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PX4 / PX4-Flow

Firmware for PX4FLOW board
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Very slow I2C update rate #87

Closed thomasck closed 7 years ago

thomasck commented 7 years ago

The flow measurements I am getting via I2C are only available at around 15 Hz, whereas I believe they should be available at about 400 Hz. Is there something I am supposed to change after I download the firmware?

Thanks!

Thomas

jgoppert commented 7 years ago

Can you give more details on your setup and how you are measuring the i2c update rate?

LorenzMeier commented 7 years ago

The optical flow is running at 400 Hz but you are getting it pre-integrated at a lower rate. There is no benefit in running the interface faster and this is not an actual issue with the sensor. You can change the interface code if you want to read it faster, but it will make no difference in terms of the result.

thomasck commented 7 years ago

What do you mean by pre-integrated? I am trying to get a current body frame x/y velocity measurement at each time step in my control loop, which is running between 80-100 Hz. But the px4flow data seems to be updating way slower than that so I get lot's of repeated values then sudden jumps and then more repeated values, which results in an overall very blockish signal that will not work as feedback. I'm begging to suspect it might be how I am reading the registers.. is there any protocol to reading or resetting the i2c _frame registers? I am using linux's mraa i2c class and reading individual words from the i2c_frame.

jgoppert commented 7 years ago

The bandwidth for position control is only ~5 Hz. So getting the data at 10 Hz is all you need. When each camera image is taken and flow is computed, it stores the pixel flow and adds it up over the interval of 0.1 seconds. If you use my latest pull request, you can change the output rate if you like in qgroundcontrol, but will have to change it in the firmware to save it. https://github.com/PX4/Flow/pull/86/files#diff-7624ab7c53eb9962990d9668d74d8608R198

Your velocity estimation should use a kalman filter and discretely update when it gets a new flow message only. If you keep updating using old flow data it will create a blocky signal. Look at the px4/firmware driver for the flow to compare how you are checking if the data is new or not.

thomasck commented 7 years ago

Hmm interesting, what about the bandwidth for velocity control? That is what I'm really trying to do. What measurement model do I use for the kalman filter?

jgoppert commented 7 years ago

5 hz should be fine, if you have an imu you should use the specific acceleration to predict the velocity: d/dt(v) = (a - g), and then the measurement is linear in the state, y = v

thomasck commented 7 years ago

Thanks for your help @jgoppert, I was able to implement the filter and get better velocity data. However I'm still trying to change the sonar update rate to around 100 hz. Is this just a matter of changing a parameter somewhere? I downloaded your updated firmware from google drive, but am unsure how to load it into my px4 module. QGroundControl won't let me load it as custom firmware.