EETE JUN 2015

SENSORS & DATA CONVERSION On-chip linearization in an angle sensor IC By Ali Husain Y. Sirohiwala and Wade Bussing Numerous applications in industries ranging from industrial automation and robotics to electronic power steering and motor position sensing require monitoring of the angle of a rotating shaft, either in an on-axis or off-axis arrangement. The design of any successful angle measurement system for such applications needs to be based on the particular user’s requirements. These may include arrangement (off-axis or onaxis), air-gap, accuracy, and temperature range, among others. In particular, minimising angle error over temperature, positional misalignment, and air-gap, is a key objective. These variables are related, in turn, to system level design choices like magnet geometry, magnet arrangement (on-axis or off-axis), magnetic material, and mechanical tolerances. As such, flexibility is required of the angle sensor IC, in order to work around these potential error sources without adding complexity and cost to the system-level design. Even the best magnetic angle sensor IC is only as good as the magnetic field that it senses. A magnetic angle measurement system has two main sources of error: • sensor IC related errors: intrinsic nonlinearity, parametric temperature drift and noise. • Magnetic input related errors: field strength variation and field non-linearity. Angle error is the difference between the actual position of the magnet and the position of the magnet as measured by the angle sensor IC. This measurement is done by reading the angle sensor IC output and comparing it with a high resolution encoder. The ‘summarized’ angle error on one full rotation is defined as angu¬lar accuracy error, and it is calculated according to the following formula: Angle accuracy error = (Emax – Emin)/2 In other words, it is the amplitude of the deviation from a perfect straight line between 0° and 360° When using a magnet in a design, the magnetic input will probably not be homogeneous over the entire range of rotation: it will have inherent errors. These magnetic input errors cause measurement errors in the system, and become especially important when considering side-shaft or off-axis designs that have higher intrinsic magnetic errors – see figure 1. Even the most accurately calibrated angle sensor IC will produce inaccurate results if the error contribution from the magnetic input dominates. In most cases, even on-axis magnetic designs suffer from relatively large misalignments that occur during the assembly of the customer module in the production line. These magnetic error sources are inevitable, and mitigating them is often impossible and almost always expensive. Fig. 1: Off-axis (left) and on-axis (right) configurations for an angle-sensor IC As far as angle sensor IC related errors are concerned, intrinsic non-linearity and parametric temperature drift are optimised by the manufacturer before shipping to the customer. Noise performance can be optimised for the customer application by using on-chip filtering. Advanced linearization This article describes an angle sensor IC (the Allegro A1332) in which this problem is solved by using advanced linearization techniques to compensate for these errors at the customer’s end-of-line manufacturing location. In particular, it shows how magnetic input related errors in excess of ±20° can be linearized to as low as ±0.3°: roughly a 65× improvement. This linearization can be performed based on data from a single rotation of the target magnet around the angle sensor IC. The angle readings from this rotation are used to generate linearization coefficients which can then be stored into on-chip EEPROM, optimising that particular angle sensor IC for that magnetic system. Two different linearization techniques are used in the A1332 angle sensor IC: segmented linearization and harmonic linearization. Ali Husain Y. Sirohiwala is Senior Systems Engineer at Allegro MicroSystems, LLC - www.allegromicro.com Wade Bussing is Engineering Technologist at Allegro MicroSystems. Fig. 2: Angle output using pre/post segmented linearization. 38 Electronic Engineering Times Europe June 2015 www.electronics-eetimes.com

EETE JUN 2015