Custom PCBThe real-time system was mounted on an STK300 development board from Atmel, but we want to have a custom board for cleanliness and smaller size reasons. Several members of the project have experience with PCB layout and design. The bottom of the Rev 1.0 board is pictured, unpopulated, on the left. There were a few big mistakes made in the Rev 1.0 board, so please do not fabricate them without checking the notes. If you want to fix any of the problems, that would be good, too.
The next rev of the boards is being design right now. It has an AT91, an FPGA, wireless serial and lots of other nice features. We're hoping to have them laid out and built by summer of 2002.
We've selected ExpressPCB for the fabrication. They have a three-day turnaround from the time that they receive an order to having the finished product delivered, for $20 a board. We can fit everything on to the small board for that special price. In addition to the PCB and AVR Mega16, you will require an IMU or AHRS. We have designed an IMU that consists of a two-axis rate gyroscopes from Gyration, and ADXL202-EB from Analog. The new design of the board fits the entire IMU, servo controller and safety pilot switch onto a single board. Of course, a remote control helicopter is also necessary.
The PCB layout program from ExpressPCB is "free-as-beer" and can be downloaded from their site. Unfortunately it only supports legacy operating systems. It can be run under Wine, so not all is lost. There are bugs in displaying thick traces, so be sure to check your layout against the Windows version.
Before the board was produced, we used a custom wiring harness that breaks out the ports on the STK300 to connect the servos, the receiver and the IMU. The wiring map for this is very simple -- A0 .. A7 connect to servo/gyro inputs and C0 .. C7 connect to the eight outgoing servos.
There is one PAL on the board that performs the Safety-Pilot switching. Should the autopilot become confused or otherwise unable to maintain control, it can give control to a ground based pilot with a standard RC transmitter. Download the source for the PAL.
The old IMU was mounted in RadioShack boxes on RadioShack protoboards and connected via Cat5 to the IMU port on the STK200 board. Visible on the left is the ADXL202-EB accelerometer and on the right is the MG100 rate gyro. More details on the IMU are available. The PCB layout for the originally planned three-axis strapdown IMU is on the left. The new design has the IMU built onto the same board as the servo controller and safety pilot switch, removing over $20 per unit cost.