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 2.

Figure 1. Wide Position Sensing

Figure 2. Sine, Cosine Waveforms

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:

 Θ = 0.5 * arctan ( ΔVa / ΔVb )

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

                       For ΔVa = 0,       Θ = 0^o
                       For ΔVb = 0 and ΔVa = < 0,    Θ = -45^o
                       For ΔVb = 0 and ΔVa = > 0,    Θ = +45^o
                       For ΔVa < 0 and ΔVb = < 0, subtract 90^o from Θ
                       For ΔVa > 0 and ΔVb = < 0, add 90^o from Θ


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.

Figure 3. HMC1512 Circuit

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