Abstract:
Power arithmetic apparatus detects a first variation amount of a voltage in proportion to a voltage of a measuring object and a second variation amount of a voltage in proportion to a current of the measuring object, and calculates power of the measuring object based on the first variation amount detected and the second variation amount detected.

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to a power arithmetic apparatus for calculating power from an AC current and voltage of a target measurement system. 
     FIG. 1 is a block diagram showing a conventional power arithmetic apparatus. 
     Referring to FIG. 1, terminals T 1  and T 2  input a voltage V 1  and a current A 1  in proportion to the voltage and current of a target measurement system. The voltages V 1  and A 1  are converted into digital values by A/D converters  1  and  2 , respectively. A CPU  3  calculates the digital values from the A/D converters  1  and  2  at a predetermined time interval. 
     The power arithmetic apparatus of this scheme serves as a wattmeter by calculating 
     
       
         
           P=V 
           1 
           ·A 
           1 
         
       
     
     and performing integration for a predetermined time, or as a watthour meter by performing infinite time integration. 
     However, such power arithmetic apparatus has the following problems. 
     (1) Since the voltage V 1  and current A 1  are multiplied by software, multiplication instruction processing takes a time. 
     (2) Since calculation is performed by software, processing is complex and time-consuming, so another processing can hardly be performed by software. 
     (3) Conversion using A/D converters takes a time, so the sampling frequency can hardly be increased. To increase the accuracy, the number of bits is increased, although this results in an increase in cost. 
     BRIEF SUMMARY OF THE INVENTION 
     The present invention has been made in consideration of the above situation, and has as its object to provide a power arithmetic apparatus capable of performing multiplication instruction processing in a short time, avoiding complex processing to allow software to perform another processing, and preventing an increase in cost even when the sampling frequency is increased. 
     To achieve the above object, according to a first aspect of the present invention, there is provided a power arithmetic apparatus comprising: 
     means for detecting a first variation amount in a voltage in proportion to a voltage of a measuring object and a second variation amount in a voltage in proportion to a current of the measuring object; and 
     means for calculating power of the measuring object based on the first variation amount detected and the second variation amount detected. 
     According to a second aspect of the present innovation, there is provided a apparatus according to first aspect, 
     wherein the means for detecting the first variation amount and second variation amount comprises: 
     a first A/D converter for converting the voltage in proportion to the voltage of the measuring object to a first digital signal indicating the voltage in proportion to the voltage of the measuring object; 
     a second converter for converting the voltage in proportion to the current of the measuring object to a second digital signal indicating the voltage in proportion to the current of the measuring object; 
     a first counter for outputting the first variation amount based on the first digital signal; and 
     a second counter for outputting the second variation amount based on the second digital signal. 
     Additional objects and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out hereinafter. 
    
    
     BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING 
     The accompanying drawings, which are incorporated in and constitute a part of the specification, illustrate presently preferred embodiments of the invention, and together with the general description given above and the detailed description of the preferred embodiments given below, serve to explain the principles of the invention. 
     FIG. 1 is a block diagram showing a conventional power arithmetic apparatus; 
     FIG. 2 is a block diagram showing a power arithmetic apparatus according to the first embodiment of the present invention; 
     FIG. 3 is a timing chart for explaining the operation of the power arithmetic apparatus shown in FIG. 2; and 
     FIG. 4 is a block diagram showing a power arithmetic apparatus according to the second embodiment of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The embodiments of the present invention will be described below with reference to the accompanying drawing. 
     FIG. 2 is a block diagram showing a power arithmetic apparatus according to the first embodiment of the present invention. 
     Referring to FIG. 2, terminals T 1  and T 2  are input terminals for inputting voltages V 1  and A 1  in proportion to the voltage and current of a target measurement system. The outputs from the input terminals T 1  and T 2  are input to positive input terminals of comparators  13  and  14  in 1-bit A/D converters  11  and  12  called delta modulators, respectively. The output from integrators  15  and  16  are supplied to the negative input terminals of the comparators  13  and  14 , respectively. The outputs from the comparators  13  and  14  are supplied to the D input terminals of flip-flops  17  and  18 , respectively. 
     The A/D converters  11  and  12  encode the above voltages V 1  and A 1  and output 1-bit pulse signals f(n) and g(n), respectively. The timing is determined by a clock φ in the A/D converter  11  and by a clock obtained by inverting the clock φ in the A/D converter  12 . 
     At the rise of the clock, output voltage F(n) or G(n) from the integrator  15  or  16  and the magnitude of the input voltage V 1  or A 1  are compared by the comparator  13  or  14 . When V 1 &gt;F(n), or A 1 &gt;G(n), a signal of high level (H) is output from the A/D converter  11  or  12 , and the integrator  15  or  16  integrates the signal by only +Δv. When V 1 &lt;F(n), or A 1 &lt;G(n), a signal of low level (L) is output, and the integrator integrates the signal by only −Δv. 
     The outputs from the A/D converters  11  and  12  are output to up-down counters  19  and  20 , respectively, to control the up-down counting operation. The clocks φ or clocks obtained by inverting the clocks φ are counted. The outputs F(n) and G(n) from the up-down counters  19  and  20  correspond to values obtained by A/D-converting the input voltages V 1  and A 1 , respectively. 
     A data selector  21  selects one of the values from the up-down counters  19  and  20 , which is to be supplied to an adder/subtracter  22 . When the clock φ is at “H” level, the data selector  21  selects data on the up-down counter  19  side. When the clock φ is at “L” level, data on the up-down counter  20  side is selected. 
     The output from the data selector  21  and the output from a latch  24  are supplied to the adder/subtracter  22 . At the same time, the output from a gate portion  23  comprising an AND gate and an OR gate which receive the output voltages f(n) and g(n) from the A/D converters  11  and  12  and the clock φ and an inverted clock of the clock φ are supplied to the adder/subtracter  22 . 
     The adder/subtracter  22  sequentially performs addition or subtraction of digital values from two input terminals A and B, i.e., an output value W(n−1) from the latch  24  and the output value F(n) from the up-down counter  19  or the output value G(n) from the up-down counter  20 . For the input terminal B, addition or subtraction is determined on the basis of the signal input to the (+/−) terminal. 
     The output from the A/D converter  11  or  12  is input to the (+/−) terminal. When the clock φ is at “H” level, the output from the A/D converter  12  is selected. When the clock φ is at “L” level, the output from the A/D converter  11  is selected. When the (+/−) terminal is at “H” level, addition is performed. When the (+/−) terminal is at “L” level, subtraction is performed. The output from the adder/subtracter  22  has a value in proportion to instantaneous V 1 ×A 1 . 
     The latch  24  latches W(n−1) immediately preceeding an output W(n) from the adder/subtracter  22 . Hence, the latch  24  outputs the value W(n−1). 
     The output W(n−1) from the latch  24  is supplied to an adder  25 . The adder  25  calculates a sum ∫W(n) before the adder  25  itself. Consequently, the integrated value ∫W(n) of the multiplied values of the instantaneous voltages V 1  and A 1  can be obtained. 
     The operation of the power arithmetic apparatus having the above arrangement will be described with reference to the timing chart in FIG. 3 showing the waveforms at the respective portions. 
     The voltages V 1  and A 1  are proportional to the voltage and current of the target measurement system. The outputs from the A/D converters  11  and  12  are represented by the pulse sequences f(n) and g(n) in FIG. 3, respectively. The pulse signal f(n) operates at the rise of the clock φ, and the pulse signal g(n) operates at the inverted clock of the clock φ. These pulse signals have a value “+1” or “−1”. 
     The signals F(n) and G(n) are obtained by integrating the outputs from the A/D converters  11  and  12 , respectively. When the signal F(n) is output, the clock φ is counted, and when the signal G(n) is output, the inverted clock of the clock φ is counted. The value F(n) equals a value obtained by A/D-converting the voltage V 1 , and the value G(n) equals a value obtained by A/D-converting the current A 1 . 
     The object of the present invention is to obtain V 1 ×A 1 . In this case, V 1  and F(n), and A 1  and G(n) have the following relations:              V1   ≈     F        (   n   )               (   1   )               A1   ≈     G        (   n   )               (   2   )                                
     Hence, F(n)×G(n)=W(n) is defined. 
     When the output from the A/D converter  11  is represented by f( 1 ), f( 2 ), . . . , f(n), the output signal F(n) from the integrator  15  at that time is                F        (   n   )       =       (       f        (   1   )       +     f        (   2   )       +     …                   f        (   n   )           )     ×              Δ                 v             (   3   )                                
     The output signal from the up-down counter  19  equals the digital code value of F(n). 
     Similarly, the output signal G(n) is 
     
       
           G ( n )=( g (1)+ g (2)+ . . .  g ( n )) xΔv   (4) 
       
     
     The value F(n)×G(n)=W(n) to be obtained is given by                      W        (   n   )       =       F        (   n   )       ×     G        (   n   )                     =       (       f        (   1   )       +     f        (   2   )       +   …   +     f        (   n   )         )     ×     (       g        (   1   )       +     g        (   2   )       +   …   +     g        (   n   )         )                     (   5   )                                
     Since the value F(n) is determined at the rise timing of the clock φ, and the value G(n) is determined at the rise timing of the inverted clock of the clock φ, i.e., at the fall timing of the clock φ, the value W(n) is obtained in two steps. 
     Let Wv(n) be the timing at which the count F(n) on the voltage side is determined, and Wa(n) be the timing at which the count G(n) on the current side is determined. At the timing Wv(n) at which the count F(n) on the voltage side is determined, rewriting equation (5) yields:                Wv        (   n   )       =       F        (   n   )       ×     G        (   n   )                     =       (       f        (   1   )       +     f        (   2   )       +   …   +     f        (   n   )         )     ×     (       g        (   1   )       +     g        (   2   )       +   …   +     g        (   n   )         )                   =       F        (   n   )       ×     (       G        (     n   -   1     )       +     g        (   n   )         )                       since                   g        (   n   )         =     ±   1       ,                 Wv        (   n   )       =         F        (   n   )       ×     G        (     n   -   1     )         ±     F        (   n   )                     =       Wa        (     n   -   1     )       ±     F        (   n   )                                      
     At the timing Wa(n) at which the count G(n) on the current side is determined, rewriting equation (5) yields:                Wa        (   n   )       =       F        (   n   )       ×     G        (   n   )                     =       (       f        (   1   )       +     f        (   2   )       +   …   +     f        (   n   )         )     ×     (       g        (   1   )       +     g        (   2   )       +   …   +     g        (   n   )         )                     =       (       F        (     n   -   1     )       +     f        (   n   )         )     ×     G        (   n   )           )                   since                   f        (   n   )         =     ±   1       ,                 Wa        (   n   )       =         F        (   n   )       ×     G        (   n   )         ±     G        (   n   )                     =       Wv        (   n   )       ±     G        (   n   )                                      
     Hence, at the timing at which the count F(n) on the voltage side is determined, the adder/subtracter  22  determines the value to be added/subtracted to/from the value held by the latch  24  on the basis of the output from the A/D converter  12  on the current side. When the output data F(n) from the voltage-side up-down counter  19  is added/subtracted, the instantaneous V 1 ×A 1 , i.e., W(n) can be obtained as the output from the adder/subtracter  22 . 
     At the timing at which the count G(n) on the current side is determined, the adder/subtracter  22  determines the value to be added/subtracted to/from the value held by the latch  24  on the basis of the output from the A/D converter  11  on the voltage side. When the output data G(n) from the current-side up-down counter  20  is added/subtracted, the instantaneous V 1 ×A 1 , i.e., W(n) can be obtained as the output from the adder/subtracter  22 . 
     Practically, the value W(n) is further integrated by the adder  25  and used power or electric energy data. 
     The second embodiment of the present invention will be described next. 
     In a delta modulator used as an A/D converter, a small difference is sometimes generated in the integration width of Δv between the upper and lower rows because of the influence of performance of an integrator in the modulator. For this reason, when the modulator is operated for a long time, the “0” point of the up-down counter may be shifted from the original “0” point. 
     FIG. 4 is a block diagram showing the arrangement of a power arithmetic apparatus according to the second embodiment for improving the point shift. The same reference numerals as in the first embodiment shown in FIG. 2 denote the same parts in FIG. 4, and a detailed description thereof will be omitted. 
     Comparators  31  and  32  are connected to terminals T 1  and T 2 , respectively, to detect a timing at which input AC voltages V 1  and A 1  become zero. Every time the comparators  31  and  32  detect the timing at which the voltages V 1  and A 1  become zero, one-shot circuits  33  and  34  generate one-shot signals to clear up-down counters  19  and  20  and adder/subtracter  22  (cleared when only the voltage V 1  becomes zero in this embodiment). 
     The outputs from the one-shot circuits  33  and  34  are supplied to timing circuits  35  and  36 , respectively. The outputs from the timing circuits  35  and  36  are supplied to integrators  15  and  16  in A/D converters  11  and  12  and also to the up-down counters  19  and  20  through OR gates  37  and  38 , respectively. 
     When the voltages V 1  and A 1  do not become zero for a long time, e.g., when the voltages do not become zero for 1 sec, the timing circuits  35  and  36  generate one-shot signals to clear the integrators  15  and  16  in the A/D converters  11  and  12  and the up-down counters  19  and  20 , respectively. 
     According to the second embodiment, the up-down counters  19  and  20  and the integrators  15  and  16  in the A/D converters  11  and  12  are simultaneously cleared every predetermined period. Hence, the “0” point shift which disables accurate measurement can be prevented. 
     As the above-described A/D converter, a delta sigma modulator may be used in place of the delta modulator. 
     According to this embodiment, the power arithmetic apparatus comprises two 1-bit A/D converters for converting voltages in proportion to the voltage and current of a target measurement system into 1-bit codes, two up-down counters whose up-down counting is controlled by the 1-bit codes output from the two 1-bit A/D converters, an addition/subtraction circuit for adding/subtracting output data from the up-down counters to/from previous output data, and a latch for sampling the previous data from the addition/subtraction circuit and outputting the data to the addition/subtraction circuit. The addition/subtraction circuit selectively performs addition or subtraction on the basis of the output data from the two 1-bit A/D converters. 
     With this arrangement, the sampling rate can be increased, and the resolving power (the number of bits) in coding an analog signal can be reduced. A compact and inexpensive power arithmetic apparatus can be provided. By minimizing processing by the CPU using software, processing by software can be simplified. A power arithmetic apparatus, wattmeter, or watthour meter can be constructed without using any software. A circuit arrangement suitable as an LSI can be obtained. 
     As has been described above, according to the present invention, the analog portion comprises only A/D converters and has a very small number of components. For this reason, a compact LSI can be realized at low cost. In addition, a wattmeter or watthour meter can be constructed using only hardware. Furthermore, since the sampling rate can be increased, the accuracy can be improved. 
     Additional advantages and modifications will readily occur to those skilled in the art. Therefore, the invention in its broader aspects is not limited to the specific details and representative embodiments shown and described herein. Accordingly, various modifications may be made without departing from the spirit or scope of the general inventive concept as defined by the appended claims and their equivalents.