Patent Publication Number: US-2023134728-A1

Title: Magnetic sensor for measuring an external magnetic field angle in a two-dimensional plane and method for measuring said angle using the magnetic sensor

Description:
FIELD 
     The present disclosure concerns a magnetic sensor for measuring an external magnetic field angle in a two-dimensional plane. The present disclosure further concerns a method for determining said angle using the magnetic sensor. 
     DESCRIPTION OF RELATED ART 
     Measuring an orientation of an external magnetic field in a 2-dimensional plane can be performed by using a magnetic sensor. Such magnetic sensor can be formed by combining 1-dimensional magnetic sensors, wherein each 1-dimensional magnetic sensors is formed from four magnetic sensor elements arranged in a full (Wheatstone)-bridge circuit configuration. One of the 1-dimensional magnetic sensors has a sensing axis being orthogonal to the sensing axis of the other 1-dimensional magnetic sensor. A constant DC voltage can be supplied to the two 1-dimensional magnetic sensors, such that each 1-dimensional magnetic sensor generates outputs being supplied to the input terminals of a respective differential amplifier in order to obtain two digitized signals. The two digitized signals are inputted into a processing unit where software routine solves the arctangent of the ratio of the two digitized signals to extract the external magnetic field angle. 
     A disadvantage of the conventional 2-dimensional magnetic sensor is that it must perform cumbersome and lengthy mathematical operations which require a powerful processing unit. This approach is therefore power, time and cost intensive. 
     In the case of angular 2-d sensors having two TMR bridges that are magnetically polarized at a difference angle of 90° can produces a sine and cosine waveform signal. However, such sensor is imperfect and the two bridges are never exactly 90° apart. This is often referred to the issue of “orthogonality”. 
     SUMMARY 
     The present disclosure concerns a magnetic sensor for measuring an external magnetic field angle in a two-dimensional plane, comprising: a first and second sensing unit outputting, respectively, a first signal sin(e) and a second signal cos(θ); a first multiplying DAC receiving the first signal and a first digital input sin(f*t) and outputting a first modulated output signal; a second multiplying DAC receiving the second signal and a second digital input cos(f*t) and outputting a second modulated output signal; a first RC filter receiving the first modulated output signal and outputting a first filtered signal sin(θ)*sin(f*t+RCd); a second RC filter receiving the second modulated output signal and outputting a second filtered signal sin(θ)*sin(f*t+RCd); an adder adding the first and second filtered signals and outputting a summed signal cos(f*t+RCd+θ); and an angle extracting unit for measuring the phase shift between the summed signal and a synchronization signal and determining the angle from the phase shift. 
     In an embodiment, the first and second sensing units comprise a plurality of TMR sensing elements arranged in full-bridge circuit. 
     The present disclosure further concerns a method for determining an rotational angle in a two-dimensional space of an external magnetic field, using the magnetic sensor. 
     The magnetic sensor and method disclosed herein allow for real-time update rates, with reduced power consumption and cost effectiveness with a compact IC solution. The magnetic sensor and method solves the issue of orthogonality. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention will be better understood with the aid of the description of an embodiment given by way of example and illustrated by the figures, in which: 
         FIG.  1    shows a TMR-based sensor comprising two sensing units, for measuring rotational angle in a two-dimensional space and an intensity of an external magnetic field; 
         FIG.  2    illustrates a possible configuration of the sensing unit; 
         FIG.  3    represents a sensing element comprising a self-referenced magnetic tunnel junction; and 
         FIG.  4    represents a portion of the magnetic sensor  10 , according to an embodiment. 
     
    
    
     DETAILED DESCRIPTION OF POSSIBLE EMBODIMENTS 
     A TMR-based magnetic sensor  10  for measuring rotational angle θ in a two-dimensional plane of an external magnetic field  60  is shown in  FIG.  1   . The magnetic sensor  10  comprises a first sensing unit  300  outputting a first signal  301  and a second field sensing unit  400  outputting a second signal  401 . 
     Each of the first sensing unit  300  and second magnetic field sensing unit  400  can comprise a plurality of TMR sensing elements arranged in full (Wheatstone)-bridge circuit, as illustrated in  FIG.  2   . In the arrangement of  FIG.  2   , the full-bridge circuit comprises two series connected TMR sensing elements  21 ,  22 , in parallel to two other series connected magnetic field sensing elements  23 ,  24 . Here, the first and second sensing units  300 ,  400  acts as a voltage divider, where the divider ratio is a function of the angle θ of the external magnetic field  60  in the two-dimensional space. Other arrangements of the TMR sensing elements are possible, such as half-bridge. 
     The sensing element  21 - 24  can comprise a self-referenced magnetic tunnel junction  2  (see  FIG.  3   ) comprising a reference layer  230  having a fixed reference magnetization  230  and a sense layer  210  having a sense magnetization  211  that is orientable relative to the reference magnetization  231 , according to a direction of the external magnetic field  60 . A sensing axis of the sensing units  300 ,  400  coincides with the fixed orientation of the reference magnetization  231 . In particular, a first sensing axis  330  of the first sensing unit  300  is set substantially orthogonal to a sensing axis  430  of the second sensing unit  400 , for example by programming the direction of the reference magnetization  231 . 
     The sensing element  21 - 24  is not limited to a self-referenced magnetic tunnel junction but can comprise a variety of elements that can sense a magnetic field. For instance, the sensing element can comprise a Hall Effect element, a magnetoresistance element or a magnetotransistor. As is known, there are different types of magnetoresistance elements, for example, a semiconductor magnetoresistance element such as Indium Antimonide (InSb), a giant magnetoresistance (GMR) element, an anisotropic magnetoresistance element (AMR), a tunneling magnetoresistance (TMR) element, a magnetic tunnel junction (MTJ), a spin-valve, etc. 
     The magnetic sensor  10  can further comprise a voltage generator  200  configured for supplying a first voltage waveform  201  to an input of the first magnetic field sensing unit  300 , and a second voltage waveform  202  to an input of the second magnetic field sensing unit  400 . The first and second voltage waveforms  201 ,  202  can comprise quadrature signals. For instance, the first voltage waveform  201  can comprise a sine waveform and the second voltage waveform  202  can comprise a cosine waveform. The first and second voltage waveforms  201 ,  202  have a periodic voltage waveform of fixed generator frequency f g  and amplitude. The first and second voltage waveforms  201 ,  202  are phase-shifted by substantially 90°. 
     The electronic circuit  10  can further comprise a clock generator  100  generating the clock synchronization signal  101 . The synchronization signal  101  synchronizes the operation of the voltage generator  200 . 
     The first sensing unit  300  outputs a first signal  301  and the second sensing unit  400  outputs a second signal  401 . The amplitude of the first and second signals  301 ,  401  is changed relative to the amplitude of the first and second voltage waveforms  201 ,  202 , depending on the orientation of the external magnetic field  60 , i.e., relative to the angle θ of the external magnetic field  60  when the sensing element  21 - 24  are operating in the linear range. 
     The magnetic sensor  10  further comprises an adder circuit  500  into which the first and second signals  301 ,  401  are inputted. The adder circuit  500  is configured for adding (or summing) the first signal  301  to the second signal  401  and outputting a summed signal  501 . 
     The magnetic sensor  10  further comprises an angle extracting unit  700 . The summed signal  501  and the clock synchronization signal  101  are supplied to an input of the angle extracting unit  700 . The synchronization signal  101  thus further synchronizes the operation of the angle extracting unit  700 . The angle extracting unit  700  is configured for measuring a phase shift between the summed signal  501  and the synchronization signal  101  and for determining the angle θ of the external magnetic field  60  from the measured phase shift. The angle extracting unit  700  outputs a digital angle output  701  comprising the information about the determined angle θ. 
       FIG.  4    represents the magnetic sensor  10 , according to an embodiment. In  FIG.  4   , the voltage generator  200  and the clock generator  100  are not visible. The first voltage waveform  201  is inputted to an input of the full-bridge first sensing unit  300  and the second voltage waveform  202  is inputted to an input of the full-bridge second sensing unit  400 . The voltage outputs −V out , V out  of each of the two branches of the first and second sensing units  300 ,  400  are inputted in a first and second adjustable gain amplifier  302 ,  402  which adjusts for offset and sensitivity variation in the voltage outputs −V out , V out  and output, respectively, the normalized first signal sin(θ)  301  and the normalized second signal cos(θ)  401 . 
     The first signal sin(θ)  301  and a first digital input sin(f*t)  303  are inputted in a first multiplying DAC  304 . The second signal cos(θ)  401  and a second digital input cos(f*t)  403  are inputted in a second multiplying DAC  404 . Here, f is a frequency and t is time, where the product f*t is larger than the angle θ (f*t&gt;&gt;θ). The first multiplying DAC  304  outputs a first modulated output signal sin(θ)*sin(f*t)  305  and the second multiplying DAC  404  outputs a second modulated output signal cos(θ)*cos(f*t)  405 . Preferably, the first and second multiplying DACs  304 ,  404  are 4-quadrant multiplying DACs. 
     The magnetic sensor  10  further comprises a first RC filter  306  receiving the first modulated output signal  305  and outputting a first filtered signal sin(θ)*sin(f*t+RCd)  307 , where RCd is a phase delay caused by the first RC filter  306 . A second RC filter  406  receives the second modulated output signal  405  and outputting a second filtered signal sin(θ)*sin(f*t+RCd)  407 , where RCd is a phase delay caused by the second RC filter  406 . The first filtered signal  307  is added to the second filtered signal  407  in the adder circuit  500 . The a summed signal  501  (sin(θ)*sin(f*t+RCd) and cos(θ)*cos(f*t+RCd)) yields cos(a)*cos(f*t+RCd)−sin(θ)*sin(f*t+RCd) corresponds to cos(f*t+RCd+θ). The summed signal cos(f*t+RCd+θ)  501  is inputted in a comparator  601 . Preferably, the first and second RC filters  306 ,  406  are configured such that ½*π*RC≈f. 
     The magnetic sensor  10  further comprises a reference multiplying DAC  504  inputted by an analog reference signal “1”  502  and a normalized reference digital input cos(f*t)  503 , such as to give a reference modulated output signal cos(f*t)  505 , where f&gt;&gt;θ. The reference modulated output signal  505  is inputted in a reference RC filter  506  such as to generate a reference output signal cos(f*t+RCd)  507 , where RCd is a phase delay caused by the reference RC filter  506 . The reference output signal cos(f*t+RCd)  507  is inputted in a reference comparator  602 . 
     The external magnetic field angle θ can be determined from the phase delay RCd. 
     Preferably, the first, second and reference RC filters  306 ,  406 ,  506  have the same roll-off frequency. 
     The comparator  601  and the reference comparator  602  are configured for finding rising zero cross of, respectively, the summed signal  501  and the reference output signal  507 . A comparator signal output  603  of the comparator  601  and a reference comparator signal output  604  of the reference comparator  602  are inputted in the angle extracting unit  700 . Here, the angle extracting unit  700  is a counter. The counter  700  runs at a clock frequency greater than f such as to determine the angle θ. 
     The counter  700  can be configured to start counting when the reference output signal cos(f*t+RCd)  507  crosses zero and to stop counting when the summed signal cos(f*t+RCd+θ)  501  crosses zero. The angle θ is then proportional to the count. 
     In an embodiment, the complementary edges of the start and stop pulses of the clock synchronization signal  101  are used. This allows for doubling the update rate of the angle extracting unit  700 . 
     In an embodiment, a method for determining an rotational angle θ in a two-dimensional space of an external magnetic field  60 , using the TMR-based magnetic sensor  10 , comprises the steps of: 
     input the first signal  301  of the first sensing unit  300  and the first digital input sin(f*t)  303  to the first multiplying DAC  304  to output the first modulated output signal sin(θ)*sin(f*t)  305 ; 
     input the second signal  401  of the second sensing unit  400  and the second digital input cos(f*t)  403  to the second multiplying DAC  404  to output the second modulated output signal cos(θ)*cos(f*t)  405 ; 
     input the first modulated output signal  305  in the first RC filter  306  and the second modulated output signal  405  in the second RC filter  406  to output, respectively, the first filtered signal sin(θ)*sin(f*t+RCd)  307  and the second filtered signal sin(θ)*sin(f*t+RCd)  407 ; 
     adding the first filtered signal ( 307 ) and the second filtered signal  407  in the adder circuit  500  to output the summed signal cos(f*t+RCd+θ) 501; 
     measuring the phase shift RCd between the summed signal  501  and the synchronization signal  101  in the angle extracting unit  700  and determining the angle θ from the measured phase shift RCd. 
     In an embodiment, the method further comprises providing inputting the summed signal  501  in the comparator  601  and finding rising zero cross of the summed signal  501 . 
     In another embodiment, the method further comprises providing a first voltage waveform  201  to the first sensing unit  300  to output the first signal sin(θ)  301  and providing a second voltage waveform  202  to the second sensing unit  400  to output the second signal cos(θ)  401 . 
     In yet another embodiment, the method further comprises inputting an analog reference signal  502  and a normalized reference digital input cos(f*t)  503  in the reference multiplying DAC  504  to output a reference modulated output signal cos(f*t)  505 ; and inputting the reference modulated output signal  505  in the reference RC filter  506  to generate the reference output signal cos(f*t+RCd)  507 . 
     In yet another embodiment, the method further comprises inputting the reference output signal  507  in the reference comparator  602  and finding rising zero cross of the reference output signal  507 . 
     One possible method is to skew (deviation, distort) the clocks that generate the digital sine and cosine modulation functions. In particular, imperfectly “orthogonal” first and second signals  301 ,  401  can be sampled and held and a programmable delay of several clock cycles can be added. This should allow the orthogonality to be corrected to the level of the angular resolution of the system. 
     REFERENCE NUMBERS AND SYMBOLS 
     
         
           10  magnetic sensor 
           100  clock generator 
           101  clock synchronization signal 
           21 - 24  TMR sensing element 
           210  sense layer 
           211  sense magnetization 
           230  reference layer 
           231  reference magnetization 
           200  periodic voltage generator 
           201  first voltage waveform 
           202  second voltage waveform 
           300  first sensing unit 
           301  first signal 
           302  adjustable gain amplifier 
           303  first digital input 
           304  first multiplying DAC 
           305  first modulated output signal 
           306  first RC filter 
           307  first filtered signal 
           330  first sensing axis 
           350  capacitor 
           400  second sensing unit 
           401  second signal 
           402  adjustable gain amplifier 
           403  second digital input 
           404  second multiplying DAC 
           405  second modulated output signal 
           406  second RC filter 
           407  second filtered signal 
           430  second sensing axis 
           500  adder circuit 
           501  summed signal 
           502  analog reference signal 
           503  reference digital input 
           504  reference multiplying DAC 
           505  reference modulated output signal 
           506  reference RC filter 
           507  reference output signal 
           60  external magnetic field 
           601  comparator 
           602  reference comparator 
           603  comparator signal output 
           604  reference comparator signal output 
           700  angle extracting unit, counter 
           701  digital angle output 
         θ magnetic field angle 
         f frequency 
         f g  fixed frequency, generator frequency 
         RCd phase delay 
         t time 
         V voltage