03/21/2017 at 03:26 •
Variable pitch rotors have been in the back of mind ever since starting #Goliath - A Gas Powered Quadcopter. The default control scheme was always vanes underneath the rotors, but variable pitch would be a more elegant solution.
The inspiration for this project started after coming across Solid-State Rotor: A Demonstration, an AIAA paper by Onur Bilgen and Thomas E. Alberts. The paper is behind an AIAA pay wall, but the the gist of the paper is that they built solid-state rotors using piezo-electric materials that could change the rotor shape. The electronics for the materials were embedded inside the rotor hub. Power was provided via a generator built into the hub.
This project differs from the prior work in a few ways. The actuation is provided via servo instead of piezo-electric actuators. It'd be really cool to go completely solid-state, but the actuators are pretty expensive and they'd make the rotor cost more than $1000. It's also for bigger rotors, and this project will be open source.
03/22/2017 at 04:38 •
One of the driving parameters for a variable pitch rotor is the torque required to actuate the blades. The servo size required directly translates to the torque required to maintain the blade position. As part of the #Inexpensive Composite Propellers/Rotors project, I recently covered the RC series of airfoils which are specifically designed for rotorcraft. This airfoils were specifically designed to have low pitching moments.
How low? The two plots below show the moment coefficients for the rc5-10, designed for inboard rotor section and the rc6-08, designed for rotor tips. For the region of interest the moments coefficients are within +/- 0.005. Comparing this to the NACA 2412, which was the airfoil used previously, the pitching moment is 5-10 times greater in magnitude for comparable angles of attack.
The shape of the blades for the EVPR will be identical the 3rd generation rotors currently being built for #Inexpensive Composite Propellers/Rotors minus the hub.
The overall blade torque required at maximum thrust was calculated. The per blade torque requirement ends up being 1.52 N-m or 1.12 ft-lbs. However the hobby servo world works in kg-cm, so the requirement is 15.5 kg-cm. However, the travel required to go from zero thrust to full thrust for the rotor blades is much less than the full travel for a servo. The gearing ratio is TBD, but a factor of 2-3 is likely, so the servo torque needs to be greater than 5-8 kg-cm.
The initial servo chosen was the Tower Pro MG996R. They were chosen because they were what I have on hand, but they have sufficient torque (10 kg-cm) and they are fairly light (55 g)
03/26/2017 at 15:55 •
The EVPR will use wireless data transmission to control the servos embedded inside of the rotor hub. An ESP32 will be at the heart of the on-board electronics, receiving the data signal and controlling the servos.
To test whether the wireless data concept will work on a spinning rotor, a Sparkfun ESP32 Thing Dev Board was placed on a fixed-pitch rotor with a battery for power. A test was run to confirm whether an LED on the board could be turned on and off remotely over WiFi.
Below are shots of the dev board setup before and after being placed on the fixed-pitch rotor. Since the built-in LED on the board would be hidden inside the rotor hub, an extra LED(rectangular blue) was connected and placed in a location that would be visible externally.
The test was successful (video below), demonstrating that the wireless system should work for controlling servos for the EVPR.
04/13/2017 at 15:32 •
The design of the hub is progressing and the CAD is taking shape. Below is a closeup of the rotor hub.
The bearings will be made from acetal bushing stock cut to length. Typically, spherical ball bearings are used, but the loads for #Goliath - A Gas Powered Quadcopter are relatively low compared to full scale helicopter rotors and spherical bearings are pricey. A potential problem using the acetal could be a short life. In case that happens, a secondary design with spherical bearings is also being created as time allows.
Below is a screenshot of the secondary design. It's gives an idea of how the spherical bearings would replace the acetal bushing. The bearings would be embedded in the blade and pivot around the two bolts. It could be possible to keep the two design paths similar so that if a switch needs to be made, it should be a minimal disruption to the process.
The first batch of hardware should be here by the weekend, so the first prototype should start taking shape soon.
04/18/2017 at 03:54 •
The first EVPR prototype is starting to take shape. Over the weekend the first set of aluminum plates and polypropylene spacers made. The plates were cut from 0.125" thick 6061-T6 plate. The spacers were cut from a 12" long piece of stock. Below are some pics with the plates and spacers bolt together. For now a short set of bolts are used just to hold the hardware together. Eventually the whole assembly will be attached to the pulleys with longer bolts.
One of the servos was placed between the plates to ensure the spacers were cut correctly. An attachment method for the servos still needs to be determined.
04/29/2017 at 18:05 •
The first bushing has been completed and mounted on the prototype. The bushings are machined from Acetal.
The bushing was cut from tubular stock. In theory starting the tubular stock might have made making the part easier, but the ID had to be bored slightly larger anyways and machining the grooves in the side was kind of pain. If I stick with Acetal bushings in the future, I may switch to rectangular stock and make through holes for the bolts.
05/05/2017 at 02:52 •
The shafts have been completed and added to the prototype completing the mechanical portion of the rotor hub.
The method for interfacing the servo to the shaft is a little more concrete now.A lever was added to the shaft that will be used to rotate the shaft. The lever is a 3/16" piano wire 2" long. A 3/16" hole was drilled through the end of the shaft on the interior side of the shaft instead of the outside of the shaft.
There are a few benefits to having the lever on the interior side:
- The hole for the lever doesn't decrease the strength of the shaft.
- The lever is now protected inside the hub and less susceptible to damage.
- If the servo or linkage fails, the lever is captive between the two plates, limiting the travel of the blades if such a failure was to occur. This could mean the difference between losing the vehicle or not.
Drilling the 3/16" hole through the hardened steel shaft was an educational experience. Typical drill bits won't do much more than scratch the surface. I was about to buy some specialty bits when the intertubes provided some very useful information. Turns out that masonry bits with a carbide tip work well at cutting hardened steel. Just go slow and use cutting fluid. Since I had some masonry bits on hand it saved me some money as well as time. The process I ended up using was using a countersink to mark the location. A 5/32" masonry bit to drill the pilot hole and a 3/16" standard metal bit to clean up the hole to the proper diameter.
With the mechanical portion of the hub complete, the next step will be integrating the servos. The mechanical portion of the assembly weighs 2.0 lbs, an encouraging number.
05/27/2017 at 05:12 •
A few weeks back I assembled the major mechanical components for the first EVPR prototype. Shown below
This prototype used a simple Acetal bushing. This was done to keep the design simple and the weight down. After assembly, it became apparent that this wouldn't work as well as hoped. The dimensional stability is less than desired. When originally assembled, the shaft and bushing fit was acceptable, but with variations in temperature and the torque on the screws, the fit is quite variable.
The second issue was the lever arm concept for rotating the shaft with the servo. The mechanical movement wasn't working smoothly and the movement range on the shaft was less than desired.
After researching different configurations, it was decided to change the bushings to bearings and use a gear drive instead of a lever arm to rotate the shafts. Even better, most of the hardware needed could be source for a reasonable price from Servo City. A new branch was created for the repository and after a quite a bit of CAD work, playing around with various configurations, a gear drive re-design was settled upon. A screenshot of the geared setup is shown below:
The servo has a 24 tooth brass gear that fits directly onto the servo spline. On the shaft is a 40 tooth steal gear giving a gear ratio of 5:3. The shaft is supported by two pillow blocks. The only piece that needs to be custom made is the steel gear. That will be machined out of gear stock from McMaster Carr. The servo, servo gear, servo mount and pillow blocks are all off the shelf hardware from Servo City.
The hardware was ordered earlier this week and most of it's already been delivered, so if things go smoothly, the goal is to have the second prototype ready by end of Memorial Day weekend. Below are the hardware bits that came from Servo City yesterday.
I also plan to have more details of the servo sizing and selection in the near future.
06/03/2017 at 01:48 •
The mechanical elements of the second prototype was assembled today and the gear drive was tested. Below is a short video of the gear drive in action (Note that the top plate is removed to share the gear movement. For this test, a temporary power supply was wired up with two 3.7V LiPo batteries to get a 7.4V power supply and a spare RC reciever was used to provide the signals.
The pillow blocks, servo mounts, and the servo gear are all off the shelf components. The shaft gear was custom made from gear stock. Below is a picture of the gear, which also includes a set screw for mounting to the shaft.
Here's an overhead view of the gear and servo on one half of the rotor.
The next step is to put the hub on the quadcopter to check the fit and the clearances. Then the quadcopter will be run with the hub in place to make sure that the assembly holds together. If that works then it'll be onto wiring up the prototype electronics and making the first set of blades.
06/09/2017 at 02:31 •
While the primary power source for the rotor will eventually be an axial flux generator, batteries will be present to provide startup power and serve as a backup. The latest servos, HS-5685MH, run at 7.4 V max and draw a maximum current of 2700 mA.
Packaging was the primary driver for the battery selection. It was desired to have batteries that would fit inside of the hub. The battery chosen was the UBP053450/PCM. This battery will tuck into the gap between the servo and the upper plate and the main mounting bolts. The batteries are highlighted in blue in the figure below.
Each of the batteries have 900 mAh, so the total capacity is 1800 mAh. This is sufficient to provide 10 min of operation time at the maximum stall current for both servos, and should be adequate for controlling the vehicle if the axial flux generators fail.
The batteries should arrive next week. In the meantime a design for the battery clip needs to be created. After that, the ESP-32 dev board can be mounted inside the hub and the electronics testing can begin in earnest.