Abstract:
A detachable current and voltage sensor provides an isolated and convenient device to measure current passing through a conductor such as an AC branch circuit wire, as well as providing an indication of an electrostatic potential on the wire, which can be used to indicate the phase of the voltage on the wire, and optionally a magnitude of the voltage. The device includes a housing that contains the current and voltage sensors, which may be a ferrite cylinder with a hall effect sensor disposed in a gap along the circumference to measure current, or alternative a winding provided through the cylinder along its axis and a capacitive plate or wire disposed adjacent to, or within, the ferrite cylinder to provide the indication of the voltage.

Description:
[0001]    The present Application is a Continuation of U.S. patent application Ser. No. 13/451,524, filed on Apr. 19, 2012, which is a Continuation of U.S. patent application Ser. No. 13/024,181, filed on Feb. 9, 2011 and issued as U.S. Pat. No. 8,680,845 on Mar. 25, 2014, and claims priority thereto under 35 U.S.C. 120. The disclosure of the above-referenced Parent U.S. Patent Application is incorporated herein by reference. 
     
    
       [0002]    This invention was made with government support under DE-EE0002897 awarded by the Department of Energy. The government has certain rights to this invention. 
     
    
     BACKGROUND OF THE INVENTION 
       [0003]    1. Field of the Invention 
         [0004]    The present invention is related to sensors providing input to power measurement systems, and more specifically to a non-contact sensor that includes an electrostatic voltage sensor and an electromagnetic current sensor that can be used to detect the voltage and current at a wire of a power distribution system. 
         [0005]    2. Description of Related Art 
         [0006]    A need to measure power consumption in AC line powered systems is increasing due to a focus on energy efficiency for both commercial and residential locations. In order to provide accurate measurements, the characteristics of the load must be taken into account along with the current drawn by the load. 
         [0007]    In order to determine current delivered to loads in an AC power distribution system, and in particular in installations already in place, current sensors are needed that provide for easy coupling to the high voltage wiring used to supply the loads, and proper isolation is needed between the power distribution circuits/loads and the measurement circuitry. 
         [0008]    Therefore, it would be desirable to provide a sensor that can provide isolated current draw information and permit load characteristics to be taken into account using outputs of a single sensor in an AC power distribution circuit. 
       BRIEF SUMMARY OF THE INVENTION 
       [0009]    The invention is embodied in a current and voltage sensing device and its method of operation. The current sensing device includes a current sensor and a voltage sensor both integrated in a housing that can be detachably coupled to a wire and provides outputs indicative of the current passing through the wire, as well as an electric potential on the wire. 
         [0010]    The housing may be a clamshell containing portions of a current sensor formed from a ferrite cylinder, which when closed around the wire, form either a complete ferrite cylinder, or one with a gap along the circumference. A semiconductor magnetic field sensor may be included in the gap and used to measure the current passing through the wire, or a winding may be provided around the ferrite cylinder along its axis. The voltage sensor may be a separate cylindrical plate, another wire or other suitable conductor either offset from the current sensor along the length of the wire, or may be a foil located inside of the ferrite sensor or a film deposited on an inside surface of the ferrite. 
         [0011]    The foregoing and other objectives, features, and advantages of the invention will be apparent from the following, more particular, description of the preferred embodiment of the invention, as illustrated in the accompanying drawings. 
     
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING 
         [0012]    The novel features believed characteristic of the invention are set forth in the appended claims. The invention itself, however, as well as a preferred mode of use, further objectives, and advantages thereof, will best be understood by reference to the following detailed description of the invention when read in conjunction with the accompanying Figures, wherein like reference numerals indicate like components, and: 
           [0013]      FIG. 1A  and  FIG. 1B  are isometric views and  FIG. 1C  is a cross-section view of a sensor according to an embodiment of the present invention. 
           [0014]      FIG. 2A  is an isometric view and  FIG. 2B  is a cross-section view of a sensor according to another embodiment of the present invention. 
           [0015]      FIG. 3A  is an isometric view and  FIG. 3B  is a cross-section view of a sensor according to yet another embodiment of the present invention. 
           [0016]      FIG. 4A  is an isometric view and  FIG. 4B  is a cross-section view of a sensor according to still another embodiment of the present invention. 
           [0017]      FIG. 5  is an electrical block diagram illustrating circuits for receiving inputs from sensors according to embodiments of the present invention. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0018]    The present invention encompasses sensors for current and voltage sensing features for providing input to power measurement systems. For example, the present invention can provide input to power monitoring equipment in computer server rooms, in which branch circuits distribute power to various electronic chassis power supplies, and in which it is beneficial to provide power usage information for the various branch circuits to power monitoring and/or system control utilities within a computer operating environment. Other applications include power monitoring for commercial and/or residential energy management. 
         [0019]    Referring now to  FIGS. 1A-1C , a sensor  10  in accordance with an embodiment of the present invention is shown. A plastic sensor body  12  encloses a current sensor and a voltage sensor, that provide information about a magnitude and phase of a current passing through a wire  3  around which sensor body is detachably secured as shown in  FIG. 1B . A latch  13  secures a top portion and a bottom portion of sensor body  12  together, along with a hinge formed on sensor body  12  at an opposite side from latch  13 . A current sensing portion of sensor  10  is formed by three ferrite pieces  14 A,  14 B that form a ferrite cylinder around wire  3 , when sensor body  12  is closed. Top ferrite piece  14 A forms a half-cylinder, while ferrite pieces  14 B define a gap between ferrite pieces  14 B and in the circumference of the ferrite cylinder, in which current sensing element  17 , which is generally a semiconductor magnetic field sensor, such as a Hall effect sensor, is disposed. Current sensing element  17  is shown as having interface wires  15  extending from its body, but other types of terminals may be used as an alternative manner of providing connections to current sensing element  17 . An aperture is formed through sensor body  12  to receive current sensing element  17 . A voltage sensor is formed by metal plates  18 A,  18 B, which provide capacitive coupling to branch circuit wire  3  and provide an output via interface wire  15 A, which may also alternatively be replaced with a terminal or other suitable electrical connector. The voltage sensor provides an AC waveform that is at least indicative of the phase of the voltage on wire  3  and may be calibrated to provide an indication of the magnitude of the voltage if needed. Electrical connection to metal plate  18 B is provided by interface wire  15 A and electrical connection to metal plate  18 A is provided by contact between metal plates  18 A and  18 B when sensor body  12  is latched closed. Metal plate  18 A includes a contact  27  and metal plate  18 B includes a mating recess  29  to improve electrical contact between metal plates  18 A and  18 B, so that connection of one of metal plates  18 A and  18 B to the measurement system is needed to provide voltage sensing. Contacts  27  and mating recesses  29  are optional and may be omitted in other embodiments of the invention, and electrical connection may be provided only by contact between metal places  18 A and  18 B, or alternatively by other suitable connection improvement techniques.  FIG. 1C  illustrates such an embodiment so that metal plates  18 A and  18 B making contact when sensor body  12  is closed, and shows the connection of interface wire  15 A to metal plate  18 B. 
         [0020]    Referring now to  FIGS. 2A and 2B , a sensor  10 A in accordance with another embodiment of the invention is shown. Sensor  10 A is similar to sensor  10  of  FIGS. 1A-1C , so only differences between them will be described below. Rather than including current sensing and voltage sensing elements that are laterally displaced along the axis of the cylinder formed by sensor body  12  as in sensor  10  shown in  FIG. 1A , in sensor  10 A, the voltage sensor and current sensors are concentrically arranged, reducing the length of sensor  10 A over that of sensor  10 , while providing similar capacitive area for the voltage sensing and ferrite volume for the current sensing. Therefore, sensor  10 A includes metal plates  18 C and  18 D having shapes differing from that of than metal plates  18 A- 18 B in sensor  10 , and ferrite pieces  14 C- 14 D differ from ferrite pieces  14 A- 14 B of sensor  10 , as well. Metal plates  18 C and  18 D may be inserts mechanically secured by sensor shell  12 A, or metal films bonded to or deposited on the interior surfaces of ferrite pieces  14 C- 14 D. In the illustrated example, metal plates  18 C and  18 D include jogs at their ends in order to provide electrical contact between them and ferrite pieces  14 C- 14 D do not make contact as in sensor  10  of  FIGS. 1A-1C , and therefore the total circumferential gap in the ferrite cylinder is increased slightly. However, in alternative embodiments, the jogs may be omitted from metal plates  18 C and  18 D and alternative electrical connection techniques may be employed, by including a second interface wire  15 A bonded to metal plate  18 C and/or additional interface metal along the edges of sensor body  12  outside of the ends of ferrite pieces  14 C- 14 D, which can then be extended to make contact as in sensor  10  of  FIGS. 1A-1C . 
         [0021]    Referring now to  FIGS. 3A and 3B , a sensor  10 B in accordance with yet another embodiment of the invention is shown. Sensor  10 B is similar to sensor  10 A of  FIGS. 2A-2B , so only differences between them will be described below. Rather than locating current sensing element  17  in a gap between two ferrite pieces  14 B as in sensor  10 A of  FIGS. 2A-2B , in sensor  10 B, current sensing element is located between two ferrite pieces  14 E and  14 F that extend around the entire circumference of sensor  10 B, excepting the thickness of current sensing element  17 , and therefore only one circumferential gap is formed provided that ferrite pieces  14 E and  14 F are in contact when sensor  10 B is closed at the area opposite the hinge in sensor body  12 B. Recesses are formed in sensor body  12 B to accept current sensing element  17 , which may be bonded to, or molded within sensor body  12 B, as may also be performed for any of the integration of current sensing element  17  in the present application. Metal plates  18 E and  18 F are shown as having jogs only opposite of the hinged portion of sensor body  12 B, to provide for ferrite pieces  14 E and  14 F extending all of the circumferential distance to the body of current sensing element  17  and since ferrite pieces  14 E and  14 F are not in contact along the hinged portion of sensor body  12 B. However, in accordance with an alternative embodiment of the invention, metal plates  18 E and  18 F may include features within the gap formed between ferrite pieces  14 E and  14 F along the hinged portion of sensor body  12 B to provide additional electrical contact between metal plates  18 E and  18 F. Further, in accordance with another embodiment of the invention, if sensor body  12 B is made of a sufficiently flexible material and/or the hinged portion of sensor body  12 B is sufficiently elastic, ferrite pieces  14 E,  14 F may extend all of the way to the inside faces of sensor body  12 B on both sides of sensor body  12 B. In such an embodiment, sensing element  17  is inserted in either the hinged side or the latching side of sensor body  12 B between the faces of ferrite pieces  14 E,  14 F to form the gap and make contact with ferrite pieces  14 E,  14 F. 
         [0022]    Referring now to  FIGS. 4A and 4B , a sensor  10 C in accordance with yet another embodiment of the invention is shown. Sensor  10 C is similar to sensor  10  of  FIGS. 1A-1C , so only differences between them will be described below. Rather than including metal plates  18 A and  18 B and the portion of sensor body  12  that extends to provide the voltage sensing portion of sensor  10  in  FIG. 1A , interface wire  15 A extends within the cylindrical cavity formed by sensor body  12 C and ferrite pieces  14 A- 14 B to provide voltage sensing, which can provide sufficient coupling to perform voltage sensing, in particular when only the phase of the voltage on wire  3  is to be measured. 
         [0023]    Referring now to  FIG. 5 , a circuit for receiving input from the current/voltage sensors of  FIGS. 1A-1C ,  2 A- 2 B,  3 A- 3 B and  4 A- 4 B is shown in a block diagram. Interface wires  15  from current sensing element  17  provide input to a current measurement circuit  108 A, which is an analog circuit that appropriately scales and filters the current channel output of the sensor. The output of current measurement circuit  108 A is provided as an input to an analog-to-digital converter (ADC)  106 , which converts the current output waveform generated by current measurement circuit  108 A to sampled values provided to a central processing unit (CPU)  100  that performs power calculations in accordance with program instruction stored in a memory  104  coupled to CPU  104 . Alternatively, current measurement circuit  108 A may be omitted and current sensing element  17  may be connected directly to ADC  106 . The power usage by the circuit associated with a particular sensor can be determined by assuming that the circuit voltage is constant (e.g., 115 Vrms for electrical branch circuits in the U.S.) and that the phase relationship between the voltage and current is aligned (i.e., in-phase). However, while the assumption of constant voltage is generally sufficient, as properly designed properly distribution systems do not let the line voltage sag more than a small amount, e.g., &lt;3%, the phase relationship between voltage and current is dependent on the power factor of the load, and can vary widely and dynamically by load and over time. Therefore, it is generally desirable to at least know the phase relationship between the branch circuit voltage and current in order to accurately determine power usage by the branch circuit. 
         [0024]    Interface wire  15 A from the voltage channel of the sensor is provided to a voltage measurement circuit  108 B, which is an analog circuit that appropriately scales and filters the voltage channel output of the sensor. A zero-crossing detector  109  may be used to provide phase-only information to a central processing unit  100  that performs power calculations, alternatively or in combination with providing an output of voltage measurement circuit to an input of ADC  106 . Alternatively, voltage measurement circuit  108 B may be omitted and interface wire  15 A connected directly to ADC  106 . An input/output (I/O) interface  102  provides either a wireless or wired connection to a local or external monitoring system. When power factor is not taken into account, the instantaneous power used by each branch circuit can be computed as: 
         [0000]    
       
      
       P 
       BRANCH 
       =V 
       rms 
       *I 
       meas  
      
     
         [0000]    where V rms  is a constant value, e.g. 115V, and I meas  is a measured rms current value. Power value P BRANCH  may be integrated over time to yield the energy use. When the phase of the voltage is known, then the power may be computed more accurately as: 
         [0000]        P   BRANCH   =V   rms   *I   meas *cos(Φ)
 
         [0000]    where Φ is a difference in phase angle between the voltage and current waveforms. The output of zero-crossing detector  109  may be compared with the position of the zero crossings in the current waveform generated by current measurement circuit  108 A and the time ΔT between the zero crossings in the current and voltage used to generate phase difference Φ from the line frequency (assuming the line frequency is 60 Hz): 
         [0000]      Φ=2π*60 *ΔT  
 
         [0000]    In general, the current waveform is not truly sinusoidal and the above approximation may not yield sufficiently accurate results. A more accurate method is to multiply current and voltage samples measured at a sampling rate much higher than the line frequency. The sampled values thus approximate instantaneous values of the current and voltage waveforms and the energy may be computed as: 
         [0000]      Σ( V   n   *I   n )
 
         [0000]    A variety of arithmetic methods may be used to determine power, energy and phase relationships from the sampled current and voltage measurements. 
         [0025]    While the invention has been particularly shown and described with reference to the preferred embodiments thereof, it will be understood by those skilled in the art that the foregoing and other changes in form, and details may be made therein without departing from the spirit and scope of the invention.