Inexpensive linear displacement transducer using a low power lock‐in amplifier

Differential capacitance displacement transducers (DCDTs) using audo frequency excitation and phase‐sensitive detection are discussed, and their sensitivities are calculated. An inexpensive, low‐power DCDT using a miniature lock‐in amplifier made of integrated circuits is described. The lock‐in amplifier has a digitally controlled phase shift between drive and reference signals, a full‐scale output of ±10 V, and total noise and drift of about 1 mV peak to peak. The DCDT system has a detection limit of 0.01 nm/√H√z, a dynamic range between 60 and 80 dB, and a power consumption of less than 0.75 W. Parts cost for the system is about $200. A simplified DCDT not requiring the full lock‐in amplifier is outlined, and improvements possible in the DCDT design are mentioned where appropriate.