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Mars helicopter employs advanced control techniques to survive in-flight anomaly

control.com

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Re: Mars helicopter employs advanced control techniques to survive in-flight anomaly

#31

Post by jcims »

ealloc wrote: post #22

The article says the problem was a dropped frame from the camera, but that just further piques my curiosity: Presumably they use some kind of Kalman Filter, but those are easy to program to account for missing frames, or frames at non-discrete timepoints, perhaps even for screwy camera images if the programmer had a reasonable prior for the likelihood of it happening. Kalman Filters by design account for measurement…

It seems like the issue wasn’t that there was a dropped frame, it’s that the time slot for that frame got filled by the next frame, then every subsequent frame was off by one resulting in a persistent timestamp offset of the vision data from reality for the remainder of the flight.

I didn’t read into it too much so I may not have all the details right, but I think this is the gist of it.

Re: Mars helicopter employs advanced control techniques to survive in-flight anomaly

#32

Post by tonyarkles »

ealloc wrote: post #22

The article says the problem was a dropped frame from the camera, but that just further piques my curiosity: Presumably they use some kind of Kalman Filter, but those are easy to program to account for missing frames, or frames at non-discrete timepoints, perhaps even for screwy camera images if the programmer had a reasonable prior for the likelihood of it happening. Kalman Filters by design account for measurement…

Thank you for your comment, because it triggered an interesting chain of thoughts about a semi-related problem I’m working on at work.

Usually with a Kalman filter, you’re taking into account the spatial measurement error (gyro-measured roll rate error, accelerometer-measured acceleration error, etc) but I don’t think I’ve ever encountered a system that explicitly modelled sensor latency variation relative to timestamps. Based on the description of the problem they encountered here, I suspect what happened is that it lost a frame but didn’t adjust the “photo timestamps” appropriately; every frame that came along afterwards would have had an incorrect timestamp? Even if the Kalman filter was set up to handle “this photo was taken 20ms ago” when doing its forward integration, if they didn’t model “this photo was taken 50ms ago but is reporting that it was taken 20ms ago” then you’d pretty readily get the kinds of oscillation they were getting.

Edit: yeah, just like the sibling comment said :)

Re: Mars helicopter employs advanced control techniques to survive in-flight anomaly

#33

Post by yupper32 »

jollybean wrote: post #23

Can anyone hint why they wouldn't use a gyro? When it's on land, they can make the gyro reliably point 'down'. Then at least during flight they know which way 'down' is. Would this be too fragile for Mars?

They have an inclinometer, so they know which way is down.

Re: Mars helicopter employs advanced control techniques to survive in-flight anomaly

#34

Post by baybal2 »

idiotsecant wrote: post #8
njoubert wrote: post #5

I see no mention of "advanced control techniques"? Sounds like there is just a limit on roll/pitch angles and a limit on distance applied. Saying "advanced control techniques [for multirotors]" sets an expectation for something along the lines of Tedrake's Underactuated Control approaches: http://underactuated.mit.edu/

Sensor fusion is part of the control system. Sensor fusion that incorporates visual information is outside of the typical classical controls sphere and is likely considered part of an advanced controls segment of the devices firmware as opposed to the bog standard deterministic controls algorithm. You need not gatekeep here, this is a real term.

> Sensor fusion that incorporates visual information is outside of the typical classical controls sphere and is likely considered part of an advanced controls segment of the devices firmware as opposed to the bog standard deterministic controls algorithm.

Not so advanced as very correctly pointed out by https://news.ycombinator.com/item?id=27554949

Motor-attitude control loop cannot, and should not be "fused" with navigation layer, whether INS, or optical flow sensor.

The bog standard deterministic controls algorithm here is "bog standard superior." Computer vision/optical flow sensors have much lower precision than gyroscopes, and accelerometers, let alone aerospace grade ones.

Re: Mars helicopter employs advanced control techniques to survive in-flight anomaly

#35

Post by tonyarkles »

jollybean wrote: post #23

Can anyone hint why they wouldn't use a gyro? When it's on land, they can make the gyro reliably point 'down'. Then at least during flight they know which way 'down' is. Would this be too fragile for Mars?

When the article refers to an IMU, they’re referring to a combination of sensors; most commonly on Earth that’d be a 3-axis gyro, a 3-axis accelerometer, and a 3-axis magnetometer (compass). The problem with just using that is accumulated error and drift. On super small aircraft like this one, we usually use MEMS parts instead of big spinning physical gyros, and they don’t have great long-term performance.

MEMS Gyros measure angular rate, not absolute angle; to compute an actual angle, you’re taking the integral of the rate from t=0 to now. Any small errors in the measurements add up quickly to give you completely nonsensical results. For drones-on-Earth, we use a variation of the Kalman filter to combine short-term and long-term measurements. As an example, an accelerometer requires a double-integral to turn into position, so errors accumulate very quickly, but we can correct those errors using GPS. The accelerometer and its integrals give us really quick acceleration, velocity, and position updates (at, say 500hz), and then the GPS is used to correct the long-term position and velocity (at, say 5hz).

Re: Mars helicopter employs advanced control techniques to survive in-flight anomaly

#36

Post by ghaff »

anvandare wrote: post #17
TheAdamAndChe wrote: post #11

Earlier quoted context omitted.

It's funny/amazing to me how we lived without GPS for thousands of years, but it's become so integral to our society that it's one of the first things we're setting up there before visiting.

> The first and most moral responsibility of Freeland will be establishing a decent brewery on the new planet. (Civilization: Beyond Earth - Hutama) Sure, our ancestors (who could make their way and thrive in the wild) would probably consider me (who gets lost the moment I make a wrong right turn and depends on the local supermarket) a complete idiot, especially since I couldn't recreate any of the technology my dail…

>(I think I'm supposed to bang rocks together to make fire, right?)

Won't work very well unless you have flint specifically :-) Even if you know the techniques, starting fire without any manufactured tools or manufactured tools to make the devices is really really hard.

Re: Mars helicopter employs advanced control techniques to survive in-flight anomaly

#37

Post by arbirk »

njoubert wrote: post #15
idiotsecant wrote: post #8

Earlier quoted context omitted.

Sensor fusion is part of the control system. Sensor fusion that incorporates visual information is outside of the typical classical controls sphere and is likely considered part of an advanced controls segment of the devices firmware as opposed to the bog standard deterministic controls algorithm. You need not gatekeep here, this is a real term.

The article title is still misleading. The sensor fusion was the root cause of the malfunction, it was not the advanced control technique that was deployed to survive the anomaly.

Agree. It's interesting that they don't treat the optical and gyroscopic systems as two separate sensors with the possibility to disregard one if it disagrees too much. A quick restart of the visual system would have been the most optimal solution #2020-hindsight

Re: Mars helicopter employs advanced control techniques to survive in-flight anomaly

#38

Post by gpm »

the8472 wrote: post #21
anvandare wrote: post #6

Earlier quoted context omitted.

Pretty sure SpaceX's first deliveries to Mars (orbit) will include just that. I think they could probably use Starlink satellites without much modification (most important one I can think of would be more panels to cope with the reduced solar irradiance)? The GPS constellation has 32 satellites. A GPS satellite weighs ~2000kg. Starship should get 100-150 tons to Mars - the math checks out! (Disclosure: I got all my r…

> satellites without much modification (most important one I can think of would be more panels to cope with the reduced solar irradiance)? atomic clocks

Are those actually necessary?

Not an expert, but the whole network is on communication (with perfect understanding of the satellites locations no less), shouldn't they be able to run an ntpd like protocol to stay in sync, and just sync ground clocks to "starling consensus time" instead of "real time".

Re: Mars helicopter employs advanced control techniques to survive in-flight anomaly

#39

Post by londons_explore »

baybal2 wrote: post #30
ncmncm wrote: post #29

The control system is clearly designed wrong. Navigation input should not be able to affect the closed-loop control system directly. It should affect only the calibration, incrementally. If that had been done, there would have been no flight instability, just disagreement between the IMU and navigation about how far they had flown. There are certainly people involved in the project who could have explained this to th…

> There are certainly people involved in the project who could have explained this to them. I hope they are learning fast. This is how nearly all modern high-end quadcopter drones fly - navigation using optical flow camera sensors short circuited directly into attitude/motor control loops. I guess there is not a small chance they entrusted helicopter autonomous operations programming to people with quadcopter backgro…

Put tape over the downward facing camera on almost any quadcopter and you'll soon find out why...

It turns out sideways drift accumulates very quickly - and so quickly that unless you are a very practiced drone operator its very hard to compensate for by hand.

GPS compensates somewhat, but obviously that isn't available on mars (or indoors).

Re: Mars helicopter employs advanced control techniques to survive in-flight anomaly

#40

Post by cryptoz »

gpm wrote: post #38
the8472 wrote: post #21

Earlier quoted context omitted.

> satellites without much modification (most important one I can think of would be more panels to cope with the reduced solar irradiance)? atomic clocks

Are those actually necessary? Not an expert, but the whole network is on communication (with perfect understanding of the satellites locations no less), shouldn't they be able to run an ntpd like protocol to stay in sync, and just sync ground clocks to "starling consensus time" instead of "real time".

There is a pretty decent description of the problem here: https://physicscentral.com/explore/writers/will.cfm

I'm also not an expert so I don't know if you can solve it with syncing. Maybe you can. But that doesn't sound like a "just" concept to me - any solution to this is going to be complicated.

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