Thermal flowmeter and method having a self-heated element controlled to operate differently under high and low phases of square wave signal

Abstract

A thermal flowmeter comprises a heating element thermally coupled to a flowing medium, a first temperature sensor and second temperature sensor. The first temperature sensor detects a flowing medium temperature in a region not affected by the heating element. The second temperature sensor detects a temperature of the heating element and also serves as a heating element. A control unit controls heating power of the heating element based on the difference between the detected temperatures at a predetermined time. The control unit provides a square wave signal to the second temperature sensor so that in a high phase it acts as heating element and in a low phase as a temperature sensor.

Claims

1. A thermal flowmeter for use in the process measuring technology at non-constant medium temperatures, comprising a heating element which is thermally coupled to the flowing medium and thus transfers its heating power to the flowing medium, and two temperature sensors (R.sub.SM, R.sub.H), which are supplied with a measuring current via a respective current source, wherein a first temperature sensor (R.sub.SM) detects the temperature of the medium in a region of the heating medium that is not affected by the heating element as a reference temperature T.sub.Medium and a second temperature sensor (R.sub.H) detects the temperature T.sub.Heiz of the heating element, wherein the second temperature sensor (R.sub.H) also serves as the heating element, wherein the two temperature sensors (R.sub.SM, R.sub.H) and, thus, also the heating element (R.sub.H) are connected to a control unit, which provides a square wave signal which is supplied to the second temperature sensor (R.sub.H) so that in the high phase it acts as a heating element and in the low phase as a temperature sensor, wherein the control unit controls the heating power of the heating element formed by the second temperature sensor (R.sub.H) so that by means of a timing element the temperature difference T=T.sub.HeizT.sub.Medium assumes a predetermined value at a predetermined time t.sub.Mess after the switch-off edge of the square wave signal, and wherein for controlling a duty cycle of the square wave signal, which is a measure of the flow rate of the flowing medium, the control unit is arranged to shorten the pulse width of the square wave signal in the case T>T.sub.Soll so that the second temperature sensor (R.sub.H) is heated for a shorter time, and to extend the pulse width of the square wave signal in the case T<T.sub.Soll, so that the second temperature sensor (R.sub.H) is heated for a longer time.

2. The flowmeter according to claim 1, wherein the control unit is configured as a microcontroller.

3. The flowmeter according to claim 2, wherein the square wave signal is configured as a PWM signal (pulse width modulated signal).

4. The flowmeter according to claim 1, wherein the square wave signal is configured as a PWM signal (pulse width modulated signal).

5. The flowmeter according to claim 1, wherein the temperature difference T is determined directly from the measurements of a voltage difference of the two temperature sensors (R.sub.SM, R.sub.H) by means of a differential amplifier and supplied to the control unit.

6. A method for operating a thermal flowmeter for application in the process measuring technology at non-constant medium temperatures, comprising a heating element which is thermally coupled to the flowing medium and thus transmits its heating power to the flowing medium, and two temperature sensors (R.sub.SM, R.sub.H), which are supplied with a measuring current via respective current sources, wherein a first temperature sensor (R.sub.SM) detects the temperature of the medium T.sub.Medium in a region of the medium that is not affected by the heating element as a reference temperature T.sub.Medium and a second temperature sensor (R.sub.H) detects the temperature T.sub.Heiz of the heating element, wherein the second temperature sensor (R.sub.H) also serves as the heating element, wherein both temperature sensors (R.sub.SM, R.sub.H) and, thus, also the heating element (R.sub.H) are connected to a control unit, which provides a square wave signal which is supplied to the second temperature sensor (R.sub.H) and controls the heating power of the heating element (R.sub.H) so that by means of a timing element the temperature difference T=T.sub.HeizT.sub.Medium assumes a predetermined value at a predetermined time t.sub.Mess after the switch-off edge of the square wave signal, wherein the temperature difference T is determined periodically only at the specified time point t.sub.Mess after the switch-off edge of the square wave signal, wherein the control unit compares the measured temperature difference T with a predetermined value T.sub.Soll, wherein for controlling a duty cycle of the square wave signal, which is a measure of the flow rate of the flowing medium the control unit in the case T>T.sub.Soll shortens the pulse width of the square wave signal so that the second temperature sensor (R.sub.H) is heated for a shorter time, and extends the pulse width of the square wave signal in the case T<T.sub.Soll the pulse width of, so that the second temperature sensor (R.sub.H) is heated for a longer time.

7. The method according to claim 6, wherein the control unit is configured as a microcontroller.

8. The method according to claim 7, wherein the square wave signal is configured as a PWM signal (pulse width modulated signal).

9. The method according to claim 7, wherein the temperature difference T is determined directly from the measurements of a voltage difference of the two temperature sensors (R.sub.SM, R.sub.H) and supplied to the control unit.

10. The method according to claim 6, wherein the square wave signal is configured as a PWM signal (pulse width modulated signal).

Description

DRAWINGS

(1) The drawings described herein are for illustrative purposes only of selected embodiments and not all possible implementations, and are not intended to limit the scope of the present disclosure.

(2) The disclosure will be described below in more detail based on an exemplary embodiment with reference to the drawings.

(3) The drawings schematically show:

(4) FIG. 1 a principal circuit structure of a flowmeter according to the disclosure;

(5) FIG. 2 a diagram representing a square wave signal and the corresponding curve of the measured temperature T in a first scenario;

(6) FIG. 2b a diagram showing a square wave signal and the corresponding curve of the measured temperature T in a second scenario (lower flow than in FIG. 2a); and

(7) FIG. 2c a diagram showing a square wave signal and the corresponding curve of the measured temperature T in a third scenario (larger flow than in FIG. 2a).

DETAILED DESCRIPTION

(8) Example embodiments will now be described more fully with reference to the accompanying drawings.

(9) In the following description of the preferred embodiments, like reference symbols designate like or similar components.

(10) FIG. 1 shows the principal circuit structure of the flowmeter according to the disclosure, wherein the focus is placed on the illustration of the disclosure and the explanation of measures obvious for a person skilled in the art, such as the supply of the control unit 10, have been omitted.

(11) At the bottom right, the measuring tube 1 is shown, through which the medium to be measured flows. The flow is indicated by the arrow. The two temperature sensors R.sub.SM and R.sub.H are disposed within the measuring tube 1 and thus in thermal contact with the medium. The first temperature sensor R.sub.SM detects the temperature of the medium in a non-heated region, i.e. in a region which is not affected by the heating element, of the medium as the reference temperature T.sub.Medium and the temperature sensor R.sub.H detects the temperature T.sub.Heiz of the heating element. Both temperature sensors R.sub.SM, R.sub.H are advantageously configured as a Pt element, preferably as Pt100.

(12) The system is operated in a pulsed way, i.e. the heating power is provided by the control unit 10 via a square wave signal, in this exemplary embodiment preferably as a PWM signal, at its output 10a. The control unit 10 is in this case preferably configured as a microcontroller.

(13) Here, the temperature sensor R.sub.H is used both for measuring the temperature and for heating the medium, which is to be indicated by the switch 12. By means of the square wave signal, which is supplied to the second temperature sensor R.sub.H, the temperature sensor R.sub.H acts as a heating element during the high phase and as a temperature sensor in the low phase. The measuring current required for the temperature detection is provided during this time by the power source 13b. The current source 13a is provided in parallel thereto for the supply of the temperature sensor R.sub.SM.

(14) The voltage drops across the Pt elements R.sub.SM and R.sub.H are supplied to a differential amplifier 14, the output signal T of which in turn is supplied to the microcontroller 10 at the input 10b. Consequently, T is determined directly from the measurement of the voltage difference between the two temperature sensors R.sub.SM and R.sub.H. This requires two similar current sources 13a and 13b, which ensure that low measuring currents of the same magnitude flow through R.sub.SM and R.sub.H. The determination of the temperature difference T is thus possible only during the time in which the heating element R.sub.H is not supplied with a high heating current, which was achieved by the measurement of the temperature difference T at the specified time t.sub.Mess after the switch-off edge of the PWM signal, i.e. in the low phase. The switching between the heating and the measuring operation is realized by the switch 12.

(15) The assignment of the respective duty cycle of the PWM signal to the corresponding flow rate is made via a family of characteristics, which is stored in the microcontroller 10. The temperature dependence of the heat transport properties (substance parameters) of the medium can also be mapped via this family of characteristics and thus compensated. To this end, the voltage drop across the first temperature sensor R.sub.SM is amplified by the amplifier 15 and then supplied to the microcontroller 10 at the input 10c where it is used to determine the medium temperature.

(16) In FIG. 2a, the PWM signal (thick line) of the microcontroller 10 and the corresponding curve of the measured temperature difference T (thin line) at an average flow rate can be seen. The following FIGS. 2b and 2c each represent situations in which the flow rate is slowerFIG. 2bor higherFIG. 2c. The three PWM signals differ only in their pulse widths or duty cycles, i.e. the ratio between the high and the low phase. In FIG. 2a, the ratio between the high and the low phase should be approximately 1:1, while in FIG. 2b compared to FIG. 2a a significantly shorter pulse width with a correspondingly longer low phase, and in FIG. 2c a significantly longer pulse width with a correspondingly shorter low phase are shown.

(17) The measurement of the temperature difference T always takes place at a certain and predetermined time T.sub.Mess after the switch-off edge of the PWM signal, i.e. from the beginning of the low phase. Thus, the system attempts to measure a predetermined overtemperature T.sub.Soll at this time point t.sub.Mess. This could, for example, be at 2K. With slow temperature sensors, the period can be several seconds and t.sub.Mess a few milliseconds. For very fast temperature sensors, the period can be reduced to a few milliseconds and also t.sub.Mess can be shortened accordingly.

(18) If, starting from the scenario in FIG. 2 a, the flow rate decreases, this would initially lead to an excess heating of the heating element R.sub.H because of the lower heat dissipation, so that after the switch-off edge the starting temperature for the cooling phase is too high to reach the expected overtemperature T.sub.Soll of, for example 2K, at the time t.sub.Mess. Indeed, at the time t.sub.Mess the expected overtemperature T.sub.Soll is initially exceeded. The microcontroller 10 recognizes this difference and readjusts the heating voltage U.sub.H accordingly by reducing the pulse width and thus the heating time. As a result of this the heating element, now energized for a shorter duration, heats the medium to a lower extent, so that the starting temperature for the cooling phase is now lower. The microcontroller 10 repeats these steps until the pulse width set by it, that is the heating time, causes that at the time t.sub.Mess the expected overtemperature T.sub.Soll of, for example 2 K, is measured again. This condition is illustrated in FIG. 2b.

(19) If, starting from the scenario in FIG. 2a, the flow rate increases, this would initially cause that the heating element R.sub.H is heated insufficiently because of the greater heat dissipation, so that after the switch-off edge the starting temperature for the cooling phase is too low to reach the expected overtemperature T.sub.Soll of, for example 2K, at the time t.sub.Mess. At the time t.sub.Mess the expected overtemperature T.sub.Soll is initially undercut. The microcontroller 10 recognizes this difference and readjusts the heating voltage U.sub.H accordingly by extending the pulse width and thus the heating time. The heating element, now energized for a longer duration, heats the medium as a result to a higher temperature, so that the starting temperature for the cooling phase is now higher. The microcontroller 10 repeats these steps until the pulse width set by it, that is to say the heating time, results in that the expected overtemperature t.sub.Soll is measured again at the time t.sub.Mess. This condition is illustrated in FIG. 2c.

(20) The duration of the heating phase, i.e. the duty cycle of the PWM signal, and thus the consumed heating power is ultimately a measure of the flow rate.

(21) The advantage of this flowmeter is, inter alia, that it can be used universally for various media, since only a software adjustment of the microcontroller 10 is necessary for an adaptation. Furthermore, there is a lower power loss of the circuit through the PWM operation. In summary, by means of the disclosure a higher measurement dynamics can be achieved, i.e. a higher measurement speed and an improved linearization of the characteristics.

(22) The foregoing description of the embodiments has been provided for purposes of illustration and description. It is not intended to be exhaustive or to limit the disclosure. Individual elements or features of a particular embodiment are generally not limited to that particular embodiment, but, where applicable, are interchangeable and can be used in a selected embodiment, even if not specifically shown or described. The same may also be varied in many ways. Such variations are to be regarded as a departure from the disclosure, and all such modifications are intended to be included within the scope of the disclosure.