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Flint McCrae

Rayven with Arduino & brushless motors

Hi! First post here, and my second real modification.

After being badly humiliated in the office Nerf skirmishes last Christmas, this year I set about my Rayven project.  It's Arduino-controlled, uses four quad-rotor type brushless outrunner motors and a stepper motor to feed the darts.

Here's a video showing the internals and some firing tests:


Link


For office warfare I wanted a compact blaster, so I set myself the limitation of keeping within the length of the stock Rayven - but as you can see I had to go much wider.

To help keep things compact the outrunner motors bear directly on the dart; there are no flywheels.  The stepper motor idea worked out, which I am pleased about, but I've found that for this motor the maximum achievable rate-of-fire is about 9 darts / second.

Hope you like it, I certainly enjoyed building it and my co-workers hated it Smile
Andrew_Aitchison

Would it be possible to make the full auto mechanism short enough that you don't have to cut the shell?
Flint McCrae

Andrew_Aitchison wrote:
Would it be possible to make the full auto mechanism short enough that you don't have to cut the shell?


Good question, I think with the crank-and-rod design I've used it would probably not be possible, I found I needed at least 30mm stroke on the "ramrod", which means the crank diameter must be a little more than 30mm. You'd need to save 20mm of length to fit the crank design into the shell, which wouldn't leave a long enough connecting rod.

However, if you used a slotted-link type of design like the Rapidstrike has then it would be possible. This would be a more complicated construction, so I went with the cut shell and simple crank.
Minky

Nice work!  Nice to see the UK throwing up guys who have the skills to put these kinds of brushless multiple stage computer controlled builds together.  From what I understand using this tech for Nerf is still very much in its prototype stage and there's still kinks to be worked out,  so it's great seeing you and others representing the UK at the bleeding edge!

Have you had opportunity to chrono it?

Is that battery pack 2s, or 3s, or more?

How long did the build take you and did your u have all the appropriate skills prior to starting (order did you learn on the fly)?

Did it always have two stages,  or did you build it with one first?

What Revs have you got the two stages running at?

Sorry,  lots of questions but very curious about these monsters
Flint McCrae

Minky wrote:
Nice work!

Thanks! I was pretty well motivated after the last office war debacle (which was mainly due to the massive arsenal that the Managing Director rocked up with Smile)

Minky wrote:
Have you had opportunity to chrono it?

Not properly, but I have put a photodiode in the barrel which I can use to time how long a dart takes to pass.  This method has all sorts of potential inaccuracies, so big pinch of salt, very approximately I'd reckon typically 30-35 m/s (or 100-115 fps) with low outliers at 25 m/s (82 fps) and high outliers at 40 m/s (130 fps).  Subjectively it feels noticeably, but not vastly, faster than my Rapidstrike with 3S, MTB Rhinos and a Snikkas flywheel kit.  I suspect the limiting factor in my design is the size of the contact patch between the dart and the outrunner, I think profiled flywheels would be the way to go next.

Minky wrote:
Is that battery pack 2s, or 3s, or more?

I used 3S because that way if it all went horribly wrong I figured 3S would be best for re-use in another project.  I did find that discharge rate is important, those brushless motors are very greedy for current as they spin up and with a lower-rated lipo they were causing the voltage to the electronics to sag.

Minky wrote:
How long did the build take you and did your u have all the appropriate skills prior to starting (order did you learn on the fly)?

Hard to say exactly how many hours because I was feeling my way with a lot of it, maybe 60 hours.  I had some of the basic skills, but I did have to learn quite a lot.  Luckily there's plenty of info out there from the quad-rotor community which was very helpful.

Minky wrote:
Did it always have two stages,  or did you build it with one first?

It was designed to be two stage from the start but I did do some testing during the build with only the first stage, especially when I was finalising the dart feeding arrangement.

Minky wrote:
What Revs have you got the two stages running at?

The motors are what the quad-rotor guys call "4200 kv", which means 4200 revs for every volt.  So with 11.1v nominal lipo voltage that's a theoretical 46620rpm.  I'm currently running both stages at the maximum setting, I did consider running the first stage slower but it doesn't seem to make any difference to performance.  I plan to do some more testing on this.

Minky wrote:
Sorry,  lots of questions but very curious about these monsters

No worries! Appreciate the interest Smile
The Dark Kitten

Blinking Heck (as we say up north)
That's a right gooden.
I really wouldn't want to be the wrong side of that monstrosity.
I think the sleeper ascetic only partially works but the idea is nicely rounded.
Is this the first of its kind ever? Because if it is i do hope us brits are at it again!
And i think the spaghetti wiring adds...character which we only partially appreciate having full lengths are spewed at us at 130fps.
Well done Flint.
Minky

Thanks for your answers to my garbled questions gorram autocorrect!
Justajolt

I'd like to know more about that pusher...
L11

This is very impressive! Allot of work has clearly gone into the blaster.
I like the pusher too and would love to see it in action at a war.
NscrupulousModefiler

Flint McCrae wrote:

I was pretty well motivated after the last office war debacle (which was mainly due to the massive arsenal that the Managing Director rocked up with


Thank god for that! I was imagining the rest of the office with Mavericks and thinking this is pretty much the most massive piece of OP-Shock-And-Awe-Bows-And-Arrows-Against-The-Lightning-Mightily-Smiteily-Slap-Down retaliation in Nerf history!

Love to see some concave shells on those outrunners to get you some SERIOUS fps...
Flint McCrae

Thanks to all for the kind words!

Justajolt wrote:
I'd like to know more about that pusher...


This is going to be a bit long-winded! Sorry! Hopefully some of what I've learned might be useful for anyone else working on pusher mechanisms.

Getting the pusher working was challenging and a lot of fun.  I wanted very precise cycle control, with no "over-run".  I found that the Rapidstrike mechanism doesn't work well with microprocessor control because the RS uses mechanical switches to brake the motor (essentially turning the motor into a generator to slow it down).  I needed a digitally controllable motor that turns at (or can be easily geared to) speeds in the tens per second region with enough torque to give the necessary shove to the darts.  Turns out this is hard to find, servo motors are a bit too slow, most other motors way too fast.  Stepper motors are usually used for accurate movement of things like printer heads and machine tools, but can be made to run at the sort of speeds I required.  The downside is that as the RPMs increase the torque drops off - it turns out I'm pretty much at the limit for rate of fire from the stepper I've got at about 9 darts / second maximum.

The stepper motor has a controller circuit ((I used this one). When your microprocessor sends the controller a signal (a simple off -> on) the stepper motor advances one step.  Each step (on the motor I used) is 1/400th of a full rotation.  So send the stepper controller 400 pulses very quickly and it should make one full rotation.  That's the theory.

In practice it's a bit more messy.  First, if you have an especially "sticky" dart in your magazine, the stepper might miss a few steps - you send the signal, but it's unable to move, and you finish up with less than a full rotation.  Second, the stepper requires some persuasion to get up to speed - you can't just start sending super-fast pulses, you need to start slow and build up speed.  Third, the stepper can "over-run" if you try to stop it from high speed.

Luckily, in the world of microprocessors you can deal with these problems in software.  What my Arduino code (called a "sketch" in Arduino-speak) does is to make the first 200 steps at a slower speed, then once everything is moving speeds up.  The first half-turn is actually completed in just over 100 milliseconds, so there's a 0.1 second delay between pulling the trigger and the first dart hitting the flywheels. After this it increases to 3600 pulses / second which gets the 9 darts / second rate.

To avoid over-run the photoreflective sensor comes into play.  It's a reasonably cheap component that contains an infra-red LED and an infra-red detector.  If there's something reflective in front of it, then it sends an "on" signal to the Arduino.  I stuck a suitable piece of reflective material to the crankwheel at the right spot.  My camcorder is actually sensitive to IR, so in the video you can see this in action as a pinkish light.  To the naked eye it's invisible.  On the final rotation of each firing cycle the Arduino actually sends fewer than the 400 pulses necessary for a full rotation, 360 seemed to work best.  This means the crank always finishes a firing cycle slightly short of its "home" position, even if it over-runs.  The Arduino then advances it at a slow rate until the sensor reports that everything's correctly lined up.

The stepper motor is sourced from Ebay and rated at 1.7 amps. The crank and rod are 3D-printed, as is the "ramrod" which is a simple long cylider running in the brass tube and connected by a pin which runs in a slot Dremmelled into the tube.  The super-technical reflective surface is made from a piece of Kit-Kat wrapper Smile

One of my future plans is to re-do this mechanism.  Next time around I won't use a stepper, I think I'll retain the sensor but go with another brushless quad-rotor type motor and a reduction gearbox.  Reasons for this are to have the option of a higher rate of fire, and to reduce the workload on the Arduino by handing off some of the micro-management to the brushless speed controller.

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