Method for operating a pressure measuring cell of a capacitive pressure sensor
Patent Information
- Authority / Receiving Office
- CN · China
- Patent Type
- Patents(China)
- Current Assignee / Owner
- IFM ELECTRONIC GMBH
- Filing Date
- 2023-03-09
- Publication Date
- 2026-06-02
Smart Images
Figure CN116735074B_ABST
Abstract
Description
Technical Field
[0001] This invention relates to a method for operating a pressure measurement unit of a capacitive pressure sensor. Background Technology
[0002] Capacitive pressure sensors or pressure measuring devices are used for pressure measurement in many industrial sectors. These devices often include a ceramic pressure measurement unit as a transducer for process pressure and evaluation electronics for signal processing.
[0003] The capacitive pressure measuring unit consists of a ceramic substrate and a diaphragm, with a glass bonding ring positioned between the substrate and the diaphragm. The cavity formed between the substrate and the diaphragm facilitates the longitudinal movement of the diaphragm due to pressure. Therefore, this cavity is also referred to as the measuring chamber. Electrodes are positioned on the underside of the diaphragm and on the opposite top of the substrate, collectively forming a measuring capacitor. The application of pressure causes deformation of the diaphragm, resulting in a change in the capacitance of the measuring capacitor.
[0004] The evaluation unit records changes in capacitance and converts them into pressure measurements. These pressure sensors are typically used to monitor or control processes. Therefore, pressure sensors are often connected to a higher-level control unit (PLC).
[0005] DE19851506C1 discloses a capacitive pressure sensor in which the pressure measurement value is determined by the quotient of two capacitance values, which are related to a measuring capacitor and a reference capacitor. Although the pressure measuring unit is not specifically described in the patent specification, the circuitry and method described are applicable to capacitive pressure measuring units. A characteristic of this pressure measuring device is that only the amplitude of the square wave signal (independent of its frequency) is related to the measurement signal at the output, which is evaluated as a metric for the recorded pressure measurement value.
[0006] EP0569573B1 discloses a circuit arrangement for a capacitive pressure sensor, which also relates to pressure assessment using the quotient method.
[0007] The quotient method is typically based on the following stress correlation:
[0008]
[0009] Among them, C M C represents the capacitance of a capacitor. R This represents the capacitance of the reference capacitor, and p represents the process pressure to be determined. Another concept to consider is C in the interchange factor. M and C R The choice. However, the denominator has C MThe example of this is the most common form of the benefit of self-linearization. Therefore, unless otherwise indicated, this embodiment is assumed below.
[0010] Due to manufacturing tolerances, capacitance evaluation requires offset compensation and gain adjustment at the factory so that the pressure sensor or its evaluation circuitry also provides process values corresponding to the measured pressure within the rated pressure range.
[0011] For example, the practice known from the cited DE19851506C1, which involves performing comparisons by laser trimming certain resistors, is increasingly finding to be overly complex. Furthermore, the incremental repositioning of analog function blocks to microcontrollers necessitates further optimization of offset compensation and gain adjustment. Summary of the Invention
[0012] The object of this invention is to provide a simplified and unaffected electronic architecture for offset compensation and gain adjustment in the pressure measurement unit of the capacitive pressure sensor described at the beginning.
[0013] This objective is achieved by a method having the features of claim 1. Advantageous configurations of the invention are specified in the dependent claims.
[0014] This invention is based on the basic circuit described in DE19851506C1 cited at the beginning, which is used to evaluate a capacitive pressure measurement unit comprising a pressure-dependent measuring capacitor and an ideal pressure-independent reference capacitor. The basic feature of this evaluation circuit is that the pressure measurement unit has an internal excitation voltage U applied to it in the form of an AC square wave signal. E0 Furthermore, the pressure measurement value p is obtained based on the capacitance values of the measuring capacitor and the reference capacitor. Then, this measurement signal is converted into an AC square wave signal U. R The signal is provided to the evaluation unit in the form of a measurement signal that is adjusted for a higher-level control unit (e.g., a PLC). The pulse level or amplitude of the signal depends on the ratio of the capacitance values of the reference capacitor and the measuring capacitor, and the duration of the signal period is determined by the capacitance value of the measuring capacitor.
[0015] The difference compared to existing circuits is a fundamentally different comparison architecture, and therefore offers a variety of advantages.
[0016] According to the present invention, in order to amplify the signal and correct the offset, the AC square wave signal U... R Provided to another amplifier unit, firstly through the internal excitation voltage U E0 Amplification is achieved by adjusting the amplitude, followed by another square wave signal U. OFTo perform offset compensation. For gain and offset correction, individual fixed voltage values have been previously stored in memory cells during the comparison process. These stored voltage values, along with a predetermined clock, are used to form two square wave signals U. E0 and U OF Finally, gain correction is first performed by multiplying the quotients of the capacitance values of the reference capacitor and the measurement capacitor, and then by applying the square wave signal U... OF It is supplied to another amplifier unit and thus added to the square wave signal U. R To perform offset correction.
[0017] Capacitive pressure sensors advantageously include a microcontroller in which, among other things, clock generation and measurement adjustment occur.
[0018] In an improvement of the present invention, two square wave signals U are used to form E0 and U OF The clock is excited by a timer, preferably integrated into a microcontroller. The timer preferably controls the conversion unit and is used to generate two square wave signals U. E0 and U OF The clock.
[0019] In another improvement of the invention, it is specified that two square wave signals U E0 and U OF The amplitude is adjusted by a corresponding digital potentiometer, which is actuated via a serial data bus that connects the two potentiometers to the microcontroller.
[0020] Gain adjustment can be applied to all sensor elements where the signal swing depends on the sensor current. This also applies to differentially operated sensor elements, and is particularly suitable for resistance measurement bridges: half-bridge (voltage divider) or full-bridge (Wheatstone bridge).
[0021] When the offset correction signal has the same signal shape as the current of the measuring element, the offset correction used for the measuring element in differential operation can be applied (added) to the single-ended process value.
[0022] This invention offers several advantages. One advantage of this novel comparison architecture is its low sensitivity to interference, as the interconnects can be designed with low impedance according to the architecture. Additionally, the circuit is highly robust to environmental impacts and aging due to the minimal number of electronic components. Furthermore, it saves on cost and space. Other advantages include the ability to adjust the gain during changes in the current of the measuring element, allowing the manufacturing-related tolerances of the pressure measuring units to be consistent with each other, thus ensuring a defined signal-to-noise ratio. Finally, it should be mentioned that the gain setting is independent of the offset setting. That is, offset correction has no effect on the previously set gain. Conversely, after a gain change, the offset correction needs to be changed again because the offset compensation requirement of the pressure measuring unit also increases. Moreover, the gain and offset settings can now be placed further away from the pressure measuring unit without compromising signal quality. This is useful, for example, in areas where high temperatures exist in the region of the pressure measuring unit or under constrained conditions. Attached Figure Description
[0023] The invention will now be explained in more detail with reference to the accompanying drawings and exemplary embodiments.
[0024] Indicatively:
[0025] Figure 1 A block diagram of a capacitive pressure measuring device is shown.
[0026] Figure 2 A schematic cross-sectional view of a capacitive pressure measurement unit is shown.
[0027] Figure 3 It shows how to use such Figure 2 The known evaluation circuit for the capacitive pressure measurement unit shown;
[0028] Figure 4 It shows Figure 3 An evaluation circuit, wherein the evaluation circuit is equipped with a microcontroller for performing a first embodiment of the method according to the invention; and
[0029] Figure 5 It shows Figure 3 An evaluation circuit, wherein the evaluation circuit is equipped with a microcontroller for performing a second embodiment of the method according to the invention. Detailed Implementation
[0030] In the following description of preferred embodiments, the same reference numerals denote the same or corresponding parts.
[0031] Figure 1A block diagram of a typical capacitive pressure measuring device for measuring process pressure p (e.g., the pressure of oil, milk, water, etc.) is shown. The pressure measuring device 1 is specifically implemented as a two-conductor device and essentially consists of a pressure measuring unit 10 and evaluation electronics 20. The evaluation electronics 20 includes an analog evaluation circuit 30 and a microcontroller μC, wherein the analog output signal from the evaluation circuit 20 is digitized and further processed. The microcontroller μC provides the evaluation result, in the form of a digital or analog output signal, to, for example, a PLC. For power supply, the pressure measuring device 1 is connected to a power supply line (12V-36V).
[0032] Figure 2 A typical capacitive pressure measuring unit 10, as commonly used in capacitive pressure measuring devices, is illustrated schematically. The pressure measuring unit 10 essentially consists of a substrate 12 and a diaphragm 14, which are connected to each other via a glass bonding ring 16. The substrate 12 and the diaphragm 14 define a cavity 19, which—preferably only in the low pressure range of up to 50 bar—is connected to the rear of the pressure measuring unit 10 via a ventilation channel 18.
[0033] Multiple electrodes are disposed on both the substrate 12 and the diaphragm 14, and these electrodes form a reference capacitor C. R and measuring capacitor C M Measuring capacitor C M Formed by a diaphragm electrode ME and an intermediate electrode M, with a reference capacitor C. R It is formed by a ring electrode R and a film electrode ME.
[0034] The process pressure p acts on diaphragm 14, which bends more or less depending on the applied pressure, wherein the distance from diaphragm electrode ME to intermediate electrode M varies substantially. This causes the measuring capacitor C to... M The corresponding change in capacitance. Since the change in distance between the ring electrode R and the diaphragm electrode ME is smaller than the change in distance between the diaphragm electrode ME and the intermediate electrode M, therefore, for the reference capacitor C... R The impact is relatively small.
[0035] The diagram below does not show the difference between the capacitor markings and their capacitance values. Therefore, C M and C R This refers to the measuring or reference capacitor itself and its respective capacitance.
[0036] Figure 3 A known evaluation circuit 30 for the pressure measuring unit 10 is shown in more detail. The measuring capacitor C... M It is arranged together with resistor R1 in the integral branch IZ, and reference capacitor C. R It is arranged together with resistor R2 in the micro-branch DZ. A square wave voltage U is applied to the input of the integral branch IZ.E0 The voltages are preferably alternated symmetrically about 0 volts. An operational amplifier OP1, operating as an integrator, is used via resistor R1 and measuring capacitor C. M Input voltage U E0 This is converted into a linearly increasing or decreasing voltage signal (depending on the polarity of the input voltage), which is output at the COM output of the integrator branch IZ. Measurement point P1 is virtually grounded through operational amplifier OP1.
[0037] The output COM is connected to a threshold comparator SG, which actuates a square wave generator RG. The voltage signal U at the output COM is... COM Once the voltage exceeds or falls below the threshold, the comparator SG changes its output signal, and the square wave generator RG reverses its output voltage under different conditions.
[0038] The micro-branch DZ also includes operational amplifier OP2, a voltage divider containing two resistors R5 and R6, and a feedback resistor R7. The output of operational amplifier OP2 is connected to the sample-and-hold circuit S&H. The output of the sample-and-hold circuit S&H is supplied with the measured voltage U. Mess According to the measured voltage U Mess The process pressure p acting on the pressure measurement unit 10 is obtained.
[0039] The operation of this measurement circuit is explained in more detail below. Operational amplifier OP1 ensures that resistor R1 and measuring capacitor C are connected. M The connection point P1 between them is virtually grounded. Therefore, a constant current I1 flows through resistor R1 and through the measuring capacitor C. M Charge until the square wave voltage U E0 Change its arithmetic symbols.
[0040] from Figure 3 As can be seen, if R1 = R2 and C M =C R Then, the measurement point P2 in the micro-branch DZ is at the same potential as the measurement point P1, that is, at ground level, even if there is no connection between the measurement point P2 and the operational amplifier OP2. This applies not only in this specific case, but also whenever the time constant R1*C M and R2*C R This applies simultaneously. Zero-point comparison allows this state to be set accordingly via variable resistors R1 and R2. If the capacitor C is being measured... MThe capacitance changes due to the pressure, so the condition that the time constants in the integral branch IZ and the micro branch DZ are the same no longer exists, and the potential at the measurement point P2 will be different from zero. However, operational amplifier OP2 directly resists this change because it continues to virtually keep the connection point P2 grounded. Therefore, a square wave voltage U is applied to the output of operational amplifier OP2. R Its amplitude depends on the quotient of two time constants. It can be easily seen that the amplitude is related to the process pressure p ~ C. R / C M The amplitude is proportional to -1, and the correlation is roughly linear. This amplitude can be set via a voltage divider formed by two resistors, R5 and R6.
[0041] The sample-and-hold circuit S&H is used to add the absolute values of the positive amplitude A+ and the negative amplitude A- of the square wave signal, so that the absolute value A is output at the output of operational amplifier OP3 as the measured voltage U. Mess This is then forwarded to the microcontroller μC (not shown). However, this can also be output directly as an analog value. Based on the measured voltage U Mess Set the input voltage U applied to the output of the square wave generator RG. E0 The amplitude is adjusted to achieve better linearity. For this purpose, a resistor R is used. 20 and R 10 A voltage divider is constructed. This voltage divider is connected to a reference voltage VREF and is advantageously comparable.
[0042] Typically, the positive operating voltage V+ is +2.5V, and the negative operating voltage V- is -2.5V.
[0043] Figure 4 A first embodiment of the invention is shown. The basic mode of operation corresponds to the previous reference. Figure 3 The method of interpretation. A square wave signal U symmetrical with respect to the reference ground potential GND_S. E0 Used for two unit capacitors C M and C R Charging and discharging. Since the two operational amplifiers OP1 and OP2 control the potential GND_S at their negative inputs, the current I... mess and I ref by I mess =U E0 / R1 and I ref =U E0 Defined as / R2. Because the feedback path contains capacitance C. M The operational amplifier OP1 acts as an integrator, thus generating a triangular waveform voltage U at its output. COM This triangular waveform voltage then acts on capacitor C. R Therefore, a current I in the form of a square wave signal is formed in this path.diff Because operational amplifier OP2 operates with current I... zusatz By keeping the potential at its negative input constant at GND_S, a ratio C is formed at its output. R / C M proportional voltage U R .
[0044] A new aspect is the amplifier circuit VE, which includes operational amplifier OP4 and resistor R. 104 and R 105 The amplifier circuit VE has two functions: first, it amplifies the signal U. R Secondly, signal U was added. OF .
[0045] U OF With the internal excitation signal U E0 Generated in the same way. Used for two square wave signals U. E0 and U OF The clock is excited by timer 60, which in turn is excited by timer 60. Figure 3 The known threshold comparator SG is fed in. Therefore, the two signals U E0 and U OF They have the same frequency. Timer 60 uses the clock to control conversion unit 70, which can be implemented, for example, as a multiplexer. Conversion unit 70 affects the activation of voltage values stored in storage unit 40. These voltage values include various fixed positive and negative gain and offset correction values, and have been determined and stored in storage unit 40 in a previous comparison process. Figure 4 As can be seen, the conversion unit 70 uses U predefined by the timer 60. E0 and U OF The clock is used to generate a corresponding square wave signal with variable amplitude. A corresponding digital-to-analog converter (DAC) is used to convert the two square wave signals U... E0 and U OF As an analog signal output, and in U E0 In this case, it is supplied to the capacitor C of the pressure measuring unit 10. M and C R Or in U OF In this case, it is supplied to the aforementioned amplifier circuit VE (i.e., operational amplifier OP4).
[0046] The amplifier circuit VE amplifies the signal U. R And add signal U OF The result was... Figure 4 The middle is represented as U R1Then it is regulated in the microcontroller μC and the CPU 50 located therein, and provided at the output of the microcontroller μC for further use.
[0047] The following transfer function reveals U E0 How can multiplication affect the ratio C? R / C M And thus it can be used as a manipulation variable for the gain setting of a sensor system - and relatively, the signal U OF It is additively applied to the useful signal—and therefore can be used as a manipulated variable for offset compensation:
[0048]
[0049] It can be used relative to U E0 Operating U in the way of phase reversal of the signal OF The negative offset is achieved by using the phase of the signal.
[0050] Figure 5 A second embodiment of the invention is shown. (Compared to...) Figure 4 The illustrated embodiment differs, with two square wave signals U E0 and U OF The amplitude modulation is not generated within the microcontroller μC, but rather by corresponding digital potentiometers DP1 and DP2 located externally to the microcontroller μC. The two potentiometers DP1 and DP2 are actuated via a serial data bus connecting them to the microcontroller μC, thus performing amplitude regulation. The serial data bus can be implemented, for example, as an I... 2 C or SPI communication protocol.
[0051] The conversion unit 70, controlled by timer 40, now comprises only one switch that toggles between "high" and "low" or between "0" and "1" solely according to a predetermined clock. An additional switch is provided for the polarity of the offset correction value, and this switch is directly actuated from memory 40, as indicated by the dashed lines. In addition to the offset polarity, memory 40 also stores the voltage value required for the desired gain or offset correction.
[0052] Now, the digital clock signal is output via the GPIO (General Purpose Input / Output) port and provided to two potentiometers, DP1 and DP2. These potentiometers use the digital clock signal and the amplitude value sent via the serial data bus to generate a signal from the GPIO port. Figure 4 The known corresponding analog square wave signal U E0 or U OF .
[0053] List of reference numerals
[0054] 1. Pressure measuring equipment
[0055] 10 pressure measurement units
[0056] 12 matrix
[0057] 14 membranes
[0058] 16 Glass welding ring
[0059] 18 ventilation ducts
[0060] 19 cavity
[0061] 20 Evaluation of Electronic Devices
[0062] 30 Evaluation Circuits
[0063] 40 memory
[0064] 50 Central Processing Units (CPUs)
[0065] 60 timers
[0066] 70 conversion units
[0067] C M Measuring capacitors
[0068] C R Reference capacitor
[0069] M intermediate electrode
[0070] R-ring electrode
[0071] ME film electrode
[0072] IZ branch path
[0073] DZ Micro Branch Circuit
[0074] SG threshold comparator
[0075] RG square wave generator
[0076] VE amplifier unit
[0077] DP1 Digital Potentiometer
[0078] DP2 Digital Potentiometer
Claims
1. A method for operating a pressure measuring unit (10) of a capacitive pressure sensor (1), in, The pressure measuring unit (10) comprises a pressure-dependent measuring capacitor (C M ) and a reference capacitor (C R ), which have an internal excitation voltage U E0 applied to them in the form of an alternating square-wave signal, and from the capacitance values of the measuring capacitor (C M ) and the reference capacitor (C R ) a pressure measurement value p is obtained, The measurement signal is an AC square wave signal U. R The AC square wave signal U is provided to the evaluation unit in the form of... R The pulse level depends on the reference capacitor (C). R ) and the measuring capacitor (C) M The quotient of the capacitance value, and the AC square wave signal U R The period duration is measured by the capacitor (C) M It is determined by the capacitance value of ). Its features are, For signal amplification and offset correction, an alternating square wave signal U R is supplied to the amplifier unit (VE), which performs signal amplification by amplitude adjustment of the internal excitation voltage U E0 and offset compensation by means of a further square wave signal U OF Furthermore, for gain and offset correction, each fixed voltage value has been stored in the memory cell (40) during the comparison process. wherein these stored voltage values are used together with a predetermined clock to form two square wave signals U E0 and U OF , And therein, through the reference capacitor (C) R ) and the measuring capacitor (C) M Gain correction is performed by multiplying the quotient of the capacitance value of the square wave signal U. OF Provided to the amplifier unit (VE) and thus added to the square wave signal U. R To perform offset correction.
2. The method according to claim 1, characterized in that, The pressure sensor (1) includes a microcontroller (μC).
3. The method according to claim 1 or 2, characterized in that, Used to generate the two square wave signals U E0 and U OF The clock is activated by a timer (60).
4. The method according to claim 3, characterized in that, The timer (60) controls the conversion unit (70) and is used to generate the two square wave signals U. E0 and U OF The clock mentioned above.
5. The method according to claim 2, characterized in that, The two square wave signals U E0 and U OF The amplitude is adjusted by corresponding digital potentiometers (DP1, DP2), which are actuated via a serial data bus connecting the two potentiometers (DP1, DP2) to the microcontroller (μC).