Electro-permanent magnetic weight release mechanism for buoyancy control of an autonomous well-logging tool

Abstract

We present the design and test of a magnetic actuator that is used to hold and release a ballast and switch buoyancy of a miniaturized autonomous well-logging tool from negative to positive. The actuator makes use of a low coercivity aluminum-nickel-cobalt (AlNiCo) magnet paired with a high coercivity neodymium-iron-boron (NIB) magnet to create and cancel a magnetic pull force on a steel plate by changing the polarization direction of the former magnet. Brief current pulses sent through a solenoid wrapped around the AlNiCo magnet by charging and discharging a capacitor. The resulting resistor-inductor-capacitor (RLC) circuit is analyzed to understand the polarization process of the AlNiCo magnet and state change of the actuator’s pull force. Our numerical solution using a nonlinear inductor model and test results showed that the repolarization can be achieved in steps using a relatively small 0.1 mF capacitor thanks to stored remnant field at each step. This made it possible to save space and energy for the miniaturized autonomous logging tool. Effects of NIB and steel weight piece on the AlNiCo polarization are experimentally analyzed. Magnetic circuit geometry is optimized using finite element analysis to maximize the pull force on the weight. The actuator occupies less than 2 cm3 and provides more than 2 kg-f to carry a ballast that weighs about 60 grams.