Limit height

Discussions about autonomous flight in general, regardless of positioning method
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JaeDeok
Beginner
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Joined: Tue Oct 06, 2020 6:22 am

Limit height

Post by JaeDeok »

Is there a recommended height limit when using only the flow deck and not the lighthouse or loco deck?
kimberly
Bitcraze
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Joined: Fri Jul 06, 2018 11:13 am

Re: Limit height

Post by kimberly »

Good question! It is depended on multiple factors actually: texture of the ground, brightness of the room, material of the floor... there is not really a recommended height limit. I guess the theoretical limit is the limit of the TOF sensor which should go up to 4 meters (but probably only under ideal conditions).

A while ago I managed to fly at 1.5 meters on a well litt floor with medium texture. It did drift a little bit more than lower to the ground (higher up, less texture to see on the ground) but definitely still good. But the best thing, is to try it out for yourself.
JaeDeok
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Re: Limit height

Post by JaeDeok »

Have you ever tried flying inside an office (an obstacle such as a desk) rather than a room with nothing?

When flying inside the office, I think you won't be able to keep hovering at high heights. What do you think?
kimberly
Bitcraze
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Joined: Fri Jul 06, 2018 11:13 am

Re: Limit height

Post by kimberly »

Indeed that will be tricky since the height estimate will be dictated by what the TOF sensor sees. It will not be able to do that. There are two ways two fix this:

- With a multiranger, use the top TOF sensor and take the difference between the room height and send that to the kalman filter instead (which require some hacking of the firmware though.). This won't work if the ceiling is not even.
- Send the TOF sensor data, take the difference in time and send it to the kalmanfilter as a velocity instead, and integrate this together with the barometer. Hopefully the change in TOF velocity won't be high enough that the height estimate will jump anyway (I tried it recently and unfortunately it does...)
- Add a stepdetector that will detect an sudden change in TOF measurement and quickly add that difference to the distance measurement.

There is currently an issue open on this: https://github.com/bitcraze/crazyflie-f ... issues/540. It's not an very immediate feature but we thought to keep it open since it will be interesting for others to see.
JaeDeok
Beginner
Posts: 18
Joined: Tue Oct 06, 2020 6:22 am

Re: Limit height

Post by JaeDeok »

Thank you.

Conversely, is there a minimum height limit?
For example, if the flow deck is affected by ground and light,
Given the following command as an example,

send_hover_setpoint(self, vx, vy, yawrate, zdistance):

When the height is set to 10cm, 20cm, or 10cm or less, the effect of the shaking is expected to be large. In fact, it's not easy to keep hovering properly at that height (when you go up sequentially from low height during takeoff)
kimberly
Bitcraze
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Joined: Fri Jul 06, 2018 11:13 am

Re: Limit height

Post by kimberly »

I think the absolute limit is the minimal detection range which is 4 cm for the downlooking tof sensor (see this forumpost).

Downwash, which causes the shaking, is another factor indeed and I think that 10 cm will still be too close to the ground. 20 cm and up is better indeed, so if you set the hover setpoint right away, it will probably go there fast enough so that down wash will not be a problem. Quick take off is the key :)
JaeDeok
Beginner
Posts: 18
Joined: Tue Oct 06, 2020 6:22 am

Re: Limit height

Post by JaeDeok »

Thank you for answering the question well.
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