CN111024554A - Measuring circuit, measuring equipment and measuring method of vibrating viscoelastic sensor - Google Patents
Measuring circuit, measuring equipment and measuring method of vibrating viscoelastic sensor Download PDFInfo
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Abstract
The invention provides a measuring circuit of a vibrating viscoelastic sensor, which comprises a regulating circuit, a controller, a voltage control current source, an analog switch, the viscoelastic sensor, a signal acquisition circuit, an analog-to-digital converter and a reference resistor, wherein the circuit makes a measuring result insensitive to the drift of most circuit parameters; the circuit measures signals in direct proportion to the impedance of the sensor, does not need to carry out complex transformation, is beneficial to improving the precision while reducing the calculation complexity, has simple circuit structure, is convenient to set the working point of the sensor, is suitable for various samples to be measured and is not easy to stop vibration. The invention also relates to a measuring method of the vibrating viscoelastic force sensor. The method balances the contradiction between the frequency resolution and the time sampling rate during scanning, and can obtain the effects of better frequency resolution and higher time sampling rate. The invention also relates to a measuring device of the vibrating viscoelastic force sensor.
Description
Technical Field
The invention relates to the technical field of viscoelastic force measuring instruments, in particular to a measuring circuit, measuring equipment and a measuring method of a vibrating viscoelastic force sensor.
Background
Vibrating viscoelastic force measuring devices typically utilize a mechanical probe that is driven to oscillate periodically. The mechanical probe is contacted with a sample to be detected, and the viscoelasticity of the sample can influence the motion state of the probe. The viscoelasticity of the sample to be tested can be analyzed by monitoring the motion state of the probe. A conventional vibrating viscoelastic sensor is similar in structure to a dynamic speaker.
The schematic structural diagram and the schematic disassembly diagram of the viscoelastic force sensor are respectively shown in fig. 1 and fig. 2. The permanent magnet generates a uniform magnetic field at an air gap between the permanent magnet and the magnetic pole; the probe adapter is respectively adhered to the inner circle of the unsupported coil and the annular spring component; the outer circle of the ring spring plays a positioning role, and is tightly combined with the magnetic pole, so that the coil is positioned in a uniform magnetic field in an air gap between the permanent magnet and the magnetic pole when the ring spring is installed.
When the coil is energized, it is exposed to a magnetic field, which causes the probe adapter and the annular spring assembly to move together. When an alternating current of a certain frequency is passed through the coil, the moving part of the sensor vibrates at this frequency.
In the measurement of the viscoelasticity, the probe adapter is brought into contact with a sample through a head-mounted disposable probe. The mechanical property of the sample measuring can influence the motion state of a moving part of the sensor, the elasticity of the sample measuring can influence the vibration resonance frequency of the sensor, and the viscosity of the sample measuring can influence the equivalent impedance of the coil in a circuit.
Therefore, the change of the viscoelasticity of the sample can be represented by monitoring the change of the resonance frequency of the sensor and the equivalent impedance of the coil in the measuring process.
The detection circuit of many vibrating viscoelastic sensors has a separate driving part and a detection part, the driving part is only responsible for driving the sensor to vibrate, the detection part detects the motion state of the sensor, and the structure is complex.
In another type of detection circuit, the driving portion and the detection portion share a set of circuits, and simultaneously drive the sensor to vibrate and detect the motion state of the sensor. The specific implementation of this type of circuit is to connect the sensor coil to an oscillating circuit, which generates self-excited oscillation and stabilizes it at resonance. The disadvantage of this type of circuit is that the circuit parameters are difficult to adjust and are easily shut down.
In addition, there are required measurement results with resolution and time sampling rate, wherein the resolution includes frequency resolution and impedance resolution. The impedance resolution is mainly determined by the measurement circuit.
The frequency resolution requirement is that the frequency step at the sweep frequency point is small enough per spectral sweep to enable the measurement to characterize a sufficiently small change in resonant frequency. The frequency resolution is increased firstly by supporting sufficiently small frequency adjustments in the direct digital frequency synthesizer hardware and secondly by setting the sweep frequency step in the measurement method to be sufficiently small.
The single spectrum sweep time depends primarily on the frequency value of the individual frequency bins, the number of setup cycles required for the sensor impedance measurement, and the number of frequency bins.
The value of the frequency point is determined by the position of the resonance frequency, which cannot be changed by the sensor.
The number of set-up cycles is determined by the sensor, and in particular high Q devices require a long set-up time, which cannot be changed.
The requirement of time sampling rate means that in the time of change of the viscoelasticity force to be measured, enough points of resonance frequency and resonance impedance data are obtained to reflect more details of the change of the viscoelasticity force along with the time. This requires that the single spectrum scan time be sufficiently short.
It follows that there is a conflict between the requirements of frequency resolution and time sampling rate: in order to improve the frequency resolution, that is, to require a smaller frequency adjustment step length, more scanning frequency points are inevitably required in the same scanning interval, thereby reducing the time sampling rate.
Disclosure of Invention
In order to overcome the defects of the prior art, the invention provides a measuring circuit of a vibrating viscoelastic force sensor. The invention eliminates the influence of the voltage gain of the digital-to-analog converter, the gain of the first buffer amplifier, the voltage gain of the multiplication type digital-to-analog converter, the gain of the second buffer amplifier and the transconductance gain drift of the voltage control current source, and is insensitive to the drift of the amplitude value of the excitation signal, thereby solving the technical problem.
The invention provides a measuring circuit of a vibrating viscoelastic sensor, which comprises a regulating circuit, a controller, a voltage control current source, an analog switch, a signal acquisition circuit, an analog-to-digital converter and a reference resistor, wherein the viscoelastic sensor is connected with the voltage control current source, the analog switch and the reference resistor in series;
the controller adjusts the adjusting circuit to enable the voltage control current source to output current with preset frequency, phase and amplitude so as to excite the viscoelastic force sensor and the reference resistor;
the signal acquisition circuit is used for acquiring and converting the measurement data signals of the reference resistor and the viscoelastic sensor, and the analog-to-digital converter is used for receiving the signals converted by the signal acquisition circuit, carrying out digital conversion on the signals and then sending the signals to the controller.
Preferably, the conditioning circuit comprises a direct digital frequency synthesizer, a digital-to-analog converter, a reconstruction filter and a multiplication type digital-to-analog converter;
the direct digital frequency synthesizer and the multiplication type digital-to-analog converter are respectively connected with the controller, and the controller enables the voltage control current source to output preset frequency, phase and amplitude by respectively adjusting the direct digital frequency synthesizer and the multiplication type digital-to-analog converter;
the digital-to-analog converter receives the digitized sinusoidal signal output by the direct digital frequency synthesizer and converts the digitized sinusoidal signal into an analog sinusoidal signal;
and the reconstruction filter filters the analog sinusoidal signal output by the digital-to-analog converter.
Preferably, the adjusting circuit further comprises a clock reference, a first buffer amplifier and a second buffer amplifier, wherein a clock signal output by the clock reference is used for a timing reference of the direct digital frequency synthesizer;
the first buffer amplifier and the second buffer amplifier are respectively connected with the reconstruction filter and the multiplication type digital-to-analog converter, and the output signal of the reconstruction filter is input into the multiplication type digital-to-analog converter after passing through the first buffer amplifier; and the output signal of the multiplication type digital-to-analog converter passes through the second buffer amplifier and then is input into the voltage control current source.
Preferably, the signal acquisition circuit comprises a first channel circuit and a second channel circuit, and one end of the first channel circuit and one end of the second channel circuit are respectively connected with the viscoelastic force sensor and the reference resistor; the first channel circuit and the second channel circuit respectively convert the measurement signals of the viscoelastic force sensor and the reference resistor into direct current voltage signals and send the converted direct current voltage signals to the analog-to-digital converter.
Preferably, the first channel circuit comprises a viscoelastic force sensor, a first instrumentation amplifier and a first true effective value converter, the first instrumentation amplifier is connected with the viscoelastic force sensor, and the first true effective value converter is located between the first instrumentation amplifier and the analog-to-digital converter;
the second channel circuit comprises the reference resistor, a second instrumentation amplifier and a second true effective value converter, wherein the second instrumentation amplifier is connected with the reference resistor, and the second true effective value converter is positioned between the second instrumentation amplifier and the analog-to-digital converter;
the first instrument amplifier and the second instrument amplifier are respectively used for amplifying voltage differences between two ends of the viscoelastic force sensor and two ends of the reference resistor to obtain the amplitude of the first sinusoidal alternating voltage and the amplitude of the second sinusoidal alternating voltage;
the first true effective value converter and the second true effective value converter are respectively used for converting the amplitude of the first sinusoidal alternating voltage and the amplitude of the second sinusoidal alternating voltage into a first direct-current voltage signal and a second direct-current voltage signal;
the analog-to-digital converter receives the first direct-current voltage signal and the second direct-current voltage signal and carries out digital conversion on the first direct-current voltage signal and the second direct-current voltage signal.
A measuring device of the vibrating viscoelastic force sensor, the measuring device comprising a measuring circuit of the vibrating viscoelastic force sensor; the measuring apparatus performs a measuring method of the vibrating viscoelastic force sensor.
Preferably, the method for measuring the vibrating viscoelastic force sensor comprises the following steps:
obtaining an offset value, disconnecting an analog switch in the circuit, enabling excitation current and voltage drop of a viscoelastic force sensor and a reference resistor on the circuit to be zero, and enabling a conversion result of an analog-to-digital converter on the circuit to be the offset value caused by the circuit;
calibrating, namely, connecting a standard resistor to a circuit instead of a viscoelastic sensor for calibration to obtain a correction coefficient under each frequency;
frequency scanning, namely obtaining a resonance frequency and a resonance impedance through cyclic scanning, wherein the next scanning is performed by taking the resonance frequency of the previous scanning as a center;
and data processing, namely filtering the resonance frequency sequence and the resonance impedance sequence obtained in the step of frequency scanning by designing a digital filter to obtain the resonance frequency and the resonance impedance.
Preferably, the step of frequency scanning further comprises: an initial wide range frequency sweep and a local fine frequency sweep; wherein the content of the first and second substances,
setting a frequency range with a first width and a first frequency step length by a controller to start the initial wide-range frequency scanning, and measuring an initial resonance frequency;
the controller sets a frequency range with a second width and a second frequency step to perform the local fine frequency scanning by taking the initial resonance frequency as a center to obtain the resonance frequency and the resonance impedance of the current scanning, wherein the second frequency step is smaller than the first frequency step, and the frequency range with the first width is larger than the frequency range with the second width.
Preferably, the step of calibrating further comprises:
the analog switch is switched on, and the controller controls the direct digital frequency synthesizer and the multiplication type digital-to-analog converter to enable the voltage control current source to output current with preset frequency, phase and amplitude to excite the viscoelastic force sensor and the reference resistor;
and subtracting the offset value from each signal value sampled by the analog-to-digital converter, and averaging to obtain a signal value, namely the final signal value of scanning.
Preferably, the sampling time length of the analog-to-digital converter is equal to the period of the excitation signal of the viscoelastic sensor and the reference resistor.
Compared with the prior art, the invention has the beneficial effects that:
the invention discloses a measuring circuit of a vibrating viscoelastic sensor, which is insensitive to the drift of the amplitude of an excitation signal by eliminating the influence of the voltage gain of a digital-to-analog converter, the gain of a first buffer amplifier, the voltage gain of a multiplication type digital-to-analog converter, the gain of a second buffer amplifier and the drift of the transconductance gain of a voltage control current source, namely the circuit makes the measuring result insensitive to the drift of most circuit parameters; the circuit measures signals in direct proportion to the impedance of the sensor, does not need to carry out complex transformation, is beneficial to improving the precision while reducing the calculation complexity, has simple circuit structure, is convenient to set the working point of the sensor, is suitable for various samples to be measured and is not easy to stop vibration. The measuring method of the vibrating viscoelastic sensor balances the contradiction between the frequency resolution and the time sampling rate during scanning, and can obtain the effects of better frequency resolution and higher time sampling rate.
The foregoing description is only an overview of the technical solutions of the present invention, and in order to make the technical solutions of the present invention more clearly understood and to implement them in accordance with the contents of the description, the following detailed description is given with reference to the preferred embodiments of the present invention and the accompanying drawings. The detailed description of the present invention is given in detail by the following examples and the accompanying drawings.
Drawings
The accompanying drawings, which are included to provide a further understanding of the invention and are incorporated in and constitute a part of this application, illustrate embodiment(s) of the invention and together with the description serve to explain the invention without limiting the invention. In the drawings:
fig. 1 is a schematic structural diagram of a conventional viscoelastic force sensor in the background art of the present invention;
fig. 2 is an exploded view of a conventional viscoelastic sensor according to the background of the invention;
fig. 3 is a schematic diagram of the overall structure of the measuring circuit of the vibrating viscoelastic sensor according to the present invention;
fig. 4 is a diagram of an impedance spectrum of a measuring circuit of the vibrating viscoelastic sensor according to the present invention;
fig. 5 is a flowchart of a measuring method of the vibrating viscoelastic force sensor according to the present invention;
reference numerals: 1. coil, 2, permanent magnet, 3, annular spring assembly, 4, probe.
Detailed Description
The present invention will be further described with reference to the accompanying drawings and the detailed description, and it should be noted that any combination of the embodiments or technical features described below can be used to form a new embodiment without conflict.
The invention provides a measuring circuit of a vibrating viscoelastic sensor, which comprises a regulating circuit, a controller, a voltage control current source, an analog switch, a signal acquisition circuit, an analog-to-digital converter and a reference resistor, wherein the viscoelastic sensor is connected with the voltage control current source, the analog switch and the reference resistor in series, the regulating circuit is respectively connected with the controller and the voltage control current source, the analog switch is connected with the controller, and the analog switch is used for controlling the current of the viscoelastic sensor and the reference resistorSwitching on and off; the controller adjusts the adjusting circuit to enable the voltage control current source to output current with preset frequency, phase and amplitude so as to excite the viscoelastic force sensor and the reference resistor; the signal acquisition circuit is used for acquiring and converting measurement data signals of the reference resistor and the viscoelastic sensor, and the analog-to-digital converter is used for receiving the signals converted by the signal acquisition circuit, carrying out digital conversion on the signals and then sending the signals to the controller. In one embodiment, the viscosity of the sample affects the equivalent impedance of the viscoelastic sensor in the circuit, denoted as ZXExciting the equivalent impedance Z by controlling the output current of the current source with a voltageXAnd a reference resistance RREFThe viscoelastic sensor is connected in series with the reference resistor. The analog switch is used for controlling the on-off of the current source, when the analog switch is closed, the exciting current of the current source is normally input to the viscoelastic force sensor (hereinafter, the impedance to be measured) and the reference resistor, and the circuit performs normal measurement; when the analog switch is turned off, the voltage difference between the two ends of the impedance to be measured and the reference resistor is zero, and the circuit cannot measure.
The adjusting circuit enables the voltage control current source to output currents with set frequency, phase and amplitude under the control of the controller to excite the impedance to be measured and the reference resistor. The analog-to-digital converter carries out digital conversion on the measured data signals of the impedance to be measured and the reference resistor and sends the converted data signals to the controller. The controller adjusts the frequency, phase and amplitude of the excitation signal to obtain the measurement result of the analog-to-digital converter, and simultaneously communicates with the outside through the communication interface.
In one embodiment, the conditioning circuit includes a direct digital frequency synthesizer, a digital-to-analog converter, a reconstruction filter, and a multiplicative digital-to-analog converter; the direct digital frequency synthesizer and the multiplication type digital-to-analog converter are respectively connected with the controller, and the controller enables the voltage control current source to output certain frequency, phase and amplitude by respectively adjusting the direct digital frequency synthesizer and the multiplication type digital-to-analog converter; the digital-to-analog converter receives the digitized sinusoidal signal output by the direct digital frequency synthesizer and converts the digitized sinusoidal signal into an analog sinusoidal signal; the reconstruction filter filters the analog sinusoidal signal output by the digital-to-analog converter. In this embodiment, the direct digital frequency synthesizer mainly includes a phase accumulator and a sine lookup table, and the output of the direct digital frequency synthesizer is a digitized sine waveform, and the frequency and the phase of the sine signal can be adjusted through an external interface; the digital-to-analog converter receives the digitized sinusoidal signal output by the direct digital frequency synthesizer and converts the digitized sinusoidal signal into an analog sinusoidal signal; the reconstruction filter is a low-pass filter, that is, a signal with a frequency lower than the cut-off frequency is allowed to pass through, because the analog sinusoidal signal spectrum output by the digital-to-analog converter contains an unnecessary image frequency component, the image frequency component is attenuated by the reconstruction filter, and the necessary sinusoidal signal component is reserved.
In a specific embodiment, the regulating circuit further comprises a clock reference, a first buffer amplifier and a second buffer amplifier, wherein a clock signal output by the clock reference is used for a timing reference of the direct digital frequency synthesizer; the first buffer amplifier and the second buffer amplifier are respectively connected with the reconstruction filter and the multiplication type digital-to-analog converter, and the output signal of the reconstruction filter is input into the multiplication type digital-to-analog converter after passing through the first buffer amplifier; the output signal of the multiplication type digital-to-analog converter passes through the second buffer amplifier and then is input into the voltage control current source. In the embodiment, a clock signal output by the clock reference circuit is used as a timing reference of the direct digital frequency synthesizer; the output of the reconstruction filter is buffered by a first buffer amplifier and then input to the reference input end of the multiplication type digital-to-analog converter; amplitude adjustment of the sinusoidal signal can be realized by utilizing a multiplication type digital-to-analog converter; the output of the multiplication type digital-to-analog converter is buffered by the second buffer amplifier and then input to the input end of the voltage control current source, so that the output current of the voltage control current source is controlled to change according to the set sinusoidal signal with frequency, phase and amplitude.
In one embodiment, the signal acquisition circuit comprises a first channel circuit and a second channel circuit, one end of the first channel circuit and one end of the second channel circuit are respectively connected with the viscoelastic force sensor and the reference resistor; the first channel circuit and the second channel circuit respectively convert the measurement signals of the viscoelastic force sensor and the reference resistor into direct current voltage signals and send the converted direct current voltage signals to the analog-to-digital converter. The first channel circuit comprises a viscoelastic force sensor, a first instrument amplifier and a first true effective value converter, the first instrument amplifier is connected with the viscoelastic force sensor, and the first true effective value converter is positioned between the first instrument amplifier and the analog-to-digital converter;
the second channel circuit comprises a reference resistor, a second instrument amplifier and a second true effective value converter, the second instrument amplifier is connected with the reference resistor, and the second true effective value converter is positioned between the second instrument amplifier and the analog-to-digital converter;
the first instrument amplifier or the second instrument amplifier is respectively used for amplifying the voltage difference between two ends of the viscoelastic force sensor and two ends of the reference resistor to obtain the amplitude of the first sinusoidal alternating voltage and the amplitude of the second sinusoidal alternating voltage;
the first true effective value converter or the second true effective value converter is respectively used for converting the amplitude of the first sinusoidal alternating voltage and the amplitude of the second sinusoidal alternating voltage into a first direct-current voltage signal and a second direct-current voltage signal; the analog-to-digital converter receives the first direct-current voltage signal and the second direct-current voltage signal and carries out digital conversion on the first direct-current voltage signal and the second direct-current voltage signal. In this embodiment, the voltage difference between the impedance to be measured and the two ends of the reference resistor is amplified by the first instrumentation amplifier and the second instrumentation amplifier respectively to obtain the sinusoidal ac voltage signals of the first channel circuit and the second channel circuit respectively, and the amplitudes of the sinusoidal ac voltages of the two channels are respectively in direct proportion to the impedance Z to be measuredXAnd a reference resistance RREF；
Measuring the amplitudes of the sinusoidal alternating voltage signals of the two channels through a first true effective value converter and a second true effective value converter, wherein the true effective value converter outputs a direct current voltage signal which is in direct proportion to the true effective value of the input signal; since the inputs to the true significance converter are all sinusoidal signals, their dc outputs are also proportional to the amplitude of the input sinusoidal signals. Proportional to the impedance Z to be measuredXAnd a reference resistance RREFThe direct current signals of the two channels are input into a digital-to-analog converter for synchronous sampling and digital conversion.
Specifically, the sine wave voltage signals which can be modulated in frequency, phase and amplitude and are generated by circuits such as a direct digital frequency synthesizer, a digital-to-analog converter and a multiplication type digital-to-analog converter are set as follows:
vSET(t)＝VVsin(2πfV+θV)
wherein VV、fV、θVThe amplitude, frequency and phase, respectively, of the sinusoidal signal are variable.
Setting transconductance gain of voltage control current source to GmThen its output current can be expressed as:
iSET(t)＝GmVVsin(2πfV+θV)
the current is used to excite the viscoelastic force sensor and the reference resistor, so that the voltages at the two ends of the viscoelastic force sensor and the reference resistor are respectively:
vX(t)＝ZXGmVVsin(2πfV+θV)
vREF(t)＝RREFGmVVsin(2πfV+θV)
let the voltage gains of the first and second instrumentation amplifiers be A1And A2Then, the outputs of the two instrumentation amplifiers are respectively:
vINA1(t)＝A1ZXGmVVsin(2πfV+θV)
vINA2(t)＝A2RREFGmVVsin(2πfV+θV)
if the transmission ratios from the true effective values of the first true effective value converter and the second true effective value converter to the output dc voltage are K1 and K2, respectively, the dc voltages output by the two converters are:
let the reference voltage of the A/D converter be VConstantTo V pairDCxAnd VDCREFPerforming analog-to-digital conversion, the analog-to-digital converter outputting digital value DataxAnd DataREFIs composed of
Wherein, N is the binary digit number of the analog-to-digital converter.
Therefore, the impedance Z to be measuredXThe calculation formula is as follows:
through the proportional measurement, the influence of the voltage gain of the digital-to-analog converter, the gain of the first buffer amplifier, the voltage gain of the multiplication type digital-to-analog converter, the gain of the second buffer amplifier and the drift of the transconductance gain of the voltage control current source is eliminated, and the circuit is insensitive to the drift of the amplitude of the excitation signal, namely the circuit enables the measurement result to be insensitive to the drift of most circuit parameters; the circuit measures signals in direct proportion to the impedance of the sensor, does not need to carry out complex transformation, is beneficial to improving the precision while reducing the calculation complexity, has simple circuit structure, is convenient to set the working point of the sensor, is suitable for various samples to be measured and is not easy to stop vibration.
By measuring the impedance of the viscoelastic force sensor at different frequencies by the measuring circuit, an impedance spectrum as shown in fig. 4 can be obtained. The frequency value corresponding to the impedance peak point in the impedance frequency spectrum is the resonance frequency, and the resonance frequency value represents the elasticity of the sample measurement; and the impedance value corresponding to the impedance peak point is the resonance impedance, and the resonance impedance represents the viscosity of the sample.
Generally, when measuring the viscoelastic force, the viscoelastic force sensor is subjected to continuous impedance spectrum measurement scanning, data is analyzed, and a curve of the resonance frequency and the resonance impedance changing along with scanning time is obtained, so that a curve representing the change of the elasticity and the viscosity of the sample measurement along with time can be finally obtained.
The measuring method of the vibrating viscoelastic force sensor, as shown in fig. 5, includes the following steps:
and S1, obtaining the offset value, disconnecting the analog switch in the circuit, and making the exciting current and voltage drop of the viscoelastic force sensor and the reference resistor on the circuit zero, wherein the conversion result of the analog-to-digital converter on the circuit is the offset value caused by the circuit. In one embodiment, a de-biasing process of the circuit and calibration using standard impedances are required before starting the sample test. When the analog switch in the circuit is turned off, the exciting currents of the impedance to be measured and the reference resistor are both zero, and the voltage drop on the exciting currents is also zero, namely the effective input signals of the instrument amplifier, the true effective value circuit and the analog-to-digital converter are zero. Two-way conversion result Data of the analog-to-digital converter obtained at the momentBias1、DataBias2I.e. the bias value caused by that part of the circuit. The purpose of the de-bias processing of the circuit is to remove the bias generated by the instrumentation amplifier, the true active circuit and the analog-to-digital converter of the impedance to be measured connected with the standard resistor.
And S2, calibration, namely, connecting the standard resistor to the circuit instead of the viscoelastic sensor, and performing frequency scanning to obtain a correction coefficient under each frequency. In one embodiment, a standard resistor is connected into the circuit instead of the impedance to be measured, frequency scanning in the working frequency range of the sensor is carried out, namely, an analog switch is switched on, the direct digital frequency synthesizer is controlled to output signals with certain frequency omega and phase, the multiplication type digital-to-analog converter is controlled to determine the amplitude of excitation, and therefore the voltage control current source outputs currents with specific frequency omega, phase and amplitude to excite the impedance to be measured and the reference resistor. After a certain number of set-up cycles, according to a certain sampling rate fSThe two paths of input of the analog-to-digital converter are synchronously sampled, and the number of sampling points is controlled, so that the sampling time length is just the period of an excitation signal, namely
Each channelThe scanning results are all subtracted by the Data obtained in the circuit de-biasing stepBias1、DataBias2Two offset values, and averaging the results of each channel after de-offset, i.e. the final scanning result of the channel, and setting the two obtained results as CalDataXOmega and CalDataREF(ω). Because the output voltage of the true effective value converter has certain ripple waves, the cycle of the ripple waves is the cycle of the excitation signal, and the ripple wave influence can be eliminated after cycle averaging. The calculation of the correction coefficient is:
wherein R isCalTo calibrate the resistance, i.e. the standard resistance.
After step S1 and this step, the impedance to be measured calculation formula is modified as follows:
calculation formula of impedance to be measured
In (A), it
Calculated from the measured analog-to-digital conversion results, for the purpose of ensuring the accuracy of the final result, the coefficients
The accuracy of the method is required. For this reason, absolute accuracy of the instrumentation amplifier gain, the voltage transfer ratio of the true effective value conversion chip, and the reference resistance is required. Wherein the coefficients
Is related to the scanning frequency, and is set as K (omega), the precision of the K (omega) is not only the precision at a certain frequency point, but also the absolute precision is required in the whole frequency scanning interval of the viscoelastic force sensorAnd (4) degree. This requirement is relatively difficult to achieve and costly to implement. By calibrating, absolute accuracy of the instrumentation amplifier gain, voltage transfer ratio of the true active conversion chip and reference resistance is not required, and they are only required to have small drift. In the calibration, a standard resistor with a high accuracy, for example a standard metal foil resistor with a ten-thousandth accuracy, is used, and the accuracy and the temperature drift of the resistor are sufficient for the measurement requirements of the viscoelastic force sensor.
And S3, frequency scanning, wherein the resonance frequency and the resonance impedance are obtained through cyclic scanning, and the scanning is carried out by taking the resonance frequency of the previous scanning as the center. In one embodiment, the resonant frequency and resonant impedance of the viscoelastic sensor are obtained by a plurality of cyclic scans, a second scan is centered on the resonant frequency obtained by the first scan, a third scan is centered on the resonant frequency obtained by the second scan, and so on until the sample test scan time ends.
Specifically, the step of frequency scanning further includes: an initial wide range frequency sweep and a local fine frequency sweep; wherein the content of the first and second substances,
setting a frequency range with a first width and a first frequency step length by a controller to start the initial wide-range frequency scanning, and measuring an initial resonance frequency;
the controller sets a frequency range with a second width and a second frequency step to perform the local fine frequency scanning by taking the initial resonance frequency as a center to obtain the resonance frequency and the resonance impedance of the current scanning, wherein the second frequency step is smaller than the first frequency step, and the frequency range with the first width is larger than the frequency range with the second width.
The frequency sweep consists of first performing a sweep of a wider frequency range, i.e. an initial wide range frequency sweep followed by a local fine frequency sweep. When the scanning of the wide frequency range is also the first scanning, the scanning is started after the initial frequency, the end frequency, the frequency step size and the excitation signal amplitude of the initial frequency scanning are written into the controller through the communication interface. After all frequency points are scanned, according to the corrected impedance to be measured calculation formulaCalculating the impedance Zx(omega) obtaining initial frequency scanning data, carrying out data analysis, and measuring initial resonance frequency omegaR(0). Scanning over a wide range of frequencies requires only an approximate range of measured resonant frequencies, and does not require great accuracy.
It should be noted that the data written to the controller via the communication interface is derived from the offline use of a large number of samples, and the data set by the controller may cover most of the possible sample parameters. In addition, the frequency range defined by the start and end frequencies may be as large as possible to ensure that all of the sample resonant frequency ranges are covered, and the frequency step may be relatively long.
In the cyclic local fine frequency scanning, the central frequency of the frequency band is the resonance frequency omega measured by the initial wide-range frequency scanning of the previous stepR(0) (ii) a The frequency band width is the product of the time required by the scanning in the step and the maximum value of the time change rate of the resonant frequency of the sample, and the scanning range is narrower relative to the initial wide-range frequency. The time rate of change maximum of the resonant frequency of the sample is generally determined by means of off-line experiments. The maximum value of the time change rate of the resonant frequency of the sample is taken, so that the resonant frequency of the system can be ensured not to exceed the range of the current scanning frequency band within the scanning time of the step. The step size of the frequency sweep is smaller than that of the first sweep, and the maximum value is determined by the frequency resolution required by the system. After all frequency points are scanned, the impedance Z is calculated according to the corrected impedance calculation formulax(omega) obtaining local fine frequency scanning data, carrying out data analysis, and measuring the resonance frequency omega obtained by the scanningR(n), where n is the number of local fine frequency sweeps.
The frequency scanning in the step needs to repeat the above process circularly, and multiple times of circulating local fine frequency scanning are carried out, namely the central frequency of each local fine frequency scanning is the resonance frequency measured by the last local fine frequency scanning; the frequency band width is the product of the time required for the sweep of this step and the maximum value of the time rate of change of the resonant frequency of the sample. The step size of the frequency sweep is determined by the frequency resolution required by the system. After all frequency points are scanned, the impedance is calculated according to the corrected impedance calculation formulaanti-Zx(omega) obtaining local fine frequency scanning data, carrying out data analysis, and measuring the resonance frequency omega obtained by the scanningR(n) of (a). Where n is the sequence number of the local fine frequency sweep.
In addition, the width of the local fine frequency scanning may need to be corrected in a few cases, for example, the maximum value of the time change rate of the resonance frequency of the sample determined offline is too large, the actually measured change range of the resonance frequency is much smaller than the range of the scanning frequency band, and most of the scanning frequency band is useless scanning; or the maximum value of the time change rate of the resonance frequency of the sample determined off line is too small, and the actually measured change range of the resonance frequency exceeds the range of the scanning frequency band, namely the frequency band range scanned this time does not cover the resonance frequency. The situations of too wide and too narrow scanning frequency band need to be corrected, and the corrected frequency band is used for the next scanning. For example: the measurement time was 45 minutes, with scans every 10 seconds, possibly 270 cycles.
In the step, a narrower scanning bandwidth is used, so that the time overhead of useless frequency scanning is avoided, only a useful frequency band is scanned, the time utilization rate is improved, and better time resolution can be obtained even if the scanning frequency step length is small; because a smaller frequency step is used, the resonance frequency can be distinguished even if the change of the resonance frequency is smaller, and better frequency resolution can be achieved.
And S4, processing data, and filtering the resonance frequency sequence and the resonance impedance sequence obtained in the step S3 by designing a digital filter to obtain the resonance frequency and the resonance impedance. In one embodiment, a sequence of resonant frequencies ω is obtained by local fine frequency scanning in step S30(n) and a resonance impedance sequence Z corresponding theretoR(n) of (a). And a proper digital filter is designed according to the change characteristic of the sample signal, so that the noise signals of the two sequences, particularly the noise signals caused by the vibration of equipment in the measurement process, are filtered, and useful signals reflecting the change of elasticity and viscosity of the sample are reserved.
The invention provides a measuring circuit and a measuring method of a vibrating viscoelastic sensor, the circuit structure is relatively simple, the working point of the sensor is convenient to set, and the circuit is suitable for various samples to be measured and is not easy to stop vibrating; the circuit structure enables the measurement result to be insensitive to the drift of most circuit parameters; the signal measured by the circuit is directly proportional to the sensor impedance, complex transformation is not needed, and the accuracy is improved while the calculation complexity is reduced. The measuring method provided by the invention further removes the bias generated by the device in the circuit; by correction, absolute accuracy of a circuit transmission link is not needed; meanwhile, the contradiction between the frequency resolution and the sampling rate in time during scanning is balanced, and the result with better frequency resolution and higher sampling rate in time can be obtained. The circuit and the method provided by the invention can meet the analysis requirement of the vibrating viscoelastic sensor with an application structure similar to that of an electrodynamic loudspeaker, and have good popularization and application values.
The invention provides a measuring device of a vibrating viscoelastic sensor, which comprises a measuring circuit of the vibrating viscoelastic sensor; the measuring apparatus performs a measuring method of the vibrating viscoelastic force sensor.
The foregoing is merely a preferred embodiment of the invention and is not intended to limit the invention in any manner; those skilled in the art can readily practice the invention as shown and described in the drawings and detailed description herein; however, those skilled in the art should appreciate that they can readily use the disclosed conception and specific embodiments as a basis for designing or modifying other structures for carrying out the same purposes of the present invention without departing from the scope of the invention as defined by the appended claims; meanwhile, any changes, modifications, and evolutions of the equivalent changes of the above embodiments according to the actual techniques of the present invention are still within the protection scope of the technical solution of the present invention.
Claims (10)
1. The measuring circuit of the vibrating viscoelastic sensor is characterized by comprising a regulating circuit, a controller, a voltage control current source, an analog switch, a signal acquisition circuit, an analog-to-digital converter and a reference resistor, wherein the viscoelastic sensor is connected with the voltage control current source, the analog switch and the reference resistor in series;
the controller adjusts the adjusting circuit to enable the voltage control current source to output current with preset frequency, phase and amplitude so as to excite the viscoelastic force sensor and the reference resistor;
the signal acquisition circuit is used for acquiring and converting the measurement data signals of the reference resistor and the viscoelastic sensor, and the analog-to-digital converter is used for receiving the signals converted by the signal acquisition circuit, carrying out digital conversion on the signals and then sending the signals to the controller.
2. The measuring circuit of a vibrating viscoelastic force sensor according to claim 1, wherein said adjusting circuit includes a direct digital frequency synthesizer, a digital-to-analog converter, a reconstruction filter, and a multiplying digital-to-analog converter;
the direct digital frequency synthesizer and the multiplication type digital-to-analog converter are respectively connected with the controller, and the controller enables the voltage control current source to output preset frequency, phase and amplitude by respectively adjusting the direct digital frequency synthesizer and the multiplication type digital-to-analog converter;
the digital-to-analog converter receives the digitized sinusoidal signal output by the direct digital frequency synthesizer and converts the digitized sinusoidal signal into an analog sinusoidal signal;
and the reconstruction filter filters the analog sinusoidal signal output by the digital-to-analog converter.
3. The measuring circuit of a vibrating viscoelastic sensor according to claim 2, wherein said adjusting circuit further comprises a clock reference, a first buffer amplifier and a second buffer amplifier, said clock reference outputting a clock signal for a timing reference of said direct digital frequency synthesizer;
the first buffer amplifier and the second buffer amplifier are respectively connected with the reconstruction filter and the multiplication type digital-to-analog converter, and the output signal of the reconstruction filter is input into the multiplication type digital-to-analog converter after passing through the first buffer amplifier; and the output signal of the multiplication type digital-to-analog converter passes through the second buffer amplifier and then is input into the voltage control current source.
4. The measuring circuit of the vibrating viscoelastic sensor according to claim 1, wherein the signal acquisition circuit includes a first channel circuit and a second channel circuit, one ends of the first channel circuit and the second channel circuit being connected to the viscoelastic sensor and the reference resistor, respectively; the first channel circuit and the second channel circuit respectively convert the measurement signals of the viscoelastic force sensor and the reference resistor into direct current voltage signals and send the converted direct current voltage signals to the analog-to-digital converter.
5. The vibrating viscoelastic sensor measuring circuit according to claim 4, wherein the first channel circuit comprises a viscoelastic sensor, a first instrumentation amplifier connected to the viscoelastic sensor, and a first true-to-significant-value converter between the first instrumentation amplifier and the analog-to-digital converter;
the second channel circuit comprises the reference resistor, a second instrumentation amplifier and a second true effective value converter, wherein the second instrumentation amplifier is connected with the reference resistor, and the second true effective value converter is positioned between the second instrumentation amplifier and the analog-to-digital converter;
the first instrument amplifier and the second instrument amplifier are respectively used for amplifying voltage differences between two ends of the viscoelastic force sensor and two ends of the reference resistor to obtain the amplitude of the first sinusoidal alternating voltage and the amplitude of the second sinusoidal alternating voltage;
the first true effective value converter and the second true effective value converter are respectively used for converting the amplitude of the first sinusoidal alternating voltage and the amplitude of the second sinusoidal alternating voltage into a first direct-current voltage signal and a second direct-current voltage signal;
the analog-to-digital converter receives the first direct-current voltage signal and the second direct-current voltage signal and carries out digital conversion on the first direct-current voltage signal and the second direct-current voltage signal.
6. Measuring device of a vibrating viscoelastic sensor, characterized in that it comprises a measuring circuit of a vibrating viscoelastic sensor according to any one of claims 1 to 5.
7. The method for measuring the vibrating viscoelastic force sensor is characterized by comprising the following steps of:
obtaining an offset value, disconnecting an analog switch in the circuit, enabling excitation current and voltage drop of a viscoelastic force sensor and a reference resistor on the circuit to be zero, and enabling a conversion result of an analog-to-digital converter on the circuit to be the offset value caused by the circuit;
calibrating, namely, connecting a standard resistor to a circuit instead of a viscoelastic sensor for calibration to obtain a correction coefficient under each frequency;
frequency scanning, namely obtaining a resonance frequency and a resonance impedance through cyclic scanning, wherein the next scanning is performed by taking the resonance frequency of the previous scanning as a center;
and data processing, namely filtering the resonance frequency sequence and the resonance impedance sequence obtained in the step of frequency scanning by designing a digital filter to obtain the resonance frequency and the resonance impedance.
8. The method of measuring a vibrating viscoelastic sensor according to claim 7, further comprising, in the step of frequency scanning: an initial wide range frequency sweep and a local fine frequency sweep; wherein the content of the first and second substances,
setting a frequency range with a first width and a first frequency step length by a controller to start the initial wide-range frequency scanning, and measuring an initial resonance frequency;
the controller sets a frequency range with a second width and a second frequency step to perform the local fine frequency scanning by taking the initial resonance frequency as a center to obtain the resonance frequency and the resonance impedance of the current scanning, wherein the second frequency step is smaller than the first frequency step, and the frequency range with the first width is larger than the frequency range with the second width.
9. The method of measuring a vibrating viscoelastic sensor according to claim 7, further comprising, in the step of calibrating:
the analog switch is switched on, and the controller controls the direct digital frequency synthesizer and the multiplication type digital-to-analog converter to enable the voltage control current source to output current with preset frequency, phase and amplitude to excite the viscoelastic force sensor and the reference resistor;
and subtracting the offset value from each signal value sampled by the analog-to-digital converter, and averaging to obtain a signal value, namely the final signal value of scanning.
10. The method of measuring a vibrating viscoelastic sensor according to claim 9, wherein the length of the sampling time of said analog-to-digital converter is equal to the period of the excitation signals of the viscoelastic sensor and the reference resistor.
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Cited By (2)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| CN111443641A (en) * | 2020-04-20 | 2020-07-24 | 英华达（上海）科技有限公司 | Sampling rate correction method, system, device and storage medium |
| CN113640377A (en) * | 2021-10-15 | 2021-11-12 | 中国科学院苏州生物医学工程技术研究所 | Blood viscoelasticity measuring method based on vibration sensor |
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2019
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Cited By (4)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| CN111443641A (en) * | 2020-04-20 | 2020-07-24 | 英华达（上海）科技有限公司 | Sampling rate correction method, system, device and storage medium |
| CN111443641B (en) * | 2020-04-20 | 2021-03-02 | 英华达（上海）科技有限公司 | Sampling rate correction method, system, device and storage medium |
| CN113640377A (en) * | 2021-10-15 | 2021-11-12 | 中国科学院苏州生物医学工程技术研究所 | Blood viscoelasticity measuring method based on vibration sensor |
| CN113640377B (en) * | 2021-10-15 | 2021-12-14 | 中国科学院苏州生物医学工程技术研究所 | Blood viscoelasticity measuring method based on vibration sensor |
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