Author: lchilamk

Over the past week we have been working on the two pending tasks on the schedule: final testing and housing. We laser cut the remainder of the pieces and attached them to the 3d printed arms and brackets we made before the break. The body is pretty sturdy and awaiting drop testing, but as a form of risk mitigation we still have our prototype. The new housing unit is completed (video) so the only other task in our schedule is to finish up testing. In today’s round of testing we had four trials:

Min: 1.27 m

Max: 2.84 m

Mean: 2.40 m

Median: 2.74

We have been making adjustments to the code as we go with our trials, like adjusting binary threshold and thrust from trial to trial. We have also been able to reduce the latency by removing a delay in the function that sends data to the Arduino via serial. The movement is much smoother after we made this change. Our goal for the next week to finish drop tests by Tuesday so we can switch focus to shooting our video and working on the final report.

This week we finished up the interim demo and continued testing to improve the propulsion system and computer vision. At the end of last week, we finished integrating the Raspberry Pi with the Arduino in the prototype we have been working with. On Sunday we finished the integration phase by calibrating our software to respond to a circle target placed below the device (where the camera is pointed). We found that the camera feed was off center. That is, the center of the frame was not exactly directly below the camera. This would have been an issue later because when the target is near the frame’s center, we want to fully deactivate the motors. Another issue was that we had to assign one of the motor directions as the origin, or 0° point. As it was, the direction derived from the computer vision was not synced to the direction of the motors. To correct for both of these problems we added an offset of about 30° to the angle and 200px to the center inside the circle detection script. 

After the demos were completed we prioritized working on our propulsion thrust because it was not enough to get our device moving enough. We fabricated another thrust testing apparatus to measure the amount of thrust being produced. 

The propulsion is exerted down onto the scale and originally measured 330 grams. We theorized that the low gauge of our wires was inhibiting current from the ESC to the motors. When we replaced these connectors with thicker ones, the thrust was 400 grams. Another measure we took was adding 600 grams of gravel to the inside of the device, as a weight. This was meant to increase the tension in the parachute straps, to reduce swinging. We did a drop test with this setup and the swing was reduced, but our propulsion was still insufficient. We found after returning to the lab that the battery was low on charge. We will be trying again with increased propulsion tomorrow, weather permitting. 

Thrust test with low vs high gauge wires:

Lastly, we held the camera over the Pausch bridge to see if our detection would work after all the changes we made over the past two weeks. This week, we reduced the shutter speed to reduce motion blur. This change increased the detection rate from about 35% to about 90% in the indoor setup. We found that the circle was too small in the camera feed when held from the 43’ height. We are working on two avenues to fix this. One is to print a 2-meter circle instead of the 1-meter target we have now. The other method is to enlarge the usable portion of the fisheye image or correct it to be equirectangular. Our mitigation strategy for this area is also to start testing on a USB webcam, which will altogether remove the issues we face with a fisheye lens, but add more weight. 

Note the lack of motion blur with the faster shutter speed, both frames taken mid-swing:

1500 microsecond exposure time

9995 microsecond exposure time (circle is not detected!)

This week, we completed several more drop tests and prepared the interim demo for next week. We inferred from last week’s tests that there was not enough thrust to move the 3 meters that we wanted. Our idea then was to use two motors on each of the three sides instead, to hopefully double the thrust. At a TA’s suggestion, we also acquired larger propellers as another way of increasing the thrust. The thrust tests, done on a scale, showed 600 grams with double propellers. It was approximately 600 grams with the large propellers as well, but these were not meant for the motors we had so they couldn’t be installed properly without vibration.

We did a few drop tests on the double propeller arrangement we put together last week. We repeated the tests several times, each time adjusting for different variables. The first time around, we noticed there was swing so we changed the Arduino code to start after the drop began. On later tests we had to address spin and lack of movement. We were unable to modify the design so that the propulsion system would move where we wanted. This is the area of our project that is the most uncertain at this point. To mitigate risk, the next time we get the chance to do a drop test we want to add balanced weights to the inside of the device. We think that a persisting problem is the lack of tension between the parachutes and the device. With this change movement will not be impeded by swing or spinning. Video

For our interim demo, we set up an apparatus that will suspend our device 1.3 meters off the ground, while we move around a target image of 10cm diameter below the camera on the bottom. This models our actual 13 meter drop height against a 1 meter target diameter. We will show that we can activate the propellers in the correct direction based on the target’s relative location.

This week we improved on the drop test from last week, the problem was that the device spun during most of the descent and the parachute ropes twisted together. To fix this, we shortened the ropes and mounted them on opposite sides of the top. This solution would ensure that the parachutes were separated during the whole drop. Spinning would otherwise interfere with our perception method, which ultimately determines the relative angular position between the device and the target. When we tested this version on Monday there was no noticeable spinning. 

We then shifted our focus on movement from our propulsion system. We wanted to see how far the propeller at full thrust would move the device. Wednesday was more appropriate for the lateral distance test, because it was not windy. By this time, another set of parachutes had arrived, so we added a third parachute to the body, so the drop time would be longer. The test showed an additional parachute did not achieve this. We would like to test this a few more times before we reject this option, especially because there is more weight that needs to be added, including the payload and extra propulsion. 

At full thrust, the device could not move three meters laterally, a key technical requirement of our project. We decided that each arm of our device should have two propellers instead  of one. We already had an extra motor, ESC, and propellers, so we put double propellers on one of the arms. We plan to test the new thrust capability on Monday, and decide whether to order more parts.

Meanwhile, we conducted a few tests on the circle detection code. We found the 1m diameter circle (pictured below) was detected between 48 feet and 10 feet of distance. It is approximately the same indoors and outdoors. We plan to find the angular range of detection at a few fixed distances next week.