Abstract:
There is provided an electromagnetic flow meter that applies magnetic fields with a first frequency and a second frequency, to a fluid to be measured, calculates a first flow rate, calculates a second flow rate, performs low-pass filtering on the first flow rate to calculate a first low-pass filtered flow rate, and performs low-pass filtering on the second flow rate to calculate a second low-pass filtered flow rate, the electromagnetic flow meter including: an abnormality detecting unit that detects an abnormal state in which, the fluid is at non-full level on the basis of at least one of the first flow rate and the second flow rate; and an abnormality removing unit that removes the abnormal state on the basis of the first low-pass filtered flow rate and the second low-pass filtered flow rate, when the abnormality detecting unit does not determine that the fluid is in the abnormal state.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
       [0001]    This application is based on and claims priority under 35 U.S.C. §119 from Japanese Patent Application No. 2009-129229 filed on May 28, 2009. 
       BACKGROUND OF THE INVENTION 
       [0002]    1. Technical Field 
         [0003]    The present invention relates to an electromagnetic flow meter, and more particularly, to a two-frequency-excitation-type electromagnetic flow meter that detects the non-full level of a fluid to be measured. 
         [0004]    2. Description of the Related Art 
         [0005]    In flow control performed in chemical plants, as the excitation type of an electromagnetic flow meter that is used to measure the flow rate of a fluid, generally, a composite excitation type (hereinafter, a ‘two-frequency excitation type’) has been known in which an excitation current component with a high frequency (first frequency) and an excitation current component with a frequency (second frequency) lower than the first frequency flow through an exciting coil at the same time to form a composite magnetic field.  FIG. 8  is a diagram illustrating the structure of a two-frequency-excitation-type electromagnetic flow meter  1 . The structure and operation of the electromagnetic flow meter  1  will be described with reference to  FIG. 1 . 
         [0006]    In  FIG. 8 , the electromagnetic flow meter  1  includes a detector  10 , an exciting circuit  20 , an amplifying circuit  30 , an A/D (analog/digital) converter  31 , a constant current circuit  40 , and a CPU (central processing unit)  50 . 
         [0007]    The detector  10  includes an exciting coil  11  and electrodes  12  and  13 . The CPU  50  includes a high frequency flow rate calculating unit  51 , a low frequency flow rate calculating unit  52 , a two-frequency flow rate calculating unit  53 , a non-full level detecting unit  54 , and an output unit  55 . 
         [0008]    The electrodes  12  and  13  are provided in the detector  10 , and the exciting coil  11  is provided such that the magnetic field generated from the electrodes is applied to a fluid R to be measured in the detector  10 . 
         [0009]    The outputs of the electrodes  12  and  13  are input to the amplifying circuit  30 , and the amplifying circuit  30  amplifies the difference between the outputs of the electrodes  12  and  13  and outputs the amplified signal to the A/D converter  31 . The A/D converter  31  converts the differential amplification signal into a digital signal and outputs the digital signal to the CPU  50 . 
         [0010]    An output terminal of the constant current circuit  40  is connected to the electrodes  12  and  13 . For example, the constant current circuit  40  includes two diodes (not shown). An anode of each diode is connected to a predetermined voltage and a cathode thereof is connected to the electrode  12  or  13 . In the constant current circuit  40 , a leakage current (hereinafter, referred to as a ‘constant current’) flows to the electrodes  12  and  13  in the opposite direction of the diode. 
         [0011]    The high frequency flow rate calculating unit  51  and the low frequency flow rate calculating unit  52  in the CPU  50  receive the digital signal from the A/D converter  31  and calculate the flow rate of the fluid R to be measured corresponding to the excitation frequency. 
         [0012]    The two-frequency flow rate calculating unit  53  receives the flow rates calculated by the high frequency flow rate calculating unit  51  and the low frequency flow rate calculating unit  52  and calculates the flow rate of the fluid R to be measured corresponding to two-frequency excitation. 
         [0013]    The non-full level detecting unit  54  receives the outputs of the electrodes  12  and  13  when a constant current flows from the constant current circuit  40  to the electrodes  12  and  13  through the amplifying circuit  30  and the A/D converter  31  and detects whether the fluid R to be measured is at a non-full level in the detector  10 . 
         [0014]    The output unit  55  receives the flow rate calculated by the two-frequency flow rate calculating unit  53  and the detection signal detected by the non-full level detecting unit  54 . Then, the output unit  55  outputs a current signal that corresponds to the flow rate or indicates the non-full level. 
         [0015]    Next, the operation of the electromagnetic flow meter  1  measuring the flow rate and detecting the non-full level will be described. The exciting circuit  20  makes an excitation current (two-frequency excitation current), which is the sum of a high frequency excitation current and a low frequency excitation current, flow to the exciting coil  11  on the basis of the excitation control signal from the CPU  50 , thereby generating a magnetic field from the exciting coil  11 . The exciting coil  11  applies a magnetic field corresponding to the excitation current to the fluid R to be measured. 
         [0016]    The electrodes  12  and  13  detect and output a signal (electromotive force) that corresponds to a flow velocity and the magnetic field and is generated by the magnetic field corresponding to the high frequency excitation current and the low frequency excitation current. 
         [0017]    The CPU  50  receives the signals output from the electrodes  12  and  13  through the amplifying circuit  30  and the A/D converter  31 . 
         [0018]    The high frequency flow rate calculating unit  51  in the CPU  50  performs a predetermined operation on the received signal in synchronization with a high frequency to calculate a flow rate eH (a first flow rate; hereinafter, referred to as a ‘high frequency flow rate’) corresponding to high-frequency excitation. The high frequency flow rate calculating unit  51  performs a low-pass operation on the high frequency flow rate eH to calculate a high frequency low-pass filtered flow rate FH (first low-pass filtered flow rate). 
         [0019]    The low frequency flow rate calculating unit  52  performs a predetermined operation on the received signal in synchronization with a low frequency to calculate a flow rate eL (a second flow rate; hereinafter, referred to as a ‘low frequency flow rate’) corresponding to low-frequency excitation. The low frequency flow rate calculating unit  52  performs a low-pass operation on the low frequency flow rate eL to calculate a low frequency low-pass filtered flow rate FL (second low-pass filtered flow rate). 
         [0020]    The two-frequency flow rate calculating unit  53  adds the high frequency low-pass filtered flow rate FH and the low frequency low-pass filtered flow rate FL in synchronization with the high frequency to calculate a flow rate eA (a third flow rate; hereinafter, referred to as a ‘two-frequency flow rate’) corresponding to two-frequency excitation. 
         [0021]    The output unit  55  outputs a current signal (for example, in the range of 4 to 20 mA) or a voltage signal (for example, in the range of 1 to 5 V) corresponding to the two-frequency flow rate eA. 
         [0022]    The following two methods are used to detect whether the fluid is at a non-full level in the non-full level detecting unit  54 . 
         [0023]    (1) First, a method of making a constant current flow from the constant current circuit  40  to the electrodes  12  and  13  will be described. When a constant current flows with the fluid R to be measured at a non-full level, the difference (differential voltage) between the output voltages of the electrodes  12  and  13  is higher than that when the fluid is at a full level. 
         [0024]    The non-full level detecting unit  54  compares the differential voltage with a predetermined detection voltage. When the differential voltage is higher than the predetermined detection voltage, it is detected that the fluid is at the non-full level. The method of detecting the non-full level is disclosed in JP-A-3-186716. 
         [0025]    In addition, an AC coupling circuit (for example, a capacitor (not shown)) for attenuating a DC component may be connected to the outputs of the electrodes  12  and  13 . The connection of the capacitor is disclosed in JP-A-6-174513. 
         [0026]    (2) A method of detecting whether the fluid is at a non-full level on the basis of a noise component overlapped with the output signals from the electrodes  12  and  13  will be described. In this method, the constant current circuit  40  may not be used. 
         [0027]    When the fluid R to be measured is at the non-full level, the level of noise overlapped with the output signals from the electrodes  12  and  13  is more than that when the fluid is at a full level. For example, the noise includes commercial power supply frequency noise and inductive noise generated by the magnetic field generated from the exciting coil  11 . 
         [0028]    The non-full level detecting unit  54  measures the level (voltage) of noise overlapped with the output signals from the electrodes  12  and  13 . When the level of noise is more than a predetermined detection voltage, it is determined that the fluid is at the non-full level. The method of detecting the non-full level is disclosed in JP-A-3-257327 and JP-A-3-60027U. 
         [0029]    Next, the operation of the output unit  55  when a non-full level detection signal is received from the non-full level detecting unit  54  will be described. 
         [0030]    When the fluid is at the non-full level, the electromagnetic flow meter  1  is in an abnormal state in which it is difficult to accurately measure the flow rate. In this case, in order to notify the abnormal state in which the fluid is at the non-full level to the outside, the output unit  55  outputs a current or voltage signal that is beyond the normal range (hereinafter, referred to as ‘burnout’) or outputs a warning signal, such as warning light or warning sound. 
         [0031]    However, the above-mentioned two methods of detecting the non-full level have the following problems. 
         [0032]    (1) In the method of making a constant current flow, when the AC coupling circuit is used, the following problems arise. When the fluid is changed from the full level to the non-full level or from the non-full level to the full level, the outputs of the electrodes  12  and  13  vary greatly. 
         [0033]    Therefore, it takes a long time for the output of the AC coupling circuit to be stabilized by a differential operation and the non-full level detecting unit  54  detects the full level and the non-full level after the outputs are stabilized. Therefore, it takes a long time to detect the full level and the non-full level (for example, about 10 minutes). 
         [0034]    (2) In the method of detecting the full level and the non-full level from the noise component, for example, when the level of noise generated by a noise source is reduced by a surrounding environment, the level of noise overlapped with the output signals from the electrodes  12  and  13  is less than a predetermined detection voltage even when the fluid is at the non-full level. Therefore, it is difficult for the non-full level detecting unit  54  to detect the non-full level. 
         [0035]    In this case, the output unit  55  does not receive a non-full level detection signal. Therefore, the output unit  55  outputs a current or voltage signal corresponding to the two-frequency flow rate eA without burning out the current or the voltage. However, actually, since the fluid is at the non-full level and the output signals from the electrodes  12  and  13  vary greatly, a hunting phenomenon in which the current or voltage output alternates between the upper limit and the lower limit occurs. 
       SUMMARY OF THE INVENTION 
       [0036]    An object of the invention provides a two-frequency-excitation-type electromagnetic flow meter capable of accurately and rapidly detecting the non-full level of a fluid to be measured and preventing output hunting due to the detection of the non-full level. 
         [0037]    In order to achieve the object, according to a first aspect of the invention, there is provided an electromagnetic flow meter that applies magnetic fields with a first frequency and a second frequency lower than the first frequency, to a fluid to be measured, calculates a first flow rate on the basis of a signal generated by a first magnetic field with the first frequency, calculates a second flow rate on the basis of a signal generated by a second magnetic field with the second frequency, performs low-pass filtering on the first flow rate to calculate a first low-pass filtered flow rate, and performs low-pass filtering on the second flow rate to calculate a second low-pass filtered flow rate, 
         [0038]    the electromagnetic flow meter including: 
         [0039]    an abnormality detecting unit that detects an abnormal state in which, the fluid to be measured is at non-full level on the basis of at least one of the first flow rate and the second flow rate; and 
         [0040]    an abnormality removing unit that removes the abnormal state on the basis of the first low-pass filtered flow rate and the second low-pass filtered flow rate, when the abnormality detecting unit does not determine that the fluid to be measured is in the abnormal state. 
         [0041]    According to a second aspect of the invention, there is provided the electromagnetic flow meter according to the first aspect, wherein 
         [0042]    the abnormality removing unit removes the abnormal state when difference between the first low-pass filtered flow rate and the second low-pass filtered flow rate is less than a predetermined value. 
         [0043]    According to a third aspect of the invention, there is provided the electromagnetic flow meter according to the second aspect, wherein 
         [0044]    the abnormality removing unit removes the abnormal state after a first predetermined time has elapsed when difference between the first low-pass filtered flow rate and the second low-pass filtered flow rate is less than the predetermined value. 
         [0045]    According to a fourth aspect of the invention, there is provided the electromagnetic flow meter according to the second or third aspect, wherein 
         [0046]    the abnormality removing unit changes the predetermined value. 
         [0047]    According to a fifth aspect of the invention, there is provided the electromagnetic flow meter according to any one of the first to fourth aspects, further including: 
         [0048]    a low-pass filtered flow rate setting unit that sets values to the first low-pass filtered flow rate and the second low-pass filtered flow rate after a second predetermined time has elapsed when the abnormality removing unit does not remove the abnormal state. 
         [0049]    According to a sixth aspect of the invention, there is provided the electromagnetic flow meter according to the fifth aspect, wherein 
         [0050]    the low-pass filtered flow rate setting unit sets the first flow rate to the first low-pass filtered flow rate and the second flow rate to the second low-pass filtered flow rate. 
         [0051]    According to a seventh aspect of the invention, there is provided the electromagnetic flow meter according to any one of the first to fourth aspects, further including: 
         [0052]    a time constant changing unit that changes a time constant used in the low-pass filtering for calculating the first low-pass filtered flow rate and the second low-pass filtered flow rate after a third predetermined time has elapsed when the abnormality removing unit does not remove the abnormal state. 
         [0053]    According to an eighth aspect of the invention, there is provided the electromagnetic flow meter according to any one of the first to seventh aspects, wherein 
         [0054]    the abnormality detecting unit detects that the fluid to be measured is in the abnormal state when at least one of the first flow rate and the second flow rate is out of a predetermined range. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0055]    A general architecture that implements the various features of the invention will now be described with reference to the drawings. The drawings and the associated descriptions are provided to illustrate embodiments of the invention and not limited the scope of the invention. 
           [0056]      FIG. 1  is a diagram illustrating the structure of an electromagnetic flow meter according to an embodiment of the invention. 
           [0057]      FIGS. 2A to 2I  are timing chart illustrating the operation of the electromagnetic flow meter shown in  FIG. 1  when a fluid to be measured is at a full level and a non-full level. 
           [0058]      FIGS. 3A and 3B  are flowchart illustrating an abnormality determining process (a) and an abnormality removing process (b) of the electromagnetic flow meter shown in  FIG. 1 . 
           [0059]      FIG. 4  is a diagram illustrating the structure of an electromagnetic flow meter according to another embodiment of the invention. 
           [0060]      FIG. 5  is a flowchart illustrating an abnormality removing process of the electromagnetic flow meter shown in  FIG. 4 . 
           [0061]      FIG. 6  is a diagram illustrating the structure of an electromagnetic flow meter according to still another embodiment of the invention. 
           [0062]      FIG. 7  is a flowchart illustrating an abnormality removing process of the electromagnetic flow meter shown in  FIG. 6 . 
           [0063]      FIG. 8  is a diagram illustrating the structure of an electromagnetic flow meter according to the related art. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0064]    Various embodiments according to the invention will be described hereinafter with reference to the accompanying drawings. 
       First Embodiment 
       [0065]      FIG. 1  is a diagram illustrating the structure of an electromagnetic flow meter  100  according to an embodiment of the invention. In  FIG. 1 , the same components as those shown in  FIG. 8  are denoted by the same reference numerals and a description thereof will be omitted. 
         [0066]    In  FIG. 1 , the electromagnetic flow meter  100  differs from the electromagnetic flow meter shown in  FIG. 8  in that it includes a predetermined value setting unit  120  and a CPU  110  includes an abnormality detecting unit  111  and an abnormality removing unit  112  instead of the non-full level detecting unit  54  (see  FIG. 8 ). 
         [0067]    The abnormality detecting unit  111  receives flow rates from a high frequency flow rate calculating unit  51  and a low frequency flow rate calculating unit  52  and determines whether a fluid is in an abnormal state, such as at a non-full level. 
         [0068]    The abnormality removing unit  112  receives the determination result from the abnormality detecting unit  111  and the flow rates from the high frequency flow rate calculating unit  51  and the low frequency flow rate calculating unit  52 , and removes the abnormal state. 
         [0069]    A predetermined value setting unit  120  sets a predetermined value input from, for example, the user to a predetermined value used by the abnormality removing unit  112 . 
         [0070]    Next, the operation of the electromagnetic flow meter  100  determining and removing abnormality will be described with reference to  FIGS. 2 and 3 . 
         [0071]    First, the operation will be described with reference to  FIGS. 2A to 2I .  FIGS. 2A to 2I  are timing chart illustrating the flow rate of the electromagnetic flow meter  100 , a removal flag, the operation of a timer, and an output at a full level and a non-full level. 
         [0072]      FIG. 2A  shows the state of a fluid R to be measured at the full level and at the non-full level,  FIG. 2B  shows a low frequency flow rate eL,  FIG. 2C  shows a high frequency flow rate eH,  FIG. 2D  shows a low frequency low-pass filtered flow rate FL,  FIG. 2E  shows a high frequency low-pass filtered flow rate FH,  FIG. 2F  shows a removal flag (described in  FIGS. 3A and 3B ),  FIG. 2G  shows a timer (described in  FIGS. 3A and 3B ),  FIG. 2H  shows an abnormal/normal state (described in  FIGS. 3A and 3B ) indicating an abnormal or normal state, and  FIG. 2I  shows a current or a voltage output from an output unit  55  (hereinafter, referred to as a ‘current output’). 
         [0073]    In  FIG. 2A , the fluid R to be measured is at a full level up to a time t 1 , at a non-full level during the period from the time t 1  to a time t 2 , and at a full level after the time t 2 . 
         [0074]    In  FIGS. 2B and 2C , the low frequency flow rate eL(b) and the high frequency flow rate eH(c) are described on the same vertical axis. 
         [0075]    Up to the time t 1 , the low frequency flow rate eL(b) and the high frequency flow rate eH(c) have substantially the same value. During the period from the time t 1  to the time t 2 , the low frequency flow rate eL(b) and the high frequency flow rate eH(c) are increased, maintained to be constant, and decreased. The low frequency flow rate eL(b) is more than the high frequency flow rate eH(c) and the difference between the low frequency flow rate eL(b) and the high frequency flow rate eH(c) is increased when the low frequency flow rate eL(b) and the high frequency flow rate eH(c) are increased. 
         [0076]    After the time t 2 , the low frequency flow rate eL(b) and the high frequency flow rate eH(c) return to substantially the same value. 
         [0077]    In  FIGS. 2D and 2E , the low frequency low-pass filtered flow rate FL(d) and the high frequency low-pass filtered flow rate FH(e) are described on the same vertical axis. 
         [0078]    Up to the time t 1 , the low frequency low-pass filtered flow rate FL(d) and the high frequency low-pass filtered flow rate FH(e) have substantially the same value. During the period from the time t 1  to the time t 2 , the low frequency low-pass filtered flow rate FL(d) and the high frequency low-pass filtered flow rate FH(e) are increased, maintained to be constant, and decreased. The low frequency low-pass filtered flow rate FL(d) is more than the high frequency low-pass filtered flow rate FH(e) and the difference between the low frequency low-pass filtered flow rate FL(d) and the high frequency low-pass filtered flow rate FH(e) is increased when the low frequency low-pass filtered flow rate FL(d) and the high frequency low-pass filtered flow rate FH(e) are increased. 
         [0079]    The low frequency low-pass filtered flow rate FL(d) and the high frequency low-pass filtered flow rate FH(e) are obtained by performing a low-pass filtering operation (for example, low-pass filtering) on the low frequency flow rate eL(b) and the high frequency flow rate eH(c), respectively. 
         [0080]    Therefore, the gradients of the low frequency low-pass filtered flow rate FL(d) and the high frequency low-pass filtered flow rate FH(e) when they are increased and decreased are less than those of the low frequency flow rate eL(b) and the high frequency flow rate eH(c). However, during the period from the time t 2  to the time t 3 , the low frequency low-pass filtered flow rate FL(d) and the high frequency low-pass filtered flow rate FH(e) return to substantially the same value. 
         [0081]      FIGS. 2F to 2I  will be described below with reference to  FIGS. 3A and 3B  on the basis of the operations of  FIGS. 2A to 2E . 
         [0082]    The states of  FIGS. 2F to 2I  up to the time t 1  are as follows. The removal flag (f) is cleared (value ‘0’), the timer (g) is in a state in which a predetermined time (first predetermined time) has elapsed, the abnormal/normal state (h) is a normal state, and the current output (i) is a current value corresponding to a two-frequency flow rate eA. 
         [0083]    Then,  FIGS. 3A and 3B  will be described.  FIG. 3A  is a flowchart illustrating an abnormality determining process and  FIG. 3B  is a flowchart illustrating an abnormality removing process. 
         [0084]    In Step S 100  of  FIG. 3A , the low frequency flow rate calculating unit  52  calculates the low frequency flow rate eL and the low frequency low-pass filtered flow rate FL. The high frequency flow rate calculating unit  51  calculates the high frequency flow rate eH and the high frequency low-pass filtered flow rate FH. 
         [0085]    In Step S 110 , the abnormality detecting unit  111  compares the low frequency flow rate eL with a normal range (predetermined range). If the low frequency flow rate eL is out of the normal range (‘No’ in Step S 110 ), the process proceeds to Step S 120 . The removal flag is cleared and it is determined that the current state is the abnormal state. 
         [0086]    Then, Step S 200  in the abnormality removing process shown in  FIG. 3B  is performed. The abnormality removing unit  112  determines whether the removal flag is set. Since the removal flag is cleared (‘No’ in Step S 260 ), the abnormality removing process ends. 
         [0087]    In  FIGS. 2A to 2I , the low frequency flow rate eL(b) is out of the normal range in a short time after the time t 1 . Therefore, at that time, the removal flag (f) is maintained in the cleared state, the timer (g) is maintained in the state in which a predetermined time has elapsed, the abnormal/normal state (h) is the abnormal state, and the current output (i) is in an abnormal state, which results in burnout. This state is maintained immediately before the time t 2 . 
         [0088]    In Step S 110  of  FIG. 3A , the high frequency flow rate eH is compared with the normal range. When the high frequency flow rate eH is out of the normal range, the process proceeds to Step S 120 . In addition, each of the low frequency flow rate eL and the high frequency flow rate eH may be compared with the normal range. When both the low frequency flow rate eL and the high frequency flow rate eH are out of the normal range, the process may proceed to Step S 120 . 
         [0089]    For simplicity of description, in Step S 110 , the low frequency flow rate eL is compared with the normal range. For example, the normal range is a flow rate range in which the electromagnetic flow meter  100  can accurately measure the flow rate. 
         [0090]    In a short time after the time t 2  in  FIGS. 2A to 2I , the low frequency flow rate eL(b) is within the normal range. Therefore, the abnormality determining process shown in  FIG. 3A  proceeds to Step S 130  without determining that the current state is an abnormal state (‘Yes’ in Step S 110 ). 
         [0091]    In Step S 130 , the abnormality detecting unit  111  sets the removal flag (value ‘1’) at the time when the process proceeds from the abnormal state in Step S 120  to Step S 130 . 
         [0092]    Then, Step S 200  in the abnormality removing process shown in  FIG. 3B  is performed. Since the removal flag is set (‘Yes’ in Step S 200 ), the process proceeds to Step S 210 . 
         [0093]    In Step S 210 , the abnormality removing unit  112  compares the absolute value of the difference between the low frequency low-pass filtered flow rate FL and the high frequency low-pass filtered flow rate FH with a comparison value (predetermined value). If the absolute value of the difference is equal to or more than the comparison value (‘No’ in Step S 210 ), the process proceeds to Step S 220 . 
         [0094]    In Step S 220 , the abnormality removing unit  112  sets the value of the timer to an initial value (for example, value ‘0’), and the abnormality removing process ends. 
         [0095]    Therefore, in  FIGS. 2A to 2I , in a short time after the time t 2 , the low frequency flow rate eL(b) is within the normal range, and the absolute value of the difference between the low frequency low-pass filtered flow rate FL and the high frequency low-pass filtered flow rate FH is more than the comparison value. Therefore, the removal flag (f) is set, the timer (g) is set to the initial value, the abnormal/normal state (h) is maintained in the abnormal state, and the current output (i) is maintained in a burnout state. This state is maintained up to the time t 3 . 
         [0096]    The fluid to be measured reaches the full level at the time t 2 , but the flow and the full level state of the fluid to be measured are unstable between the time t 2  and the time t 3 . In this case, at the time t 2 , when the abnormal state is immediately changed to the normal state, the normal state and the abnormal state are alternated due to the instability of the fluid at the full level, and hunting occurs in the current output. The following operation is performed to prevent the hunting. 
         [0097]    At the time t 3 , since the difference between the low frequency low-pass filtered flow rate FL(d) and the high frequency low-pass filtered flow rate FH(e) is less than the comparison value in  FIGS. 2A to 2I , the abnormality removing process shown in  FIG. 3B  proceeds to Step S 230  (‘Yes’ in Step S 210 ). 
         [0098]    In Step S 230 , the abnormality removing unit  112  counts the value of the timer and increases the value. 
         [0099]    In Step S 240 , the abnormality removing unit  112  determines whether a predetermined time (first predetermined time) of the timer has elapsed. For example, the abnormality removing unit  112  compares the value of the timer with a predetermined threshold value. When the value of the timer is equal to or less than the threshold value, the abnormality removing unit  112  determines that the predetermined time has not elapsed (‘No’ in Step S 240 ), and the abnormality removing process ends. 
         [0100]    Therefore, in  FIGS. 2A to 2I , during the period from the time t 3  to the time t 4 , since the value of the timer (g) is less than the predetermined threshold value, the removal flag (f) is maintained in the set state, the timer (g) counts the time, the abnormal/normal state (h) is maintained in the abnormal state, and the current output (i) is maintained in the burnout state. 
         [0101]    At the time t 4 , since the value of the timer (g) is equal to or more than the predetermined threshold value and a predetermined time has elapsed, the abnormality removing process shown in  FIG. 3B  proceeds to Step S 250  (‘Yes’ in Step S 240 ). 
         [0102]    In Step S 250 , the abnormality removing unit  112  removes the abnormal state and clears the removal flag. Then, the abnormality removing process ends. 
         [0103]    Therefore, in  FIGS. 2A to 2I , after the time t 4 , since the value of the timer (g) is more than the predetermined threshold value and the predetermined time has elapsed, the removal flag (f) is cleared, the timer (g) is in the state in which the predetermined time has elapsed, the abnormal/normal state (h) is the normal state (removal of abnormality), and the current output (i) is a current value corresponding to the two-frequency flow rate eA. 
         [0104]    For example, since the difference between the low frequency low-pass filtered flow rate FL(d) and the high frequency low-pass filtered flow rate FH(e) is less than the comparison value, the predetermined time (first predetermined time) may be the time until the flow and the full level state of the fluid to be measured are sufficiently stabilized. 
         [0105]    The abnormality determining process and the abnormality removing process have been described above. 
         [0106]    According to this embodiment, when at least one of the low frequency flow rate eL and the high frequency flow rate eH is out of the normal range, the abnormality detecting unit  111  determines that the fluid is in an abnormal state. When the flow rates are within the normal range and it is determined that the fluid is not in the abnormal state, the abnormality removing unit  112  removes the abnormal state after the difference between the low frequency low-pass filtered flow rate FL and the high frequency low-pass filtered flow rate FH is less than the comparison value and a predetermined time has elapsed. In this way, it is possible to accurately detect the abnormal state, that is, the non-full level of the fluid to be measured and thus prevent output hunting. In addition, it is possible to rapidly detect the abnormal state, that is, the non-full level using the method of using an AC coupling circuit to make a constant current flow according to the related art. 
         [0107]    Even when noise is intermittently mixed and the low frequency flow rate eL and the high frequency flow rate eH vary (are suddenly changed), it is possible to prevent the variation in the low frequency low-pass filtered flow rate FL and the high frequency low-pass filtered flow rate FH using low-pass filtering. Therefore, it is possible to prevent an operation error due to noise by removing the abnormal state using the low frequency low-pass filtered flow rate FL and the high frequency low-pass filtered flow rate FH. 
         [0108]    In Step S 210  of  FIG. 3B , when the difference between the low frequency low-pass filtered flow rate FL and the high frequency low-pass filtered flow rate FH is less than the comparison value (‘Yes’ in Step S 210 ), the abnormal state may be removed in Step S 250  without waiting for the elapse of a predetermined time (that is, without performing Steps S 230  and  240 ). 
         [0109]    In this way, at the time t 3  in  FIGS. 2A to 2I , the abnormal state is removed and it is possible to more rapidly return to the normal state. 
         [0110]    The predetermined value setting unit  120  may change the comparison value (predetermined value) used by the abnormality removing unit  112 . In this case, for example, the user can change the comparison value to a value at which the non-full level can be effectively detected on the basis of, for example, the flow state of the fluid to be measured. For example, the comparison value may be set to a value corresponding to the percentage (%) of a flow rate span. 
       Second Embodiment 
       [0111]    In  FIGS. 2A to 2I , at the non-full level, when the difference between the low frequency low-pass filtered flow rate FL(d) and the high frequency low-pass filtered flow rate FH(e) is excessively large or when a time constant used for the low-pass filtering is excessively large, it takes a long time for the difference between the low frequency low-pass filtered flow rate FL(d) and the high frequency low-pass filtered flow rate FH(e) to be less than the comparison value. 
         [0112]    That is, the period from the time t 2  to the time t 3  is increased. As a result, the time until the abnormal state is removed at the time t 4  is increased. This embodiment is for significantly reducing the time. 
         [0113]    The second embodiment when the difference between the low frequency low-pass filtered flow rate FL and the high frequency low-pass filtered flow rate FH is excessively large will be described with reference to  FIG. 4 .  FIG. 4  is a diagram illustrating the structure of an electromagnetic flow meter  200  according to this embodiment. In  FIG. 4 , the same components as those shown in  FIG. 1  are denoted by the same reference numerals and a description thereof will be omitted. 
         [0114]    In  FIG. 4 , the electromagnetic flow meter  200  differs from the electromagnetic flow meter shown in  FIG. 1  in that a CPU  210  includes a low-pass filtered flow rate setting unit  220  in addition to the components of the CPU  110  (see  FIG. 1 ). 
         [0115]    The low-pass filtered flow rate setting unit  220  receives the process result from the abnormality removing unit  112  and sets the high frequency low-pass filtered flow rate FH of the high frequency flow rate calculating unit  51  and the low frequency low-pass filtered flow rate FL of the low frequency flow rate calculating unit  52 . 
         [0116]    The operation of the low-pass filtered flow rate setting unit  220  will be described with reference to  FIG. 5 .  FIG. 5  is a flowchart illustrating an abnormality removing process including the operation of the low-pass filtered flow rate setting unit  220 . In  FIG. 5 , the same components as those shown in  FIG. 3B  are denoted by the same reference numerals and a description thereof will be omitted. 
         [0117]    In Step S 210  of  FIG. 5 , the absolute value of the difference between the low frequency low-pass filtered flow rate FL and the high frequency low-pass filtered flow rate FH is more than a comparison value (‘No’ in Step S 210 ) and the abnormal state is not removed. The process proceeds to Step S 220 . Here, the process after the time t 2  in  FIGS. 2A to 2I  will be described. 
         [0118]    The abnormality removing unit  112  sets the value of the timer to an initial value (Step S 220 ). Then, the abnormality removing unit  112  counts the number of times Step S 220  is performed with the timer and increases the value in Step S 300 . 
         [0119]    In Step S 310 , the low-pass filtered flow rate setting unit  220  receives the count value and determines whether the value is more than a predetermined threshold value. If it is determined that the value is equal to or less than the threshold value (‘No’ in Step S 310 ), the abnormality removing process ends. 
         [0120]    If it is determined that the value is equal to or more than the threshold value (‘Yes’ in Step S 310 ), the process proceeds to Step S 320 . That is, after a second predetermined time has elapsed in Step S 310 , the process proceeds to Step S 320 . In addition, a timer different from that in the first embodiment, not the counter, may be used to determine whether the second predetermined time has elapsed. 
         [0121]    In Step S 320 , the low-pass filtered flow rate setting unit  220  sets the current high frequency flow rate eH to the high frequency low-pass filtered flow rate FH and the current low frequency flow rate eL to the low frequency low-pass filtered flow rate FL. 
         [0122]    Since the current high frequency flow rate eH and the current low frequency flow rate eL have substantially the same value, the absolute value of the difference between the set low frequency low-pass filtered flow rate FL and the set high frequency low-pass filtered flow rate FH is less than the comparison value. Therefore, the determination result of Step S 210  which will be performed after this process is ‘Yes’ and the abnormal state is removed in Step S 250  after a predetermined time (first predetermined time) has elapsed. 
         [0123]    This operation will be described with reference to  FIGS. 2A to 2I . When the period from the time t 2  to a time t 3   a  in  FIGS. 2A to 2I  is referred to as the second predetermined time, the timer (g) starts to count the time at the time t 3   a . That is, the time t 3  is advanced to the time t 3   a.    
         [0124]    At a time t 4   a , the predetermined time (first predetermined time) of the timer (g) has elapsed, the removal flag (f) is cleared, the abnormal/normal state (h) is a normal state (removal of an abnormal state), and the current output (i) is a current value corresponding to the two-frequency flow rate eA. That is, the time t 4  is advanced to the time t 4   a.    
         [0125]    According to this embodiment, when the abnormal state is not removed, the low-pass filtered flow rate setting unit  220  sets the current high frequency flow rate eH to the high frequency low-pass filtered flow rate FH and the current low frequency flow rate eL to the low frequency low-pass filtered flow rate FL after the second predetermined time has elapsed. In this way, the time until the abnormal state is removed is reduced and it is possible to rapidly change the current state to the normal state. 
         [0126]    Next, an embodiment when a time constant used in low-pass filtering is excessively large will be described with reference to  FIG. 6 .  FIG. 6  is a diagram illustrating the structure of an electromagnetic flow meter  300  according to this embodiment. In  FIG. 6 , the same components as those shown in  FIG. 1  are denoted by the same reference numerals and a description thereof will be omitted. 
         [0127]    In  FIG. 6 , the electromagnetic flow meter  300  differs from the electromagnetic flow meter shown in  FIG. 1  in that a CPU  310  includes a time constant changing unit  320  in addition to the components of the CPU  110  (see  FIG. 1 ). 
         [0128]    The time constant changing unit  320  receives the process result from the abnormality removing unit  112 , and changes a time constant used in the calculation of the high frequency low-pass filtered flow rate by the high frequency flow rate calculating unit  51  and a time constant used in the calculation of the low frequency low-pass filtered flow rate by the low frequency flow rate calculating unit  52 . 
         [0129]    Next, the operation of the time constant changing unit  320  will be described with reference to  FIG. 7 .  FIG. 7  is a flowchart illustrating an abnormality removing process including the operation of the time constant changing unit  320 . In  FIG. 7 , the same components as those shown in  FIG. 3B  are denoted by the same reference numerals and a description thereof will be omitted. 
         [0130]    In Step S 210  of  FIG. 7 , the absolute value of the difference between the low frequency low-pass filtered flow rate FL and the high frequency low-pass filtered flow rate FH is more than a comparison value (‘No’ in Step S 210 ) and the abnormal state is not removed. Therefore, process proceeds to Step S 220 . Here, the process after the time t 2  in  FIGS. 2A to 2I  will be described. 
         [0131]    The abnormality removing unit  112  sets the value of the timer to an initial value (Step S 220 ). Then, the abnormality removing unit  112  counts the number of times Step S 220  is performed with the timer and increases the value in Step S 300 . 
         [0132]    In Step S 310 , the time constant changing unit  320  receives the count value and determines whether the value is more than a predetermined threshold value. If it is determined that the value is equal to or less than the threshold value (‘No’ in Step S 310 ), the abnormality removing process ends. 
         [0133]    If it is determined that the value is equal to or more than the threshold value (‘Yes’ in Step S 310 ), the process proceeds to Step S 400 . That is, after a third predetermined time has elapsed in Step S 310 , the process proceeds to Step S 400 . In addition, a timer different from that in the first embodiment, not the counter, may be used to determine whether the third predetermined time has elapsed. 
         [0134]    In Step S 400 , the time constant changing unit  320  reduces the time constants used in the calculation of the high frequency low-pass filtered flow rate and the calculation of the low frequency low-pass filtered flow rate. 
         [0135]    In this way, the low frequency low-pass filtered flow rate FL and the high frequency low-pass filtered flow rate FH rapidly approach (converge on) the same value and the difference therebetween is less than the comparison value. Therefore, the determination result of Step S 210  which will be performed after this process is ‘Yes’ and the abnormal state is removed in Step S 250  after a predetermined time (first predetermined time) has elapsed. 
         [0136]    This operation will be described with reference to  FIGS. 2A to 2I . Similar to the operation of the low-pass filtered flow rate setting unit  220 , the time t 3  is advanced to the time t 3   a  and the time t 4  is advanced to the time t 4   a.    
         [0137]    The second predetermined time and the third predetermined time may be set such that the returning time to the normal state (the removal time of the normal state) is shorter than that in the first embodiment by advancing the time t 3  to the time t 3   a  and advancing the time t 4  to the time t 4   a.    
         [0138]    According to this embodiment, when the abnormal state is not removed, the time constant changing unit  320  reduces the time constants used in the calculation of the high frequency low-pass filtered flow rate and the calculation of the low frequency low-pass filtered flow rate after the third predetermined time has elapsed. In this way, the time until the abnormal state is removed is reduced and it is possible to more rapidly change the current state to the normal state. 
         [0139]    As another embodiment, the method of making a constant current flow or the method of detecting a noise component may be combined with the first embodiment or the second embodiment. In this case, it is possible to detect a non-full level and the constant current circuit  40  is not required, which results in a low cost. 
         [0140]    The high frequency flow rate calculating unit  51 , the low frequency flow rate calculating unit  52 , the two-frequency flow rate calculating unit  53 , the abnormality detecting unit  111 , the abnormality removing unit  112 , the low-pass filtered flow rate setting unit  220 , and the time constant changing unit  320  are provided in the CPU  110 ,  210 , or  310  and are executed by a predetermined program. However, they may be implemented by, for example, a logic circuit that is provided separately from the CPU  110 ,  210 , or  310 . 
         [0141]    In addition, a timer may be provided in the CPU  110 ,  210 , or  310  separately from the timer according to the first embodiment and the counter according to the second embodiment, or it may be independently provided. The removal flag may be stored in a storage unit (not shown) and it may be read from or written to the storage unit. 
         [0142]    According to the above-mentioned embodiments of the invention, the two-frequency-excitation-type electromagnetic flow meter determines the abnormal state in which the fluid to be measured is at the non-full level on the basis of at least one of the first flow rate and the second flow rate and removes the abnormal state on the basis of the first low-pass filtered flow rate and the second low-pass filtered flow rate when it is determined that the fluid to be measured is in the abnormal state. In this way, it is possible to accurately and rapidly detect the abnormal state in which the fluid to be measured is at the non-full level and prevent output hunting due to the detection of the non-full level. 
         [0143]    The invention is not limited to the above-described embodiments, but various modifications and changes of the invention can be made without departing from the scope and spirit of the invention. In addition, the invention may include combinations other than the above-mentioned combinations of the components.