High-Temperature Anisotropic Magnetoresistive (AMR) Sensors

Abstract

Magnetic field sensors are employed in downhole oil and gas well drilling applications for azimuth sensing, orientation/rotation sensing, and magnetic anomaly detection. As the wells get deeper there is demand from industry to increase the operating temperature from ~175°C to ~225°C and higher. We have extended the operating regimen of silicon-based anisotropic magnetoresistive sensors to higher temperatures to address this demand. The low-frequency minimum detectable field of these sensors monotonically increases with increasing temperature. At room temperature it is 2.2 μG/√Hz@1 Hz reaching a value of 26 μG/√Hz@1 Hz at 225°C. Signal and noise density both increase with increasing sensor bias voltage such that low-frequency signal-to-noise ratio does not vary in the bias voltage range of 2.5–10 V. We achieve excellent linearity of transfer function in the ±0.8 Gauss range in a closed-loop configuration. Deviation from linearity increases monotonically with increasing temperature but remains <0.002% of full scale or 29 μGauss at 225°C. Using low-noise electronics, closed loop operation of a typical sensor shows 1 – σ measurement variability of 21 μGauss at 220°C. By a combination of averaging and closed-loop operation, an input step from 0 to 75 μGauss is replicated at the output to within 0.1 μGauss at 225°C. Initial measurements suggest survivability of these sensors at 225°C to 2,000 h.