Abstract:
An embodiment of a method for measuring an RMS voltage of an arbitrary time-varying waveform includes: a) coupling said arbitrary time-varying waveform to said thermally sensitive device; b) decoupling said arbitrary time-varying waveform from said thermally sensitive device; c) coupling said thermally sensitive device to said controlled DC voltage source; d) measuring a current through said thermally sensitive device at least two points in time; e) determining a change in said current over said at least two points in time; f) if said current is increasing between said at least two points in time, then decrease said controlled DC voltage source responsive to determining that said current is increasing; g) if said current is decreasing between said at least two points in time, then increase said controlled DC voltage source responsive to determining that said current is decreasing; and h) repeating steps a)-g).

Description:
BACKGROUND  
       [0001]     RMS stands for root of the mean of the square and is a standard for measuring arbitrary time-varying waveforms. Currently, there are three primary methods for measuring an RMS voltage. The first method employs a peak detector and an averaging circuit that scales the average value to equal the RMS voltage. However, this method yields inaccurate results especially for non-sine wave arbitrary time-varying signals. The second method employs a computational method with either analog computation using log diodes or digital computation using dedicated hardware. This method has difficulty with linearity and crest factor accuracy. A third method employs a thermal method of measurement that typically uses balanced thermal sensors. This method is accurate and it can handle a wide frequency range and crest factor. However, this thermal method may be difficult to implement, delicate, expensive, and slow. Therefore there exists a need for systems and methods that address these and/or problems associated with existing thermal methods of measuring RMS voltage. For example, there exists a need for more cost effective systems and methods for accurately measuring RMS voltages.  
       SUMMARY  
       [0002]     Systems and methods for measuring an RMS voltage value using a thermally sensitive device are provided. An embodiment of a method for measuring an RMS voltage of an arbitrary time-varying waveform includes: a) coupling said arbitrary time-varying waveform to said thermally sensitive device; b) decoupling said arbitrary time-varying waveform from said thermally sensitive device, c) coupling said thermally sensitive device to said controlled DC voltage source; d) measuring a current through said thermally sensitive device at least two points in time; e) determining a change in said current over said at least two points in time; f) if said current is increasing between said at least two points in time, then decrease said controlled DC voltage source responsive to determining that said current is increasing; g) if said current is decreasing between said at least two points in time, then increase said controlled DC voltage source responsive to determining that said current is decreasing; and h) repeating steps a) - g).  
         [0003]     An embodiment of an RMS measurement circuit includes: a resistance selection circuit coupled to a controlled DC voltage source circuit and to a current measurement circuit; a microcontroller coupled to said controlled DC voltage source circuit and to said current measurement circuit; wherein said resistance selection circuit comprises: an input of a arbitrary time-varying waveform coupled to a first switch; a controlled DC voltage source coupled to a second switch; a thermally sensitive device coupled to said first and second switches; and wherein said current measurement circuit is configured to measure said current through said thermally sensitive device.  
         [0004]     Other systems, methods, features, and advantages of the present invention will be or become apparent to one with skill in the art upon examination of the following drawings and detailed description. It is intended that all such additional systems, methods, features, and advantages be included within this description, be within the scope of the present invention, and be protected by the accompanying claims.  
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0005]     Many aspects of the invention can be better understood with reference to the following drawings. The components in the drawings are not necessarily to scale, emphasis instead being placed upon clearly illustrating the principles of the present invention. Moreover, in the drawings, like reference numerals designate corresponding parts throughout the several views.  
         [0006]      FIG. 1  is a block diagram depicting an embodiment of an RMS measurement system.  
         [0007]      FIG. 2A-2B  are example circuit diagrams of a resistance selection system for an RMS measurement system.  
         [0008]      FIG. 3  is an example circuit diagram of a current measuring system, a DC voltage mirror, and a microcontroller for an RMS measurement system.  
         [0009]      FIG. 4  is an example circuit diagram of a current measurement system.  
         [0010]      FIG. 5  is an example timing diagram of an RMS measurement system.  
         [0011]      FIG. 6  is a method for measuring an RMS voltage value of an unknown time-varying waveform with an RMS measurement system.  
         [0012]      FIG. 7  is an example method of an operation of an RMS measurement system.  
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0013]     As will be described in greater detail herein, systems and method(s) for measuring an RMS amplitude of an unknown time-varying waveform are disclosed. RMS is mathematically the root of the mean of the square of a waveform and is equal to the heating value of a waveform. Systems and method(s) are disclosed for a thermal method to measure RMS.  
         [0014]     A system and method(s) are provided for measuring the RMS voltage of an unknown time-varying waveform (“unknown source”) by converting the RMS of the unknown source to a DC voltage value equal to the RMS value of the unknown source. Systems and method(s) for an RMS measurement system comprise an unknown source, a controlled DC voltage value (“DC voltage mirror”), a thermally sensitive device (e.g., negative temperature coefficient thermistor) for measurement, a selection system to toggle between the sources, and a microcontroller to take measurements and to apply the feedback system. The thermally sensitive device is switched between the unknown source and the DC voltage mirror. The switch normally connects the unknown source to the thermally sensitive device causing a certain power to dissipate equal to the square of the voltage divided by the resistance of the thermally sensitive device. The switch is then momentarily switched to the DC voltage mirror and the DC current value is measured and recorded. Over a brief interval, the DC current value is measured again to see if it is increasing or decreasing. If the measured current with the DC voltage mirror is decreasing it indicates the thermally sensitive device is cooling, and then the DC voltage mirror value is less than the applied RMS of the unknown source. To compensate and bring the DC voltage mirror value closer to the RMS, the DC voltage mirror value is increased. Alternatively, if the current is increasing it indicates the DC voltage mirror is higher than the RMS and the DC voltage mirror value is decreased. This process continues in a feedback loop until the current change is zero. At this point, the DC voltage mirror value equals the RMS value of the unknown source and the value may be provided by an analog output or a digital output. The microcontroller may be configured to control the selection of the sources, take the current measurements, record the current values, compute the difference in current values, adjust the DC voltage mirror, and repeat the process.  
         [0015]     Referring now to the drawings,  FIG. 1  is a block diagram depicting an embodiment of an RMS measurement system  100 . The RMS measurement system  100  comprises an input  101 , a resistance selection system  110 , a current measuring system  120 , a DC voltage mirror  130 , a microcontroller  140 , an analog output  102 , and a digital output  111 . The DC voltage mirror  130  comprises a controlled DC voltage source. The resistance selection system  110  comprises a thermally sensitive device (e.g., negative temperature coefficient thermistor) which may be switched between the input  101  of an unknown source and the DC voltage mirror  130 . The thermally sensitive device (e.g., thermistor) may have a resistance which varies in a non-linear fashion as a function of temperature. Each voltage across the thermally sensitive device represents a unique temperature and equal voltages across the thermally sensitive device will result in the same resistance of the thermally sensitive device. The resistance selection system  110  is configured to switch the thermally sensitive device between an unknown source on the input  101  and the DC voltage mirror  130 . The current measuring system  120  comprises circuitry which is configured to measure a current value through the thermally sensitive device, and store the value in the microcontroller  140  (e.g., converting analog current value with an analog-to-digital converter and providing to microcontroller input). The microcontroller  140  is configured to control the resistance selection system  110 , to store the current values from the current measuring system  120 , to determine if the current values are changing, to adjust the DC voltage mirror  130  value, and to continue until the DC voltage mirror  130  value converges with the RMS value. The current measuring system  120  is configured to measure the current through the thermally sensitive device in the resistance selection system  110  while the DC voltage mirror  130  is connected. The RMS measurement system may provide an analog output  102  which provides the DC voltage mirror value and may be connected to the resistance selection system  110 . Additionally, the microcontroller  140  may provide a digital output  121  which may provide a digital value of the DC voltage mirror value.  
         [0016]      FIGS. 2A-2B  are example circuit diagrams of a resistance selection system  110  for an RMS measurement system  100 . The resistance selection system  110  comprises an input  101 , a switch S 1 A and S 1 B, a switch S 2 A and S 2 B, a thermistor  210 , and an analog output  102 . An unknown source input  202  may be attached to the input  101  of the resistance selection system  110 . The thermistor  210  is an example of a thermally sensitive device with the resistance varying as a function of voltage. The thermistor  210  is connected to the switch S 1 A and S 1 B and to the switch S 2 A and S 2 B via a junction  207  and a junction  208 . The switch S 1 A and S 1 B connects the unknown source input  202  to the thermistor  210  when closed as depicted in  FIG. 2A . The switch S 2 A and S 2 B connects the DC voltage mirror  130  (not shown) to the thermistor  210  when closed as depicted in  FIG. 2B . An electrical ground  212  is connected to the circuit at junction  209 , and a current measuring system  120  (not shown), a DC voltage mirror  130  (not shown), and microcontroller  140  (not shown) may connect to the resistance selection system  110  at the junction  208 . The microcontroller  140  (not shown) may operate the switch S 1 A and S 1 B and the switch S 2 A and S 2 B to alternate connecting the unknown source input  202  to the thermistor  210  and connecting the DC voltage mirror  130  (not shown) to the thermistor  210 .  
         [0017]      FIG. 3  is an example circuit diagram of a current measuring system  120 , a DC voltage mirror  130 , and a microcontroller  140  for an RMS measurement system  100 . The current measuring system  120  comprises a resistor  307 , an analog-to-digital converter  302 , and a differential amplifier  304 . The DC voltage mirror comprises an operational amplifier  305 , a low-pass filter  306 , a digital-to-analog converter  303 , and a transistor  308 . The microcontroller  140  comprises a digital input  311 , a-digital output  312 , and a digital output  310 . The current measuring system  120  is configured to measure the voltage across the resistor  307  with the differential amplifier  304  and to convert this voltage representing the current value with the analog-to-digital converter  302 . The differential amplifier  304  is configured to have its inputs  321  and  322  in a parallel connection with the resistor  307  at a junction  331  and a junction  332 . The voltage value across resistor  307  changes according to the resistance of the thermistor  210  (not shown) in  FIGS. 2A and 2B . The current measuring system  120  provides a digital value for the current through resistor  307  to the input  311  of the microcontroller  140 . The microcontroller  140  may be programmed to store the current value received at input  311  and to compare this value to subsequent values. The microcontroller  140  may be, for example, a Motorola 6811© or an Intel 8051©. The microcontroller  140  may use various feedback algorithms such as, for example, a Proportional Integral Derivative (PID) to provide rapid response times to changing current values. The microcontroller  140  is configured to provide the DC voltage mirror  130  value on the output  312 . The output  312  is connected to a digital-to-analog converter  303 . The output of the digital-to-analog converter  303  may be connected to a low-pass filter  306  to improve the accuracy of the DC voltage mirror  130  value by using an averaging technique of the DC voltage mirror  130  value. The output of the low-pass filter  306  is connected to a non-inverting input  324  of the operational amplifier  305 . An inverting input  325  of the operational amplifier  305  is connected in a feedback loop to a junction  333  which in turn is connected to a transistor  308  at the output of the operational amplifier  305 . The DC voltage mirror  130  value is provided at junction  333 . The microcontroller  140  may have an output  310  which provides a digital output of the DC voltage mirror  130 , which will equal the RMS value of the unknown source when the current values are no longer changing. A power source  330  may be connected to the current measuring system  120  at a junction  331 . The junction  333  may connect to a junction  208  (not shown) on the resistance selection system  110  (not shown) in  FIGS. 2A and 2B .  
         [0018]      FIG. 4  is an example circuit diagram of an RMS measurement system  100 . The RMS measurement system  100  comprises a resistance selection system  110 , a current measuring system  120 , a DC voltage mirror  130 , and a microcontroller  140 . The resistance selection system  110  is depicted in  FIGS. 2A and 2B . The current measuring system  120 , the DC voltage mirror  130 , and the microcontroller  140  are depicted in  FIG. 3 . The resistance selection system  110  is connected at junction  208  to the current measuring system  120  at junction  333 . The DC voltage mirror value is provided at junction  333 .  
         [0019]      FIG. 5  is an example timing diagram  500  of an RMS measurement system  100 . The timing diagram  500  depicts three operations  510 ,  520 , and  530  of an RMS measurement system to measure the RMS value of an unknown source. The timing diagram  500  comprises the switch S 1  which connects an unknown source to a thermistor, the switch S 2  which connects a DC voltage mirror to the thermistor, a current feedback which is the change in current while the DC voltage mirror is connected to the thermistor, an A/D strobe  1  and  2  where the microcontroller takes the current measurement, a difference which shows whether the current is decreasing or increasing, and the DC voltage mirror value which is adjusted according to whether the current is decreasing or increasing. In operation  510 , the switch S 1  is initially closed to connect the unknown source to the thermistor. Switch S 1  is then opened and Switch S 2  is closed to connect the DC voltage mirror to the thermistor. The current through the thermistor is increasing which indicates the DC voltage mirror value is greater than the RMS value of the unknown source. The A/D strobes twice at different times and the difference in current value is recorded in the microcontroller. The microcontroller decreases the DC voltage mirror value because the difference is positive, and opens switch S 2  and closes switch S 1  to connect the unknown source to the thermistor for another operation. The operation  520  begins with switch S 1  initially closed to connect the unknown source to the thermistor. Switch S 1  is then opened and switch S 2  is closed to connect the DC voltage mirror to the thermistor. The current through the thermistor is decreasing which indicates the DC voltage mirror value is less than the RMS value of the unknown source. The A/D strobes twice at different times and the difference in current value is recorded in the microcontroller. The microcontroller increases the DC voltage mirror value because the difference is negative, and opens switch S 2  and closes switch S 1  to connect the unknown source to the thermistor for another operation. The operation  530  begins with switch S 1  initially closed to connect the unknown source to the thermistor. Switch S 1  is then opened and switch S 2  is closed to connect the DC voltage mirror to the thermistor. The current through the thermistor is increasing which indicates the DC voltage mirror value is greater than the RMS value of the unknown source. The A/D strobes twice at different times and the difference in current value is recorded in the microcontroller. The microcontroller decreases the DC voltage mirror value because the difference is positive, and opens switch S 2  and closes switch S 1  to connect the unknown source to the thermistor for another operation.  
         [0020]      FIG. 6  is a method  600  for measuring an RMS voltage value of an unknown time-varying waveform with an RMS measurement system  100 . The method  600  comprises switching a thermistor to the unknown source (step  601 ); switching the thermistor to a DC voltage mirror (step  602 ); and measuring the current through the thermistor during at least two points in time (step  603 ). The method  600  comprises a feedback loop that will cause the voltage mirror to substantially converge with the RMS voltage value. If the current is decreasing (step  604 ), then the resistance of the thermistor is increasing and the temperature is decreasing indicating the DC voltage mirror value is less than the RMS voltage of the unknown source. The DC voltage mirror value is increased responsive to determining that the current is decreasing (step  605 ). If the current is increasing (step  606 ), then the resistance of the thermistor is decreasing and the temperature is increasing indicating the DC voltage mirror value is greater than the RMS voltage of the unknown source. The DC voltage mirror value is decreased responsive to determining that the current is increasing (step  607 ).  
         [0021]      FIG. 7  is an example method  700  of an operation of an RMS measurement system  100 . The method  700  comprises opening of a switch S 1 A and S 1 B (step  701 ); closing of a switch S 2 A and S 2 B (step  702 ); strobing of an A/D converter and measuring the current feedback and storing this value as a measurement of current # 1  in a microcontroller (step  703 ); strobing of the A/D converter and measuring the current feedback and storing this value as a measurement of current # 2 , and subtracting the measurement of current # 1  from the measurement current # 2  and storing as the difference (step  704 ); opening of the switch S 2 A and S 2 B (step  705 ); closing of the switch S 1 A and S 1 B (step  706 ); and increasing/decreasing the voltage mirror if the difference is negative/positive (step  707 ).  
         [0022]     It should be emphasized that the above-described embodiments of the present invention are merely possible examples, among others, of the implementations, setting forth a clear understanding of the principles of the invention. Many variations and modifications may be made to the above-described embodiments of the invention without departing substantially from the principles of the invention. All such modifications and variations are intended to be included herein within the scope of the disclosure and present invention.