A simple measurement scheme for multiple capacitors and its application to an ophthalmic anesthesia training system

Capacitive sensors are widely used due to their robustness and accuracy. In several applications such as proximity detection, multiple capacitances have to be measured simultaneously. This could involve multiple transmitter electrodes and a receiver electrode connected to the body whose proximity is to be detected. In order to excite the transmitter electrodes, a single sinusoidal source is multiplexed to each electrode one at a time, while capacitance measurements are made between the excited electrode and the receiver. This process is slow and limited by the switching speed of the multiplexer used and hence all the capacitances involved cannot be measured simultaneously. To address this issue, a simple multiple-frequency capacitive measurement scheme is proposed in this paper. Sinusoidal signals of unique frequencies are produced using a binary counter and used as excitation for the transmitter electrodes. The displacement current received by the receiver is converted to a voltage signal and fast Fourier transform is performed on it to extract the magnitudes of the frequency components corresponding to the excitation frequencies. Thus, all the capacitances involved can be measured simultaneously. A simulation study performed, showed that the proposed multiple frequency scheme has low cross-excitation sensitivity compared to the multiplexer based scheme. Finally, the proposed scheme was implemented on an ophthalmic anesthesia training system to detect the proximity of the needle to important structures of the eye model. Test results from a prototype developed show that the scheme is effective in detecting the needle proximity and touch to various ocular structures thus demonstrating its use for practical proximity detection purposes.