Micro Capacitive Sensor

The goal of this project is to develop a printed circuit board (PCB) that utilizes capacitive sensors and an off-the-shelf capacitance-to-digital-converter (CDC) to detect and monitor micro entities. The monitoring process is an autonomous process that displays a plot of real-time capacitance values.

Background
The counting and classifying of particles on the micrometer scale is expensive if automated and difficult if done using non-automated methods. An inexpensive automated counting and classification method for small particles needs to be created to lower the cost and increase the ease of use for this process.

Deliverables

 * PCB designed with CDC chips and multiple capacitive sensor designs
 * An easy to use graphing method that accurately displays capacitance vs time on a PC from the CDC chip
 * A capacitive sensor design, or multiple designs, that successfully detects micro-particles

Assembly Operations/Schematic
The overall system layout will consist of a capacitive sensor (shown as a capacitor in the schematic) that is read by the AD7746 capacitive to digital converter chip. This chip will be interfaced with a host Arduino microcontroller to autonomously gather data. The communication between the two utilizes I2C communication protocol depicted as SDA and SCL.



In the preliminary stages of system development we are using an off-the-shelf development board that hosts its on microcontroller and AD7746 capacitive to digital converter. This will be used to test the PCB sensor design to ensure accurate functionality.



Sensor Design
The capacitive sensors can be designed in different configurations. The figure below shows a co-planer and inter-digital design. For the purpose of our project, the co-planer design was chosen so that we can utilize the "channel" down to the middle to fasten a microfluidic device.



Placing the microfluidic device over the capacitive sensor allows us to pump a sample over the sensor. As particles pass through over the sensor, the measured capacitance value will change.



Utilizing the co-planer design template and varying the parameters of channel width, number of fingers, finger width and length and finger spacing, several different testable designs were made. Varying these parameters will change the E-field between both plates and can cause certain sensor designs to be more sensitive and accurate than other designs. The purpose of the testing process is to find which sensor design will be the most responsive to micro-particles passing through the "channel".

Sensor Testing
The sensors can be first tested using the microcontroller on the AD7746 Evaluation board. The picture shown (left) displays this configuration where the evalutaion board microcontroller interfaces with the prototype PCB and mounted CDC chip. Once a suitable design for the sensors is chosen, the next step will be to interface the on PCB AD7746 with our own microcontroller that will then display the capacitance values onto a monitor.



Spring Semester Timeline
The timeline for the spring semester will consist largely in testing the PCB design and debugging. The end of Fall semester is focused on the design of the prototype sensor design so that by mid Spring semester a final sensor can be manufactured and interfaced with the overall complete system.