Thermal flow sensor

Abstract

Thermal sensor comprising: - A bridge circuit, with a first voltage divider (11) in which an electrically controllable heating element (111) is connected, and a second voltage divider (12) in which a temperature-dependent resistor (121) is connected at the corresponding position of the heating element (111) in the first voltage divider (11), wherein the temperature-dependent resistor (121) is associated with an impedance converter circuit (123), which is designed as an impedance converter, which impedance converter is divided into two voltage dividers (17, 18) connected in series, wherein the temperature-dependent resistor (121) forms one of the resistors in one of the voltage dividers (13, 14), and wherein the impedance converter circuit (123) is further designed to determine the impedance of the resistor (121) in the second voltage divider. (12) to multiply by a defined factor (X).

Description

[0001] The invention relates to a thermal flow sensor for determining the flow velocity of a flowing measuring medium in a pipe section.

[0002] In automation technology, particularly in process automation, field devices are frequently used to acquire various measured variables. These variables can include, for example, fill level, flow rate or velocity, pressure, temperature, pH value, redox potential, conductivity, or dielectric constant of a medium in a process plant. To acquire these measurements, field devices incorporate suitable sensors or are based on appropriate measurement principles. A wide range of thermal flow sensors are manufactured and distributed by Innovative Sensor Technology (IST), a subsidiary of the Endress+Hauser Group.

[0003] Thermal flow sensors are used to determine the flow rate or flow velocity of fluid-like media, such as gases or gas mixtures. This type of sensor is based on the effect that flowing media conduct heat away from a heated surface. Thermal flow sensors operating on the anemometric principle typically consist of at least one low-resistance, temperature-dependent heating element (e.g., a Pt50), which heats up during flow measurement, and a high-resistance, temperature-dependent resistance element (e.g., a Pt1000), which serves as the temperature sensor. In analog circuits, the heating element and the resistance element are each connected in a voltage divider, either after or before a series resistor, with the two voltage dividers themselves forming a Wheatstone bridge.

[0004] Such an anemometric thermal flow sensor is typically operated using one of the following two control methods: In the "Constant-Current Anemometry (CCA)" control method, the heating element is supplied with a constant current. The flow of the measured medium changes the resistance of the heating element and thus the voltage drop across it, which represents the measurement signal. In the "Constant-Temperature Anemometry (CTA)" control method, the temperature difference between the resistance element and the heating element is maintained at a relatively constant value. For this purpose, the current supplied to the heating element, or the applied voltage, is increased with increasing flow velocity by adjusting the voltage at the Wheatstone bridge accordingly. Relatively high flow velocities can be measured using this control method.

[0005] A significant disadvantage of a Wheatstone bridge is the comparatively high voltage that must be applied to the resistive element or the voltage divider in which the resistive element is located. This causes a relatively high self-heating of the resistive element. This, in turn, prevents the resistive element from being used to measure the temperature of gaseous media. This problem can be solved by using a second, additional temperature sensor that measures only the temperature of the gaseous medium. The obvious drawbacks of this solution are the increased cost and complexity of the flow sensor.

[0006] Based on this problem, the invention aims to provide a thermal flow sensor with the simplest possible design, which can determine the flow velocity without measurement errors.

[0007] This task is solved by a thermal sensor that includes at least the following components: - A Wheatone bridge circuit, with ◯ a first voltage divider in which an electrically controllable heating element is connected in series with a first series resistor, and ◯ a second voltage divider in which a temperature-dependent resistance element is connected in series to a second series resistor at the corresponding position to the heating element in the first voltage divider.

[0008] According to the invention, the temperature-dependent resistive element comprises an impedance converter circuit designed to multiply the impedance of the resistive element in the second voltage divider by a defined factor. The impedance converter circuit can, for example, be designed as an impedance converter. This increases the sensitivity of the flow measurement because less self-heating occurs at the temperature-dependent resistive element.

[0009] Based on the thermal sensor according to the invention, a thermal flow meter with improved measurement accuracy can be realized, which serves to determine the flow velocity of a flowing medium in a pipe section and is based on the CTA principle. For this purpose, the flow meter comprises, in addition to the thermal sensor according to one of the preceding embodiments: - A measuring probe that can be arranged in the line section in this way, by means of which the heating element and the resistance element can be brought into thermal contact with the measuring medium, - a controllable voltage control unit, by means of which the supply voltage dropping across the bridge circuit and / or the current flowing across the bridge circuit can be adjusted, - a control unit designed to control the voltage control unit in such a way that a temperature difference between the temperature-dependent resistance element and the heating element is regulated to a constant value according to the CTA method, and - an evaluation unit designed to determine the flow velocity based on the set supply voltage, for example using a lookup table.

[0010] With regard to the CTA principle, it is necessary that a defined, constant temperature difference is maintained between the heating element and the temperature-dependent resistor element. This can be achieved by appropriately dimensioned series resistors in the voltage dividers. Alternatively or additionally, the heating element can be designed with a correspondingly lower resistance than the temperature-dependent resistor element. It is also essential for the CTA principle that the heating element is designed as a temperature-dependent resistor. Accordingly, both the temperature-dependent resistor element and the heating element can be made of materials such as platinum, nickel (positive temperature coefficient), or polysilicon (negative temperature coefficient).

[0011] An advantage of the invention is that both the temperature-dependent resistance element and the heating element can be designed with a low resistance value in the range of 100 ohms – possibly the same – without impairing the sensitivity of the flow measurement. In this regard, it is possible for the resistance value and / or the temperature coefficient of the heating element to correspond to the resistance value or the temperature coefficient of the temperature-dependent resistance element. Conveniently, both the heating element and the temperature-dependent resistance element can thus be designed, for example, as PT100 sensors.

[0012] The control unit of the flow meter can, for example, be implemented as an operational amplifier whose inputs are connected to the center taps of the two voltage dividers. Correspondingly, the voltage control unit can be designed as a bipolar transistor, with the base or gate of the transistor connected to the output of the operational amplifier.

[0013] Corresponding to the thermal flow meter, the problem underlying the invention is also solved by a method for its operation. Accordingly, this method for determining the flow velocity of a flowing measuring medium in a pipe section using the thermal flow meter comprises the following process steps: - Applying a supply voltage to the two voltage dividers via the voltage control unit, whereby the resistance of the resistor element is multiplied by a defined factor, - such regulation of the supply voltage that a defined temperature difference is established between the heating element and the resistance element, and - Determination of the flow velocity based on the set supply voltage. Within the scope of the invention, the term "flow velocity," also called "flow rate" or "flow speed," represents a generic term for various specifications of the measured flow. For example, the term "flow velocity" encompasses the mass flow rate and / or the volume flow rate of the measuring medium.

[0014] The invention is explained in more detail using the following figures. It shows Fig. 1: a schematic arrangement of a thermal flow meter on a pipe section; Fig. 2: a circuit diagram of a CTA-based thermal flow meter according to the state of the art; Fig. 3: a circuit diagram of a thermal flow meter according to the invention, and Fig. 4: a circuit diagram of an impedance converter.

[0015] For a general understanding of the invention, see in Fig. Figure 1 shows a thermal flow meter 1 attached to a pipe section 2. To determine the flow velocity γ of the measuring medium, the flow meter 1 includes a measuring probe 13 which, in its mounted state, extends perpendicular to the flow direction into the interior of the pipe section 2 and is thus surrounded by the measuring medium. The measuring medium can be any liquid or gaseous fluid.

[0016] Since the thermal flow meter 1 operates on the anemometric principle, an electrically controlled heating element 111 and a temperature-dependent resistance element 121 are arranged in the measuring probe 13 such that they are in sufficient thermal contact with the measuring medium. In order to generate sufficient heating power, the heating element 111 must be designed with a correspondingly low resistance, for example in the range of 100 ohms.

[0017] According to the CTA principle, the resistance element 111 and the heating element 121 are designed to be temperature-dependent. In this respect, the resistance element 111 and the heating element 121 are designed such that their temperature coefficients have the same sign. In the case of a positive temperature coefficient ("PTC"), the heating element 111 or the resistance element 121 can accordingly be made of platinum or nickel. In the case of a negative temperature coefficient ("NTC"), the heating element 111 and the resistance element 121 can, for example, be made of polysilicon. For mechanical and chemical protection against the measuring medium, the heating element 111 and the resistance element 121 are typically mounted on a common substrate and coated with a passivation layer that maintains thermal contact with the medium.

[0018] The electrical wiring of the heating element 111 and the resistance element 121 is shown in Fig. Figure 2 shows the core of the circuit: a Wheatstone bridge circuit consisting of two voltage dividers 11 and 12. A supply voltage V is applied to the first voltage divider 11 of the bridge circuit. s With respect to ground (GND) at a center tap 112, 122 is divided by the heating element 111 and a series resistor R1. The second voltage divider 12 is formed by the resistor element 121 and a second series resistor R2. This divides the supply voltage V s With respect to ground (GND), the voltage is divided at the intermediate center tap 122. For the CTA method, it is irrelevant whether the series resistors R1 and R2 are as shown in... Fig. 2 in relation to the supply voltage V swhether the heating element 111 or the resistance element 121 is actually located before or after it. The only crucial factor is that the heating element 111 and the resistance element 121 are positioned at corresponding locations within the voltage dividers. In the case of the Fig. Furthermore, the embodiment of the flow meter 1 shown in Figure 2 does not explicitly show that, according to the prior art, an adjustment resistor can also be arranged serially on the resistance element 121 in order to adjust the temperature difference to the heating element 111 to a defined value.

[0019] The supply voltage V applied to the bridge circuit sThe current flowing through the voltage dividers 12 and 13 is regulated by a controllable voltage control unit 14. This can be implemented simply using a transistor whose source (collector) is connected to a voltage source of, for example, 10 V, while the drain (emitter) is connected to the voltage dividers and their series resistors R1 and R2.

[0020] The base or gate of transistor 14 is connected to the output of an operational amplifier 15. It must be ensured that the subsequent voltage dividers 11 and 12 can be driven adequately by the operational amplifier 15 or an additional amplifier.

[0021] For CTA-compliant control, the inputs of the operational amplifier 15 are connected to the center taps 112 and 122 of voltage dividers 11 and 12, respectively. In this way, the operational amplifier acts as a control unit 15 for the voltage control unit 14: As the flow velocity of the measuring medium increases, the temperature induced by the heating element 111 at the temperature-dependent resistor 121 decreases, and the potential difference between the center taps 112 and 122 of the voltage dividers 11 and 12 changes. Consequently, the operational amplifier 15 changes the potential at the base and gate of the transistor 14 accordingly, thus adjusting the supply voltage V. s or the current flow through the voltage dividers 11, 12 increases. This in turn increases the heating power at the heating element 111, which in turn also increases the measured temperature at the temperature-dependent resistance element 121 due to self-heating.

[0022] This control loop maintains a defined target potential difference V. m The voltage dividers 11 and 12 are set between the center taps 112 and 122, thereby establishing a temperature difference between heating element 111 and resistance element 121 that is as constant as possible, according to the CTA method. This type of control ensures that the supply voltage V s monotonically dependent on the flow rate of the measuring medium, so that based on the prevailing supply voltage V s The instantaneous flow velocity γ can be determined. The in Fig. The embodiment shown in 2 includes a corresponding evaluation unit 16, which supplies the supply voltage V s for example, by means of a lookup table created via calibration or a conversion function of the flow velocity γ.

[0023] Due to the undesirable self-heating of the resistive element 121, the second voltage divider 12 is often dimensioned with a higher overall resistance, for example by a factor of 10, compared to the first voltage divider 11, according to the prior art. A disadvantage of this is... Fig. As shown in the circuit 2, the thermal resistance element 121 must therefore be designed with a very high resistance to avoid self-heating and thus measurement errors. However, this reduces the sensitivity of the flow measurement. According to the invention, this is prevented by connecting the temperature-dependent resistance element 121 in the second voltage divider 12 within an impedance converter circuit 123. The impedance converter circuit 123 multiplies the impedance or resistance of the resistance element 121 in the second voltage divider 12 downstream of the second center tap 122 by a defined factor X of, for example, X = 10. With respect to the second voltage divider 12, this means a tenfold increase in the resistance value of the resistance element 121. This reduces the effective power consumption.the self-heating at the resistance element 121, which allows for high sensitivity of the flow measurement.

[0024] The inventive design is shown in Fig. 3, where the flow meter 1 there is otherwise functionally the same as in Fig. This corresponds to the circuit shown in section 2.

[0025] One possible implementation variant of the impedance converter circuit 123, which is designed as an impedance converter, is described in Fig.Figure 4 shows that the impedance converter 123 is divided into two voltage dividers 17, 18 connected in series, with the temperature-dependent resistor 121 forming one of the resistors in one of the voltage dividers 13, 14. Each voltage divider 13, 14 is assigned an operational amplifier 19, 20, whose inputs are connected to the end taps of the respective voltage divider 13, 14. The output of each operational amplifier 19, 20 is connected to the center tap of the other voltage divider 13, 14. The two voltage dividers 13, 14 are grounded in series by a high-impedance resistor 21 of, for example, 50 kΩ. At the serially opposite input of the impedance converter 123, if the resistor element 121 is designed as a PT1000 and the other resistors of the voltage dividers 17, 18 are each designed with 5 kOhms, an adequate input impedance of 10 kOhms results, which corresponds to a multiplication factor X of 10.This reduces the current draw at the resistance element 121 in the flow meter 1 by approximately one hundred times, so that measurement errors in the measurement of the flow velocity γ due to self-heating of the resistance element 121 practically no longer play a role. Reference symbol list 1 Thermal flow meter 2 Line section 11 First voltage divider of the bridge circuit 12 Second voltage divider of the bridge circuit 13 Measuring probe 14 Voltage control unit 15 rule unit 16 Evaluation Unit 17,18 Voltage divider of the impedance converter 19, 20 Operational amplifiers in the impedance converter 21 Impedance 111 Heating element 112 Center tap of the first voltage divider 121 Temperature-dependent resistance element 122 Center tap of the second voltage divider 123 Impedance converter circuit GND Circuit Ground R1, R2 are series resistors of the voltage divider R x Offset resistor V m Potential difference between the center taps V s Operating voltage X Resistance multiplication factor of the resistance element γ Flow velocity of the measuring medium

Claims

[1] Thermal sensor, comprising: - A bridge circuit, with ◯ a first voltage divider (11) in which an electrically controllable heating element (111) is connected, and ◯ a second voltage divider (12) in which a temperature-dependent resistance element (121) is connected at the corresponding position of the heating element (111) in the first voltage divider (11), wherein an impedance converter circuit (123) is assigned to the temperature-dependent resistance element (121), which is designed to multiply the impedance of the resistance element (121) in the second voltage divider (12) by a defined factor (X). [2] Thermal sensor according to claim 1, wherein the impedance converter circuit (123) is designed as an impedance converter. [3] Thermal sensor according to claim 1 or 2, wherein the heating element (111) is in particular designed as a temperature-dependent resistor such that its resistance value and / or its temperature coefficient corresponds to the resistance value or the temperature coefficient of the resistance element (121). [4] Thermal flow meter (1) for determining the flow velocity (γ) of a flowing measuring medium in a pipe section (2), comprising: - A thermal sensor (1) according to any of the preceding claims, - a measuring probe (13) which can be arranged in the line section (2) and by means of which the heating element (111) and the resistance element (121) can be brought into thermal contact with the measuring medium, - a controllable voltage control unit (14) by means of which the supply voltage (V) dropping across the bridge circuit is controlled s ) and / or the current flowing through the bridge circuit is adjustable, - a control unit (15) designed to control the voltage control unit (14) such that a defined temperature difference is established between the heating element (111) and the resistance element (121), and - an evaluation unit (16) designed to use the set supply voltage (V s ) to determine the flow velocity (γ). [5] Thermal flow meter (1) according to claim 4, wherein the control unit (15) is implemented as an operational amplifier, the inputs of which are connected to the center taps (112, 122) of the voltage dividers (11, 12), wherein the voltage control unit (14) is designed as a bipolar transistor in particular, and wherein the base or gate of the transistor is connected to the output of the operational amplifier. [6] Method for determining the flow velocity (γ) of a flowing measuring medium in a pipe section (2) using a thermal flow meter (1) according to any one of claims 4 to 6, comprising the following method steps: - Applying a supply voltage (V s ) to the two voltage dividers (11, 12) via the voltage control unit (14), whereby the resistance of the resistive element (122) is multiplied by a defined factor (X), - such regulation of the supply voltage (V s ), that a defined temperature difference is established between the heating element (111) and the resistance element (121), and - Determination of the flow velocity (γ) based on the set supply voltage (V) s ).

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