Abstract:
Current flowing into and out of a battery installed in a vehicle (V) with the required cable between one of the battery terminals and a reference point on the vehicle is determined by measuring the voltage across the cable and computing the current from the digital value of the measured current. The resistance value of the cable can be known in which case the current is computed using Ohms law and a differential current sensor can be used to respond to the voltage measured across the cable to accommodate for different ranges of current. In an embodiment where the cable resistance is unknown, a reference current source produces a known voltage that is used to set the input value to an amplifier of fixed gain for the voltage measured across the cable by controlling the output of a potentiometer so that the amplifier output voltage can be set to match the reference current generator output voltage, thereby establishing the voltage output of the amplifier as a measurement of the current flow in the cable.

Description:
FIELD OF THE INVENTION 
     The present invention relates to the measurement of electrical current and particularly to measuring current in a vehicle using one or both of the cables of a vehicle connected to its battery. 
     BACKGROUND OF THE INVENTION 
     In many applications an electrical current has to be measured and monitored. For example, in vehicles having a battery used for various purposes, such as starting an engine or powering electrical systems such as lights and instruments, current is measured and monitored for instrumentation control purposes. For example, U.S. Pat. No. 4,937,523 entitled “Method for Monitoring Automotive Battery Status”, which is assigned to the assignee of the subject application, describes an analysis system in which current is measured and monitored during operation of an automobile to determine capacity, state of charge and certain fault conditions of the vehicle&#39;s battery. Such a system can also be used for other types of vehicles, such as aircraft, buses, etc. 
     One technique for electrical current measurement and/or monitoring the current flow in the electrical systems of automotive, aircraft and other vehicles requires a sensor that effectively measures the magnetic field caused by a current flowing through a conductor. Some of the commonly used sensor types are Hall effect and inductive devices. Another technique, used in the aforesaid patent, is that of a precision resistance shunt of a known resistance value which is placed in series with a part of the conductor carrying the current to be measured. By measuring the voltage across the shunt and knowing its resistance, the current can be calculated. 
     While the use of a shunt is adequate for many purposes, it has disadvantages in that it requires the cost of the shunt itself and additional electrical connections. More importantly, the shunt results in a power loss which is a product of the square of the current value times the resistance value of the shunt. For example, if a one ohm shunt is used to measure current flow in the electrical system of an automobile, the power loss due to the resistance heating of the shunt would be substantial and could cause unnecessary fuel consumption in the vehicle. 
     Accordingly, it would be desirable to be able to measure current in a vehicle having a battery without the necessity of using an additional element such as a shunt or another type of sensor. 
     SUMMARY OF THE INVENTION 
     The battery used in a vehicle typically has two terminals, one of which is connected to a vehicle electrical reference point (ground) such as its metal chassis and the other to provide the operating voltage to a takeoff supply point. In most vehicles, a cable is provided between each of the battery terminals and its connecting point, such as ground or the vehicle system voltage supply takeoff point. In accordance with one embodiment of the invention, one or both of the battery cables is made to have a known resistance value. For example, a typical value would be 1.0 millohm for the cable connected to the positive battery terminal and 0.1 millohm for the cable connected to the battery negative terminal. 
     In accordance with the invention, the current flow through a cable connected to the battery into the battery, such as during charging by the vehicles alternator, or the flow from the battery, such as during engine starting or operation of any other vehicle electrical system component, is determined by measuring the voltage across the existing and necessary vehicle cable. The current is determined by measuring the voltage drop across the cable of known resistance value and computing, such as by using a computer, the current by the well known Ohm&#39;s law formula I=E/R. 
     In one embodiment of the invention, a differential current sensor is used. The current sensor is an electronic amplifier type device whose characteristics can be modified to accommodate different current ranges. In a preferred embodiment, to accommodate for the possibility of the cable resistance being different from the desired value and other circuit tolerance variations, an auto-calibration circuit is used which permits an accurate measurement of battery current to be made without the need to know the exact resistance of the cable. Also, provision is made to automatically compensate for component tolerances that would cause an offset when the system input current is zero. 
     It is therefore an object of the present invention to provide for the monitoring and measurement of current in a vehicle having a battery using one of the existing and necessary battery cables as a part of a sensor arrangement. 
     In accordance with one aspect of the invention a cable of known resistance value is connected to one of the terminals of a vehicle battery through which current flows is used to provide the input to a differential current sensor to measure and monitor battery current flow. 
     In accordance with another aspect of the invention a current sensor for a vehicle in which one or both of the cables connected to the vehicle&#39;s battery terminals is made to be of a known resistance value and is used as shunt for measuring current. 
     In accordance with a further aspect of the invention the voltage across a cable connected to one of the terminals of a vehicle battery through which current flows is provided to a circuit for measuring and monitoring the battery current having auto-calibration capability which permits an accurate measurement of the current to be made without the need to know the exact resistance of the cable. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Other objects and advantages of the present invention will become more apparent upon reference to the following specification and annexed drawings in which: 
     FIG. 1 is a schematic diagram of one illustrative embodiment of the invention comprising a current measurement and monitoring circuit using the existing battery cable in a vehicle; 
     FIG. 2 is a schematic diagram of another specific illustrative embodiment of the invention in wherein a current measuring and monitoring circuit uses an existing vehicle battery cable with a differential amplifier; and 
     FIG. 3 is a schematic diagram of another illustrative embodiment of the invention wherein the circuit has an auto-calibration capability. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring to FIG. 1, the body of a vehicle, such as an automobile, aircraft, bus, or boat is designated V. The vehicle body is generally of metal and has an electrical ground reference  8  which can be the vehicle frame or chassis. Within the vehicle is a battery  1  of any conventional type, such as lead-acid, used to supply power for normal purposes such as starting, lighting and instrumentation (SLI). Battery  1  has the usual positive and negative terminals. Reference numeral  2  designates the electrical load of the vehicle that is serviced by the battery. The load  2  is shown connected between the battery positive terminal and the electrical reference ground  8  and can be of any suitable type, such as a starter motor, lights, air conditioning system, etc. The battery charging system, which is of any conventional type, is not shown. 
     An electrical conductor cable  3  is connected between the battery negative terminal and the ground reference  8 . This is commonly called the ground return cable. Cable  3  is a necessary component of the electrical system found in virtually all vehicles having a battery. In accordance with one embodiment of the invention, cable  3  is made to have a known resistance value. This can be, for example, 0.1 millohms, although any other suitable value can be used depending upon the overall requirements of the vehicle electrical system. Any current flowing into battery  1 , such as during charging, or flowing out of the battery, such as during operation of an electrical system, will flow through the cable  3 . 
     In many cases, such as for operation of instrumentation and control systems, it is desired to measure and monitor the current flow into and out of the battery. To accomplish this in the embodiment of FIG. 1 the voltage drop across cable  3  is used. This voltage will vary depending upon the current flow to and from battery  1 . The voltage appearing across cable  3 , which is an analog quantity, is applied over lead  14  to the input of an analog-digital converter  20  which converts it to a digital quantity. The digital quantity is supplied to a computer  22  which has programmed therein the resistance value of the cable  3 . Thus, the computer is able to compute the current flow to and from the battery  1  by using the usual well-known Ohm&#39;s law formula 
     
       
         I=E/R, where 
       
     
     I=the current 
     E=the value of the voltage measured across cable  3   
     R=the resistance value of cable  3 . 
     Accordingly, the arrangement shown in FIG. 1 is capable of monitoring the current without the need for any additional component, such as a shunt used with cable  3 , which would consume power, or another type of sensor and any connections that they would require. The computed current value is available for any desired purpose. There can be a continuous monitoring of the current flow so that information can be supplied to a battery monitoring system of the type disclosed in the aforesaid patent. All of this is accomplished simply and efficiently. 
     FIG. 2 shows another embodiment of the invention that uses a differential current sensor. The same reference numerals are used for the common elements shown in FIG.  1 . Element  1  is the vehicle battery,  2  is the vehicle electrical system load supplied by the battery and  3  is the cable of known resistance value between the battery&#39;s negative terminal and the vehicle electrical ground reference  8 . 
     An instrumentation amplifier  30  has input terminals  32  and  33  connected respectively to the battery negative terminal and the vehicle electrical ground reference  8 . The instrumentation amplifier  30  can be, for example, of type AWA  118  manufactured by Burr-Brown. The input terminals of such a device have a high input impedance. The device  30  also has a resistor  34  connected between appropriate terminals to set its gain. Device  30  is also shown as receiving a positive operating voltage from a source Vcc and a negative voltage from a source Vdd. Device  30  has an output terminal  38  and a reference terminal  32 . 
     The voltage across cable  3 , corresponding to the current flowing through it, is sensed at the high impedance input terminal  32  of the device  30 . The gain of device  30  is set by resistor  34  to any suitable value, for example in a range of ±5 volts input (the range of the voltage change across cable  3 ) to correspond to a range of ±1000 Amperes of current flowing through the cable. Thus, as the voltage across cable  3  varies, corresponding to a variation in current flow, the signal at device  30  input terminal  32  changes. 
     The output on device terminal  38  will be an analog voltage which corresponds to the current flowing through cable  3  into and out of the battery. This is supplied to the A/D converter  20  whose output digital signal is supplied to the computer  22  which is programmed to be able to make the current flow computation. 
     The reference terminal  39  of device  30  can be adjusted to offset the device  30  output range. If a bipolar A/D converter  20  is used, the reference terminal  39  of device  30  would be set near zero depending on the internal offset of the amplifier device itself. If the converter  20  is unipolar, the reference terminal  39  would be set at the center of the voltage range of the converter. 
     The output  38  of the amplifier device  30  can be amplified further to obtain smaller current ranges such as, for example, ±100 Amperes and ±10 Amperes. That is, circuit resolution is increased for a higher voltage input to A/D converter  20  since it can be resolved into finer increments. The computer  22  can be programmed with an algorithm to automatically determine which of the ranges to use for the battery current at any given time. 
     FIG. 3 shows another embodiment of the invention which operates without knowing the exact resistance value of the cable. This is to provide three main functions for monitoring the battery current of a vehicle. The first function is to measure the current going both in and out of the battery. The second is an auto-calibration circuit which calibrates the battery&#39;s return cable used as a shunt to allow accurate measurement of the battery&#39;s current. The auto-cal circuit eliminates the need for knowing the exact impedance (resistance) of the cable. This can vary with manufacturing tolerances and also varies depending on the model of the car. The third function is to provide for automatic compensation for component tolerances that would cause an offset when the system input current is zero. 
     In FIG. 3 the following elements are used with the same reference numerals applied as used in FIGS. 1 and 2. As before,  1  indicate the car battery,  2  the vehicle electrical load,  3  the ground return cable and  8  the ground reference connection point of the vehicle frame. There is an auto-zero relay  104  whose operation is controlled by digital signals from computer  22  in accordance with an application program or by a dedicated digital controller having an embedded program. In the following description the term controller is used but it should be understood that the controller can be part of a computer. One contact of relay  104  is connected to the negative terminal of battery  1 , to which cable  3  is connected, and the other contact to the ground reference point  8 . The relay  104  center arm is connected to a resistor  105  which serves as the input to a variable voltage divider which includes an adjustable digitally controlled potentiometer  106  having a resistor  107  connected between its output terminal and ground  8 . The center arm pick off point of potentiometer  106  can be set by applying digital signals from the controller to its input terminals. 
     A resistor  108  connects the output of the voltage divider  105 - 107  to the positive input terminal of an instrumentation amplifier  111  which has an input resistor  109  between its negative input terminal and the negative battery terminal to which cable  3  is connected. Amplifier  111  can be of type INA  118  made by Burr-Brown. There is a resistor  110  at the input of amplifier  111  to set its gain. 
     Resistors  112  and  113  connected between the circuit positive voltage supply point and ground are part of a voltage divider for the auto-zero circuit which includes another adjustable digital potentiometer  114  controlled by the controller. Resistor  115  at the output of potentiometer  114  is connected to the upper end of a voltage divider formed by resistors  116  and  117  whose junction receives voltage from the circuit negative supply. The upper end of this voltage divider is connected to the input of a buffer amplifier  119  whose output is connected to ground by resistors  120  and  121 . This provides a negative bias for the auto-zero circuit enabling the voltage at the input of amplifier  119  to swing negative without the need of the output voltage of potentiometer  114  to go negative. 
     There is a connection from the junction of resistors  120  and  121  to the reference terminal  111 R of amplifier  111  to provide a divided down low impedance drive for the amplifier  111  reference voltage. 
     Each of amplifiers  122  and  123  has its input connected to the output V o  of amplifier  111 . Each of the amplifiers  122  and  123  has a selected gain to have an output to correspond to a current range. For example, amplifier  122  has a gain of  100  representing a current range of ±10 Amps and amplifier  123  a gain of  10  to provide an output representing ±100 Amps. The output of each of the amplifiers  122  and  123  is connected to the ADC  20  of FIG. 1 which is a data input to the controller or computer  22 . 
     A transistor  125 , such as a PMOS device, along with its bias resistors  126  and  127  is a reference current source. A signal is applied from the controller to the lower end of resistor  127  to turn on the device  125  and produce a voltage across a resistor  128  which is a precision resistor of known value, for example 10 ohms. The output voltage of device  125  across resistor  128  corresponds to a current value that can be computed since the value of resistor  128  is known. This voltage on line  129  is applied to the ADC and is used by the controller for calibrating the system gain. 
     In the operation of the circuit of FIG. 3, current from the car&#39;s battery  1  flows through the car&#39;s load  2  and back to the battery via the battery return cable  3 . By measuring the voltage across the return cable  3  the current flow in and out of the battery can be calculated. The voltage at the battery negative terminal, which is the voltage across cable  3 , is applied from the lower contact of relay  104  across voltage divider  105 ,  106 ,  107 . A part of this voltage is taken from the center arm terminal of the digital potentiometer  106  and applied to the positive input of amplifier  111 . Amplifier  111  has an output swing, for example over a range ±5V, that corresponds to a range of current, for example ±1000 Amps, flowing through cable  3 . This is provided to the input of ADC  20 . The output of amplifier  111  also is applied to the inputs of amplifiers  122  and  123  of different gain and the outputs of these amplifiers are applied to ADC  20 . The outputs of amplifiers  122  and  123 , which also can be over a ±5V range, correspond to other ranges of the current flow as indicated above. 
     To make an accurate measurement requires properly setting the voltage divider  105 - 107  feeding the fixed gain amplifier  111  to correspond to the resistance of cable  3 , whose value is unknown. This is accomplished by the controlled voltage divider  105 ,  106 ,  107  in the path leading from cable  3  to the input to amplifier  111 . The fixed gain of amplifier  111  is set by resistor  110 . The range of the voltage output of divider  105 ,  106 ,  107  combined with the gain of amplifier  111  will cover the range of voltage variation applied to the input of amplifier  111  caused by variations in the resistance of cable  3 . 
     Adjustment of the divider  105 - 107  is accomplished by using the reference current from source  125  that can be switched on and off. As previously described, since the value of precision resistor  128  is known the controller can calculate a current from the measured voltage across resistor  128 . The controller periodically switches the current from source  125  on and off, and the difference between the voltage across resistor  128  and that at the output of the ±10 Amp amplifier  122  is measured. Based on this difference the controller adjusts divider  105 - 107 , by moving the arm of the digitally controlled potentiometer  106 , so as to obtain an output difference equal to the switched reference current. For example, if the reference current is a 1 amp pulse the voltage pulse at the output of amplifier  122  (±10 amp range) would be 500 mv peak to peak since ±5 volts (the range of the amplifier output) represents ±10 amps. 
     With the output of amplifier  111  having been set by the divider  105 - 107  to a reference current, during the time that the controller operates the circuit to measure the voltage across cable  3 , the voltage outputs of the amplifiers  111 ,  122  and  123  will be accurate measurements of the current flowing through the cable. In the circuit of FIG. 3 it is not necessary to know the resistance of cable  3 . 
     Once the auto-cal is completed the controller starts the auto-zero function. It first turns on relay  104 , i.e., the center arm would go up. This disconnects the input from the car&#39;s cable  3  to the voltage divider  105 - 107  and places a short circuit across the input to the current measuring circuit. The controller then operates to adjust the center arm of digital potentiometer  114  to set the input voltage at the amplifier  111  reference terminal  111 R. This allows the output of amplifier  111  to be adjusted to compensate for internal offsets. This is adjusted until the output of amplifier  111  makes the output at the ±10 Amp amplifier  123  at zero, within a predetermined tolerance. The auto-zero voltage at the output of potentiometer  114  is combined with negative voltage from the junction of voltage divider resistors  117  and  118  to provide a voltage centered on zero for the input to the reference terminal  111 R of amplifier  111 . Amplifier  119  along with resistors  120  and  121  provides a low impedance drive for the reference terminal  111 R of amplifier  111 . By using the biasing network of resistors  117  and  118  a positive to negative swing is obtained for the amplifier  111  reference terminal without having the output of potentiometer  114  go negative. The center arm of the digital potentiometer  106  cannot go below the voltage at terminal  114 G of digital potentiometer  114  in accordance with manufacturing specifications. 
     With the correct values of resistors  115  through  118  the reference voltage needed for amplifier  111  can be set to be at the center of the range of digital potentiometer  114 . In the selection of components around both digital potentiometers  106  and  114  the adjustment range is set to cover the expected tolerance variations of the ground return cable  3  and various offset voltages caused by component tolerances. 
     Each of the circuits of FIGS. 1-3 is able to measure and monitor the current flow into and from the vehicle battery without the need for any additional components such as a current shunt. 
     Modifications of the circuits shown include the use of the cable connected to the battery positive terminal or the use of both cables. For the latter, there can be measurement of the voltage across one cable for battery current flow input and across the other terminal for current flow output. Also, the computer  22  can be provided with a data lookup table to directly convert voltage input data to a current value rather than making the Ohm&#39;s law type calculation. 
     Specific features of the invention are shown in one or more of the drawings for convenience only, as each feature may be combined with other features in accordance with the invention. Alternative embodiments will be recognized by those skilled in the art and are intended to be included within the scope of the claims.