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Unread 09-08-2018, 03:17 PM
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Post New Motion Sensor

In the summer of 2017, a company called Bitcraze, makers of the Crazyflie drone, announced a new motion sensor for general purpose use. This product is their Flow Breakout board. We tried to use this sensor on our robot for the 2018 season. The attached white paper describes what we learned about how to get started using the sensor. In total it took several months to figure out how to use the sensor correctly, because the sample program from the vendor was incorrect (still is), and because we didn't pay proper reverence to the sensor chip's SPI interface timing requirements, assuming the RoboRio SPI interface would be just fine. If you decide to use this sensor, and we do recommend it, you will be months ahead if take advantage of what we learned the hard way. We would love to hear what you learn if you use this new sensor, too.
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Unread 09-08-2018, 04:29 PM
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Re: New Motion Sensor

This sensor seems extremely useful, thank you for sharing! I read through your white paper and it seems like you have been able to get readings and should have been able to use it on the real field for an auto, but viewing your most recent matches it seems your team still only had a simple drive forward auto. Have you been able to develop this further and create accurate auto modes or is just testing the sensor as far as you have gotten?
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Unread 09-08-2018, 04:40 PM
sailorjoe sailorjoe is offline
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Re: New Motion Sensor

Thank you for your interest, Brian. We had the sensor mounted on the competition robot, but because of the problems described in the paper, we really never got it working correctly until after the season was finished. It was in the post-season school year that we added the timing delays, rearranged the register read sequence, and added the squal and shutter variables. Because the Arduino code "appeared" to work, and because we "appeared" to get good readings three times out of four, the diagnostic process took us down a lot of dead ends. So you're seeing about six months of trial and error learning collapsed into a paper. I hope the team decides to use the sensor this year for better autonomous performance.
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Unread 09-19-2018, 11:22 PM
sailorjoe sailorjoe is offline
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Re: New Motion Sensor

The attachment herein has the spreadsheet we used to capture and analyze the test data for the Flow Motion sensor on the Arduino. I have it in xls format, but we did it originally with OpenOffice. Not sure if that will cause problems.
By way of explanation, Sheets 1 through 7 show the data from the sensor with different loop delay values. You can see the spikey data, but at this point, these can be ignored.
Sheet 8 is where most of the good stuff resides. At this point the software library problem had been solved. The left columns are the ascii data from the sensor via the Arduino. The next columns are the same data in numeric form. The column called "Rounds" is an Arduino count of times through a "sensor ready to read" loop, and its not likely useful any longer.
Then there is the same data through a low pass filter with half second time constant that can be adjusted. The graph shows motion going out and back to zero.
Under the top graph is a sorted list of runs made sequentially, in one direction only. The table is sorted by the number of readings, which goes down as the speed goes up for the fixed distance. To the left of the table is a graph of the same data.
Sheet 9 is the data from just one run of the Y axis, and we see no surprises.
Sheet 10 is a single run by turning (spinning by hand) the sensor. No surprises there either, so far.
I hope this provides some enlightenment on how to begin to interpret the data from the sensor. We certainly have more work to do to understand it before build season.
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File Type: xls FlowMotionTests.xls (284.5 KB, 4 views)
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