For geological “down-hole” exploration tilt and vibration measurement, precision sensors that can take the heat in a tiny, constrained, and brutal environment are critical. But these sensors must do more than withstand harsh operating conditions. They also must deliver an accurate and reliable output in a small package and keep working at low power levels.
The Analog Devices ADXL206 dual-axis iMEMS accelerometer can tackle these tasks with its signal-conditioned analog outputs (see the figure). It performs measurements while drilling, at several hours in each well during its creation, and continuously keeps on running. The packaged product integrates the sensing element and the signal-conditioning circuitry on a single monolithic chip.
The accelerometer measures acceleration over a full-scale range of ±5 g with a resolution of 1 mg. It also measures both dynamic acceleration such as tilt and static acceleration such as gravity, all while operating from –40°C to 175°C. Shock survival is rated at 3500 g. All of this comes in a RoHS-compliant (Restrictions on Hazardous Substances) eight-lead side-brazed ceramic dual-inline package that’s just 13 by 8 by 2 mm.
Although the upper operating temperature of 175°C is guaranteed for 1000 hours, the sensor can recover 100% of the measured data beyond that limit, at diminished performance, thanks to innovative compensation techniques. This is ensured through the design of the signal-processing ASIC whose size is minimized, compared to other competitive approaches that usually employ large and bulky circuitry that constrains maneuverability in down-hole operations.
“We ensure performance stability and repeatability at high temperatures through the design of the signal-processing circuitry in the ASIC,” explains Wayne Meyer, marketing and applications manager for the Analog Devices MEMS and Sensor Technologies Division. “Instead of using additional temperature-compensation circuitry, innovative design techniques have been used to ensure that high performance is built in. As a result, there is essentially no quantization error or non-monotonic behavior, and temperature hysteresis is very low, typically 2 mg over the entire operating-temperature range of –40°C to 175°C.”
Performance parameters include a resolution of 1 mg at 60 Hz and high 0-g bias repeatability of ±10 g. The ADXL206’s noise floor is specified at 110 µg/√Hz, allowing signals below l mg (0.06° of inclination) to be resolved in tilt-sensing applications using narrow bandwidths of less than 60 Hz. The accelerometer is also skimpy on power. The low-power device requires a single supply and draws just 700 µA at 5 V typical.
“As designers put together smart drilling systems that go miles and miles under the earth’s surface, they are challenged to balance power and space requirements against the need for more information,” says Pam Aparo, segment marketing manager for instrumentation at Analog Devices. “The ADXL206 is not only extremely compact, but it also lowers power consumption by an order of magnitude from over 10 mA to under 0.5 mA per axis.”
The accelerometer’s bandwidth is user-selectable with capacitors at the chip’s X and Y outputs. A bandwidth of 0.5 Hz to 2.5 kHz is available to suit the application. The ADXL206 is in production and costs $789.00 each in lots of 1000 pieces.