Good day everyone,
A modern drone or flying robot tends to have quite a number of different hardware units onboard - Autopilot, Gimbal, GPS, ESC, OSD, PDB, RC just to name a few. As there is no common method to interconnect that variety of devices, and for sure each hardware unit tends to implement its very own interface, obvious difficulties arise:
- Implementing support for new peripherals is hard (e.g. consider PX4Flow driver for APM; 3DR GPS; MinimOSD).
- Autopilots need to accommodate variety of physical interfaces for different peripherals - this imposes some penalties on the device size, cost and weight (e.g. Pixhawk and its so called "connector farm").
- MOST of currently used interfaces are simply not designed to be reliable nor scalable, thus moving towards more reliable systems while sticking to the current onboard communication methods is absolutely impossible (e.g. RC PWM - pet peeve of mine, absolutely worst case; or ubiquitous UART).
Properly designed standard for hardware interconnection in UAV systems may encourage different manufacturers to produce compatible hardware and software, this is why I consider it important to offer such a standard for community. Previously I took some steps trying to rectify the current state of affairs with the hardware compatibility: first, I implemented the CANaerospace protocol support in portable C library - libcanaerospace: http://diydrones.com/profiles/blogs/open-source-canaerospace-library-for-uav (BTW this article covers the CAN bus features in general, those who are unfamiliar with CAN may be interested to read); second, with cooperation with Lorenz Meier, there was proposed a convention for connector compatibility: http://www.diydrones.com/profiles/blogs/can-bus-conventions-for-small-uav.
However, CANaerospace is apparently not a silver bullet, here's why:
- CANaerospace has more than 50% of payload overhead which makes it unfit for high speed data streams that are so common for UAV (especially for light machines where the controlled system requires high update rates and low latency).
- CANaerospace does not provide an easy way to pass multiple values in one bunch, which is also very common task in the UAV field.
- CANaerospace does not provide adequate means for some extremely common higher-level tasks, such as node configuration handling, firmware update, time synchronization, etc.
Thus, there seem to be a need for some protocol that could be used as a backbone network on UAV and flying robots, and that could address the issues listed above. So meet the UAVCAN protocol specification draft: https://docs.google.com/document/d/1IgOZ2proS0Bwx3_nc8u7CM81ldHcxWzlgMVjNfW5Has/pub
UAVCAN core design goals:
- Provide a suitable protocol standard for UAV hardware manufacturers.
- Support redundant physical interfaces and redundant nodes for highly reliable systems.
- Address the issues inherent to CANaerospace, as listed above.
- Make it easy to implement and easy to use in deeply embedded systems with limited computational capabilities (i.e. ensure small memory footprint, low computational complexity).
- Explicitly define support for some common tasks (node configuration handling, firmware update, time synchronization, etc).
All of this is described in-depth in the RFC linked above. I'd like to ask the community for comments on this standard, especially regarding the standard data type definitions (see the corresponding chapter), the "std/equipment/" part. It is important to make sure that the standard types are extensive enough to cover most possible use cases for the majority of UAV applications. Also I'd like to make sure that the RFC language is comprehensive enough so that other engineers can get at least general idea of how to implement it, and absolutely clear understanding of how to use it.
As a closing remark I'd like to add that the situation with UAV hardware is changing: the next revision of Pixhawk is going to get the full support for UAVCAN, as well as PX4ESC is; also there are some other compatible hardware to be announced pretty soon.
P.S. This is a cross-post from drones-discuss: https://groups.google.com/forum/#!topic/drones-discuss/DMjfnaWViUw; please leave your comments there.