Abstract:
A flow meter includes a flow sensor for conducting fluid in a measuring direction and an opposite measuring direction, alternatively. Evaluation electronics coupled to the flow sensor generate a sensor signal corresponding to the flow rate of the fluid. A first subcircuit converts the sensor signal to a flow rate signal and a second subcircuit coupled to the first subcircuit generates an output signal representing the flow rate in the measuring direction. A third subcircuit fed by the flow rate signal delivers a control signal for controlling the generation of the output signal. The third subcircuit determines a flow when the fluid flows in the measuring direction and a counterflow when the fluid flows in the opposite direction, and calculates a balanced flow therefrom. Depending on the balanced flow, the third subcircuit controls the second subcircuit with the control signal to generate the output signal in a predetermined manner.

Description:
FIELD OF THE INVENTION 
     The invention relates to a volume or mass flowmeter with a flow sensor and with evaluation electronics. 
     BACKGROUND OF THE INVENTION 
     For the purpose of measuring the flow rate of a fluid flowing in a pipeline or the like there are several principles which are each based on a physical regularity. Irrespective of the electric conductivity of the fluid, its volume flow rate can be measured, for example, by vortex flowmeters based on the Kármán vortex street or by ultrasonic flowmeters or its mass flow rate can be measured, for example, by mass flowmeters which are based on the Coriolis principle, by thermal mass flowmeters or by mass flowmeters which are based on the determination of a pressure difference over an orifice plate. The volume flow rate of electrically conductive fluids can further be measured with electromagnetic flowmeters based on Faraday&#39;s law of induction. 
     The evaluation electronics convert a signal as generated according to one of the aforementioned principles into an output signal proportional to the volume or mass flow rate with high measuring accuracy. For example, this can be the DC current with 4 mA to 20 mA for a given measuring range which has long been used in industrial metrology. However, the invention does not deal with this kind of output signal. 
     Rather, the invention is aimed at the elimination of a disadvantage which can occur in an alternating output signal, which has also been standardized for a long time in industrial metrology and whose frequency is proportional to the flow rate in the given measuring range, whenever the fluid flowing in a measurement direction due to the mechanical arrangement of the flow sensor flows occasionally, and for a short time in particular, in the opposite direction of the measurement. Such a counterflow rate cannot be converted by the evaluation electronics into a respective counterfrequency because, as is well known, an alternating signal cannot have a negative frequency. 
     The output at which the standardized output signal aforementioned at last lies is usually designated in technical manuals and data sheets as pulse/frequency output. 
     The aforementioned counterflow rate occurs, for example, when the flow rate is to be measured in a pipeline in which the fluid is not moved continuously but instead in a pulsating manner, e.g. by metering pumps or, in the case of fluids placed under pressure, by valves which are triggered to be open or closed. Although metering pumps have a very high precision of up to 0.5%, return flows, which also includes counterflow rates, occurring during each metering step cannot be avoided as a result of their principle of design (e.g. reciprocating pump, bellow-type pump or diaphragm pump). 
     Efforts were made up until now to compensate counterflow rates by damping in the evaluation electronics. As a result, the flow rate measurement only reacts with delays to changes in the flow rate and in addition it is necessary that the time constant of the damping has to be adjusted to the flow rate, i.e. the time constant must be changed and must be changeable. 
     SUMMARY OF THE INVENTION 
     It is an object of the present invention to provide a different and more advantageous approach for the elimination of the aforementioned disadvantage. 
     In order to achieve this object, the invention therefore consists in a volume or mass flow meter with a flow sensor and with evaluation electronics, comprising: 
     a first subcircuit for the generation of a flow rate signal, 
     which in the given measuring range is proportional to the flow rate of the fluid to be measured, 
     a second subcircuit for generating an output signal, 
     whose frequency in the given measuring range is proportional to the flow rate of the fluid flowing in a measuring direction determined by the constructional arrangement of the flow sensor, and 
     a third subcircuit, 
     which determines a counterflow rate in an opposite measurement direction during a scanning interval, saves the same and subtracts it from the next measured flow rate. 
     In accordance with a preferred development of the invention the evaluation electronics add up the saved counterflow rates separately and make them available for further processing and/or display. 
     In accordance with a preferred embodiment of the invention and/or its preferred development, the third subcircuit comprises: 
     a clock generator, 
     which periodically generates clock pulses with a predetermined clock period, 
     an averager stage controlled by the clock generator, 
     whose input is supplied with the flow rate signal and whose ouptut supplies an average flow-rate value signal representative of an average taken over the clock period, 
     a write-read memory, 
     a first divider, 
     of which a divisor input is fed with a clock period signal representative of the clock period and 
     of which a dividend input is connected to an output of the write-read memory, 
     a first summer, 
     of which a first input is connected to an output of the averager stage and 
     of which a second input is connected to an output of the first divider, 
     a first multiplier, 
     of which a first input is connected to an output of the first summer, 
     of which a second input is supplied with a setting signal representative of a reciprocal mass or a reciprocal volume and 
     of which an output supplies a frequency signal representative of a frequency, 
     a triple comparator, 
     of which an input is connected to output of the first multiplier and 
     of which an output is connected to an input of the second subcircuit and supplies a signal which 
     represents zero when the frequency signal represents values smaller than zero, 
     is equal to the frequency signal when the same represents values between zero and an adjustable maximum value and 
     represents the maximum value when the freqeuncy signal represents values larger than the maximum value, 
     a second multiplier, 
     of which a first input is connected to the output of the triple comparator and 
     of which a second input is supplied with the clock period signal, 
     a second divider, 
     of which a first input is connected to an output of the second multiplier and 
     of which a second input is supplied with the setting signal, 
     a third multiplier, 
     of which a first input is connected to the output of the averager stage and 
     of which a second input is supplied with the clock period signal, 
     a summer/subtracter of which 
     a subtrahend input is connected to the output of the second divider, 
     a first addend input is connected to an output of the third multiplier, 
     a second addend input is connected to the output of the write-read memory and 
     an output is connected to an input of the write-read memory which is enabled by the clock signal. 
     One advantage of the invention is that a counterflow rate occurring at a specific time will already be considered by balancing during the measurement of the next flow rate value, i.e. in real time. Accordingly, in accordance with the invention the balancing follows a change in the flow rate without any delay, so that it is also not necessary to change any time constants. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention and further advantages are now explained in detail by reference of embodiments shown in the figures and the drawings. The same parts in different figures are provided with the same reference numerals. If required for clarity of the drawings, already previously mentioned reference numerals have been omitted in subsequent figures. Moreover, details that have already been described will not be explained any further in the following figures. 
     FIG. 1 shows a highly simplified block diagram for explaining the principle on which the invention is based; 
     FIG. 2 shows a diagram for illustrating one of the properties of the second subcircuit which property is relevant for the invention; 
     FIG. 3 shows a diagram for illustrating the problem of the invention; 
     FIG. 4 shows, among other things, a block diagram of a preferred embodiment of the second subcircuit, and 
     FIG. 5 shows a diagram for illustrating the properties of the output signal of the triple comparator of the second subcircuit. 
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     The block diagram represented in FIG. 1 shows in a highly simplified and abstract manner the principal arrangement of a volume and mass flowmeter in accordance with the invention. If in the following it should be irrelevant as to whether a volume flowmeter or mass flowmeter is concerned, reference will only be made by using the term flowmeter. The preferred types of flowmeters in which the invention can be employed have already been stated above. 
     The main components of such a flowmeter are as usual a flow sensor  1  and evaluation electronics  2 . The flow sensor  1  substantially comprises mechanical parts (not shown) which are required for the respective measuring principle and its operation. These are at least one measuring tube through which flows the fluid to be measured and, optionally, a housing. 
     At least one exciter arrangement which is specific to the measuring principle and/or at least one sensor arrangement are attached on or in the measuring tube, with the sensor arrangement being used as a physical-electric transducer and generating an output signal which is representative, particularly already directly, of the flow rate. 
     The evaluation electronics  2  process this output signal or the respective output signal of several sensor arrangements in such a way that the flow rate can be displayed on a display for example. In addition, the evaluation electronics  2  transform the flow rate for further electronic processing in at least one suitable other form of signal, in particular the form of signal of the aforementioned frequency output standard relevant to the invention. 
     In the case of electromagnetic flowmeters, the exciter arrangement specific to the measuring principle is an arrangement for producing a magnetic field which penetrates the measuring tube, such as a coil arrangement with an associated coil current generator, and the sensor arrangement specific to the measuring principle comprises at least two electrodes with which a voltage is tapped which is induced as a result of Faraday&#39;s law of induction on the flowing fluid which must be electrically conductive. 
     In the case of vortex flowmeters the exciter arrangement specific to the measuring principle is a bluff body placed against the fluid flowing in the measuring tube from which vortices separate and thus pressure fluctuations occur, and the sensor arrangement specific to the measuring principle comprises at least one sensor element which responds to said pressure fluctuations. 
     In the case of Coriolis mass flowmeters the exciter arrangement specific to the measuring principle acts on at least one measuring tube and excites vibrations of it, in particular resonant vibrations, and the sensor arrangement specific to the measuring principle comprises sensor elements which tap a phase shift between the measuring tube movements on the inlet and outlet sides. 
     In the case of ultrasonic flowmeters the exciter arrangement specific to the measuring principle and the sensor arrangement specific to the measuring principle are operatively interlinked in such a way that a first ultrasonic transducer which is coupled with the measuring tube and a second ultrasonic transducer which is coupled with the measuring tube and is arranged offset from the first ultrasonic transducer are operated periodically alternatingly in such a way that the one acts as transmitter and the other simultaneously as a receiver. Accordingly, the ultrasound is sent through the fluid alternatingly in the direction of flow and in the direction opposite of the flow, so that the flow velocity and thus the flow rate can be determined from the difference in the running time of the ultrasound. 
     In the case of thermal mass flowmeters the exciter arrangement specific to the measuring principle is a heating element arranged in the fluid and the sensor arrangement specific to the measuring principle comprises a temperature probe which detects a temperature difference between itself and the heating element. The temperature difference is dependent on the mass flow rate. 
     In the case of pressure difference mass flowmeters the exciter arrangement specific to the measuring principle is an orifice plate placed in the fluid and restricting the flow cross section, and the sensor arrangement specific to the measuring principle comprises a pressure pick-up in front of the orifice plate and a pressure pick-up behind the orifice plate each leading to a pressure-difference sensor element. 
     In FIG. 1 the evaluation electronics  2  comprise a first subcircuit  21  for generating a flow rate signal “q”. In a given measuring range m the same is proportional with a high precision to the flow rate q of a fluid flowing in the flow sensor  1 . 
     The evaluation electronics  2  further comprise a second subcircuit  22  which according to the aforementioned standard produces an output signal “q c ” whose frequency, in the given measuring range m, is proportional with a high precision to the flow rate q c  which occurs in a measuring direction of the flowing fluid determined by the constructional arrangement of the flow sensor. The second subcircuit  22  can be a square-wave generator for example, whose frequency f is controlled by the flow rate q c  and whose output signal is provided with a predetermined, but fixed, mark-to-space ratio. 
     These connections are shown schematically in FIG.  2 . For the sake of simplicity it is assumed for FIG. 2 that the axes of coordinates each have a linear division, so that the dependence q−f is strictly linear, i.e. it results in a family of straight lines whose parameter is a measuring range m of the flow rate q which is assigned to a maximum value f max  of the frequency f. 
     In FIG. 1, a third subcircuit  23  is disposed between the first and second subcircuit, which third subcircuit determines a counterflow rate −q′ occurring in the opposite measurement direction during a scanning interval δt, stores the same and subtracts it from the next measured flow rate. 
     FIG. 3 is used to explain the disadvantage due to the occurrence of this counterflow rate −q′. FIG. 3 is a diagram in which the flow rate q is entered as ordinate over the abscissa for the time t. 
     The curve of FIG. 3 extends not only above the abscissa, where the q values are positive, but also below the same, so that there are also negative q values −q′. These are caused by the reasons as illustrated above and falsify the result of the measurement and reduce its accuracy. Moreover, FIG. 3 shows a duration at during which the flow rate has an average value q m . 
     The block diagram of FIG. 4 shows the details of a preferred embodiment of evaluation electronics which substantially relate to the third subcircuit  23 . A clock generator  23   1  is used for producing periodic clock pulses which have a predetermined or predeterminable clock period δt, i.e. are equal to the duration δt of FIG.  3 . 
     The variation in time of the clock pulses is shown at the left below the clock generator  23   1 . In this case they are rectangular pulses with a unity mark-to-space ratio and with a suitable frequency. 
     An averager stage  23   2  is controlled by the clock generator  23   1 . The input of the averager stage  23   2  is supplied with the flow rate signal “q”. Its output supplies at the end of each clock pulse an average flow-rate value signal “q m ” which is representative of the average q m  taken over the clock period δt. 
     A divisor input of a first divider  23   3  is supplied with a clock period signal “δt” which is representative of the clock period δt. A dividend input of the divider  23   3  is connected to an output of a write-read memory  23   4  which can be a common RAM or even an EEPROM for example. 
     A first input of a first summer  23   5  is connected to an output of the averager stage  23   2 , a second input to an output of the first divider  23   3  and an output to a first input of a first multiplier  23   6 . A second input of the latter is supplied with a setting signal “k” which is representative of a reciprocal mass or a reciprocal volume. The multiplier  23   6  generates a frequency signal “F” representative of a frequency F at an output. 
     An input of a triple comparator  23   7  is connected to the output of the multiplier  23   6 . One of its outputs is connected to an input of the second subcircuit  22  and supplies the same with a signal s as follows: 
     If the frequency signal “F” represents values lower than zero, the signal s represents zero. 
     If the frequency signal “F” represents values between zero and a maximum value F max , signal s is equal to the frequency signal “F” per se. 
     If the frequency signal “F” represents values higher than the maximum value F max , signal s represents the maximum value F max . 
     This course of the signal s is shown in FIG.  5 . Notice should be taken that the above statement that the frequency signal “F” can be smaller than zero is no contradiction to the statement made above that there are no negative frequencies. The last statement relates to the property of subcircuit  22  which assigns the variable frequency of an alternating current or voltage generator to the flow rate q. The frequency of alternating currents or voltages, however, can only be positive. 
     The situation is the opposite with the frequency signal “F”. It represents a value which is the result of the multiplication of the average flow-rate value signal “q m ” representative of the average q m  with the setting signal “k” representative of a reciprocal volume or a reciprocal mass. The aforementioned value thus represents a frequency. The frequency signal “F” can undoubtedly assume negative values, namely in cases when the aforementioned counterflow rates −q′ occur. 
     In FIG. 4, a first input of a second multiplier  23   8  is connected to the output of the triple comparator  23   7 , of which multiplier a second input receives the clock period signal “δt”. It is followed by a second divider  23   9  of which a first input is connected to an output of the multiplier  23   8  and a second input is supplied with the setting signal “k”. A first input of a third multiplier  23   10  is connected to the output of the averager stage  23   2  and a second input is supplied with the clock period signal “δt”. 
     A subtrahend input of a summer/subtracter  23   11  is connected to an output of the divider  23   9 , a first addend input to an output of the multiplier  23   10  and a second addend input to the output of the write-read memory  23   4 . An output of the summer/subtracter  23   11  is connected to an input of the write-read memory  23   4  which is enabled by the clock signal. 
     The output signal of the summer/subtracter  23   11  is always read into the write-read memory  23   4  during falling edges of the clock pulses, see the arrows. As the average flow-rate value signal “qm” is always made available during the rising edges of the clock pulses, a value which is read into the write-read memory  23   4  in a directly preceding clock period is applied to an input of the summer  23   5  for half of a directly following clock pulse period until a new value is written into the write-read memory  23   4 . 
     Accordingly, a counterflow rate −q′ is already deducted from the flow rate after a single clock pulse period. As opposed to the aforementioned damping there is an immediate balancing. 
     When switching on the flowmeter a suitable initial value such as preferably a zero value is written into the write-read memory  23   4 . Even during the current measurements it may be appropriate to write the zero value into, and to thus perform a reset of, the write-read memory  23   4 . 
     FIG. 4 shows in a broken line that the counterflow rates −q′ can be added up and, optionally, displayed and further processed in other ways by means of a comparator  23   12  with a following accumulator  23   13 . The comparator  23   12  is set by the dimensioning of a switching threshold for example in such a way that (positive) values of its input signals are suppressed. 
     Notice is to be taken in respect to the reference numerals which are marked with quotation marks and do not consist of numbers that it is thereby expressed that they concern reference numerals for signals which represent the respective values bearing no quotation marks. The information content of these signals is therefore the respective value, but not the signal per se. 
     The functions of the individual stages of the block diagram of FIG. 4 can also be realized by means of a respectively programmed microprocessor which will be preferably applied when the evaluation electronics are already provided with a microprocessor for processing the flow rate signal.