Calibration of a sensor comprising a heating element

Abstract

Described herein is a sensor comprising a heating element and a control circuit. The control circuit is configured to: set a bias applied to the heating element to a calibration bias; obtain a measurement of a calibration parameter while the heating element is at the calibration bias, the calibration bias corresponding to a heating element temperature at which the calibration parameter is substantially independent of a stimulus applied to the sensor; set the bias applied to the heating element to a sensing bias; obtain a measurement of a sensing parameter while the heating element is at the sensing bias, the sensing parameter corresponding to an interaction between the sensor and the stimulus; and obtain a sensor reading using the sensing parameter and the calibration parameter. A method of operating a sensor is also described.

Description

[0001] Calibration of a Sensor Comprising a Heating Element

[0002] TECHNICAL FIELD

[0003] The present disclosure relates to sensors comprising heating elements, for example, but not limited to, thermal conductivity sensors, fluid sensors, and / or fluid flow sensors.

[0004] BACKGROUND

[0005] A sensor, for example a thermal sensor, such as a thermal conductivity sensor, a fluid sensor, and / or a fluid flow sensor, may comprise a heating element configured to operate at high temperatures.

[0006] For example, a heating element of a thermal conductivity fluid sensor may operate at a temperature in the range 100 °C - 500 °C. When the heating element is heated at a predefined temperature and in contact with a fluid or fluid mixture, and / or a fluid flow, heat transfer from the heating element to the fluid, for example a gas or gas mixture, will take place which can be measured in terms of power loss of the heating element which, in turn, can be correlated to the thermal conductivity of the gas(es), so that one or more target gases can be identified and their concentration measured, or to the flow rate of a flowing fluid.

[0007] Any one of a fluid or fluid mixture, a variation in their concentration or composition, a fluid flow, and / or a variation in fluid flow may be defined as a “stimulus" to which the sensor is exposed.

[0008] If the composition of the fluid(s) surrounding the heating element changes, the power loss from the heating element changes accordingly. This change can be determined either by a change in temperature of the heating element, or by a change in the power required to keep the heating element at the predefined temperature. When the heating element is a resistive heating element, these measurements are based on the resistance of the heating element. The resistance of a heating element varies with temperature, so that, to obtain reliable measurements, the resistance of the heating element should be known at any time. At high operating temperature values, the heating element may also undergo physical properties modification or degradation, in time, so that, for example, its resistance may become different from the expected values at specific temperatures. The progressive change in the heating element resistance over time is also known as “drift”.

[0009] The resistance of the heating element may change over time due to phenomena such as electromigration, chemical reaction with any of the surrounding materials, stress or thermal degradation, which may cause changes in the crystal structure of the material of the heating element. Any of these phenomena may result in change in resistance of the heating element, and this change may affect the accuracy of the sensor’s readings or outputs.

[0010] The resistance of the heating element is measured during production, as part of a calibration of the sensor. The calibration is generally carried out at one or more fixed temperatures and in the presence of a controlled gas, or gas mixture, in controlled conditions (besides temperature, also pressure, gas composition, humidity and the like).

[0011] If a change in the heating element resistance occurs, a sensor reading due to this change in resistance cannot be distinguished from a sensor reading caused by a stimulus.

[0012] To ensure reliability and accuracy of measurements, especially when small concentrations, small flow rates, or small changes in gas composition or concentration or flow rate have to be determined, any drift of the resistance of the heating element should be minimised.

[0013] Choosing particularly stable materials, such as platinum or tungsten, for the heating element may reduce the effect of drift but cannot remove it completely as even a highly stable material will eventually suffer from drift effects with time.

[0014] To reduce drift, a high temperature anneal may be carried out to try and stabilise the properties of the heating element - though this procedure may not be fully effective. Another method which may be used is based on an estimation of the expected drift for the lifetime of the device, and application of a time based compensation. EP1692748B1 gives an example where the lifetime drift is tested for sample devices from a lot, and all devices of the same lot are applied the same time based compensation.

[0015] Another option is to repeat manual calibration when drift is identified - but this may not be feasible for many applications as specific controlled calibration conditions have to be replicated.

[0016] For example, A. Hewes et.al. “Drift compensation in thermal anemometry” Meas. Sci. Technol., Vol. 31 , No. 4, 045302, proposes equations to compensate for drift effects in anemometric flow sensors without the need to re-calibrate the sensor, therefore assuming that the resistance of the heating element at 25 °C remains constant. Nevertheless, the authors identify cases where those equations require a new calibration to be run to compensate for significant changes in the resistance of the heating element at 25 °C.

[0017] US8779777 proposes to use calibration resistors to do a dynamic compensation based on a comparison of a reading of the calibration resistor with a reading of the secondary resistor. This requires a secondary resistor, so adding complexity and cost to the device.

[0018] There is, therefore, a need for more effective, less impactful and less costly ways of compensating for drift without having to re-run a calibration in the same conditions as the calibration run at production stage.

[0019] SUMMARY

[0020] According to the present disclosure, a calibration parameter may be obtained with respect to a sensor comprising a heating element (also referred to as a heater). The calibration parameter may be used in obtaining a sensor reading, and may enable compensation of any drift in the response (e.g. resistance) of the heating element to an applied bias over the lifetime of the heating element.

[0021] A sensor as described herein comprises a heating element and a control circuit (also referred to as a control and readout circuit).

[0022] The control circuit is configured to set the temperature of the heating element, by applying a corresponding electrical bias.

[0023] The control circuit is configured to: set a bias applied to the heating element to a calibration bias; obtain a measurement of a calibration parameter while the heating element is at the calibration bias, the calibration bias corresponding to a heating element temperature at which the calibration parameter is substantially independent of a stimulus applied to the sensor; set the bias applied to the heating element to a sensing bias; obtain a measurement of a sensing parameter while the heating element is at the sensing bias, the sensing bias corresponding to a heating element temperature at which a measurable interaction between the sensor and the stimulus takes place; the sensing parameter corresponding to an interaction between the sensor and the stimulus; and obtain a sensor reading using the sensing parameter and the calibration parameter.

[0024] The heating element temperature corresponding to the calibration bias (i.e. at which the calibration parameter is substantially independent of a stimulus applied to the sensor) may be referred to as a calibration temperature. The heating element temperature corresponding to the sensing bias may be referred to as a sensing temperature. It will be understood that sensor readings using a sensor according to the present disclosure may be obtained at one or more than one, different sensing temperatures and / or in transient conditions, i.e. at a time, or time interval, characterised by the temperature of the heating element changing from a first temperature to a second temperature.

[0025] The calibration parameter may be any parameter connected to the heating element resistance, such as current, voltage or power. The calibration parameter may be the resistance itself. At a calibration temperature, a calibration parameter is not affected by the presence of a fluid (e.g. a gas) and its changes in composition and concentration, or of a fluid flow.

[0026] The calibration parameter may be used to adjust the sensing parameter to account for any drift in the response of the heating element to the applied bias. For example, a measured sensing parameter value may be adjusted to provide a sensor reading or output that is more representative of the actual stimulus to which the sensor is exposed. For example, the control circuit may be configured to obtain the sensor reading by adjusting the sensing parameter based on the measurement of the calibration parameter.

[0027] Alternatively, the sensing bias (e.g. the sensing bias applied to the heating element to achieve a particular heating element temperature) may be adjusted, based on the measurement of the calibration parameter, to take into account the effects of any drift in the response of the heating element to the applied bias. For example, the control circuit may be configured to determine the sensing bias based on the measurement of the calibration parameter.

[0028] The sensor described herein may, advantageously, enable compensation of the effects of any drift over time in the response of the heating element to the applied bias (e.g. any drift over time in the temperature reached by the heating element for a given bias applied to the heating element). For example, in the case of a resistive heating element, the drift in the response of the heating element may correspond to a change in the resistance of the heating element over time, e.g. due to phenomena such as electromigration, chemical reaction with any of the surrounding materials, stress on the heating element or thermal degradation causing changes in the crystal structure of the material of the heating element.

[0029] The calibration parameter is measured by the control circuit, at a chosen calibration temperature, preferably when the heating element has reached a steady state at the calibration temperature, and the value so obtained is used to compensate for the drift of the resistance of the heating element from the value which would have been expected at the calibration temperature, in accordance with the calibration process as carried out at production stage. The value of the calibration parameter may be used by the control circuit either to change a set point of the heating element resistance or to modify the power loss reading, or in any other suitable way.

[0030] The sensor and methods described herein enable a calibration (e.g. a maintenance calibration) to be performed in situ, rather than under the controlled conditions applied at production stage.

[0031] It will be understood that the measurement of the calibration parameter according to the disclosure is distinct from the calibration step which is held at production stage. The measurement of the calibration parameter according to the disclosure does not require exposing the heating element to a controlled environment in terms of specific gas composition and / or flow conditions. It may be carried out in situ as a self-calibration routine. It is, therefore, less impactful to the functioning of the sensor and quicker and easier to perform. The sensors provided with a control and readout circuit configured to implement the drift compensation strategy here proposed, may provide reliable readings without having to undergo a full calibration routine, allowing to save time and costs and, in turn, to prolong the operational lifetime of the sensor.

[0032] In some examples, in preparation for the measurement of the calibration parameter, the control circuit may set a calibration bias in order to set the temperature of the heating element to a calibration temperature, for example 25 °C. It will be understood that any other suitable temperature value may be chosen as long as, at the chosen temperature, the measurement of the calibration parameter is not affected by the composition, concentration and / or flow rate of the gas(es) to which the heating element is exposed. The control circuit may be configured to measure the calibration parameter after a set period of time has elapsed so that the temperature of the heating element may stabilise at the calibration temperature.

[0033] For example, the control circuit may set the temperature of the heating element at ambient temperature by turning the heating element off (i.e. by applying a bias equal to zero) and letting the heating element reach ambient temperature. In this case, there will be no heat flow from the heating element, and so the value of its resistance at ambient temperature will not be affected by the fluid, but only by any drift that may have occurred in the heating element over time. So, by measuring the resistance of the heating element at ambient temperature, the drift may be determined, and then used to compensate or correct the sensor output. It should be noted that, although the heating element will be turned off by applying a bias equal to zero, during the measurement of the calibration parameter, a small bias may be required to measure the heating element resistance. In some examples, the heating element is configured to function as a sensing element (e.g. a temperature sensing element). That is, the heating element may be both a heating element and a sensing element.

[0034] Alternatively, a separate sensing element in thermal contact with the heating element may be used to carry out the measurement. In some examples, the sensor further comprises a sensing element (e.g. a temperature sensing element), e.g. separate from the heating element.

[0035] In some examples, the control circuit is configured to obtain the measurement of the calibration parameter at a predetermined time after setting the heating element to the calibration bias. For example, the control circuit may be configured to wait long enough for the heating element temperature to stabilise at the calibration temperature before obtaining the measurement of the calibration parameter. The predetermined time may preferably be in the range of 1 ms to 1 s, for example between 10 ms and 100 ms.

[0036] In some examples, the control circuit is configured to monitor an actual temperature of the heating element at the calibration bias, and to obtain the measurement of the calibration parameter when a difference between the actual temperature and a nominal calibration temperature value is at or below a predetermined threshold value, for example at or below 1 °C, at or below 0.5 °C, or less.

[0037] It will be understood that the nominal calibration temperature value corresponds to an expected temperature of the heating element at the calibration bias. It will further be understood that the control circuit may be configured to adjust the calibration bias to allow the temperature of the heating element to get closer, or substantially at, the nominal calibration temperature value.

[0038] In some examples, the control circuit is configured to monitor an actual sensing temperature of the heating element when the heating element is at the sensing bias, and to adjust the sensing bias to allow the actual temperature of the heating element to be brought to and to be maintained at or about a nominal sensing temperature value. A difference between the actual temperature of the heating element and the nominal sensing temperature value may be, for example, at or below 1 °C, at or below 0.5 °C, or less.

[0039] It will be understood that the nominal sensing temperature value corresponds to a target temperature of the heating element at which it is desirable to obtain the measurement of the sensing parameter.

[0040] As already mentioned, in some examples, the heating element is configured to function as a sensing element (e.g. a temperature sensing element). That is, the heating element may be both a heating element and a sensing element.

[0041] In some examples, the sensor further comprises a sensing element (e.g. a temperature sensing element), e.g. separate from the heating element.

[0042] The control and readout circuit may be configured to monitor the temperature of the heating element and to measure the calibration parameter after the sensing element (as a separate sensor or otherwise) has returned substantially the same reading of the temperature for a set period of time so that the temperature of the heating element may be considered to have stabilised. It will be understood that there may be some variation in the temperature readings, and that the temperature will be considered to have stabilised when the difference between two or more consecutive readings is at or below a predetermined threshold, for example at or below 1 °C, at or below 0.5 °C, or less.

[0043] In some examples, setting the temperature of the heating element at a calibration temperature where the calibration parameter is not affected by the presence of a gas and its changes in composition and concentration, may comprise turning the heating element off, or, in other words, applying a bias of zero.

[0044] In those cases, the ambient temperature may be considered the calibration temperature at which the calibration parameter is to be measured and the control circuit may be configured to measure the calibration parameter after a set time has elapsed since the heating element has been applied a bias equal to zero, so as the ambient temperature may be reached with a sufficient degree of confidence without the need to monitor the temperature of the heating element.

[0045] In those cases the temperature of the heating element may, in time, reach ambient temperature or it may reach the temperature of the part of the sensor which supports the heating element, for example, but not limited to, a substrate. The temperature of the substrate may be different from the ambient temperature. In some implementations, the temperature of the substrate may provide a sufficiently good approximation of the ambient temperature.

[0046] In some examples, the sensor further comprises a temperature sensor. In examples wherein the sensor comprises the sensing element described above, the temperature sensor described herein may be a separate component from the sensing element.

[0047] Additionally, or instead, the temperature sensor may be used to measure the temperature of the heating element in cases where the calibration bias is set to zero (i.e. the heating element is off during calibration).

[0048] The control circuit may be configured to obtain, using the temperature sensor, a measured temperature of the heating element while the heating element is at the calibration bias, and to obtain the sensor reading based, additionally, on the measured temperature.

[0049] Additionally, the sensor may be provided with an ambient temperature sensor to monitor the ambient temperature. In some examples, the control circuit is configured to obtain, using the ambient temperature sensor, a measured ambient temperature. The control circuit may be configured to obtain a measurement of the calibration parameter, and, in turn the sensor reading based, additionally, on the measured ambient temperature. For example, a sensor described herein may have been initially calibrated (e.g. during production) at a particular temperature (e.g. room temperature), but subsequently a calibration parameter may be measured, over the lifetime of the sensor and heating element, at a different ambient temperature. The measurement of the ambient temperature may therefore be used in the measurement of the calibration parameter as well, and / or in obtaining of a sensor reading.

[0050] In other implementations, where higher precision in the measurement of the calibration parameter is required, the sensor may be provided with an additional temperature sensor located on the substrate so that the actual temperature of the substrate is used as the calibration temperature for the measurement of the calibration parameter.

[0051] For example, if the production stage calibration was done at 25 °C, and if at the time of the measurement of the calibration parameter, the ambient temperature sensor measures a temperature of the substrate of 50 °C, the control circuit may be configured to calculate the heating element resistance at 25 °C, based on the measurement of the calibration parameter at 50 °C, and to use this calculated value for drift compensation.

[0052] It should be noted that, during operation of the sensor, the temperature of the immediate surroundings of the sensor and / or heating element may be higher than the ambient temperature or the temperature of the rest of the fluid to which the sensor is exposed due to the self-heating caused by the operation of the sensor or by the circuitry used to control the sensor. This may increase the temperature of the sensor circuit board, package and / or chip. In this case the ambient temperature may refer to the temperature of the immediate surroundings of the sensor and / or heating element and the ambient temperature sensor may measure the temperature of the immediate surroundings of the sensor and / or heating element. The calibration of the heating element at production stage may comprise measuring the heating element resistance at one or more predefined temperature(s). Whenever the measurement of the calibration parameter, in accordance with this disclosure, is made at any temperature different from the one or more predefined temperature(s) at which the calibration was performed at production stage, the control and readout circuit may be configured to compensate for the temperature difference between the calibration temperature and the one or more predefined temperature(s) and modify the value of the calibration parameter accordingly. For example, the compensation may be carried out by calculating the expected resistance at the relevant calibration temperature using a known temperature coefficient of resistance of the material of which the heating element is made.

[0053] The measurement of the calibration parameter, in accordance with this disclosure, may be carried out as a direct measurement of the resistance (or another correlated calibration parameter) of the heating element, for example by applying a current and measuring the voltage, or applying a voltage and measuring current, or other well-known methods. Alternatively, the sensor may further comprise a reference element and a measurement of the calibration parameter may be carried out as a differential measurement of the resistance (or another correlated calibration parameter) of the heating element against a respective parameter of the reference element. The reference element may, preferably, be identical to the heating element in terms of material, shape, size and resistance. The reference element may be a heating element that is only biased during the calibration. In this way, its resistance is less likely to drift as the reference heating element is not biased during normal operation of the sensor. Alternatively the reference heating element may be designed or arranged such that it does not heat up during normal operation of the sensor. In some examples, the reference element may be an ambient temperature sensor. An instrumentation amplifier may be used in the differential measurement to obtain more accurate values of the heating element resistance.

[0054] In some implementations of the sensor, the reference heating element may be isolated from the target stimulus. For example in a gas sensor, the reference heating element may be sealed from the external atmosphere, while the heating element may be exposed to it.

[0055] In a first mode, the control circuit may be configured to set the electrical bias, applied to both the heating element and the reference heating element, to a calibration bias and obtain a differential measurement of the calibration parameter.

[0056] The control circuit may further be configured to set the electrical bias, applied to both the heating element and the reference heating element, to a sensing bias and obtain a differential measurement of a sensing parameter.

[0057] The calibration parameter and the sensing parameters are then used to obtain a sensor reading.

[0058] In a second mode, the control circuit may be configured to set the electrical bias, applied to both the heating element and the reference heating element, to a calibration bias and obtain a measurement of a first calibration parameter from the heating element, and a measurement of a second calibration parameter from the reference heating element.

[0059] The first calibration parameter and the second calibration parameter may be used to determine a calibration parameter.

[0060] The calibration parameter may then be combined with a sensing parameter (which may have been obtained as a differential measurement between the heating element and reference heating element both biased at a sensing bias) to determine the sensor reading.

[0061] The first and second mode may be used separately or in combination. When the first and second modes are combined the control circuit may be configured to obtain a measurement of a first calibration parameter from the heating element, a measurement of a second calibration parameter from the reference heating element and a measurement of a third calibration parameter as a differential measurement from the heating element and the reference heating element. The first, second and third calibration parameters may be used to determine the calibration parameter.

[0062] The control circuit may further be configured to store the value of the calibration parameter, or the modified calibration parameter, when compensation of the measured calibration parameter is required, and to use it to modify one reading or two or more readings. In other words, the measurement of the calibration parameter may be made before or after one or each sensor measurement, or before a series of measurements so that the same value of the calibration parameter is used to modify a plurality of sensor readings. The control circuit may be configured to set the temperature of the heating element at a suitable calibration temperature for the measurement of the calibration parameter and measure the calibration parameter periodically, at set time intervals or before or after each reading of the sensor or before or after a fixed number of readings have been taken.

[0063] In some examples, the calibration parameter may be measured for every sensor reading. For example, the control circuit may be configured to obtain a measurement of the calibration parameter for each sensor reading to be obtained.

[0064] In other examples, the calibration parameter may be measured periodically, for example at regular time intervals, and / or after a certain number of sensor readings has been obtained using the previously measured calibration parameter. For example, the control circuit may be configured to obtain two or more sensor readings based on an obtained measurement of the calibration parameter.

[0065] For example, the calibration parameter may be measured every hour, while the sensor provides a reading or output every minute. In this case, a single measurement of the calibration parameter is used to compensate or adjust 60 sensor readings.

[0066] In some examples, the calibration temperature is lower than the sensing temperature.

[0067] In some examples, the calibration temperature is an ambient temperature (e.g. room temperature). In some examples, the calibration bias is zero, or approximately zero.

[0068] Additionally, the control circuit may be configured to set the bias applied to the heating element to more than one calibration biases. For example, the control circuit may set the bias applied to the heating element to a first calibration bias substantially equal to zero, or low enough as not to cause any self-heating of the heating element, and obtain a measurement of a first calibration parameter substantially independent of (or unaffected by) a target stimulus. The control circuit may then set the bias applied to the heating element to a second calibration bias, which second calibration bias may be chosen such that it is higher than the first calibration bias and lower than a sensing bias. The control circuit may obtain a measurement of a second calibration parameter which may be affected by the target stimulus, but to a lesser extent than the sensing parameter. The first calibration parameter and the second calibration parameter may be used to determine a combined calibration parameter so that the control circuit may obtain a sensor reading using the sensing parameter and the combined calibration parameter. This configuration may be useful in a case where the heating element drifts over time in such a way that the resistance of the heating element, when an electric bias substantially equal to zero is applied to it, does not change over time.

[0069] In some examples, the sensor is a thermal conductivity based sensor.

[0070] In some examples, the sensor is a thermal fluid flow sensor.

[0071] In some examples the sensor is a resistive or catalytic gas sensor.

[0072] The heating element may be provided on a chip which is part of a MEMS device. The control circuitry may be on the same chip as the heating element, or on a separate chip but within the same package, or it may be outside the package. The control circuitry may comprise various portions in different locations - for example, some parts of the circuitry on the same chip, and some outside the package on a printed circuit board (PCB) where the chip is soldered. In some examples, the sensor comprises: a substrate comprising a cavity and a dielectric layer disposed over the substrate and the cavity, wherein the dielectric layer comprises at least one dielectric membrane located over the cavity and wherein the heating element is on or within the dielectric membrane.

[0073] The heating element may comprise platinum, tungsten, polysilicon, and / or another suitable material. The type of heating element, its shape and the material it is made of are not limited and any heating element known in the art may be used in the implementation of the sensors and methods described herein.

[0074] It will be understood that the control circuit may also be configured to output the sensor reading.

[0075] Also described herein is a method of operating a sensor, the method comprising: setting a bias applied to a heating element of the sensor to a calibration bias; obtaining a measurement of a calibration parameter while the heating element is at the calibration bias, the calibration bias corresponding to a heating element temperature at which the calibration parameter is substantially independent of a stimulus applied to the sensor; setting the bias applied to the heating element to a sensing bias; obtaining a measurement of a sensing parameter while the heating element is at the sensing bias, the sensing bias corresponding to a heating element temperature at which a measurable interaction between the sensor and the stimulus takes place; the sensing parameter corresponding to an interaction between the sensor and the stimulus; and obtaining a sensor reading using the sensing parameter and the calibration parameter.

[0076] The method described herein may advantageously enable compensation of the effects of any drift over time in the response of the heating element to the applied bias (e.g. any drift over time in the temperature reached by the heating element for a given bias applied to the heating element). For example, in the case of a resistive heating element, the drift in the response of the heating element may correspond to a change in the resistance of the heating element over time, e.g. due to phenomena such as electromigration, chemical reaction with any of the surrounding materials, stress on the heating element or thermal degradation causing changes in the crystal structure of the material of the heating element.

[0077] The heating element temperature corresponding to the calibration bias (i.e. at which the calibration parameter is substantially independent of a stimulus applied to the sensor) may be referred to as a calibration temperature. The heating element temperature corresponding to the sensing bias may be referred to as a sensing temperature.

[0078] The calibration parameter may be used to adjust the sensing parameter to account for any drift in the response of the heating element to the applied bias. For example, a measured sensing parameter value may be adjusted to provide a sensor reading or output. For example, obtaining the sensor reading may comprise adjusting the sensing parameter based on the measurement of the calibration parameter.

[0079] Alternatively, the sensing bias (e.g. the sensing bias applied to the heating element to achieve a particular heating element temperature) may be adjusted based on the measurement of the calibration parameter, to take into account the effects of any drift in the response of the heating element to the applied bias. For example, the method may comprise determining the sensing bias based on the measurement of the calibration parameter.

[0080] In some examples, the calibration temperature is an ambient temperature, or room temperature, e.g. 25 °C.

[0081] In some examples, the calibration bias applied to the heating element is zero, or approximately zero.

[0082] In some examples, the method comprises obtaining the measurement of the calibration parameter at a predetermined time after setting the heating element to the calibration bias. For example, the method may comprise waiting for a suitable amount of time for the heating element temperature to stabilise at the calibration temperature before obtaining the measurement of the calibration parameter. The predetermined time may preferably be in the range of 1 ms to 1 s, for example between 10 ms and 100 ms. The method may comprise monitoring an actual temperature of the heating element at the calibration bias, and obtaining the measurement of the calibration parameter when a difference between the actual temperature and a nominal calibration temperature value is at or below a predetermined threshold value, for example at or below 1 °C, at or below 0.5 °C, or less.

[0083] It will be understood that the nominal calibration temperature value corresponds to an expected temperature of the heating element at the calibration bias. It will further be understood that the method may comprise adjusting the calibration bias to allow the temperature of the heating element to get closer, or substantially at, the nominal calibration temperature value.

[0084] The method may comprise monitoring an actual sensing temperature of the heating element when the heating element is at the sensing bias, and adjusting the sensing bias to allow the actual temperature of the heating element to be brought to and to be maintained at or about a nominal sensing temperature value. A difference between the actual temperature of the heating element and the nominal sensing temperature value may be, for example, at or below 1 °C, at or below 0.5 °C, or less.

[0085] It will be understood that the nominal sensing temperature value corresponds to a target temperature of the heating element at which it is desirable to obtain the measurement of the sensing parameter.

[0086] The calibration bias may be lower than the sensing bias.

[0087] The calibration bias may correspond to a heating element temperature that is substantially equal to an ambient temperature.

[0088] The method may comprise obtaining the sensor reading using additionally a measurement of the temperature of the heating element while the heating element is set to the calibration bias. The method may comprise obtaining the measurement of the calibration parameter and, in turn, of the sensor reading using, additionally, a measurement of the ambient temperature.

[0089] The method may comprise obtaining a measurement of the calibration parameter for each sensor reading to be obtained. That is, the measurement of the calibration parameter may be performed for every sensor reading.

[0090] In other examples, the calibration parameter may be measured periodically, for example at regular time intervals, and / or after a certain number of sensor readings has been obtained using the previously measured calibration parameter. For example, the method may comprise obtaining two or more sensor readings based on an obtained measurement of the calibration parameter.

[0091] In some examples, obtaining the reading of the calibration parameter and / or of the sensing parameter comprises obtaining a reading from the heating element, the heating element being configured to function as a sensing element.

[0092] In some examples, obtaining the reading of the calibration parameter and / or of the sensing parameter comprises obtaining a reading from a sensing element.

[0093] It will be understood that the method may comprise outputting the sensor reading.

[0094] BRIEF DESCRIPTION OF THE DRAWINGS

[0095] The present invention will now be described, by way of example only, with reference to the following drawings:

[0096] Figure 1A: top view of an example of a sensor according to the present disclosure;

[0097] Figure 1 B: cross-section of the sensor illustrated in Figure 1A;

[0098] Figure 2A: example of a method of operating a sensor according to the present disclosure; Figure 2B: example of a method of operating a sensor according to the present disclosure;

[0099] Figure 2C: example of a method of operating a sensor according to the present disclosure;

[0100] Figure 3: flow-chart of an implementation of a method according to the present disclosure, which may be implemented as a maintenance routine;

[0101] Figure 4: flow-chart of an implementation of a method according to the present disclosure, which may be implemented as a maintenance routine; and

[0102] Figure 5: flow-chart of an implementation of a method according to the present disclosure, which may be implemented as a maintenance routine.

[0103] DETAILED DESCRIPTION

[0104] An example of a sensor 100 according to the present disclosure is shown in Figure 1A in top view and in Figure 1 B in cross-section.

[0105] As shown in Figure 1A, the sensor 100 comprises a control circuit 110, a dielectric layer 103 above a substrate 101 having a cavity 102. The portion of the dielectric layer 103 above the cavity 102 is referred to as a membrane 104. The membrane 104 is provided with a heating element 105. The sensor also comprises an ambient temperature sensor 106.

[0106] Figure 2A illustrates an example of a method 200 of operating a sensor (for example the sensor 100 illustrated in Figures 1A and 1 B) according to the present disclosure.

[0107] At a step S202, the method 200 comprises setting a bias applied to a heating element of the sensor to a calibration bias. The heating element may be the heating element 105 illustrated in Figures 1A and 1 B. The bias may be applied via a control circuit, such as the control circuit 110 illustrated in Figures 1A and 1 B.

[0108] For example, the calibration bias may correspond to a baseline, or calibration, temperature of the heating element, such as room temperature or ambient temperature. In some examples, the method 200 comprises waiting a set interval of time to lapse to allow the temperature of the heating element to stabilise at the calibration temperature.

[0109] At a step S204, the method 200 comprises obtaining a value of a calibration parameter (e.g. a resistance of the heating element, or a current or voltage across the heating element) while the heating element is at the calibration bias, the calibration bias corresponding to a heating element temperature at which the calibration parameter is substantially independent of a stimulus applied to the sensor.

[0110] At a step S206, the method 200 comprises setting the bias applied to the heating element to a sensing bias.

[0111] At a step S208, the method 200 comprises obtaining a value of a sensing parameter while the heating element is at the sensing bias, the value of the sensing parameter corresponding to an interaction between the sensor and the stimulus.

[0112] At a step S210, the method 200 comprises obtaining a sensor reading using the sensing parameter and the calibration parameter. Obtaining the sensor reading may comprise modifying the value of the sensing parameter based on the value of the calibration parameter, and / or obtaining the sensor reading may comprise modifying the sensing bias based on the value of the calibration parameter.

[0113] It will be understood that steps S202, S204, S206, S208, S210 of the method 200 may be performed in any suitable order.

[0114] As shown in Figure 2A and described herein, the calibration parameter (e.g. the resistance) of the heating element at the calibration bias (e.g. corresponding to the calibration or baseline temperature) is measured and the measured value of the calibration parameter is used to modify a sensor reading or output, resulting from a measurement at a sensing temperature. Modifying the sensor reading so as to reduce the effect of the ageing of the heating element may comprise modifying the measured value or it may comprise modifying the bias applied to the heating element to reach a sensing temperature.

[0115] For example, the heating element may operate, in constant temperature mode, at a sensing temperature of 300 °C. The sensing temperature may be measured by the change in resistance of the heating element from the value of the resistance as determined during the calibration at production stage, taking into account the variation of temperature of the heating element with respect to the temperature at which the calibration was carried out at production stage. In use, the control circuit may keep the heating element at a resistance which corresponds to a heating element temperature of 300 °C.

[0116] A drift in heating element resistance would mean that the resistance corresponding to 300 °C has changed with respect to the expected value based on the calibration of the sensor at production stage, and, unless a correction is applied, the heating element may be operating at a slightly different temperature with respect to the chosen sensing temperature of 300 °C. This, in turn, may lead to inaccuracy in the final sensor reading. By measuring the calibration parameter and, hence, the actual heating element resistance at ambient temperature, the bias applied to the heating element to work at a chosen sensing temperature and, hence, resistance, may be corrected and / or adjusted, so as to keep the heating element at a sensing temperature of 300 °C, as desired.

[0117] Figures 2B and 2C show two different examples of part 220 of the method 200 of Figure 2A.

[0118] Figure 2B shows part 220’ which may be used as part 220 of the method 200 of Figure 2A.

[0119] At a step S202’, the bias applied to the heating element as the calibration bias is set to zero, or, in other words, the heating element is switched off. At a step S202a’, a period of time is waited so that the heating element temperature is stabilised at the calibration temperature. At a step S204’, in order to obtain a measurement of the calibration parameter, a measurement bias is applied to the heating element, the measurement bias being low enough, e.g. at or below a predefined threshold, to substantially prevent selfheating of the heating element. For example, a bias as a power of 1 mW or below, 1 pW or below, may be applied. The person skilled in the art will understand that the value of the applied bias will depend on the size of the heating element, its design and the material of which it is made.

[0120] Figure 2C shows part 220” which may be used as part 220 of the method 200 of Figure 2A. Part 220” is similar to part 220’ shown in Figure 2B except that at a step S204”, in order to obtain a measurement of the calibration parameter, a measurement bias is applied to the heating element for a short period of time, e.g. as a pulse, e.g. of duration at or below a predefined threshold, so as to substantially prevent self-heating of the heating element. For example, a bias may be applied for 10 ms or less, 1 ps or less. The person skilled in the art will understand that the time during which the bias will be applied will depend on the size of the heating element, its design and the material of which it is made. To improve accuracy of measurement within a short period of time, circuitry such as a sample and hold circuit or a flash analog to digital converter (ADC) may be used.

[0121] Figure 3 shows a flow-chart of an implementation of a related method 300, which may be implemented as a maintenance routine.

[0122] At step S301 , the heating element is set to a sensing temperature (e.g. a bias applied to the heating element may be set to a sensing bias).

[0123] At step S302, the resistance (or a correlated sensing parameter) of the heating element is measured at the sensing temperature.

[0124] At step S303, the temperature of the heating element is set at a calibration or baseline temperature, by applying a calibration bias to the heating element.

[0125] At step S304 a time period is waited for the temperature of the heating element to stabilise at the calibration temperature. When the temperature of the heating element is stabilised, at step S305, the resistance of the heating element (or a correlated calibration parameter) is measured and, at step S306, the measured value is combined with the value measured at the sensing temperature, so that the value measured at sensing temperature is modified using the value measured at the calibration temperature. An output of the sensor is based on the modified value.

[0126] At steps S302 and S305, it is not necessary that the parameter which is measured is the resistance of the heating element. Any other electrical parameter affected by resistance may be measured, for example voltage, current or power. Alternatively, instead of measuring directly a parameter of the heating element, a sensing element, for example a temperature sensor, may be used to measure the temperature of the heating element. The sensing element may be a resistive temperature sensor, a diode or a transistor, or a temperature sensing circuitry. A temperature, resistance, voltage or current reading of the sensing element may be used for the purpose of the method. The sensing element may either be in close proximity to the heating element so as to have a temperature similar, or substantially equal, to the temperature of the heating element, or it may be further away and have a temperature which is different than the temperature of the heating element, but still affected by the temperature of the heating element.

[0127] Further, there may also be an ambient temperature sensor present, and a reading from the ambient temperature sensor may be obtained at any time, for examples at steps S302, and / or S305, and can be used for determining the sensor reading or output along with the sensing parameter and the calibration parameter.

[0128] Figure 4 shows an implementation of a method 400 as a maintenance routine.

[0129] At step S401 of the method 400, the temperature of the heating element is set at a calibration temperature by applying a calibration bias to the heating element. At step

[0130] 5402 the heating element is allowed to stabilise at the calibration temperature. At step

[0131] 5403 the resistance (or another correlated calibration parameter) of the heating element is measured at the calibration temperature. At step S404 the temperature of the heating element is set at a sensing temperature by applying a sensing bias to the heating element. At step S405 the resistance (or another correlated sensing parameter) of the heating element at the sensing temperature is measured.

[0132] At step S406 the value measured at step S403 is combined with the value measured at step S405 to produce a sensor reading or output. Steps S405 and S406 may be repeated a plurality of times so that the same value measured at step S403 is combined with each value measured at step S405. When a set interval of time has passed, or when a set number of sensor outputs has been reached, or whenever otherwise required, the method loops back to step S401 so that a new value of the resistance (or another correlated calibration parameter) is measured at step S403, to be used with a new set of measurements at the sensing temperature. This is shown in Figure 4 with a loop which repeats until a quantity “q” is lower or equal to “Q”. The quantity “q” may represent a single occurrence of measurement out of a plurality of measurements defined by “Q”. For example, the quantity “q” may represent a point in time when a measurement is obtained within a set interval of time “Q” during which the plurality of measurements are obtained, e.g. q = 1s, 2s, 3s etc. and Q = 10s. In other examples, “q” may represent a “q-th” measurement of a set number of measurements “Q” to be obtained, e.g. q = 1st, 2nd, 3rd, etc. measurement and Q = 10 measurements. It will be appreciated that any other quantity “q” may be chosen as appropriate according to the conditions or application in which the method is used.

[0133] It will be understood that steps S405 and S406 may happen concurrently, that is, step S406 need not wait for all measurements at step S405 to be complete, rather a sensor output may be generated immediately for each measurement.

[0134] As an example, the value of the resistance (or another correlated calibration parameter) of the heating element at the calibration temperature may be measured every 24 hours as a self-calibration routine. During the 24 hours interval between two self-calibration routines, the heating element may be operated at sensing temperature and one or many measurements of heating resistance (or another correlated sensing parameter) may be taken, and each of them may be combined with the value measured at the latest selfcalibration routine. It will be understood that the time interval between two consecutive self-calibration routines may be varied, to be smaller or larger than 24 hours, for example an hour, a week, a month, or any other suitable time period.

[0135] It will further be understood that, alternatively, a new value of the calibration parameter may be measured after each occurrence of steps S405 and S406. For example, if a sensing parameter is to be measured once a minute, the heating element may be kept at sensing temperature for 1 s to take the measurement, and be kept at calibration temperature for the remaining 59 s.

[0136] Similarly to what has been discussed in relation to S302 and S305 of Figure 3, it will be appreciated that, at steps S403 and S405 of Figure 4, it is not necessary that the parameter which is measured is the resistance of the heating element. Any other electrical parameter affected by resistance may be measured, for example voltage, current or power. Alternatively, instead of measuring directly a parameter of the heating element, a sensing element, for example a temperature sensor, may be used to measure the temperature of the heating element. The sensing element may be a resistive temperature sensor, a diode or a transistor, or a temperature sensing circuitry. A temperature, resistance, voltage or current reading of the sensing element may be used for the purpose of the method. The sensing element may either be in close proximity to the heating element so as to have a temperature similar, or substantially equal, to the temperature of the heating element, or it may be further away and have a temperature which is different than the temperature of the heating element, but still affected by the operation of the heating element.

[0137] Figure 5 illustrates a further example of a method according to the present disclosure, which may be implemented as a maintenance routine.

[0138] The method 500 of Figure 5 requires a temperature sensor arranged so that the temperature sensor senses any temperature variation occurring at the heating element. The temperature sensor may be in close proximity to the heating element and sensing substantially the temperature of the heating element or the temperature sensor may be removed from the heating element as long as the temperature sensor is affected by the temperature (or variations thereof) of the heating element. At step S501 the heating element is operated at a sensing temperature (here defined a target temperature) by applying a sensing bias and controlling it so as to maintain the heating element resistance at a predetermined value R. The predetermined value R corresponds to the target temperature, as determined by using a calibration parameter value obtained from a previous measurement, e.g. a previous in situ calibration or a calibration carried out at production stage. At step S502 the method requires measuring a parameter of the temperature sensor, which is used, at step S502a, to generate a sensor output. The heating element is set at a calibration temperature at step S503 by applying a calibration bias to the heating element. The temperature of the heating element is allowed to stabilise at step S504. At step S505 the resistance (or another correlated calibration parameter) of the heating element at the calibration temperature is measured and at step S506 the value measured at step S505 and the value of the ambient temperature, as measured by an ambient temperature sensor (which may be located on the same chip as the sensor or it may be external), are used to determine the actual value of the resistance of the heating element, e.g. R’, corresponding to the target temperature. R’ may be different from R due to drift of the resistance of the heating element.

[0139] In this way, the value of the calibration parameter measured at the calibration temperature is used to keep the heating element sensing temperature (e.g. the target temperature) from drifting over time.

[0140] It will be understood that, in some implementations of the method, two or a plurality of measurements may be taken at step S502 and combined with a same heating element resistance at calibration temperature, as measured at step S505, as shown by the broken arrows in Figure 5.

[0141] It will be understood that, in some examples, the heating element will provide both the heating element function and the temperature sensor function.

[0142] At steps S502 and S502a, instead of a temperature sensor, it is also possible to use a different sensor that requires to be heated at the sensing temperature to operate. For example a resistive gas sensor may be provided with a sensing material in contact with the heating element and which may be heated by the heating element. At the sensing temperature the electrical resistance of the sensing material may change due to the presence of a target gas. The resistance measurement is used to determine the presence or concentration of the target gas.

[0143] It will be further understood that in examples 300, 400 and 500 of the method of the present disclosure, especially in cases where the heating element also functions as a sensing element, applying a calibration bias to the heating element, may comprise switching the heating element off (i.e. applying a bias equal to zero) and then, in order to obtain a measurement of the calibration parameter, and preferably once the heating element temperature is stabilised, applying a measurement bias such that the measurement bias prevents self-heating of the heating element. The measurement bias may be low enough, e.g. at or below a predefined threshold, to substantially avoid selfheating of the heating element, and / or the measurement bias may be applied for a short time, e.g. a pulse, of duration at or below a predefined threshold, so as to substantially avoid self-heating of the heating element. For example, a bias as a power of 1 mW or below, 1 pW or below, may be applied. For example, a bias may be applied for 10 ms or less, 1 ps or less. The person skilled in the art will understand that value of the applied bias and / or the time during which the bias will be applied will depend on the size of the heating element, its design and the material of which it is made.

[0144] In some implementations, the sensor of the disclosure is further provided with a sensing element, for example a temperature sensor in close proximity, e.g. in thermal contact, with the heating element so that the control and readout circuit is configured to take a reading from the sensing element and, from that, to infer the calibration parameter (e.g. the resistance or anther correlated calibration parameter) of the heating element, so that the maintenance method is based on an indirect measurement rather than on a direct measurement of the calibration parameter. The sensing element may be a resistor, or it may be another device, such as a diode, a thermopile, a transistor, or a temperature measurement circuit. It will be understood that any of the methods 200, 300, 400, 500 described herein and illustrated in Figures 2 to 5 may be implemented using software, firmware, hardware (e.g., fixed logic circuitry), or a combination thereof.

[0145] For example, a non-transitory computer readable medium may comprise instructions which, when executed by a processor of a computing device, cause the computing device to carry out any of the methods 200, 300, 400, 500 described herein and illustrated in Figures 2 to 5.

[0146] The instructions may be provided on one or more carriers. For example, there may be one or more non-transient memories, e.g. a EEPROM (e.g. a flash memory) a disk, CD- or DVD-ROM, programmed memory such as read-only memory (e.g. for Firmware), one or more transient memories (e.g. RAM), and / or a data carrier(s) such as an optical or electrical signal carrier. The memory / memories may be integrated into a corresponding processing chip and / or separate to the chip. Code (and / or data) to implement embodiments of the present disclosure may comprise source, object or executable code in a conventional programming language (interpreted or compiled) such as C, 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.

[0147] The control circuit 110 may comprise any type of circuitry such as a differential amplifier, buffer, analog to digital converter, a microcontroller and / or microprocessor, volatile or non-volatile memory, etc.

[0148] In some examples, the control circuit 110 comprises the non-transitory computer readable medium described above.

[0149] The control circuitry may be on the same substrate or chip as the heating element, or on a separate chip but within the same package, or it may be outside the package. The control circuitry may comprise various portions in different locations - for example, some parts of the circuitry on the same chip, and some outside the package on a PCB where the chip is soldered. It will be understood that a “sensor output” referred to herein may correspond to an obtained sensor reading, as described herein. While the description above has focused in particular on thermal conductivity sensors, it will be understood that the present disclosure may also be applicable to other kinds of sensors comprising heating elements.

[0150] Although the disclosure has been described in terms of preferred embodiments as set forth above, it should be understood that these embodiments are illustrative only and that the claims are not limited to those embodiments. Those skilled in the art will be able to make modifications and alternatives in view of the disclosure, which are contemplated as falling within the scope of the appended claims. Each feature disclosed or illustrated in the present specification may be incorporated in any embodiments, whether alone or in any appropriate combination with any other feature disclosed or illustrated herein.

Claims

CLAIMS:

1. A sensor comprising: a heating element; and a control circuit; wherein the control circuit is configured to: set a bias applied to the heating element to a calibration bias; obtain a measurement of a calibration parameter while the heating element is at the calibration bias, the calibration bias corresponding to a heating element temperature at which the calibration parameter is substantially independent of a stimulus applied to the sensor; set the bias applied to the heating element to a sensing bias; obtain a measurement of a sensing parameter while the heating element is at the sensing bias, the sensing parameter corresponding to an interaction between the sensor and the stimulus; and obtain a sensor reading using the sensing parameter and the calibration parameter.

2. The sensor according to claim 1 , wherein the control circuit is configured to obtain the sensor reading by adjusting the sensing parameter based on the measurement of the calibration parameter.

3. The sensor according to claim 1, wherein the control circuit is configured to determine the sensing bias based on the measurement of the calibration parameter.

4. The sensor according to any one of the preceding claims, wherein the control circuit is configured to obtain the measurement of the calibration parameter at a predetermined time after setting the heating element to the calibration bias.

5. The sensor according to any one of the preceding claims, wherein the control circuit is configured to monitor an actual temperature of the heating element at the calibration bias, and to obtain the measurement of the calibration parameterwhen a difference between the actual temperature and a nominal calibration temperature value is at or below a predetermined threshold value.

6. The sensor according to any one of the preceding claims, wherein the control circuit is configured to monitor an actual sensing temperature of the heating element when the heating element is at the sensing bias, and to adjust the sensing bias so that the actual sensing temperature is brought to and maintained at, or about, a nominal sensing temperature value.

7. The sensor according to any one of the preceding claims, wherein the heating element is configured to function as a sensing element.

8. The sensor according to any one of claims 1 to 6, further comprising a sensing element.

9. The sensor according to any one of the preceding claims, further comprising a temperature sensor.

10. The sensor according to claim 9, wherein the control circuit is configured to obtain, using the temperature sensor, a measured temperature of the heating element while the heating element is at the calibration bias, and to obtain the sensor reading based additionally on the measured temperature.

11. The sensor according to any one of the preceding claims, further comprising an ambient temperature sensor.

12. The sensor according to claim 11 , wherein the control circuit is configured to obtain, using the ambient temperature sensor, a measured ambient temperature, and to obtain the sensor reading based additionally on the measured ambient temperature.

13. The sensor according to any one of the preceding claims, wherein the control circuit is configured to obtain a measurement of the calibration parameter for each sensor reading to be obtained.

14. The sensor according to any one of claims 1 to 12, wherein the control circuit is configured to obtain two or more sensor readings based on an obtained measurement of the calibration parameter.

15. The sensor according to any one of the preceding claims, wherein the sensor is a thermal conductivity based sensor.

16. The sensor according to any one of the preceding claims, wherein the sensor is a thermal fluid flow sensor.

17. The sensor according to any one of the preceding claims, further comprising: a substrate comprising a cavity; and a dielectric layer disposed over the substrate and the cavity, wherein the dielectric layer comprises at least one dielectric membrane located over the cavity; wherein the heating element is on or within the dielectric membrane.

18. A method of operating a sensor, the method comprising: setting a bias applied to a heating element of the sensor to a calibration bias; obtaining a measurement of a calibration parameter while the heating element is at the calibration bias, the calibration bias corresponding to a heating element temperature at which the calibration parameter is substantially independent of a stimulus applied to the sensor; setting the bias applied to the heating element to a sensing bias; obtaining a measurement of a sensing parameter while the heating element is at the sensing bias, the sensing parameter corresponding to an interaction between the sensor and the stimulus; andobtaining a sensor reading using the sensing parameter and the calibration parameter.

19. The method according to claim 18, wherein obtaining the sensor reading comprises adjusting the sensing parameter based on the measurement of the calibration parameter.

20. The method according to claim 18, comprising determining the sensing bias based on the measurement of the calibration parameter.

21. The method according to any one of claims 18 to 20, calibration bias is approximately zero.

22. The method according to any one of claims 18 to 20, wherein the calibration bias is zero.

23. The method according to claim 22, wherein obtaining a measurement of the calibration parameter comprises waiting for the temperature of the heating element to stabilise at the calibration temperature and setting the bias applied to the heating element to a measurement bias.

24. The method according to claim 23, wherein the measurement bias is at or below a predefined threshold so as to prevent self-heating of the heating element.

25. The method according to any one of claims 22 to 24, wherein the measurement bias is applied for a period of time at or below a predefined threshold so as to prevent self-heating of the heating element.

26. The method according to any one of claims 18 to 25, comprising obtaining the measurement of the calibration parameter at a predetermined time after setting the heating element to the calibration bias.

27. The method according to any one of claims 18 to 26, comprising monitoring an actual temperature of the heating element at the calibration bias, and obtaining the measurement of the calibration parameter when a difference between the actual temperature and a nominal calibration temperature value is at or below a predetermined threshold value.

28. The method according to any one of claims 18 to 27, wherein the calibration bias is lower than the sensing bias.

29. The method according to any one of claims 18 to 28, wherein the calibration bias corresponds to a heating element temperature that is substantially equal to an ambient temperature.

30. The method according to any one of claims 18 to 29, comprising obtaining the sensor reading using additionally a measurement of the temperature of the heating element while the heating element is set to the calibration bias.

31. The method according to any one of claims 18 to 30, comprising obtaining a measurement of the calibration parameter for each sensor reading to be obtained.

32. The method according to any one of claims 18 to 30, comprising obtaining two or more sensor readings based on an obtained measurement of the calibration parameter.

33. The method according to any one of claims 18 to 32, comprising monitoring an actual sensing temperature of the heating element when the heating element is at the sensing bias, and adjusting the sensing bias so that the actual sensing temperature is brought to and maintained at, or about, a nominal sensing temperature value.

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