A Current-Mode Analog-Front-End for Capacitance-to-Voltage Conversion of Length Transducers in Pneumatic Artificial Muscles

This paper reports on the design, implementation, and characterization of an analog-front-end circuit for capacitance-to-voltage conversion that can be used with a large variety of sensors and actuators in industrial and medical applications. The proposed circuit converts the variation of the capacitance of any capacitive sensor/transducer into a DC voltage signal. The solution has been designed in the current-mode approach by employing the second-generation current conveyor as the basic blocks. Even if the circuit has been presently implemented by using commercial discrete components, its circuitry has been designed using an architecture allowing its integration in a small Si area for a system-on-chip (SoC) in standard Si-CMOS technology. The circuit allows for gain/sensitivity tunability, offset compensation and/or regulation, and capability to manage different ranges of variations of the input capacitance. For a circuit gain of 1000, the measured circuit sensitivity has been found equal to 154.95 mV/pF with a resolution of 4 fF. The implemented circuit has been employed to measure in real time the capacitance of a developed McKibben pneumatic muscle during its elongation and contraction phases. The reported experimental measurements prove that the pneumatic muscle length depends linearly on the circuit output voltage. In these conditions, the effective system sensitivity has been determined to be equal to 70 mV/mm with a standard deviation error of the measured muscle length of about 0.008 mm.