Patent Publication Number: US-2023160990-A1

Title: Methods and systems for diagnosing magnetic sensors

Description:
CROSS REFERENCE TO RELATED APPLICATION 
     This application is a Divisional Application of U.S. Pat. Application No. 17/161,009 filed Jan. 28, 2021, which claims priority to U.S. Provisional Application No. 63/030,601, filed May 27, 2020, entitled “Methods for Run Time Diagnostics in Magnetic Sensors”, which Applications are hereby incorporated by reference herein in their entireties. 
    
    
     TECHNICAL FIELD 
     This description relates generally to magnetic sensors. 
     BACKGROUND 
     A magnetic sensor such as a Hall-effect sensor is a device used to measure the strength of a magnetic field. The magnetic sensor provides an output voltage that is directly proportional to the magnetic field strength. Magnetic sensors are used for proximity sensing, position and speed detection, and current sensing. A Hall-effect sensor can be combined with a threshold detection circuit so that it acts as a switch. 
     Due to safety requirements in automotive applications, run time diagnostics are performed on Hall-effect sensors to validate their integrity. A known magnetic field is typically created and isolated from an external magnetic field for run time diagnostics. Current systems include on-chip coils built inside integrated circuits to create a local magnetic field. Current systems require additional on-chip area and consume significant amount of power. Also, current systems typically are not reliable due to challenges associated with isolating the local magnetic field from the external magnetic field. 
     SUMMARY 
     In one aspect, a method includes generating a reference voltage by periodically switching direction of current flow in a diagnostic sensor, where the reference voltage is a nonsinusoidal differential voltage of which an amplitude alternates between minimum and maximum values, and where the reference voltage includes a diagnostic sensor output voltage component responsive to an external magnetic field and a diagnostic sensor offset voltage component responsive to a mismatch of the diagnostic sensor. The method also includes amplifying the reference voltage to produce an amplified reference voltage, where the amplified reference voltage is a differential voltage having an amplifier offset voltage component. Additionally, the method includes demodulating the amplified reference voltage by filtering the diagnostic sensor offset voltage component and the amplifier offset voltage component to produce a demodulated voltage. Also, the method includes digitizing the demodulated voltage to produce a digitized voltage. 
     In another aspect, a circuit includes a magnetic sensor having a first sensor terminal, a second sensor terminal, a third sensor terminal, and a fourth sensor terminal and a diagnostic sensor having a first diagnostic terminal, a second diagnostic terminal, a third diagnostic terminal, and a fourth diagnostic terminal. The circuit also includes a first multiplexer coupled to the first sensor terminal and to the first diagnostic terminal and a second multiplexer coupled to the second sensor terminal and the second diagnostic terminal. Additionally, the circuit includes a third multiplexer coupled to the third sensor terminal, the fourth sensor terminal, the first diagnostic terminal, the second diagnostic terminal, the third diagnostic terminal, and the fourth diagnostic terminal, the third multiplexer having a first differential output terminal and a second differential output terminal. 
     In another aspect, a vehicle includes a magnetic sensor circuit. The magnetic sensor circuit includes a magnetic sensor having a first sensor terminal, a second sensor terminal, a third sensor terminal, and a fourth sensor terminal and a diagnostic sensor having a first diagnostic terminal, a second diagnostic terminal, a third diagnostic terminal, and a fourth diagnostic terminal. The magnetic sensor circuit also includes a first multiplexer coupled to the first sensor terminal and to the first diagnostic terminal and a second multiplexer coupled to the second sensor terminal and the second diagnostic terminal. Additionally, the magnetic sensor circuit includes a third multiplexer coupled to the third sensor terminal, the fourth sensor terminal, the first diagnostic terminal, the second diagnostic terminal, the third diagnostic terminal, and the fourth diagnostic terminal, the third multiplexer having a first differential output terminal and a second differential output terminal. Also, the magnetic sensor circuit includes an analog front end (AFE) coupled to the third multiplexer, a demodulator coupled to the AFE, and an analog to digital converter (ADC) coupled to the demodulator. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG.  1    is a block diagram of a magnetic sensor circuit of an example embodiment. 
         FIG.  2    illustrates a timing diagram. 
         FIG.  3    is a schematic diagram of a magnetic sensor of an example embodiment. 
         FIG.  4    is a block diagram of a test circuit of an example embodiment. 
         FIG.  5    is a flow diagram of an example embodiment. 
     
    
    
     DETAILED DESCRIPTION 
       FIG.  1    is a block diagram of a magnetic sensor circuit  100  of an example embodiment. The magnetic sensor circuit  100  operates in two modes: a diagnostic mode and a normal mode. In the diagnostic mode, the magnetic sensor circuit  100  performs a self-diagnosis to validate the signal chain integrity of the circuit  100 . In the normal mode, the magnetic sensor circuit  100  measures an external magnetic field and provides an output voltage representative of the external magnetic field. 
     The magnetic sensor circuit  100  operates in duty cycles having a sleep state and an active state. As illustrated in a timing diagram  200  of  FIG.  2   , in a sleep state  204  the magnetic sensor circuit  100  is inactive, and in an active state  208  the magnetic sensor circuit  100  performs a signal chain diagnostic check  210  and a sensor diagnostic check  214 . Thereafter, the magnetic sensor circuit  100  performs a normal operation  218  which is also referred to as Hall-effect sensor operation. 
     The magnetic sensor circuit  100  includes three magnetic sensors  104 A,  104 B, and  104 C oriented to measure external magnetic fields in x, y, and z directions, respectively. The magnetic sensor circuit  100  may be constructed with any suitable number of magnetic sensors. The magnetic sensors  104 A,  104 B, and  104 C may, for example, be Hall-effect sensors which provide an output voltage representative of the strength of the external magnetic fields. 
     The magnetic sensor circuit  100  includes a diagnostic sensor  108  which provides an output voltage that is immune to the external magnetic field. In an example embodiment, the diagnostic sensor  100  is built using resistors (e.g., poly resistors) that do not produce a voltage responsive to the external magnetic field. The resistors in the diagnostic sensor  100  can be connected in a wheatstone bridge network. The diagnostic sensor  108  is used to perform a self-diagnosis to check the integrity of the signal chain of the circuit  100 . The diagnostic sensor  108  may, for example, be a resistor network insensitive to the external magnetic field. 
     The magnetic sensors  104 A,  104 B, and  104 C include respective bias input terminals  110 A,  110 B and  110 C configured to receive a bias current. During the normal operating mode, a switch S 1  couples the bias input input terminals  110 A,  110 B and  110 C to a current source I bias  which provides the bias current. The switch S 1  may be implemented with a multiplexer. 
     The magnetic sensors  104 A,  110 B, and  104 C include respective bias output terminals  112 A,  112 B and  112 C. During the normal operating mode, a switch S 2  couples the bias output terminals  112 A,  112 B and  112 C to a first termnial  116  of a switch M 1 . The switch S 2  may, for example, be a multiplexer. The switch M 1  has a second terminal  118  coupled to a ground terminal. The ground terminal may be coupled to a ground voltage. The switch M 1  may, for example, be an n-channel field effect transistor (NFET) of which the first terminal  116  is a drain and the second terminal  118  is a souce. The switch M 1  also has a gate. When M 1  is turned on, a conduction path is provided for the bias current to flow from the current source I bias  to ground. 
     The magnetic sensor  104 A includes measurement terminals  122 A and  122 B, the magnetic sensor  104 B includes measurement terminals  124 A and  124 B, and the magnetic sensor  104 C includes measurement terminals  126 A and  126 B. Responsive to the external magnetic field, the magnetic sensors  104 A,  104 B and  104 C provide output voltages at the measurement terminals. The output voltage at the measurement terminals is representative of the strength of the external magnetic field. A switch S 3  (e.g., a multiplexer) selectively couples the measurement terminals to differential output terminals  128  and  130 . During the normal operating mode, the output voltages generated by the magnetic sensors  104 A,  104 B and  104 C are available at the differential output terminals  128  and  130 . 
     The diagnostic sensor  108  includes a bias input terminal  134  and a bias output terminal  136 . The diagnostic sensor  108  includes measurement terminals  138  and  140 . During the diagnostic mode, the switch S 1  couples the bias input terminal  134  to the current source I bias  and the switch S 2  couples the bias output terminal  136  to the first terminal  116  of the transistor M 1 , thus providing a conduction path between the current source I bias  and ground. Also, during the diagnostic mode, the switch S 3  couples the measurement terminals  138  and  140  to the differential output terminals  128  and  130 . The diagnostic sensor  108  provides an output voltage immune to the external magnetic field at the measurement terminals  138  and  140 . 
     The magnetic sensor circuit  100  includes an analog front end (AFE)  150  which may be an amplifier. The AFE  150  includes differential inputs  152  and  154  coupled to the differential outputs  128  and  130 , respectively. During the diagnostic mode, a switch S 4  connects a current source, I diagsrc , to the  152  input of the AFE  150  and a switch S 5  connects a current sink, I diagsnk  to the input  154  of the AFE  150 . The AFE  150  applies a predetermined gain to the differential voltage provided by the magnetic sensors  104 A- 104 C or the diagnostic sensor  108  and provides an amplified differential signal at outputs  156  and  158 . The magnetic sensor circuit  100  includes a demodulator  160  coupled to receive the amplified differential signal at inputs  162  and  164 . The demodulator  160  demodulates the amplifed signal and provides a filtered signal at an output  166 . An analog-to-digital converter (ADC)  168  digitizes the filtered signal. 
     In an example embodiment, the magnetic sensor circuit  100  includes an operational amplifier  170  having first and second input terminals  172  and  174 , respectively, coupled to the respective differential output terminals  128  and  130 , of the third switch S 3  and a third input terminal  176  coupled to a common mode terminal to which a common mode voltage can be applied. The operational amplifier  170  also includes an output terminal  178  coupled to a gate of the switch M 1 . Responsive to the differential voltages at the terminals  128  and  130  and a common mode voltage, the operational amplifier  170  applies a gate voltage to the switch M 1  to control the current through M 1 , and thus control the current in the magnetic sensors  104 A- 104 C and the diagnostic sensor  108 . 
     In an example embodiment, the magnetic sensors  104 A- 104 C and the diagnostic sensor  108  are implemented with four resistors connected in a wheatstone bridge configuration.  FIG.  3    illustrates a sensor  300  which may be one of the magnetic sensors  104 A- 104 C or the diagnostic sensor  108 . The sensor  300  comprises four resistors R 1 , R 2 , R 3  and R 4  connected in a bridge configuration defining first, second, third and fourth terminals, T 1 , T 2 , T 3  and T 4 , respectively. The sensor  300  is operated in four phases, and in each phase a different pair of opposed terminals is selected as the bias input and bias output terminals while the other pair of opposed terminals is selected as the measurement terminals. For example, in phase 1, terminals T 1  and T 3  may be selected as the bias input and output terminals, respectively, while the two opposed terminals T 2  and T 4  may be selected as the measurement terminals. During phase 1, the switch S 1  couples the current source I bias  to terminal T 1 . Thus, the bias current flows through the resistors R 1 , R 2 , R 3  and R 4  and out via terminal T 3 . Responsive to an external magnetic field H 1 , the sensor  300  provides an output voltage at the measurement terminals T 2  and T 4 . The switch S 3  couples the measurement terminals T 2  and T 4  during phase 1 to the differential output terminals  128  and  130 . 
     In phase 2, terminals T 2  and T 4  may be selected as the bias input and output terminals, respectively, while the two opposed terminals T 1  and T 3  may be selected as the measurement terminals. During phase 2, the switch S 1  couples the current source I bias  to terminal T 2 . Thus, the bias current flows through the resistors R 1 , R 2 , R 3  and R 4  and out via terminal T 4 . Responsive to an external magnetic field H 1 , the sensor  300  provides an output voltage at the measurement terminals T 1  and T 3 . The switch S 3  couples the measurement terminals T 1  and T 3  during phase 2 to the differential output terminals  128  and  130 . 
     In phase 3, terminals T 3  and T 1  may be selected as the bias input and output terminals, respectively, while the two opposed terminals T 2  and T 4  may be selected as the measurement terminals. During phase 3, the switch S 1  couples the current source I bias  to terminal T 3 . Thus, the bias current flows through the resistors R 1 , R 2 , R 3  and R 4  and out via terminal T 1 . Responsive to an external magnetic field H 1 , the sensor  300  provides an output voltage at the measurement terminals T 2  and T 4 . The switch S 3  couples the measurement terminals T 2  and T 4  during phase 2 to the differential output terminals  128  and  130 . 
     In phase 4, terminals T 4  and T 2  may be selected as the bias input and output terminals, respectively, while the two opposed terminals T 1  and T 3  may be selected as the measurement terminals. During phase 4, the switch S 1  couples the current source I bias  to terminal T 4 . Thus, the bias current flows through the resistors R 1 , R 2 , R 3  and R 4  and out via terminal T 2 . Responsive to an external magnetic field H 1 , the sensor  300  provides an output voltage at the measurement terminals T 1  and T 3 . The switch S 3  couples the measurement terminals T 1  and T 3  during phase 2 to the differential output terminals  128  and  130 . 
     By coupling the current source I bias  to a different bias input terminal during each phase, the direction of current flow in the sensor  300  is periodically changed. As a result, a periodic non-sinusoidal voltage is generated at the measurement terminals of the sensor  300 . The amplitude of the non-sinusoidal voltage at the measurement terminals alternates between minumum and maximum values. 
     During the diagnostic mode, the magnetic sensor circuit  100  is configured to check the integrity of the magnetic sensors  104 A- 104 C by measuring the magnetic sensor offset and the offset of the AFE  150 . In this mode the current souce I bias  provides a current with a predetermined value to the magnetic sensors. The direction of current flow in the magnetic sensor  104 A- 104 C is periodically switched. Responsive to the external magnetic field the magnetic sensor provides a periodic non-sinusoidal voltage, which is referred to as a hall voltage, at the differential output terminals  128  and  130 . The output voltage comprises a diagnostic sensor offset voltage component and a magnetic sensitive diagnostic sensor output voltage component corresponding to the current source, I bias , the resistance of the magnetic sensor and the external magnetic field. The offset component is generated due to a mismatch of the resistors of the sensor, and the magnetic sensitive voltage component is generated by the magnetic sensor which is responsive to the external magnetic field. Since the external magnetic field might be an unknown value during the diagnostic mode it is necessary to ignore its effect. The signal at the differential output terminals  128  and  130  is amplified by the AFE  150 . At the output of the AFE  150 , an offset component is added due to a mismatch in the AFE  150 . The signal at the output of the AFE  150  can be represented as: 
     V ph(i)  = (-1) i+1  V Hall  + V OS,Hall,ph(i)  + V OS,AFE , where:   V ph(i)  = AFE output signal for each phase (1, 2, 3 and 4)   V Hall  = Hall-effect voltage component   V OS,Hall,ph(i)  = Hall sensor offset voltage component   V OS,AFE  = AFE offset voltage component   

     Based on the above: 
     
       
         
           
             
               V 
               
                 ph 
                 
                   1 
                 
               
             
             
               
                 +V 
               
               
                 ph 
                 
                   2 
                 
               
             
             
               
                 +V 
               
               
                 ph 
                 
                   3 
                 
               
             
             
               
                 +V 
               
               
                 ph 
                 
                   4 
                 
               
             
             =4 
             
               
                 
                   V 
                   
                     OS,Hall,ph 
                     
                       i 
                     
                   
                 
                 
                   
                     +V 
                   
                   
                     OS,AFE 
                   
                 
               
             
           
         
       
     
     The magnetic sensor integrity may be determined from the sum of the offset of the magnetic sensors and the analog front end as shown below even in the presence of an unknown external magnetic field. 
     
       
         
           
             
               
                 
                   V 
                   
                     OS,Hall,ph 
                     
                       i 
                     
                   
                 
                 
                   
                     +V 
                   
                   
                     OS,AFE 
                   
                 
               
             
             = 
             
               
                 
                   1 
                   / 
                   4 
                 
               
             
             
               
                 
                   V 
                   
                     ph 
                     
                       1 
                     
                   
                 
                 
                   
                     +V 
                   
                   
                     ph 
                     
                       2 
                     
                   
                 
                 
                   
                     +V 
                   
                   
                     ph 
                     
                       3 
                     
                   
                 
                 
                   
                     +V 
                   
                   
                     ph 
                     
                       4 
                     
                   
                 
               
             
           
         
       
     
     In normal operation, the sensor output corresponding to the external field is demodulated using the demodulator  160  as below: 
     
       
         
           
             
               V 
               
                 ph 
                 
                   1 
                 
               
             
             
               
                 -V 
               
               
                 ph 
                 
                   2 
                 
               
             
             
               
                 +V 
               
               
                 ph 
                 
                   3 
                 
               
             
             
               
                 -V 
               
               
                 ph 
                 
                   4 
                 
               
             
             =4 
             
               
                 
                   V 
                   
                     Half 
                   
                 
               
             
           
         
       
     
     During the diagnostic mode, the circuit  100  is configured to verify the signal chain integrity using the dianostic sensor  108 . In an example emodiment, in the diagnostic mode, in addition to the bias current I bias , diagnostic current sources I diagsrc  and sink I diagsnk  are applied to the diagnostic sensor  108 . The diagnostic current source I diagsrc  can be connected to the differential terminals  128  and  130  by a switch S 4  and the diagnostic current sink, I diagsnk  can be connected to the differential terminals  128  and  130  by a switch S 5 . The switch S 3  connects the differential terminals  128  and  130  to the diagnostic sensor  108  and thus applies the diagnostic current source I diagsrc  and the diagnostic current sink, I diagsnk  to the diagnostic sensor  108 . The diagnostic current source I diagsrc  and the diagnostic current sink, I diagsnk  have predetermined values and can be referred to as reference currents to the diagnostic sensor  108 . 
     The direction of current flow of I bias , I diagsrc  and I diagsn  is periodically switched in the diagnostic sensor  108 . In phase 1 and phase 3, a terminal  180  of the diagnostic current source is switched to the terminal  152  and a terminal  182  of the diagnostic current sink is switched to the terminal  154 . In phase 2 and phase 4, the terminal  180  of the diagnostic current source is switched to the terminal  154  and the terminal  182  of the diagnostic current sink is switched to the terminal  152 . Immune to the external magnetic field and I bias  the diagnostic sensor  108  provides a periodic non-sinusoidal voltage, which is referred to as a diagnostic reference voltage at the differential output terminals  128  and  130 . Both the diagnostic currents (I diagsrc , I diagsnk ) may have the same value (I diagsrc ). The diagnostic reference voltage comprises a diagnostic sensor offset component and a known diagnostic reference voltage component corresponding to the diagnostic current sources (I diagsrc ) and the resistance of the diagnostic sensor, R diagsns . The diagnostic sensor offset component is generated due to a mismatch of the resistors of the diagnostic sensor  108 , and the known diagnostic reference voltage component is generated by the diagnostic sensor  108  immune to the external magnetic field and due to the voltage drop created by the reference diagnostic currents flowing through the diagnostic sensor. Since the diagnostic reference currents have a known value, the resulting diagnostic reference voltage component also has a known value. The signal at the differential output terminals  128  and  130  is amplified by the AFE  150 . At the output of the AFE  150 , an offset component is added to the signal due to a mismatch in the AFE  150 . The signal at the output of the AFE  150  can be represented as: 
     V ph(i)  = (-1) i+1  V ref,diag  + V OS,Hall,ph(i)  + V OS,AFE , where:   V ph(i)  = AFE output signal for each phase (1, 2, 3 and 4)   V ref,diag  = I diagsrc *R diagsns  (Diagnostic Reference Voltage Component)   V OS,diag,ph(i)  = Diagnostic sensor offset voltage component   V OS,AFE  = AFE offset voltage component   

     Based on the above: [0048] 
     
       
         
           
             
               V 
               
                 ph 
                 
                   1 
                 
               
             
             
               
                 -V 
               
               
                 ph 
                 
                   2 
                 
               
             
             
               
                 +V 
               
               
                 ph 
                 
                   3 
                 
               
             
             
               
                 -V 
               
               
                 ph 
                 
                   4 
                 
               
             
             =4 
             
               
                 
                   V 
                   
                     ref,diag 
                   
                 
               
             
           
         
       
     
      Thus, the signal chain integrity may be determined by obtaining a known output reference voltage based on demodulation of the four different phases: 
     
       
         
           
             
               
                 
                   V 
                   
                     ref,diag 
                   
                 
               
             
             = 
             
               
                 
                   1 
                   / 
                   4 
                 
               
             
             
               
                 
                   V 
                   
                     ph 
                     
                       1 
                     
                   
                 
                 
                   
                     -V 
                   
                   
                     ph 
                     
                       2 
                     
                   
                 
                 
                   
                     +V 
                   
                   
                     ph 
                     
                       3 
                     
                   
                 
                 
                   
                     -V 
                   
                   
                     ph 
                     
                       4 
                     
                   
                 
               
             
           
         
       
     
     In an example embodiment, the magnetic sensor circuit  100  is configured to perform a sensor integrity check to verify the sensitivity of the magnetic sensors  104 A- 104 C.  FIG.  4    illustrates a simplified circuit  400  for the sensor integrity check of the magnetic sensor  104 A. The bias input terminal  110 A of the magnetic sensor  104 A is coupled to the current source and the bias output terminal  112 A is coupled to the drain  116  of the transistor M 1 . During the sensor integrity check, the current source generates a current I diag  using a known voltage V diag  and a resistor R diag . Responsive to I diag , which has a known value, the magnetic sensor  104 A provides a differential output voltage V(d1-d2) which can be representated as: 
     
       
         
           
             V 
             
               
                 d1-d2 
               
             
             = 
             
               
                 
                   I 
                   
                     diag 
                   
                 
               
             
             * 
             
               
                 
                   R 
                   
                     Hall 
                   
                 
               
             
           
         
       
     
     Where R Hall  is the equivalent resistance of the magnetic sensor  104 . 
     After substituting (V bg /R diag ) for I diag : 
     
       
         
           
             V 
             
               
                 d1-d2 
               
             
             = 
             
               
                 
                   
                     
                       V 
                       
                         bg 
                       
                     
                   
                   / 
                   
                     
                       R 
                       
                         diag 
                       
                     
                   
                 
               
             
             
               
                 *R 
               
               
                 Hall 
               
             
             
               
                 =K*V 
               
               
                 bg 
               
             
           
         
       
     
     Where K=R Hall /R diag  and is defined as the sensitivity constant. 
     Thus, by measuring the differential voltage V(d1-d2) responsive to a known current value, the sensitivity of the magnetic sensor  104 A can be determined. As discussed before, the differential voltage can be determined from the output of the ADC  168  converter which provides a digital signal representative of the differential voltage. 
       FIG.  5    is a flow diagram of a method of diagnosing the signal chain of a magnetic sensor circuit of an example embodiment. In a block  504 , a reference voltage is generated by periodically switching direction of current flow in a diagnostic sensor. The reference voltage is a non-sinusoidal differential voltage of which the amplitude alternates between minimum and maximum values. The reference voltage comprises a diagnostic sensor output voltage component responsive to a magnetic field and a diagnostic sensor offset voltage component resulting from a mismatch of the diagnostic sensor. In a block  508 , the reference voltage is amplified by an analog front end. The amplified voltage is a differential voltage which includes an amplifier offset voltage component. In a block  512 , the amplified reference voltage is demodulated by filtering the diagnostic sensor offset voltage component and the amplifier offset voltage component. In a block  516 , the demodulated signal is digitized. The signal chain is diagnosed using the digitized signal and comparing to the reference voltage. 
     Various illustrative components, blocks, modules, circuits, and steps have been described above in general terms of their functionality. Whether such functionality is implemented as hardware or software depends upon the particular application and design constraints imposed on the overall system. The described functionality may be implemented in varying ways for each particular application, but such implementation decision should not be interpreted as causing a departure from the scope of the present disclosure. 
     For simplicity and clarity, the full structure and operation of all systems suitable for use with the present disclosure is not being depicted or described herein. Instead, only so much of a system as is unique to the present disclosure or necessary for an understanding of the present disclosure is depicted and described.