Pressure measuring cell

Abstract

A pressure measuring cell (5) is described, comprising a support body (5.3), a membrane (5.2) supported by the support body and a measurement circuit (5.1), the measurement circuit comprising a plurality of strain sensitive resistors (R1a-b, R2, R3, R4a-b) connected in a Wheatstone bridge circuit, wherein the measurement circuit comprises a temperature-balancing network comprising a first temperature-balancing auxiliary resistor (R61) connected to a first strain sensitive resistor (R1, R1a) of the Wheatstone bridge circuit and a second temperature-balancing auxiliary resistor (R62) connected to a second strain sensitive resistor (R2) of the Wheatstone bridge circuit, the temperature-balancing network further comprising a joint thermistor (PTC) connected between the first and second temperature-balancing auxiliary resistors.

Description

[0001] PRESSURE MEASURING CELL

[0002] Technical field

[0003] The present invention relates to a pressure measuring cell and a method for balancing a measurement circuit of a pressure measuring cell .

[0004] Background of the invention

[0005] The pressure of a measurement medium such as a gas or a liquid can be measured using strain sensitive resistors arranged on a deflectable membrane , where a surface of the membrane is facing the measurement medium . The strain sensitive resistors are typically arranged in a Wheatstone bridge circuit providing a simple and sensitive configuration for pressure sensing .

[0006] Due to variations in fabrication of the resistors of the Wheatstone bridge circuit , a pressure measuring cell using the Wheatstone bridge circuit usually generates non-vanishing of fset signals which are furthermore temperature dependent . In order to provide an accurate pressure measurement , balancing of the measurement circuit is therefore desired .

[0007] A way to balance a strain gauge circuit using a plurality of balancing resistors is for example described in EP 2 175 252 A2 . The pressure sensor described therein comprises a cylindrical body on which a strain gauge circuit is printed, comprising a plurality of trimming resistors which electrically connect two conductor tracks to one another . At least some of these trimming res istors have curved shape in the circumferential direction . In order to carry out an analog ( or continuous ) adj ustment , individual adj ustment resistors are cut , for example by means of a laser beam, as a result of which their electrical resistance value changes . Furthermore , it is possible to connect balancing resistors in parallel in order to carry out digital ( or discreet ) balancing . With digital ( or discreet ) adj ustment , individual adj ustment resistors can be completely severed by appropriate cuts , resulting in a change of resistance in predetermined steps .

[0008] Summary of the invention

[0009] When balancing a measurement circuit in an analog and / or digital fashion, stability and / or precision can represent a challenge .

[0010] It is therefore an obj ect of the invention to provide a pressure measuring cell which at least partially improves the prior art and avoids at least part of the disadvantages of the prior art .

[0011] According to the present invention, this obj ect is achieved by the features of the independent claims . In addition, further advantageous embodiments follow from the dependent claims and the description as well as the figures .

[0012] According to an aspect of the invention, this obj ect is particularly achieved by a pressure measuring cell comprising a support body, a membrane supported by the support body and a measurement circuit , the measurement circuit comprising a plurality of strain sensitive resistors connected in a Wheatstone bridge circuit , wherein the measurement circuit comprises a temperature-balancing network compris ing a first temperature-balancing auxiliary resistor connected to a first strain sensitive resistor of the Wheatstone bridge circuit and a second temperature-balancing auxiliary resistor connected to a second strain sensitive resistor of the Wheatstone bridge circuit , the temperature-balancing network further comprising a j oint thermistor connected between the first and second temperature-balancing auxiliary resistors .

[0013] Using the j oint thermistor, the temperature dependence of the of fset signal generated by the Wheatstone bridge circuit can be compensated owing to the temperature dependent resistance of the j oint thermistor . The first and second temperaturebalancing auxiliary resistors can advantageously be used to control to which extent the thermistor signal is applied to the of fset signal to be compensated . For example , the first temperature-balancing auxiliary resistor may be varied in order to add the thermistor signal to a negative or decreasing of fset signal of the Wheatstone bridge circuit . Alternatively or additionally, the second temperature-balancing auxiliary resistor may be varied in order to subtract the thermistor signal from a positive or increasing of fset signal of the Wheatstone bridge circuit . Furthermore , the si ze of the thermistor signal to be applied onto the of fset signal may be controlled by varying the first and / or second temperaturebalancing auxiliary resistor .

[0014] By connecting the j oint thermistor between the first and second temperature-balancing auxiliary resistors , an ef ficient temperature-balancing network can therefore be provided . The j oint thermistor provides the advantage that balancing can be provided by exploiting the temperature dependent characteristics of the j oint thermistor without having to trim the j oint thermistor in a continuous fashion . Furthermore , as balancing of the temperature dependence can be achieved by the j oint thermistor connected between the first and second temperature-balancing auxiliary resistors , the complexity of the temperature-balancing can be reduced . In particular, a temperature-balancing network with a plurality of thermistors which may be trimmed away in a discreet fashion for balancing, may be avoided .

[0015] Preferably, the measurement circuit is at least partially arranged on the membrane . In some embodiments , the strain sensitive resistors and the auxiliary resistors are arranged on the membrane . In some embodiments , the strain sensitive resistors are arranged on the membrane whereas the one or more of the auxiliary resistors are arranged of f the membrane on the support body . Typically, the sensing and feed terminals may be arranged of f the membrane on the support body .

[0016] In some embodiments , the j oint thermistor is a positive temperature coef ficient thermistor .

[0017] In some embodiments , the j oint thermistor is a negative temperature coef ficient thermistor .

[0018] In some embodiments , the j oint thermistor is connected between the first and second strain sensitive resistor of the Wheatstone bridge circuit .

[0019] In particular, a first leg of the Wheatstone bridge circuit may comprise the first strain sensitive resistor and a second leg of the Wheatstone bridge circuit may comprise the second strain sensitive resistor . The j oint thermistor may therefore be connected between the first leg and the second leg of the Wheatstone bridge circuit . In particular, an end of the first temperature-balancing auxiliary resistor, an end of the first strain sensitive resistor and a first end of the j oint thermistor may therefore be connected together at a first node . Accordingly, an end of the second temperature-balancing auxiliary resistor, an end of the second strain sensitive resistor and a second end of the j oint thermistor may be connected together at a second node . Thus , an end of each of the first and second temperature-balancing auxiliary resistors , an end of each of the first and second strain sensitive resistors and a first and second end o f the j oint thermistor may each be connected together at a first and second node . Due to fabrication-caused variations in the resistances of the strain sensitive resistors of the Wheatstone bridge circuit , a potential di f ference may exist between the first leg and the second leg for constant pressure , which may be temperature dependent . By connecting the j oint thermistor between the first leg and the second leg, temperature-balancing between the first leg and the second leg can therefore be achieved whereby the first temperature-balancing auxiliary resistor and the second temperature-balancing auxiliary resistor can be used to control the weight of the compensation introduced by the j oint thermistor to the first leg and / or the second leg .

[0020] In some embodiments , the temperature-balancing network comprises a first auxiliary sensing terminal arranged at a first side of the j oint thermistor and a second auxiliary sensing terminal arranged at a second side of the j oint thermistor .

[0021] Using the first and second auxiliary sensing terminals , the resistance of the j oint thermistor can be measured at di f ferent temperatures . Furthermore , using first and second sensing terminals of the Wheatstone bridge circuit and first and second feed terminals of the measurement circuit , the resistances of the strain sensitive resistors of the Wheatstone bridge circuit and of the temperature-balancing auxiliary resistors can be measured . The so-obtained resistance values can be used for varying the temperature-balancing auxiliary res istors to resistance values where the measurement circuit is balanced .

[0022] In some embodiments , the first and second temperaturebalancing auxiliary resistors are configured for adj ustment by laser trimming of resistor material .

[0023] The first and second temperature-balancing auxiliary resistors or the resistance values thereof , respectively, can therefore be varied in a continuous fashion by laser trimming of the resistor material .

[0024] In some embodiments , the first and second temperaturebalancing auxiliary resistors each exhibit a rectangular shape with two longer edges and two shorter edges , wherein the first and second temperature-balancing auxiliary resi stors are electrically contacted along the longer edges and trimmable by one or more laser cuts parallel to the shorter edges .

[0025] Using this design of the temperature-balancing auxiliary resistors and the electrical contacts , the advantages of discreet and continuous adj ustment of the resistances can be combined . On the one hand, stability of the resistances after trimming can be increased since the cut created by the laser does not end within the resistor material . Thus , post trim dri ft ef fects can be reduced . On the other hand, precision of the adj ustment can be increased compared to a discreet adj ustment . Furthermore , the required space for the auxiliary resistors can be decreased compared to a discreet trimming scheme .

[0026] In an adj ustment scheme of the temperature-balancing network, the first and / or second temperature-balancing auxiliary resistors may be trimmed parallel to the longer edges up to a threshold value of a target value of the resistance of the first and / or second temperature-balancing auxiliary resistors while monitoring the signal of the measurement circuit by a control circuit . The control circuit may determine the rate of change of the signal of the measurement circuit induced by trimming parallel to the longer edges and then determine at which position a trim parallel to the shorter edges of the first and second temperature-balancing auxiliary resistors shall be executed such that the target value of the resistance of the first and / or second temperature-balancing auxiliary resistors can be obtained .

[0027] By using this scheme , a target value of the resistance of the first and / or second temperature-balancing auxiliary resistors can therefore precisely be reached due to the monitoring of the signal of the measurement circuit while trimming parallel to the longer edges . At the same time , a stable resistance value of the first and / or second temperature-balancing auxiliary resistors can be reached due to the trim parallel to the shorter edges . The adj ustment scheme of the temperature-balancing network may be applied to the of fset-balancing network in an analogous fashion .

[0028] In some embodiments , the temperature-balancing network comprises a first array of first temperature-balancing auxiliary resistors connected in parallel and a second array of second temperature-balancing auxiliary resistors connected in parallel .

[0029] The first and / or second temperature-balancing auxiliary resistors of the first and / or second array can selectively be disconnected from the measurement circuit by laser trimming of the conductive paths connecting the temperature-balancing auxiliary resistors of the first and / or second array to the measurement circuit . Thus , discreet adj ustment of the temperature-balancing network can be achieved .

[0030] In some embodiments , the measurement circuit comprises an of fset-balancing network comprising a first of fset-balancing auxiliary resistor connected to a third strain sensitive resistor of the Wheatstone bridge circuit and a second of fsetbalancing auxiliary resistor connected to a fourth strain sensitive resistor of the Wheatstone bridge circuit .

[0031] By varying the first of fset-balancing auxiliary resistor and / or the second of fset-balancing resistor, a nonzero of fset signal of the Wheatstone bridge circuit can be compensated . The measurement circuit can therefore comprise a temperaturebalancing network for balancing the temperature dependence of the of fset signal of the Wheatstone bridge circuit and an of fset-balancing network for compensating the of fset signal of the Wheatstone bridge circuit to zero . In some embodiments , the of fset-balancing network comprises a third auxiliary sensing terminal arranged between the first of fset-balancing auxiliary resistor and the third strain sensitive resistor of the Wheatstone bridge circuit , and a fourth auxiliary sensing terminal arranged between the second of fset-balancing auxiliary resistor and the fourth strain sensitive resistor of the Wheatstone bridge circuit .

[0032] Using the third and the fourth auxiliary sensing terminals , as well as the first and second sensing terminals of the Wheatstone bridge circuit and the first and second feed terminals of the measurement circuit , the resistances of the third and fourth strain sensitive resistors of the Wheatstone bridge circuit and of the of fset-balancing auxiliary resistors can be measured . The so-obtained resistance values can be used for varying the of fset-balancing auxiliary res istors to resistance values where the of fset signal is compensated and the measurement circuit is balanced, respectively .

[0033] In some embodiments , the first and second of fset-balancing auxiliary resistors are configured for adj ustment by laser trimming of resistor material .

[0034] The first and second of fset-balancing auxiliary resistors or the resistance values thereof , respectively, can therefore be varied in a continuous fashion by laser trimming of the resistor material .

[0035] In some embodiments , the first and second of fset-balancing auxiliary resistors each exhibit a rectangular shape with two longer edges and two shorter edges , wherein the first and second of fset-balancing auxiliary resistors are electrically contacted along the longer edges and trimmable by one or more laser cuts parallel to the shorter edges.

[0036] Using this design of the offset-balancing auxiliary resistors and the electrical contacts, the advantages of discreet and continuous adjustment of the resistances can be combined. On the one hand, stability of the resistances after trimming can be increased since the cut created by the laser does not end within the resistor material. Thus, post trim drift effects can be reduced. On the other hand, precision of the adjustment can be increased compared to a discreet adjustment. Furthermore, the required space for the auxiliary resistors can be decreased compared to a discreet trimming scheme.

[0037] In some embodiments, the offset-balancing network comprises a first array of first offset-balancing auxiliary resistors connected in parallel and a second array of second offsetbalancing auxiliary resistors connected in parallel.

[0038] The first and / or second offset-balancing auxiliary resistors of the first and / or second array can selectively be disconnected from the measurement circuit by laser trimming of the conductive paths connecting the offset-balancing auxiliary resistors of the first and / or second array to the measurement circuit. Thus, discreet adjustment of the offset-balancing network can be achieved.

[0039] In some embodiments, one or more of: the strain sensitive resistors, the temperature-balancing auxiliary resistors, the thermistor and offset-balancing auxiliary resistors are thick film resistors. Fabrication-caused variations in the resistance of the strain sensitive resistors and in the temperature dependence thereof can be of increased relevance for thick film resistors . The temperature-balancing network and / or the of fset-balancing network may therefore particularly be beneficial for a measurement circuit where the resistors are thick film resistors .

[0040] In some embodiments , the membrane is circular-shaped and the plurality of strain sensitive resistors comprises at least one arc-shaped resistor with an arc-shaped surface area that is radially delimited by an inner circular arc and an outer circular arc .

[0041] Using a circular-shaped membrane and arc-shaped resistors provides the advantage that the measurement circuit can be reduced in si ze and miniaturi zation of the pressure measuring cell be improved . Further, distribution of strain and / or stress of the membrane due to pressure variations in the measurement medium may exhibit a symmetry corresponding to the rotational symmetry of the membrane . By providing at least one strain sensitive resistor as an arc-shaped resistor, the at least one strain sensitive resistor can better be conformed to the symmetry of the circular-shaped membrane . Further, the si ze of the arc-shaped strain sensitive resistor can be increased with respect to the membrane area compared to e . g . rectangular strain sensitive resistors , which improves the signal-to-noise ratio . As the area of the arc-shaped strain sensitive resistor does not have to be limited with respect to the membrane , miniaturi zation of the membrane and therefore the pressure measuring cell can be facilitated . In some embodiments , the at least one arc-shaped resistor comprises a first electrical contact extending along the inner circular arc and a second electrical contact extending along the outer circular arc .

[0042] Arranging the first electrical contact at the inner circular arc and the second electrical contact at the outer circular arc provides the advantage that a predominantly radial current flow can be achieved . Compared to an azimuthal current flow, where the current density is strongly enhanced in radially inner sections of an arc-shaped resistor, a more homogeneous and suf ficiently low current density can be maintained .

[0043] In some embodiments , the at least one arc-shaped resistor is a sheet resistor with a filled surface area .

[0044] Providing the arc-shaped resistor as a sheet resistor with a filled surface area has the advantage that a homogeneous and suf ficiently low current density can be maintained across the area of the arc-shaped resistor .

[0045] In some embodiments , at least one of the temperature-balancing auxiliary resistors , the of fset-balancing auxiliary resistors and the j oint thermistor is arc-shaped .

[0046] Therefore , at least one of the temperature-balancing auxiliary resistors , the of fset-balancing auxiliary resistors and the j oint thermistor can better conform to the rotational symmetry of the circular-shaped membrane .

[0047] According to a further aspect , the present invention is also directed to a pressure sensor comprising the pressure measuring cell according to the present disclosure . According to a further aspect , the present invention is also directed to a measurement circuit for measuring a pressure or force , the measurement circuit comprising a plurality of strain sensitive resistors connected in a Wheatstone bridge circuit , wherein the measurement circuit comprises a temperaturebalancing network comprising a first temperature-balancing auxiliary resistor connected to a first strain sensitive resistor of the Wheatstone bridge circuit and a second temperature-balancing auxiliary resistor connected to a second strain sensitive resistor of the Wheatstone bridge circuit , the temperature-balancing network further comprising a j oint thermistor connected between the first and second temperaturebalancing auxiliary resistors .

[0048] According to a further aspect , the present invention is also directed to a force sensor comprising a measurement circuit according to the present disclosure .

[0049] According to a further aspect , the present invention is also directed to a method for balancing a measurement circuit of a pressure measuring cell , wherein the of fset and the temperature dependence of the output signal of the Wheatstone bridge circuit is balanced by performing the steps of : a ) providing a pressure measuring cell according to the present disclosure ; b ) determining at a first temperature the resistance of the temperature-balancing auxiliary resistors , the of fsetbalancing auxiliary resistors , the j oint thermistor and the strain sensitive resistors using supply and sensing terminals of the Wheatstone bridge circuit and auxiliary sensing terminals ; c ) determining at a second temperature the resistance of the temperature-balancing auxiliary resistors , the of fset-balancing auxiliary resistors , the j oint thermistor and the strain sensitive resistors using the supply and sensing terminals of the Wheatstone bridge circuit and the auxiliary sensing terminals ; d) adj usting the resistance o f the first and / or the second temperature-balancing auxiliary resistor and the resistance of the first and / or the second of fset-balancing auxiliary resistor by laser trimming to a resistance balancing the measurement circuit using the resistances of the temperature-balancing auxiliary resistors , the of fsetbalancing auxiliary resistors and the strain sensitive resistors determined at the first and the second temperatures .

[0050] The measurement circuit according to the present disclosure therefore provides an ef ficient scheme to determine the resistances of the strain sensitive resistors of the Wheatstone bridge circuit , the temperature-balancing auxiliary resistors , the of fset-balancing auxiliary resistors and the j oint thermistor . Using the so-obtained resistance values at two di f ferent temperatures , the measurement circuit can precisely be balanced by adj usting the temperature-balancing auxiliary resistors and the of fset-balancing resistors by laser trimming .

[0051] According to a further aspect , the present invention is also directed to a method for balancing a measurement circuit of a pressure measuring cell , wherein the of fset and the temperature dependence of the output signal of the Wheatstone bridge circuit is balanced by performing the steps of : a ) providing a pressure measuring cell according to the present disclosure ; b ) determining at a first temperature the output voltage of the Wheatstone bridge circuit at an applied supply voltage ; c ) determining at a second temperature the output voltage of the Wheatstone bridge circuit at the applied supply voltage ; d) adj usting the resistance of the first and / or the second temperature-balancing auxiliary resistor and the resistance of the first and / or the second of fset-balancing auxiliary resistor by laser trimming to a resistance balancing the measurement circuit using the output voltages of the Wheatstone bridge circuit determined at the first and the second temperatures ; e ) repeating one or more times the steps b ) to d) .

[0052] The method provides the advantage that the measurement circuit can be balanced by determining the output voltages of the Wheatstone bridge circuit at two di f ferent temperatures without the need to separately measure the resistance values of the resistors of the measurement circuit . Instead, preestimated values of the resistances can be used, for example obtained by a Monte Carlo simulation of the measurement circuit . Since the exact resistance values are not known, an iterative approach may be used by repeating one or more times the steps b ) to d) . It has turned out that two iterations may typically be suf ficient to obtain a satis factory precision in balancing of the measurement circuit . However, additional iterations may be used to increase the balancing precision .

[0053] Starting from the pre-estimated resistance values , target values of the first and / or second temperature-balancing auxiliary resistor and of the first and / or the second of fsetbalancing auxiliary resistor can be determined using Newton' s method to determine the zeroes of the di f ferential output voltage of the Wheatstone bridge circuit . Laser trimming of the first and / or second temperature-balancing auxiliary resistor and of the first and / or the second of fset-balancing auxiliary resistor can then be performed using the pre- estimated resistance values and the determined target values to resistances balancing the measurement circuit .

[0054] Compared to conventional balancing schemes where compensation of the of fset signal and balancing of the temperature dependence of the of fset signal are performed us ing several separate processes , the measurement circuit according to the present disclosure therefore advantageously allows to integrate compensation of the offset signal and balancing in a j oint balancing scheme . The complexity of balancing the measurement circuit can therefore be reduced .

[0055] Brief description of the drawings

[0056] The present invention will be explained in more detail , by way of exemplary embodiments , with reference to the schematic drawings , in which :

[0057] Fig . l shows a circuit diagram of an embodiment of a measurement circuit comprising a temperaturebalancing network and an of fset-balancing network;

[0058] Fig . 2 shows a circuit diagram of an embodiment of a measurement circuit comprising a temperaturebalancing network;

[0059] Fig . 3 shows an exemplary layout of an embodiment of a measurement circuit of a pressure measuring cell comprising a temperature-balancing network and an of fset-balancing network; Fig . 4 shows a top view of an embodiment of a pressure measuring cell ;

[0060] Fig . 5 shows a top view of an embodiment of a pressure measuring cell ;

[0061] Fig . 6 shows a top view of an embodiment of a pressure measuring cell .

[0062] Detailed description of exemplary embodiments

[0063] For some embodiments in the description of the Figures , same reference numerals may be used for corresponding parts of the embodiments .

[0064] Figure 1 shows a circuit diagram of an embodiment of a measurement circuit 1 . 1 comprising a temperature-balancing network and an of fset-balancing network . The measurement circuit 1 . 1 comprises four strain sensitive resistors Rl , R2 , R3 , R4 connected in a Wheatstone bridge circuit . The temperature-balancing network comprises a first temperaturebalancing auxiliary resistor R61 connected to the first strain sensitive resistor Rl , a second temperature-balancing auxiliary resistor R62 connected to the second strain sensitive resistor R2 and a j oint thermistor PTC connected between the first and second temperature-balancing auxiliary resistors R61 and R62 . The j oint thermistor PTC is a positive temperature coef ficient thermistor . In some embodiments , the j oint thermistor may be a negative temperature coef ficient thermistor . The of fset-balancing network comprises a first of fset-balancing auxiliary resistor R51 connected to the third strain sensitive resistor R3 and a second of fset-balancing auxiliary resistor R52 connected to the fourth strain sensitive resistor R4 .

[0065] The measurement circuit 1 . 1 further comprises sensing terminals S+ and S- of the Wheatstone bridge circuit and feed terminals Hi and Lo . The temperature-balancing network further comprises a first auxiliary sensing terminal T1+ arranged at a first side of the j oint thermistor PTC and a second auxiliary sensing terminal Tl- arranged at a second side o f the j oint thermistor PTC . The of fset-balancing network further comprises a third auxiliary sensing terminal T2+ arranged between the first of fset-balancing auxiliary resistor R51 and the third strain sensitive resistor R3 and a fourth auxiliary sensing terminal T2- arranged between the second of fset-balancing auxiliary resistor R52 and the fourth strain sensitive resistor R4 . The strain sensitive resistors Rl , R2 , R3 , R4 of the Wheatstone bridge circuit , the temperature-balancing auxiliary resistors R61 , R62 and the of fset-balancing auxiliary resistors R51 , R52 may be thick film resistors .

[0066] In Figure 1 , it can be recogni zed that a first ( right-hand side ) leg of the Wheatstone bridge circuit comprises the first strain sensitive resistor Rl and a second ( left-hand side ) leg of the Wheatstone bridge circuit comprises the second strain sensitive resistor R2 . The j oint thermistor PTC is connected between the first strain sensitive resistor Rl and the second strain sensitive resistor R2 and, in particular, between the first leg and the second leg of the Wheatstone bridge circuit . It can furthermore be recogni zed in the measurement circuits of the Figures 1- 6 that an end of the first temperaturebalancing auxiliary resistor R61 , an end of the first strain sensitive resistor Rl and a first end of the j oint thermistor PTC are connected together at a first node . Accordingly, an end of the second temperature-balancing auxiliary resistor R62 , an end of the second strain sensitive resistor R2 and a second end of the j oint thermistor PTC are connected together at a second node . Thus , an end of each of the first and second temperature-balancing auxiliary resistors R61 , R62 , an end of each of the first and second strain sensitive resistors Rl , R2 and a first and second end of the j oint thermistor PTC are each connected together at a first and second node .

[0067] Temperature-balancing between the first leg and the second leg is achieved by us ing the first and second temperature-balancing auxiliary resistors R61 , R62 to control the weight of the compensation introduced by the j oint thermistor PTC to the first leg and / or the second leg .

[0068] Using the first and second auxiliary sensing terminals T1+ and T1- , first and second sensing terminals S+ and S- of the Wheatstone bridge circuit , first and second feed terminals Hi and Lo of the measurement circuit 1 . 1 and third and fourth auxiliary sensing terminals T2+ and T2- , the resistance of the j oint thermistor PTC, the resistances of the strain sensitive resistors Rl , R2 , R3 , R4 of the Wheatstone bridge circuit , the resistances of the temperature-balancing auxiliary resistors R61 and R62 and the resistances of the of fset-balancing auxiliary resistors R51 and R52 can be measured at di f ferent temperatures . The so-obtained resistance values can be used for varying the temperature-balancing auxiliary res istors and the of fset-balancing auxiliary resistors to resistance values where the measurement circuit 1 . 1 is balanced . The temperaturebalancing auxiliary resistors and the of fset-balancing auxiliary resistors can be varied by laser trimming in a continuous or discreet fashion. For discreet adjustment of the auxiliary resistances, arrays of auxiliary resistors may be foreseen at the positions where the auxiliary resistors R61, R62, R51, R52 are shown.

[0069] With the shown measurement circuit 1.1, it is also possible to perform iterative balancing by measuring the output voltage of the Wheatstone bridge circuit between the sensing terminals S+ and S- at a supply voltage applied at the feed terminals Hi and Lo at two different temperatures without separately measuring the resistances involved, as described above.

[0070] Figure 2 shows a circuit diagram of an embodiment of a measurement circuit 2.1 comprising a temperature-balancing network without an offset-balancing network. Using the temperature-balancing network comprising the temperaturebalancing auxiliary resistors R61, R62 and the thermistor PTC, the temperature dependence of the offset signal of the Wheatstone bridge circuit can be balanced.

[0071] Figure 3 shows an exemplary layout of an embodiment of a measurement circuit 3.1 of a pressure measuring cell comprising a temperature-balancing network and an offset-balancing network. At least the strain sensitive resistors Rl, R2, R3, R4 of the Wheatstone bridge circuit are arranged on a deflectable membrane. In some embodiments, one or more of the auxiliary resistors R61, R62, R51, R52 and the joint thermistor PTC may also be arranged on the deflectable membrane. In some embodiments, the auxiliary resistors R61, R62, R51, R52 and the joint thermistor PTC may not be arranged on the deflectable membrane, but on the support body of the pressure measuring cell. Similar to the circuit scheme as shown in Figure 1, a joint thermistor PTC is connected between the first and second temperature-balancing auxiliary resistors R61 and R62 as well as between the first and second strain sensitive resistors R1 and R2. The first offset-balancing resistor R51 is connected in series to the third strain sensitive resistor R3 and the second offset-balancing resistor R52 is connected in series to the fourth strain sensitive resistor R4. The feed terminals Hi and Lo, sensing terminals S+ and S-, auxiliary sensing terminals T1 + , T1-, T2 + , T2- are arranged in an analogous fashion as shown in the circuit scheme of Figure 1.

[0072] Figure 4 shows a top view of an embodiment of a pressure measuring cell 4 with a measurement circuit 4.1 and a circularshaped deflectable membrane 4.2 supported by a support body 4.3. The strain sensitive resistors Rl, R2, R3 and R4 are designed as arc-shaped resistors with an arc-shaped surface area that is radially delimited by in inner circular arc and an outer circular arc. The arc-shaped resistors are thick film sheet resistors with a filled surface area. The feed terminals Hi and Lo, sensing terminals S+, S- and the auxiliary terminals T1+, T1-, T2+, T2- are arranged off the deflectable membrane on the support body 4.2.

[0073] The temperature-balancing auxiliary resistors R61, R62, the offset-balancing auxiliary resistors R51, R52 and the joint thermistor PTC are arranged on a circle with minimal strain.

[0074] The arc-shaped strain sensitive resistors Rl, R2, R3, R4 and the auxiliary resistors R61, R62, R51, R52 and the joint thermistor PTC each comprise a first electrical contact arranged at the inner circular arc and a second electrical contact arranged at the outer circular arc. In particular, the first electrical contact of each of the arc-shaped strain sensitive resistors Rl, R2, R3, R4 and the auxiliary resistors R61, R62, R51, R52 and the joint thermistor PTC extends along the entire respective inner circular arc and the second electrical contact extends along the entire respective outer circular arc. Due to the configuration of the first and second electrical contacts, the current flows predominantly in radial direction .

[0075] The strain sensitive resistors Rl and R4 are arranged at an inner circle where the combined surface strain of the membrane 4.2 due to pressure variations of the measurement medium is positive whereas the strain sensitive resistors R2 and R3 are arranged at an outer circle where the combined surface strain of the membrane 4.2 due to pressure variations of the measurement medium is negative.

[0076] The pair or arrows at each auxiliary resistor R61, R62, R51 and R52 symbolizes the possible directions of laser trimming for adjusting the resistances of the auxiliary resistors R61, R62, R51 and R52.

[0077] Figure 5 shows a top view of an embodiment of a pressure measuring cell 5 with a measurement circuit 5.1 and a circularshaped deflectable membrane 5.2 supported by a support body 5.3. Compared to the pressure measuring cell 4 as shown in Figure 4, the pressure measuring cell 5 comprises a smaller membrane 5.2. Accordingly, the available space for the strain sensitive resistors is reduced. The Wheatstone bridge circuit comprises strain sensitive resistors Ria, Rib, R2, R3, R4a,

[0078] R4b. The strain sensitive resistors R2 and R3 are rectangular strain sensitive resistors and arranged at an inner region of the membrane .

[0079] The arc-shaped resistors Ria and Rib are connected in series and correspond to one arc-shaped resistor, similar to the arcshaped resistor R1 of the embodiment shown in Figure 4 . Accordingly, the arc-shaped resistors R4a and R4b are connected in series and correspond to one arc-shaped resistor, similar to the arc-shaped resistor R4 of the embodiment of Figure 4 . The four arc-shaped resistors Ria, Rib, R4a, R4b are arranged on a circle parallel to and near the circular edge of the membrane 5 . 2 .

[0080] The rectangular strain sensitive resistors R2 , R3 are arranged at positions where the combined surface strain of the membrane 5 . 2 due to pressure variations o f the measurement medium is positive , whereas the arc-shaped resistors Ria, Rib , R4a, R4b are arranged at positions where the combined surface strain of the membrane 5 . 2 due to pressure variations of the measurement medium is negative .

[0081] The measurement circuit 5 . 1 compri ses a temperature-balancing network comprising a first temperature-balancing auxiliary resistor R61 and a second temperature-balancing auxiliary resistor R62 and a j oint thermistor PTC connected between the first temperature-balancing auxiliary resistor R61 and the second temperature-balancing auxiliary resistor R62 .

[0082] The first and second temperature-balancing auxiliary resistors R61 , R62 each exhibit a rectangular shape with two longer edges and two shorter edges . The first and second temperaturebalancing auxiliary resistors R61 , R62 are electrically contacted along the longer edges and trimmable by one or more laser cuts parallel to the shorter edges , as symbol i zed by the vertical arrays of arrows . In particular, the first and second temperature-balancing auxiliary resistors R61 , R62 are electrically contacted along the longer edges from the same short edge side such that the electrical contact to the temperature-balancing auxiliary resistors R61 , R62 can not be interrupted by trimming .

[0083] Figure 6 shows a top view of an embodiment of a pressure measuring cell 6 with a measurement circuit 6 . 1 and a circularshaped deflectable membrane 6 . 2 supported by a support body 6 . 3 . The Wheatstone bridge circuit comprises rectangular strain sensitive resistors Rl , R2 , R3 , R4 arranged at the membrane 6 . 2 .

[0084] The measurement circuit 6 . 1 compri ses a temperature-balancing network comprising a first temperature-balancing auxiliary resistor R61 and a second temperature-balancing auxiliary resistor R62 and a j oint thermistor PTC connected between the first temperature-balancing auxiliary resistor R61 and the second temperature-balancing auxiliary resistor R62 . The measurement circuit 6 . 1 further comprises an of fset-balancing network with a first of fset-balancing auxiliary resistor R51 connected to the third strain sensitive resistor R3 and a second of fset-balancing auxiliary resistor R52 connected to the fourth strain sensitive resistor R4 .

[0085] The first and second temperature-balancing auxiliary resistors R61 , R62 each exhibit a rectangular shape with two longer edges and two shorter edges . The first and second temperaturebalancing auxiliary resistors R61 , R62 are electrically contacted along the longer edges and trimmable by one or more laser cuts parallel to the shorter edges , as symbol i zed by the hori zontal arrows . Besides that , the first and second temperature-balancing auxiliary resistors R61 , R62 are trimmable parallel to the longer edges , as symboli zed by the vertical arrows . The first and second temperature-balancing auxiliary resistors R61 , R62 are therefore designed that trimming can be executed both parallel to the direction of the current flow ( i . e . parallel to the shorter edges ) and transverse to the direction of the current flow ( i . e . parallel to the longer edges ) .

[0086] In an adj ustment scheme of the temperature-balancing network, the first and / or second temperature-balancing auxiliary resistors R61 , R62 can be trimmed parallel to the longer edges up to a threshold value ( for example 80% ) of a target value of the resistance of the first and / or second temperaturebalancing auxiliary resistors R61 , R62 , while monitoring the signal of the measurement circuit by a control circuit . The control circuit can determine the rate of change of the signal of the measurement circuit induced by trimming parallel to the longer edges and then determine at which position a trim parallel to the shorter edges of the first and second temperature-balancing auxiliary resistors R61 , R62 shall be executed such that the target value of the resistance of the first and / or second temperature-balancing auxiliary resistors R61 , R62 can be obtained . By using this scheme , a target value of the resistance of the first and / or second temperaturebalancing auxiliary resistors R61 , R62 can therefore precisely be reached due to the monitoring of the signal of the measurement circuit while trimming parallel to the longer edges . At the same time , a stable resistance value of the first and / or second temperature-balancing auxiliary resistors R61, R62 can be reached due to the trim parallel to the shorter edges. As recognizable in Fig.6, the scheme with a vertical and horizontal trim can be executed twice per first and second temperature-balancing auxiliary resistors R61, R62, since the trims can be executed from either side of the first and second temperature-balancing auxiliary resistors R61, R62.

[0087] The first and second offset-balancing auxiliary resistors R51, R52 can be trimmed in an analogous fashion as the first and second temperature-balancing auxiliary resistors R61, R62 for adjusting the offset-balancing network.

[0088] Furthermore, the adjustment scheme as shown in Figure 6 can analogously be applied to the measurement circuits of the present disclosure, for example the one as shown in Figure 5.

Claims

Claims1. A pressure measuring cell (4, 5, 6) comprising a support body (4.3, 5.3, 6.3) , a membrane (4.2, 5.2, 6.2) supported by the support body and a measurement circuit (1.1-6.1) , the measurement circuit comprising a plurality of strain sensitive resistors (R1-R4, Rla-b, R4a-b) connected in a Wheatstone bridge circuit, wherein the measurement circuit comprises a temperature-balancing network comprising a first temperature-balancing auxiliary resistor (R61) connected to a first strain sensitive resistor (Rl, Ria) of the Wheatstone bridge circuit and a second temperature-balancing auxiliary resistor (R62) connected to a second strain sensitive resistor (R2) of the Wheatstone bridge circuit, the temperature-balancing network further comprising a joint thermistor (PTC) connected between the first and second temperaturebalancing auxiliary resistors.

2. The pressure measuring cell (4, 5, 6) according to claim1, wherein the joint thermistor (PTC) is a positive temperature coefficient thermistor.

3. The pressure measuring cell (4, 5, 6) according to claim1, wherein the joint thermistor (PTC) is a negative temperature coefficient thermistor.

4. The pressure measuring cell (4, 5, 6) according to one of the preceding claims, wherein the joint thermistor (PTC) is connected between the first and second strain sensitive resistor (R61, R62) of the Wheatstone bridge circuit.

5. The pressure measuring cell (4, 5, 6) according to one of the preceding claims, wherein the temperature-balancing network comprises a first auxiliary sensing terminal (T1+) arranged at a first side of the joint thermistor (PTC) and a second auxiliary sensing terminal (T1-) arranged at a second side of the joint thermistor.

6. The pressure measuring cell (4, 5, 6) according to one of the preceding claims, wherein the first and second temperature-balancing auxiliary resistors (R61, R62) are configured for adjustment by laser trimming of resistor material .

7. The pressure measuring cell (5, 6) according to claim 6, wherein the first and second temperature-balancing auxiliary resistors (R61, R62) each exhibit a rectangular shape with two longer edges and two shorter edges, wherein the first and second temperature-balancing auxiliary resistors are electrically contacted along the longer edges and trimmable by one or more laser cuts parallel to the shorter edges.

8. The pressure measuring cell according to one of the preceding claims, wherein the temperature-balancing network comprises a first array of first temperaturebalancing auxiliary resistors connected in parallel and a second array of second temperature-balancing auxiliary resistors connected in parallel.

9. The pressure measuring cell (4, 6) according to one of the preceding claims, wherein the measurement circuit (1.1, 3.1, 4.1, 6.1) comprises an offset-balancing network comprising a first offset-balancing auxiliaryresistor (R51) connected to a third strain sensitive resistor (R3) of the Wheatstone bridge circuit and a second offset-balancing auxiliary resistor (R52) connected to a fourth strain sensitive resistor (R4) of the Wheatstone bridge circuit.

10. The pressure measuring cell (4) according to one of the preceding claims, wherein the offset-balancing network comprises a third auxiliary sensing terminal (T2+) arranged between the first offset-balancing auxiliary resistor (R51) and the third strain sensitive resistor (R3) of the Wheatstone bridge circuit, and a fourth auxiliary sensing terminal (T2-) arranged between the second offset-balancing auxiliary resistor (R52) and the fourth strain sensitive resistor (R4) of the Wheatstone bridge circuit.

11. The pressure measuring cell (4, 6) according to claim 9 or 10, wherein the first and second offset-balancing auxiliary resistors (R51, R52) are configured for adjustment by laser trimming of resistor material.

12. The pressure measuring cell (6) according to claim 11, wherein the first and second offset-balancing auxiliary resistors (R51, R52) each exhibit a rectangular shape with two longer edges and two shorter edges, wherein the first and second offset-balancing auxiliary resistors are electrically contacted along the longer edges and trimmable by one or more laser cuts parallel to the shorter edges.

13. The pressure measuring cell according to one of the claims 9 to 12, wherein the offset-balancing network comprisesa first array of first offset-balancing auxiliary resistors connected in parallel and a second array of second offset-balancing auxiliary resistors connected in parallel .

14. The pressure measuring cell (4, 5, 6) according to one of the preceding claims, wherein one or more of: the strain sensitive resistors (R1-R4, Rla-b, R4a-b) , the temperature-balancing auxiliary resistors (R61, R62) , the thermistor and offset-balancing auxiliary resistors (R51, R52) are thick film resistors.

15. The pressure measuring cell (4, 5) according to one of the preceding claims, wherein the membrane is circularshaped and the plurality of strain sensitive resistors (R1-R4, Rla-b, R4a-b) comprises at least one arc-shaped resistor with an arc-shaped surface area that is radially delimited by an inner circular arc and an outer circular arc .

16. The pressure measuring cell (4, 5) according to claim 15, wherein the at least one arc-shaped resistor (R1-R4, Rla- b, R4a-b) comprises a first electrical contact extending along the inner circular arc and a second electrical contact extending along the outer circular arc.

17. The pressure measuring cell (4, 5) according to claim 15 or 16, wherein the at least one arc-shaped resistor (Rl- R4, Rla-b, R4a-b) is a sheet resistor with a filled surface area.

18. The pressure measuring cell (4, 5) according to one of the claims 15 to 17, wherein at least one of thetemperature-balancing auxiliary resistors (R61, R62) , the offset-balancing auxiliary resistors (R51, R52) and the joint thermistor (PTC) is arc-shaped.

19. A pressure sensor comprising the pressure measuring cell(4, 5, 6) according to one of the preceding claims.

20. A measurement circuit (1.1-6.1) for measuring a pressure or force, the measurement circuit comprising a plurality of strain sensitive resistors (R1-R4, Rla-b, R4a-b) connected in a Wheatstone bridge circuit, wherein the measurement circuit comprises a temperature-balancing network comprising a first temperature-balancing auxiliary resistor (R61) connected to a first strain sensitive resistor (Rl, Ria) of the Wheatstone bridge circuit and a second temperature-balancing auxiliary resistor (R62) connected to a second strain sensitive resistor (R2) of the Wheatstone bridge circuit, the temperature-balancing network further comprising a joint thermistor (PTC) connected between the first and second temperature-balancing auxiliary resistors.

21. A force sensor comprising a measurement circuit according to claim 20.

22. A method for balancing a measurement circuit (1.1-6.1) of a pressure measuring cell (4, 5, 6) , wherein the offset and the temperature dependence of the output signal of the Wheatstone bridge circuit is balanced by performing the steps of: a) providing a pressure measuring cell according to one of the claims 1 to 18;b) determining at a first temperature the resistance of the temperature-balancing auxiliary resistors (R61, R62) , the offset-balancing auxiliary resistors (R51, R52) , the joint thermistor (PTC) and the strain sensitive resistors (R1-R4, Rla-b, R4a-b) using supply and sensing terminals (Hi, Lo, S+, S-) of theWheatstone bridge circuit and auxiliary sensing terminals (T1+, T1-, T2+, T2-) ; c) determining at a second temperature the resistance of the temperature-balancing auxiliary resistors, the offset-balancing auxiliary resistors, the joint thermistor and the strain sensitive resistors using the supply and sensing terminals of the Wheatstone bridge circuit and the auxiliary sensing terminals; d) adjusting the resistance of the first and / or the second temperature-balancing auxiliary resistor and the resistance of the first and / or the second offsetbalancing auxiliary resistor by laser trimming to resistances balancing the measurement circuit using the resistances of the temperature-balancing auxiliary resistors, the offset-balancing auxiliary resistors and the strain sensitive resistors determined at the first and the second temperatures.

23. A method for balancing a measurement circuit (1.1-6.1) of a pressure measuring cell (4, 5, 6) , wherein the offset and the temperature dependence of the output signal of the Wheatstone bridge circuit is balanced by performing the steps of:a ) providing a pressure measuring cell according to one of the claims 1 to 18 ; b ) determining at a first temperature the output voltage of the Wheatstone bridge circuit at an applied supply voltage ; c ) determining at a second temperature the output voltage of the Wheatstone bridge circuit at the applied supply voltage ; d) adj usting the resistance of the first and / or the second temperature-balancing auxiliary resistor (R61 , R62 ) and the resistance of the first and / or the second of fset-balancing auxiliary resistor (R51 , R52 ) by laser trimming to resistances balancing the measurement circuit using the output voltages of the Wheatstone bridge circuit determined at the first and the second temperatures ; e ) repeating one or more times the steps b ) to d) .

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