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EVPR: Electric Variable Pitch Rotor

An electrically actuated variable pitch rotor with a wireless interface

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The Electric Variable Pitch Rotor (EVPR) is a variable pitch rotor that is electrically actuated. The design is self contained, eliminating the need for heavy mechanical linkages or costly slip rings. An on-board controller, the ESP32, receives commands via wireless data and controls the blade pitch via servos placed inside of the rotor hub.

The primary power source for the on-board electronics will be a 3-phase axial flux generator, rectified to DC. The frequency of the alternator will be used to measure the rotor speed (RPM).

The EVPR could enable new multi-rotor vehicle designs, enhance current multi-rotor designs and potentially even be applied to small conventional aircraft.

The design files are open-source under the GPLv3 License.

Background

Multi-rotor vehicles are opening up new possibilities everyday. While much of what's promised regarding agriculture, shipping, flying cars, etc. is years, or even decades away, if just some of their capabilities can be achieved, the world will be a different place. Variable pitch rotors can make the promises of multi-rotors come to fruition.

Fixed pitch rotors are typically optimized for a single condition, namely hovering. This means that for other conditions, the design is less than optimal. Variable pitch rotors change the pitch of the blades as required and provide better overall efficiency.

Variable Pitch Examples

Most multi-rotor vehicles use fixed pitch rotors, but there are a few exceptions. The Stingray 500 and the MIT ACL Variable Pitch Quadrotor are two such examples. The maneuvering capabilities of these two vehicles are simply amazing.

EVPR: Electric Variable Pitch Rotor

The EVPR is a self contained mechanism. Servos inside the hub are used to actuate the blade pitch. Power for the servos is provided by an axial flux generator built into the hub. Control is provided via wireless signals. Once completed, the project will be demonstrated on #Goliath - A Gas Powered Quadcopter.

Mechanical

Unlike traditional variable pitch rotors, the blade actuation hardware is self-contained inside the rotor hub. Each blade is mounted to a shaft, held in place by two pillow-blocks. The shaft is actuated using a geared servo.

Below is the gear drive in action.

The servos and gear ratio should be application specific. For #Goliath - A Gas Powered Quadcopter, the required torque and a variety of servos are shown in the figure below.

The gear ratio chosen was 5:3 and the servo chosen was the Hitec HS-5685MH.

Electronics

The heart of the EVPR will be the ESP32. The chip will receive the data signal wirelessly from the flight controller and be used to command the servos via pulse with modulation (PWM) signals.

A simple test was conducted to ensure that data could be transmitted to the spinning rotor. The test was successful and the same wireless system should work for controlling servos for a variable-pitch rotor.

Power Source

Primary power for the on-board electronics will be provided via an axial flux generator with battery for startup and backup power.

The axial flux generator will have permanent magnetsfixed to the vehicle frame. and the coils will be built into the rotor hub and provide 3-phase alternating current. The current will be rectified to provide a DC power source for the on-board electronics and servos. An example of an axial flux generator is shown in the below video. The axial flux generator for the EVPR will be based off of that design.

Power Required

The power required will be dependent of the blade torque requirements for a specific vehicle. A simple power budget is shown below for the Goliath quadcopter as an example.

Item mA Voltage Count Watts
Servos 2700 7.4 2 38.5
ESP32 260 3.3 1 0.9








Total 39.3

The axial flux generator example shown in the video was outputting 45W at about 2000 RPM. For Goliath, the EVPR rotors will be operated nominally between 3000 and 4000 RPM, so the design will need to be scaled down to keep too much power from being generated.

The total power required will be about 160W. Since the gas engine is capable of providing 22kW (30 Hp). So the percentage of power used for the rotors would be 0.7%. Even if the power budget doubles, it still only be 2% of the total power available.

  • 1 × ESP32 Controller
  • 2 × Tower Pro MG996R Servo, Metal Gear
  • 6 × Hex Bolt Grade 8 Steel, 1/4"-20 Thread Size, 3" Long, Partially Threaded
  • 1 × Aluminum Plate 6061-T6 0.125" Plate
  • 6 × Polypropylene Spacers 0.5" OD x 0.25" ID x 0.78" long

  • Prototype Repair, Working Power Supply

    Peter McCloud13 hours ago 0 comments

    While it's impossible to know the exact sequence of failures during the previous test, it's clear that losing power was a contributing factor. Using small gauge solid wires with soldered connections and standard servo connectors was a poor choice considering the high vibration environments.

    The power supply board was re-worked afterwards to come up with a better design. The connectors for the servos and the batteries are all screw terminals now and all of the solid core wires have been eliminated. The image below shows the power supply board with the screw terminals.

    Another test was conducted, this time with just the electronics powered and the blades and axle removed (The replacement parts aren't ready quite yet, but I wouldn't have put them on for this test anyways). 

    The test was a success. The start-up and shut-down sequence was conducted a total of four times and the electronics never lost power. I should have done a test similar to test before attaching the blades in the first place, which would have eliminated some of the re-work I'm having to do now.

    The new blades are nearly complete and a spare set is in-work to ensure that if there is another failure, the project won't get slowed down. Below is the two epoxied blades and the cores for two more space blades.

    One other design change in the works is the addition of thrust bearings. The hardware for that will get here tomorrow and then the blades will be ready to be installed with the thrust bearings. If everything goes smoothly, the full prototype will be assembled again and ready for testing by the end of the week.

  • First Full Prototype Tests, Prototype Broken

    Peter McCloud07/16/2017 at 16:27 0 comments

    The first full prototype had been assembled and some basic firmware was written to do some simply tests. The first was a self test where the on-board ESP32 command the servos to sweep through the servo travel range. Below is the video of the self-test.

    Note that the servos have way more range than is actually required. More gearing could be added, but that would mean a loss in response time. It was decided to keep the current travel range, but limit the range in software.

    For the next test, the firmware was setup simply hold the blades at a specified angle of attack. The blade angle was set to a near zero lift condition (Due to the blade twist, the isn't really a true zero lift condition). The position hold functionality was tested by attempting to twist the blades by hand and the blades maintained their specified position. With the firmware functionality confirmed, the vehicle was turned on to test out the full prototype similar to the hub spin up test done a few weeks ago.


    The test didn't go well. The vehicle had an unusually hard start-up, causing the vehicle to flex more than usual. Shortly afterwards the electronics on the rotor lost power. Afterwards, it's hard to determine exactly what happened next. Even at 60 fps, the blades are spinning too fast to see what's going on. However, without power, the servos can't maintain the blades. It's possible that they rotated into the belts and were subsequently ripped off.

    Here's a photo of the rotor after the test. One of the blades was simply ripped from the axle. It appears that the bonding between the shaft and the blade wasn't that great in places. The black wire is the battery wire that became disconnected during the test.

    The other blade (the one that bounces page into the frame in the video) took the axle with it. To do this, one of the retaining rings and the steel gear had to be stripped off (I still haven't found either of those pieces in the shop). It appears that when the blade impacted the wall, the axle was kept going and popped out through the blade.

    Repairs are in-work and shouldn't take too long. More robust power connections are needed. I'll start with slightly bigger wires and screw-down style terminal blocks. Mechanical limit stops will also be added to prevent the blades from rotating too far. It'd probably also make sense to add some additional clearance between the rotor and the belts.

    UPDATE (7/16/17 10PM PDT)

    I finally found the missing gear today! The gear was embedded in the foam enclosure around the CNC router. The 2" thick foam was the best possible material for the gear to impact and the steel gear is completely unharmed.

  • Blade Construction

    Peter McCloud07/11/2017 at 01:20 0 comments

    The previous project log showed the assembled prototype complete with rotor blades. However, the construction of the blades for EVPR hadn't been documented. The rotor blades use a similar process as what's documented in #Inexpensive Composite Propellers/Rotors, but with a few tweaks.The blades are built using a composite layup process. The image above shows all of the materials that go into making the rotor blade. The process starts with the pink object, which is the foam core. The core is cut into the shape of the rotor blade using a CNC router. The core as shown above is after the supports have been trimmed off.

    To start, epoxy is mixed and a thin layer is applied to the core. Next, the two strips of uni-directional carbon fiber tape (The two black strips on the far right) are applied over the quarter chord on the top and bottom of the blade. Next, 3 layers of fiberglass (white woven fabric in the image) are applied individually and wetted out with epoxy as they are applied. The fiberglass is then covered with Aeroveil (gauze looking material below the fiberglass fabric in the image) which gives the composite a smooth finish when complete.

    The next step is to wrap the blade with peel ply (green plastic) which keeps the bagging material from sticking to the epoxy and then with breather cloth (white felt) and then everything is placed it n the vacuum bag (yellowish plastic).

    When vacuum is applied to the bag, the air is removed and the excess resin is sucked through tiny holes in the peel ply and into the breather cloth. This makes the part as light as possible. Below is the part after the epoxy has cured and the item has been removed from the vacuum bag.

    Finally the ends are trimmed off, and the axle is attached. This is done by drilling a hole into the root side and using epoxy to adhere the axle to the blade. With the axle attached, the blades can then be installed onto the hub.



  • Blades and electronics added to the first prototype

    Peter McCloud07/05/2017 at 03:56 0 comments

    The rotor blades and the electronics are now complete and have been added to the hub.

    For now, the prototype will just be operated on battery power. Once the rotor has been tested, and shown to work, then the primary power source will be added.

    Two 3.7V LiPo batteries provide the power (and will be the backup source for the final version). The batteries are tied together to provide the 7.4V needed to drive the servos. A voltage regulator is mounted on one half of the hub and provides power for the ESP32 dev board on the other side of the hub.

    Small proto-board with battery connections and voltage regulator.

    ESP32 dev board.
    The hardware is now ready to test all of the capabilities, except for the primary power source. The next step is to develop the Firmware so that more complex testing can begin

  • Prototype Hub Spin Up Test

    Peter McCloud06/21/2017 at 04:46 0 comments

    The prototype hub was assembled with just the mechanical components. (No batteries, controller or blades). It was decided that it'd be good to do a quick test to ensure that the assembly would hold together before proceeding further with the rest of the project. Therefore the hub was mounted onto the vehicle and a quick spin up test was performed.

    The hub held together, so the next step will be adding the batteries and the controller board. I've also started making the first set of rotor blades, which I'll document in a future build log.

  • Batteries selected

    Peter McCloud06/09/2017 at 02:31 0 comments

    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.

  • Second Prototype Assembled, Gear Drive Test

    Peter McCloud06/03/2017 at 01:48 0 comments

    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.

  • Gear Drive Re-design

    Peter McCloud05/27/2017 at 05:12 0 comments

    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.

  • Prototype Mechanical Components Assembled

    Peter McCloud05/05/2017 at 02:52 0 comments

      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:

      1. The hole for the lever doesn't decrease the strength of the shaft.
      2. The lever is now protected inside the hub and less susceptible to damage.
      3. 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.

  • First Bushing Complete

    Peter McCloud04/29/2017 at 18:05 0 comments

    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.

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Discussions

Mal'oo wrote 6 days ago point

When thinking about flight critical systems in aircraft, they tend to be set up in a way so that they have failsafes. Electric pitch adjustment, for example, use long screws, since they hold position without power to the motor. Fixed wing aircraft seem to use oil pressure to actuate the prop pitch, and in the event of loss of oil pressure (engine failure) the prop's failsafe position is full feathered: Minimal drag to aid in safely landing.

So, there's a need to engineer for failure here. Strong return springs might be out of the question, as it's additional force for the servo to overcome, but it's the best fail-safe option, as failure will, ideally, revert back to stable flight. 

Another possibility might be worm gears, as they hold position without any power input, similarly to the screws mentioned above. They're in the same ballpark for speed, even: A tiny bit of research puts servo gear reductions in the 100-600 to 1 range, with a tiny, low torque motor running at 10k RPM or so. Worm gear reduction is dependent on the number of teeth on the gear you're driving. At a guess, leaving the gear on your props unchanged would result in a 50:1 reduction.

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Peter McCloud wrote 20 hours ago point

Mal'oo, thanks for the feedback. I completely agree, flight critical systems need to have fail-safes. Since this project is in the early stage, there is a balance between adding fail-safes and just trying to get something to work. Honestly, I had considered the failure scenario that broke the blades before it happened, but I felt that the risk was low enough to move forward.

I like the idea of using worm gears to hold the blade positions in the case of a power failure. I did a quick search, but I don't see any off-the-shelf hardware that'd be a quick replacement. I'll be sure to keep it in mind for future iterations.

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Gravis wrote 07/05/2017 at 13:10 point

I like your concept but I have some tips to help.

1) Gear both rotors so that they can never go out of sync.

2) When in flight, it will need significantly more torque than a typical servo can provide.  Changing the gear ratios in a continuous servo might work.

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Peter McCloud wrote 07/05/2017 at 14:55 point

Thanks for the feedback.

1) Not locking the shafts together actually leaves open some intriguing design possibilities, such as the ability to act as a virtual swash plate.

2) The airfoils were chosen for their low pitching moment and the servos have more than sufficient torque for the current application. This is covered in one of the early logs.

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vsohal2 wrote 07/05/2017 at 05:37 point

You might look at the magnetic awash-late concept too: https://www.rcgroups.com/forums/showthread.php?969632-The-MAGNETIC-SWASHPLATE-4-Coil-Push-Pull-Concept-Experiment

What sort of power draw do you think the servos on your design will have? It seems that you will also have losses from wireless power transmission as well as holding torque for the servo. You might dramatically boost efficiency if you added some sort of mechanical locking mechanism to allow holding torque to be zero when you don't need to change pitch.

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Peter McCloud wrote 07/05/2017 at 14:49 point

Thanks for the swash plate info. That's a whole nother level beyond what I'm trying to do, but I'll be sure to keep it in mind in the future.

The two servos draw about 40W at stall. You're right, a locking mechanism or spring mechanism could reduce the power consumption. Once I've demonstrated the concept works, then I can l look at improving the efficiency.

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Yusuf Khan wrote 05/08/2017 at 20:22 point

Hi. This is a fascinating project, and congrats on becoming a finalist.

I was curious about 2 things. Why not slip rings for transferring power and control signals?
Also, I see the torque required has been calculated, but what about the servo's angular velocity and acceleration . What's the formula for rotor rotational speed to servo pitch speed?

I may be using the wrong lingo...

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Peter McCloud wrote 05/08/2017 at 20:48 point

Yusef, thanks for the compliments.

I haven't been able to find any off the shelf slip rings that can handle 3000-4000 rpm. There are some aerospace grade models that can be custom ordered, but they cost thousands of USD.

I haven't documented the servo details yet as I'm still working that out. Ideally I want 20 deg of blade pitch rotation for the full servo travel. The servos I'm using right now will travel the full range in 0.9s

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EngineerAllen wrote 05/01/2017 at 09:49 point

im going to be using variable pitch props on a future version of my asymmetric uav

having large rotors around CoM to generate 100% of vertical thrust eliminates the issue my current design has with asymmetric propulsion on pitch axis control.

since smaller variable pitch rotors can then control pitch axis without surrounding CoM.

it also maximises efficiency 

so im interested to see how yours works out

especially with the control system side

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Peter McCloud wrote 05/01/2017 at 14:59 point

Cool, glad to hear others are interested in it. I'm trying to keep in mind scalability as the design proceeds. How big are your smaller rotors?

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EngineerAllen wrote 05/04/2017 at 08:53 point

6"

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Daren Schwenke wrote 04/03/2017 at 19:12 point

This idea scares me.  You are introducing multiple single points of failure into a system where you already have no room for any.  I would look at using near field communication instead if you insist on going wireless for your control surfaces.  There is just too much traffic in the 2.4 Ghz band to make any system 100% reliable.  First time this powers up at a Makerfaire, you'll soon learn the entire band becomes a really small place.

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Yann Guidon / YGDES wrote 04/03/2017 at 20:26 point

My head spun a few times when I read that too... People just put 2.4GHz gizmos everywhere !

If you inject power through HF fields, ike contactless smartcards do, you can also modulate them to transmit data.

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Daren Schwenke wrote 04/03/2017 at 22:04 point

In this case, he could just modulate it directly with the inverse of the PWM signal the servo requires anyway.  A couple of analog bits would do it then.

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Peter McCloud wrote 04/03/2017 at 20:59 point

Daren, I'm not sure where "multiple" single points of failure are being added. I understand that the wireless data adds one extra single point failure. If the 2.4 GHz band is being relied upon to provide primary control remotely, via the transmitter, it doesn't seem too much of a stretch to use it for intra-vehicle communication.

I went with Wifi to start with because it's cheap and easy to implement as it's already part of the ESP32. I think your right that the NFC might be a better choice, though it would require 4 TX/RX pairs. The nice thing is that once the hardware and firmware are in place, it should be easy to implement other wireless technologies. Having two different ones would probably be best for redudency.

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Daren Schwenke wrote 04/03/2017 at 21:39 point

Each rotor will rely on it's own connection.  4 rotors.  4 single points of failure. 

The law enforcement way of bringing down drones is to flood them with wifi/false GPS signals.  In the case of your drone, that would also result in compromising flight stability and a pretty much instant uncontrolled plummet to the ground.

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daigakusei wrote 03/21/2017 at 11:10 point

Hi Peter,

is the power generator not a little bit over engeniering. The generation of electric power in the hub reduce the rotation for the rotor. Every change of magnetic field in the coils produce an reverse force on the magnet. 

Why not an Qi power induction like the loader for mobilephones. 

The sender coil axial around the axis and receiver coild in hub parallel.

The AC of the system whouldn't interferenc with the rotation of the hub.

P.S.: Excuse my bad english its not my native language.

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Peter McCloud wrote 03/21/2017 at 18:53 point

I had steered away from wireless power induction because I thought it'd be less efficient. Looking at Qi after you mentioned it, that may not be the case. I'll have to look into that more.

One advantage to a generator based system is that it could potentially be more fault tolerant. If the vehicle suffers a power loss the rotor could still operate and a backup flight control system could attempt to autorotate by feathering the rotors.

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Benjamin Hab wrote 04/08/2017 at 20:59 point

When talking about wireless power: I am not entirely sure if this would work, but how about tying the pwm line of the servos to V+ and using a pwm signal on the Qi? At least for analog servos that sounds like a feasible idea to me. 
Anyway, is there any particular reason not to take the rotor head of a big model helicopter? It is at least proven to work and would greatly enhance your yaw dynamics (I havent understood by now, how they are supposed to work with variable pitch anyway?)...
Best wishes,
Ben

P.S.: Your projects are really impressing!

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