Capacitive Linear Displacement Sensors With High Accuracy and Long Range Absolute Positioning Capability Based on a Splicing Technique

Abstract

Considerable progress has been made in the development of absolute capacitive linear displacement sensors based on time grating that are capable of providing highly accurate absolute position measurements in conjunction with easy fabrication using standard printed circuit board (PCB) technology. The working range of currently available sensor designs has, however, been rather limited. The present work addresses this issue by greatly expanding the measurement range via a technique for splicing multiple identical stationary rulers with individual measurement lengths of 600 mm. Here, absolute positioning is realized in a manner similar to that of Vernier calipers using stationary segments composed of periodic rectangular excitation electrodes with orthogonal excitation signals and a moving component composed of sinusoidal-shaped induction electrodes. The sensor wiring is greatly simplified by transferring the induction signals output by the moving ruler to the stationary rulers via special transmitting and receiving electrodes. Splicing errors are eliminated by applying two sets of induction electrodes, including a front sensing unit (FSU) and a rear sensing unit (RSU) along the measurement direction on the moving ruler. The performance of the proposed design is evaluated experimentally using a two-segment prototype sensor fabricated by PCB technology. The results demonstrate that interference between the FSU and RSU generates a first harmonic error. This is then addressed by increasing the distance between the FSU and RSU, and enhancing the shielding layer between the two units. This yields a measurement accuracy of <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\pm 3.5~\mu \text{m}$ </tex-math></inline-formula> over the full 988 mm measurement range of the sensor.