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It's all coming together! We have a rig that allows us to attach an ultrasound detector and recorder to a UAV (have a look here on how we did this), and we can use gliding flight to record without interference from the engine. So we gave it a go: Flying the drone around on a loop we used a ultrasound source to produce noises that the on board recorders would pick up. The results are promising. When flying overhead at around 20 meters the detector was able to pick up the ultrasound signals emitted from ground-level. Since bats are often flying at tree height this gives us a good chance of picking them up in flight. The glide slope we experienced was approximately 3.5:1, meaning that for every 1 meter altitude lost we flew 3.5 meters laterally. This isn't particularly good and the glide slope was reduced by the fact we were flying into the wind whilst gliding. Although this provides additional lift, the reduced ground speed has a large impact on the slope. In future flights we'll fly the glides downwind as much as possible. The next step will be to plan and fly an autopilot mission using the gliding method to try and record an ultrasound source. It's worth noting that we don't think this is the ideal solution and would much prefer to be able to record all the time so we're looking at other options for this.
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In our previous tests we have shown that ultrasound interference is an issue (here and here) and that suspending the detector away from the engine is difficult (here, here, and here). One way to beat this problem is to turn off the engine and glide. If we take this approach then the detector rig can be mounted on the aircraft directly. A couple of things to consider:
We then placed Velcro strips on either side of the plane where we are going to attach the detector and recorder, and on the devices. This makes it easy to attach and remove the devices. First we found the center of gravity and marked it on the side of the plane 'CG' The detector and recorder fitted on really nicely to the Velcro which seemed pretty secure. To make sure that these don't fall off we put the lanyards around the pins that secured the wings, when the wings are in place this means there is no way they can fall off even if the Velcro fails. Next up we'll try testing the rig in flight and seeing if we can record some ultrasound!
Having pretty much decided now that a quadcopter is just too noisy for this project, we've taken the plunge and are now the proud owners of a Hobby King Bix 3: This plane was chosen primarily because it fulfills some basic requirements:
Once we'd put the plane together we needed to get into some flight testing. The first flights used manual mode. These went badly due to a poorly managed centre of gravity (CG) which needs to be accurately placed. Here's the result which we had to patch up: First successful flight now done we can move on to testing more features of the flight controller. Most important of these is the autopilot which will fly between a set of waypoints. Here's a map of the first route we'd set for our plane: When we turned on stabilisation mode (something to assist with flying) this was somewhat of a disaster as it turns out that the aileron controls were reversed in the flight controller leading to further crashes. A somewhat large amount of gluing and fixing later and we finally we worked out how to use our plane and using Fly-By-Wire-A mode. We achieved successful stable and in control flight with this mode. We were pretty chuffed as after the numerous crashed we'd given ourselves about a 25% chance of achieving a decent flight this time round. And below is the plane actually flying the route. Note that the waypoints are are in three dimensions so not only are they points on the map but they also have altitudes. We'd designed a route for our plane that took it to 50m altitude (relative to the ground level where we took off from). So what have we achieved? We now have a functional platform which can perform autopilot routes! Despite the crashes t's been a very successful weekend. We're looking to move onto longer autopilot routes next time and in-flight testing of the ultrasound recording equipment. When we tested the quadcopter we found it produced a lot of ultrasound interference. This time we tested our new plane (Bix-3, Hobby King) to see how it compared. We had three different propellers to test, and we also tried out some acoustic foam to try and dampened the noise from the engine. In the graph you can see each of the propellors types across the top and the results with and without foam along the side. The three lines in each panel show three distances from the nose of the plane, 0cm, 50cm, and 100cm, the top line being the closest (0cm). You can clearly see that across the three propeller types the foam makes a big difference to the volume of interference (note that y-axis has a log scale). All the top panels (with foam) have a lower amount of interference than the paired bottom panels (without foam). When we compare across the propeller types the Bix3 clearly outperforms the others, and the volume levels at 50cm with foam is similar to that at 300cm with the quadcopter, a great improvement.
What do we know so far: Hanging a bat detector from a single pivot point with string doesn't work, have a look at the blog post here for what happens. Next up the bat detector was attached to each of the four quadcopter arms using string at a distance of 3m. There's no video of this unfortunately but it didn't work. There was still huge oscillation with the load which would have lead to a crash if it had continued. The second option that was tried was a solid rod which could pivot with the pitch of the quadcopter. The idea here was to try and fly the quadcopter with pitch and yaw only. Here's a few photos of the design: The pivot is made from meccano and a few bolts from B&Q. The rod was meant for fishing and I think it's a glass/carbon fibre composite. Here's what happened when it was flown: So this didn't work either. It's not particularly clear from the video but the fact that there's almost no roll component to the flight makes the quad very difficult to control. Once a load was added to the extention of the rod it would be practically unflyable. As a short aside it may be possible to fly using the autonomous flight controller and limiting inputs to pitch and yaw but by human control it's too difficult not to use roll which is muscle memory by now. What's next? By using a dampener on the rod and allowing pivoting in two axes we believe that this would allow freer flight. Watch this space. UPDATE: at the present moment (October 2015) we've moved onto working with a plane and have achieved mild success using this vehicle. It's worth adding that the work performed here on the quadcopter is no longer necessary as our detector is now much less that 300g. I do believe that a quadcopter would be a very good vehicle to use for conducting these surveys though the cost is prohibitive to us as to achieve a flight time of even 30 minutes is beyond our budget. We also worked on the dampener in the form of a rubber golf tee (the sort you find at driving ranges). This proved to be very effective. If we hadn't investigated the plane, we may well be using this method now. I'd like to write a little on how we're going to capture the ultrasound that the bats are chatting and hunting with. In the last post we thought that we'd be using a Dodotronic ultramic 200k but as it's somewhat expensive (about £200) we thought it would be worth investigating other options. We're really getting into the nitty gritty of the project here so if you're not fussed by the technical aspects then we'll be putting up a Part 2 which will update everyone with which option we've chosen. Let's start with what we know so far. We'll be flying around with this thing so our system needs to be as light and small as possible as it might end up a bit of distance from the aircraft (due to the ultrasonic interference from the motor and propeller). There's a few different ways to do this let's take a look at them: Direct recording to digital recorder from an ultrasonic mic - for this option we need to fulfil a few requirements. The recorder must be able to sample at circa 200kHz as the highest frequency we'll be looking to capture will be around 100kHz and the sampling rate must be at least double the frequency you're looking to record. We would also need a microphone that is sensitive to ultrasound at these high frequencies. In theory this is an excellent option as it can be very small and light. The problem is that we haven't been able to find a small digital recorder that can record at ~200kHz. Aside from this, the ones that can are expensive (£thousands) which definitely takes this option out of our price range. Recording the output of a bat detector - we can use any type of bat detector for this (have a google for heterodyne, frequency division or time expansion detectors if you'd like to know more). The detector can be tied up with a digital recorder using a standard sampling rate (~96kHz) as the output of the bat detector is in the human audible range. The bat detector we were thinking of is the Batbox Baton (http://www.batbox.com/baton.asp). The cost of the Baton is around £85 and we already have a digital recorder so this does provide a good option. The only concern is the size and weight which, whilst smaller and lighter than other bat detectors is still considerable. Whether we use this option will depend on whether we see a large amount of interference close to the plane or whether we can work out a method for the quadcopter. Digital microphone paired with phone/raspberry pi - The final option was discussed briefly in the last post which is the Dodotronic Ultramic 200k. This does the sampling for you at 200kHz and can be recorded directly to a .wav file on a phone. This provides probably the lightest and smallest option but at a cost of around £200 for the microphone.
We'll make the decision for which option we're going after doing some interference testing with the plane in order to find out where we can place the microphone. If it's on the plane or underneath then we'll be able to go with the cheaper Batbox baton option if it needs to be a good distance away from the plane then it'll likely be the ultramic option. As a quick aside you might be wondering why the interference drops off so quickly with distance from the motors and propellers. As the frequency of sound increases so does the attenuation of that sound. This means that the attenuation of ultrasound in air is high so by 3m away from the quadcopter there is virtually no interference (see the earlier post when we measured this). Fortunately, some bats are very noisy with their ultrasound calls so despite the high attenuation we should be able to detect them from up to 30m away. So the distance at which we'll be able to detect species depends on the intensity of their call and the frequency at which they emit the call. The major factor here is the intensity of the call as due to specialisations they can be very different. For example, noctules are super loud as they fly in open areas and they need to see long distances. Can't wait to get testing and out into the field. Stay tuned for updates! So, as we know from the experiment with measuring the ultrasonic noise output of the quadcopter at different distances that the microphone needs to be at least 3m away from the quadcopter. As shown from the string suspended weight this produces stability problems when the 'detector' swings about. There seem to be a number of solutions to these problems:
What we're actually going to try: All of them! Might as well try everything as only by experimentation are we likely to find the best option. Firstly, we've bought a plane and will be doing the same noise test with the propeller interference. We'll also be trying wooden propellers to reduce this. The plane is a Bix3 from Hobbyking: Secondly, we're trying to reduce the weight of the recording equipment. Instead of using a bat detector and digital recorder we'll try using this configuration: Happily, we can use a mobile phone instead of a tablet to help keep the weight down. The microphone is an ultramic from Dodotronics.
Thirdly, we'll attempt to mount a lightweight (~100g) carbon fibre rod below the quad which will pivot with changes in pitch. This may provide problems with the roll of the quad but we'll have to see. Finally, we will be shielding any microphone with noise insulation foam. Hopefully through the combination of these efforts we'll have a viable vehicle to do some real world testing! We had a second go at suspending a pretend detector from our drone, but high-winds mean't that the test was probably doomed from the start! We got some nice shots flying over Wittenham Clumps, but once again the swinging cargo proved problematic ending in a pretty spectacular crash. It looked pretty cool but orientation was a serious problem. It was also difficult to see where we were landing so one landing was in a big puddle! Attaching the torch to the drone was a little tricky - to start with we had it attached at the front but as you can see from the end of the video, this made it flip over on take-off. We ended up attaching it right in the center. Flying our quad at night is key to the whole idea of recording bats since they only come out at night. This poses lots of difficulties such as knowing the orientation of the quad from a distance. We had a go in a place where we knew there wouldn't be people walking about.
As you can see we had some issues with this method, the bottle made the drone very hard to fly, and the bottle soon started oscillating. This doesn't seem to be a method that would work in practice as it would be very hard to pilot the drone, especially in the dark. Our previous work showed that the drone kicks of a serious amount of ultrasound, enough to make recording from a ultrasound recorder on the drone useless. Our plan was to see if we could suspend a detector under the drone, far enough away to avoid interference. We used a water bottle, weighing the same as the detector in place of an expensive ultrasound detector, to avoid unnecessary accidents!
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