Extension of Linear Operating Range for Linear Variable Differential Transformer Using Its Inverse Transfer Characteristic

An analog circuit technique to realize an inverse transfer characteristic of a linear variable differential transformer (LVDT) is presented in this paper. Practically, the structure of the LVDT causes a narrow linear operating range compared with its full stroke range. However, a large linear operating range requires a huge structure for the LVDT, making it unsuitable for a small or compact measurement system. The proposed technique can be used in a commercial LVDT to extend the linear operating range to its full stroke range. The technique utilizes an inherent behavior of an operational transconductance amplifier (OTA) to emulate the LVDT transfer characteristic. The LVDT transfer characteristic generated by the OTA is used as a feedback path of the inverting amplifier formed by an operational amplifier (opamp) to realize the inverse transfer characteristic. The residual error due to the OTA behavior is very small and can be neglected without adversely affecting the performance of the proposed technique. All devices used in the proposed scheme are commercially available. The attractive features of the proposed technique are its simple configuration, small size, low cost, and high accuracy. The performance of the proposed technique is discussed in detail and confirmed by its experimental implementation. Measurement results demonstrate that the linear operating range of the commercial LVDT used in this study can be extended by a factor of more than 2.4, and a full-scale percentage error of about 0.068% was obtained.