Measuring circuit, measuring device and measuring method of vibration type viscoelastic force sensor

Abstract

The present invention provides a measuring circuit for a vibrating viscoelastic force sensor, including a regulating circuit, a controller, a voltage-controlled current source, an analog switch, a viscoelastic force sensor, a signal acquisition circuit, an analog-to-digital converter, and a reference resistor. The circuit makes the measurement result insensitive to the drift of most circuit parameters. The signal measured by the circuit is proportional to the sensor impedance, and no complex transformation is required. This reduces the computational complexity and is also beneficial to improving the accuracy. The circuit structure is simple, and the sensor working point is easy to set. It is suitable for a variety of samples to be tested and is not easy to stop. The present invention also relates to a measuring method for a vibrating viscoelastic force sensor. The method balances the contradiction between the frequency resolution during scanning and the sampling rate in time, and can obtain the effect of having better frequency resolution and higher time sampling rate. The present invention also relates to a measuring device for a vibrating viscoelastic force sensor.

Description

Technical Field

[0001] The present invention relates to the technical field of viscoelastic force measuring instruments, and in particular to a measuring circuit, measuring equipment and a measuring method of a vibration type viscoelastic force sensor. Background Art

[0002] The vibrating viscoelastic force measuring device usually uses a mechanical probe that is driven to oscillate periodically. The mechanical probe contacts the sample to be tested, and the viscoelasticity of the sample affects the motion state of the probe. By monitoring the motion state of the probe, the viscoelasticity of the sample to be tested can be analyzed. An existing vibrating viscoelastic force sensor has a structure similar to that of an electrodynamic speaker.

[0003] The structural diagram and splitting diagram of the viscoelastic force sensor are shown in Figure 1 , Figure 2 As shown. The permanent magnet generates a uniform magnetic field in the air gap between it and the magnetic pole; the probe adapter is respectively adhered to an unsupported coil and the inner circle of the annular spring assembly; the outer circle of the annular spring plays a positioning role, and it is tightly combined with the magnetic pole. When installed, the above coil is in the uniform magnetic field in the air gap between the permanent magnet and the magnetic pole.

[0004] When the coil is passed through by current, it is in a magnetic field and thus is acted upon by magnetic force, thereby driving the probe adapter and the annular spring assembly to move together. When an alternating current of a certain frequency passes through the coil, the moving part of the sensor will vibrate at this frequency.

[0005] When measuring viscoelastic force, the probe adapter contacts the sample through a disposable probe installed on the head. The mechanical properties of the sample will affect the motion state of the sensor's moving parts, the elasticity of the sample will affect the resonant frequency of the sensor's vibration, and the viscosity of the sample will affect the equivalent impedance of the coil in the circuit.

[0006] Therefore, by monitoring the changes in the resonant frequency of the sensor and the equivalent impedance of the coil during the measurement process, the changes in the viscoelastic force of the sample can be characterized.

[0007] The detection circuits of many vibration viscoelastic force sensors have separate driving and detecting parts. The driving part is only responsible for driving the sensor to vibrate, while the detecting part detects the motion state of the sensor, and the structure is relatively complex.

[0008] Another type of detection circuit uses a common circuit for the driving part and the detection part, which drives the sensor to vibrate and detects its motion state at the same time. The specific implementation of this type of circuit is to connect the sensor coil to an oscillation circuit, so that the circuit generates self-excited oscillation and stabilizes in a resonant state. The disadvantage of this type of circuit is that the circuit parameters are difficult to adjust and it is easy to stop oscillating.

[0009] In addition, for the measurement results with requirements for resolution and time sampling rate, the resolution includes frequency resolution and impedance resolution. The impedance resolution is mainly determined by the measurement circuit.

[0010] The frequency resolution requirement is that each time the spectrum is scanned, the frequency step of the scanning frequency point is small enough so that the measurement result can represent a sufficiently small resonant frequency change. To improve the frequency resolution, the direct digital frequency synthesizer hardware must first support sufficiently small frequency adjustment, and secondly, the scanning frequency step set in the measurement method must be small enough.

[0011] The single spectrum scan time mainly depends on the frequency value of a single frequency point, the number of settling cycles required for the sensor impedance measurement, and the number of frequency points.

[0012] The value of the frequency point is determined by the resonant frequency position, which is determined by the sensor and cannot be changed.

[0013] The number of settling cycles is determined by the sensor. In particular, high-Q devices require a longer settling time and this value cannot be changed.

[0014] The requirement of time sampling rate is to obtain enough resonant frequency and resonant impedance data within the measured viscoelastic force change time to reflect more details of the viscoelastic force change with time. This requires that the single spectrum scanning time is short enough.

[0015] It can be seen from this that there is a contradiction between the requirements for frequency resolution and time sampling rate: to improve the frequency resolution, a smaller frequency adjustment step is required. Under the same scanning interval, more scanning frequency points are inevitably required, which leads to a decrease in the time sampling rate. Summary of the invention

[0016] In order to overcome the deficiencies of the prior art, the present invention provides a measurement circuit for a vibrating viscoelastic force sensor. The present invention eliminates the influence of the drift of the digital-to-analog converter voltage gain, the first buffer amplifier gain, the multiplication type digital-to-analog converter voltage gain, the second buffer amplifier gain, and the voltage-controlled current source transconductance gain, and is insensitive to the drift of the excitation signal amplitude, so as to solve the above technical problems.

[0017] The present invention provides a measuring circuit of a vibrating viscoelastic force sensor, comprising a regulating circuit, a controller, a voltage-controlled current source, an analog switch, a signal acquisition circuit, an analog-to-digital converter, and a reference resistor. The viscoelastic force sensor is connected in series with the voltage-controlled current source, the analog switch, and the reference resistor. The regulating circuit is connected to the controller and the voltage-controlled current source, respectively. The analog switch is connected to the controller. The analog switch is used to control the on-off of the current of the viscoelastic force sensor and the standard resistor.

[0018] The controller adjusts the regulating circuit so that the voltage-controlled current source outputs a current of a preset frequency, phase and amplitude to excite the viscoelastic force sensor and the reference resistor;

[0019] The signal acquisition circuit is used to acquire and convert the measurement data signals of the reference resistor and the viscoelastic force sensor. The analog-to-digital converter receives the signal converted by the signal acquisition circuit, performs digital conversion on it, and then sends it to the controller.

[0020] Preferably, the regulating circuit comprises a direct digital frequency synthesizer, a digital-to-analog converter, a reconstruction filter and a multiplication-type digital-to-analog converter;

[0021] The direct digital frequency synthesizer and the multiplication type digital-to-analog converter are respectively connected to the controller, and the controller adjusts the direct digital frequency synthesizer and the multiplication type digital-to-analog converter respectively so that the voltage-controlled current source outputs a preset frequency, phase and amplitude;

[0022] The digital-to-analog converter receives the digitized sinusoidal signal output by the direct digital frequency synthesizer and converts it into an analog sinusoidal signal;

[0023] The reconstruction filter filters the analog sinusoidal signal output by the digital-to-analog converter.

[0024] Preferably, the regulating circuit further comprises a clock reference, a first buffer amplifier and a second buffer amplifier, and the clock signal output by the clock reference is used as a timing reference of the direct digital frequency synthesizer;

[0025] The first buffer amplifier and the second buffer amplifier are connected to the reconstruction filter and the multiplication type digital-to-analog converter respectively. The output signal of the reconstruction filter passes through the first buffer amplifier and then inputs the signal into the multiplication type digital-to-analog converter; the output signal of the multiplication type digital-to-analog converter passes through the second buffer amplifier and then inputs the voltage-controlled current source.

[0026] Preferably, the signal acquisition circuit includes a first channel circuit and a second channel circuit, and one end of the first channel circuit and the second channel circuit are respectively connected to 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 DC voltage signals, and send the converted DC voltage signals to the analog-to-digital converter.

[0027] Preferably, 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 to the viscoelastic force sensor, and the first true effective value converter is located between the first instrument amplifier and the analog-to-digital converter;

[0028] The second channel circuit includes the reference resistor, a second instrument amplifier and a second true effective value converter, the second instrument amplifier is connected to the reference resistor, and the second true effective value converter is located between the second instrument amplifier and the analog-to-digital converter;

[0029] The first instrument amplifier and the second instrument amplifier are used to amplify the voltage difference between the two ends of the viscoelastic force sensor and the two ends of the reference resistor, respectively, to obtain the amplitude of the first sinusoidal alternating voltage and the amplitude of the second sinusoidal alternating voltage;

[0030] The first true effective value converter and the second true effective value converter are used to convert 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 respectively;

[0031] The analog-to-digital converter receives the first DC voltage signal and the second DC voltage signal and performs digital conversion on the two.

[0032] A measuring device for a vibrating viscoelastic force sensor comprises a measuring circuit for the vibrating viscoelastic force sensor; the measuring device executes a measuring method for the vibrating viscoelastic force sensor.

[0033] Preferably, the measuring method of the vibrating viscoelastic force sensor comprises the following steps:

[0034] Obtain the bias value, disconnect the analog switch in the circuit, and the excitation current and voltage drop of the viscoelastic force sensor and the reference resistor in the circuit are zero. At this time, the conversion result of the analog-to-digital converter in the circuit is the bias value caused by the circuit;

[0035] Calibration: connect a standard resistor to the circuit instead of the viscoelastic force sensor for calibration to obtain the correction coefficient at each frequency;

[0036] Frequency scanning, obtaining the resonance frequency and resonance impedance by cyclic scanning, wherein the next scan is performed with the resonance frequency of the previous scan as the center;

[0037] Data processing: by designing a digital filter, the resonance frequency sequence and the resonance impedance sequence obtained in the step frequency scanning are filtered out to obtain the resonance frequency and the resonance impedance.

[0038] Preferably, the frequency scanning step further includes: initial wide range frequency scanning and local fine frequency scanning; wherein,

[0039] The controller sets a frequency range of a first width and a first frequency step to start the initial wide range frequency scan, and measures an initial resonant frequency;

[0040] The controller sets a frequency range of a second width and a second frequency step to perform the local fine frequency scan with the initial resonant frequency as the center to obtain the resonant frequency and resonant impedance of the current scan, wherein the second frequency step is smaller than the first frequency step, and the frequency range of the first width is larger than the frequency range of the second width.

[0041] Preferably, the calibration step also includes:

[0042] The analog switch is turned on, and the controller controls the direct digital frequency synthesizer and the multiplication type digital-to-analog converter to make the voltage-controlled current source output a current with a preset frequency, phase and amplitude to excite the viscoelastic force sensor and the reference resistor;

[0043] The signal value obtained by subtracting the offset value from each signal value sampled by the analog-to-digital converter and taking the average value is the final signal value of the scan.

[0044] Preferably, the sampling time length of the analog-to-digital converter is equal to the period of the excitation signal of the viscoelastic force sensor and the reference resistor.

[0045] Compared with the prior art, the present invention has the following beneficial effects:

[0046] The present invention discloses a measuring circuit of a vibrating viscoelastic force sensor. The circuit is insensitive to the drift of the excitation signal amplitude by eliminating the influence of the drift of the digital-to-analog converter voltage gain, the first buffer amplifier gain, the multiplication type digital-to-analog converter voltage gain, the second buffer amplifier gain, and the voltage-controlled current source transconductance gain. That is, the circuit makes the measurement result insensitive to the drift of most circuit parameters. The signal measured by the circuit is proportional to the sensor impedance, and no complex transformation is required. While reducing the calculation complexity, it is also beneficial to improve the accuracy. The circuit structure is simple, the sensor working point is easy to set, and it is suitable for a variety of samples to be tested without easy to stop vibration. The measuring method of the vibrating viscoelastic force sensor balances the contradiction between the frequency resolution during scanning and the sampling rate in time, and can obtain the effect of having better frequency resolution and higher time sampling rate.

[0047] The above description is only an overview of the technical solution of the present invention. In order to more clearly understand the technical means of the present invention and implement it according to the contents of the specification, the following is a detailed description of the preferred embodiments of the present invention in conjunction with the accompanying drawings. The specific implementation of the present invention is given in detail by the following embodiments and their accompanying drawings. BRIEF DESCRIPTION OF THE DRAWINGS

[0048] The drawings described herein are used to provide a further understanding of the present invention and constitute a part of this application. The exemplary embodiments of the present invention and their descriptions are used to explain the present invention and do not constitute an improper limitation of the present invention. In the drawings:

[0049] Figure 1 It is a structural schematic diagram of an existing viscoelastic force sensor in the background technology of the present invention;

[0050] Figure 2 It is a schematic diagram of an exploded view of an existing viscoelastic force sensor in the background technology of the present invention;

[0051] Figure 3 It is a schematic diagram of the overall structure of the measurement circuit of the vibration type viscoelastic force sensor of the present invention;

[0052] Figure 4 The impedance spectrum diagram of the measuring circuit of the vibration type viscoelastic force sensor of the present invention;

[0053] Figure 5 It is a flow chart of the measuring method of the vibration type viscoelastic force sensor of the present invention;

[0054] Figure numerals: 1. coil, 2. permanent magnet, 3. annular spring assembly, 4. probe. DETAILED DESCRIPTION

[0055] The present invention is further described below in conjunction with the accompanying drawings and specific implementation methods. It should be noted that, under the premise of no conflict, the various embodiments or technical features described below can be arbitrarily combined to form a new embodiment.

[0056] The present invention provides a measuring circuit of a vibrating viscoelastic force sensor, such as Figure 3 As shown, it includes a regulating circuit, a controller, a voltage-controlled current source, an analog switch, a signal acquisition circuit, an analog-to-digital converter, and a reference resistor. The viscoelastic force sensor is connected in series with the voltage-controlled current source, the analog switch, and the reference resistor. The regulating circuit is connected to the controller and the voltage-controlled current source respectively, and the analog switch is connected to the controller. The analog switch is used to control the on-off of the current of the viscoelastic force sensor and the reference resistor; the controller adjusts the regulating circuit so that the voltage-controlled current source outputs a current of a preset frequency, phase, and amplitude to excite the viscoelastic force sensor and the reference resistor; the signal acquisition circuit is used to collect and convert the measurement data signal of the reference resistor and the viscoelastic force sensor, and the analog-to-digital converter receives the signal converted by the signal acquisition circuit, and converts it into a digital form before sending it to the controller. In one embodiment, the viscosity of the sample will affect the equivalent impedance of the viscoelastic force sensor in the circuit, which is recorded as Z X , use the output current of the voltage-controlled current source to excite the equivalent impedance Z X and reference resistor R REF, the viscoelastic force sensor and the reference resistor are in series. The analog switch is used to control the on and off of the current source. When the analog switch is closed, the excitation current of the current source is normally input to the viscoelastic force sensor (hereinafter referred to as the impedance to be measured) and the reference resistor, and the circuit performs normal measurement; when the analog switch is disconnected, the voltage difference between the impedance to be measured and the reference resistor is zero, and the circuit cannot perform measurement.

[0057] Under the control of the controller, the regulating circuit enables the voltage-controlled current source to input a current with a set frequency, phase and amplitude to excite the impedance to be measured and the reference resistor. The analog-to-digital converter converts the measurement data signal of the impedance to be measured and the reference resistor into digital form and sends it 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 communicates with the outside through the communication interface.

[0058] In a specific embodiment, the regulating circuit includes 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 to a controller, and the controller adjusts the direct digital frequency synthesizer and the multiplication type digital-to-analog converter respectively so that the voltage-controlled current source outputs a certain frequency, phase and amplitude; the digital-to-analog converter receives the digitized sinusoidal signal output by the direct digital frequency synthesizer and converts it 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 sinusoidal lookup table, and its output is a digitized sinusoidal waveform, and the frequency and phase of the sinusoidal 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 it into an analog sinusoidal signal; the reconstruction filter is a low-pass filter, that is, it allows signals below the cutoff frequency to pass. Since the analog sinusoidal signal spectrum output by the above digital-to-analog converter contains unnecessary image frequency components, the reconstruction filter is used to attenuate the image frequency components and retain the necessary sinusoidal signal components.

[0059] In a specific embodiment, the adjustment circuit also includes a clock reference, a first buffer amplifier and a second buffer amplifier. The clock signal output by the clock reference is used as a timing reference for a direct digital frequency synthesizer; the first buffer amplifier and the second buffer amplifier are respectively connected to a reconstruction filter and a 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 is input into a voltage-controlled current source after passing through the second buffer amplifier. In this embodiment, the clock signal output by the clock reference circuit is used as a timing reference for a direct digital frequency synthesizer; the output of the reconstruction filter is buffered by the first buffer amplifier and then input into a reference input terminal of the multiplication type digital-to-analog converter; the amplitude adjustment of the sinusoidal signal can be realized by using the 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 into the input terminal of the voltage-controlled current source, thereby controlling the output current of the current source to change according to the sinusoidal signal of the set frequency, phase and amplitude.

[0060] In a specific embodiment, the signal acquisition circuit includes a first channel circuit and a second channel circuit, one end of the first channel circuit and the second channel circuit are respectively connected to 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 DC voltage signals, and send the converted DC voltage signals to the analog-to-digital converter. The first channel circuit includes the viscoelastic force sensor, a first instrument amplifier and a first true effective value converter, the first instrument amplifier is connected to the viscoelastic force sensor, and the first true effective value converter is located between the first instrument amplifier and the analog-to-digital converter;

[0061] The second channel circuit includes a reference resistor, a second instrument amplifier and a second true effective value converter, the second instrument amplifier is connected to the reference resistor, and the second true effective value converter is located between the second instrument amplifier and the analog-to-digital converter;

[0062] The first instrument amplifier or the second instrument amplifier is used to amplify the voltage difference between the two ends of the viscoelastic force sensor and the two ends of the reference resistor, respectively, to obtain the amplitude of the first sinusoidal alternating voltage and the amplitude of the second sinusoidal alternating voltage;

[0063] The first true effective value converter or the second true effective value converter is used to convert the amplitude of the first sinusoidal AC voltage and the amplitude of the second sinusoidal AC voltage into the first DC voltage signal and the second DC voltage signal respectively; the analog-to-digital converter receives the first DC voltage signal and the second DC voltage signal and performs digital conversion on the two. In this embodiment, the voltage difference between the impedance to be measured and the reference resistor is amplified by the first instrument amplifier and the second instrument amplifier respectively, and the sinusoidal AC voltage signals of the first channel circuit and the second channel circuit are obtained respectively. The amplitudes of the sinusoidal AC voltages of the two channels are respectively proportional to the impedance to be measured ZX and reference resistor R REF ;

[0064] The amplitude of the sinusoidal AC voltage signal of the above two channels is measured by the first true RMS converter and the second true RMS converter. The true RMS converter outputs a DC voltage signal proportional to the true RMS value of the input signal. Since the input of the true RMS converter is a sinusoidal signal, its DC output is also proportional to the amplitude of the input sinusoidal signal. X With reference resistor R REF The DC signals of the two channels are input to the digital-to-analog converter for synchronous sampling and digital conversion.

[0065] Specifically, the sine wave voltage signal that can be modulated in frequency, phase and amplitude by a direct digital frequency synthesizer, a digital-to-analog converter and a multiplication-type digital-to-analog converter is:

[0066] v SET (t) = V V sin(2πf V +θ V )

[0067] Where V V 、f V ,θ V They are the amplitude, frequency and phase of the sinusoidal signal, all of which are variable.

[0068] Assume the transconductance gain of the voltage controlled current source is G m , then its output current can be expressed as:

[0069] i SET (t) = G m V V sin(2πf V +θ V )

[0070] This current is used to excite the viscoelastic force sensor and the reference resistor, so the voltages across the viscoelastic force sensor and the reference resistor are:

[0071] v X (t) = Z X G m V V sin(2πf V +θ V )

[0072] v REF (t) = R REF G m V V sin(2πf V +θV )

[0073] Assuming the voltage gains of the first instrumentation amplifier and the second instrumentation amplifier are A1 and A2 respectively, the outputs of the two instrumentation amplifiers are:

[0074] v INA1 (t) = A1Z X G m V V sin(2πf V +θ V )

[0075] v INA2 (t) = A2R REF G m V V sin(2πf V +θ V )

[0076] Assuming that the transmission ratios of the true effective value to the output DC voltage of the first true effective value converter and the second true effective value converter are K1 and K2 respectively, the DC voltages output by the two are respectively:

[0077]

[0078]

[0079] Assume the reference voltage of the analog-to-digital converter is V Constant , for V DCx and V DCREF Perform analog-to-digital conversion, and the analog-to-digital converter outputs the digital value Data x and Data REF for

[0080]

[0081]

[0082] Where N is the number of binary bits of the analog-to-digital converter.

[0083] Therefore, the impedance to be measured is Z X The calculation formula is:

[0084]

[0085] Through the above-mentioned proportional measurement, the influence of the drift of the digital-to-analog converter voltage gain, the first buffer amplifier gain, the multiplication type digital-to-analog converter voltage gain, the second buffer amplifier gain, and the voltage-controlled current source transconductance gain is eliminated, and it is insensitive to the drift of the excitation signal amplitude, that is, the circuit makes the measurement result insensitive to the drift of most circuit parameters; the signal measured by the circuit is proportional to the sensor impedance, and does not require complex transformation. While reducing the calculation complexity, it is also beneficial to improve the accuracy. The circuit structure is simple, the sensor working point is easy to set, and it is suitable for a variety of samples to be tested without easy oscillation.

[0086] By using the above measurement circuit, the impedance of the viscoelastic force sensor at different frequencies can be measured, as shown in the following figure: Figure 4 The impedance spectrum shown in the figure. The frequency value corresponding to the impedance peak point in the impedance spectrum is the resonant frequency, which represents the elasticity of the sample; the impedance value corresponding to the impedance peak point is the resonant impedance, which represents the viscosity of the sample.

[0087] Generally, when viscoelastic force is measured, the viscoelastic force sensor is continuously scanned by impedance spectrum measurement, and the data is analyzed to obtain the curves of the resonant frequency and resonant impedance changing with the scanning time, so as to finally obtain the curves representing the changes of the elasticity and viscosity of the sample with time.

[0088] The measurement method of the vibrating viscoelastic force sensor, such as Figure 5 As shown, the following steps are included:

[0089] S1. Obtain the bias value, disconnect the analog switch in the circuit, and the excitation current and voltage drop of the viscoelastic force sensor and the reference resistor on the circuit are zero. At this time, the conversion result of the analog-to-digital converter on the circuit is the bias value caused by the circuit. In one embodiment, before starting the sample test, the circuit needs to be debiased and calibrated using a standard impedance. Disconnect the analog switch in the above circuit, then the excitation current of the impedance to be measured and the reference resistor are both zero, and the voltage drop across them is also zero, that is, the effective input signal of the instrument amplifier, the true RMS circuit, and the analog-to-digital converter circuit is zero. The two-way conversion result Data of the analog-to-digital converter obtained at this time Bias1 、Data Bias2 , which is the offset value caused by this part of the circuit. The purpose of debiasing the circuit is to remove the offset caused by the instrument amplifier, true effective circuit and analog-to-digital converter that connects the impedance to be measured to the standard resistor.

[0090] S2, calibration, connect the standard resistor to the circuit instead of the viscoelastic force sensor and perform frequency scanning to obtain the correction coefficient at each frequency. In one embodiment, connect the standard resistor to the above circuit instead of the impedance to be measured, perform frequency scanning within the operating frequency range of the sensor, that is, turn on the analog switch, control the direct digital frequency synthesizer to output a signal of a certain frequency ω and phase, and control the multiplication type digital-to-analog converter to determine the amplitude of the excitation, so that the voltage-controlled current source outputs a current with a specific frequency ω, phase and amplitude to excite the impedance to be measured and the reference resistor. After a certain number of establishment cycles, according to a certain sampling rate f S The two inputs of the analog-to-digital converter are sampled synchronously, and the number of sampling points is controlled so that the sampling time length is just the period of the excitation signal, that is, The scan result of each channel is subtracted from the Data obtained in the circuit debiasing step. Bias1 、Data Bias2 Two offset values ​​are obtained, and the average value of the debiased results of each channel is taken as the final scan result of the channel. The two results are CalData X (ω) and CalData REF (ω). Because the output voltage of the true RMS converter has a certain ripple, and the period of the ripple is the period of the excitation signal, the ripple effect can be eliminated by period averaging. The calculation formula of the correction coefficient is:

[0091]

[0092] Among them, R Cal It is the calibration resistor, that is, the standard resistor.

[0093] After step S1 and this step, the calculation formula of the impedance to be measured is corrected to:

[0094]

[0095] In the calculation formula of the impedance to be measured Among them Calculated from the measured analog-to-digital conversion result, in order to ensure the accuracy of the final result, the coefficient To this end, the instrument amplifier gain, the voltage transfer ratio of the true RMS conversion chip, and the reference resistor must have absolute accuracy. It is related to the scanning frequency, let it be K(ω). Its accuracy is not only the accuracy at a certain frequency point, but also requires absolute accuracy in the entire frequency scanning range of the viscoelastic force sensor. This requirement is difficult to achieve and the cost of implementation is high. Through calibration, the instrument amplifier gain, the voltage transfer ratio of the true effective conversion chip and the reference resistor do not need to have absolute accuracy, but only require them to have a small drift. During calibration, use a standard resistor with higher accuracy, such as a standard metal foil resistor with an accuracy of one ten-thousandth. The accuracy and temperature drift of this resistor are sufficient for the measurement requirements of the viscoelastic force sensor.

[0096] S3, frequency scanning, obtaining the resonant frequency and resonant impedance through cyclic scanning, wherein the next scan is performed with the resonant frequency of the previous scan as the center. In one embodiment, the resonant frequency and resonant impedance of the viscoelastic force sensor are obtained through multiple cyclic scanning, the second scan is performed with the resonant frequency obtained by the first scan as the center, the third scan is performed with the resonant frequency obtained by the second scan as the center, and so on until the sample test scanning time ends.

[0097] Specifically, the frequency scanning step also includes: initial wide range frequency scanning and local fine frequency scanning; wherein,

[0098] The controller sets a frequency range of a first width and a first frequency step to start the initial wide range frequency scan, and measures an initial resonant frequency;

[0099] The controller sets a frequency range of a second width and a second frequency step to perform the local fine frequency scan with the initial resonant frequency as the center to obtain the resonant frequency and resonant impedance of the current scan, wherein the second frequency step is smaller than the first frequency step, and the frequency range of the first width is larger than the frequency range of the second width.

[0100] Frequency scanning includes first performing a wider frequency range scan, i.e., initial wide range frequency scan, and then performing local fine frequency scan. When scanning a wider frequency range, i.e., the first scan, the controller is written with the start frequency, end frequency, frequency step, and excitation signal amplitude of the initial frequency scan through the communication interface before starting the scan. After scanning all frequency points, the impedance Z is calculated according to the modified impedance calculation formula to be measured. x (ω), obtain the initial frequency scanning data, perform data analysis, and measure the initial resonance frequency ω R (0) A wider frequency range scan only requires measuring the approximate range of the resonant frequency, not the accuracy.

[0101] It should be noted that the data written to the controller through the communication interface is obtained through offline use of a large number of samples, and the data set by the controller can cover most possible sample parameters. In addition, the frequency range determined by the start frequency and the end frequency can be as large as possible to ensure that all sample resonant frequency ranges are covered, and the frequency step can be relatively long.

[0102] In the cyclic local fine frequency scan, the center frequency of this frequency band is the resonant frequency ω measured by the initial wide range frequency scan in the previous step. R (0); The frequency band width is the product of the time required for scanning in this step and the maximum time rate of change of the sample resonance frequency, which is narrower than the initial wide frequency range. The maximum time rate of change of the sample resonance frequency is generally determined by offline experimental methods. Taking the maximum time rate of change of the sample resonance frequency can ensure that the resonance frequency of the system will not exceed the frequency band of this scan during the scanning time of this step. The step size of the frequency scan is smaller than the step size of the first scan, and its maximum value is determined by the frequency resolution required by the system. After scanning all frequency points, calculate the impedance Z according to the above-mentioned corrected impedance calculation formula. x (ω), obtain local fine frequency scanning data, perform data analysis, and measure the resonance frequency ω obtained by this scan R (n), where n is the sequence number of the local fine frequency scan.

[0103] The frequency scan in this step needs to repeat the above process cyclically, and perform multiple cyclic local fine frequency scans, that is, the center frequency of each local fine frequency scan is the resonant frequency measured by the previous local fine frequency scan; the frequency band width is the product of the time required for the scan in this step and the maximum time rate of change of the sample resonant frequency. The step size of the frequency scan is determined by the frequency resolution required by the system. After scanning all frequency points, calculate the impedance Z according to the above modified impedance calculation formula x (ω), obtain local fine frequency scanning data, perform data analysis, and measure the resonance frequency ω obtained by this scan R (n). Where n is the sequence number of the local fine frequency scan.

[0104] In addition, the width of the local fine frequency scan may need to be corrected in a few cases. For example, the maximum time change rate of the resonance frequency of the sample determined offline is too large, the measured resonance frequency change range is much smaller than the scanning frequency band, and most of the scanning frequency band is useless; or the maximum time change rate of the resonance frequency of the sample determined offline is too small, the measured resonance frequency change range exceeds the scanning frequency band, that is, the frequency band of this scan does not cover the resonance frequency. This situation where the scanning frequency band is too wide or too narrow needs to be corrected, and the corrected bandwidth will be used for the next scan. For example: the measurement time is 45 minutes, and the scan is performed every 10 seconds, and the scan may be cyclically scanned 270 times.

[0105] In this step, a narrower scanning bandwidth is used to avoid worthless frequency scanning time overhead, and only useful frequency bands are scanned, thereby improving time utilization. In this way, even if the scanning frequency step is very small, good time resolution can still be obtained; because a smaller frequency step is used, even small changes in the resonant frequency can be distinguished, that is, better frequency resolution can be achieved.

[0106] S4, data processing, filtering out the resonance frequency sequence and resonance impedance sequence obtained in step S3 by designing a digital filter to obtain the resonance frequency and resonance impedance. In one embodiment, a resonance frequency sequence ω0(n) and a corresponding resonance impedance sequence Z are obtained by the local fine frequency scanning in step S3. R (n). According to the characteristics of sample signal changes, a suitable digital filter is designed to filter out the noise signals of the two sequences, especially the noise signals caused by the vibration of the equipment during the measurement process, and retain the useful signals reflecting the changes in the elasticity and viscosity of the sample.

[0107] The present invention provides a measurement circuit and method for a vibrating viscoelastic force sensor, which has a relatively simple circuit structure, is convenient for setting the sensor working point, and is adaptable to a variety of samples to be tested without the risk of stopping vibration. The circuit structure makes the measurement result insensitive to the drift of most circuit parameters. The signal measured by the circuit is proportional to the sensor impedance, and no complex transformation is required, which reduces the computational complexity and is beneficial to improving accuracy. The measurement method provided by the present invention further removes the bias generated by the components in the circuit. Through correction, the circuit transmission link does not need absolute accuracy. At the same time, the contradiction between the frequency resolution during scanning and the sampling rate in time is balanced, so that a result with better frequency resolution and higher time sampling rate can be obtained. The circuit and method provided by the present invention can meet the requirements for analyzing vibrating viscoelastic force sensors with an application structure similar to that of electric speakers, and have great value for promotion and application.

[0108] The invention provides a measuring device for a vibrating viscoelastic force sensor. The measuring device comprises a measuring circuit for the vibrating viscoelastic force sensor. The measuring device executes a measuring method for the vibrating viscoelastic force sensor.

[0109] The above are only preferred embodiments of the present invention and are not intended to limit the present invention in any form. Any ordinary technician in the industry can smoothly implement the present invention as shown in the drawings and above. However, any equivalent changes, modifications and evolutions made by technicians familiar with the profession without departing from the scope of the technical solution of the present invention using the technical content disclosed above are all equivalent embodiments of the present invention. At the same time, any equivalent changes, modifications and evolutions made to the above embodiments based on the essential technology of the present invention are still within the protection scope of the technical solution of the present invention.

Claims

1. A measuring circuit of a vibrating viscoelastic force sensor, characterized in that: Including regulation circuit, control A controller, a voltage-controlled current source, an analog switch, a signal acquisition circuit, an analog-to-digital converter and a reference resistor, the viscoelastic force sensor is connected in series with the voltage-controlled current source, the analog switch and the reference resistor, the regulating circuit is connected to the controller and the voltage-controlled current source respectively, the analog switch is connected to the controller, and the analog switch is used to control the on-off of the current of the viscoelastic force sensor and the reference resistor; The controller adjusts the regulating circuit so that the voltage-controlled current source outputs a current of a preset frequency, phase and amplitude to excite the viscoelastic force sensor and the reference resistor; The signal acquisition circuit is used to acquire and convert the measurement data signals of the reference resistor and the viscoelastic force sensor. The analog-to-digital converter receives the signal converted by the signal acquisition circuit, performs digital conversion on it, and then sends it to the controller.

2. The measuring circuit of the vibrating viscoelastic force sensor according to claim 1, characterized in that: The regulating circuit includes 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 to the controller, and the controller adjusts the direct digital frequency synthesizer and the multiplication type digital-to-analog converter respectively so that the voltage-controlled current source outputs a preset frequency, phase and amplitude; The digital-to-analog converter receives the digitized sinusoidal signal output by the direct digital frequency synthesizer and converts it into an analog sinusoidal signal; The reconstruction filter filters the analog sinusoidal signal output by the digital-to-analog converter.

3. The measuring circuit of the vibrating viscoelastic force sensor according to claim 2, characterized in that: The regulating circuit also includes a clock reference, a first buffer amplifier and a second buffer amplifier, and the clock signal output by the clock reference is used as a timing reference of the direct digital frequency synthesizer; The first buffer amplifier and the second buffer amplifier are connected to the reconstruction filter and the multiplication type digital-to-analog converter respectively. The output signal of the reconstruction filter passes through the first buffer amplifier and then inputs the signal into the multiplication type digital-to-analog converter; the output signal of the multiplication type digital-to-analog converter passes through the second buffer amplifier and then inputs the voltage-controlled current source.

4. The measuring circuit of the vibrating viscoelastic force sensor according to claim 1, characterized in that: The signal acquisition circuit includes a first channel circuit and a second channel circuit, one end of the first channel circuit and the second channel circuit are respectively connected to 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 DC voltage signals, and send the converted DC voltage signals to the analog-to-digital converter.

5. The measuring circuit of the vibrating viscoelastic force sensor according to claim 4, characterized in that: The first channel circuit includes a viscoelastic force sensor, a first instrument amplifier and a first true effective value converter, the first instrument amplifier is connected to the viscoelastic force sensor, and the first true effective value converter is located between the first instrument amplifier and the analog-to-digital converter; The second channel circuit includes the reference resistor, a second instrument amplifier and a second true effective value converter, the second instrument amplifier is connected to the reference resistor, and the second true effective value converter is located between the second instrument amplifier and the analog-to-digital converter; The first instrument amplifier and the second instrument amplifier are used to amplify the voltage difference between the two ends of the viscoelastic force sensor and the two ends of the reference resistor, respectively, 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 used to convert 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 respectively; The analog-to-digital converter receives the first DC voltage signal and the second DC voltage signal and performs digital conversion on the two.

6. A measuring device for a vibrating viscoelastic force sensor, characterized in that: The measuring device comprises a measuring circuit of the vibrating viscoelastic force sensor according to any one of claims 1 to 5.

7. A method for measuring a vibrating viscoelastic force sensor, characterized in that: The measuring method comprises the measuring device of the vibrating viscoelastic force sensor according to claim 6, and further comprises the following steps: Obtain the bias value, disconnect the analog switch in the circuit, and connect the viscoelastic force sensor on the circuit to the reference voltage. The excitation current and voltage drop of the resistor are zero. At this time, the conversion result of the analog-to-digital converter on the circuit is the bias value caused by the circuit. Calibration: connect a standard resistor to the circuit instead of the viscoelastic force sensor for calibration to obtain the correction coefficient at each frequency; Frequency scanning, obtaining the resonance frequency and resonance impedance by cyclic scanning, wherein the next scan is performed with the resonance frequency of the previous scan as the center; Data processing: by designing a digital filter, the resonance frequency sequence and the resonance impedance sequence obtained in the step frequency scanning are filtered out to obtain the resonance frequency and the resonance impedance.

8. The measuring method of the vibrating viscoelastic force sensor according to claim 7, characterized in that: The frequency scanning step also includes: initial wide range frequency scanning and local fine frequency scanning; wherein, The controller sets a frequency range of a first width and a first frequency step to start the initial wide range frequency scan, and measures an initial resonant frequency; The controller sets a frequency range of a second width and a second frequency step to perform the local fine frequency scan with the initial resonant frequency as the center to obtain the resonant frequency and resonant impedance of the current scan, wherein the second frequency step is smaller than the first frequency step, and the frequency range of the first width is larger than the frequency range of the second width.

9. The measuring method of the vibrating viscoelastic force sensor according to claim 7, characterized in that: The calibration step also includes: The analog switch is turned on, and the controller controls the direct digital frequency synthesizer and the multiplication type digital-to-analog converter to make the voltage-controlled current source output a current with a preset frequency, phase and amplitude to excite the viscoelastic force sensor and the reference resistor; The signal value obtained by subtracting the offset value from each signal value sampled by the analog-to-digital converter and taking the average value is the final signal value of the scan.

10. The measuring method of the vibrating viscoelastic force sensor according to claim 9, characterized in that: The sampling time length of the analog-to-digital converter is equal to the period of the excitation signal of the viscoelastic force sensor and the reference resistor.

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