A current-controlled variable inductor

Our development of an Automated Test System (ATS) for Proximity Sensor Electronic Units (PSEUs) in the aircraft industry required the implementation of a variable inductor. The variable inductor simulates inductive proximity sensors so the ATS can measure and verify the switch points of the electronics as the sensors move between their near and far states. Though developed for PSEU testing, the presented methodology and technology may be applied to other test equipment and applications that require a variable inductor. The paper begins by looking at different techniques for implementing a variable inductor: moving cores, switched decade boxes, gyrator circuitry and saturable core reactors. The paper presents the pros and cons of the different technologies and then focuses on the development of a saturable core reactor as the chosen technology. The paper presents fundamental formulae used during the development of the variable inductor and test results for a number of developed prototypes. The presentation includes the development of a highly-accurate control loop to precisely hold the value of the controlled inductance. Finally, the paper concludes with a brief discussion of the ATS that ultimately housed the variable inductor.