The biggest disadvantage of capacitive sensors is that the distance between the sensor and the measuring device produces noise at the measurement output. Therefore, the measuring device and the capacitive sensor must be as close as possible to each other. In a system with increased inter-sensor spacing, it is best practice to establish separate circuits from each other to avoid noise.
Fig. 1 Measurement and transmission design of capacitive sensors
This post presents a system where the inter-sensor distances are increased, the data can be collected from a single point and the system can work as one piece. As shown in Fig. 1, the system consists of mobile phone, transmitter, bus, microcontrollers (MC) and sensors. The microcontrollers are placed to the closest point to the sensors and take measurements. At that location, the measured values are transmitted digitally to the transmitter from the microcontrollers via the bus. The transmitter requests data from microcontrollers with a sampling frequency of 50 Hz, and then it transfers the received data to the mobile device via Bluetooth. The entire system is fed with a 3.7V lipo battery connected to the transmitter and power is transferred to the microcontrollers from the transmitter via the bus.
Four-wire bus structure has been integrated to provide power and data transmission between microcontrollers and transmitter. The I2C protocol is preferred because of the less cable requirement for data transmission, speed and the capability to communicate with a dozen of devices using two parallel wires at the same time. By establishing a Bluetooth connection between the transmitter and the mobile phone, the collected data can be transferred wirelessly.
In this setup, since the analogue measurement is made at the nearest point to the sensors and the data is transmitted digitally in the rest of the circuit, the data can be transmitted less prone to noise.