Allegro MicroSystems Magnetic Position Sensors

Magnetic position sensors offer significant advantages over optical encoders: Environmental robustness - immune to dust, dirt, oil, and moisture that can foul optical discs

operates reliably in harsh industrial and automotive environments. Reliability - contactless operation eliminates mechanical wear, providing unlimited rotational life

no bearings to fail or discs to crack. Shock and vibration - solid-state construction withstands high shock (up to 50g) and vibration without performance degradation. Integration - smaller package sizes available

can be integrated directly on motor PCBs or in compact assemblies. Cost - lower total cost when considering wiring, connectors, and installation

no precision alignment required. Speed - no mechanical speed limitations

operates at very high speeds limited only by electrical bandwidth (typically > 10,000 RPM). The main limitation is absolute position at power-up - magnetic sensors typically require movement to establish position, while absolute encoders retain position information.

Use magnetic sensors for harsh environments and high reliability; optical encoders only when absolute position at power-up is required.

Magnet selection is critical for magnetic sensor performance: Magnet material - Neodymium (NdFeB): Highest field strength, good for small air gaps

Samarium Cobalt (SmCo): High temperature capability (up to 300°C), expensive

Ferrite (ceramic): Low cost, lower field strength, larger sizes needed

Alnico: Stable over temperature, moderate cost. Field strength requirements - Typical requirement: 300-800 Gauss at sensor location

Higher field strength improves noise immunity

Excessive field (> 1000G) may cause saturation. Magnet geometry - Diametrically magnetized disc: Best for angle sensors

Axially magnetized disc: Common for linear sensors

Multipole ring: For incremental position sensing. Temperature considerations - Neodymium loses ~0.1%/°C above 80°C

SmCo stable to 300°C

Consider temperature coefficient for precision applications. Size and mounting - Larger magnets provide stronger fields

Ensure secure mounting to prevent position shift

Consider back-iron for field concentration. Allegro provides magnet selection guidelines and can recommend specific magnets for your application.

Use NdFeB for high performance; SmCo for high temperature; ensure 300-800G field strength at sensor.

Allegro angle sensor accuracy specifications: Resolution - 8-bit to 14-bit depending on device (0.09° to 0.02°)

Higher resolution provides finer position information. Accuracy - Total error: ±0.5° to ±2° depending on device and conditions

Includes offset, gain, and linearity errors. Sources of error: Sensor nonlinearity: ±0.1° to ±0.5°

Magnet misalignment: ±0.1° to ±1° depending on tolerance

Temperature drift: ±0.01°/°C typical

External fields: Depends on shielding and proximity. Improving accuracy: Use precision-machined magnet mounts

Implement calibration for offset and gain

Shield from external magnetic fields

Use differential sensing for common-mode rejection. Device comparison: A1335: ±1° accuracy, 12-bit resolution, cost-effective

AAS33001: ±0.5° accuracy, 14-bit resolution, ISO 26262 support

For highest accuracy, consider end-of-line calibration.

Use A1335 for ±1° accuracy; AAS33001 for ±0.5° with safety support; implement calibration for best results.

External magnetic field protection strategies: Shielding - Mu-metal or ferrite shields redirect external fields away from sensor

Place shield between sensor and interference source

Ground shield to prevent capacitive coupling. Distance - Increase separation between sensor and interference sources

Field strength decreases with cube of distance

2× distance = 8× field reduction. Differential sensing - Use differential Hall sensors (A1335) to reject common-mode fields

External fields affect both elements equally and cancel out. Magnet strength - Use stronger magnets (higher coercivity) to dominate over external fields

Neodymium magnets resist demagnetization from external fields. Orientation - Orient sensor to minimize sensitivity to expected interference direction

Some sensors have directional sensitivity patterns. System design - Route high-current cables away from sensors

Use twisted pair wiring to reduce field generation

Implement software filtering for slow-varying interference. For automotive applications with high-current cables (starter, alternator), shielding and differential sensing are typically required.

Use differential sensors and shielding for noisy environments; increase distance from interference sources; use strong magnets.

Hall-effect vs TMR (Tunnel Magnetoresistance) magnetic sensors: Hall-effect technology - Measures magnetic flux perpendicular to chip surface

Mature, proven technology with excellent reliability

Moderate sensitivity (mV/G)

Higher power consumption (mA range)

Good temperature stability

Wide availability, lower cost. TMR technology - Measures in-plane magnetic fields

Higher sensitivity (mV/V/G) than Hall-effect

Lower power consumption (μA range)

Excellent temperature stability

Higher resolution possible

Newer technology, higher cost. Performance comparison: Sensitivity: TMR 10-100× higher than Hall-effect

Power: TMR 10-100× lower than Hall-effect

Bandwidth: Hall-effect typically higher (> 1MHz)

Temperature drift: Both excellent with proper design

Cost: Hall-effect lower, TMR higher. Application guidance: Use Hall-effect for general applications, high bandwidth, cost-sensitive designs

Use TMR for battery-powered devices, high sensitivity requirements, precision applications. Allegro offers both technologies with the A1335 (Hall-effect) and A31301 (TMR) families.

Use Hall-effect for general applications and high bandwidth; TMR for low-power and high-sensitivity requirements.