A Drone-Borne Thermal Imager Assembly

Richard Heurtley
2018-12-21
richard@heurtley.com

Overview

The Melexis MLX90640 is a 32x24 pixel infrared thermal imager with a temperature range of -40 to 300 degrees Celsius. It uses the two pin I2C bus for communication and draws 23ma at 3.3 volts:

https://www.melexis.com/-/media/files/documents/datasheets/mlx90640-datasheet-melexis.pdf

Single units are $45 from DigiKey:

https://www.digikey.com/en/product-highlight/m/melexis/mlx90640-fir-sensor

A convenient breakout board, with an MLX90640 installed, is $70 from Sparkfun:

https://www.sparkfun.com/products/14844

I was evaluating the MLX90640 as a non-contact temperature sensor for boiler room monitoring when, after seeing a demo, a friend inquired if one could be mounted on a drone. Knowing his enthusiasms I call the result CoyoteCam (TM).

System Hardware

  • A thermal imager assembly.
  • A drone capable of carrying the thermal imager assembly.
  • A WiFi wireless access point.
  • A computer running Microsoft Windows.

System Software

  • A program running on the thermal imager assembly to get temperature data from the thermal imager and transmit it to the Windows computer.
  • A program running on the Windows computer to receive the temperature data and calculate and render thermal images. The sequence of images may also be saved as a video file.

Network Architecture

The thermal imager assembly and the Windows computer both attach to the wireless access point which serves as an intermediary relaying data between the two ends. The wireless access point does not need to have Internet access to work in this application.

The completed assembly weighs six ounces. The antenna is six inches long:

A Windows computer screenshot:

Hardware

Thermal Imager Assembly Components

  • Raspberry Pi 2 Model B
  • microSD card
  • cheap plastic case
  • LiPo battery
  • LiPo battery charge controller/power supply module
  • USB WiFi adapter with an external antenna
  • MLX90640

Some models of the Raspberry Pi have built-in WiFi. These models have an antenna engraved into the circuit board which, while extremely clever, does not have great range. Range is important in a drone application so I chose a Raspberry Pi model with USB ports and no built-in WiFi, and a USB WiFi adapter with an external antenna.

The LiPo battery I chose is this $15 unit from Adafruit:

https://www.adafruit.com/product/328

The LiPo charge controller/power supply module is this $20 unit from Adafruit:

https://www.adafruit.com/product/2465

The USB WiFi adapter is this $20 unit from Adafruit:

https://www.adafruit.com/product/1030

The same WiFi adapter *may* be only $8 from Amazon. The reviews say that it isn't always the same model:

https://www.amazon.com/Long-Range-WiFi-Antenna-Raspberry/dp/B00FADTN14

$8 plastic case from Adafruit:

https://www.adafruit.com/product/2258

Power Supply

In the interest of brevity the LiPo charge controller/power supply module will be abbreviated "CC/PS".

The CC/PS has the following connections:

  • 5 volt micro USB connector (input)
  • 3.7 volt LiPo battery connector (storage)
  • 5.2 volt power supply and ground lines (output)
  • enable line

The 5 volt micro USB connector is used to charge the LiPo battery and power the assembly from an external power supply. If the Raspberry Pi's built-in micro USB connector is used the assembly will be powered but the LiPo battery will not be charged.

The Adafruit LiPo battery comes with a connector that conveniently plugs into the Adafruit CC/PS.

The 5.2 volt output power and ground lines are connected to the Raspberry Pi GPIO connector's 5 volt and ground lines, pins 6 and 4 respectively.

The CC/PS is on by default. There is an enable line that turns of the CC/PS off when the line is connected to ground. I used a toggle switch to do this, a small slide switch would be lighter, and connected it to GPIO connector ground pin 14. Do not connect the battery until the CC/PS enable switch is wired up.

Component Mounting

The toggle switch and CC/PS are mounted inside the cheap plastic case. I drilled a hole and mounted the toggle switch above and between the HDMI and audio jacks:

The CC/PS is mounted upside-down on the case cover above the Raspberry Pi's micro SD card slot. I used a nibbling tool to cut out a section of plastic to expose the CC/PS's micro USB connector.

I used a 40 pin ribbon cable header:

https://www.jameco.com/z/8000-40-R-Connector-Socket-Strip-40-Position-0-1-2-54MM-IDC-Right-Angle-Cable-Mount_32679.html

to attach wires to the following GPIO lines:

Connected to the CC/PS:

  • 4: 5 volt power
  • 6: ground

Connected to the CC/PS enable switch:

  • 14: ground

Brought outside the case to the thermal imager:

  • 1: 3.3 volt power
  • 3: I2C bus SDA
  • 5: I2C bus SCL
  • 9: ground

The LiPo battery and thermal imager module are mounted outside the case. I used double sided Velcro tape to secure the LiPo battery to the chase cover and to secure the thermal imager module to the LiPo battery.

Orientation

The thermal imager assembly is intended to be attached to the underside of the drone with the WiFi adapter's antenna pointing down. I intend to use more double sided Velcro tape to attach it. The thermal imager has a small tab that indicates image "down". The tab should point to the "tail" of the drone.

Software

Thermal Imager Assembly

The program running on the thermal imager assembly is written in the C language and is called "drone". It has parameters to specify:

  • The Windows computer's IP address.
  • The port on which the Windows computer is listening for a TCP connection.

The drone program does the following. Error recovery is implemented as three nested loops: 


Parse parameters
Initialize the I2C subsystem
Initialize the networking subsystem
Connection loop:
 Connect to the Windows server
 Thermal imager initialization loop:
  Initialize the thermal imager
  Frame loop:
   Acquire a frame from the thermal imager
   Transmit the frame to the Windows computer
If there's an error acquiring a frame from the thermal imager then the program exits the frame loop and reinitializes the thermal imager.

If there's an error transmitting a frame to the Windows computer then the program jumps out of the frame and thermal imager initialization loops and reconnects to the Windows computer.

The source code of the drone program is here. The drone program compiles and runs under both Windows and Linux:

drone.c

The drone program is automatically started when the Raspberry Pi is turned on. This line in the /etc/rc.local file: 


su - hardack -c "/home/hardack/droneloop &"
assumes the user identity "hardack" and invokes script /home/hardack/droneloop which is: 

#!/bin/sh
cd /home/hardack
rm -f dronehalt
while true
do
 nohup ./drone -host 192.168.7.127 -freq 8
done
The infinite while loop guarantees the drone program is always running in case there's an error condition the program doesn't catch.

Windows Computer

The program running on the Windows computer is also written in the C language and is called "therm". It reads a configuration file to specify the IP address and port to which it listens for a connection. This is the same IP address and port given as parameters to the drone program.

The therm program is structured as several independent threads:

  • main: Initialization and window event processing.
  • acq: Acquire thermal images from an MLX90640 directly, not from the thermal imager assembly.
  • net: Acquire thermal images over the network from the thermal imager assembly.
  • render: Render thermal images to the screen.
  • video: Write a sequence of thermal images as a video file.

The net thread does the following: 


Initialize the networking subsystem
Listen for a connection
Connection loop:
 Accept a connection from the thermal imager assembly
 Frame loop:
  Receive a frame from the thermal imager assembly
  Signal the render thread that a new frame has been received
The source code to the net thread is here:

net.c

When running the therm program on a Windows computer for the first time the Microsoft Windows operating system will warn you that the program is attempting to use the networking subsystem and ask if you want to add an exception to the Windows firewall. The therm program should be granted full access to the networking subsystem.

Future Considerations

The thermal imager could be replaced with any other 3.3 volt I2C sensor like the BME680 temperature/humidity/pressure/VOC sensor. Of course the software would be different.

More GPIO lines could be bought outside like:

  • 5 volt power
  • the SPI bus
  • the serial bus

to support other modules and sensors. In particular a GPS receiver would provide important information in a drone application.

Once a set of sensors has been standardized a custom case that exposes the sensors while protecting the circuit board(s) could be designed and made with a 3D printer.