Hex redundancy on 3.6.X

GootyS · October 19, 2019, 12:20pm

Hi All,

A few years ago there was talk about Hexacopters falling out the sky due to no redundancy support on a Hex with a motor failure. Some had a solve of reducing YAW to 0 and using simple mode to bring the Hex back all this on 5 Motors. Is this a standard in the latest code? Apologies for this question but its been some time for me since using Ardupilot.

wn0x · October 19, 2019, 2:01pm

github.com/ArduPilot/ardupilot

Copter: Detect and handle motor loss for improved redundancy

opened 02:23AM - 11 Jan 18 UTC

closed 03:15AM - 03 Aug 19 UTC

lthall

Enhancement Copter Safety

Motor mixers are based on a fixed mixing that limits the roll, pitch, yaw and th…rust to prevent saturation of the motor outputs. When a esc/motor/propeller fails there that motor tends to go to maximum limiting the average motor output (thrust) and the roll, pitch, and yaw authority. The motor mixers are designed to prioritise roll and pitch first, then yaw, followed by thrust. This then represents the order in which control is lost. First the aircraft will start to descend, more severe loss will result in loss of yaw control, followed finally by complete loss of attitude control. The severity may be limited by ensuring the aircraft is well tuned with the correct CG location and excess thrust. Yaw control can be given up earlier by reducing MOT_YAW_HEADROOM to 0. This may mean that yaw control is temporarily lost during aggressive maneuvers or extreme disturbances. The most vulnerable frame is the hex as any loss of motor control will make yaw control limited or impossible depending on the precise location of the CG. This may be mitigated using a new propeller configuration: [Copter: Additional Hex propeller configuration #7524](https://github.com/ArduPilot/ardupilot/issues/7524) The solution has two parts, detection and handling. **Detection**: -This is most reliably done using ESC feedback. This should be a basic requirement of any CAN esc implementation (both failure and maximum thrust limitation). -This could also be done much less reliably by looking at the maximum thrust output to any motor and trigger a failure if a motor stays at full throttle for an extended period. This may have many other implications in the case of a false positive and I do not recommend this approach. **Handling**: - The simplest approach is to remove the ability for the compromised motor to limit the thrust of the remaining motors. This will ensure we achieve a significantly higher maximum thrust and also maximise roll and pitch authority. - The more complex and complete solution is to completly recalculate the motor mixing matrix based on the remaining motors potentially even including the limited thrust output of the compromised motor. This is a much more ambitions goal and I would not recommend this approach in the first instance. This is an issue that encompases this issue: [Copter: ensure hexacopter with failed motor can make safe landing #838](https://github.com/ArduPilot/ardupilot/issues/838) [3.3.1 Hexacopter motor redundancy #3294](https://github.com/ArduPilot/ardupilot/issues/3294) To quote @rmackay9 - > I had a chat with @lthall and there's two things we need: > > single motor failure detection. Leonard suggested we modify the AP_Motors_Matrix class so that it looks for a single motor >95% while all other motors are <75% (those percentages are guesses and could be modified based on testing). We should probably use a counter and/or bitmask so that the situation must continue for some amount of time (0.5sec?) before we decide the motor has failed and avoid false positives. > based on the above checks, exclude the "failed" motor from the saturation handling code. So for example, we should exclude from the calculation of "rpy_low" and "rpy_high" here in AP_MotorsMatrix.cpp. https://github.com/ArduPilot/ardupilot/blob/master/libraries/AP_Motors/AP_MotorsMatrix.cpp#L231 > According to Leonard, doing this will theoretically allow the overall throttle of the motors to climb to 66% instead of the current 33% while still maintaining attitude control.

GootyS · October 19, 2019, 3:10pm

Thank you wn0x, this is exactly the info im looking for.

imrj · October 22, 2019, 2:07am

this doesnt work if your copter is too heavy…it wont be able to sustain altitude even if it tracks properly…Ive had this happen…at best it will rotate around yaw but track its course and maintain altitude IF it isnt too heavy or headwind not too strong…otherside its going down…

GootyS · October 22, 2019, 8:24am

Thanks, The hex can fly fine on 4 motors at full payload.

imrj · October 22, 2019, 2:02pm

must be pretty lite…mine is 5 kilos…

wn0x · October 22, 2019, 3:28pm

Or… some pretty big motors

GootyS · October 22, 2019, 6:01pm

Roughly 35kg AUW maybe more.