Position sensor is immune to magnetic stray fields

June 11, 2015 // By Christoph Hammerschmidt
Rotary position sensors cover multiple application fields in vehicles. From electric power steering (EPS) to pumps, double-clutch transmissions, starter motors and alternators to windscreen wiper motors, in all these cases position sensing is an indispensible element of closed-loop control approaches. With its AS5147P, Austrian chip vendor ams AG covers those applications where high speed is required. Compatible with the requirements of the ISO 26262 functional safety standard, the device can be deployed in safety-relevant applications.

Its appeal to automotive system designers rests on a combination of high measurement accuracy at high rotation speeds (up to 28.000 rpm), high reliability, low system cost and support for standards compliance, the company says.

The AS5147P draws on patented DAEC (Dynamic Angle Error Correction) technology to deliver accurate position measurements even at very high rotation speeds. The DAEC compensation scheme reduces the propagation delay inherent in the sensor’s signal chain to almost zero. As a result, the AS5147P’s angle error is a negligible 0.02° at 1.7krpm, 0.17° at 14.5krpm and 0.34° at 28krpm. The DAEC function also enables the device to refresh its measurement output every 1.9µs.

Block diagram of the AS5147P. For higher resolution click here .

High reliability and low system cost in automotive applications are assured by the AS5147P’s intrinsic immunity to stray magnetic fields. Strong stray magnetic fields are present in vehicles, particularly those with electric powertrain such as hybrid and battery electric vehicles. These stray magnetic fields are much stronger than that of the small target magnet with which a magnetic position sensor is paired.

The Austrian chipmaker claims to have a unique differential sensing principle of the magnetic position sensors that makes them immune by design to stray magnetism. Thus, automotive system designers can avoid the need to incorporate bulky and expensive magnetic shielding into their products. The high sensitivity of the AS5147P also enables the use of a small (6mm diameter), low-cost target magnet.

In addition, the AS5147P supports automotive OEMs’ ISO26262 compliance programmes in various ways:

  • The AS5147P was developed in accordance with the ISO26262 flow, following the SEooC (Safety Element out of Context) and AoU (Assumption of Use) guidelines specified in the standard, thus helping manufacturers to achieve the requirements of any ASIL grade
  • ams provides a safety manual and failure mode effects diagnostic analysis (FMEDA) document for the AS5147P, guiding customers on the way to achieve their target ASIL