Method and apparatus for determining a stray magnetic field in the vicinity of a sensor
View Patent ↗An apparatus ( 10 ) for determining a stray magnetic field in the vicinity of a sensor is described. The apparatus ( 10 ) has a multipole permanent magnet ( 60 ) with four or more poles and an axis of rotation ( 70 ). The multipole permanent magnet ( 60 ) produces a magnetic field ( 65 ) with magnetic field vectors ( 67 ). Two vertical Hall sensors ( 40 a and 40 ) are so arranged in two positions on a circular path ( 50 ) about the axis of rotation, such that the sum of the magnet field vectors ( 67 ) measured at the two positions is substantially zero.
1. A method for computing a stray magnetic field in the vicinity of an apparatus, the stray magnetic field being due to a current-carrying interfering conductor, the apparatus comprising a multipole permanent magnet with four or more poles and an axis of rotation, wherein the multipole permanent magnet produces a magnetic field with magnetic field vectors; and
two pairs of vertical Hall sensors for producing measurement signals, wherein the current-carrying interfering conductor is located at a first distance (d) from the two pairs of vertical Hall sensors, wherein the first distance is at least twice as great than a second distance (x) between the axis of rotation and either one of the two pairs of vertical Hall sensors, and
wherein the two pairs of vertical Hall sensors are arranged in two positions on a circular path about the axis of rotation in the magnetic field of the multipole permanent magnet, such that the sum of the magnetic field vectors measured at the two positions is substantially zero, the method comprising:
measuring the values of the magnetic field at the vertical Hall sensors; computing the values of |V a | and |V b | of the stray magnetic field according to the formula:
γ
n
=
arctan
(
x
·
sin
α
α
d
-
x
·
cos
α
α
)
α
n
=
α
α
+
γ
n
V
a
→
=
x
·
sin
∝
∝
sin
γ
n
γ
f
=
γ
n
α
f
=
α
α
-
γ
f
V
b
→
=
x
·
cos
α
f
+
d
·
cos
γ
f
subtracting the computed values |V a | and |V b | of the stray magnetic field from the measured values of the magnetic field at the vertical Hall sensors; and
determining the rotation angle.
2. The method of claim 1 , further comprising calculating a rotation angle from the measurement signals and the computed stray magnetic field.
3. The method of claim 1 , further comprising processing the measurement signals of the Hall sensors by a microprocessor in order to determine a strength of the stray magnetic field.
4. The method of claim 1 , further comprising determining an angle value.