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PX4 allow controlling attitude only in OFFBOARD, it is impossible to combine modes.

But depending on control type you may hold altitude. Easiest is when you send position setpoints, just send same Z. You want rpy control, it is done via attitude setpoints (in quaternion form), but in that mode you only control throttle, so altitude controller should be implemented by user.

FCU-OBC connection depends on what computer you use. In general you need 3.3V UART from OBC and connect it to TELEM2. Example: Raspberry Pi UART pins may be directly used; Odroid U3 need level converter 1.8<->3.3; Intel NUC need FTDI USB-UART.

Then you should setup SYS_COMPANION FCU parameter to 921600 or 57600 (depends on maximum baud allowed by your computer). Tip for raspberry pi 2 (and 1 too): place in /boot/config.txt (alternate: /boot/firmware/config.txt) this lines:

# Higher UART Speed
init_uart_baud=921600
init_uart_clock=14745600