RU2654905C1 - Device for converting the resistance changes into voltage - Google Patents

Abstract

FIELD: measuring equipment.

SUBSTANCE: invention relates to electrical measurement techniques, in particular to electrotensometry, and can be used in the aircraft industry, engineering, construction for the study of the structures strength with the help of a single strain gage in the frequency range from 0 to 5,000 Hz and more at an increased level of interfering factors – electromagnetic interference and thermoEMF. Device for converting changes of resistance to voltage comprises a strain gage, a resistor, a switch, a differential amplifier, a bipolar rectangular current source and a subtraction circuit, the current source outputs through the normally closed switch contacts are connected in parallel to the strain gage and through normally open contacts – parallel to the resistor, the inputs of the differential amplifier are connected through normally closed contacts in parallel to the strain gage and through normally open contacts – parallel to the resistor. Output of the differential amplifier is connected to the input of the subtraction circuit. In addition, sequentially coupled synchronous inverting amplifier, a low-pass filter and an output amplifier, the output of the subtracting circuit being connected to the input of a synchronous inverting amplifier, and the output of the output amplifier generates the output of the device. Input of the low-pass filter receives an additive mixture of a unipolar sequence of pulses modulated by a useful signal according to AIM-1, and a bipolar sequence of pulses modulated by an interference signal. Low-pass filter rates allocate a spectrum of the useful signal and effectively compensates for the noise pulses in a wide frequency range, while the amplitude of the useful signal at the output of the filter does not depend on the frequency of the signal. Output amplifier enhances the signal from the low pass rates filter to the desired level, the output of the output amplifier is the output of the device. Result of the conversion is the amplified signal of the strain gage, while the frequency range of the conversion is extended and the non-informative signal components that determine the basic measurement errors are compensated.

EFFECT: technical result consists in increasing the measurement accuracy in the extended frequency range of the measured processes up to 5,000 Hz and more against the background of disturbing factors – electromagnetic interference and termoemf at a clock rate not more than 16 kHz.

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Description

The invention relates to electrical engineering, in particular to electrotensometry, and can be used in the aviation industry, engineering, construction to study the strength of structures using a single strain gauge in the frequency range from 0 to 5000 Hz or more with an increased level of interfering factors - electromagnetic interference and thermoelectric power .ds

Known are converters of resistance to voltage conversion by connecting a single strain gauge in a three-wire circuit to one of the bridge arms, a power source is connected to the supply diagonal of the bridge, the output signal is taken from the output diagonal of the bridge (M.L.Daichik, N.I. Prigorovsky, G.Kh. Khurshudov Methods and means of full-scale tensometry: a reference book “Fundamentals of designing machines.” - M.: Mashinostroenie, 1989, p. 51, Fig. 6). The use of a three-wire inclusion of a strain gauge with equal resistance of the wires of the communication line in the bridge eliminates the influence of the initial resistances of the wires, but there remain errors from the difference in their resistances, which is most affected when working in a wide range of changes in ambient temperature. In addition, such converters are unprotected from external interference due to the asymmetry of the strain gauge switching circuit.

A strain gauge device with a single strain gauge connected in a three-wire circuit is known, comprising three passive bridge arms, a programmable gain amplifier, an analog filter, an analog-to-digital converter, a digital filter, and a control unit. The passive shoulders of the bridge are made on stable resistors and are located on the board of the measuring channel. The tensor bridge is powered from a constant voltage source (Operation manual for the KAM-500 ACRA control ltd data collection system, 2012).

The disadvantage of this device is the effect of changes in the resistance of the wires of the communication line and the three resistances of the bridge on the measurement error when changing the temperature conditions of operation and the low noise immunity of the device due to the asymmetry of the measuring line.

There are various schemes of converters with a strain gauge connected in a three-wire circuit, which solves the problem of reducing the error from the influence of the resistance of the wires of the communication line, correcting the error from nonlinearity, subtracting a constant voltage value corresponding to the initial voltage value on the strain gauge, etc., but is not eliminated error from the influence of external noise on the measurement result (Gutnikov BC Integrated electronics in measuring devices. - 2nd ed., revised and additional - L.: Energoat mizdat, Leningrad Dep-tion, 1988. - p. 76, Figure 2.11)...

Known converters of resistance to voltage with a single strain gauge connected in a four-wire circuit, in which a more complete correction of errors caused by the active resistance of the wires of the connecting line is achieved. The best performance is ensured when the strain gauge is powered by a current source (Gutnikov BC Integrated electronics in measuring devices. - 2nd ed., Revised and additional - L .: Energoatomizdat, Leningrad Department, 1988. - Pages 78 ÷ 80 , Fig. 2.12, 2.13). When using two identical current sources (or one weighted one) and a reference resistor, the initial voltage value on the strain gauge is compensated, but the non-informative components of the signal, including the error from external interference, are not compensated.

A device for converting a change in resistance to voltage is described in Russian patent for invention No. 2366966 G01R 27/02, 2006. The device comprises a strain gauge, a resistor and a current source connected in series, two differential amplifiers and a switch, while the inputs of one differential amplifier through normally closed switch contacts are connected parallel to the strain gauge and through normally open contacts are connected parallel to the resistor, and the inputs of the second differential amplifier are connected via rmalno closed contacts parallel resistor and through normally open - parallel to the strain gauges, the outputs of differential amplifiers are output devices.

In the absence of external interference, the device does not contain uninformative components and provides high accuracy of measuring the low-frequency useful signal.

The disadvantage of this device is low immunity from external electromagnetic interference. The interference voltage at the device output is absent only if the level of interference voltages at the inputs of differential amplifiers is equal. In the absence of interference at the input of one of the differential amplifiers, an amplified interference at the input of the second differential amplifier is present at the output of the device. Therefore, it is necessary to place the strain gauge and resistor in close proximity and to ensure the identity of the parameters of the connecting lines.

Closest to the proposed invention is a device for converting a change in resistance to voltage, patent RU No. 2473919 C1, 09/27/2011, containing a strain gauge, a resistor, a bipolar rectangular current source, a switch, a differential amplifier and a subtraction circuit, while the outputs of the current source through normally closed contacts of the switch are connected in parallel with the strain gauge and through normally open contacts - in parallel with the resistor, the inputs of the differential amplifier are connected through normally closed Tide gages parallel contacts and through normally open - parallel to the resistor and the differential amplifier output is connected to the input of the subtractor, the subtractor circuit forms the output of the output device.

The device provides high accuracy in measuring low-frequency processes against low-frequency interference.

The disadvantage is the low accuracy of measuring high-frequency processes - from tens of Hz to 5000 Hz or more.

Indeed, in the process of operation of the device, the positive and negative sequences of pulses obtained in the 2nd and 4th clock cycles follow from the output of the subtraction circuit (Fig. 3, c). The resulting transformation formula is the difference between the voltage values of the 2nd and 4th measurement cycles. In this case, the useful signals of two clocks are added up, and the interference signals are subtracted. Thus, for further processing, a sequence of unipolar pulses free of interference arrives, the amplitude of which is proportional to the amplitude of the measured process only in the case of static processes (Fig. 3, e). When measuring dynamic processes, the amplitude of the resulting pulses drops sharply with increasing frequency of the process.

The drop in amplitude is associated with the process of calculating the difference of the second and fourth measurement clocks spaced in time. To calculate, for example, in analog form, it is necessary to expand the instantaneous voltage value of the 2nd cycle to the 3rd-4th cycle. As a result, the amplitude of the extended pulses is proportional to the instantaneous value of the measured process and remains constant during the pulse, which corresponds to the modulation of AIM-2 (Fig. 2, d).

The amplitude of each pulse of the 4th work cycle follows the changes in the measured process during the entire time the pulse exists according to the amplitude pulse modulation of AIM-1 (Fig. 2, d).

Thus, the pulses of two different-polarity sequences are subjected to the calculation of the difference, one of which is modulated by AIM-1, and the second by AIM-2.

If the measured process (modulating voltage) is a sinusoidal function r (e) = r _max sin (Ωt + ϕ), then in the AIM-1 spectrum it is contained in the form (Yu.P. Borisov, PI Penin. Basics of multichannel information transfer. M., Communication, 1967, p. 223, Fig. 5.12):

,

,

where B (Ω, t) is the measured process;

- коэффициент амплитудной импульсной модуляции;

- coefficient of amplitude pulse modulation;

A _PAM ^_ constant coefficient that determines the slope of the modulation characteristic;

U ₀ - the amplitude of the modulated pulses of the sequence;

r _max ^_ maximum amplitude of the measured process

T _P and τ - period and pulse duration of the sequence;

Ω is the angular frequency of the measured process.

The same process in the AIM-2 spectrum is contained in the form (Yu.P. Borisov, P.I. Penin. "Fundamentals of multichannel information transfer." M., Communication, 1967, p. 253.31,

fig. 2.4):

where q is the coefficient characterizing the elongation of the pulses.

резко падает, и, как следствие, падает амплитуда полезной компоненты.It is easy to establish that pulse expansion leads to the appearance of a dependence of the amplitude of the useful component of the spectrum on the modulation frequency Ω, and with increasing frequency, the coefficient

drops sharply, and, as a result, the amplitude of the useful component decreases.

Obviously, the amplitude of the useful component of the spectrum of the resulting sequence will also decrease with increasing frequency of the measured process. It is possible to reduce the drop in the amplitude of the useful component by increasing the clock frequency of the device, but in real conditions of on-board measurements, the clock frequency is limited by the duration of the transient processes in the “strain gauge-device” communication line. With a communication line length of 40-50 meters, the clock frequency does not go beyond 16 ÷ 20 kHz, otherwise the measurement error from transients will reach an unacceptable value.

The technical result to which the invention is directed is to increase the measurement accuracy in an extended frequency range of the measured processes to 5000 Hz or more against the background of interfering factors - electromagnetic interference and thermoelectric power. at a clock frequency not exceeding 16 kHz.

To achieve a technical result in a device for converting a change in resistance to voltage, including a strain gauge, a resistor, a switch, a differential amplifier, a rectangular bipolar current source and a subtraction circuit, the current source outputs are connected parallel to the strain gauge through normally closed contacts of the switch and parallel to the resistor through normally open contacts , the inputs of the differential amplifier are connected through normally closed contacts parallel to the strain gauge and through normally open - parallel to the resistor, the differential amplifier output is connected to the input of the subtraction circuit, the current source is made bipolar with a clock frequency not exceeding 16 kHz, a synchronous inverting amplifier, a low-pass filter and an output amplifier are additionally introduced, and the output of the subtraction circuit is connected to the synchronous input inverting amplifier, and the output of the output amplifier forms the output of the device.

The proposed device is illustrated and illustrated by the drawings:

in FIG. 1 is a block diagram of the proposed device;

in FIG. 2 - (a, b, c, d, e) - diagram of the work of the prototype;

in FIG. 3 - (a, b, c, d, d, e, g) is a diagram of the operation of the proposed device.

In FIG. 1 shows a block diagram of the proposed device for converting changes in resistance to voltage.

The device contains a strain gauge Rtr, resistor Ro, switch 1, a bipolar current source of rectangular shape 7, a differential amplifier 2 connected in series by a subtraction circuit 3, a synchronous inverting amplifier 4, a low-pass filter 5 and an output amplifier 6, the outputs of the current source 7 and the inputs of the differential amplifier 2 are connected in parallel by Rtp and Ro through a switch, the output of the differential amplifier 2 is connected to the input of the subtraction circuit 3, the output of the output amplifier 6 is the output of the device for converting resistance to voltage.

A device for converting a change in resistance to voltage works as follows.

In the first state of the switch (closed K5 ÷ K8, K1 ÷ K4 open), the inputs of the differential amplifier 2 and the outputs of the current source 7 are connected in parallel Ro. In the second state (closed K1 ÷ K4, K5 ÷ K8 open), the inputs of the differential amplifier 2 and the outputs of the current source 7 are connected in parallel Rtp.

The measurement process consists of four measures. The first two cycles are carried out with a positive current value of the source 7, the third and fourth cycles - with a negative (Fig. 3, a, b). In this case, the voltages at Ro and Rtp change their sign to the opposite, and the voltage of the low-frequency noise does not depend on the magnitude of the current and its polarity and does not change its sign.

During the first cycle, contacts K5 ÷ K8 are closed, contacts K1 ÷ K4 are open. In this case, the resistor Ro is connected to the current source and the inputs of the differential amplifier 2, the voltage at the output of the differential amplifier 2 is equal to:

Uout1 = Ky⋅I⋅Ro,

where Ku is the gain of the differential amplifier 2; I is the magnitude of the current of the current source. Ro is a constant value, the output voltage Uout1 is also constant and positive during the first measurement step. Uout1 is stored by subtracting 3 until the end of the second measurement step.

In the second cycle, contacts K1 ÷ K4 are closed, contacts K5 ÷ K8 are open. In this case, the resistor Rtp is connected to the current source 7 and the inputs of the differential amplifier 2. The voltage at the output of the differential amplifier 2 is equal to:

Uout ₂ = Кy⋅I⋅ (Rtp + Δ Rtp ₂ ),

where Rtp = Ro is the initial resistance of the strain gauge; Δ Rtp ₂ - a value characterizing the continuous increment of the resistance of the strain gauge under the influence of the measured load during the second measurement step. The voltage Uout _{2 is} positive, and changes its value according to changes in the measured load.

The output voltage of the subtraction circuit 3 in the second cycle:

Ucx ₂ = Uout ₂ -Uout ₁ = Кy⋅I⋅Δ Rtp _2.

The output voltage of the subtraction circuit in the second cycle of operation Ucx ₂ is a positive pulse, modulated in amplitude according to the law of the measured process, and with a duration equal to the duration of the second measurement cycle (Fig. 3, c). The pulse is then fed to the input of the synchronous inverting amplifier.

The measurement process in steps 3 and 4 is carried out at a negative current value and is similar to the work in steps one and two.

The output voltage of the subtraction circuit in the fourth cycle:

Ucx ₄ = -Ky⋅I⋅ΔRtp ₄ ,

where ΔRtp ₄ is a value characterizing the continuous increment of the resistance of the strain gauge under the influence of the measured load during the fourth measurement step.

The output voltage of the subtraction circuit 3 in the fourth cycle of operation Ucx ₄ is a negative pulse, modulated in amplitude according to the law of the measured process, and with a duration equal to the duration of the fourth measurement cycle (Fig. 3, c). The pulse is then fed to the input of the synchronous inverting amplifier 4.

During the measurement, the input of the synchronous inverting amplifier 4 continuously receives the positive and negative sequences of pulses received in the second Ucx2 and fourth Ucx4 clock cycles and modulated in amplitude by the modulating function - the measured process. The amplitude of each pulse in the sequences follows the changes in the measured process over the entire duration of the pulse. The synchronous inverting amplifier 4 passes the pulses received in the second clock cycle of the Ucx2 converter without changes to the input of the low-pass filter, and inverts the pulses of the fourth clock Ucx4. As a result, the input of the low-pass filter 5 receives a unipolar sequence of pulses, modulated in amplitude according to the amplitude pulse modulation of the first kind AIM-1 (Fig. 3, d). The low-pass filter 5 extracts from the sequence spectrum the signal of the measured process, the amplitude of which does not depend on its frequency. The output amplifier 6 amplifies the signal to the desired level (Fig. 3, d). Under the conditions of the Kotelnikov theorem, the frequency of the measured process in the proposed device can be close to the excitation frequency of the current source 8 kHz (clock frequency 16 kHz) and in real conditions of on-board measurements reach values of 5 kHz or more. Further signal processing is not limited by the frequency of excitation of the current source, the sampling frequency of the signal is determined by the required accuracy.

The interference signal Uп does not depend on the measured process and at the output of the subtraction circuit 3 does not change its sign (Fig. 3, f). Synchronous inverting amplifier 4, according to the work in the second cycle, passes the interference signal Uп without changes, and in the fourth cycle of work it inverts (Fig. 3, g). In this case, the input of the low-pass filter 5 receives a sequence of bipolar pulses modulated by an interference signal Uп. The low-pass filter 5 averages the incoming pulses and reduces the level of low-frequency noise Uп to a negligible value.

In fact, the additive mixture of the low-pass filter 5 receives an additive mixture of a unipolar sequence of pulses modulated by a useful signal according to AIM-1 and a bipolar sequence of pulses modulated by an interference signal. The low-pass filter selects the spectrum of the useful signal and effectively compensates for interference pulses in a wide range of frequencies, while the amplitude of the useful signal at the filter output is independent of the signal frequency.

Thus, the proposed device in comparison with the prototype provides a much greater increase in measurement accuracy in the extended frequency range of the measured processes against the background of interfering factors - electromagnetic interference and thermoelectric power.

The proposed device will be used to create on-board multi-channel strain gauge equipment with a single strain gauge to study the strength characteristics of structural elements of aircraft in flight in the frequency range from 0 to 5000 Hz.

Claims (1)

A device for converting a change in resistance to voltage, containing a strain gauge, a resistor, a bipolar rectangular current source, a switch, a differential amplifier and a subtraction circuit, the outputs of the current source through the normally closed contacts of the switch are connected in parallel to the strain gauge and through the normally open contacts of the switch are connected in parallel to the resistor, the inputs of the differential the amplifier is connected through normally closed contacts parallel to the strain gauge and through the normal they are open, parallel to the resistor, and the output of the differential amplifier is connected to the input of the subtraction circuit, characterized in that it is additionally connected with a series-connected synchronous inverting amplifier, a low-pass filter, and an output amplifier, the output of the subtraction circuit being connected to the input of a synchronous inverting amplifier, the output is output amplifier forms the output of the device.

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