Capacitance measurement with MSP430 microcontrollers

Capacitive sensors are widely used, e.g. for measurement of humidity and various physical movement (proximity, linear or angular acceleration, speed or position) of some element in the environment or in the material or structure. MSP430 micro-controllers are often used in wireless sensor node applications, as being low-power low-cost mixed signal systems on chip with various (including wireless) connectivity options. So, among other applications, the development of MSP430-based sensor nodes for smart materials and structures could be highly interesting, with capability of capacitive sensor interfaces. MSP430-s have been much used with capacitive touchpad inputs, but less investigated for the use from the instrumentation viewpoint. In the current paper two implementation ways of the capacitive measurement were investigated, from practical and from the metrological aspects, one implementation as the pin-relaxation-oscillator (possible with MSP430G2 family) and another solution, implemented as the internal comparator-based relaxation-oscillator solution (with MSP430F5 family). The results indicate that MSP430 micro-controllers can be easily (by simple interface) connected to the capacitive sensors for reasonably precise measurements (measurement error is below ± 1 pF in the range of 10 to 600 pF. One of the future research challenges could also be the implementation of the sensor nodes with conductive fibre or layer (with changing resistance and capacitance to be measured) for structural health monitoring.