the control input of I / O 31.

The lack of sensitivity of the prototype due to its conversion function:

UBUX Uo r t o

where ioh is the value of the output voltage of the converter,

Cx is the value of the capacitance of the measured capacitor,

Co is the value of the capacitance of the reference capacitor,

Uo is the value of the output voltage of the reference source.

In this case, the relative value of the output voltage increment of the Divine transducers, when the measured capacitance changes by the value of ACx, is determined from the expression:

Lee GCX CX ACX

Aiayh uo ip- -rJ vr)

LoOoLo

.. And Shhgl

 Uo-Ј- ..11)

The conversion function of the proposed device is described by the expression: i r 11 Cx - Co

UBBIX Uo 7-GT5

-X VO

Using the proposed device allows you to provide a relative value of the increment of the output voltage L1) output, when changing the value of the measured capacitance by the value of DSx: Uebix

Co - Cx Co - Cx H- A Cx- | / gCO - Cx1

Co + CXJ

U ° Co + Cx

 UC

Co + Cx -SC:

2 DSH

US1

Cx

(a-dxx) - (cx-dxx)

about

(2)

From the analysis of expressions (1) and (2) it follows that for a value of Cx of the same order with the value of the capacity Co. the proposed structure provides a higher sensitivity, prototype.

Using the proposed structure makes it possible to provide a linear conversion characteristic when working with differential capacitive sensors with a varying gap value, as well as with sensors with a variable area of plates. The capacitances of the measured Cx and reference Co capacitors are described by the expression:

g Ј Sx g Ј§o

-h ;;

dxdo

where Ј is the dielectric constant of the dielectric of the capacitive sensor.

Sx, So - respectively, the area of mutual overlap of the plates of the measured and reference capacitors of the sensor,

dx, do, respectively, the gaps between the plates of the measurements and the reference capacitance of the sensor.

Ј So Ј S

Co - Cx do dx

C0 + Cx

ЈSo eSx do dx

 So dx Sx do So dx + Sx d0

(3)

Expression (3) indicates that the conversion function of the new device will be linear both when working with differential sensors with a variable area of plates, and differential sensors with a varying amount of clearance. Therefore, we can conclude that the proposed device can be used as a universal tool with any type of differential capacitive sensors.

The claimed device is a compensation converter with

balancing charges. The structural diagram of the measuring transducer is shown in figure 1. The operation algorithm of the device is set by the control unit (CU) 1.

5 The output signal of the voltage generator of rectangular shape 2 through the integrating circuit 3 is supplied to the clock inputs of the counter - decoder 5 and T-trigger b. (The output voltage of the generator 2 is shown in Fig. 2.1). The device operation cycle is divided into two conversion clock cycles and is set by the sequence of output pulses of the counter - decoder 5 and T-trigger b. To synchronize their work in

15, the device is integrated circuit 4. The time constant of this circuit has a value:

R4 C4 # (1-2) g, where t is the trigger time b,

20 R 4d is the time constant of the integrating circuit 4.

The integration circuit 3 is designed to generate a phase shift between the control signals of the switch 15,

25 keys 16, 17, 18 and control signals LED 25 and I / O 31. Gating control signals LED 25 and I / O 31 is carried out using the inverter 10 and logic elements 11, -12 and 13. Control signals

30, the operation of the switch 15 and the keys 16, 17, 18 are shown in figures 2.3, 2.5 and 2.2, respectively. The voltages supplied to the first and second control inputs of LEDs 25 and I / O 31 are shown respectively in FIG.

35 2.7, 2.6 and 2.8. As a result of this algorithm of operation of the switch 15 and the switches 16, 17 and 18, the voltages shown in Figs. 2.9 (Voltage across capacitors

40 19 and 20 are shown by solid and dash-dotted lines, respectively). As noted above, the process of balancing the charges on the capacitors of the capacitive sensor 19 and 20 is divided into two clock cycles. In the first

On the 45th conversion step, switch 15, switches 17 and 18 are involved, the voltage taken from the output of the charge amplifier 21 (shown in Fig. 2.10) is described by the expression:

fifty

A I I, i I Cl9C19. , C20

AU, -Uo- -ive- -ive.

This voltage is fed to the input of the synchronic detector (LED) 25. At the moment of switching the switch 27 (see Fig. 2.6), the voltage Di-i is scaled with the coefficient -C2b / C29. As a result of this, the voltage is generated at the output of the op-amp 30:

Cae

(and 0C19-IOutS19-IOutS20).

(4)

When the key 27 is opened, the LED 25 enters the information storage mode, and the capacitor 26 is recharged using the key 28 (see Fig. 27).

In the second conversion cycle, the keys 16 and 18 take part in the work. As a result, a rectangular pulse with an amplitude Uo is formed on the capacitor 20.

In this case, the negative voltage increment at the output of the charge amplifier 21 has the value:

.

and consequently, the voltage increment AU4 at the output of the LED 25, after the operation of the key 27, can be described by the expression:

,,, Sao С26 Щ Uo

C22 C29

The result of the algebraic summation of the voltage increments ALJ2 and AU4, during one conversion cycle, is recorded in the I / O 31. The process of recording the result of this mathematical operation is achieved when the key 32 is closed. The control signal of the operation of the key 32 is shown in FIG. 2.8. Based on the operation algorithm of this device, it follows that the balancing process in the claimed device ends when the equality of the increments AU2 and AU4 is reached. Therefore, the following equality holds: A Ub A LM,

 (UoCl9-UBb, xCl9-UBb, xC2o) С20 С26

Uo

and

ooh

Uo

C19 + C20

Physically, the balancing process consists in setting the output voltage of the UBUX, which meets the requirements of equality (6). In FIG. 2.11, the solid line shows the output voltage of the LED 25, and the dotted line shows the output voltage of the UVX 31. It should be noted that the timing diagrams shown in FIG. 2 reflect the equilibrium state in the measuring circuit of the device.

(eleven)

C22 C29

which as a result of the reduction of members C26, C22 and C29 has the form:

UoCl9-UBbixCl9-UBbixC20 UoC20. (6)

From the expression (6) it is easy to determine the conversion function of this device:

C19 - C20g-l

To evaluate the transients, it is advisable to refer to the difference equation describing the balancing process in the device:

i i Cl9C26,, g ,, C19 C26

 Uo -oggogg - new

0

5

0

5

C22 C29

C20 C26

C22 C29

- live

C22 C29

11 CZQ C26, ,, r, Uoo-. + live inj,

(8)

 C22 C29

where is the value in the previous step of converting the output voltage of the device,

the output voltage value of the device in the subsequent conversion cycle.

As a result of the Z-transformation, practical expression (8) takes the form:

H ", H-u.C CfiZ

-U

C22C2) Cgo Cj6

TIN

SlchSge SgoS

gS2, CggS

yy-gh

° С СГЧг-ф-И

CvfCze, ЈгоЈ | б Щ СггСгч СггСстЩ

 Iwm

-G, (With hSGB P

- (СггСгч СггСг, |

where is the initial value of the output

voltage before the start of the balancing process (This value can lie in the range (-E) - (+ E), where E is the value of the bipolar supply voltage of the device);

The solution of the above difference equation has the form:

gg

40

5

fifty

+ "4-s (

/

where а р-Щт- (Gig + С2о) - coefficient

U22

transmitting a negative feedback loop of the device.

Analysis (9) led to the conclusion that:

with a-1, the device is balanced in one conversion cycle. In this case, the balancing time of the converter is eight periods of the signal of the reference oscillator 2, and the transient is critical;

at a 1, the transient response of the converter is approximated by the transition characteristic of the inertial link of the first order, and the transition process

in this case is aperiodic in nature;

at 2 a 1, the transient is oscillatory-damped in nature, and the transient response of the device is approximated by the transient response of the second order link;

at a 2, the transmitter is self-excited. In this case, the drive process is caused by an increased value of the gain of the negative feedback circuit.

at 0 a k, where k is any negative number. In this case, the feedback becomes positive and the device self-energizes. The state of the device at a 0 corresponds to the feedback circuit and loss of operability.

The above analysis allows the creation of measuring transducers with desired dynamic properties.

This converter was prototyped at the department of information-measuring equipment of the Leningrad Polytechnic Institute named after M.I. Kalinin. Currently, experimental design bureaus EPO Signal of the city of Engels, Saratov Region, have made prototypes and conducted bench tests of this device. The results of practical studies when operating the converter with differential capacitive sensors with a measuring range of capacities of 1.2-5.6 pF showed that the conversion accuracy of this device is not worse than 0.05% with a balancing time of the measuring circuit of 0.8 ms, the ego is three times higher than with known devices.

As the element base of the converter, K 544, K 590, K 140, and K 561 series microcircuits were used. Counter - decoder 5 is made on the K 561 IE8 microcircuit.

The economic effect was not calculated, since the aim of the proposed invention is to increase the accuracy achieved by increasing the resolution due to high

sensitivity value of the claimed device.

Claims (1)

SUMMARY OF THE INVENTION A measuring transducer of a differential capacitive sensor comprising a reference voltage source whose output is connected to the first input of the switch, first and second capacitors of the capacitive sensor, the first terminals of which are connected through the charge amplifier to the information input of the synchronous detector, and the second terminal of the first capacitor of the capacitive sensor with a switch output, characterized in that, with the aim of increasing the sensitivity and expanding the functionality due to obtaining a linear conversion characteristic as when working with differential capacitive sensors with a varying gap between the plates, and when working with differential capacitive sensors with a changing area of the plates, a control unit and a device are introduced into it sample-storage and three keys, the first output of the first key is connected to the output of the reference voltage source, the first output of the second key is connected to the second input of the switch and the output of the device sample-storage, which is the output of the device, the first output of the third key is connected to the common bus of the device, the second conclusions of the keys through the second capacitor of the capacitive sensor are connected to the input of the charge amplifier, and the output of the synchronous detector is connected to the information input of the sample-storage device, the control input of the switch is connected to the second output of the control unit, inputs the controls of the first, second and third keys are connected respectively to the third first and fifth outputs of the control unit, the first and second control inputs of the synchronous detector are connected to the sixth and fourth inputs of the control unit, respectively, and the seventh output of the control unit is connected to the control inputs of the fetch-storage device. pdshshshpshshppsh: 2.1  p.pl: I U 4 2.2 2.3 .2.4