Gain Switching Device With Reduced Error for Watt Meter

This invention relates to watt hour apparatus for switching a signal to pass through paths having different gain characteristics, and more particularly to such apparatus including therein structure for reducing errors caused by interaction of common frequency components present in the signal and in a switching function used to determine the path for the signal.

Electronic instruments for measurement electrical energy are known in the art. In such instruments, it is known to use electronic components for measurement of analog electrical parameters, such as current and voltage, and for conversion of the analog quantities to digital signals. The use of digital circuitry enables the resultant digital signals to be multiplied with great accuracy, thereby to provide accurate representations of the electrical energy transfer rate in a circuit.

The conversion of analog signals to digital values is typically achieved by analog-to-digital converters. Such converters are capable of providing accurate digital signals representative of the analog input value, for adjustable ranges of peak or average values of the input parameters. Accordingly, prior art circuits require manual intervention to change the range of measurement in accordance with the parameter values. Alternatively, auto-ranging electrical systems are known wherein different ranges of measurement are automatically determined by internal circuitry, the specific range being shown to the user by a visual display, for example. Such auto-ranging systems are typically responsive to peak or average values of alternating electrical parameters. These devices, however, do not address the question of providing accurate instantaneous conversion of the low magnitudes of a high range alternating electrical quantity, such as typically found in the vicinity of the zero crossings thereof

Moreover, in prior art systems utilizing electronic signals it is known that various offset voltages of electronic components tend to result in inaccuracies. In the prior art there are provided suggestions for overcoming such offset errors. In one approach, described in U.S. Pat. No. 4,058,768, for example, errors caused by offset are averaged out utilizing a complicated system. Therein is proposed the phase switching of voltage variable signals and of a polarity detector whenever an output pulse is generated, thereby to average out offset caused errors. An integrator is caused alternately to integrate upwardly or downwardly, at a repetitious cycle having a frequency substantially higher than the frequency of the electrical parameter being measured. The patent, however, fails to address the issue of inaccurate instantaneous conversions in the zero crossing area. Moreover, the correction of errors due to interaction between common frequency components of a switching function and of a signal being measured is similarly not addressed, whether such errors are due to offset voltages or to other causes.

There is thus a need to provide structures for correcting internal errors in an electronic measurement apparatus due to interactions between the signal being measured and a switching function used in measurement of the signal. Moreover, there is a need to provide correction for inaccurate instantaneous conversions of analog values for signals having wide instantaneous variations although remaining in a particular measurement range.

It is accordingly an object of the present invention to overcome the difficulties of the prior art and to provide apparatus for reducing errors resulting from interaction between common frequency components of a switching function and a signal being operated on by said switching function.

It is another object of the invention to provide a gain switching arrangement for a varying input signal as a function of the instantaneous value of that signal and for causing the signal to be conveyed through alternate paths having different gains therefor.

It is a more specific object of the invention to provide an apparatus for generating a switching signal responsive to an input signal, and to provide means for reducing errors caused by interactions between common frequency components of the switching signal and the input signal.

Still another object of the invention is the elimination of a DC error resulting from interactions between two signals having common frequency components by doubling the frequency of one of the components.

In accordance with the objects of the invention, there is accordingly provided an error reducing means for reducing errors resulting from interactions between common frequency components of a switching function, applied in a switching apparatus to an input signal, and the input signal In a broad sense, the invention includes a first means for receiving the switching function and a second means for receiving another function. Additionally, there is provided a combining means for combining the switching function with the other function to produce as a result a substitute switching function substantially devoid of the common frequency component and, instead, having a substitute component at a substitute frequency which, upon interaction with the input signal component of the common frequency, results in a reduced error signal, as compared with the error generated by the original switching function.

Preferably, the reduction in error signal is achieved by changing the switching function to include components at multiples of the common frequency, but not at the common frequency itself. The multiple frequency components of the substitute switching function are generated by a structure providing both the switching function and a delayed version thereof. The delay is provided via a delay means which is selected to provide a delay equivalent to a submultiple of the period of the common frequency. Thus, by using a delay of one half the period of the common frequency, the resultant substitute switching function includes two repetitions of the original switching function in each cycle. That is, a double frequency switching function is obtained.

In accordance with another feature of the invention, there is provided an analog-to-digital converter which receives the input signal after its switched passage through one of two alternate paths and its amplification at one of two different gain levels thereby. A multiplier operates on digital signals output by the analog-to-digital converter. The multiplier further receives a signal from the structure used to generate the substitute switching function in order properly to scale the resultant product as a function of the appropriate gain provided to the input signal.

Preferably, the analog-to-digital converter receives both voltage and current signals, the multiplier thus providing an output product indicative of the power associated with the voltage and current.

In accordance with another feature of the invention, the generating structure for the switching function includes a comparator for determining the instantaneous amplitude of the input signal and a latching circuit, or flip-flop, for retaining the result of the comparison for a predetermined time period. The output of the latch circuit is provided to an OR gate, as well as to a shift register functioning as the delay used in generating the substitute switching function. The output of the shift register is combined in the OR gate with the output of the latch circuit to generate the substitute switching function.

In accordance with another aspect of the invention, there is described a gain switching arrangement for providing a plurality of gains to a varying input signal. The arrangement includes a plurality of paths for conveying the input signal therethrough at differing gains. Switching means is included for selectively switching the input signal through the plurality of paths, while a control means generates a switching signal to control the switching means. The control means includes a comparator for comparison of an instantaneous value of the signal with a threshold, and a circuit responsive to the comparator for providing the switching signal output by the control means. Preferably, there is provided an operational amplifier whose output is input to the comparator.

The error reducing feature of the invention is achieved as previously described by providing to the output signal of the comparator a delay corresponding to a submultiple of a frequency present in the switching signal and in the input signal. The comparator output signal and its delayed version are combined to produce a double frequency representation thereof, thus eliminating the common frequency with the inputsignal which otherwise results in a DC error.

In yet another feature of the invention, there is provided a sample and hold circuit for selective provision of samples of two input signals to the analog-to-digital converter so that a single converter provides digital values representative of the signals, preferably current and voltage signals. The outputs of the converter are provided to a multiplier, as is a control signal indicative of the specific gain provided to the input signal, thereby to result in an appropriately scaled output signal from the multiplier representing electrical power associated with the voltage and current.

Still other objects and features of the present invention will become more readily apparent to those skilled in the art from the following description when read in conjunction with the accompanying drawing, wherein there is shown and described a preferred embodiment of the invention simply by way of illustration of one of the best modes contemplated for carrying out the invention. As will be realized, the invention is capable of other, different, embodiments and its several details are capable of modifications in various obvious aspects, all without departing from the invention. Accordingly, the drawings and descriptions will be regarded as illustrative in nature and not as restrictive.

The accompanying drawings, incorporated in and forming a part of the specification, illustrate several aspects of the present invention, and, together with the description, serve to explain the principles of the invention. In the drawings:

FIG. 1 illustrates a prior art switching apparatus useful in measurement of electrical power;

FIG. 2 illustrates an improvement of the prior art apparatus of FIG. 1 in accordance with the present invention;

FIG. 3 shows a block diagram of the inventive improvement shown in FIG. 2;

FIG. 4 provides a block diagram representation of a portion of the structure in FIG. 3; and

FIG. 5 shows a detailed logic diagram of the structure of FIG. 3.

Typical prior art measurement of electrical power is illustrated by the structure in FIG. 1. As shown therein, electrical voltage and current signals are input at leads 10 and 12, respectively. The voltage and current are represented by analog values which, after conversion to digital form in an analog-to digital converter (ADC) 14 are multiplied in a multiplier 16. The resultant product is provided to a utilization device 18 which typically includes a display.

The arrangement of FIG. 1 shows the use of a single ADC to convert both the voltage and current signals. Such an approach is enabled by use of a controlled switch 20, effectively multiplexing the voltage and current signals for input to the ADC. Not shown in the circuit is a pair of storage devices which may be necessary for storing the digital output of ADC 14 to provide a simultaneous multiplier and multiplicand to multiplier 16. Of course, if two ADC units are used, the controlled switch 20 is not required and the voltage and current signals are provided directly to the converters.

In order to assure that the voltage and current signals are accurate representations of contemporaneous electrical parameters, there is provided a pair of synchronized sample-and-hold units 22 and 24. Moreover, although the circuitry in FIG. 1 illustrates a single pair of input terminals for voltage and current, the structure is equally applicable to multi-phase systems by utilization of a multiplexer (not shown) for periodically sampling the voltage and current of each of the phases, and providing signals representative of the same on leads 10 and 12.

A problem with the prior art structure shown in FIG. 1 is that the analog-to-digital converter 14 has a limited input operating range within which accurate conversion may be obtained. Thus, independently of the range of input parameters to which the device is set, it will be appreciated that for alternating current signals there will be a portion of the signal cycle for which the input signals have low instantaneous magnitudes. Thus, power determination in the vicinity of the zero crossings of the current or voltage waveforms will not be as accurate as those obtained during the higher magnitude instantaneous values of the input waveforms.

In order to overcome this deficiency in the prior art, there is thus provided a modification of the structure in FIG. 1 as shown in FIG. 2. More specifically, as seen in FIG. 2 there is provided a gain switching network 26 in the path of the input current lead 12. It is the function of the gain switching network 26 to provide an increased amplitude of the current for conversion in ADC 14 when the instantaneous current magnitudes are low. Thus, ADC 14 provides a more accurate digital representation of the current during those instants of time when its magnitude is low, in order to provide a more accurate product by multiplier 16. Of course, in order to retain the appropriate scale for the product, multiplier 16 is required to undo the effects of the amplification of the instantaneous current magnitude by gain switching network 26. Towards that end, a control signal is provided by gain switching network 26 on a lead 28. The signal indicates to multiplier 16 the specific gain provided by network 26 to the current signal on lead 12.

For simplicity in operation, where multiplier 16 may represent a function performed by a microprocessor, or by another binary coded device, gain switching network 26 may provide different gains representable by different powers of 2. Multiplier 16 may then simply compensate for different gains provided to the current by shifting the binary point of its output product.

The system of FIG. 2 illustrates a single gain switching network 26 in the current lead. It should be appreciated, however, that a similar network may be provided in the voltage lead 10. However, inasmuch as typical power measurements are made with a fixed voltage (e.g., 115 volts at 60 Hz) and with varying current, the voltage conversions will be of sufficient accuracy. Inasmuch as the current value may change, however, and its instantaneous value is thus not predictable, the preferred embodiment of the invention contemplates compensation of the reduced accuracy of an ADC only for the current values.

Referring now to FIG. 3, the gain switching network 26 is shown in greater detail in order further to explain the operation of the invention. As shown in the figure, there is provided for the input signal on lead 12 a first path 30, having a first gain therein, symbolically represented by an amplifier 32, and a second path 34, having a second gain represented by an amplifier 36. It should be appreciated that the illustration of two alternate paths for the input signal is illustrative only. That is, there may be provided a larger number of alternate paths for the input signal. The specific gain given to the input signal prior to its passage to ADC 14 is determined by the instantaneous value thereof. Thus, where ADC 14 (or other utilization apparatus receiving the output of gain switching network 26) includes a number of operating ranges, each requiring a separate signal amplitude for proper operation, it is clear that a number of such paths, with an appropriate number of gain factors provided therein, may be used in the invention.

The signals passed by the first and second paths are provided to a controlled switch 38, which provides an output on its output lead 39 to the sample-and-hold circuit 24 shown in FIG. 2. A control circuit 40 is provided for controlling switch 38 in order to pass the input signal after amplification in path 1 or path 2 to the ADC.

In accordance with the preferred embodiment, the control circuit 40 includes a signal providing circuit 42, for providing a switching function control signal for switch 38. Additionally, control circuit 40 includes a comparator 44. The comparator is used to compare the input signal (either directly or, as shown in FIG. 3, after amplification in one of the paths) with a threshold to determine which gain should be provided to the signal prior to its passage through control switch 38 to the utilization device represented by ADC 14. Upon determination of the appropriate gain, comparator 44 causes circuit 42 to provide a control signal for switch 38 in order to switch the appropriate path into the circuit.

Preferably, the circuit 42 for providing a signal to the controlled switch 38 and to the multiplier 16 includes a latch 46 for temporarily storing the results of the comparison conducted at comparator 44. The output of latch 46 may be used directly to provide the desired output switching function and the output control signal to multiplier 16. However, such a connection may lead to an error for the following reason.

Ideally, for a situation wherein ADC 14 is set to accept 60 Hz signals of a particular (high) signal range, the instantaneous signal magnitude is at low levels requiring further amplification (in the vicinity of a zero crossing) at a 120 Hz rate. Thus, the output of latch 46 should include only a 120 Hz component, and should not include a 60 Hz component. However, because of possible asymmetries in the input waveform, in the offset voltages in the positive and negative comparisons conducted at comparator 44, or because of differences in offset voltages in paths 30 and 34, a 60 Hz component may be included in the switching signal output by latch 46. This component thus affects the switching of switch 38 and provides a 60 Hz error component on output lead 39. Since this signal is ultimately multiplied in multiplier 16 by a voltage signal having the same frequency, the result of the multiplication may include an appreciable DC error.

It is known that a multiplication of signals of two frequencies results in signals having sum and difference frequency components. Where the two multiplied signals include a common frequency component, the difference frequency component is thus at zero, or DC frequency. Accordingly, the present invention includes an error reducing circuit 48 within the circuit 42 providing the output switching function. It is the function of error reducing circuit 48 to provide a substitute switching function for the function output by latch 46, wherein the substitute function is substantially devoid of components at the common input frequency, thus reducing or eliminating the DC error hereinabove described. Thus, rather than providing the switching function generated in latch 46 to switch 38 and to multiplier 16, the inventive circuit substitutes a switching function at a substitute frequency, so that the interaction with the 60 Hz input frequency will not result in the undesirable DC component.

Referring now to FIG. 4, the error reducing circuit 48 is shown to include a delay circuit 50 and a logic gate 52 for providing the substitute switching function. The output of latch 46 is input to the delay circuit 50, and combined with the output of delay circuit 50 in logic gate 52 to provide the substitute switching function.

In operation, delay circuit 50 is selected to provide a delay of 1/2 period of the offending common frequency. Thus, where it is desired to eliminate a 60 Hertz component, an 8.33 millisecond delay is provided. By combining the output of latch 46 with its output 1/2 cycle previous thereto, there results at the output of logic gate 52 a signal of a frequency double the undesired frequency. That is, logic gate 52 outputs a switching function at twice the frequency of the function output by latch 46, and any errors therein are similarly provided at double frequencies. Thus, a 60 Hz error component in the switching function, due to the various offset voltages for example, is replaced by a 120 Hz waveform in the substitute switching function generated by logic gate 52. Subsequent correlation between the signal passed by switch 38 and the 60 Hz input voltage signal might thus result in an easily ignored 60 Hz error component, but without the objectionable DC error. It should be understood that instead of being delayed by 1/2 the period of the frequency common to the input signal and the switching function, the output function of the latch may be delayed by an appropriate plurality of any submultiple of the period of the common frequency. For example, to provide a tripling of the output error frequency the error reducing circuit may be altered to provide logic gate 52 with three inputs originating at the latch output. A first input would be passed through a delay circuit of 1/3 the common period, a second input would be provided a delay of 2/3 of the common period, and the third input would be the direct output of the latch itself. As will be appreciated, this approach results in tripling of the frequencies in the output of the latch circuit.

Referring now to FIG. 5, there is shown a logic diagram illustrating in detail the preferred embodiment of the present invention. Preferably, the gain of path 34 is unity and the gain of path 30 is represented by a circuit arrangement 54 wherein the gain of operational amplifier 32 is adjusted by resistors R1 and R2 to equal a power of 2, such as 8, in accordance with the previous description. The output of operational amplifier 32 is provided to a pair of comparators 56 and 58 which form the structure shown as comparator 44 in FIG. 3.

Comparators 56 and 58 provide output signals indicative of whether the output of amplifier 32 is within or without a range between +V.sub.1 and -V.sub.2. Typically, V.sub.1 =V.sub.2, so that for values of the input signal in a symmetric low range the signal is amplified by amplifier 32 prior to passage through switch 38 to the ADC.

Operation of the circuit thus provides for control of switch 38 to pass the input signal to the sample-and-hold circuitry 24 when its instantaneous amplitude is sufficiently high. When the amplitude is low, however, and is within, a predetermined range, the signal passed to sample-and-hold circuit 24 is the amplified version of the input signal, as provided by operational amplifier 32. The determination of whether the input signal is within or without the predetermined range is made by comparators 56 and 58, each of which provides a high level output signal when the input signal is within the low range. The resultant low level overrange signals output by the open collector output circuits of the comparators are wire OR'ed to provide a high level input signal to a D-type flip-flop 60 only when the input signal is within the low range requiring increased gain. The output of flip-flop 60 is provided to the data input of a shift register 62, performing the function of delay circuit 50 in FIG. 4. The shift register output and the flip-flop output are both input to a NAND circuit which provides a high level control signal for the switching circuit whenever either input thereto is low.

Thus, the controlled switch 38, which may be a type DG 303, manufactured by Siliconix, for example, provides a closed switch for path 34 if the input signal is now, or was 1/2 period previously, beyond the low level range. Alternatively, the signal on path 30 is passed to the ADC if the input signal is now, and was 1/2 period previously, within the low level range requiring additional amplification.

Preferably, the flip-flop 60, the shift register 62, and the sample-and-hold units 22 and 24 are clocked by a common signal provided on lead 66. The shift register is selected to have an appropriate number of stages to store a sequence of outputs of the flip-flop covering half the common period at the clock rate of the signal on lead 66.

Where a microprocessor is used to perform the multiplication function, as well as to control sequencing of operations in the circuit, the substitute switching function output by control circuit 40 on lead 28 may be applied to an interrupt line thereof. Thus, since a low value on line 28 is indicative of provision of a signal to the ADC which represents an amplified version of the input signal, the presence of a low level on line 28 may be used to trigger the microprocessor (not shown) to correct for the gain of amplifier 32. Where the gain of amplifier 32 is eight, the microprocessor will provide a product after a leftward shift of the binary point by three binary places.

Although the invention has been described in terms of a watt-hour environment, it should be appreciated that the described structure as recited in the claims is applicable in other environments as well. Thus, in addition to providing a reduced gain switching function for a watt-hour meter, the present invention more broadly provides an error reducing apparatus for environments wherein a switching function is generated as a function of a particular input signal, and results in a component having a common frequency with the input signal. By changing the frequency of the switching function components, errors due to correlation of the frequency component with similar components of the input signal are thus reduced.

The foregoing description of the preferred embodiment of the invention has been presented for purposes of illustration and description and is not intended to be exhaustive or to limit the invention to the precise form disclosed, since many obvious modifications and variations are possible in light of the above teaching. The embodiment was chosen and described in order best to explain the principles of the invention and its practical application, thereby to enable others skilled in the art best to utilize the invention in various embodiments and with various modifications as are suited to the particular use contemplated. It is intended that the scope of the invention be defined by the claims appended hereto, when interpreted in accordance with the full breadth to which they are fairly and legally entitled.