When I am investigating something new, I like to try and imagine an ideal application for whatever it is I am investigating. This project grew out of one of these brainstorms. I was evaluating some capacitive sensors for work and was impressed at the range at which it could sense things. If I could move the sensor in a controlled manner, I could produce images representing the capacitance of an object.

Electronics

I drew up a board and created a mounting bracket so that it would fit into my 3d printer. The sensor is an FDC2212 mated to a MSP430F5528. I followed the TI evaluation module example close enough to make use of their example firmware with minor changes.

I added a header for additional I2C sensors and two SMA connectors for the capactive sensor. A quick bake in the oven later and it was ready for testing.

The sensing elements are two 20mm squares side by side. Each square is one side of the FDC2122 resonant tank, meaning that this is reading in a differential capacitance configuration. The sensing elements are surrounded by individual shields on 3 sides and above.

First scans

I knew that temperature would be a factor but didn’t realize just how important it would be. Both the air and the electronics themselves are effected by the temperature. My first scan looked like the following

As you can see I did a good job of tracking the changes in room temperature (driven by the thermostat). Luckily this thermal “noise” can be somewhat improved by simply subtracting the average of each column

Capacitive scan with columns subtracted

The dark spots are artifacts of the filtering. Luckily I anticipated the temperature dependence and placed an I2C port on the main sensing PCB for additional sensors.

Files and source can be found on my github

Future goals

In the coming weeks I’m planning on working on the following:

Adding a temperature sensor to the I2C port Calibrate for temperature drift

Develop more image processing tools DFT analysis for easier filtering High and low pass digital filters

Adding a time of flight sensor to the I2C port

Add inductive sensing

Make a larger sensing element

Make a circular sensing element

Improve the sensitivity by removing/improving the shield drivers on the coax cables

Evaluate scanning speed Faster= better for temperature drift Too fast = smearing

