Digital angle encoder for motors works at 200,000rpm

Melexis is aiming at electric motors with family of angle-sensing absolute magnetic encoder ICs that can work at high speed and typically deliver 1° accuracy or better.

“By detecting the three components of the magnetic flux density – Bx, By and Bz – and measuring the differential magnetic field in the z-axis, MLX90382 enables tracking of absolute angle position and speeds of up to 200,000rpm,” said the company.

There are four devices with part numbers of the form ‘MLX90382LLW-ABA-xxx-RE’, where xxx is:

000 for the 10mT on-axis version
010 for the 10mT off-axis version
100 for the 40mT on-axis version
600 for the stray field rejecting on-axis (X-Y only) version

These all sense 360° of rotation, operate over -40 to +150°C and come in a 4 x 4mm QFN-24 package.

Maximum magnet strength is 70mT for 10mT versions, and 120mT (x or y axis) and 70mT (z) for the 40mT version.

There are also dual-die variants for higher reliability applications. These come in TSSOP-16 packaging, and their part number has ‘GO’ where ‘LW’ sits for the single-die versions.

Inside is a complex digital signal processing chain that provides customer-programmable filtering and interpolated break-point linearisation amongst other functions.

On axis and off-axis sensing

On-axis sensing, in Melexis’ definition, is where the IC can be placed in-line with the axis of rotation. It defines three versions of on-axis sensing depending on the orientation of the IC (diagram right).

The stray-field suppressing ‘600’ version is a special case than can only be used in on-axis X-Y mode. It has a maximum error of ±0.15° when exposed to a field of 4kA/m from any direction (or ±0.25° for dual-die TSSOP-16 types).

Inherently the on-axis arrangements needs the IC to be at one end of a rotating shaft. If the sensor cannot be at the end of the shaft, or wires have to pass up a hollow shaft, Melexis offers its ‘off-axis’ variant (version ‘010’) which has the IC next to a magnet threaded through by the shaft. There is only one possible IC orientation for off-axis MLX90382 sensing (diagram left).

Five different output interface modes are available: industry-standard differential ABI/UVW for incremental angular output, emulation of UVW typically used in brushless dc motors, a PWM interface, as well as configurable SPI and SSI interfaces.

Operation is from 5V or 3.3V, and parts are AEC-Q100 (Grade 0) qualifified. “Furthermore, the on-axis configuration supports safety standards with SIL2 integrations”, said Melexis.

There is far more going on inside this IC than is covered above – take a look at this MLX90382 data sheet for more.