Hall-based motor position sensor for electric mobility

Antoine DELAPORTE |

Accurate motor position sensors are required to ensure the efficient and reliable control of electric machines.

Various technical solutions are available, but despite the widespread use of magneto-sensitive sensors in end-of-shaft configurations, such a technology was not seen in through-shaft configurations, until now. MMT has developed an innovative and simple magnetic solution that consists only of a rotor magnet and two 2D Hall ICs. At its core, the technology, uses a specific sinusoidal magnetization pattern that can be repeated over multiple periods.  This principle already confers an excellent intrinsic accuracy as a single-probe sensor; however, requirements for motor control are stricter than for traditional applications, and the accuracy commonly required is ±1 electrical degree in all conditions. This is where the second Hall IC and a simple algebraic signal cross-combination come into play.

The benefits of such a design are the following:

  • Improvement of the accuracy to under ±1 electrical degree and increased robustness to mechanical tolerances by reducing the parasitic harmonics in the probe signal
  • Mitigation of the influence of external magnetic fields generated by the motor by use of signal cross-combination
  • Possibility of single-probe measurement in degraded mode, bringing an intrinsic safety state in case of failure
  • Reduced mass and overall dimensions even for large electric machines

The raw output signals can be fed directly to the electronics used to drive the electric machine, requiring only simple signal processing (cross-combination & CORDIC algorithm). Alternatively the signal processing can easily be carried out inside the position sensor itself to get a standalone device with the desired communication protocol.

This technology has the benefits of the Hall Effect sensors on which it is based. Indeed, It is well known that Hall Effect sensors are at their most versatile when it comes to measurement of position or rotational speed, since they can withstand rough environmental conditions (e.g., high temperature, shocks, dust, particle contamination), and at the same time offer optimized system costs and miniaturization. And contrary to some other alternatives, Hall Effect position sensors can be used with large airgaps between the rotor and the stator portions of the sensor and do not need tight mechanical tolerances. Moreover, different mechanical integrations are possible e.g. measurement on the outer side, on the inside of a ring magnet or even axial measurement on a disk magnet. In addition, the technology is adaptable to a wide range of rotor sizes (from a few 10mm to over 100 mm) and for low to high speeds, which makes this sensor technology flexible for a wide range of applications in electric vehicles and auxiliaries (Starter-Generator, traction motor…).

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