Electric/Magnetic

360 Degree Rotary Position Sensing with Novel Hall Effect Sensors

March 1, 2006 By: Vincent M. Hiligsmann, Melexis Sensors

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A new breed of Hall sensors can sense all three magnetic flux density components at a single point.

The block diagram in Figure 2 shows that both raw Hall signals VX and VY are amplified through a multiplexed chopper amplifier prior to being digitized. A microcontroller-based digital signal processing (DSP) core further processes the signals and computes the angular information from them. The angle (α) is output as an analog signal (after passing through a DAC) or as a digital PWM or serial signal.

Figure 3. A diametrically magnetized magnet rotating above the IC

When a diametrically magnetized magnet (one that is magnetized through the plane of a disk magnet) rotates above the IC (Figure 3), the flux density components BX and BY will describe two sine waves in quadrature (Figure 4) with BX proportional to cosine(α) and BY proportional to sine(α).

Figure 4. The rotating magnet produces two sine waves in quadrature, giving the magnetic flux densities in the X and Y axes, BX and BY

The raw Hall signals VX and VY are proportional to BX and BY. After amplification, the MLX90316's embedded DSP performs the following operation to get the angular information:

where:

A = gain

VX = raw Hall signal in the X direction

VY = raw Hall signal in the Y direction

α= angle

Because the MLX90316 directly provides the angular position (up to 360°) of the magnet rotating above it, it is intrinsically a rotary position sensor IC.

The relationship in Equation 1 highlights one of the key features offered by the Triaxis Hall technology: After amplification, the two Hall signals are divided; any matched variations of both signals are compensated for and so do not impact the accuracy of the angular output. A Triaxis Hall IC is not influenced by the magnet's thermal coefficient or to changes in air gap whereas conventional Hall technology is directly affected by such variations. The MLX90316 also features an EEPROM to store all the necessary parameters linked to the chip functionality and output characteristic, allowing the adjustment of the output transfer characteristic once the module is completely mounted.

Rotary Position Sensor Performance

The main figure-of-merit for any position sensor is the linearity error. This term includes all the deviations from the ideal output transfer characteristic (i.e., straight line). The deviations are coupled to electrical, mechanical, magnetic, thermal, and aging tolerances. In the case of a rotary position sensor, performance is mainly governed by the intrinsic IC linearity error and additional errors introduced by the sensor module assembly.

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