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Magnetic Angle Sensor for BLDC Brushless DC Motor

Brushless DC (BLDC) motors

Brushless DC motors have become gained profound popularity among engineers and designers in the recent past. This growing popularity can be attributed to its design. The lack of brushes makes it long lasting since wear and tear is reduced. BLDC motors also have better efficiency compared to their brushed counter parts. More importantly, they are smaller hence can fit well into small spaces unlike brushed motors.

Some motor applications have no tolerance for unpredictable or irregular motor behavior. Designers of such applications are using BLDC motors to meet this requirement. This is because angle position sensing in BLDC motors is easier to achieve using a magnetic position sensor.

Magnetic Angle Sensor for BLDC Brushless DC Motor

Magnetic angle sensing is responsible for ensuring that the motor provides a smooth start-up, achieves highest possible efficiency, and maintains continuous commutation. A magnetic angle sensor is made up of multiple highly sensitive hall sensor elements integrated on a die alongside an analog, signal processing and digital circuitry. All these can be implemented in a single chip paired with a simple magnet. This chip is then fixed at the end of the motor’s shaft in parallel with a circular two pole magnet mounted on the end of the motor’s shaft.

This simple chip solution enables the acquisition of the absolute position data from start-up to a maximum rpm of 28000.Dynamic Angle Correction is used to internally compensate for propagation delays at high speeds thus reducing the dynamic angle error down to no more than 0.36 at 28000 rpm. The absolute position data update is 222ns hence data is available for use in real-time across a wide range of rpm.

Benefits of a single chip sensor solutions

  • Accurate absolute position information always
  • Simple to assemble
  • It costs less since an IC benefits from the economies of scale accrued in the semiconductor fabrication process
  • It is immune contamination (by chemicals, particles or other materials)

Example

A good example is the AS5047 from ams. The AS5047 has the following features;

  • 1 to 2mm range between the magnet and the IC thus allowing for a large amount of tolerance in the production process.
  • Permanent absolute alignment of the rotor and stator is achieved electronically after mechanical assembly using an on-chip OPT memory programming.
  • The implemented differential sensing scheme implemented in the chip provides for extremely high levels of stray field immunity hence the special magnetic shielding arrangements are not required

When paired with a low-cost circular diametrically magnetized magnet, the AS5047 converts magnetic field strength to position data. It can provide data in the UVW output signals for steady state commutation of motors while offering absolute 14-bit position data incremental position data alongside other position information needed.

Constant monitoring of the absolute position at defined intervals enables the system to switch to absolute angle and computed drive when starting up or when handling varying loads. Absolute position sensing can be implemented in all BLDC motor management schemes.

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