Method and controller for controlling a fluid flow sensor

Abstract

A method for controlling a fluid flow sensor in the presence of a flowing fluid, the method comprising: applying an electrical bias to a heating element of the fluid flow sensor; determining a phase of the flowing fluid; and modifying the electrical bias in dependence on the phase of the flowing fluid. A controller for a fluid flow sensor is also described.

Description

Technical Field

[0001] This disclosure relates to a method for controlling a fluid flow sensor and a controller for the fluid flow sensor. The fluid flow sensor can be a micromechanical sensor capable of measuring multiphase flows of liquids and gases. In particular, but not exclusively, this disclosure relates to a fluid flow sensor having a heater formed within a discontinuous dielectric film for sensing the characteristics of the fluid flow or the composition of the fluid based on thermal conductivity properties. Background Technology

[0002] Thermal fluid flow sensors utilize the thermal interaction between the sensor itself and the fluid. Based on the physical phenomena governing this interaction, flow sensors can be classified into the following three categories:

[0003] (i) An anemometer sensor that measures convective heat transfer caused by fluid flow through a heated element;

[0004] (ii) A heat measurement sensor that detects the asymmetry in temperature distribution generated by the heated element and caused by forced convection of the fluid flow; and

[0005] (iii) Time-of-flight (ToF) sensor, which measures the time elapsed between the application of a thermal pulse and its sensing.

[0006] A review of thermal fluid flow sensors has been published in (B. Van Oudheusden, “Silicon Flow Sensors,” *Control Theory & Applications*, IEE Proceedings D, 1988, pp. 373-380; B. Van Oudheusden, “Silicon Thermal Flow Sensors,” *Sensors & Actuators A: Physics*, Vol. 30, pp. 5-26, 1992; N. Nguyen, “Micromechanical Flow Sensors—A Review A,” *Flow Measurement & Instrumentation*, Vol. 8, pp. 7-16, 1997; Y.-H. Wang et al., “MEMS-Based Gas Flow Sensors,” *Microfluidics & Nanofluidics*, Vol. 6, pp. 333-346, 2009; JTKuo et al., “Micromechanical Thermal Flow Sensors—A Review A,” *Micromechanics*, Vol. 3, pp. 550-573, 2012). Further background information can also be found in US6460411 by Kersjes et al.

[0007] A. Van Putten and S. Middelhoek, “Integrated Silicon Anemometer,” *Electronic Letters*, Vol. 10, pp. 425-426, 1974; and A. Van Putten, “Integrated Silicon Dual-Bridge Anemometer,” *Sensors and Actuators*, Vol. 4, pp. 387-396, 1983, proposed a resistor-based anemometer integrated on a chip within a Wheatstone bridge configuration. B. Van Oudheusden and J. Huijsing, “Integrated Flow Friction Sensor,” *Sensors and Actuators*, Vol. 15, pp. 135-144, 1988, proposed a thermal flow sensor calibrated for friction measurements, in which a thermocouple, in addition to a heating resistor and an ambient temperature monitoring transistor, is also integrated on a chip. JH Huijsing et al., “Monolithically Integrated Direction-Sensitive Flow Sensor,” *Electronic Devices*, IEEE Transactions on Electronics, Vol. 29, pp. 133-136, 1982; WSKuklinski et al., “Integrated Bipolar Transistor Arrays for Fluid Velocity Measurement,” *Medical and Bioengineering & Computation*, Vol. 19, pp. 662-664, 1981; Platzer and T. Qin-Yi and H. Jin-Biao, US3992940, “Novel CMOS Flow Sensor with Constant Chip Temperature (CCT) Operation,” *Sensors & Actuators*, Vol. 12, pp. 9-21, 1987, are examples of transistor-based anemometers. A drawback of the aforementioned citations is that they exhibit high power dissipation, low sensitivity, and slow dynamic response of the sensors.

[0008] In D. Moser et al., "Silicon Gas Flow Using Industrial CMOS and Bipolar IC Technologies"

[0009] In the article "Quantity Sensors," published in *Sensors and Actuators A: Physics*, Vol. 27, pp. 577-581, 1991, an array of seven npn transistors is used as a heating element and suspended on a crystalline silicon cantilever beam for effective thermal isolation. Ordinary pn diodes measure the temperature on the beam. When the heater is driven at a constant power, the voltage across 19 silicon / aluminum thermocouples (with hot junctions on the beam and cold junctions on the substrate) is related to the gas flow rate. The device exhibits mechanical fragility and vibration sensitivity.

[0010] Similarly, L. Lofdahl et al., “Silicon-based sensors for turbulence measurements”,

[0011] The journal *Physics Journal E: Scientific Instruments*, Vol. 22, p. 391, 1989, describes a heating resistor and heater temperature sensing diode integrated onto a cantilever beam. Polyimide is used as the thermal insulation material between the beam and the substrate, which affects the mechanical robustness of the beam.

[0012] In R. Kersjes et al.'s "Integrated Sensor for Invasive Blood Flow Velocity Measurement," *Sensors and Actuators A: Physics*, Vol. 37, pp. 674-678, 1993, a polycrystalline silicon heater driven by a constant heating power and a first diode for heater temperature monitoring were placed on a silicon film. A second diode was placed on the substrate for ambient temperature monitoring. A. Van der Wiel et al., "Liquid Velocity Sensor Based on Hot-Wire Principle," *Transportation and Actuation Technology*, Vol. 37, pp. 674-678, 1993, described a similar invention.

[0013] The journal *Sensors and Actuators A: Physics*, Vol. 37, pp. 693-697, 1993, also describes a similar sensor in which more transistors in a diode configuration are connected in series to improve the sensor's temperature sensitivity. The use of silicon as the film material results in high power dissipation, low sensitivity, and slow dynamic response in the sensor.

[0014] In US6460411 by Kersjes et al., a silicon film with slot perforations made of thermal insulation material was proposed, but its manufacturing process is more complex.

[0015] US20160216144A1 discloses a CMOS flow sensor including a heating element and multiple thermocouples. The thermocouples provide additional heat dissipation paths within the membrane, thereby increasing power dissipation, reducing sensitivity, and slowing down the sensor's dynamic response.

[0016] In E. Yoon and KDWise's "Integrated Mass Flow Sensor with On-Chip CMOS Interface Circuit," *Electronic Devices*, IEEE Transactions on Electronic Devices, Vol. 39, pp. 1376-1386, 1992, a multi-measurement flow sensor was proposed. However, the fabrication process was not fully CMOS compatible, and therefore more expensive than a fully CMOS process.

[0017] N. Sabaté et al., “Multi-range silicon micromechanical flow sensors”, Sensors and Actuations

[0018] The journal *Apparatus A: Physics*, Vol. 110, pp. 282-288, 2004, describes a multi-range flow sensor that uses a nickel resistor as a temperature sensor, positioned at different distances from a nickel resistance heater. Nickel is not a standard CMOS material, making the sensor manufacturing process more expensive than a fully CMOS process.

[0019] A structure is described in G. De Graaf and RF Wolffenbuttel, “Surface Micromechanical Thermal Conductivity Detector for Gas Sensing,” Proceedings of the 2012 IEEE International Conference on Instrumentation and Measurement Technology, pp. 1861-1864. This structure includes a heater for temperature control and two thermopile for sensing embedded in a dielectric perforated film. The film is obtained by front-facing etching through holes that have no effect on the device's operation. The effect of this process on flow rate is not compensated.

[0020] US20180143051A1 describes a structure using four resistors in a full-bridge configuration, wherein at least one external element unaffected by current is coupled to any of the previous external elements. This design requires complex circuitry for readout and uses large resistances to increase the output signal, which severely compromises the insulation provided by the film.

[0021] Traditional flow sensors based on heat wires embedded in membranes are known. Efforts are also being made to quantify the composition of fluids using thermal conductivity sensors. Summary of the Invention

[0022] Currently available sensors have the following drawbacks:

[0023] • It is impossible to accurately measure the flow rate of multiphase flow that may unexpectedly switch between liquid and gas.

[0024] • High power dissipation, especially when measuring liquid flow rate.

[0025] • Poor accuracy and precision, especially when measuring gas flow rate.

[0026] • Vulnerability when exposed to liquids

[0027] • Vulnerability to large changes in pressure

[0028] • Complex manufacturing process;

[0029] • Manufacturing processes not fully compatible with CMOS; and

[0030] • Expensive manufacturing process.

[0031] Due to these drawbacks, currently available flow sensors are primarily designed to measure only gases or, alternatively, only liquids. When a thermal gas flow sensor is exposed to liquid, it will cease to measure accurately, and in some cases, it will be permanently damaged. When a thermal liquid flow sensor is exposed to gas, the reading will be inaccurate (or zero).

[0032] The apparatus of this disclosure is superior to the apparatus of the prior art for at least the following reasons:

[0033] • The sensor can accurately measure the flow rate of liquids or gases.

[0034] • When the flow suddenly changes from gas to liquid or from liquid to gas, the transition time for inaccurate readings is abnormally short.

[0035] • The sensor will not be damaged even when exposed to large amounts of liquid.

[0036] • The sensor is robust to sudden, large changes in pressure (e.g., due to a change in fluid in the system from gas to liquid).

[0037] • The sensor exhibits very low power dissipation when measuring both liquid and gas flow rates.

[0038] • Sensor power dissipation varies little between liquids and gases being measured.

[0039] • These devices are fully CMOS compatible and therefore can be manufactured using fully CMOS compatible processes.

[0040] The sensor disclosed herein, referred to as a multiphase flow and thermal conductivity sensor, is capable of measuring (i) convective heat transfer caused by multiphase flow through a heated element; and (ii) determining whether the fluid flow is a liquid or a gas based on the different thermal conductivity and / or heat capacity of each component of the fluid flow.

[0041] The aspects and preferred features are set forth in the appended claims.

[0042] This disclosure relates to a sensor and a connected controller. The controller may be integrated with the sensor on the same substrate. Alternatively, the controller may be integrated with the sensor within the same component package. Alternatively, the controller may be separate from the sensor component but connected to the sensor component via wires, traces, or other electrical connections.

[0043] According to this disclosure, a method for controlling a fluid flow sensor in which a flowing fluid is present is provided. The method may include: applying an electrical bias voltage to a heating element of the fluid flow sensor; determining the phase of the flowing fluid; and modifying the electrical bias voltage according to the phase of the flowing fluid.

[0044] For example, the phase of the flowing fluid can be liquid or gas. In some examples, the flowing fluid may undergo a phase change during flow (e.g., from liquid to gas, or from gas to liquid). Advantageously, this method enables the fluid flow sensor to determine the rate and / or direction of fluid flow, regardless of the phase of the flowing fluid.

[0045] It should be understood that modifying the bias voltage according to the phase of the flowing fluid can include changing the bias voltage when a phase change of the flowing fluid is detected.

[0046] In some examples, applying an electrical bias to the heating element of the fluid flow sensor includes driving the fluid flow sensor in a first drive mode.

[0047] In some examples, modifying the electrical bias includes driving the fluid flow sensor in a second drive mode.

[0048] One or more of the first drive mode and / or the second drive mode may be phase-specific. That is, in some examples, the first drive mode may be a drive mode specific to the first phase of the flowing fluid, and the second drive mode may be a drive mode specific to the second phase of the flowing fluid (e.g., the first drive mode may be particularly suitable for measuring fluid flow rate when the fluid is in the first phase, while the second drive mode may be particularly suitable for measuring fluid flow rate when the fluid is in the second phase).

[0049] As an example only, one of the first and second phases can be a liquid, while the other of the first and second phases can be a gas.

[0050] In some examples, applying an electrical bias to the heater may include driving the fluid flow sensor in an initial drive mode. The initial drive mode may correspond to a phase-specific drive mode (e.g., the initial drive mode may correspond to a first drive mode or a second drive mode). Therefore, modifying the electrical bias according to the phase of the flowing fluid may include modifying the electrical bias (and thus the drive mode) only if the determined phase of the flowing fluid does not correspond to the initial drive mode.

[0051] Alternatively, the initial drive mode may correspond to neither the first drive mode nor the second drive mode. Therefore, in all cases after the phase of the flowing fluid is determined, the electrical bias can be modified according to the phase of the flowing fluid.

[0052] In some examples, the method includes monitoring the phase of the flowing fluid. For example, the phase of the flowing fluid may be monitored continuously or at regular intervals. The method may also include modifying the electrical bias voltage to drive the fluid flow sensor in different phase-specific drive modes in response to a phase change in the flowing fluid. For example, when a phase change in the fluid is detected while driving the fluid flow sensor in a first drive mode, the method may include modifying the electrical bias voltage to drive the fluid flow sensor in a second drive mode (or even a third drive mode), and vice versa.

[0053] In some examples, the phase of a flowing fluid can be determined via the output of a fluid flow sensor (e.g., from a heating element, and / or a temperature sensor, or the output signal of a fluid flow sensor). Similarly, monitoring the phase of a flowing fluid can include monitoring the output of a fluid flow sensor.

[0054] The method disclosed herein includes applying an electrical bias to a heating element of a fluid flow sensor. Applying an electrical bias to the heating element of the fluid flow sensor may mean applying a drive mode to the fluid flow sensor.

[0055] It should be understood that different driving modes can be applied to fluid flow sensors, including different driving modes that depend on the phase of the flowing fluid.

[0056] As those skilled in the art will understand, optimal performance of a fluid flow sensor with a first phase of flowing fluid may require applying a first driving mode specific to the first phase of flowing fluid to the fluid flow sensor. Similarly, optimal performance of a fluid flow sensor with a second phase of flowing fluid different from the first phase of flowing fluid may require applying a second driving mode specific to the second phase of flowing fluid and different from the first driving mode to the fluid flow sensor, as described below.

[0057] The method disclosed herein can allow modification of the electrical bias applied to the heating element based on the phase of the flowing fluid, or in other words, switching from a first driving mode specific to the first phase of the flowing fluid to a second driving mode specific to the second phase of the flowing fluid when the phase of the flowing fluid changes from a first phase of the flowing fluid to a second phase of the flowing fluid.

[0058] Additionally or alternatively, the method may include applying an electrical bias to the heating element to apply a drive mode, which is not necessarily a drive mode specific to a first phase or a second phase of the flowing fluid, but may include, for example, but not limited to, a drive mode applied only at the start of a flow sensing operation or after a flow sensing operation has been paused when the phase of the flowing fluid is unknown or uncertain (e.g., an initial drive mode). The method may also include determining the phase of the flowing fluid and modifying the applied electrical bias according to the phase of the flowing fluid to apply a drive mode specific to the determined phase (either the first phase or the second phase of the flowing fluid).

[0059] Therefore, it will be understood that, depending on the application or use of the fluid flow sensor, the driving mode prior to the flow fluid phase determination step can be any of a first driving mode specific to the first phase of the flow fluid, a second driving mode specific to the second phase of the flow fluid, or a driving mode not specific to either the first or second phase of the flow fluid. In any case, after the step of determining the phase of the flow fluid, a phase-specific driving mode can be applied according to the flow fluid phase.

[0060] It should also be understood that the method may further include monitoring the output of the fluid flow sensor (e.g., continuously or at regular intervals), for example, during at least a portion of the flow sensing process, and applying different electrical biases when a phase change in the flowing fluid is detected, so as to drive the fluid flow sensor in different phase-specific drive modes.

[0061] Those skilled in the art will further understand that a drive mode suitable for one phase of a flowing fluid may not be suitable for different phases of the flowing fluid, and in some cases, using a drive mode suitable for the first phase of the flowing fluid in the presence of a second phase of the flowing fluid may damage the fluid flow sensor.

[0062] In some examples, the method may include applying a phase-specific driving mode that includes reducing the bias voltage to a safe level or shutting off the fluid flow sensor (i.e., reducing the bias voltage to zero) when the fluid is in a liquid or gas phase. For example, modifying the bias voltage may include reducing the bias voltage to a safe level or to zero when the phase of the flowing fluid is liquid. Advantageously, for example, in the case where the fluid flow sensor is a gas flow sensor, reducing the bias voltage to a safe level or shutting off the fluid flow sensor in the presence of a liquid can prevent damage to the sensor.

[0063] It should be understood that driving methods, including reducing the electrical bias to a safe level or shutting off the fluid flow sensor (i.e., reducing the electrical bias to zero) when a specific phase of the flowing fluid is detected, can still be considered specific phase driving methods.

[0064] This method may include determining the phase of a flowing fluid via the output of a fluid flow sensor. The output of the fluid flow sensor is phase-sensitive, meaning that any value of the output can be associated with a specific phase of the flowing fluid. In other words, this output will undergo a specific change as the phase of the flowing fluid changes from a first phase to a second phase. It should be understood that, depending on the application or specific use of the fluid flow sensor, the first phase of the flowing fluid can be a gas or a liquid, and the second phase of the flowing fluid can be a gas or a liquid, as long as the second phase of the flowing fluid is different from the first phase of the flowing fluid.

[0065] The method may include determining the power dissipated in the heating element. Determining the phase of the flowing fluid may include determining the phase of the flowing fluid based on the power dissipated in the heating element.

[0066] Determining the power dissipated in the heating element may include determining the average and / or root mean square (RMS) power dissipated in the heater unit.

[0067] In some examples, the method includes determining the temperature of the heating element. For example, the temperature of the heating element may be determined based on the resistance of the heating element. In some examples, the temperature of the heating element may be determined by a temperature sensor, or based on a signal from a temperature sensor.

[0068] In some examples, the method may include determining the rate of temperature change of the heating element. In some examples, the method includes dividing the rate of temperature change of the heating element by the temperature at which it is heated.

[0069] The power dissipated in the element; and comparing the quotient of the rate of temperature change of the heating element and the power dissipated in the heating element with one or more known first values. Determining the phase of the flowing fluid may include determining the phase of the flowing fluid based on the comparison between the quotient of the rate of temperature change of the heating element and the power dissipated in the heating element with one or more known first values.

[0070] One or more known first values ​​may include a threshold of the rate of temperature change of the heating element divided by the power dissipated in the heater. In some examples, one or more known first values ​​may include a database of known values ​​of the rate of temperature change of the heating element divided by the power dissipated in the heater for different known fluids.

[0071] In some examples, the method includes determining the temperature of the flowing fluid.

[0072] In some examples, the method may include determining the rate of temperature change of the flowing fluid. In some examples, the method includes dividing the rate of temperature change of the flowing fluid by the power dissipated in the heating element; and comparing the quotient of the rate of temperature change of the flowing fluid and the power dissipated in the heating element with one or more known second values. Determining the phase of the flowing fluid may include determining the phase of the flowing fluid based on a comparison between the quotient of the rate of temperature change of the flowing fluid and the power dissipated in the heating element and one or more known second values.

[0073] One or more known second values ​​may include a threshold value for the rate of temperature change of the flowing fluid divided by the power dissipated in the heater. In some examples, one or more known first values ​​may include a database of known values ​​for the rate of temperature change of the flowing fluid divided by the power dissipated in the heater for different known fluids.

[0074] In some examples, the method includes determining the heat transfer coefficient of the heating element. The method may further include comparing the heat transfer coefficient with one or more known third values. Determining the phase of the flowing fluid may include determining the phase of the flowing fluid based on a comparison between the heat transfer coefficient and one or more known third values.

[0075] One or more known third values ​​may include a threshold heat transfer coefficient (heat transfer coefficient threshold). In some examples, one or more known third values ​​may include a database of known heat transfer coefficients for different known fluids.

[0076] This paper also describes a controller for a fluid flow sensor, which is configured to: apply an electrical bias to the heating element of the fluid flow sensor when the fluid flow sensor is in a flowing fluid; determine the phase of the flowing fluid; and modify the electrical bias according to the phase of the flowing fluid.

[0077] For example, the phase of the flowing fluid can be liquid or gas. In some examples, the flowing fluid may undergo a phase change during flow (e.g., from liquid to gas, or from gas to liquid). Advantageously, the controller can enable the fluid flow sensor to determine the rate and / or direction of fluid flow, regardless of the phase of the flowing fluid.

[0078] In some examples, the controller can be configured to shut off the fluid flow sensor when the fluid is in the liquid or gas phase (i.e., reduce the bias voltage to zero). For example, when the flowing fluid is in the liquid phase, the controller can be configured to modify the electrical bias voltage by reducing it to zero. Advantageously, for example, in the case where the fluid flow sensor is a gas flow sensor, shutting off the fluid flow sensor in the presence of liquid can prevent damage to the sensor.

[0079] Determining the power dissipated in the heating element may include determining the average value and / or RMS of the power dissipated in the heater unit.

[0080] In some examples, the controller is configured to determine the temperature of the heating element. For example, the temperature of the heating element may be determined based on the resistance of the heating element. In some examples, the temperature of the heating element may be determined by a temperature sensor, or based on a signal from a temperature sensor.

[0081] In some examples, the controller is configured to determine the rate of temperature change of the heating element. The controller may be further configured to divide the rate of temperature change of the heating element by the power dissipated in the heating element; and to compare the quotient of the rate of temperature change of the heating element and the power dissipated in the heating element with one or more known first values. Determining the phase of the flowing fluid may include determining the phase of the flowing fluid based on a comparison between the quotient of the rate of temperature change of the heating element and the power dissipated in the heating element and one or more known first values.

[0082] One or more known first values ​​may include a threshold of the rate of temperature change of the heating element divided by the power dissipated in the heater. In some examples, one or more known first values ​​may include a database of known values ​​of the rate of temperature change of the heating element divided by the power dissipated in the heater for different known fluids.

[0083] In some examples, the controller can be configured to determine the rate of temperature change of the flowing fluid. The controller can be further configured to divide the rate of temperature change of the flowing fluid by the power dissipated in the heating element; and to compare the quotient of the rate of temperature change of the flowing fluid and the power dissipated in the heating element with one or more known second values. Determining the phase of the flowing fluid can include determining the phase of the flowing fluid based on the comparison between the quotient of the rate of temperature change of the flowing fluid and the power dissipated in the heating element with one or more known second values.

[0084] One or more known second values ​​may include a threshold value for the rate of temperature change of the flowing fluid divided by the power dissipated in the heater. In some examples, one or more known first values ​​may include a database of known values ​​for the rate of temperature change of the flowing fluid divided by the power dissipated in the heater for different known fluids.

[0085] In some examples, the controller is configured to determine the heat transfer coefficient of the heating element. The controller may be further configured to compare the heat transfer coefficient with one or more known third values. Determining the phase of the flowing fluid may include determining the phase of the flowing fluid based on a comparison between the heat transfer coefficient and one or more known third values.

[0086] One or more known third values ​​may include the threshold heat transfer coefficient. In some examples, one or more known third values ​​may include a database of known heat transfer coefficients for different known fluids.

[0087] This article also describes an apparatus that includes a fluid flow sensor and controller as described herein.

[0088] According to some aspects of this disclosure, a flow sensor including a heater and a controller is provided, wherein the controller is capable of determining, based on a signal from the heater, whether the fluid in contact with the flow sensor is a liquid or a gas, and modifying the bias applied to the heater according to whether the fluid is a liquid or a gas.

[0089] The controller is configured in a way that enables it to perform specific functions. The controller is connected to a heating element inside the sensor. Through these connections, the controller can apply an electrical bias to the heating element to drive the fluid flow sensor in drive mode.

[0090] The controller is configured to apply different electrical bias voltages to the heating element based on the application and use of the fluid flow sensor and the phase of the flowing fluid, so as to drive the fluid flow sensor in different drive modes. The controller is also configured to change, modify, or adjust the drive mode when a specific phase of the flowing fluid is detected, or when a phase change of the flowing fluid is detected through the output of the fluid flow sensor.

[0091] As those skilled in the art will understand, the electrical bias applied to the heating element by the controller will allow a drive mode to be applied to the fluid flow sensor, and the drive mode is designed to set a controlled bias to the heating element. Therefore, in some examples, the drive mode can be defined by at least the following:

[0092] a. The type of controlled bias (constant voltage, constant current, constant resistance, constant power, constant temperature, or constant temperature difference); b. The controlled bias level (i.e., the bias setpoint, which depends on the type of controlled bias); c. The type of waveform used to apply the controlled bias (e.g., a constant bias waveform, a pulse bias waveform, a square waveform, a sine waveform, or any other suitable waveform of any type). Those skilled in the art will further understand that applying the electrical bias may include applying the electrical bias directly to the heating element or through additional circuitry, such as a Wheatstone bridge or any other suitable circuitry known in the art.

[0093] The electrical bias voltage is controlled by a controller. The controller can control the electrical bias voltage in various ways, including but not limited to controlling the voltage, controlling the current, or controlling the duty cycle of the pulse width modulation signal.

[0094] The controller can use the applied electrical bias voltage to determine certain characteristics of the heating element inside the sensor. The controller can determine the power dissipated in the heating element and the temperature of the heating element.

[0095] In one embodiment, the heating element is made of tungsten metal, and the electrical bias is controlled by modulating the duty cycle of a pulse-width modulated electrical signal. The power dissipated in the heating element is determined by calculating the RMS voltage across the heating element and the RMS current flowing through the heating element. By multiplying these values, the average power dissipated in the heating element can be determined.

[0096] In another embodiment, the sensor is driven using a constant controlled voltage. In another embodiment, the heater is driven using a constant controlled current. In yet another embodiment, the heater is driven in series with a current-limiting resistor.

[0097] Furthermore, the resistance of the heating element can be determined by taking the average voltage across its terminals and dividing it by the average current flowing through it. For tungsten metal heaters, the relationship between resistance and temperature is known, and this can be used to determine the temperature of the heated element.

[0098] The controller is also connected to a temperature sensing element inside the sensor. Through these connections, the controller is able to determine the temperature of the temperature sensing element.

[0099] In one embodiment, the temperature sensing element is made of tungsten metal, and the controller measures its temperature in the same way as the heating element. In this case, the electrical bias voltage applied to the temperature sensing element must be controlled to minimize the amount of self-heating that occurs, thereby avoiding interference with the temperature measurement.

[0100] The electrical bias can be controlled in a variety of ways, including but not limited to using current-limiting resistors, pulse width modulation of the drive signal, constant controlled voltage, constant controlled current, constant power, or constant temperature.

[0101] The controller can use the temperature of the heating element and / or a temperature sensing element to control the electrical bias applied to the heating element. In one embodiment, the controller uses the temperature of the heating element to control the electrical power dissipated in the heating element in order to maintain the heating element at a constant temperature.

[0102] The controller monitors the relationship between heater power, heater temperature, and the temperature of the temperature sensing element. The controller uses these relationships to determine whether the sensor is exposed to a liquid or gas flow.

[0103] In another embodiment, proxy parameters affected by power or temperature are used to control the heater and temperature sensing element, rather than the power or temperature itself. This can include voltage, current, or resistance.

[0104] If the sensor is exposed to a liquid flow, the electrical power dissipated in the heating element will be much higher for a given heater temperature and fluid temperature than when the sensor is exposed to a gas flow.

[0105] In one approach, the temperature difference is found by subtracting the temperature of the temperature sensing element from the temperature of the heating element. The average RMS power dissipated in the heating element is divided by this temperature difference to determine the heat transfer coefficient. This heat transfer coefficient is compared to a threshold to determine whether the flow in the sensor is liquid or gas.

[0106] It is important to note that the actual heat transfer coefficient does not need to be calculated, but any value corresponding to the heat transfer coefficient can be used. Instead, if a relevant signal is received from the heater or temperature sensing element, that signal or its derivative will correspond to the heat transfer coefficient. Such a signal can be voltage, current, power, temperature, or resistance.

[0107] In another method, the temperature of the heating element is measured over time, and the power dissipated in the heating element is also measured over time. The rate of temperature change is calculated and divided by the average RMS power dissipated in the heating element. This value is compared to a threshold to determine whether the flow in the sensor is liquid or gas.

[0108] It should be noted that what needs to be calculated or measured is not the actual rate of temperature change. Any other value corresponding to the actual rate of temperature change can also be used, such as the rate of change of voltage, current, resistance, etc.

[0109] In another method, the temperature of one or more heating elements or temperature sensors is measured over time, and the power dissipated in the heating elements is also measured over time. The rate of temperature change in one or more of these elements is calculated and divided by the average RMS power dissipated in the heating elements. This value is compared to a threshold to determine whether the flow in the sensor is liquid or gas.

[0110] Once the controller determines whether the flow is liquid or gas, it can adjust the control of the heating element accordingly.

[0111] To obtain a sufficiently large signal for accurate and precise measurement of gas flow, the controller needs to drive the heated element to a high temperature of approximately 200 degrees Celsius. In contrast, the required temperature for the heated element is much lower, around 60 degrees Celsius, compared to measuring liquid flow. This can pose a problem when switching between measuring gas and liquid flow. Contacting the liquid with the heated element at its high temperature can create a large thermal gradient, causing the liquid to boil.

[0112] Boiling liquids can generate physical and thermal stresses on dielectric films, which can lead to film failure or shorten its service life.

[0113] Large thermal gradients on the heated element can lead to localized hot spots, whose temperatures may be much higher than the heater's average temperature. This can cause localized damage to the heater. To address this, the controller can limit the maximum amount of power that can be dissipated in the heated element once liquid is detected. In the case of closed-loop temperature control of the heating element, the controller can lower the target heater temperature. The controller can also shut off the heater for a period of time. Alternatively, the controller can switch to a mode that controls the heater's power to a safe level and uses the heater's temperature and temperature sensors to determine the flow rate.

[0114] Corrosion is another common problem when exposed to liquids. Liquids containing dissolved ions that come into contact with electrically powered metal parts can cause corrosion through electrolysis of the metal components. This can significantly shorten the lifespan of sensors. In some cases, a mixture of different fluids may be worse than either fluid alone when it comes to corrosion rates.

[0115] The controller can mitigate corrosion damage by operating the sensor in pulse mode while detecting liquid. The frequency and length of the pulses can vary. For example, the pulse width can be a few milliseconds, and the time interval between pulses can be several seconds, a minute, several minutes, or longer. For instance, when using a flow sensor to detect gas flow, sometimes some liquid may enter the flow sensor—in this case, the sensor can detect when liquid is present in the sensor, i.e., the sensor switches to pulse mode. By energizing the sensor only for short periods and only periodically, the rate of electrolytic corrosion can be reduced, and the sensor's lifespan can be extended. By periodically energizing and measuring the power dissipated in the heated element, as well as the temperature of the heated element and the temperature sensing element, the controller can determine when the sensor is no longer in contact with liquid. Based on this information, the controller can immediately return to normal operating mode or after the sensor has dried for a certain period of time.

[0116] In a simpler embodiment, when the controller detects liquid, the sensor is powered off and remains powered off until an external signal from the controller commands it to restart the sensor.

[0117] In some non-limiting examples, the method and / or controller according to this disclosure may be adapted for use with a fluid flow sensor according to one or more of the following examples:

[0118] A flow and thermal conductivity sensor includes: a semiconductor substrate including an etched portion; a dielectric region on the substrate, wherein the dielectric region includes at least one dielectric film located above the etched portion of the semiconductor substrate; and a heating element located within the dielectric film. The film may also have one or more recessed regions.

[0119] The dielectric region may include a dielectric layer or multiple layers including at least one dielectric layer. The heating element may be fully or partially embedded within the dielectric film. The at least one recessed region may include one or more discontinuous regions in which the thickness of the dielectric film is discontinuous or differs from the average or most common dielectric film thickness.

[0120] Generally, dielectric film regions can be located immediately adjacent to the etched portion of the substrate. A dielectric film region corresponds to a region of dielectric area above an etched cavity portion of the substrate. Each dielectric film region can be located above a single etched portion of the semiconductor substrate.

[0121] The disclosed sensor can also be used as a gas sensor and is applicable to a variety of gases and liquids, but we specifically mention carbon dioxide (CO2), methane, and hydrogen because these specific gases have thermal conductivity properties that are significantly different from air. Although we specifically mention thermal conductivity as a thermomechanical property that allows for differentiation between fluids, the disclosed device can utilize any other thermomechanical property. The disclosed device can be used, for example, in breathalyzers where flow rate and CO2 concentration can be measured simultaneously. The disclosed device can also be used in other healthcare, fluid, consumer, environmental, or smart home applications.

[0122] The flow sensor can be contained within the same device or chip and optionally within the same membrane, based on a thermal conductivity sensor with at least one temperature sensing element. The device is capable of simultaneously sensing characteristics of the fluid flow, such as velocity, mass, volume, shear stress, and the composition of the flow (e.g., for a fluid, in this case, a gas, whether it has a certain percentage of CO2, hydrogen, or methane in the air / ppm).

[0123] The heater temperature can be adjusted by applying different power levels according to the fluid phase, thereby increasing the sensitivity and selectivity to different fluid phases based on the change in thermal conductivity of different fluid phases with temperature.

[0124] The heater can be operated in pulse mode (e.g., driven by square wave, sine wave, pulse width modulation wave, pulse density modulation, etc.) or continuous mode. Pulse mode has advantages such as reduced power dissipation, reduced electromigration to enhance device reliability / lifespan, and improved fluid characteristic sensing capability.

[0125] Heating elements can be configured to operate as sensing elements, for example, by sensing changes in resistance due to temperature variations. A heating element can also operate as both a heating element and a sensing element simultaneously. Electrically, a heating element is equivalent to a resistor. The thermal conductivity of most heater materials (tungsten, titanium, platinum, aluminum, polycrystalline silicon, monocrystalline silicon) changes with temperature. This change is primarily linear, characterized by the TCR (temperature coefficient of resistance). TCR can be positive or negative, but most metals have a positive and stable TCR, meaning their resistance increases as temperature rises. When current flows through a heating element, the element heats up, heating the film surrounding it. If the heater operates at the same power, when fluid flows over the heater, it cools the heater due to convection, thus changing its resistance (lower resistance for a positive TCR). The heater can also be driven in constant resistance or constant temperature modes, and the power change required to maintain the same heater resistance or temperature in the presence of flow can be correlated. Sensors are capable of measuring characteristics of flow, such as flow rate, velocity, mass or volumetric flow rate, and the composition of the fluid. The device can be configured to measure flow characteristics, such as flow rate, velocity, mass flow rate, or volumetric flow rate, by sensing changes in temperature, voltage (when supplied with a constant current), or power of the heater when it operates in a constant temperature or constant resistance mode.

[0126] Alternatively, flow rate can be measured using one or more sensing elements (e.g., temperature-sensitive elements or temperature sensors). Preferably, two sensing elements can be placed on either side of the heater within the same dielectric film and optionally used as a differential pair. The differential pair can be formed by an upstream sensing element and a downstream sensing element. Optionally, an aperture or discontinuity can be provided between the heater and the sensing elements.

[0127] One or more temperature sensing elements can be configured to measure the temperature difference across the heating element. For example, they can be used to measure temperature changes across the heating element. At least one thermopile can be placed symmetrically around the heater or at both ends of the heater, and the voltage difference between the thermopile terminals can indicate the characteristics of the flow, while the sign of the voltage can indicate the direction of the flow.

[0128] At least one temperature sensing element can be configured to measure the difference between the dielectric film and the dielectric region above the semiconductor substrate. For example, the thermopile can be arranged to have a hot junction located on the dielectric film and a cold junction located on the dielectric region above the semiconductor substrate, i.e., outside the dielectric film region.

[0129] Two thermopile structures can be arranged on either side of the heating element, with their hot junctions located on the dielectric film and their cold junctions located outside the dielectric film region. Since the two sets of cold junctions outside the dielectric film will be at substantially the same temperature, the temperature difference between the two hot junctions can be used to measure the temperature change across the heating element. The cold junctions of the at least two thermopile structures can be placed outside the film and physically or electrically connected together.

[0130] Temperature sensing elements may include resistive temperature detectors, diodes, and / or thermopile. Thermopiles can be used to measure the temperature difference between a dielectric film and a dielectric region above a substrate, or to measure the temperature difference across a heating element. Compared to thermopile, diodes and detectors reduce heat loss from the semiconductor substrate because they are located entirely on or within the dielectric film. One type of sensing element may be used, or a combination of different types of sensing elements may be used.

[0131] The temperature sensing element can be any one of a resistive temperature detector, a calorimeter, a diode, a transistor, or a thermopile, or an array of them in series or in parallel, or a combination thereof.

[0132] Temperature sensing elements can also be made of thermopile. A thermopile comprises one or more thermocouples connected in series. Each thermocouple may comprise two different materials that form a junction at a first region of the film, while the other end of the materials forms a junction at a second region of the film or in a heat dissipation region (on the substrate outside the film region), where they are electrically connected to adjacent thermocouples or to pads for an external reader. Thermocouple materials may include metals such as aluminum, tungsten, titanium, or combinations of these metals or any other metal available in the process. Alternatively, thermocouple materials may include thermocouples based on n-type and p-type silicon or polycrystalline silicon or a combination of metals and semiconductors. The location of each junction of the thermocouples, as well as the number and shape of the thermocouples, can be any location, number, and shape required to fully map the temperature distribution curve on the film to achieve specific performance.

[0133] The selection of the shape, location, and number of temperature sensing elements, heating elements, and recesses within the membrane can generate temperature profiles and / or map temperature distribution profiles on the membrane to achieve specific performance, and can produce multi-directional, multi-range, and multi-characteristic sensing capabilities. For example, a flow sensor can be designed to sense flow rate and flow direction, or flow rate, flow direction, and fluid composition, based on any other combination of thermal conductivity or fluid properties.

[0134] The sensing element formed within the dielectric film can be configured as a temperature resistive detector (TRD) or a calorimeter, a diode, a transistor, or an array of transistors or diodes to enhance sensitivity and selectivity.

[0135] Temperature sensing elements can be used differentially to sense (i) characteristics of a flow, such as flow velocity, flow rate, volume, or mass flow rate (by measuring the signal difference between upstream and downstream sensing elements) or (ii) to sense the composition of a flow based on the difference in thermal conductivity between different components of the fluid (e.g., hydrogen has a much higher thermal conductivity than air; CO2 has a lower thermal conductivity than air).

[0136] Heaters or heating elements can also be used as temperature sensing devices. The heat exchange between the heater and the fluid can be measured by the change in the resistance of the heater itself and is associated with at least one characteristic of the fluid, such as velocity, flow rate, mass or volumetric flow rate, applied wall shear stress, pressure, temperature, or direction.

[0137] In use, the heating element can extend in a direction perpendicular to the direction of flow through the sensor. The heating element may not be precisely perpendicular to the flow direction, and can extend diagonally or at an acute angle to the flow direction; however, a component of the extension of the heating element may be perpendicular to the flow. Optionally, the heating element may be substantially perpendicular to the direction of flow through the sensor, or may be arranged at an angle of less than 10° to the direction perpendicular to the flow through the sensor.

[0138] Temperature sensing elements can be formed as long elements that can be aligned with a first heater or an additional / second heater, depending on whether their primary purpose is to sense flow characteristics such as flow rate or velocity, or whether their primary purpose is to sense the composition of the fluid and the concentration of different components of the fluid, respectively.

[0139] The dielectric film can be circular. The heating element and sensing element can also be circular. This improves the mechanical stability of the film.

[0140] The sensor may further include an application-specific integrated circuit (ASIC) coupled to the sensor. The ASIC may be located beneath the sensor, for example, using die stacking technology. Alternatively, the ASIC may be located elsewhere. The ASIC may be connected to the sensor using wire bonding and pads or using through-silicon vias (TSVs) extending through the semiconductor substrate.

[0141] An ASIC can be housed within the same system or package, or on a chip, to provide electronic circuitry for driving, reading out, and processing signals from a sensor. The ASIC can be placed in a stacked die configuration below the sensor, and the sensor and ASIC can be housed within a manifold.

[0142] Analog / digital circuitry can be integrated on-chip. The circuitry may include IPTAT, VPTAT, amplifiers, analog-to-digital converters, memory, RF communication circuitry, timing blocks, filters, or any other components used to drive heating elements, read out temperature sensing elements, or electronically manipulate sensor signals. For example, it has been demonstrated that driving heating elements in a constant temperature mode results in enhanced performance, and having an on-chip device for implementing such a driving method will lead to significant advancements in prior art flow sensors. The driving method, referred to as 3ω, can be implemented on-chip or by any other driving method, such as constant temperature difference and time of flight, to achieve specific performance characteristics (e.g., power dissipation, sensitivity, dynamic response, range, fluid characteristic detection, etc.). In the absence of on-chip circuitry, this disclosure also covers off-chip implementations of such circuit blocks when applied to flow sensors having one or more features described in any of the preceding embodiments. Such off-chip implementations can be accomplished in an ASIC or by discrete components or a combination of both.

[0143] The device can be packaged in a metal TO package, ceramic, metal, or plastic SMD (surface mount device) package. It can also be packaged directly on a PCB or using a flip-chip approach. Alternatively, the device can be embedded in a substrate (e.g., a custom version of the aforementioned package, rigid PCB, semi-rigid PCB, flexible PCB, or any other substrate) so that the device surface is flush with the substrate surface. The package can also be a chip or wafer-level package, for example, formed through wafer bonding.

[0144] The device can also be assembled within a manifold that provides inlet, outlet, and predefined channels through which fluid flow occurs. The manifold provides protection for the device and allows for easier and more controlled measurement of flow rate or fluid composition. ASICs or external readout circuitry can also be placed in the same manifold in a lateral or die-stack configuration.

[0145] Flow sensors can have through-silicon vias (TSVs) to avoid the presence of bonding leads near the sensitive area of ​​the device that could affect flow sensor readings. Advantageously, flow sensors with TSVs can be implemented using 3D stacking techniques. For example, the flow sensor chip can be located on top of an ASIC, thereby reducing the size of the sensor system.

[0146] The semiconductor substrate may be silicon, and the dielectric film may be formed primarily of oxide and nitride materials, wherein the heater is made of metals such as tungsten, titanium, copper, aluminum, gold, platinum or combinations thereof, or of semiconductors such as highly doped n-type or p-type silicon or polycrystalline silicon, and wherein the heater has a tortuous, spiral or hot wire shape.

[0147] The starting substrate can be any semiconductor, such as silicon, silicon-on-insulator (SOI), silicon carbide, sapphire, or diamond. The use of silicon is particularly advantageous because it ensures the manufacturability of sensors with high volume, low cost, and high reproducibility. The use of silicon substrates also enables on-chip circuitry for sensor performance enhancement and system integration facilitators. This on-chip circuitry can be implemented using analog, digital, or mixed-signal blocks placed outside the dielectric film.

[0148] Dielectric films or multiple films can be formed by back etching using depth reactive ion etching (DRIE) of the substrate, which produces vertical sidewalls, enabling a reduction in sensor size and cost. However, back etching can also be performed using anisotropic etching, such as KOH (potassium hydroxide) or TMAH (tetramethylammonium hydroxide), which produces sloping sidewalls. The dielectric layer within the film, formed by oxidation or oxide deposition, can be used as an etch stop layer during DRIE or wet etching processes. Films can also be formed by front etching or a combination of front and back etching to produce a suspended film structure supported by only two or more beams. The film can be circular, rectangular, or rectangular with rounded corners to reduce stress at the corners, but other shapes are also possible. Additionally, holes can be formed within the film to reduce heat dissipation through thermal conduction via the dielectric film and to enhance heat loss through heat exchange and conduction in the regions below and above the film, and optionally in the fluid path (above the film). Optionally, holes or discontinuities can be formed by front etching after film formation.

[0149] The dielectric film may include silicon dioxide and / or silicon nitride. The film may also include one or more spin-coated glass layers, and a passivation layer on one or more dielectric layers. Using materials with low thermal conductivity (e.g., dielectrics) can significantly reduce power consumption and increase the temperature gradient within the film, which has direct benefits in terms of sensor performance (e.g., sensitivity, frequency response, range, etc.). Temperature sensing elements or heaters made of materials such as single-crystal or polycrystalline semiconductors or metals can be suspended or embedded in the dielectric film.

[0150] The membrane may also have other structures made of metals or other conductive materials or other materials with high mechanical strength. These structures may be embedded within the membrane, or embedded above or below the membrane, to design the membrane's thermomechanical properties (e.g., stiffness, temperature distribution profile, etc.) and / or the hydrodynamic interactions between the fluid and the membrane. More generally, these structures may also be located outside the membrane and / or bridged between the inside and outside of the membrane.

[0151] The sensed fluid can be a gas, which can be made from air, and the component of interest can be CO2, methane, or hydrogen, or any other gas with a thermal conductivity different from that of air. The fluid can also be a liquid.

[0152] The substrate may include: more than one etched portion; and a dielectric region located on the substrate, wherein the dielectric region comprises a dielectric film on each region of the etched portion of the substrate. At least one film may include any combination of the above features. The second film may employ more holes or discontinuities, larger-area holes or discontinuities, or holes or discontinuities at different locations. A differential signal can be measured between a sensing element on the first film and a sensing element placed on the second film to detect the composition of the fluid in addition to its flow characteristics.

[0153] In some examples, the methods and / or controllers disclosed herein may be suitable for use with sensors of the type described in US 2021 / 0116281, the contents of which are incorporated herein by reference in their entirety. Attached Figure Description

[0154] Some embodiments of this disclosure will now be described by way of example and with reference to the accompanying drawings, in which:

[0155] Figure 1 shows a cross-section of a prior art flow sensor based on heating and self-sensing elements;

[0156] Figure 2 shows a top view of a prior art flow sensor based on heating and self-sensing elements;

[0157] Figure 3 schematically illustrates a controller integrated with existing technology sensors;

[0158] Figure 4 schematically illustrates a stand-alone controller connected to a prior art sensor;

[0159] Figure 5 illustrates the control flow diagram for calculating the heat transfer coefficient and comparing it with a threshold to determine whether a liquid or gas is used.

[0160] Figure 6 illustrates the control flow diagram for calculating the rate of temperature change and comparing it with a threshold to determine whether a liquid or gas is being used; and

[0161] Figure 7 illustrates the method according to this disclosure. Detailed Implementation

[0162] Figures 1 (cross-section) and 2 (top view) illustrate a prior art flow sensor based on heating and self-sensing elements. The device has a substrate 1, a membrane 4, and a heater (e.g., a heating element 3). The substrate 1 may be based on a semiconductor material such as silicon, and the membrane 4 includes one or more dielectric layers 2 (e.g., oxides and / or nitrides). The heating element 3 may include a hot wire (e.g., tungsten, platinum, titanium). The membrane 4 is defined by back-side etching (as shown) or front-side etching using dry or wet etching techniques. As fluid passes over the top of the membrane 4, the heating element 3 cools due to heat convection losses. This can be easily measured by correlating the change in the heater's resistance with flow rate, velocity, volumetric flow rate, or mass flow rate. Alternatively, as illustrated in Figures 1 and 2, the sensor may include a first temperature sensing element 6a and a second temperature sensing element 6b. For example, as illustrated in Figures 1 and 2, the first temperature sensing element 6a may be located upstream of the heating element 3, and the second temperature sensing element 6b may be located downstream of the heating element 3. The first temperature sensing element 6a and the second temperature sensing element 6b can be configured to measure the difference (e.g., temperature change) across the heating element 3. The heating element 3, the first temperature sensing element 6a, and the second temperature sensing element 6b are externally connected via connectors and / or pads 5 (as shown in FIG. 2), such as bias traces. Alternatively, the heating element 3 can be maintained in a constant temperature or constant resistance mode by varying the power supplied to it. In this case, power changes due to flow rate, velocity, volumetric flow rate, or mass flow rate can be measured.

[0163] In the example, the first temperature sensing element 6a and the second temperature sensing element 6b may include a resistive temperature detector, a diode and / or a thermopile.

[0164] like Figure 1 and Figure 2 As further illustrated, the membrane 4 may include one or more recessed regions (also described herein as pores and / or discontinuities) 7a, 7b, which can reduce heat dissipation via heat conduction through the membrane 4 and enhance heat loss via heat exchange and conduction in the regions below and above the membrane 4. A first recessed region 7a may be located upstream of the heating element 3. A second recessed region 7b may be located downstream of the heating element 3.

[0165] Figures 3 and 4 illustrate exemplary embodiments according to the present invention. The apparatus illustrated in Figures 3 and 4 includes, for example, a device connected to a controller 8. Figure 1 and Figure 2The illustrated prior art flow sensor. A controller 8 controls the electrical bias applied to the heating element 3, the first temperature sensing element 6a, and the second temperature sensing element 6b, and measures the RMS voltage and RMS current applied to the heating element 3, the first temperature sensing element 6a, and / or the second temperature sensing element 6b to determine the power and temperature of the heating element 3, the first temperature sensing element 6a, and / or the second temperature sensing element 6b. In Figure 3, the controller 8 is integrated with the sensor in the same housing. In Figure 4, the controller 8 is separate from the sensor but connected to the heating element 3, the first temperature sensing element 6a, and the second temperature sensing element 6b via wires or PCB traces.

[0166] As described herein, “element” may refer to heating element 3, first temperature sensing element 6a, and / or second temperature sensing element 6b.

[0167] Figure 5 illustrates a flowchart detailing how the controller applies an electrical bias and determines the power consumed in each element and the element's temperature by measuring the voltage and current applied to each element. This power and temperature are then used to calculate the element's heat transfer (h) coefficient. This h coefficient is then compared to an experimentally determined threshold to identify whether the sensor is exposed to a liquid or a gas. Finally, once the controller has determined whether the sensor is exposed to a liquid or a gas, it uses this information to adjust the electrical bias applied to the sensor elements to prevent overheating and optimize the sensor's sensitivity to flow rate changes.

[0168] Figure 6 illustrates a flowchart detailing how the controller applies an electrical bias to the heater and determines the power consumed in each element and the temperature of each element. The controller applies an electrical bias that heats the heater and the surrounding fluid. As the heating elements raise the temperature, the temperature sensing elements measure the fluid temperature. The controller calculates the rate of temperature change. The controller compares the relationship between the rate of temperature change and the electrical bias applied to the heater with previously collected known information to identify the fluid present. The controller can then use this information to adjust the electrical bias used during flow sensing to prevent element overheating and optimize the sensor's sensitivity to flow changes.

[0169] Figure 7 illustrates an example of method 700 according to the present disclosure. Method 700 can be executed by a controller according to the present disclosure. For example, one or more steps of method 700 can be provided to the controller as instructions, for example, in the form of computer code.

[0170] In step S702 of method 700, an electrical bias voltage is applied to the heating element of the fluid flow sensor.

[0171] In step S704 of method 700, the phase of the flowing fluid is determined.

[0172] In step S706 of method 700, the electrical bias is modified according to the phase of the flowing fluid. As described herein, controller 8 can be configured to perform the methods described herein (e.g., method 700 shown in FIG. 2) and / or processes (e.g., processes illustrated in FIG. 5 and FIG. 6). For example, controller 8 can be configured to execute instructions as described herein (e.g., in the form of computer program code). Instructions can be provided on one or more carriers. For example, one or more non-transient memories, such as EEPROM (e.g., flash memory), disks, CD or DVD-ROMs, programmable memories, such as read-only memories (e.g., read-only memories for firmware), one or more transient memories (e.g., RAM), and / or (one or more) data carriers, such as optical or electrical signal carriers. The memories (one or more) can be integrated into and / or separated from the corresponding processing chip. Code (and / or data) used to implement embodiments of this disclosure may include source code, object code, or executable code in a conventional programming language such as C (interpreted or compiled), or assembly code, code for setting up or controlling an ASIC (Application-Specific Integrated Circuit) or FPGA (Field-Programmable Gate Array), or code for a hardware description language.

[0173] This disclosure may be further defined according to one or more of the following non-restrictive provisions:

[0174] 1. A flow sensor including a heater and a controller, wherein the controller is capable of determining whether the fluid in contact with the flow sensor is a liquid or a gas, and modifying the bias applied to the heater based on whether the fluid is a liquid or a gas.

[0175] 2. The flow sensor as described in Clause 1, wherein the controller uses a heat transfer coefficient to determine whether the fluid is a liquid or a gas.

[0176] 3. The flow sensor according to Clause 1, wherein the controller uses the rate of change of heater temperature to determine whether the fluid is a liquid or a gas.

[0177] 4. The flow sensor according to Clause 1, wherein when the fluid is a gas, the controller operates the heater at a higher temperature, and when the liquid is a liquid, the controller operates the heater at a lower temperature.

[0178] 5. The flow sensor according to Clause 1, wherein the controller operates the heater in the presence of gas, but shuts off the heater when liquid is detected.

[0179] 6. The flow sensor according to Clause 5, wherein the controller shuts off the heater in the presence of liquid and then turns the heater on after receiving an external signal.

[0180] 7. The flow sensor as described in Clause 5, wherein the controller periodically energizes the heater to determine whether liquid is still present.

[0181] 8. The flow sensor according to Clause 1 includes a substrate having an etched portion, a dielectric over the substrate, a region of the dielectric over the etched portion being a dielectric film, and a heater embedded within the dielectric film.

[0182] 9. The flow sensor according to Clause 1, wherein the controller uses a signal from the heater to determine whether the fluid in contact is a liquid or a gas.

[0183] 10. The flow sensor according to Clause 1, wherein one or more temperature sensing elements are included within the sensor.

[0184] 11. The flow sensor according to Clause 10, wherein the controller uses signals from the one or more temperature sensing elements within the sensor.

[0185] 12. The flow sensor according to Clause 10, wherein the controller uses signals from both the heater and the temperature sensing element to determine whether the fluid in contact is a liquid or a gas.

[0186] Typically, any of the functions described herein can be implemented using software, firmware, hardware (e.g., fixed logic circuitry), or a combination of these implementations.

[0187] Although this disclosure has been described with reference to preferred embodiments as described above, it should be understood that these embodiments are merely illustrative and the claims are not limited to those embodiments. In view of the content of this disclosure, those skilled in the art will be able to make modifications and substitutions, which are considered to fall within the scope of the appended claims. Each feature disclosed or illustrated in this specification may be combined in this disclosure individually or in any suitable combination with any other feature disclosed or illustrated herein.

[0188] Those skilled in the art will conceive of many other effective alternatives. It should be understood that this disclosure is not limited to the described embodiments, but includes all modifications falling within the spirit and scope of this disclosure.

[0189] List of reference signs 1. Substrate

[0190] 2 (one or more) dielectric layers

[0191] 3 Heating element

[0192] 4. Membrane

[0193] 5 Connectors

[0194] 6a First temperature sensing element

[0195] 6b Second temperature sensing element

[0196] 7a, 7b Depression areas

[0197] 8. Controller.

Claims

1. A method for controlling a fluid flow sensor in which a fluid is flowing, the method comprising: An electrical bias voltage is applied to the heating element (3) of the fluid flow sensor to drive the fluid flow sensor in the initial drive mode; Determine the phase of the flowing fluid; and The method is characterized by further comprising: The electrical bias is modified according to the phase of the flowing fluid to drive the fluid flow sensor in a second driving mode, which is a phase-specific driving mode.

2. The method according to claim 1, comprising determining the power dissipated in the heating element (3), and wherein, Determining the phase of the flowing fluid includes determining the phase of the flowing fluid based on the power dissipated in the heating element (3).

3. The method according to any one of the preceding claims further includes determining the temperature of the heating element (3).

4. The method according to claim 3 further includes determining the temperature change rate of the heating element (3).

5. The method according to claim 4, further comprising: Determine the power dissipated in the heating element (3), and wherein determining the phase of the flowing fluid includes determining the phase of the flowing fluid based on the power dissipated in the heating element (3); Divide the rate of temperature change of the heating element (3) by the power dissipated in the heating element (3); and The ratio of the temperature change rate of the heating element (3) to the power dissipated in the heating element is compared with one or more known first values; Determining the phase of the flowing fluid includes determining the phase of the flowing fluid based on a comparison between the ratio of the rate of temperature change of the heating element (3) to the power dissipated in the heating element (3) and one or more known first values.

6. The method according to claim 1 or 2, further comprising determining the temperature of the flowing fluid.

7. The method of claim 6, further comprising determining the rate of temperature change of the flowing fluid.

8. The method according to claim 7, further comprising: Determine the power dissipated in the heating element (3), and wherein determining the phase of the flowing fluid includes determining the phase of the flowing fluid based on the power dissipated in the heating element (3); Divide the rate of temperature change of the flowing fluid by the power dissipated in the heating element (3); and The ratio of the rate of temperature change of the flowing fluid to the power dissipated in the heating element (3) is compared with one or more known second values; Determining the phase of the flowing fluid includes determining the phase of the flowing fluid based on a comparison between the quotient of the rate of temperature change of the flowing fluid and the power dissipated in the heating element (3) and one or more known second values.

9. The method according to claim 1 or 2, further comprising: Determine the heat transfer coefficient of the heating element (3); and The heat transfer coefficient is compared with one or more known third values; Determining the phase of the flowing fluid includes determining the phase of the flowing fluid based on a comparison between the heat transfer coefficient and one or more known third values.

10. The method according to claim 1 or 2, wherein, Modifying the electrical bias voltage includes reducing the electrical bias voltage to zero when the phase of the flowing fluid is liquid.

11. A controller (8) for a fluid flow sensor, the controller (8) being configured to: apply an electrical bias to a heating element (3) of the fluid flow sensor to drive the fluid flow sensor in an initial drive mode when a flowing fluid is present in the fluid flow sensor; Determine the phase of the flowing fluid; and The controller is characterized in that it is further configured to The electrical bias is modified according to the phase of the flowing fluid to drive the fluid flow sensor in a second driving mode, which is a phase-specific driving mode.

12. The controller (8) according to claim 11 is configured to determine the power dissipated in the heating element (3) and is further configured to determine the phase of the flowing fluid based on the power dissipated in the heating element (3).

13. An apparatus comprising: Fluid flow sensor; and The controller (8) according to claim 11 or claim 12.

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