A Closed-Loop Signal Conditioning Scheme for Core-Less Planar LVDT

In this article, we present an efficient signal conditioning scheme for a previously developed planar core-less linear variable differential transformer (LVDT). The stationary coils are excited with variable-amplitude complementary sinusoidal current signals through a Howland source using a closed-loop controller in such a way that the moving coil voltage is maintained close to zero. The ratio of the difference to the sum of the amplitudes of the applied voltages is taken as the output, which is proportional to the moving coil position. The highlight is that any external interference can be detected by monitoring the null voltage, which allows taking remedial measures like switching of operating frequency. Tests on a prototype sensor with this scheme implemented show improvement in positioning performance by more than one order of magnitude under strong interference near the operating frequency. Other benefits include insensitivity to variations in the forward path gain and a better update rate. The new scheme also retains the characteristics of tolerance to vertical displacement, good linearity, and high resolution previously expounded.