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DerekF posted a blog entry in Digital Flight Control System for Foiling DinghiesAnother quick update, A productive weekend ended up with an IMU (BNO055) and a waterproof sonar range sensor (JSN-SR04T) being integrated to my Arduino Mega board. Both are yielding good data and easy to work with. I also am able to write commands successfully to a spare servo I from an RC Tricopter. I am still in the process of selecting the correct servo and identifying the correct attachment point(s) for the servo to flap linkage (please see and comment on servo math below). Additionally, I now have a PID library running giving servo commands based on the measured elevation. Tuning the loop gains will take some tweaking and I am beginning to realize a few buttons and a display of some sort may be necessary. I am trying to avoid scope creep but tuning control loops will most certainly need to be done on the water and doing so on a laptop will not be possible. More fun to come on UX... Lastly, it's time to think about packaging and power. The whole system runs on 5V and would be easy to power off of a set of AA or C cells. Eventually a lithium battery would work well and be rechargeable. I may need to add a power distribution board as the Arduino Mega won't provide enough current for the servo I am guessing. We'll see once the servo is selected. Servo Math Section Speaking of servos, my thinking here is to calculate how much torque is being imparted by the wand and using that as a minimum value required for servo torque. Being conservative my math is as follows: Calculate force on paddle: F = ma from Newton Finding a: Paddle will push flap down at minimum boat speed of about 3 knots or 1.55 m/s -- below that boat speed the flap rebound spring overwhelms the paddle drag. Therefore paddle must accelerate oncoming water from 0 m/s to 1.55 m/s. I am going to assume that this acceleration takes place over the course of 0.2 seconds. I have no scientific basis for this duration of acceleration. In reality this is probably a very complex fluid dynamics problem, but I think 0.2 seconds is a safe and conservative number. Therefore a = (1.55 m / s) / 0.2s = 7.75 m / s^2 Finding m: The paddle is around 4 square inches (~2 x 2 inches). (The wand also provides some drag that we will ignore for simplicity -- hopefully this isn't fatal =) Assuming that the amount of water accelerated from 0 knots to 3 knots is 4 square inches and 1 inch deep (yes another assumption), we have a total volume of water decelerated of 4 cubic inches or 65.5 cm^3 or 6.55e-5 m^3. The density of luke warm seawater is nominally 1027 kg / m^3. Therefore m = 6.55e-5 m^3 * 1027 kg / m^3 = 0.06726kg Finding F: Putting it all together total minimum force on the paddle to push the flap down is: F = ma = 0.06726 kg * (7.75 m / s^2) = 0.52 Newtons To be clear, this is the minimum force it takes to push the flap down when traveling slowly. As speed increases, it will take more force to actuate the wing as the speed of the water rushing by the wing is faster and will increase resistance to movement. Not to fear however, we can use the same approach to calculate the torque imparted on the wing by the paddle at higher speeds by varying a. At 18 knots (about as fast as my GPS has ever recorded me traveling) and assuming acceleration takes place over 0.2 s, our formula becomes F = ma = 0.06726 kg * 46.3 m / s^2 = 3.1 Newtons 18 knots = 9.26 m / s ; (9.26 m / s) / (0.2s) = 46.3 m / s^2 Finding Torque: Putting it all together to get torque, I assume that we are operating on the end of a 1m arm (our wand length). Therefore our torque values would be between 0.52 N-m and 3.1 N-m. A few caveats -- I made a lot of assumptions here and tried to be conservative. I also notice that at high foiling speeds (18 knots) the paddle depresses the flap with extreme ease indicating that while it imparts 3.1 N-m, a far smaller value would be sufficient. The last consideration comes with how to mount the servo and how many meters of moment arm we will use. I am thinking the servo will be mounted close to the fulcrum therefore our moment arm will likely be 0.2 m or similar. Therefore torque will need to increase by a factor of 5 to deal with the fact we only have a fifth the lever arm as the paddle. With all this math out of the way, I want to say that I am not a fluid dynamics or mechanics expert. I intend to take some empirical measurements next time I'm out foiling to verify my math. Please check my work and shoot holes in it wherever possible.
DerekF posted a blog entry in Digital Flight Control System for Foiling DinghiesTime for another blog installment of the Digital Flight Control System for my UFO. I have been busy at work on the project and making good progress toward a first flight in early 2020! Also attached is an updated block diagram with details on various subjects below: Data Logging: In my last blog post I mentioned that we needed some sort of data logging to really get into the meat of how well our control and filter algorithms are working. Post processing data after getting off the water will also allow us to see if there is data being generated by sensors that could improve control. For example, I received a suggestion that I take vertical acceleration from the IMU into consideration and use it to derive vertical position. This vertical position would then be fed into a Kalman filter along with the ultrasonic range sensor's vertical position measurements to determine a better estimate for boat altitude. Logging and crunching data after getting off the water would allow us to take these types of ideas into consideration easily. Datalogging would also greatly improve troubleshooting of all types. To this end, I have added an 8GB micro SD card to the design as well as written some simple code to log data to a CSV file for post processing. My current code logs speed (GPS), acceleration (XYZ axes), attitude (XYZ planes), and servo control input. Any other thoughts on datalogger inputs? PID Loop Tuning: It has also occurred to me that tuning PID loops while on the water will not be possible with a laptop. As such, I have integrated a wireless, 4-button, key fob (in a waterproof bag), a wireless receiver and an LCD screen. This will allow for wireless adjustment of PID constants and LCD feedback to the user. This setup should allow for easy adjustment while out on the water between foiling runs. The enclosure chosen also has a clear plastic top to make reading the LCD screen possible. Mounting, Wiring and Power Distribution: The Arduino board I am using only has so many pin outputs for 3.3V and 5V. While my sensors easily fall within the max current output of the board, I am short on pin space. I also have a 3D model of the enclosure that requires at least one mounting plate made from plastic / metal / fiberglass / FR-4. It occurred to me that rather than mess with mounting plates and a wiring rat's nest, I should just burn a simple PCB to handle mounting and power distribution. This PCB may also contain some custom circuitry--we'll see. The PCB will minimize the wiring mess and be a mounting plate in one shot. Many PCB shops will burn custom sized, one-off boards for ~$150. It's probably a small price to pay for much better reliability and maintainability (not to mention the time saved on creating a mounting plate(s) and hand wiring all of the modules). Enclosure: Now that I have selected a battery (2S LiFePo, 8000mAh) and all other components, I created a 3D CAD model of how the boards will lay into an enclosure. It appears that a 4x6 inch case will fit everything we need to hold the system (see photo of selected case). This enclosure will mount onto a mounting plate (probably custom fiberglass or carbon fiber) that will bolt on to click bond studs bonded to the spirit. The servo and ultra sonic sensors (both waterproof) will live outside of the case and be mounted to features on the mounting plate. This section of the design is the most open at the moment and will certainly require more CAD work. That's all for now. Comments are welcome!
DerekF posted a blog entry in Digital Flight Control System for Foiling DinghiesA few more updates on the project with the selection of a servo and battery. Servo Sizing: I decided to actually measured the torque required to actuate the flap. I did my measurement out of the water and therefore I only measured the resistance of the linkage and the resistance of the rubber "spring" that mates the foil flap to the mainfoil. I did the measurement using a pull scale attached to the wand 6 inches from the pivot point of the wand. I found the required torque to be ~32 oz. at 6 inches -- or 192 in-oz's. Converting this to kg-cm which is how servos are spec'd, we are looking at 13.8 kg-cm for the linkage and rubber "spring" resistance. Note: I did this experiment without the paracord attached as it will not be necessary once the servo and linkage is installed. As discussed earlier, determining the amount of force imparted by water rushing past the flap is a complex fluid dynamics problem however there are several great calculators for RC aircraft servos online. I used the one at the link below: http://www.radiocontrolinfo.com/information/rc-calculators/rc-airplane-calculator/#Torque I had to make a few assumptions regarding servo deflection as I have not fully designed the linkage but all results were within a pretty tight range. If we were foiling through air we would need ~0.2 oz-in of torque to operate our flap at 18 knots airspeed. Given that water is ~825 times as dense as air, I will assume we need to multiply by 825 to get 165 oz-in of torque in addition to the 192 oz-in to overcome the rubber spring force. This gives us ~356 oz-in required or 25.6 kg-cm. Therefore, our total theoretical torque required to actuate the flap will be 25.6 kg-cm. The other feature we will need is self-locking. Looking at my GPS logs, even on high wind days, I am spending about a third of my time not on the foils. On lighter days, it can be over fifty percent. During this time, the main foil flap is at max lift when the boat is moving forward and the wand is being pushed back. This does add more drag but allows the boat to enter foil-assisted planing modes. But having to provide constant servo torque even when not foiling will likely kill the battery pretty quickly and seems wasteful. To be on the safe side, I have selected a 32 kg-cm part that has metal gears and is self locking. Some waterproofing will need to be done but there are countless tutorials on how to do this online. This part should have plenty of torque to actuate the foil flap and the self locking feature will save battery juice. Power Budget: With all parts now chosen, it's time to choose a battery that will meet our needs. As the battery is likely to be the physically largest component of the system, the battery size will drive the enclosure sizing as well. My power budget is below and the result looks ugly! Amps Volts Power (W) Arduino Mega 0.5 5 2.5 IMU 0.1 5 0.5 Sonar 0.1 5 0.5 GPS (optional) 0.5 5 2.5 Servo (Stall Current) 5 7.4 37 Total 43 Runtime 4 hours Wh required 172 2S LiPo Operating V 7.4 Ah required 23.24324324 The largest 2S RC battery I could find was 8Ah max. There are bigger batteries out there but options are few. The 800 pound gorilla in the room is the servo. While we might be able to get away with a smaller servo, having the torque headroom makes me feel much better especially on a prototype. Not to mention that worm gear, locking servos aren't too common so choices are limited. A linear actuator might also be an option but they typically respond much slower than rotary servos and are heavy and expensive--I will do some research in this area. The other design difficulty if we try to mimic the current mechanical system, is that even while not trying to foil, we will constantly be using servo torque / battery power just to fight the flap's rubber "spring" to max out flap deflection on the main foil. Soooo.....this leads me back to insisting on a self-locking servo so that we aren't using battery charge to max out flap deflection even when in irons. If a suitable self-locking servo was not able to be identified another options is to use a GPS to determine boat speed over ground and assume that it is equal to boatspeed through the water. Yes, I know that doesn't take ocean current into account, but I think it's a small price to pay right now to extend battery life. Ocean current could be taken into account at a later date with tide charts built into the firmware as well as heading data from the GPS or IMU. We could also implement a button on the tiller extension that allowed the sailor to activate the foiling system when ready to fly. My worry with a button is placement and wiring / cable harness. Any thoughts here from the peanut gallery? If we limit current draw to the servo until we are actually trying to foil, an 8Ah standard battery will give us about 90 minutes of foiling time under (close to) worst case power draw. I would still like to see more battery life, but 90 minutes of time actually on the foils is quite a bit. Hopefully, real world testing will tell us that we don't use as much power and can get closer to 2 hours of foiling time--we'll see. Sampling and Filtering of Ultrasonic Sensor: While out foiling a few days back it occured to me that simply using the sonar to look at current elevation is not really what we want. We want to know what the "average" elevation is over a ~1m stretch of water. This reduces the effect of wavelets or even small boat wake (big boat wake is a different story). The other thing we need to be cautious of is false readings. When looking at the sonar sensor output, I occasionally see 0.0cm readings. Clearly, these erroneous readings need to be filtered. As a result, I have decided to take 5 samples over a 1m distance (assuming 12 knots of boatspeed). My code then takes the median value. This mid-level vote (MLV) system comes close to the average of the readings and automatically eliminates outliers. MLV systems are often used in redundant aircraft altimeters for this very reason. The code is pretty flexible and if sample size or sample frequency needs to be updated it is not hard. Data Logging: Last thought of the post: we will probably need some data logging for the prototype phase. Specifically, it will be useful to understand the delay between commanding a deflection on the foil flap and seeing the boat actually pitch up. Understanding control loop delay will really help in tuning PID loops and eventually allow testing of algorithms to deal with more complex and challenging wave terrain. I have put zero thought energy into how to implement the data logger, but it needs to be on the tech roadmap. Thoughts?
DerekF posted a blog entry in Digital Flight Control System for Foiling DinghiesOk team, here goes my first blog post. My goal here is to build a Digital Flight Control System (DFCS) for my UFO using a sonar range sensor as opposed to the wand. The design needs to completely mimic the performance of the wand system currently in the boat. The system needs to be fully removable to allow for One Design Racing if desired. A few design guidelines... Improvements to current control system: 1) Sensing wave terrain out ahead of the boat 2) Not engaging the mainfoil flap until the boatspeed hits 7 knots to reduce drag while floating 3) Toggle settings for upwind vs. downwind (maybe a button press?) 4) Data acquisition 5) Auto rear foil rake adjustment (this is a maybe feature, this would be lots of extra hardware for limited improvement) Questions to answer: 1) Do I need a GPS to get good boat speed or is the accel, gyro, magnetometer IMS good enough? 2) How and where will I couple the servo to the flap control push rod 3) How big a challenge will marinization be? Is enough to buy waterproof parts and a waterproof box? 4) Is it useful to have more than two sonar sensors to create an accurate map of oncoming wave motion? Does wave motion traveling orthogonal to the direction of the boat matter? Or are waves going so slow compared to the speed of the boat that it doesn't matter? Design Philosophy: I will endeavour to use parts that have excellent tutorials available online. I will be concerned about the cost of my time even when it costs a bit more money to buy parts that are easier to integrate. Basic Block Diagram Below: https://drive.google.com/open?id=1fPYXJLlrcZufAwTNtmjy9HG_BbAYtpas