Measure angles on rotating systems with high resolution

A common approach to measuring distance or angle in a rotating system is by using an incremental encoder or resolver. The majority of industrial applications, such as machines or drives, are dominated by optical encoder—either with incremental or absolute output of the distance or angle.

However, more often the mechanical size and cost are demanding new alternatives. The end users are also pushing for higher resolution and at the same time want more robustness with higher operating temperatures.

Increasingly users and manufacturer are implementing magnetic systems to meet this demand and in this approach Hall sensors are of specific interest. The principal is well known and different sensors have been used in the automotive and measurement equipment successful for a number of years. Most applications use Hall sensors as simple switches, pulse generators, or field sensors.

The system integration of the Hall sensor and the encoder function onto a single silicon chip is now providing a lower cost solution. Using multiple Hall sensors on-chip allows signal processing with good noise immunity. The distance and angle information are output as sine and cosine signals.

For rotating applications integrated encoder ICs, such as the iC-MA, are suitable as it includes all functions to sense and process the rotating magnetic field of the permanent magnet placed on top of the IC.

Innovation is provided by a single device, such as the iC-ML which integrates linear Hall sensors and encoder to measure rotating motions. Instead of a single permanent magnet, a "pole wheel" is used to apply measurement scale as needed.

Pole wheel multiplies resolution
A rotating permanent magnet generates one cycle per rotation. With a pole wheel, it can create multiple magnetic field cycles per revolution.

The figure above shows an iC-ML application with a pole wheel having eight pole pairs. This means that it will generate eight magnetic cycles and eight electronic signal periods per revolution of the pole wheel—thus increasing the resolution for the angle measurement by a factor of eight for the complete system.

A linear motion of eight cycles represents for the iC-ML a length of 8 x 5.12 mm = 40.96 mm. This is equivalent to the pole-wheel circumference.

The effective circumference includes the distance h between the sensor IC and the pole wheel. As a rule of thumb it can be said that distance h should not exceed half of the magnetic pole width. This is to ensure that there is a sufficient magnetic field at the chip surface. For the iC-ML pole width of 2.56 mm, it works out to h = 1.3 mm. With the TSSOP20 package thickness typically 0.4 mm on the top side, the maximum working distance h should be 0.9 mm.