Abstract:
An apparatus for measuring contact current includes data acquisition circuitry with at least two sensor contacts to measure the voltage drop across an animal or human body. A portable data processing unit is connected to the data acquisition circuitry to process the voltage data to produce current flow data.

Description:
BRIEF DESCRIPTION OF THE INVENTION  
         [0001]    This invention relates generally to measuring and analyzing current flow through an animal or human body. More particularly, this invention relates to a portable contact meter analysis device.  
         BACKGROUND OF THE INVENTION  
         [0002]    For the purposes of this patent, contact current is defined as the current that flows in an animal or human subject when the subject touches (contacts) two different electrically conductive points in the subject&#39;s environment. Different conductive points with different voltage potentials cause current to flow in the living body.  
           [0003]    Most current measurement systems use one of two basic methods. The first method is to insert a known impedance into the current path and measure the voltage across the known impedance. (Current, voltage, and impedance are all related by Ohm&#39;s law, which states that Voltage=Current×Impedance. With two known quantities, one can calculate the unknown quantity, current in this case.) The second method is to attach a coupling transformer around the current path and measure the current via the magnetic field generated from the current flow.  
           [0004]    Both methods have significant disadvantages for a contact meter; and at present there are no devices available to measure this contact current. The first method requires that the current path be broken with a fixed impedance and that the fixed impedance not significantly alter the measured current. For contact current, the points of contact can be anywhere on the body. The current flow will generally occur throughout the volume of the body. Contact points can occur at more than two locations with different voltages at each point. Consequently, it is difficult to gather and process this information.  
           [0005]    The second measurement method (using a transformer) also has a number of disadvantages. Current transformers of this type are usually insensitive and must be used with large primary currents (Amps). Contact currents are typically small, milliamps (mA) and microamps (uA), and need a sensitive measurement device. Sensitivity can be increased by increasing the size and weight of the transformer, but this makes the devices too cumbersome for human use. Thus, prior art devices have not successfully measured contact current.  
           [0006]    An additional problem is that the transformer method measures the magnetic field created by the current. Environments with potential contact currents often have ambient magnetic fields. The transformer measures both the magnetic field from the current flow and the ambient magnetic field, and cannot differentiate the two sources.  
           [0007]    Accordingly, it is desirable to provide an improved technique for measuring current flow in an animal or human body.  
         SUMMARY OF THE INVENTION  
         [0008]    An apparatus for measuring contact current includes data acquisition circuitry with at least two sensor contacts to measure the voltage drop across an animal or human body. A portable data processing unit is connected to the data acquisition circuitry to process the voltage data to produce current flow data.  
           [0009]    The invention provides a compact, lightweight, low-power device that can be used to flexibly and non-invasively determine the magnitude of current flowing through an animal or human body. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0010]    For a better understanding of the invention, reference should be made to the following detailed description taken in conjunction with the accompanying drawings, in which:  
         [0011]    [0011]FIG. 1 illustrates a contact meter analysis apparatus in accordance with an embodiment of the invention.  
         [0012]    [0012]FIG. 2 illustrates the data acquisition circuitry channels in accordance with an embodiment of the invention.  
         [0013]    [0013]FIG. 3 illustrates a channel of the acquired data conditioning circuitry in accordance with an embodiment of the invention 
     
    
       [0014]    Like reference numerals refer to corresponding parts throughout the drawings.  
       DETAILED DESCRIPTION OF THE INVENTION  
       [0015]    [0015]FIG. 1 illustrates a contact meter analysis apparatus  100  constructed in accordance with an embodiment of the invention. The contact meter analysis apparatus  100  includes data acquisition circuitry  102  and a portable data processing unit  104 . To make the unit portable, a preferred embodiment of the invention includes an autonomous power source  124 . In an embodiment of the invention, the portable data processing unit  104  includes acquired data conditioning circuitry  112  to receive data from the data acquisition circuitry  102 . As its name implies, the acquired data conditioning circuitry  112  pre-processes and digitizes the data so that it is in a format useful to the remainder of the circuitry in the portable data processing unit  104 .  
         [0016]    As indicated in FIG. 1, the acquired data conditioning circuitry  112  and data input interface circuitry  114  apply digital signals to the system bus  126 . The data input interface circuitry  114  preferably includes an input mechanism, such as a keypad. A central processing unit (CPU)  116 , data output interface circuitry  118 , and memory  130  are also connected to the system bus  126 . The CPU  116  executes the programs stored in memory  130 . The data output interface circuitry  118  receives digital signals from the system bus  126  and is connected to a display  122 , preferably an LCD, and a removable memory  120 , preferably a compact flash memory. The memory  130  stores a set of executable programs and data, including: an initiate logging routine  132 , an adjust threshold routine  134 , a parameter calculator  136 , generic body impedance data  138 , accumulated body impedance data  140 , current flow data  142 , voltage data  144 , and an output module  146 . The programs and data stored in memory  130  are described in connection with FIG. 2 and FIG. 3.  
         [0017]    [0017]FIG. 2 illustrates an embodiment of the data acquisition circuitry  102  and acquired data conditioning circuitry  112  that utilizes four channels. The electrically conductive skin contacts  210 A,  210 B,  210 C, and  210 D, hereinafter referred to collectively as contacts  210 , correspond to respective channels of the data acquisition circuitry  102 . The contacts  210  are preferably made with skin patches commonly used for medical measurements. The contacts  210  are placed on an animal or human body  202  and are connected via connections  220 A,  220 B,  220 C, and  220 D, hereinafter referred to collectively as connections  220 , to the conditioning circuitry channels  230 A,  230 B,  230 C, and  230 D, hereinafter referred to collectively as channels  230 , or individually as a channel  230 . The channels  230  make up the acquired data conditioning circuit  112 . In one embodiment of the invention, the connections  220  are cables. The invention encompasses both wireless and physical connections.  
         [0018]    In general, an overall model of the body&#39;s impedance would have high impedance and high variability for the contact areas, skin, and extremities (fingers, hands, feet, toes, head, etc.); and low impedance and low variability for the torso, legs, and arms. In a preferred embodiment of the invention, the contacts  210  are placed on the legs, torso, or arms so that the voltage measured is across the arms, legs, and torso to reduce the impedance variability. Note that in FIG. 2 the contacts  210  are placed on both arms and both legs of living body  202 .  
         [0019]    [0019]FIG. 3 illustrates an embodiment of a channel  230  (see FIG. 2). The channel  230  includes a low noise amplifier  302  that buffers the signal and protects the circuitry from outside noise and disturbances. In addition, the low input current (e.g., 10 picoamps (pA)) of the instrument reduces errors from the high impedance contacts  210  (see FIG. 2). The low noise amplifier  302  amplifies the signal. The signal is bandpass filtered through a high pass filter  304  and a low pass filter  306 . After the low pass filter  306 , the processed signal is fed to an analog to digital converter (A/D)  310  to digitize the signal and to an adjustable threshold detector  308 . The adjust threshold routine  134 , discussed below, operates to adjust the adjustable threshold detector  308  for each channel  230 . The adjustable threshold detector  308  preferably disregards signals between the positive and negative threshold levels at which it is set. The digital output of the analog to digital converter  310 , a basic voltage measurement, is applied to the system bus  126  for use by the CPU  116 .  
         [0020]    In a preferred embodiment of the invention, the output module  146  generates menus that are displayed on display  122 . A user selects items on the menu or enters input through the data input interface circuitry  114  to control the behavior of the executable programs stored in memory  130 . When so ordered by the user, the output module  146  preferably writes data to the removable memory  120  or to the display  122 . In this way, the user can, for example, write current flow data  142  to the removable memory  120  and access that data on a different machine.  
         [0021]    In a preferred embodiment of the invention, when the adjust threshold routine  134  is selected, the output module  146  prompts the user through the display  122  for a threshold value for a channel  230 . The user may enter threshold values for one or all of the channels  230  through the data input interface circuitry  114 . The adjust threshold routine  134  sets the adjustable threshold detector  308  to the threshold values input by the user.  
         [0022]    Studies have measured impedances at various locations on the body and the ranges of values and averages for different sized individuals are available. The memory  130  preferably contains generic body impedance data  138  from such studies. Thus, generic body impedance data  138  contains estimated impedances for the living body  202 . Preferably, this data includes data for bodies of different heights and weights.  
         [0023]    When a user indicates through the data input interface circuitry  114  that the logging is to begin by pressing the appropriate key or sequence of keys, the data input interface circuitry applies a signal to the system bus  126  that the CPU  116  recognizes as an initiate logging command. The CPU  116  then executes the initiate logging routine  132 . The initiate logging routine  132  starts logging voltages measured through the data acquisition circuitry  102  and conditioned by the acquired data conditioning circuitry  112 . The voltages are stored as voltage data  144  in memory  130 . The voltage data  144 , as well as the current flow data  142  and accumulated body impedance data  140 , could also be processed immediately, without storing to memory  130 . In that case, the parameter calculator  136  would be run concurrently with the initiate logging routine  132 , rather than subsequently, as discussed below for illustrative purposes.  
         [0024]    Utilizing Ohm&#39;s Law, current=voltage/impedance, the voltage data  144 , and the generic body impedance data  138 , the parameter calculator  136  calculates current through an animal or human body  202 . Voltage from the voltage data  144  and impedance from the generic body impedance data  138  are available. The results of the calculation are stored as current flow data  142 . Note that “known impedances,” for the purposes of this calculation, are from the generic body impedance data  138  and are, in the strictest sense, estimates.  
         [0025]    The parameter calculator  136  also utilizes Ohm&#39;s Law for the purpose of measuring impedance for the living body  202 . The user sets current flow data  142  to a known value and runs a current with that known value through the living body  202 . Preferably, additional contacts are used for this purpose. Voltage data  144  is accumulated as previously described. The parameter calculator  136  calculates impedance=voltage/current for the current represented in current flow data  142  and the voltage represented in voltage data  144 . The calculated impedance is stored in accumulated body impedance data  140 .  
         [0026]    Utilizing Ohm&#39;s Law, current=voltage/impedance, the voltage data  144 , and the accumulated body impedance data  140 , the parameter calculator  136  calculates current through an animal or human body  202 . Voltage from the voltage data  144  and impedance from the accumulated body impedance data  140  are available. The results of the calculation are stored as current flow data  142 . Because impedance for each living body  202  is slightly different, calculating known impedances for an animal or human body  202  will generally result in more accurate current flow measurements than using the generic body impedance data  138 .  
         [0027]    Preferably, the data input interface circuitry  114  includes a button to stop the logging process initiated by the initiate logging routine  132 . This button functions as a kill switch or cancel button.  
         [0028]    The foregoing description, for purposes of explanation, used specific nomenclature to provide a thorough understanding of the invention. However, it will be apparent to one skilled in the art that the specific details are not required in order to practice the invention. In other instances, well known circuits and devices are shown in block diagram form in order to avoid unnecessary distraction from the underlying invention. Thus, the foregoing descriptions of specific embodiments of the present invention are presented for purposes of illustration and description. They are not intended to be exhaustive or to limit the invention to the precise forms disclosed, obviously many modifications and variations are possible in view of the above teachings. The embodiments were chosen and described in order to best explain the principles of the invention and its practical applications, to thereby enable others skilled in the art to best utilize the invention and various embodiments with various modifications as are suited to the particular use contemplated. It is intended that the scope of the invention be defined by the following claims and their equivalents.