Abstract:
In an air-velocity sensor and a method for operating an air-velocity sensor, the air-velocity sensor includes a temperature sensor arranged in an air flow and sequential electronics. The sequential electronics are used to cyclically impinge upon the temperature sensor with a define heat output and to determine the temperature rise time required by the temperature sensor to heat up to a given temperature difference after being impinged upon with the heat output. Temperature measurements are performed at a defined measuring frequency. The velocity of the air flow is determined on the basis of temperature rise time.

Description:
FIELD OF THE INVENTION 
   The present invention relates to an air-velocity sensor and to a method for operating an air-velocity sensor. 
   BACKGROUND INFORMATION 
   Air-velocity sensors, which are based on the so-called constant-temperature or excess-temperature method and are normally referred to as hot-film anemometers, are conventional for measuring air-flow velocities. Such air-velocity sensors are marketed by the Applicant hereof under, for instance, model designations EE60, EE61, EE62, EE65 or EE70. Such air-velocity sensors include, inter alia, two separate sensors, which are positioned in the flowing medium, e.g., air, whose flow velocity should be determined. In this connection, a first sensor, the temperature sensor, is used for determining the temperature of the flowing medium and tracks its temperature as accurately as possible. The second sensor, the heat sensor, constitutes, in principle, a temperature sensor as well and is adjusted to a constant temperature difference with respect to the first sensor by supplying electric power. The electrical heating power necessary for this represents a direct measure of the mass flow rate or the flow velocity to be determined. 
   Air-velocity sensors constructed in such a manner require a relatively high heating power. Thus, if such an air-velocity sensor is intended to be operated by a battery, then the result is only very short operating times. When two batteries having each an operating voltage of 1.5 V and a capacity of approximately 2600 mAh are used, the result is typically a possible operating time between just 10 and 20 hours. 
   An operation, which saves as much power as possible, is, for example, possible, when such an air-velocity sensor is cyclically heated and repeated cools down in the air flow in question. The current air velocity may then be deduced from the required heating time. Reference is made, for example, to U.S. Pat. No. 4,501,145, German Published Patent Application No. 36 37 497 or German Published Patent Application No. 199 39 942 for such variants of measuring air velocity. 
   It is an aspect of the present invention to provide a further improved air-velocity sensor, as well as an improved method for operating an air-velocity sensor in a power-saving manner. 
   SUMMARY 
   The foregoing and other beneficial aspects may be achieved by providing a air-velocity sensor having the features described herein. 
   Example embodiments of the air-velocity sensor according to the present invention are described herein. 
   In addition, the foregoing aspect may be achieved by providing a method for operating an air-velocity sensor, having the features described herein. 
   Example embodiments of the method according to the present invention are described herein. 
   It is provided, that only one single sensor element in the form of a temperature sensor is to be used in the air-velocity sensor of an example embodiment of the present invention, which is cyclically or periodically acted upon by a certain heating power and, therefore, not heated for most of the time during the actual measuring operation. To determine the air velocity, the heating time needed by the temperature sensor to heat up by a particular temperature difference is measured. The ascertained heating time represents the measure for the specific flow velocity. The relationship between heating time and flow velocity evaluated in this connection is ascertained in one or more calibration measurements prior to the actual measuring operation, and a corresponding calibration curve is stored in the sequential electronics. 
   Therefore, since the temperature sensor only has to be heated within the specific heating cycles, the result may be a significant reduction in the current consumption or power consumption, which allows, in turn, markedly longer operating times, e.g., during battery operation. 
   In addition, a simple overall system may be produced, since, in comparison with the constant-temperature variant of an air-velocity sensor mentioned at the outset, a controller may no longer be necessary. 
   In addition, a simple interface may result between the air-velocity sensor and any post-connected microcontrollers, since only the signals of a single sensor element must be further processed. 
   The sensor and method may also be used for measuring the flow velocity of other gaseous media. Furthermore, there are diverse possibilities regarding how and where the sequential electronics of the air-velocity sensor may be positioned. Thus, it is equally possible, for instance, to position them in direct proximity to the temperature sensor, or to position them spatially further away via a suitable signal connection. 
   In addition, the sensor and method may be used for both the measurement of air velocity explained below and the measurement of mass flow rate. 
   In an example embodiment of the present invention, an air-velocity sensor includes: a temperature sensor configured to be arranged in an air flow; and sequential electronics configured to cyclically apply a predetermined heating power to the temperature sensor and to determine a heating time required by the temperature sensor to heat up by a predetermined temperature difference after being acted upon by the heating power, the sequential electronics configured to perform a regular temperature measurement at a predetermined measuring frequency to determine the heating time, the measuring frequency selected as a function of a flow velocity of the air flow and adapted to the flow velocity during a continuous measuring operation, the flow velocity determinable from the heating time. 
   In accordance with an example embodiment of the present invention, a method for operating an air-velocity sensor, which includes a temperature sensor arranged in an air flow and sequential electronics, includes: cyclically acting upon the temperature sensor by a predetermined heating power by the sequential electronics; determining by the sequential electronics a heating time required by the temperature sensor to heat up by an amount of a predefined temperature difference after being acted upon by the heating power, including performing a regular temperature measurement by the sequential electronics at a predetermined measuring frequency to determine the heating time; determining a flow velocity of the air flow from the heating time; and selecting the measuring frequency as a function of the flow velocity and adapting the measuring frequency to the current velocity during a continuous measuring operation. 
   In accordance with an example embodiment of the present invention, an air-velocity sensor includes: a temperature sensing means configured to be arranged in an air flow; and sequential electronics means for cyclically applying a predetermined heating power to the temperature sensor and for determining a heating time required by the temperature sensor to heat up by a predetermined temperature difference after being acted upon by the heating power, the sequential electronics means for performing a regular temperature measurement at a predetermined measuring frequency to determine the heating time, the measuring frequency selected as a function of a flow velocity of the air flow and adapted to the flow velocity during a continuous measuring operation, the flow velocity determinable from the heating time. 
   Further aspects and details pertaining to the sensor and method are set forth in the following description of an exemplary embodiment, on the basis of the appended Figures. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a schematic view of an exemplary embodiment of the air-velocity sensor according to the present invention, positioned on a tube through which the flowing medium travels. 
       FIG. 2  illustrates the relationship between heating time t j  and excess temperature T j  of the temperature sensor in the case of flow velocity v S =0 m/sec. 
       FIG. 3  illustrates the relationship between heating time t j  and excess temperature T j  of the temperature sensor in the case of flow velocity v S =35 m/sec. 
       FIG. 4  is a block diagram of the air-velocity sensor illustrated in  FIG. 1  for explaining the functioning principle. 
   

   DETAILED DESCRIPTION 
   An exemplary embodiment of the air-velocity sensor of the present invention, which is used to determine the speed of the air flowing through a tube  100 , is shown in  FIG. 1  in a highly schematized form. The air-velocity sensor used for this includes a sensor element in the form of a temperature sensor  10 , and sequential electronics  20 , which are connected to temperature sensor  10  by a connecting line  50 . 
   In the represented example, temperature sensor  10  is situated in the air flow, i.e., in tube  100 , whereas sequential electronics  20  are situated in a housing, which is attached to the outer wall of tube  100 . 
   As a alternative to such a design, it is possible to position the air-velocity sensor on a probe tube  11 , which is then situated, on its part, in the air flow in question. In this case, the temperature sensor may be situated, for example, at the tip of the probe tube, and the sequential electronics may be positioned spatially separately from this, on the housing of the probe tube  11 . 
   The air-velocity sensor is operated in such a manner, that temperature sensor  10  is cyclically acted upon by a specific heating power, via sequential electronics  20 . Heating time Δt j , which temperature sensor  10  requires for heating up by the amount of specific, predefined temperature difference ΔT j  after being acted upon by the heating power, is determined by sequential electronics  20 . Flow velocity v S  in question may then be determined from ascertained heating time Δt j . 
   For a further explanation of this procedure, reference is made to  FIGS. 2 and 3 , which each illustrate the relationship between heating time t j  as of the start of measuring, and temperature or excess temperature T j  of the temperature sensor, for different flow velocities v S . In this connection, the relationship in the case of flow velocity v S =0 m/sec is represented in  FIG. 2 , while  FIG. 3  illustrates the corresponding relationship in the case of flow velocity v S =35 m/sec. As is apparent from the two representations, the temperature sensor may require, in the case of higher flow velocity v S =35 m/sec, considerably more time to reach a specified excess temperature T j =10 K. Thus, according to  FIG. 2 , heating time Δt j  is only approximately equal to 0.1 sec for a temperature difference ΔT j =10 K at flow velocity v S =0 m/sec, while, according to  FIG. 3 , corresponding heating time Δt j  approximately equals 0.19 sec at a flow velocity v S =35 m/sec. 
   Therefore, flow velocity v S  in question may be determined from heating time Δt j  needed by the temperature sensor to heat up by the amount of a particular, specified temperature difference ΔT j . Thus, at least one calibration curve, which indicates the relationship between heating time Δt j  and corresponding flow velocity v S  for the instance of predefined heating by the amount of a specific temperature difference ΔT j  of the temperature sensor, must be recorded prior to the actual measurement. In the course of the actual measurement, the calibration curve is used for determining flow velocity v S  from measured heating times Δt j . The calibration curve is stored in a suitable storage medium of the sequential electronics. 
   As long as the heating pulse in question is applied to the temperature sensor, the temperature of the temperature sensor is cyclically determined at defined measuring times t M,i , until the temperature sensor is heated by the amount of predefined temperature difference ΔT j . As is explained in detail in the following, the temperature may be determined here, from the measured resistance values of the temperature sensor, which is why a suitable voltage measurement is carried out. In order to determine the temperature of the temperature sensor as precisely as possible, it may be provided to amplify the measured voltage with the aid of an amplifier circuit, before the measured voltage is transmitted to a microprocessor in the sequential electronics for further processing. 
   In this connection, the mentioned, cyclical temperature measurement is carried out at specific measuring times t M,i , at a specific measuring frequency f M . In this connection, it may be provided that, when measuring frequency f M  is selected as a function of flow velocity v S  or continuously adapted to it during the measuring operation, for instance, sharply changing flow velocities render this necessary. In this respect, this is practical, since it takes a relatively long time, particularly in the case of low flow velocities v S  (&lt;0.5 m/sec), for the heated temperature sensor to cool down again to the ambient temperature. Therefore, a lower measuring frequency f M  may be used for the cyclical temperature measurement at low flow velocities v S , whereas a higher measuring frequency f M  is used at higher flow velocities v S . Thus, measuring frequency f M  is selected to be proportional to flow velocity v S . For example, a measuring frequency f M =0.2 sec −1  may be provided, for instance, for flow velocities v S &lt;0.5 m/sec, whereas a measuring frequency =1.0 sec −1  may be provided for flow velocities v S =30 m/sec. Therefore, a linear function describes the relationship between flow velocity v S  and measuring frequency f M  in the present example. 
   In continuous measuring operation, such an adjustment of measuring frequency f M  to specific, current flow velocity v S  is accomplished, in that, on the basis of a flow velocity v S  just ascertained, measuring frequency f M  for the subsequent measurements may be modified when ascertained flow velocity v S  requires this. 
   Furthermore, it may be provided that, after the start of the temperature sensor being acted upon by the heating power, the regular temperature measurement is only begun after a defined delay time t DEL . In this connection, delay time t DEL  is selected to be large enough, that sequential electronics in a steady-state condition are available after delay time t DEL  has elapsed. Otherwise, the measuring uncertainty is high. In an example embodiment, corresponding delay time t DEL  is selected to be 200 μsec. 
   Since the exact resistance and, therefore, the temperature of the temperature sensor is known for each of the individual measuring times t M,i , heating time Δt j  may also be precisely determined by suitably interpolating between adjacent measuring times t M,i , t M,i+1 , when, for instance, predefined temperature difference ΔT j  is reached between adjacent measuring times t M,i , t M,i+1 . 
   As soon as the temperature sensor is heated up by the amount of predefined temperature difference ΔT j  during the measuring operation, after being acted upon by the specific heating power, the heating of the temperature sensor is ended. After a certain cycle time, e.g., after one second, the temperature sensor is acted upon again by the heating power in question, and so on. Therefore, the temperature sensor is not heated for most of the time during the measuring operation, i.e., the resulting current consumption may be considerably less than conventional air-velocity sensors, which function according to the constant-temperature method. 
   After flow velocity v S  has been determined in the explained manner, it may be visualized, for example, on a display unit or converted into a transmittable signal and transmitted to a post-connected evaluation unit for further processing. 
   The construction of an exemplary embodiment of the air-velocity sensor according to the present invention, including temperature sensor  10  and sequential electronics  20 , is explained on the basis of the block diagram in  FIG. 4 . 
   Compared to a minimal variant, the variant of the air-velocity sensor shown in the figure represents an example embodiment, which is supplemented on the side of sequential electronics  20  with an amplifier stage  23  having several components R 1  to R 4 ,  22 . 
   In addition to temperature sensor  10 , it may be necessary for the air-velocity sensor to have at least one series resistor Rv connected in series with it, a voltage supply Us, as well as a switch element S 1 , which is used to apply voltage supply Us to temperature sensor  10  at specific times. 
   In order to determine the temperature at temperature sensor  10 , a voltage measurement, which supplies measuring voltage U 1 , and from which the temperature of temperature sensor  10  may be determined on the basis of the known resistance-temperature characteristic in a conventional manner, is carried out at node K between temperature sensor  10  and series resistor Rv. 
   As previously explained above, an amplifier stage  23  is additionally provided in the represented exemplary embodiment, on the side of sequential electronics  20 ; measuring voltage U 1  being amplified via the amplifier stage by a desired amplification factor V, e.g., V=20, before the voltage is transmitted via output Out to microprocessor  21  for further processing. This measure may ensure that no high-resolution A/D converter may be necessary in front of microprocessor  21  to measure voltage in a highly precise manner. Rather, A/D converters  21 . 1 , which are already integrated in microprocessor  21  on the input side and provide resolutions between 8 and 12 bits, may be used. 
   Both temperature sensor  10  and series resistor Rv may have a conventional design. In this connection, temperature sensor  10  includes, more or less, a thin, glass supporting plate, to which conductor tracks made of material having a temperature-dependent resistance, e.g., molybdenum, nickel, platinum, etc., are applied. The temperature sensor  10  need not be a standard temperature sensor, such as a Pt1000 element. Rather, it may be sufficient when utilized temperature sensor  10  has a defined resistance-temperature characteristic. Series resistor Rv situated on the side of the sequential electronics is designed to be a standard resistor having good electrical stability, in the form of a metallic-film resistor of the model MiniMELF or 0805. 
   A semiconductor switch element, e.g., an FET, is used as switch element S 1 , and one or more batteries are provided as voltage supply Us. 
   In addition, sequential electronics  20  include microprocessor  21 , which may assume a number of functions. These include the operation of switch element S 1 , the cyclical determination of the resistance of temperature sensor  10 , the determination of the temperature of temperature sensor  10  from the resistance values, as well as the determination of the heating time required until the predefined temperature difference is reached. In the present exemplary embodiment, the determination of flow velocity v S  from previously ascertained heating time Δt j  is also accomplished by microprocessor  21 . 
   At the beginning of a measurement, temperature sensor  10  and series resistor Rv are connected to voltage supply Us by switch element S 1 , which is operated by microprocessor  21 . As soon as switch element S 1  is closed the first time, measuring voltage U 1  is measured for the first time. The electrical resistance of temperature sensor  10  may be ascertained from measuring voltage U 1  and the measuring current in a conventional manner. Since the resistance of temperature sensor  10  changes in proportion with the temperature of temperature sensor  10 , predefined temperature difference ΔT may be directly expressed as resistance change ΔR. This resistance change ΔR is only systematically dependent on the specific ambient temperature to a small extent. The influence of the ambient temperature may be disregarded when the accuracy requirements are low. In the case of higher accuracy requirements, it may be easily possible to mathematically compensate for this. 
   In principle, the air-velocity sensor may be regarded as a mass flow sensor. This means that a calibration curve generated once only indicates the relationship between heating time Δt j  and air velocity v S  to be determined, for a specific temperature. In the case of higher temperatures, the density of the air and, therefore, the measured mass flow decreases in reverse proportion to the absolute temperature. This known relationship may be utilized for arithmetic compensation, since the temperature of the air may be ascertained from the resistance of temperature sensor  10 . Therefore, the device may operate as an air-velocity sensor in the described manner, when such compensation is automatically carried out in the case of changing temperatures. However, it is also possible, in principal, to dispense with such compensation, so that the device actually operates as a mass-flow sensor and indicates the amount of air flowing through a defined cross-section per unit time. 
   At specific measuring times t M , the determination of measuring voltage U 1  is repeated and the respective resistance of temperature sensor  10  is ascertained in fixed time segments, i.e., at measuring frequency f M  previously discussed, e.g., every 5 msec, while being controlled by microprocessor  21 . The heating of temperature sensor  10  is monitored by microprocessor  21 , until resistance change ΔR of temperature sensor  10  exceeds a particular threshold value, i.e., until temperature sensor  10  is heated up by the amount of predefined temperature difference ΔT j . As soon as this is the case, temperature sensor  10  is separated again from voltage supply Us by opening switch element S 1 . Required and measured heating time Δt j  between the beginning of the measurement and the attainment of predefined resistance change ΔR is, in defined form, a function of flow velocity v S  and the specific heating power at temperature sensor  10 . In the case of a constant series resistance Rv, the heating power is only a minimal function of the heating of temperature sensor  10 . This dependence may be disregarded in practice. The dependence on the supplied voltage supply is more marked. Therefore, if the constancy of the voltage supply cannot be adequately ensured, then it is possible to arithmetically compensate for its influence. 
   Furthermore, the suitable dimensioning of series resistor Rv may ensure that, even in the case of a minimal heating power and a maximum expectable ambient temperature, the predefined temperature difference may be reached during the heating. 
   Therefore, ascertained heating time Δt j  may be used as a direct measure of flow velocity v S . In microprocessor  21 , flow velocity v S  may be determined from heating time Δt j  with the aid of one or more calibration curves, and the mentioned, possibly necessary corrections and compensation are carried out. 
   The result of this may be a simple system for determining flow velocities, which additionally may render the required, power-saving operation possible.