Abstract:
An apparatus for measuring current carried by a conductor has a differential voltage amplifier having inputs for connection to the conductor at two spaced-apart points thereof, constituting the ends of a segment. A circuit which converts an output voltage of the amplifier into a current value incorporates a microcontroller associated with a programmable memory storing a calibration table in digital form.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    The invention relates to apparatus for measuring current flowing in an electrical conductor, whether DC or AC.  
           [0002]    A conventional solution for measuring such a current, particularly when it is DC, consists in cutting the cable, in interposing a known resistance between the cut ends, and in measuring the potential difference across the terminals of the resistance.  
           [0003]    That solution requires the cable to be cut and a resistive element to be inserted between its cut ends.  
         SUMMARY OF THE INVENTION  
         [0004]    It is an object of the invention to provide apparatus of a type avoiding that requirement and for this purpose making use of the performance of recent voltage amplifiers and the possibility of digitally processing the analog data supplied by such an amplifier.  
           [0005]    To this end, it is provided apparatus for measuring DC or AC current carried by a conductor, comprising a voltage differential amplifier having two inputs for connection to the conductor each at one of two spaced-apart points of the conductor thereof, constituting the ends of a segment and a measurement circuit. That circuit is apt to convert an output voltage of the amplifier into an electrical current value. It further includes a microcontroller and a programmable memory for storing a calibration table in digital form. It may further include a generator for generating a calibration current at a frequency sufficiently different from a frequency of the current to be measured for enabling the calibration current to be separated from the current to be measured by filter means, and means for determining or correcting the calibration table responsive to a voltage measured at the calibration current frequency.  
           [0006]    With DC, the circuit provides the magnitude of the current directly. With AC, a converter is provided. The invention is particularly adapted to measuring high currents.  
           [0007]    Such measurement apparatus avoids any need to cut the cable and does not have any incidence on the connections between successive lengths of the conductor.  
           [0008]    Measurement is based on the relationship which exists between the output voltage Vs from the amplifier and the current I carried by the conductor: 
             Vs =(1+ε) A.Vc   
           with  
           Vc=R.I 
           [0009]    where:  
           [0010]    I is the current in the conductor;  
           [0011]    R is the resistance of the segment of conductor;  
           [0012]    ε is a coefficient that depends on static errors, is always much less than 1, and it stored; and  
           [0013]    A is the current/voltage amplification factor.  
           [0014]    When the temperature of the conductor is liable to vary sufficiently to have significant influence on the resistivity and thus on the resistance of the segment, provision is made for a temperature sensor and for a correction to be calculated that takes account of the way resistance varies as a function of temperature.  
           [0015]    Static errors can be measured in a workshop when the measurement apparatus is definitively installed on the conductor. It is also possible to perform periodic calibration by providing in the apparatus a current generator delivering a calibration current of known magnitude at a frequency that enables the calibration current to be distinguished from the current that is to be measured, which current may be DC or AC. The voltage measured when passing a known current that comes from a generator which is not subject to temperature drift makes it possible to determine the required calibration factors and possibly correction factors as a function of the temperature. A temperature sensor is no longer necessary if calibration by means of the current generator is performed often enough or just before each measurement.  
           [0016]    The circuits of the apparatus advantageously constitute an assembly which is molded onto the conductor segment after that segment has been stripped to allow the connections to be made thereto.  
       
    
    
       [0017]    The above characteristics, and others, will appear more clearly on reading the following description of particular embodiments, given as non-limiting examples;  
       BRIEF DESCRIPTION OF THE DRAWINGS  
       [0018]    [0018]FIG. 1 is a block diagram showing the parameters involved in the measurement; and  
         [0019]    [0019]FIG. 2 is a diagram showing one possible physical structure of the apparatus.  
     
    
     DETAILED DESCRIPTION  
       [0020]    The apparatus schematically shown in FIG. 1 comprises a differential preamplifier  10  whose inputs are connected to the conductor at opposite ends of a segment  12  of length L. In the embodiment shown diagrammatically, the preamplifier is followed by two amplifiers  14  and  16 , one or other of which is put into operation depending on the magnitude of the current so as to increase the range over which measurements can be made. This number is not limiting. The number of amplifiers depends on the measurement range and also on whether or not the current, if DC, can reverse. Selection can be performed by a selector circuit  16  as a function of the voltage output by the preamplifier  10 . An analog output stage  18  feeds a microcontroller  22  which includes an input analog-to-digital converter and which is associated with a memory  20  for storing a calibration table. A voltage-stabilized power supply (not shown) is connected to the active components.  
         [0021]    The resistance R of the conductor segment can be written as follows: 
           R=ρ   0 (1+α.Δθ) L/S   
         [0022]    where:  
         [0023]    ρ 0  is its resistivity at a calibration temperature, e.g. 20° C.;  
         [0024]    S is its cross-section;  
         [0025]    α is the mean temperature coefficient in the intended operating range; and  
         [0026]    Δθ is the difference between the measurement temperature and the calibration temperature.  
         [0027]    Variations in the length L of the cable are generally negligible over the intended measurement range.  
         [0028]    The current Ic carried by the cable is related to the output voltage Vs after amplification by the nominal factor A according to the following relationship:  
           Vs =(1+ε) A.Ic (1+α.Δθ). R   0   (1) 
         [0029]    where R 0  is the resistance at the reference temperature and ε is a static error stored in the form of a table since it can depend on Vs. The value of Ic is thus written as follows: 
           Ic=Vs /[(1+ε) A (1+α.Δθ)]. R   0   (2) 
         [0030]    To enable the microcontroller  22  to perform temperature correction, the apparatus includes a temperature sensor  24  placed in the vicinity of the segment  12 . The temperature measured in this way, put into digital form, is taken into account by the microcontroller to calculate the correction term 1+α.Δθ in equation (2).  
         [0031]    In general, the apparatus is mounted in permanent manner on a segment of cable and calibration is performed in factory. This implies storing R 0  and the mean value α of the temperature coefficient over the intended measurement range. With copper, for example, the mean temperature coefficient is 0.004 over the range going from −20° C. to 60° C. In the common circumstance of a copper conductor, variations in length over this temperature range are negligible since the linear expansion coefficient is then 1.7×10 −5  mm/° C.  
         [0032]    Static errors ε generally change little as a function of temperature so it suffices to measure the value thereof over a series of current values that are sufficiently close and to store them. This operation can be performed by causing well-determined currents to pass along the segment at a stabilized temperature and by calculating each time the corresponding value for ε.  
         [0033]    During each measurement, the microcontroller  22  determines an appropriate value for ε responsive to Vs, possibly interpolating between the values stored in the memory  20 . It also calculates the temperature correction factor 1+α.Δθ and it delivers the value of the current on its output  25  in digital form. By way of example, this output can be applied to the input circuit of a display. It can also be used as an input to a regulation or control member.  
         [0034]    An important application lies in measuring battery current, in particular for a motor vehicle having an alternator-starter.  
         [0035]    In a modified embodiment, the apparatus further includes a current generator  26  connected to the segment of cable so as to cause it to carry a determined current or a current that has a time sequence of determined levels. A component  28  enables the apparatus to be given two operating modes. One of the two modes is a calibration mode during which the generator supplies an alternating voltage at a frequency that is quite different from the frequency to be measured in the conductor, for example a frequency in the kHz range when the apparatus is intended to measure DC or AC at 50 Hz or 60 Hz. When calibration is to be performed while a current is being carried by the conductor, a frequency filter is inserted at the input to the preamplifier  10 .  
         [0036]    The component  28  can be manually controlled so as to authorize calibration on demand, or it can be constituted by a sequencer.  
         [0037]    The physical structure of the apparatus can be as shown in FIG. 2. The conductor  12  is stripped over a portion which receives voltage-taking clamps  30 . A card  32  carriers the active components of the system. At the ends of the segment, the insulation of the cable is connected to the card by molded gaskets  34 . A cover (not shown) can close the apparatus and the cover can carry a display unit.