Signal conversion circuit and measuring system for differential capacitance sensor

Abstract

The utility model provides a signal conversion circuit and a measurement system for a differential capacitance sensor, which relate to the technical field of sensor measurement and comprise an alternating current excitation signal source, a first resistor unit, a second resistor unit and a transconductance amplification unit. The alternating current excitation signal source is connected with the common end of the differential capacitance sensor; the first resistor unit is connected with one capacitance bridge arm of the differential capacitance sensor, and the second resistor unit is connected with the other capacitance bridge arm of the differential capacitance sensor; the positive and negative phase input ends of the transconductance amplification unit are respectively connected with the first resistor unit and the second resistor unit. According to the utility model, the AC excitation signal source is arranged, so that the risk of charging saturation of the capacitor is avoided, the capacitance of the differential capacitance sensor is converted into output differential current, the change of the capacitance can be reflected in real time, long-distance transmission and common-mode noise suppression are facilitated, and the measurement accuracy of the measurement system on the capacitance sensor is improved.

Description

Technical Field

[0001] The utility model relates to the technical field of sensor measurement, in particular to a signal conversion circuit and a measurement system for a differential capacitance sensor. Background Art

[0002] A differential capacitance sensor is a sensor that uses capacitance changes to detect physical quantities (such as displacement, pressure, or liquid level), and its working principle is based on the relationship between capacitance changes and electrode spacing, dielectric constant, and electrode area. The signal conversion circuit of existing differential capacitance sensors has the risks of charging saturation, slow measurement speed, and lack of accuracy. Summary of the Utility Model

[0003] The utility model provides a signal conversion circuit and a measurement system for a differential capacitance sensor to solve the defects that the signal conversion circuit of the differential capacitance sensor in the prior art has the risks of charging saturation, slow measurement speed, and lack of accuracy.

[0004] The utility model provides a signal conversion circuit for a differential capacitance sensor, including an AC excitation signal source, a first resistor unit, a second resistor unit, and a transconductance amplification unit;

[0005] The AC excitation signal source is connected to the common terminal of the differential capacitance sensor and is used to provide an AC excitation signal to the differential capacitance sensor;

[0006] The first resistor unit is connected to one capacitance bridge arm of the differential capacitance sensor, and the second resistor unit is connected to the other capacitance bridge arm of the differential capacitance sensor;

[0007] The positive and negative input terminals of the transconductance amplification unit are respectively connected to the first resistor unit and the second resistor unit, and are used to convert the differential voltage between the first resistor unit and the second resistor unit into a current to obtain an output differential current.

[0008] According to the signal conversion circuit for a differential capacitance sensor provided by the utility model, the AC excitation signal source is a sine AC excitation signal source, and provides a sine AC excitation signal to the differential capacitance sensor.

[0009] According to the signal conversion circuit for a differential capacitance sensor provided by the utility model, the frequency range of the AC excitation signal is 10 kHz to 200 kHz.

[0010] According to the signal conversion circuit for a differential capacitance sensor provided by the utility model, the value range of the current flowing through the two capacitance bridge arms of the differential capacitance sensor is 0.1 mA to 20 mA.

[0011] According to a signal conversion circuit for a differential capacitance sensor provided by the present utility model, both the first resistor unit and the second resistor unit include a resistor network, and the first resistor unit and the second resistor unit have a symmetric structure.

[0012] According to a signal conversion circuit for a differential capacitance sensor provided by the present utility model, the transconductance amplification unit includes a fully differential transconductance operational amplifier, the fully differential transconductance operational amplifier includes a transconductance element, and the transconductance element is used to convert the differential voltage between the first resistor unit and the second resistor unit into a current to obtain an output differential current.

[0013] According to a signal conversion circuit for a differential capacitance sensor provided by the present utility model, the transconductance element includes an external resistor or a resistor network.

[0014] According to a signal conversion circuit for a differential capacitance sensor provided by the present utility model, the relationship expression between the output differential current and the current flowing through the transconductance element is as follows:

[0015] Iout = K × (Vin / Zt);

[0016] Wherein, Iout represents the output differential current, Vin / Zt represents the current flowing through the transconductance element, Vin represents the differential voltage between the first resistor unit and the second resistor unit, Zt represents the resistance value of the transconductance element, and K represents a linear coefficient determined based on the electrical parameters of the fully differential transconductance operational amplifier.

[0017] According to a signal conversion circuit for a differential capacitance sensor provided by the present utility model, the value range of the output differential current is 10 μA to 10 mA.

[0018] The present utility model also provides a measurement system, including the signal conversion circuit for a differential capacitance sensor described above.

[0019] The signal conversion circuit and measurement system for a differential capacitance sensor provided by the present utility model include an AC excitation signal source, a first resistor unit, a second resistor unit, and a transconductance amplification unit; the AC excitation signal source is connected to the common terminal of the differential capacitance sensor for providing an AC excitation signal to the differential capacitance sensor; the first resistor unit is connected to one capacitance bridge arm of the differential capacitance sensor, and the second resistor unit is connected to the other capacitance bridge arm of the differential capacitance sensor; the positive and negative input terminals of the transconductance amplification unit are respectively connected to the first resistor unit and the second resistor unit for converting the differential voltage between the first resistor unit and the second resistor unit into a current to obtain an output differential current. By providing an AC excitation signal source, compared with using a DC signal as the excitation signal source, the present utility model can avoid the risk of capacitor charging saturation, reduce the circuit for capacitor discharging and the discharge control circuit, convert the capacitance of the differential capacitance sensor into an output differential current, and use a current signal to represent the change in capacitance. Compared with the voltage signal representation method, the change speed of the current signal is faster than that of the voltage signal, which can reflect the change in capacitance in real time, is beneficial to long-distance transmission and suppression of common-mode noise, and improves the measurement accuracy. Description of the Drawings

[0020] In order to more clearly illustrate the technical solutions in the present utility model or the prior art, the following will briefly introduce the drawings required for use in the description of the embodiments or the prior art. Obviously, the drawings in the following description are some embodiments of the present utility model. For those of ordinary skill in the art, without creative efforts, other drawings can also be obtained based on these drawings.

[0021] Figure 1 is a schematic block diagram of the signal conversion circuit for a differential capacitance sensor provided by the present utility model.

[0022] Figure 2 is a schematic circuit diagram of the signal conversion circuit for a differential capacitance sensor provided by the present utility model.

[0023] Figure 3 is a schematic block diagram of the measurement system provided by the present utility model.

[0024] Reference Signs:

[0025] 1 - Signal conversion circuit, 101 - AC excitation signal source, 102 - Differential capacitance sensor, 103 - First resistor unit, 104 - Second resistor unit, 105 - Transconductance amplification unit, 2 - Current sampling circuit, 3 - Data processing circuit. Detailed Embodiments

[0026] To make the objectives, technical solutions and advantages of the present utility model clearer, the technical solutions in the present utility model will be clearly and completely described below with reference to the accompanying drawings in the present utility model. Apparently, the described embodiments are some but not all of the embodiments of the present utility model. All other embodiments obtained by those of ordinary skill in the art based on the embodiments in the present utility model without making creative efforts belong to the scope of protection of the present utility model.

[0027] Existing technical solutions for differential capacitance sensor signal conversion mostly convert capacitance into voltage form, and this method directly reads the voltage change across the capacitance sensor. Since the voltage change on capacitive devices lags behind the current change, the conversion speed is relatively low and the change amount of capacitance cannot be reflected in real time. In addition, the voltage signal does not have the ability of long-distance transmission, and there is signal attenuation during the transmission of the voltage signal. The attenuation conditions of lines with different lengths also vary greatly, resulting in inaccurate signal reading.

[0028] To solve the above technical defects, the present utility model provides a signal conversion circuit and a measurement system for a differential capacitance sensor. The technical solutions of the present utility model will be described in detail below with reference to the accompanying drawings.

[0029] Figure 1 is the principle block diagram of the signal conversion circuit for the differential capacitance sensor 102 provided by the present utility model. As Figure 1 shown, the present utility model provides a signal conversion circuit for a differential capacitance sensor 102, which includes an AC excitation signal source 101, a first resistor unit 103, a second resistor unit 104, and a transconductance amplification unit 105.

[0030] The AC excitation signal source 101 is connected to the common terminal of the differential capacitance sensor 102 for providing an AC excitation signal to the differential capacitance sensor 102; the common terminal of the differential capacitance sensor 102 refers to the common connection point between the two capacitors inside the sensor. The common terminal is used to provide a stable reference potential, maintain signal balance, and reduce the influence of external interference. The AC excitation signal includes sine wave, square wave, and triangular wave. According to the waveform smoothness and frequency spectrum characteristics, a sine wave is preferably used.

[0031] The first resistor unit 103 is connected to one capacitance bridge arm of the differential capacitance sensor 102, and the second resistor unit 104 is connected to the other capacitance bridge arm of the differential capacitance sensor 102; the change amount of the capacitance of the differential capacitance sensor 102 is in differential form.

[0032] The non-inverting input terminal and the inverting input terminal of the transconductance amplification unit 105 are respectively connected to the first resistor unit 103 and the second resistor unit 104, and are configured to convert the differential voltage between the first resistor unit 103 and the second resistor unit 104 into a current to obtain an output differential current.

[0033] Optionally, the differential capacitance sensor 102 in the present invention may be a MicroelectroMechanical Systems (MEMS) differential capacitance sensor 102. The MEMS differential capacitance sensor 102 uses capacitance changes to measure physical quantities (such as displacement, pressure, acceleration, etc.) and has characteristics such as miniaturization, high sensitivity, high precision, low power consumption, and easy integration, and is widely used in fields such as the automotive industry, consumer electronics, medical devices, and industrial automation. Correspondingly, the signal conversion circuit provided by the present invention for the differential capacitance sensor 102 is a weak signal acquisition system for the MEMS system differential capacitance sensor 102 of the material in-situ mechanics research experimental platform. The weak capacitance change of the MEMS capacitance sensor is converted into a differential current signal with a linear correspondence relationship and transmitted to the subsequent analog / digital measurement system for digital acquisition and analysis.

[0034] The working principle of the present invention is as follows.

[0035] The AC excitation signal source 101 loads an AC excitation signal to the common terminal of the differential capacitance sensor 102. The AC excitation signal causes currents to be generated in the two capacitance bridge arms of the differential capacitance sensor 102 respectively. The currents flowing through the two capacitance bridge arms of the differential capacitance sensor 102 form loops with the ground terminal through the first resistor unit 103 and the second resistor unit 104 respectively. The current difference flowing through the two capacitance bridge arms of the differential capacitance sensor 102 can be defined as a differential current. Utilizing the capacitive reactance characteristic of capacitance blocking DC and passing AC, the present invention uses an AC signal as the excitation signal. When the two capacitance bridge arms of the differential capacitance sensor 102 are in a balanced state, the effective values of the two output currents are equal and the differential current value is 0. When the two capacitance bridge arms are in an unbalanced state, the capacitive reactance of the capacitance with a smaller capacitance also increases, the current passing through the capacitance decreases, while the capacitive reactance of the other capacitance with an increased capacitance decreases, and the current passing through the capacitance increases accordingly, resulting in a change in the differential current.

[0036] The current flowing through the first resistor unit 103 forms a first voltage signal at the non-inverting input terminal of the transconductance amplification unit 105, and the current flowing through the second resistor unit 104 forms a second voltage signal at the inverting input terminal of the transconductance amplification unit 105. The voltage difference between the first voltage signal and the second voltage signal can be defined as a differential voltage.

[0037] The relationship between the output current and the input voltage of the transconductance amplification unit 105 is linear. It can convert the differential voltage into an output differential current, and the output differential current is related to the capacitance change of the differential capacitance sensor 102. Therefore, the present invention uses the output differential current as the measurement value of the differential capacitance sensor 102, realizing the signal acquisition of the differential capacitance sensor 102.

[0038] It can be understood that by setting the AC excitation signal source 101, compared with using a DC signal as the excitation signal source, the risk of capacitor charging saturation can be avoided, the circuit for capacitor discharging and the discharge control circuit are reduced. The capacitance of the differential capacitance sensor 102 is converted into an output differential current, and the change in capacitance is characterized by a current signal. Compared with the voltage signal characterization method, the change speed of the current signal is faster than that of the voltage signal, which can reflect the change in capacitance in real time, is beneficial to long-distance transmission and suppression of common-mode noise, and improves the measurement accuracy.

[0039] Figure 2 is the circuit schematic diagram of the signal conversion circuit for the differential capacitance sensor 102 provided by the present invention. As Figure 2 shown, Figure 2 In the figure, Exc is a sinusoidal AC excitation signal, which is input from the common terminal of the differential capacitance sensor 102. Cs1 and Cs2 are the two capacitance bridge arms of the differential capacitance sensor 102, and their capacitance changes are in a differential form. i1 and i2 are the current values flowing through the two capacitance bridge arms of the differential capacitance respectively. A loop is formed through the first resistor unit in+ and the second resistor unit in- and the system GND. The differential current is defined as ∆i = i1 - i2. The currents passing through the first resistor unit in+ and the second resistor unit in- form a first voltage signal Vin+ and a second voltage signal Vin- at the positive and negative input terminals of the transconductance amplification unit 105. The differential voltage can be defined as Vin = Vin+ - Vin-.

[0040] Based on the above embodiments, as an optional embodiment, the AC excitation signal source 101 is a sinusoidal AC excitation signal source 101, which provides a sinusoidal AC excitation signal to the differential capacitance sensor 102.

[0041] Optionally, the sinusoidal AC excitation signal source 101 can adopt a sine wave generator, which is used to generate a sinusoidal AC excitation signal, and the output terminal of the sine wave generator is connected to the common terminal of the differential capacitance sensor 102.

[0042] Optionally, the frequency range of the AC excitation signal is 10 kHz to 200 kHz.

[0043] Optionally, the value range of the currents flowing through the two capacitance bridge arms of the differential capacitance sensor 102 is

[0044] A sinusoidal AC excitation signal is applied to the common end of the differential capacitance sensor 102. The frequency and amplitude of the sinusoidal AC excitation signal are adjusted according to the capacitances of the two capacitance bridge arms of the differential capacitance sensor 102. The frequency is selected in the range of 10 kHz to 200 kHz. When the current value of the current flowing through any one of the capacitance bridge arms of the differential capacitance sensor 102 is less than 0.1 mA, the frequency of the sinusoidal AC excitation signal is increased. When the current value of the current flowing through any one of the capacitance bridge arms of the differential capacitance sensor 102 exceeds 20 mA, the frequency of the sinusoidal AC excitation signal is decreased. After the frequency of the sinusoidal AC excitation signal is determined, the amplitude of the sinusoidal AC excitation signal can be adjusted according to the current values flowing through the two capacitances, so that the effective value of the current flowing through any one of the capacitance bridge arms of the differential capacitance sensor 102 is between 0.1 mA and 20 mA.

[0045] It can be understood that the present utility model uses a sinusoidal AC signal as the excitation signal of the capacitance sensor, avoiding the risk of capacitor charging saturation when using a DC signal as the excitation, reducing the loop for capacitor discharging and the discharging control circuit. Using a current signal to characterize the change in capacitance has the advantage of faster speed compared to using a voltage signal. The change in the voltage signal requires an integration time, so the speed is slower. Using a differential current form to transmit signals takes advantage of the characteristic of no attenuation of the current signal during long-distance transmission and the characteristic of the differential transmission form to suppress common-mode noise.

[0046] Based on the above embodiments, as an optional embodiment, both the first resistor unit 103 and the second resistor unit 104 include a resistor network. The first resistor unit 103 and the second resistor unit 104 have a symmetrical structure. Among them, the resistor network is a circuit structure formed by connecting multiple resistors in a specific manner. The first resistor unit 103 and the second resistor unit 104 form a ground loop at the input end of the transconductance amplification unit 105, and a differential voltage signal is generated between the positive and negative input ends of the transconductance amplification unit 105.

[0047] Optionally, the first resistor unit 103 and the second resistor unit 104 have a symmetrical structure, specifically including selecting resistors with the same resistance value and using resistor networks with the same structure, and the connection methods of the first resistor unit 103 and the second resistor unit 104 are symmetrical.

[0048] It can be understood that the present utility model respectively inputs the output currents of the two capacitance bridge arms of the differential capacitance sensor 102 to the positive and negative input ends of the transconductance amplification unit 105, and forms a ground loop at the input end using a symmetrical resistor network. At this time, a differential voltage signal is generated between the positive and negative input ends of the transconductance amplification unit 105, improving the anti-interference ability.

[0049] Based on the above embodiments, as an optional embodiment, the transconductance amplification unit 105 includes a fully differential transconductance operational amplifier, and the fully differential transconductance operational amplifier includes a transconductance element for converting the differential voltage between the first resistor unit 103 and the second resistor unit 104 into a current to obtain an output differential current.

[0050] Optionally, the transconductance element includes an external resistor or a resistor network.

[0051] Optionally, the relationship expression between the output differential current and the current flowing through the transconductance element is as follows:

[0052] Iout = K × (Vin / Zt);

[0053] Where, Iout represents the output differential current, Vin / Zt represents the current flowing through the transconductance element, Vin represents the differential voltage between the first resistor unit 103 and the second resistor unit 104, Zt represents the resistance value of the transconductance element, and K represents a linear coefficient determined based on the electrical parameters of the fully differential transconductance operational amplifier.

[0054] Optionally, the value range of the output differential current is 10 μA to 10 mA.

[0055] The present invention selects an appropriate external resistor or resistor network according to the magnitude of the output differential current to construct a transconductance element with resistance characteristics for the fully differential transconductance operational amplifier. Generally, an output differential current in the range of dozens of μA to several mA is selected, and the current value flowing through the transconductance element has a linear multiple relationship with the output current of the fully differential transconductance operational amplifier. The current formed by the positive and negative output terminals of the fully differential transconductance operational amplifier through a closed loop is the differential current value reflecting the capacitance change of the differential capacitance sensor 102.

[0056] It can be understood that the present invention uses a sinusoidal AC signal as the excitation signal of the capacitance sensor, avoiding the risk of capacitor charging saturation when using a DC signal as the excitation, reducing the loop for capacitor discharging and the discharge control circuit; using a current signal to characterize the capacitance change has the advantage of faster speed compared with the voltage signal characterization method. The change of the voltage signal requires an integration time, so the speed is slower; using the differential current form to transmit the signal utilizes the characteristic of no attenuation of the current signal during long-distance transmission and the characteristic of suppressing common-mode noise of the differential transmission form.

[0057] Next, the measurement system provided by the present invention will be described. The measurement system described below can be mutually corresponded and referred to the signal conversion circuit for the differential capacitance sensor described above.

[0058] Figure 3is the principle block diagram of the measurement system provided by the present utility model. As Figure 3 shown, the present utility model also provides a measurement system, including the signal conversion circuit for the differential capacitance sensor. Optionally, the measurement system is an analog / digital measurement system, used for digital acquisition and analysis of the differential capacitance sensor.

[0059] The measurement system includes a signal conversion circuit 1 for the differential capacitance sensor, a current sampling circuit 2, and a data processing circuit 3. The signal conversion circuit 1 is used to convert the capacitance change of the differential capacitance sensor into an output differential current. The current sampling circuit 2 is used to collect the output differential current at the output end of the signal conversion circuit. The data processing circuit 3 includes a microcontroller or a microprocessor, used to analyze and process the output differential current to obtain the measurement signal of the differential capacitance sensor.

[0060] As an embodiment, the signal conversion circuit for the differential capacitance sensor includes an AC excitation signal source, a first resistor unit, a second resistor unit, and a transconductance amplification unit;

[0061] The AC excitation signal source is connected to the common terminal of the differential capacitance sensor, used to provide an AC excitation signal to the differential capacitance sensor;

[0062] The first resistor unit is connected to one capacitance bridge arm of the differential capacitance sensor, and the second resistor unit is connected to the other capacitance bridge arm of the differential capacitance sensor;

[0063] The inverting and non-inverting input terminals of the transconductance amplification unit are respectively connected to the first resistor unit and the second resistor unit, used to convert the differential voltage between the first resistor unit and the second resistor unit into a current to obtain an output differential current.

[0064] As an embodiment, the AC excitation signal source is a sine AC excitation signal source, used to provide a sine AC excitation signal to the differential capacitance sensor.

[0065] As an embodiment, the frequency range of the AC excitation signal is 10 kHz to 200 kHz.

[0066] As an embodiment, the value range of the current flowing through the two capacitance bridge arms of the differential capacitance sensor is 0.1 mA to 20 mA.

[0067] As an embodiment, both the first resistor unit and the second resistor unit include a resistor network, and the first resistor unit and the second resistor unit have a symmetric structure.

[0068] As an embodiment, the transconductance amplification unit includes a fully differential transconductance operational amplifier, and the fully differential transconductance operational amplifier includes a transconductance element for converting a differential voltage between the first resistor unit and the second resistor unit into a current to obtain an output differential current.

[0069] As an embodiment, the transconductance element includes an external resistor or a resistor network.

[0070] As an embodiment, the relationship expression between the output differential current and the current flowing through the transconductance element is as follows:

[0071] Iout = K × (Vin / Zt);

[0072] Wherein, Iout represents the output differential current, Vin / Zt represents the current flowing through the transconductance element, Vin represents the differential voltage between the first resistor unit and the second resistor unit, Zt represents the resistance value of the transconductance element, and K represents a linear coefficient determined based on the electrical parameters of the fully differential transconductance operational amplifier.

[0073] As an embodiment, the value range of the output differential current is 10 μA to 10 mA.

[0074] It can be understood that the measurement system provided by the present invention has the same technical effects as the signal conversion circuit for the differential capacitance sensor, and will not be elaborated herein.

[0075] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and are not intended to limit them; although the present invention has been described in detail with reference to the foregoing embodiments, those of ordinary skill in the art should understand that they can still modify the technical solutions described in the foregoing embodiments, or perform equivalent replacements for some of the technical features; and these modifications or replacements do not make the essence of the corresponding technical solutions deviate from the spirit and scope of the technical solutions of the embodiments of the present invention.

Claims

1. A signal conversion circuit for a differential capacitance sensor, characterized in that It includes an AC excitation signal source, a first resistor unit, a second resistor unit, and a transconductance amplification unit; The AC excitation signal source is connected to the common terminal of the differential capacitance sensor and is used to provide an AC excitation signal to the differential capacitance sensor; The first resistor unit is connected to one capacitance bridge arm of the differential capacitance sensor, and the second resistor unit is connected to the other capacitance bridge arm of the differential capacitance sensor; The positive and negative input terminals of the transconductance amplification unit are respectively connected to the first resistor unit and the second resistor unit, and are used to convert the differential voltage between the first resistor unit and the second resistor unit into a current to obtain an output differential current.

2. The signal conversion circuit for a differential capacitance sensor according to claim 1, characterized in that, The AC excitation signal source is a sine AC excitation signal source and provides a sine AC excitation signal to the differential capacitance sensor.

3. The signal conversion circuit for a differential capacitance sensor according to claim 1 or 2, characterized in that, The frequency range of the AC excitation signal is 10 kHz to 200 kHz.

4. The signal conversion circuit for a differential capacitance sensor according to claim 3, characterized in that, The value range of the current flowing through the two capacitance bridge arms of the differential capacitance sensor is 0.1 mA to 20 mA.

5. The signal conversion circuit for a differential capacitance sensor according to claim 1, characterized in that, Both the first resistor unit and the second resistor unit include a resistor network, and the first resistor unit and the second resistor unit have a symmetric structure.

6. The signal conversion circuit for a differential capacitance sensor according to claim 1, wherein, The transconductance amplification unit includes a fully differential transconductance operational amplifier, and the fully differential transconductance operational amplifier includes a transconductance element, and the transconductance element is used to convert the differential voltage between the first resistor unit and the second resistor unit into a current to obtain an output differential current.

7. The signal conversion circuit for a differential capacitance sensor according to claim 6, characterized in that, The transconductance element includes an external resistor or a resistor network.

8. The signal conversion circuit for a differential capacitance sensor according to claim 6 or 7, characterized in that The relationship expression between the output differential current and the current flowing through the transconductance element is as follows: Iout = K×(Vin / Zt); Wherein, Iout represents the output differential current, Vin / Zt represents the current flowing through the transconductance element, Vin represents the differential voltage between the first resistor unit and the second resistor unit, Zt represents the resistance value of the transconductance element, and K represents a linear coefficient determined based on the electrical parameters of the fully differential transconductance operational amplifier.

9. The signal conversion circuit for a differential capacitance sensor according to any one of claims 1, 2, 4-7, characterized in that, The value range of the output differential current is 10 μA to 10 mA.

10. A measurement system, characterized in that, It includes the signal conversion circuit for a differential capacitance sensor according to any one of claims 1-9.

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