Abstract:
A clamping current and voltage sensor provides an isolated and convenient technique for measuring 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 body formed from two handle portions that contain the current and voltage sensors within an aperture at the distal end, which may be a ferrite cylinder with a hall effect sensor disposed in a gap along the circumference to measure current, or alternatively 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. When the handles are compressed the aperture is opened to permit insertion of a wire for measurement.

Description:
[0001]    The present Application is a Continuation-in-Part (CIP) of U.S. patent application Ser. No. 13/024,181, filed on Feb. 9, 2011 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. 
     
    
     BACKGROUND OF THE INVENTION 
       [0002]    1. Field of the Invention 
         [0003]    The present invention is related to sensors providing input to power measurement systems, and more specifically to a clamping 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. 
         [0004]    2. Description of Related Art 
         [0005]    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. 
         [0006]    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. 
         [0007]    It is also necessary to provide a safe environment for electrical workers and other personnel in the vicinity of the installations where power is being measured, because installation may be required in an electrical panel that is operational. Installation of current sensors in a live panel requires the use of insulating gloves that make it difficult to perform fine work with the fingers. 
         [0008]    Therefore, it would be desirable to provide a combined voltage and current sensor that can provide isolated current draw information and voltage information so that power can be measured with a single sensor in an AC power distribution circuit. It would further be desirable to provide such a non-contact sensor that is easy to operate while an installer is wearing insulating gloves. 
       BRIEF SUMMARY OF THE INVENTION 
       [0009]    The invention is embodied in a current-sensing and voltage-sensing clamp and method. A current-sensing device including a current sensor and a voltage sensor both integrated in a clamping body that can be detachably coupled to a wire. The sensor thus provides outputs indicative of the current passing through the wire as well as an electric potential on the wire, so that power can be computed. 
         [0010]    The clamping body has two handles at a proximal end, which when squeezed, open an aperture containing a current sensor formed from ferrite cylinder portions in which the wire is inserted. When the handles are released, the current sensor closes around the wire to 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 portion(s). 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. 1  is a side view of a current and voltage sensor according to an embodiment of the present invention. 
           [0014]      FIG. 2  is a side view of a current and voltage sensor according to another embodiment of the present invention. 
           [0015]      FIG. 3  is a side view of a current and voltage sensor according to another embodiment of the present invention. 
           [0016]      FIG. 4  is an electrical block diagram illustrating a circuit according to an embodiment of the present invention. 
           [0017]      FIG. 5  is an electrical block diagram illustrating a circuit according to another embodiment of the present invention. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0018]    The present invention encompasses a clamping current and voltage sensor and methods of operating such sensors 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. The clamping feature makes it possible to attach and detach the sensor with gloved hands, for example while measuring branch circuit power in live electrical panels. 
         [0019]    Referring now to  FIG. 1 , a sensor  10 A, in accordance with another embodiment of the invention is shown. Sensor  10 A includes a clamp body  26 A that has two handle portions  22  at a proximal end, and at the distal end defines an aperture in which multiple ferromagnetic cylinder portions  12 A,  12 B, a current-sensing element  17 , and portions of a metallic voltage-sensing element  20  are disposed. Handle portions  22  are joined by a hinge  24  which is spring-loaded. When handle portions  22  are compressed together, the aperture opens, separating ferromagnetic cylinder portions  12 A,  12 B and voltage-sensing element portions  20  and permitting one or more wires to be introduced to sensor  10 A. When handle portions  22  are released, the ends of ferromagnetic cylinder portions  12 A,  12 B opposite current-sensing element  17  make contact and a side of current-sensing element  17  makes contact with ferromagnetic cylinder portion  12 A, closing the magnetic flux sensing loop formed by flexible cylinder portions  12 A,  12 B and current-sensing element  17 . Current-sensing element  17 , is generally a magnetic field sensor, such as a Hall effect sensor, current-sensing transformer, anisotropic magnetoresistance (AMR) sensor, ordinary magnetoresistance (CMR) sensor, giant magnetoresistance (GMR) sensor, or other suitable current-sensing device. 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 . Current-sensing element  17  provides information about a magnitude and phase of a current passing through a wire  3 , which can include multiple wires/conductors, around which flexible ferromagnetic cylinder portions  12 A,  12 B extend to form a conduction loop for magnetic flux, with a gap defined by current-sensing element  17  which senses the magnetic flux to measure the current passing through wire  3 . Sensor  10 A additionally includes a voltage-sensing element  20  provided by metal foil segments, which may be plated onto, formed within, or adhered to the inner surface of ferromagnetic cylinder portions  12 A,  12 B. Voltage-sensing element  20  provides capacitive coupling to branch circuit wire  3  and provides an output via an interface wire  15 A, which may also alternatively be replaced with a terminal or other suitable electrical connector. Voltage-sensing element  20  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. Interface wires  15 A and  15  extend through one of handle portions  22  for connection of measurement electronics, but alternatively the measurement electronics can be integrated within clamp body  26 A or handle portions  22 . Details of electronic measurement systems and the measurements that can be performed with the sensors disclosed herein are described below with reference to  FIG. 4  and  FIG. 5 . 
         [0020]    Referring now to  FIG. 2 , details of yet another sensor  10 B in accordance with another alternative embodiment of the present invention are shown. Sensor  10 B has a clamp body  26 B similar to clamp body  26 A of sensor  10 A of  FIG. 1 , and incorporates both current and voltage sensors. Therefore, only differences between sensor  10 A and  10 B are described below. Sensor  10 B includes a plurality of high-permeability bodies  18  replacing the ferromagnetic cylinder portions in sensor  10 A of  FIG. 1 , that cause the field around a cable inserted through the opening in housing  14 A to be concentrated in the vicinity of semiconductor magnetic field sensors  17 . High-permeability bodies  18  thereby improve the signal strength and signal-to-noise ratio (SNR) of the output voltages of sensors  17 . In accordance with alternative embodiments of the invention, high-permeability bodies  18  may be located between sensors  17  and the center of the opening formed in housings  14 A and  14 B, or may be a high-permeability coating deposited on one or more faces of sensors  17 . Further, high-permeability bodies  18  may be omitted altogether. The exemplary sensor  10 B also includes multiple voltage-sensing elements  20 A, which are functionally independent from sensors  17 , but may be plated onto, attached as a film, or otherwise disposed on the outer surfaces of sensors  17 . Voltage-sensing elements  20 A are coupled to measurement electronics by corresponding interface wires  15 A, not all of which are shown for clarity. Details of the field generated around multiple conductors and the measurements that are made with multiple sensors are disclosed in U.S. patent application Ser. No. 13/159,554 entitled “MULTI-CONDUCTOR CABLE CURRENT AND VOLTAGE SENSORS”, which is incorporated herein by reference. Differing positions of sensors  17  with respect to the conductor(s) inserted in the claim, for example, conductors of a multi-phase power cable, will lead to different resulting current values sensed at the various sensors  17 . Similarly, the differing positions of the conductor (s) to the various voltage-sensing elements  20 A will yield different values of sensed electric potential. 
         [0021]    Referring now to  FIG. 3 , details of yet another sensor  10 C in accordance with another alternative embodiment of the present invention, is shown. Sensor  10 C has a clamp body  26 C similar to sensor  10 A of  FIG. 1 , and incorporates both current and voltage sensors. Therefore, only differences between sensor  10 A and  10 C are described below. Sensor  10 C of  FIG. 3  includes a conductor  15 A that extends through and around ferrite cylinder portion  12 B and thereby provides two terminals for connection to the measurement circuit. The loop formed by conductor  15 A is used to provide self-calibration of the current and voltage-sensing elements of sensor  10 C as will be described in further detail below. 
         [0022]    Referring now to  FIG. 4 , a circuit for receiving input from sensor  10 A of  FIG. 1  and/or sensor  10 C of  FIG. 3  is shown in a block diagram. All or a portion of the depicted circuit may be integrated within clamp body  26 A of sensor  10 A/ 10 C, or sensor  10 A/ 10 C may be interfaced to the depicted circuit via a multi-conductor cable and connector. 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  100 . 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 power 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. 
         [0023]    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. If interface wire  15 A provides two terminals as in sensor  10 C of  FIG. 3 , then selector S 1  may be included to select between a measurement and calibration mode as described below. 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  108 B 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*Δ T  
 
         [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. 
         [0024]    If sensor  10 C of  FIG. 3  is connected to the circuit of  FIG. 4 , the voltage sensor wires  15 A from each end of the voltage-sensing conductor are provided to a selector S 1  that is controlled by a control signal measure provided from CPU  100 . When control signal measure is asserted, the circuit is in measurement mode, and the voltage sensor wires  15 A from each end of sensor  10 A are coupled together and provided to the input of voltage measurement circuit  108 B. When control signal measure is de-asserted, the circuit is in calibration mode, and voltage sensor wires  15 A from each end of sensor  10 A are coupled to a current source  101  that generates a predetermined calibration current through voltage sensor wire  15 A. Also in calibration mode, a current measurement is made to determine an indication of the magnetic field generated by the current passing through voltage sensor wire  15 A as indicated by the output of current measurement circuit  108 A, which receives the output of the current sensor. Since the predetermined current level generated by current source  101  is known, the output of current measurement circuit  108 A provides a scale factor that can be used to correct subsequent measurements of current by current sensor  10 A, e.g., the current passing through wire  3 . Current source  101  may be a DC current source, so that CPU  100  can use a low-pass filter or integrator algorithm to remove AC noise from the calibration measurement, or alternatively, current source  101  may be an AC current source and a bandpass filter or algorithm can be used to remove other noise and offset from the measurement. The DC calibration measurement may be performed while the current is being passed through wire  3 . 
         [0025]    An exemplary set of measurements provide illustration of a calibration technique in accordance with the above-described calibration mode. In calibration mode, if the predetermined current level generated by current source  101  is given by I CAL , and the output voltage of current measurement circuit  108 A is given by V CAL , then, as long as sensor  10 C is linear and all of the circuits in  FIG. 4  remain linear, the output of current measurement circuit  108 A for an unknown current level I UNK  is given by: 
         [0000]        V   MEAS   =I   UNK ( V   CAL   /I   CAL ) 
         [0000]    Therefore, unknown current level I UNK  can be determined from: 
         [0000]        I   UNK   =K·V   MEAS , 
         [0000]    where calibration value K=I CAL/ V CAL . Further, if in calibration mode V CAL  does not exceed a predetermined threshold, the system can indicate a sensor failure, which may be a connection failure in one of wires  15  or voltage-sensing conductor  15 A, or may be a failure of sensor  17  or the measurement circuit. Further, while the above equations assume linear behavior, current source  101  may be an adjustable current source that in a linearity measuring mode is adjusted according to a control value Adjust, which controls the magnitude of the current injected in voltage-sensing conductor  15 A when control signal measure is de-asserted. A table of calibration values may be stored and/or coefficients may be determined to form a piecewise linear or other approximation that permits non-linear computation of I UNK  from V MEAS . A saturation level may be detected for sensor  10 A when increases in the adjustable current level commanded by control value Adjust no longer lead to consequent increases in measured voltage level V MEAS . For example, operation of the sensing system may be restricted to current levels that have less than a predetermined error due to non-linearity in the sensor, or the measurement range may extend to levels at which correction has high error due to the measured voltage level V MEAS  changing by small fractions of the value expected if sensor  10 A were linear. Other details of current calibration in voltage/current sensors are described in U.S. patent application Ser. No. 13/159,536 entitled “CALIBRATION OF NON-CONTACT CURRENT SENSORS”, the disclosure of which is incorporated herein by reference. 
         [0026]    Referring now to  FIG. 5 , a system in accordance with another embodiment of the present invention is shown, and which is suitable for use with sensor  10 B of  FIG. 2 . A multiplexer  101 A receives signals from the individual magnetic field sensors  16  and selects a sensor for measurement, providing input to a current measurement circuit  108 A, which is an analog circuit that appropriately scales and filters the output of sensors  16 . The output of current measurement circuit  108 A is provided as an input to an analog-to-digital converter (ADC)  106 ,which converts the output waveforms generated by current measurement circuit  108 A to sampled values provided to a central processing unit (CPU)  100  that performs calculations in accordance with program instructions stored in a memory  104  coupled to CPU  100 . Alternatively, a separate magnetic field measurement circuit  108 A and multiplexer  101 A may not be necessary, and sensors  16  may be coupled directly to ADC  106 . Similarly, multiplexer  101 B receives signals from voltage sensing elements  20 A and selects a voltage-sensing element for measurement, providing input to a voltage measurement circuit  108 B, which is an analog circuit that appropriately scales and filters the output of voltage sensing elements  20 A. The output of voltage measurement circuit  108 B is provided as an input to an analog-to-digital converter (ADC)  106 , which converts the output waveforms generated by voltage measurement circuit  108 B to sampled values provided to a central processing unit (CPU)  100 . An input/output (I/O) interface  102  provides either a wireless or wired connection to an external monitoring system, such as a wireless local area network (WLAN) connection or wired Ethernet connection. An integrated display  105  may be additionally or alternatively provided to indicate a direct measure of current in a conductor. CPU  100  can perform computations to discover and map the phases of conductors in a cable, as the invention is not limited to 2-phase systems such as that depicted in above-incorporated U.S. Patent Application “MULTI-CONDUCTOR CABLE CURRENT AND VOLTAGE SENSORS.” Further, cable configurations such as multiple conductors corresponding to a single return conductor may be measured and a net current magnitude value determined. 
         [0027]    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.