Design and Performance Evaluation of a Simple Resistance-to-Digital Converter for Tunneling Magneto-Resistance-Based Angular Position Sensor with 180° Range

Numerous applications require angular position measurement of a rotating shaft up to 180 degrees range. Some examples include engine throttle-valve, joint angle of robotic arms, skid loaders, etc. Angle sensors based on tunneling magneto-resistance (TMR) effect can be used for many such applications due to their features such as compact and cost-effective design, low powered operation, and high sensitivity. The resistances of TMR angle sensor vary as sine and cosine function of the shaft angle. In this paper, a simple Resistance-to-Digital Converter for TMR angle sensor (RDCT) is presented. The proposed RDCT processes the nonlinear natured TMR resistances and render a linear digital output for 180 degrees range. The digital indication of the shaft angular position is achieved without using any explicit analog-to-digital converter (ADC) and look-up table (LUT). The RDCT is based on standard dual-slope principle and hence possesses all the merits of dual-slope-based measurement method. It can also be employed to detect angular position of the target shaft rotating between 0 and 180 degrees at slow speeds. The working principle of the RDCT is explained first in a detailed manner. Later, the effects of practical parameters of TMR angle sensor and the RDCT circuitry are analyzed. An effective offset-error reduction method is then discussed that reduces the output error due to the mismatch in TMR resistances. Extensive simulation studies are carried out to quantify the RDCT performance under ideal as well as non-ideal conditions. The effectiveness of the offset-reduction method is also proved in simulation studies.