Abstract:
A current sensor system that uses a sensor resistor to sense current flow and in which the level of voltage drop across that resistor is used to control the current flow through another resistor, the input resistor, into a current integrator. The ratio of input resistor to sensor resistor resistance values determines the ratio of sense current to integrator input current level. By matching the temperature coefficients of the resistors the effects of temperature are reduced. The integrator output provides either directly or indirectly a voltage level, frequency, or duty cycle output signal to indicate the sensor resistor current level.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
   The benefits of filing this invention as Provisional application for patents “Temperature stable current sensor system”, U.S. PTO 61/000,617 filed Oct. 26, 2007 and “Temperature stable current sensor system”, U.S. PTO 60/964,902 filed Aug. 15, 2007 by Fred Mirow are claimed. 

   BACKGROUND OF THE INVENTION 
   This invention relates to current sensor system, and more particularly, methods for using matched temperature coefficient resistors to reduce measurement error. 
   BRIEF SUMMARY OF THE INVENTION 
   According to this invention, a sensor resistor is used to sense current flow in which the voltage drop across that resistor is used to determine the current flow through another resistor, the input resistor, into a current integrator. The voltage level across the input resistor is proportional to the voltage level across sensor resistor which along with the ratio of input resistor to sensor resistor resistance values determines the ratio of sense resistor current to integrator input current level. 
   By constructing the resistors of the same material, as is easily done in integrated circuits, the temperature performance of the sensor resistor and input resistor can be made substantially identical. Since the resistance ratio of these two resistors remain substantially constant, the ratio of current through current sensor resistor to the integrator input current also remains substantially constant over temperature variations. Also, since the ratio of sensor current to integrator input current depends on the resistance ratio and not the absolute resistor values, the need for resistor trimming is reduced. It is understood that constant current or voltage gain amplifiers maybe used to additionally adjust the ratio of the sensor resistor to input resistor current level without changing the basic principle of circuit operation. 
   Current integrators can be built using many well known in the art circuits including those using op-amps in combination with capacitors. The integrator output voltage equals the integral over time of the current level into it divided by the capacitor value. The integrator can provide very stable performance over temperature since the integration time period and the capacitor can both be made relatively temperature stable. Capacitors normally have much greater stability then that of integrated circuit resistors. Also the active circuit amplifier gain variations have little effect on the integrator performance since normally a large level of negative feedback is used. 
   The integrator output voltage level can be used to indicate the sensor resistor current level. In other application the integrator can be used as a section of oscillators or pulse generators so as to provide a digital output signal that uses frequency or duty cycle to indicate the sensor resistor current level. 
   In voltage level output systems, the integrator has a stable integral of input current to output voltage ratio. The integration time period is controlled by an oscillator with a stable pulse width and frequency. The ratio of current into the integrator to current flow through the sense resistor is primarily determined by the resistance ratio of the sense resistor to input resistor. By maintaining the capacitor value, oscillator pulse width and frequency, and resistance ratio of the resistors substantially constant, the ratio of output voltage level of the current integrator to current level through the sensor resistor remains substantially constant over temperature variations. 
   In frequency or duty cycle output systems, the integrator has a stable time integral of input current to output voltage ratio. Two well known methods for varying the frequency or duty cycle dependant on the current level into the integrator are by measuring the time required for the integrator output voltage level to substantially equal a reference voltage level and the other is by maintaining the integrator output voltage level at a substantially constant level by using charge balancing techniques. As described above in the voltage level output system, the integrator has a stable integral of input current to output voltage ratio and by maintaining the capacitor value, and resistance ratio of the resistors substantially constant the output frequency or duty cycle for a given current level through the sensor resistor remains substantially constant over temperature variations. 
   An objective of this invention is to provide a current sensor system with a voltage level output signal that has a high temperature stability due to its reliance on stable oscillator pulse width and capacitors along with resistor ratios to set input current to output voltage ratios values. This circuit is substantially immune to the effects of temperature change. 
   Another objective is to provide a current sensor system with a frequency or duty cycle output signal that has a high temperature stability due to its reliance on stable current integrator circuits along with resistor ratios to set sensor resistor current level to integrator input current level. This circuit is substantially immune to the effects of temperature change. 
   Another objective is to provide an accurate over temperature current sensor system which can be built using commonly available integrated circuit components. 
   Also, another objective is to provide an accurate current sensor system without the need for resistor trimming which can be built using commonly available integrated circuit components. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Reference will be made in detail to preferred embodiments of the invention, examples of which are illustrated in the accompanying drawings. The drawings are intended to be illustrative, not limiting. Although the invention will be described in the context of these preferred embodiments, it should be understood that it is not intended to limit the spirit and scope of the invention to these particular embodiments. 
       FIG. 1  shows a block diagram of current sensor system  1 ; 
       FIG. 2  shows timing diagram of the current sensor system  1 ; 
       FIG. 3  shows details of integrator  30 A; 
       FIG. 4  shows details of integrator  30 B; 
       FIG. 5  shows details of integrator  30 C; 
       FIG. 6  shows a block diagram of current sensor system  30 D; 
       FIG. 7  shows timing diagram of the current sensor system  30 D; 
       FIG. 8  shows a block diagram of current sensor oscillator system  2 ; 
       FIG. 9  shows timing diagram of the current sensor oscillator system  2 . 
       FIG. 10  shows details of integrator  30 E 
       FIG. 11  shows a block diagram of current sensor oscillator system  3 ; and 
       FIG. 12  shows timing diagram of the current sensor oscillator system  3 . 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   The current sensor system in  FIG. 1  consist of sensor resistor  11  which is connected between the terminals  10  and  9 . The external current level to be sensed flows into terminal  10  and out of terminal  9  or vice a versa. Line  13  connects one of the integrator  30  inputs to input resistor  12  and the other end of resistor  12  is connected to terminal  10 . The other input of integrator  30  is by line  8  connected to terminal  9 . Input resistor  12  is normally much larger in value than both of sensor resistor  11  or the input impedance of integrator  30 . The current though input resistor  12  and also on line  8  has negligible effect on the voltage drop across sensor resistor  11 . Oscillator  18  provides a constant frequency and pulse width output signal on line  19  that is applied to the control input of integrator  30 . Integrator  30  integrates during the pulse width of the output signal on line  19  the current level flowing on line  13 , and provides a voltage output on line  16  representing the value of the integrated current value. After the pulse ends the integrator  30  output is reset to a starting value which is normally zero and the integration cycle then repeats again. In most cases the percentage of time used for integration is much greater than the reset time. In the timing diagram shown in  FIG. 2  examples of the wave forms and time relationship are given. Between time  0  and time  1  the output of oscillator  18  is low placing integrator  30  in the reset mode and causes it&#39;s output on line  16  to be zero volts. Between time  1  and time  2  the pulse output of oscillator  18  is high placing integrator  30  in the integrate mode and it&#39;s output voltage magnitude to increase with time in proportion to the current level on line  13 . Between time  2  and time  3  the output of oscillator  18  is again low placing integrator  30  in the reset mode and it&#39;s output to be zero volts. 
   The voltage level between terminals  10  and  9  is substantially equal to the current level through sensor resistor  11  times the resistance of sensor resistor  11 , since input resistor  12  has a much greater resistance than that of sensor resistor  11 . The input impedance of integrator  30  between line  13  and  8  is low and effectively zero in comparison to the resistance of input resistor  12 . The current level through input resistor  12  is substantially equal to the current level through sensor resistor  11  times the value of resistors  11  divided by that of input resistor  12 . In effect resistor input resistor  12  and sensor resistor  11  can be considered to be in parallel to calculate their current levels. The integrator output on line  16  is applied to the input of filter  17 . Filter  17  is a low pass filter that provides a DC voltage at output terminal  20  in proportion to the voltage level on line  16  that is substantially free of ripple related to oscillator  19 . 
   Integrator  30  has a stable input current to output voltage ratio that is varied only by the pulse width and repetition rate of oscillator  18 . By maintaining the output of oscillator  18  constant, the output voltage level of integrator  30  with a constant external current level through sensor resistor  11  is substantially dependant on only the resistance ratio of sensor resistor  11  to input resistor  12 . Over temperature the time period of oscillator  18  can be maintained reasonably accurate however the value of the resistors  11  and  12  when constructed of materials commonly used in integrated circuits such as polysilicon have substantial value changes over temperature. By constructing the resistors of the same material, as is easily done in integrated circuits, the temperature performance of resistors  11  and  12  can be made substantially identical. Since the resistance ratio of resistors  11  and  12  remain substantially constant, the ratio of current through sensor resistor  11  to voltage on line  16  also remains substantially constant over temperature variations. If desired the ratio of sensor current to output voltage can be changed by varying the oscillator  18  pulse width and or frequency. 
   One form of integrator  30  is shown in  FIG. 3 . Integrator  30 A consisting operational amplifier  40 , capacitor  14  and relay  15 . Relay  15  is connected across capacitor  14  and controlled by the signal on line  19 . Capacitor  14  is connected by line  16  to the output of operational amplifier  40  and by line  13  to the negative input of operational amplifier  40 . Line  13  also connects input resistor  12  to the negative input of operational amplifier  40 . Line  8  connects the positive input of operational amplifier  40  to terminal  9 . 
   This integrator circuit is a well known in the art and is normally used when the input terminal  9  is connected to ground. It is obvious that the relay can be replaced by an semiconductor switching device such as a FET. The voltage level on line  13  is maintained substantially at 0 volts by using a high gain operational amplifier  40  and the current level on line  13  is substantially equal to the current level through sensor resistor  11  times the resistance of sensor resistor  11  divided by the resistance of input resistor  12 . 
   In the timing diagram shown in  FIG. 2  between time  0  and time  1  the output of oscillator  18  is low placing integrator  30 A in the reset mode with relay  15  shorting out capacitor  14  and causing it&#39;s output on line  16  to be zero volts. Between time  1  and time  2  the pulse output of oscillator  18  is high placing integrator  30 A in the integrate mode with relay  15  open and capacitor  14  being charged by the current through input resistor  12  and in response it&#39;s output voltage on line  16  becoming more negative with time assuming terminal  10  is positive with respect to terminal  9 . Between time  2  and time  3  the output of oscillator  18  is again low placing integrator  30 A in the reset mode with relay  15  shorting out capacitor  14  and causing it&#39;s output on line  16  to be zero volts. 
   When resistors  11  and  12  have the same temperature coefficient, temperature change has substantially no effect on the line  16  voltage level for a given current. The value of capacitor  14  and the time period and pulse width of oscillator  18  are substantially constant over temperature variations when compared to the resistors, so by reducing the error caused by resistor temperature coefficient the current sensor accuracy over temperature is improved. 
   Referring now to the integrator system  30 B in  FIG. 4 . Differential input integrators are well known in the art. The differential input allows integrator  30 B to have both terminal  10  and  9  at a different voltage than ground level. Differential input integrator  30 B is created by adding capacitor  46 , resistor  35  and relays  47  to integrator  30 A. Line  8  of integrator  30 A is connected to resistor  35 , capacitor  46 , and relay  47 . The other end of resistor  35  is connected to terminal  9  and sensor resistor  11 . The other ends of capacitor  46  and relay  47  are connected to ground. Relay  47  is controlled by the signal on line  19  and closes and opens when relay  15  does the same. Resistor  35  is matched in value and electrical characteristics to input resistor  12  and capacitor  46  is matched in value and electrical characteristics to capacitor  14 . The value of capacitor  14  and  46 , and the time period and pulse width of oscillator  18  are substantially constant over temperature variations when compared to the resistors. When resistors  11 ,  35 , and  12  have the same temperature coefficient, temperature change has substantially no effect on the line  16  voltage level for a given input current. 
   An addition source of error is caused by the offset voltage of non-ideal operational amplifiers used as part of the integrator. There are were known in the art means for reducing the resultant offset voltage error, one of the methods is to use auto-zero.  FIG. 5  shows as an example integrator  30 A to which one circuit version of auto-zero has been added to form an reduced offset voltage error integrator, integrator  30 C Integrator  30 C consist of integrator  30 A with inverter  51 , capacitor  52 , and relays  53 ,  54 ,  55 ,  56  added. One end of capacitor  52  and relay  55  is connected to line  8 . The other end of relay  55  is connected to ground. The other end of capacitor  52  is connected to one side of relays  53  and  56 . The other end of relay  53  is connected to ground. The other end of relay  56  is connected to line  13 . One side of relay  54  is connected to terminal  10  and the other side to input resistor  12 . The input of inverter  51  is connected to line  19  and it&#39;s output controls relays  54  and  53 . 
   In the timing diagram shown in  FIG. 2  between time  0  and time  1  the output of oscillator  18  is low placing integrator  30 C in the reset and offset reduction mode with relay  15  shorting out capacitor  14 , relay  55  connecting line  8  to ground, and relay  56  connecting capacitor  52  to line  13 . This results in capacitor  52  being charged to level of the offset voltage which is on line  16 . The output of inverter  51  is high keeping the contacts of relays  54  and  53  open. 
   Between time  1  and time  2  the output of oscillator  18  is high placing integrator  30 C in the integrate mode with relay  15 ,  55 , and  56  open. The output of inverter  51  is low keeping the contacts of relays  54  and  53  closed. On side of input resistor  12  is now connected through relay  54  to terminal  10  to allow capacitor  14  to be charged by the current through input resistor  12  with the voltage on line  16  becoming more negative with time assuming terminal  10  is positive with respect to terminal  9 . One side of capacitor  52  is now connected to ground through relay  53 . The other side of capacitor  52  is connected to line  8  with the voltage level on line  8  now of a value to substantially cancel that of the offset voltage. Between time  2  and time  3  the output of oscillator  18  is again low placing integrator  30 C in the reset and offset reduction mode and it&#39;s output on line  16  at the level of the offset voltage. 
     FIG. 6  shows as an example integrator  30 B to which an other circuit technique for auto zero has been added to form an reduced offset voltage error current sensor system, current sensor  30 D. Current sensor  30 D can also be used to reduce the output signal  16  error due to the mismatch in values between input resistor  12  and  35  and also between capacitor  14  and  46 . Current sensor  30 D consist of integrator  30 B with frequency divider  61 , resistors  11 ,  12 , relays  64 ,  65  and detector  67  added. The input of frequency divider  61  is connected to line  19  and it&#39;s output controls relays  64  and  65 . When the frequency divider  61  output is high relay  64  connects input resistor  12  to terminal  10 , and relay  65  connects input resistor  35  to terminal  9 . When the of frequency divider  61  output is low relay  64  connects input resistor  12  to terminal  9  and relay  65  connects input resistor  35  to terminal  10 . The other end of input resistor  12  is connected to line  13  and the other end of resistor  35  is connected to line  8 . Sensor resistor  11  is connected between terminal  10  and  9 . The output of frequency divider  61  changes state every other time oscillator  18  changes state (high/low). Line  16  of integrator  30 B is also connected to the input of detector  67 . The output of detector  67  is proportional to the magnitude of it&#39;s input signal and is applied to terminal  68 . For example when the voltage level on line  16  is negative the terminal  68  voltage is positive and when the voltage level on line  16  is positive the terminal  68  voltage is also positive as shown in  FIG. 7 . The terminal  68  signal level difference between when the frequency divider  61  output is low and high and integrator  30 B is in the integrate mode can be processed to substantially remove the errors due to mismatch in values between input resistor  12  and  35  and also between capacitor  14  and  46  to more accurately determine the true current level through sensor resistor  11 . For example in  FIG. 7  at time  2  the voltage at terminal  68  is 1.6 volts and at time  4  the voltage at terminal  68  is 2 volts the voltage at terminal  68  without being effecting by the errors would be at time  2 , 1.8 volts and at time  4  the voltage 1.8 volts. The accurate voltage value of 1.8 v could be obtained from the voltage levels containing the error by taking the average value of the measurements at time  2  and  4 . 
   In the timing diagram shown in  FIG. 7  between time  0  and time  1  the output of oscillator  18  is low placing integrator  30 B in the reset mode with relay  15  shorting out capacitor  14 , and relay  47  connecting line  8  to ground. The frequency divider  61  output is low and relay  64  connects input resistor  12  to terminal  9  and relay  65  connects input resistor  35  to terminal  10 . 
   Between time  1  and time  2  the output of oscillator  18  is high placing integrator  30 B in the integrate mode with relay  15 , and  47  open. The frequency divider  61  output is high and relay  64  connects input resistor  12  to terminal  10  and relay  65  connects input resistor  35  to terminal  9 . 
   Between time  2  and time  3  the output of oscillator  18  is again low placing integrator  30 B in the reset mode. The frequency divider  61  output remains high and relay  64  connects input resistor  12  to terminal  10  and relay  65  connects input resistor  35  to terminal  9 . 
   Between time  3  and time  4  the output of oscillator  18  is again high placing integrator  30 B in the integrate mode with relay  15 , and  47  open. The frequency divider  61  output is again low and relay  64  connects input resistor  12  to terminal  9  and relay  65  connects input resistor  35  to terminal  10 . 
   The level of current flow can also be indicated by varying the frequency of the output signal. The current sensor oscillator system  2  in  FIG. 8  shows one method of doing this. It consist of sensor resistor  11  which is connected between the terminals  10  and  9 . The external current level to be sensed flows into terminal  10  and out of terminal  9 . Terminal  9  is normally connected to ground. Line  13  connects one of the integrator  30  inputs to one side of input resistor  12  with the other side of input resistor  12  connected to terminal  10 . The other input of integrator  30  is by line  8  connected to terminal  9 . Input resistor  12  is normally much larger in value than both sensor resistor  11  and also the input impedance of integrator  30 . The current though input resistor  12  has substantially negligible effect on the voltage drop across sensor resistor  11 . During integration the voltage output on line  16  represents the value of the integrated current value of the current level flowing on line  13 . For an example, when the current flow direction into terminal  10  causes terminal  10  polarity to be positive in respect to terminal  9  the integrator output voltage level on line  16  goes negative and positive when terminal  10  polarity is negative in respect to terminal  9 . 
   When the voltage level on line  16  goes negative and substantially reaches the level of voltage reference  62  the one shot pulse generator  60  provides a output signal which is a narrow pulse going from high to low level of stable time duration at terminal  68  and the input of AND gate  66 . When the voltage level on line  16  goes positive and substantially reaches the level of voltage reference  65  the one shot pulse generator  64  provides a output signal which is a narrow pulse going from high to low level of stable time duration at the other input of AND gate  66 . 
   When a pulse is applied to either input of AND gate  66 , the output of AND gate  66  on line  19  has a substantially identical pulse width as at it&#39;s input. The pulse on line  19  is applied to the control input of integrator  30 . Integrator  30  output is reset to a starting value which is normally zero and begins to integrate again at the end of the pulse. The frequency of the pulses on line  19  varies in relationship to the current level that flows into terminal  10 , the higher the current level magnitude the faster the integrator  30  output level reaches that of voltage reference  62  or  65 . In addition the presence or absence of a pulse at terminal  68  indicates the polarity of the current flow into terminal  10 . Thus the pulse frequency on line  19  and the presence or absence of pulses at terminal  68  indicates the magnitude and polarity of the current level into terminal  10 . 
   In the timing diagram shown in  FIG. 9  examples of the wave forms and time relationship are given. The current flow into terminal  10  is such that the integrator  30  output level goes negative with time. At time  0  the output of the one shot pulse generator  60  goes low and a pulse is present at terminal  68  and on line  19 . Between time  0  and time  1  integrator  30  is in the reset mode causing it&#39;s output on line  16  to be zero volts. At time  1  the one shot pulse generator  60  pulse ends. Between time  1  and time  2  integrator  30  is in the integrate mode and it&#39;s output voltage becomes more negative with time in proportion to the current level on line  13  which is proportional to the current level through sensor resistor  11 . At time  2  the voltage level substantially equals the voltage level of voltage reference  62  and the one shot pulse generator  60  output goes low again. Between time  2  and time  3  the output of the one shot pulse generator  60  remains low placing integrator  30  in the reset mode and it&#39;s output to be zero volts. 
   Another form of integrator  30  is shown in  FIG. 10 . Integrator  30 E consisting of operational amplifier  40 , capacitor  14 , FET  75 , and relay  15 . Relay  15  is connected across capacitor  14  and controlled by the signal on line  19 . Capacitor  14  is connected between ground and line  16  and the drain of P channel FET  75 . The gate of FET  75  is connected to the output of operational amplifier  40  and the source of FET  75  is connected to the negative input of operational amplifier  40  and one side of input resistor  12 . The other side of input resistor  12  is connected to terminal  9  and battery  76 . The negative terminal of battery  76  is connected to ground. Sensor resistor  11  is connected between terminal  9  and  10 . Terminal  10  is also connected to the positive input of operational amplifier  40 . 
   This integrator circuit  30 E is a well known in the art. The voltage level between operational amplifier  40  positive and negative inputs is maintained substantially at 0 volts by using a high gain operational amplifier  40  and the current level through input resistor  12  is substantially equal to the current level through sensor resistor  11  times the resistance of sensor resistor  11  divided by the resistance of input resistor  12 . When the contact of relay  15  is open and negligible current is being drawn by an external load connected to line  16 , the current flow into capacitor  14  is substantially equal to that of input resistor  12 . 
   The level of current flow can also be indicated by varying the duty cycle of the output signal. The current sensor oscillator system  3  in  FIG. 11  shows one method of doing this by trying to maintaining the integrator output voltage level at a substantially constant level by using charge balancing techniques. It consist of sensor resistor  11  which is connected between the terminals  10  and  9 . The external current level to be sensed flows into terminal  10  and out of terminal  9 . Terminal  9  is normally connected to ground. Line  13  connects one of the integrator  30  inputs to one side of input resistor  12  and also to the output of constant current source  81 . The other side of input resistor  12  connected to terminal  10 . The other input of integrator  30  is by line  8  connected to terminal  9 . Input resistor  12  is normally much larger in value than both sensor resistor  11  and also the input impedance of integrator  30 . The voltage level across line  13  and  8  is zero. The current though input resistor  12  has substantially negligible effect on the voltage drop across sensor resistor  11 . Integrator  30  output is on line  16  and is applied to one input of comparator  82 . The output of comparator  82  is on line  83  and applied to the input of constant current source  81 . The output of voltage reference  84  is applied to the other input of comparator  82 . 
   During integration the voltage output level on line  16  represents the integrated current value of the current level flowing on line  13 . For an example, when the current flow direction into terminal  10  causes terminal  10  polarity to be positive in respect to terminal  9  the integrator output voltage level on line  16  goes decreases with time and the constant current source  81  output polarity is the opposite to cause the voltage level on line  16  to increase with time. 
   Comparator  82  has an input hysteresis centered around the voltage level of voltage reference  84  which may be set at zero volts as used in this example. When the voltage level on line  16  goes negative and substantially reaches the level of voltage reference  84  minus the level of input hysteresis the comparator  82  output on line  83  goes high causing constant current source  81  to output a fixed current level to line  13 . This causing the voltage level on line  16  to increase with time. 
   When the voltage level on line  16  goes positive and substantially reaches the level of voltage reference  65  plus the input hysteresis, the comparator  82  output on line  83  goes low and constant current source  81  output is now zero current. This causing the voltage level on line  16  to now decrease with time. 
   The duty cycle of the pulses on line  83  varies in relationship to the ratio of current level that flows into terminal  10  to that of the constant current source  81  output level. The greater the current level magnitude on line  13  the faster the integrator  30  output level reaches the switching point of comparator  82 . 
   In the timing diagram shown in  FIG. 12  examples of the wave forms and time relationship are given. The current flow into terminal  10  is such that the integrator  30  output level decreases with time. At time  0  the voltage level on line  16  substantially equals the switching level of comparator  82  causing the output on line  83  to go high. When the signal on line  82  is high between time  0  and time  1 , constant current source  81  output sources current to line  13  causing integrator  30  output on line  16  to increase with time in proportion to the current level on line  13 . At time  1  the voltage level on line  16  substantially equals the switching level of comparator  82  causing the output on line  83  to go low. Between time  1  and time  2  only input resistor  12  supplies current to line  13  with constant current source  81  output at zero current level. Integrator  30  output voltage level decreases with time in proportion to the current level on line  13  which is proportional to the current level through sensor resistor  11 . At time  2  the voltage level on line  16  substantially equals the switching level of comparator  82  causing the output on line  83  to again go high. 
   Although the above description has been directed to preferred embodiments of the invention, it will be understood and appreciated by those skilled in the art that other variations and modifications may be made without departing from the spirit and scope of the invention, and therefore the invention includes the full range of equivalents of the features and aspects set forth in the claims.