Vibration sensor with a piezoelectric stack drive for high operating temperatures without additional temperature compensation elements.

Abstract

Vibration sensor (100) with a mechanical vibration unit (7) arranged on a diaphragm (90) that can be set into vibration and a connecting section for connecting the diaphragm (90) to a drive unit (1) such that vibrations of the drive unit (1) are transmitted to the diaphragm (90) and vibrations of the diaphragm (90) to the drive unit (1), wherein the drive unit (1) has a drive receptacle (93) for mounting the drive unit (1) in the vibration sensor (100), wherein within the drive receptacle (93) a piezo stack drive (11) with a stack consisting of a piezo unit (13) comprising at least one piezo element (15), an adaptation ceramic (17) above and below the piezo unit (13), and a preferably metallic pressure piece (19) at least below the adaptation ceramic (17) is arranged, characterized in that the drive receptacle (93) is cylindrical,a metallic sleeve formed and made of a material with a linear coefficient of thermal expansion reduced by at least 25% compared to stainless steel.

Description

The present invention relates to a vibration sensor according to the preamble of claim 1. Piezoelectric transmitters and / or receivers, vibration sensors with such piezoelectric transmitters and / or receivers, and methods for manufacturing piezoelectric transmitters and / or receivers are known from the prior art. Piezoelectric transmitters and / or receivers can be used, for example, in vibration sensors, which are frequently used as limit level sensors in level measurement technology. Such piezoelectric transmitters and / or receivers are often also referred to as actuators. Typically, a vibration sensor has a diaphragm that can be excited to vibrate by a drive mechanism. This diaphragm, in turn, excites a mechanical oscillator mounted on it. Depending on the degree of coverage of the mechanical oscillator with a substance and the viscosity of that substance, the mechanical oscillator vibrates at a characteristic frequency, which is detected by the vibration sensor and converted into a measurement signal. In the prior art, two different types of actuators are commonly used. In the first variant, a multi-segmented piezoelectric element is bonded to the diaphragm. Applying an electrical voltage to one or more segments of the piezoelectric element excites it to bend or torsion, which it transmits to the diaphragm. This causes the diaphragm to vibrate, in turn causing the mechanical oscillator to vibrate. This type of actuator produces only a limited stroke and can only be used with vibration sensors operating at temperatures significantly below the glass transition temperature of the adhesive and below the Curie temperature of the piezoelectric material. These sensors are not suitable for high-temperature applications above 150°C. If a sensor with a larger stroke is required for an application, or if use at higher temperatures is necessary, a second type of drive, so-called piezo stack drives, is used. Here, a stack consisting of a piezoelectric unit (comprising one or more piezoelectric elements), an adapter ceramic arranged above and below the piezoelectric unit, and pressure pieces arranged above and below the adapter ceramics is clamped against a clamping bolt and a clamping nut located on the sensor's diaphragm. Applying an electrical voltage to the piezoelectric elements causes them to change their axial expansion along the bolt, thus setting the diaphragm into vibration. A known vibration sensor with a piezoelectric stack drive is shown in Fig. 2. Fig. 2 shows a section of a vibration sensor 100 known from the prior art, which is used, for example, in level measurement technology as a limit level sensor. The vibration sensor 100 has a membrane 90 that can be excited to oscillation via a piezo stack drive 11, which acts as a piezoelectric transmitting and / or receiving device, by means of which a mechanical vibration unit 7 arranged on the membrane 90 can be excited to oscillation. The vibration sensor 100 has a piezoelectric stack drive 11 consisting of stacked piezoelectric elements 15 contacted via electrodes 16, with an adapting ceramic element 17 and a pressure piece 19, in this case made of a metallic material, arranged above and below the stack. The adapting ceramic elements 17 serve to adjust the coefficient of thermal expansion between the piezoelectric elements 15 and the pressure pieces 19. The pressure pieces 19 are designed such that a force from the piezoelectric stack is absorbed over its entire surface. The piezoelectric stack drive 11 is clamped against the diaphragm 90 by means of a clamping nut 92 via a clamping bolt 91 integrally molded onto the diaphragm 90, so that vibrations of the piezoelectric stack drive 11 are effectively transmitted to the diaphragm 90 and vice versa. Electrodes 16 for contacting the piezoelectric elements 15 are connected to connecting leads 31, which run along the outside of the piezoelectric stack drive 11 and are led to sensor electronics on the rear side. A sensor housing 98 is arranged on the diaphragm 90 via an outer, circumferential rim extending in the rear direction, and is connected to or integrally formed with the rim of the diaphragm 90. The piezo stack drive 11 is insulated from the clamping bolt 91 by means of an insulating sleeve 89 made of plastic. The known vibration sensors 100 exhibit a temperature dependence, since the clamping bolt 91, via which the drive 11 is clamped to the membrane 90, has a larger coefficient of thermal expansion α than the piezo stack drive 11, so that with increasing temperature the preload of the drive 11 decreases in the direction of the membrane 90. To reduce the temperature dependence of these known sensors 100, so-called thermal expansion compensation elements 95 are used in the prior art. Figure 3 shows a vibration sensor 100 in which the temperature-related change in length of the sleeve-shaped drive receptacle 93, which is used there instead of the clamping bolt 91 for clamping and which simultaneously serves to clamp the drive 11 against the diaphragm 90, is compensated by such a thermal expansion compensation element 95. The thermal expansion compensation elements 95 are made of a material with a higher coefficient of thermal expansion α than the drive mount 93 or the clamping bolt 91. They are arranged in series with the drive and clamped against the diaphragm 90. In this way, a thermally induced change in length Δl and the resulting change in the preload of the drive 11 can be compensated. If the clamping bolt 91 or the drive mount 93 is made of stainless steel, for example, the thermal expansion compensation elements 95 can be made of aluminum, for example. Complete compensation of the change in length Δl is only possible with relatively long thermal expansion compensation elements 95. Furthermore, it is considered a disadvantage that the thermal expansion compensation elements 95 must be very long. As shown in the embodiment of Fig. 3, the dimensions of the thermal expansion compensation element 95 can double the length of the entire drive 11 in the axial direction A. Further state of the art is known from WO 2009 / 114 073 A1 , DE 10 2016 108 553 A1 , US 2011 / 0 001 395 A1 and EP 0 869 278 A1 . It is therefore the object of the present invention to further develop a drive unit for a vibration sensor and a vibration sensor with such a drive unit in such a way that it has compact dimensions and a reduced temperature dependence. This problem is solved by a vibration sensor with a drive unit having the features of claim 1. Advantageous further developments are the subject of dependent claims. A vibration sensor according to the invention, comprising a mechanical vibration unit arranged on a diaphragm capable of being set into vibration and a second connecting section for connecting the diaphragm to a drive unit such that vibrations of the drive unit are transmitted to the diaphragm and vibrations of the diaphragm to the drive unit, has a drive unit with a drive receptacle for mounting the drive unit in the vibration sensor, wherein within the drive receptacle a piezoelectric stack drive is arranged with a stack consisting of a piezoelectric unit comprising at least one piezoelectric element, an adapting ceramic above and below the piezoelectric unit, and a preferably metallic pressure piece at least below the adapting ceramic, and is characterized in that the drive receptacle is cylindrical,a metallic sleeve formed and made of a material with a linear coefficient of thermal expansion reduced by at least 25% compared to stainless steel. In this way, compared to drive units known from the prior art, which have a clamping bolt or drive mount made of stainless steel, a significant reduction in the thermally induced decrease in preload and thus in the efficiency of the drive can be achieved without additional compensatory measures such as thermal expansion compensation elements. The linear coefficient of thermal expansion, or coefficient of length expansion α, of a solid with length l is the constant of proportionality between a temperature change dT and a relative change in length dL / l. It describes the relative change in length in response to a temperature change. Since the thermal expansion of solids is also temperature-dependent, in this application, unless otherwise specified, the term "coefficient of thermal expansion" refers to the linear coefficient of thermal expansion at a reference temperature of 20°C. Stainless steel is a designation for alloyed or unalloyed steels with a particularly high degree of purity. Due to the possible variations in the alloy composition, these materials also exhibit different coefficients of linear expansion, ranging from approximately 11 × 10⁻⁶ K⁻¹ to 16 × 10⁻⁶ K⁻¹. Therefore, the present application refers to V2A / V4A steel X2CrNi18-9 (DIN EN 10088-1: 1.4307, AISI: 304L, Acidur 4307) and X2CrNiMo17-12-2 (DIN EN 10088-1: 1.4404, AISI: 316L) as well as X8CrNiS18-9 (DIN EN 10088-1: 1.4305, AISI: 303, Acidur 4305). These steels exhibit a linear coefficient of thermal expansion of α = 16 · 10⁻⁶ K⁻¹ at 20°C. A linear coefficient of thermal expansion reduced by at least 25% is therefore less than 12 · 10⁻⁶ K⁻¹.Advantageously, the coefficient of thermal expansion of the material used is even smaller, preferably reduced by at least 30%, more preferably by 40% and particularly preferably by about 50% compared to the coefficient of thermal expansion of stainless steel. To reduce the weight of the sleeve and save material, the sleeve can have multiple axially oriented slots. These slots can partially or fully penetrate the sleeve's outer surface in the axial direction, either along one side or completely to its end. In the former case, the sleeve is stabilized by a continuous ring at both ends; in the latter case, only at one end. In a preferred embodiment, the sleeve is made of a material with a coefficient of thermal expansion of less than 12 × 10⁻⁶ K⁻¹. A coefficient of thermal expansion of 12 × 10⁻⁶ K⁻¹ already achieves a significant reduction in thermal expansion in the temperature range relevant for drive units for vibration sensors. The piezoelectric stack drive of the drive unit is advantageously pre-tensioned, at least in the assembled state, by means of a pressure screw that interacts with the drive mount, wherein the pressure screw is at least partially made of a metal with a linear coefficient of thermal expansion that is greater, in particular at least 25% greater, than the coefficient of thermal expansion of the drive mount. Such a design ensures that the pressure screw expands relatively more, particularly in the axial direction, than the drive mount, so that any change in length caused by the thermal expansion of the drive mount can be at least partially compensated without the use of additional components. In a preferred embodiment, the pressure screw has an engagement section, a threaded section and an expansion section oriented away from the engagement section and extending in the axial direction, wherein at least the expansion section is made of the metal with a higher coefficient of thermal expansion compared to the drive receptacle. This design of the pressure screw, featuring an axially extending section (i.e., a section extending particularly towards the drive), allows for further improvement in compensating for the thermal expansion of the drive housing. The expansion section can be integrally formed with the pressure screw or attached to it. A separate expansion section is also possible. For example, the expansion section can be designed as a shoulder or a circumferential step. The material and axial dimensions of the expansion section are preferably matched to each other and to the thermal expansion coefficients of the drive housing material and the piezoelectric drive itself, such that temperature-induced changes in length are compensated. For the piezo stack drive, the coefficient of thermal expansion can be determined empirically or calculated based on the materials used. To ensure simple assembly with as few individual parts as possible, the pressure screw can preferably be made entirely of metal with an increased coefficient of thermal expansion. Preferably, the increased coefficient of thermal expansion and the dimensions of the pressure screw, as described above, are adapted to the coefficient of thermal expansion of the drive housing and the piezoelectric stack drive. The pressure screw can be made of stainless steel, preferably 316L steel. If the material of the pressure screw is known, the expansion section can be dimensioned to compensate for the different thermal expansion of the drive housing and the piezoelectric stack drive. Stainless steel also has the advantage of being a commercially available and therefore inexpensive material. The drive housing can be made of titanium or nickel steel, preferably 47Ni or 36Ni. This particular selection allows for a very low coefficient of thermal expansion for the drive housing. Titanium has a coefficient of thermal expansion of 8.2 × 10⁻⁶ K⁻¹, 47Ni steel a coefficient of thermal expansion of 7.5 × 10⁻⁶ K⁻¹, and 36Ni steel a coefficient of thermal expansion of 1.0 × 10⁻⁶ K⁻¹. By selecting a suitable material, the thermally induced linear expansion of the drive housing can be significantly reduced compared to, for example, stainless steel, thus considerably increasing the reliability and operational capability of the drives and vibration sensors equipped with such drives. The drive unit can be directly or indirectly screwed, welded, soldered, glued, or connected to the diaphragm via a bayonet fitting. These connection techniques ensure that even materials with different coefficients of thermal expansion can be securely joined. The present invention is explained in detail below with reference to exemplary embodiments and the accompanying figures. Figure 1 shows a longitudinal section through an exemplary embodiment of a vibration sensor with a drive unit according to the present application, Figure 2 shows a longitudinal section through a vibration sensor with a drive unit according to the prior art (already discussed), and Figure 3 shows a longitudinal section through a vibration sensor with a thermal expansion compensation element according to the prior art (already discussed). In the figures, unless otherwise stated, the same reference symbols denote the same components with the same function. Fig. 1 shows a longitudinal section through an embodiment of a vibration sensor 100. The vibration sensor 100 has a membrane 90 that can be excited to oscillation via a piezoelectric stack drive 11, which acts as a piezoelectric transmitting and / or receiving device, by means of which a mechanical vibration unit 7 arranged on the membrane 90, which in the present embodiment is designed as two paddles arranged on the membrane, can be excited to oscillate. The piezoelectric stack actuator 11 is formed from stacked piezoelectric elements 15 contacted via electrodes 16, with an adaptation ceramic 17 arranged above and below each stack. The adaptation ceramic 17 serves to adapt the coefficient of thermal expansion between the piezoelectric elements 15 and a pressure piece 19 arranged below in the stack, as well as a pressure screw 92 arranged above for preloading. The pressure piece 19 is designed such that a force from the piezoelectric stack is absorbed over its entire surface. The piezo stack drive 11 is clamped against the diaphragm 90 by means of a sleeve-shaped drive receptacle 93 which is attached to a circumferential edge of the diaphragm 90 and into which a pressure screw 92 is screwed on the rear side, so that vibrations of the piezo stack drive 11 are effectively transmitted to the diaphragm 90 and vice versa. In the present embodiment, the drive mount 93 is screwed to the diaphragm 90; that is, an external thread of the drive mount 93 engages in an internal thread formed on the circumferential rim molded onto the diaphragm and extending axially A to the rear. However, the drive mount 93 could also be attached to the diaphragm in a different way, e.g., welded, soldered, connected via a bayonet fitting, or otherwise. To reduce thermal influences on the sensor 100, the drive mount 93 is made of titanium in the present embodiment. Titanium has a coefficient of thermal expansion of αtitanium = 8.2 × 10-6 K-1. The pressure screw 92 is made of 316L steel with a coefficient of thermal expansion of α316L = 16 × 10-6 K-1 and has an engagement section 21 for tightening the pressure screw 92 using a tool, a threaded section 22 that interacts with a thread of the drive mount 93, and an expansion section 23 that extends away from the engagement section 21 and along the axial direction A. The threaded section 22 has an external thread which interacts with an internal thread of the drive receptacle 93 in such a way that the pressure screw 92 can be screwed into the drive receptacle 93 in axial direction A and thus presses the drive 11 against the diaphragm 90.In the direction of the drive 11, the pressure screw 92 has the expansion section 23, which in the present embodiment is designed as an annular shoulder that connects radially inwards to the threaded section 22 in axial direction A. The expansion section 23 is not engaged with the internal thread of the drive receptacle 93, so that the expansion section 23 can expand independently of any thermal expansion of the drive receptacle 93. The effect of the present arrangement upon an increase in temperature is described below. For this analysis, a surface of the membrane 90 oriented towards the drive 11 is assumed to be the zero point. If, for example, the sensor 11 heats up from 20°C to 120°C, i.e., the temperature increases by 100 K, the individual components of the drive 11 expand. Due to the present problem, in which the preload of the drive 11 against the diaphragm 90 is essential, only expansions in the axial direction A are considered in more detail. The extension of the circumferential edge of the membrane 90 and the equally high proportion of the pressure piece 19 in axial direction A are neglected, since they are made of the same material. The drive mount has a longitudinal extent of 15 mm from its attachment to the diaphragm 90 to the start of the thread engagement between the drive mount 93 and the pressure screw 93. Due to a temperature increase ΔT of 100 K, the drive mount 93 expands in this area by an amount in the axial direction A. At the same time, the pressure piece 19, the adapting ceramics 17 and the piezo unit 13 expand with an average thermal expansion coefficient αdrive of 5.83 · 10-6K-1 and a length l of 6.7 mm by Δldrive = l × αdrive × ΔT = 11.5 mm × 5.83 · 10-6K-1 × 100 K = 6.7 µm in axial direction A. This results in a difference in length expansion in the axial direction A of 5.6 µm. The pressure screw 92 has an expansion section 93 with an axial length A of 3.5 mm. The pressure screw is made of 316L steel and therefore has a coefficient of thermal expansion of α316L = 16 · 10⁻⁶ K⁻¹, so that a temperature increase of 100 K results in a change in length of the expansion section of . The thermally induced length changes Δl of the drive and expansion section 23 of the pressure screw 92 thus correspond to the thermally induced length change Δl of the drive mount. Because the drive has a lower mean coefficient of thermal expansion α than the drive mount 93, and because the coefficient of thermal expansion α of the pressure screw 92 is greater than that of the drive mount 93, a thermally induced reduction in the preload of the drive 11 can be completely compensated. Reference symbol list 1 Drive unit 7 Vibration unit 11 Piezo stack drive / drive 15 Piezo elements 16 Electrodes 17 Adapting ceramic 19 Pressure piece 31 Connecting leads 89 Insulating sleeve 90 Diaphragm 91 Clamping bolt 92 Clamping nut / pressure screw 93 Drive mount 94 Expansion section 95 Thermal expansion compensation element 98 Sensor housing 100 Vibration sensor α Coefficient of thermal expansion Δl Change in length ΔT Temperature change l Length

Claims

Vibration sensor (100) with a mechanical vibration unit (7) arranged on a diaphragm (90) that can be set into vibration and a connecting section for connecting the diaphragm (90) to a drive unit (1) such that vibrations of the drive unit (1) are transmitted to the diaphragm (90) and vibrations of the diaphragm (90) to the drive unit (1), wherein the drive unit (1) has a drive receptacle (93) for mounting the drive unit (1) in the vibration sensor (100), wherein within the drive receptacle (93) a piezo stack drive (11) with a stack consisting of a piezo unit (13) comprising at least one piezo element (15), an adaptation ceramic (17) above and below the piezo unit (13), and a preferably metallic pressure piece (19) at least below the adaptation ceramic (17) is arranged, characterized in that the drive receptacle (93) is cylindrical,a metallic sleeve formed and made of a material with a linear coefficient of thermal expansion reduced by at least 25% compared to stainless steel. Vibration sensor (100) according to claim 1, characterized in that the sleeve has a plurality of axially extending slots. Vibration sensor (100) according to claim 2, characterized in that the sleeve is made of a material having a coefficient of thermal expansion of less than 12 · 10-6K-1. Vibration sensor (100) according to one of the preceding claims, characterized in that the piezo stack drive (11) is pre-tensioned at least in the assembled state via a pressure screw which interacts with the drive receptacle (93), wherein the pressure screw is formed at least sectionally from a metal with a linear coefficient of thermal expansion of at least 10 · 10-6K-1. Vibration sensor (100) according to claim 4, characterized in that the pressure screw has an engagement section (21), a threaded section (22) and an expansion section (23) oriented towards the threaded section (22) away from the engagement section (21) and extending in the axial direction, wherein at least the expansion section (23) is made of the metal. Vibration sensor (100) according to one of claims 4 or 5, characterized in that the pressure screw is made entirely of metal. Vibration sensor (100) according to one of claims 4 to 6, characterized in that the pressure screw is made of stainless steel, preferably X2CrNiMo17-12-2 steel or V2A steel. Vibration sensor (100) according to one of the preceding claims, characterized in that the drive mount (93) is made of titanium or a Ni-steel, preferably 47Ni or 36Ni. Vibration sensor (100) according to one of the preceding claims, characterized in that the drive mount (93) is screwed, welded, soldered, glued or connected via a bayonet fitting to the diaphragm either directly or indirectly.

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