Capacitive pressure measuring cell and pressure measuring device comprising such a pressure measuring cell

Abstract

The present invention relates to a capacitive pressure measuring cell for detecting the pressure of a medium adjacent to the pressure measuring cell. The pressure measuring cell comprises an elastic membrane, a cylindrical main body, and a Microfused Strain Gauge (MSG) chip placed on the outer side of the main body. The MSG chip enables the measurement of gauge pressure and temperature.

Description

[0001] Capacitive pressure measuring cell and pressure measuring device with such a pressure measuring cell

[0002] The invention relates to a capacitive pressure measuring cell for detecting the pressure of a medium adjacent to the pressure measuring cell, according to the preamble of claim 1, and to a pressure measuring device with such a pressure measuring cell.

[0003] Capacitive pressure sensors or pressure gauges are used for pressure measurement in many industrial sectors. They often feature a ceramic pressure measuring cell, which acts as a transducer for the process pressure, and evaluation electronics for signal processing.

[0004] Capacitive pressure sensors consist of a ceramic base and a membrane, with a glass ring positioned between the base and the membrane. The resulting cavity between the base and membrane allows the membrane to move longitudinally in response to pressure. This cavity is therefore also referred to as the measuring chamber. Electrodes are located on the underside of the membrane and on the opposite upper side of the base, together forming a measuring capacitor. Pressure causes the membrane to deform, resulting in a change in the capacitance of the measuring capacitor.

[0005] An evaluation unit detects the change in capacitance and converts it into a pressure reading. These pressure sensors are typically used to monitor or control processes and are therefore often connected to higher-level control units (PLCs).

[0006] From DE 19851 506 C1, a capacitive pressure sensor is known in which the pressure measurement is determined from the quotient of two capacitance values, a measuring capacitor and a reference capacitor. While this patent does not specifically describe a pressure measuring cell, the circuit and method described are suitable for capacitive pressure measuring cells. The special feature of this pressure measuring device is that, for evaluating the output signal as a measure of the detected pressure value, only the amplitude of the square wave signal is relevant, regardless of its frequency. P2024.1361-WQ / TT - 2 -

[0007] From EP 0569573 B1 a circuit arrangement for a capacitive pressure sensor is known, in which a quotient method is also used for pressure evaluation.

[0008] Quotient methods generally assume the following pressure dependencies:

[0009] CR, CR, CM - CR

[0010] P - or p - 1 or p - ,

[0011] CM

[0012]

[0013] CM CM + CR

[0014] where CM denotes the capacitance of the measuring capacitor, CR the capacitance of the reference capacitor, and p the process pressure to be determined.

[0015] It is also conceivable to interchange CM and CR in the quotient. However, the given example with CM in the denominator represents the most common form for the sake of eigenlinearization. Therefore, this embodiment will be assumed in the following unless otherwise stated.

[0016] In addition to the "normal" stress on the pressure sensor, especially the pressure measuring cell, caused by the nominal pressure of the medium being monitored, short-term overpressures frequently occur in many applications, which are significantly higher than the maximum nominal pressure of the medium. Such an overpressure can occur, for example, in the food industry when a container normally used to store milk is cleaned with a high-pressure liquid.

[0017] From DE 10221 219 A1, it is known to construct the pressure measuring cell such that the diaphragm rests against the base body or parts thereof in the event of overload, thus ensuring overload and burst resistance. However, in this case, when the diaphragm rests against the base body, pressure measurement is no longer possible or is correspondingly limited, so that two sensors with different nominal pressure ranges, e.g., 10 bar and 100 bar, had to be provided.

[0018] German patent DE 102007063 116 A1 describes a ceramic pressure measuring cell which, in addition to a capacitive main sensor, has a second measuring sensor on the back of the ceramic base body for the detection of measurement errors. This second measuring sensor, which can be implemented as a thick-film resistor or strain gauge, serves to determine a second pressure value, which is compared with the capacitively determined pressure value. A measurement error is signaled if the difference between the measured values ​​exceeds a predefined reference value.

[0019] US patent 2016 / 0252418 A1 discloses a pressure sensor housing with a microfused silicon strain gauge (MSG) sensor element. The sensor element is mounted on a carrier and electrically connected to an electronic unit. The design of the housing and the carrier incorporates decoupling features to isolate the sensor element from parasitic forces and mechanical influences from the housing environment, thus enabling a compact sensor housing design.

[0020] The object of the invention is to propose a pressure measuring cell and a pressure measuring device that enable sufficient pressure measurement even in overload conditions and are suitable for recording a further measured variable. This object is achieved according to the invention by a pressure measuring cell with the features of claim 1 and a pressure measuring device with the features of claim 7. Advantageous embodiments of the invention are specified in the dependent claims.

[0021] In a first aspect, according to the invention, an electromechanical transducer designed as a Microfused Strain Gauge (MSG) chip is arranged on the outside of the base body of the pressure measuring cell, which is positioned in an area of ​​mechanical stress caused by deformation of the base body, advantageously in the strongest compression or bending zones, and which is suitable to provide a pressure measurement signal when the membrane is bent in the case of overload such that it rests against the base body and as a result the membrane electrode on the inside of the membrane is short-circuited with at least the central electrode on the inside of the base body.

[0022] An advantageous further development provides that a printed circuit board with an evaluation circuit, for example in the form of an ASIC, is arranged on the outside of the base body. The MSG chip is electrically connected to the printed circuit board, preferably by means of bond wires, and the evaluation circuit is designed to determine a pressure value based on a pressure measurement signal dependent on the deflection of the diaphragm when the pressure p is within the nominal pressure range of the pressure measuring cell, and to determine a pressure value based on a pressure measurement signal from the MSG chip when the pressure p is outside the nominal pressure range of the pressure measuring cell.

[0023] The advantage is that the MSG chip can determine the pressure value even if the pressure measuring cell is defective or if overpressure severely bends the membrane with the membrane electrode and causes a short circuit with the counter-current or counter-current.

[0024] The central electrode is attached to the base body. In such cases, an evaluation of the actual measuring cell is no longer possible, and the MSG chip takes over the recording of the pressure value, as it is comparatively sensitive and can therefore also detect slight compression and bending movements on the base body.

[0025] Additionally, the MSG chip enables temperature measurement via its temperature-sensitive total resistance. The determined temperature value can be used for compensation, diagnostic purposes, and / or as a secondary process value. This increases the functionality and reliability of the pressure sensor.

[0026] A second aspect of the invention relates to a pressure measuring device comprising a previously described pressure measuring cell and evaluation electronics, wherein the MSG chip is electrically connected to the evaluation electronics.

[0027] Preferably, the evaluation electronics comprise the evaluation circuitry, and more preferably, the evaluation circuitry is configured to determine a pressure value based on a pressure measurement signal dependent on the deflection of the diaphragm when the pressure p is within the nominal pressure range of the pressure measuring cell, and to determine a pressure value based on a pressure measurement signal from the MSG chip when the pressure p is outside the nominal pressure range of the pressure measuring cell. The evaluation circuitry can therefore be optionally arranged directly on the pressure measuring cell or remotely inside the pressure gauge.

[0028] The pressure gauge can advantageously signal the measured overpressure via a display or output signals and transmit the values ​​to external systems or store them internally for diagnostic purposes. This provides an additional safety feature for detecting and verifying the correct functioning of the pressure sensor. The invention is explained in more detail below with reference to exemplary embodiments and the drawings.

[0029] They show schematically:

[0030] Figure 1 shows a block diagram of a capacitive pressure gauge;

[0031] Figure 2 shows a schematic sectional view of a capacitive pressure measuring cell according to the invention;

[0032] Figure 3 shows a perspective view of a capacitive pressure measuring cell according to the invention.

[0033] In the following description of preferred embodiments, identical reference numerals denote identical or comparable components.

[0034] Figure 1 shows a block diagram of a typical capacitive pressure gauge 1, which is used to measure a process pressure p (e.g., of oil, milk, water, etc.). The pressure gauge 1 is designed as a two-wire device and essentially consists of a pressure sensor 10 and evaluation electronics 20. The evaluation electronics 20 comprise an analog evaluation circuit 30 and a microcontroller pC. In the evaluation circuit 30, the sensor signals from the pressure sensor 10 are processed. The microcontroller pC digitizes the processed signal, further processes it, and provides the evaluation result as a digital or analog output signal, for example, for a higher-level control unit (PLC). The pressure gauge 1 is connected to a power supply line for its power supply.

[0035] Figure 2 shows a schematic representation of a typical capacitive pressure measuring cell 10, as used in a variety of capacitive pressure gauges. The pressure measuring cell 10 essentially consists of a base body 12 and a diaphragm 14, which are connected to each other via a glass solder ring 16. The base body 12 and the diaphragm 14 define a cavity 19, which—preferably only in low pressure ranges up to 50 bar—is connected to the rear of the pressure measuring cell 10 via a vent channel 18.

[0036] Both the base body 12 and the membrane 14 are provided with several electrodes, forming a reference capacitor CR and a measuring capacitor CM. The measuring capacitor CM is formed by the membrane electrode ME and the center electrode M, the reference capacitor CR by the ring electrode R and the membrane electrode ME.

[0037] The process pressure p acts on the membrane 14, which deflects to a greater or lesser degree depending on the pressure applied, essentially changing the distance between the membrane electrode ME and the center electrode M. This leads to a corresponding change in the capacitance of the measuring capacitor CM. The influence on the reference capacitor CR is less significant, since the distance between the ring electrode R and the membrane electrode ME changes less than the distance between the membrane electrode ME and the center electrode M.

[0038] An electromechanical transducer designed as a Microfused Strain Gauge (MSG) chip 40 is arranged on the outside of the base body 12 and positioned in an area of ​​mechanical stress caused by deformation of the base body 12, advantageously in the areas of greatest compression or bending. The pressure measuring cell 10 is thus able to provide a pressure measurement signal when, in the case of overload, the diaphragm 14 is deflected to such an extent that it abuts the base body 12 and, consequently, the diaphragm electrode ME on the inside of the diaphragm 14 is short-circuited with at least the central electrode M on the inside of the base body 12.

[0039] Figure 3 shows a perspective view of the pressure measuring cell 10, illustrating the compact and integrated design of the invention. Visible is the base body 12 with the membrane 14 arranged at the bottom. Several contact pins 17 are arranged on the upper or outer surface of the base body 12, establishing an electrical connection to the electrodes M, R, and ME located inside.

[0040] Also mounted on the outside of the base body 12 is a circuit board with the evaluation electronics 20. This circuit board carries the evaluation circuit 30, which can be implemented, for example, as an application-specific integrated circuit (ASIC). The MSG chip 40, also located on the outside of the base body 12, is electrically connected to the evaluation electronics 20, preferably via bond wires. This arrangement enables the processing of the signals from both measurement principles—the capacitive signal from the pressure sensor 10 and the piezoresistive signal from the MSG chip 40—in a single component integrated on the base body 12.

[0041] Evaluation unit. Reference symbol list

[0042] 1 pressure gauge

[0043] 10 pressure measuring cells

[0044] 12 basic shapes

[0045] 14 Membran

[0046] 16 Glass solder ring

[0047] 17 Contact pin

[0048] 18 Ventilation duct

[0049] 19 Cavity

[0050] 20 Evaluation electronics 30 Evaluation circuit 40 MSG chip

[0051] CM measuring capacitor CR reference capacitor p pressure, process pressure M center electrode

[0052] R Ring electrode

[0053] M Membrane electrode

Claims

Patent claims 1. Capacitive pressure measuring cell (10) for detecting the pressure p of a medium adjacent to the pressure measuring cell (10), comprising an elastic membrane (14) whose first side is at least partially in contact with the medium, and a cylindrical base body (12) arranged opposite the membrane (14), wherein a spacer ring (16) is arranged between the base body (12) and the membrane (14), and the base body (12) has a central electrode (M) and a ring electrode (R) for detecting the deflection of the membrane (14), which together with a membrane electrode (ME) on the membrane (14) form a measuring capacitance (CM) or a reference capacitance (CR), characterized by that an electromechanical transducer designed as a Microfused Strain Gauge (MSG) chip (40) is arranged on the outside of the base body (12), which is positioned in an area of ​​mechanical stress caused by deformation of the base body (12), and which is suitable to provide a pressure measurement signal when the membrane (14) is bent in such a way as to come into contact with the base body (12) in the event of an overflow, and as a result the membrane electrode (ME) and the center electrode (M) are short-circuited.

2. Pressure measuring cell according to claim 1 , characterized in that a printed circuit board with an evaluation circuit (30) is arranged on the outside of the base body (12), wherein the MSG chip (40) is electrically connected to the circuit board and the evaluation circuit (30) is designed to determine a pressure value based on a pressure measurement signal dependent on the deflection of the diaphragm (14) when the pressure p is within the nominal pressure range of the pressure measuring cell (10), and to determine a pressure value based on a pressure measurement signal from the MSG chip (40) when the pressure p is outside the nominal pressure range of the pressure measuring cell (10).

3. Pressure measuring cell (10) according to claim 1 or 2, characterized in that the MSG chip (40) determines the pressure value when the ceramic pressure measuring cell (10) has a defect.

4. Pressure measuring cell (10) according to one of the preceding claims, characterized in that the MSG chip (40) is connected to the circuit board by means of bond wires.

5. Pressure measuring cell (10) according to one of the preceding claims, characterized in that the MSG chip (40) additionally determines the temperature value, which is done via its temperature-sensitive total resistance.

6. Pressure measuring cell (10) according to claim 5, characterized in that the determined temperature value is used for compensation purposes, diagnostic purposes and / or as a second process value.

7. Pressure measuring device (1), comprising a pressure measuring cell (10) according to one of the preceding claims and an evaluation electronics (20), wherein the MSG chip (40) is electrically connected to the evaluation electronics (20).

8. Pressure gauge (1) according to claim 7, further comprising a display or output signals for signaling an overload and / or overpressure event and means for transmitting the measured overpressure to external systems or for internal storage for diagnostic purposes.

9. Pressure gauge (1) according to claim 7 or 8, wherein the evaluation electronics (20) comprises the evaluation circuit (30).

10. Pressure measuring device (1) according to claim 9, in which the evaluation circuit (30) is designed to determine a pressure value based on a pressure measurement signal depending on the deflection of the diaphragm (14) when the pressure p is within the nominal pressure range of the pressure measuring cell (10), and to determine a pressure value based on a pressure measurement signal from the MSG chip (40) when the pressure p is outside the nominal pressure range of the pressure measuring cell (10).

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