WIDER POSITION SENSING CIRCUIT

WIDER POSITION SENSING CIRCUIT

To go from 45° to 90° requires two HMC1501
sensors or a single HMC1512 dual sensor part. By
using two bridges with 45° displacement from each
other, the two linear slopes can be used additively.
Figure 8 shows a typical configuration.
From Figure 8, as the shaft rotates around, magnetic
flux from a magnet placed at the end of the shaft exits
the north pole and returns to the south pole. With a
HMC1512 placed on the shaft axis, just above the
magnet, the flux passing through the sensor bridges
will retain the orientation of the magnet. From this
rotation, the output of the two bridges will create sine
and cosine waveforms as shown in Figure 9.

Because the sine (sensor bridge A) and cosine
(sensor bridge B) will match after the offset error
voltages are subtracted, the ratio of bridge A to bridge
B creates a tangent 2O function and the amplitude A
values cancel. Thus the angle theta is described
as:

However because there are some trigonometric
nuances with the arctangent function when gets
close to _45° and beyond, these special cases apply:

Because most trigonometric functions are performed
as memory maps in microcontroller integrated circuits,
these kinds of special case conditions are easily dealt
with. The resultant angle theta is the relative
position of the magnetic field with respect to the
sensor. It should be noted that if rotation is permitted
beyond _90°, the theta calculation will replicate again
with postive and negative 90° readings jumping at the
end points. Further performance to 360° or _180° can
be mapped into a microcontroller by using this circuit
plus a Hall Effect sensor to determine which side of
the shaft is being positionally measured via magnetic
polarity detection. Figure 10 shows the basic circuit
interface for the HMC1512.

Source http://www.ssec.honeywell.com/magnetic/datasheets/an211.pdf

HMC1501 / HMC1512
Linear / Angular / Rotary
Displacement Sensors
High resolution, low power MR sensor capable of measuring the angle
direction of a magnetic field from a magnet with <0.07>

HMC1501 Datasheet pdf