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[0:00] I’ve been messing around making PCB coils, but instead of drawing them by hand,

[0:04] which would be pretty frustrating I’ve automated the entire process.
[0:09] I’m working on a new project with one of my friends and we’ve taken

[0:12] inspiration from Carl Bugeja’s amazing work on PCB Motors.
[0:17] If you haven’t seen his channel then check it out it’s very very cool.
[0:21] Now it’s not clear if what we’re doing will actually work as well as his projects,

[0:26] but there’s only one way to find out.
[0:28] As always my PCBs will be manufactured by PCBWay more about them later.
[0:34] So, making coils on a PCB. You might think this

[0:38] is pretty trivial and for a basic coil, it actually is very simple.
[0:42] So we’ll start off doing this first and then move

[0:45] on to the more challenging problem of creating arbitrary coil shapes.
[0:49] For our PCB coils we want to create as many turns as possible.
[0:53] Our main constraint is going to be our track width

[0:56] and track spacing along with the number of layers that we can use.
[0:59] Looking at PCBWay we can see that prices start to go up very quickly as soon as

[1:05] we go below 5 mil for the track width and spacing.
[1:08] So this will be the smallest size we’ll try and use. This

[1:11] is equivalent to a track width and spacing of 0.127 millimeters each.
[1:16] Staying on the theme of keeping it cheap, we’ll also be using a

[1:20] two layer board. Going up to a four layer board would increase the cost considerably.
[1:24] It is possible that these constraints will make our coils too weak to do anything,

[1:28] but we’ll see once we have the pcbs manufactured.
[1:32] So how do we make a coil?
[1:34] Well all we need to do is generate a spiral.
[1:37] If we were just trying to make a circular track, then we would simply use these two equations.
[1:42] We would then generate points on the circle by sweeping the angle from 0

[1:46] to 360 degrees and keeping the radius constant.
[1:50] The X and Y coordinates of the points on the circle can be calculated using basic trigonometry.
[1:55] To make a spiral we just need to make the radius increase proportionally to the current Angle.
[2:01] Now we can calculate the coordinates on a spiral by sweeping our angle from 0 to

[2:05] 360 Degrees multiplied by the number of turns that we need.
[2:09] We’re going to also want to have a coil on the bottom layer of the board.
[2:12] So we’re going to need some space in the center for a via to connect the top and bottom layers.
[2:17] We can do this by starting our radius off with a small offset. We can now

[2:22] just join our top and bottom chords together using a via in the center.
[2:26] If we look at the direction of the magnetic field that will be created

[2:30] from the two coils we can see that they will cancel each other out.
[2:33] To fix this we just need to flip the bottom coil.
[2:36] The current will now flow into the top coil and out of the bottom coil in the same direction.
[2:40] The top and bottom coils will now reinforce each other.
[2:43] Generating this and turning it into tracks and vias results in this very nice looking

[2:48] PCB layout in KiCad. It’s pretty cool. We have the top layer coil and the bottom layer coil.
[2:54] If we apply power to the top coil the current will flow in

[2:57] this direction and it will also flow in the same direction on the bottom coil.
[3:01] Now we’re going to be trying to repeat some of Carl’s work and make a three-phase PCB motor.
[3:06] For this kind of motor we’re going to need three coils spaced 120 degrees around our PCB.
[3:12] The motor can be made more powerful by duplicating each

[3:16] coil on the opposite side of the PCB. So we end up with six coils in total.
[3:21] To make the wiring up of these coils easier I’m adding a 180 degree offset

[3:25] to the angle on the bottom coil so that the end of the coil finishes on

[3:29] the opposite side to the start of the coil on the top layer.
[3:31] We can now easily connect the pairs of coils

[3:34] together by connecting the ends of the coils on the bottom layer.
[3:37] To make the current flow in the correct direction we just

[3:40] need to flip the Y coordinates of the opposite coils again.
[3:43] Connecting the ends of the opposite coils together

[3:45] to a common Point gives us the classic star wiring pattern.
[3:49] If we connect A to the positive rail and B to the negative rail leaving C unconnected

[3:55] then the A coils will create north magnets and the B coils will create south magnets.
[3:59] Similarly connecting C to the negative rail will make the C coils create south magnets.
[4:04] If we now connect A to the negative rail and B to

[4:07] the positive rail we’ll get north from the B coils and south from the A coils.
[4:11] And so on as we drive the coils appropriately.
[4:14] So this is great, but if we look at some images of

[4:17] other people’s PCB stators we can see that they don’t always stick to circular coils.
[4:22] How about generating arbitrarily shaped coils?
[4:26] Now this turned out to be a surprisingly interesting problem.
[4:29] I’ve managed to get something that works pretty well, but I’m sure there

[4:33] must be other solutions - let me know in the comments if you know a good way of doing it.
[4:37] My initial attempts produce some very interesting shapes. I tried to take

[4:42] the existing code for creating spirals, but calculated the radius by sweeping an angle

[4:46] around and casting a ray out to find the intersection point with the template shape.
[4:51] I used the distance from the origin to this

[4:53] intersection point as the radius and then tried offsetting this.
[4:57] Now obviously, this doesn’t work very well. And if we think about

[5:00] what is happening with a normal spiral we can intuitively see that we are always

[5:04] offsetting points at approximately a normal to the template circle shape.
[5:08] So, what we need to do is offset the intersection point at right angles to the line it belongs to.
[5:14] If we do this then we get something that looks much better the only issue is though

[5:19] it doesn’t seem to handle corners - they are getting chopped off.
[5:22] My first attempt to solve this was simply to smooth the corners of the template shape.
[5:27] There’s a really nice algorithm we can use to do this automatically

[5:30] called Chaikin’s corner cutting algorithm.
[5:32] This iteratively smooths lines and gives a really nice smooth

[5:36] result after just a couple of iterations.
[5:39] If we do this first and then run our algorithm we get quite reasonable results.
[5:43] However, we’re not really ending up with the shape that we originally wanted.
[5:48] An alternative way to fix the corner cutter problem is to detect when we

[5:52] switch to a new line segment and create an intermediate point where the corner should be.
[5:56] We do this by calculating the intersection of the

[5:59] previously generated points with the next two generated points.
[6:03] This algorithm works really well and we get a nice looking coil.
[6:06] The only issue we face now is that we can miss small details in our template shape.
[6:11] We are sweeping a ray around a circle and unless

[6:14] we step with very small increments we can easily miss a line segment.
[6:18] But, if we step with really small increments then it takes a very long time to process.
[6:23] So, what I’ve ended up doing is taking each point on the template. I calculate the angle

[6:28] from the point to the origin and from that I can work out how much offset would be required.
[6:32] This generates pretty much the same results as our original sweeping ray algorithm,

[6:37] but it’s a lot faster and it doesn’t miss any line segments.
[6:40] This works really well and we can create pretty cool wedge-shaped coils.
[6:44] Here’s the version with six coils, and we can even generate 12 coils instead.

[6:48] The wiring up is a bit more complicated but it can still all be done programmatically.
[6:53] The 12 coil version still creates the same star-shaped circuit as the six coil version.
[6:58] Everything is fully automated using a KiCad plugin.
[7:00] Making a keycard plugin was an adventure in itself. In

[7:04] many ways it was almost as hard as actually getting the coil algorithm.
[7:07] There are a couple of issues with making KiCad plugins - they are pretty hard to develop and

[7:12] test - you need to edit your code, refresh the plugins, run the plugin, see if it works.
[7:17] It’s a slow process.
[7:19] It’s also quite hard to include additional Python libraries that your plugin might rely on.
[7:24] KiCad comes with its own installation of Python. In theory you can add new

[7:28] python libraries using the bundled pip installer.
[7:31] However, on modern Mac computers this causes all sorts of problems.
[7:35] Macs have an additional security layer that prevents applications from being

[7:39] modified. This does help prevent viruses and malware from infecting your computer,

[7:44] but unfortunately running pip install in the KiCad directory modifies the KiCad python installation.
[7:50] So the operating system thinks it’s no longer valid and won’t let you run it.
[7:55] To avoid all these issues I’ve developed all the complicated code in a Python notebook.

[8:00] This lets me develop the coil generation algorithms much more quickly and I can

[8:04] easily preview what it will look like by plotting the output directly in the notebook.
[8:08] The notebook produces very simple JSON representation of the tracks, the vias,

[8:13] silk and pads. And my KiCad plugin simply reads this and creates the required objects.
[8:18] It’s a really nice decoupling and it means we aren’t really tired to KiCad at all.
[8:22] We could use any PCB layout tool that supports scripting that can read JSON data.
[8:28] The KiCad python API is pretty difficult to get started with. The documentation is

[8:33] auto generated from header files, so it doesn’t really tell you how to use it.
[8:37] there are quite a few examples, but the API is pretty unstable and seems to change fairly often.
[8:43] You often have to spelunk into the C++ code to see how things actually work.
[8:47] A great example of this was just trying to work out how to draw a circle.
[8:51] I could not work out how to set the radius - there’s no setRadius function!
[8:56] Digging into the code it transpires that you

[8:59] use the distance between the start and end points to set the radius.
[9:02] This is one of the big issues with auto-generating documentation from header

[9:05] files - there’s no context to help you understand how to use the API.
[9:09] Having said that, it’s an open source and free project. So I’m not going to complain too much.

[9:15] If I really wanted to make it better I should contribute some documentation back to the project.
[9:20] I won’t go into too much detail on the KiCad plugin itself. It’s surprisingly easy to

[9:25] create everything we need including tracks, edge cuts, vias, pads and mounting holes.
[9:30] It’s very very cool and definitely worth investing some time in learning how it works.
[9:34] I’ve put my code up on GitHub, hopefully, it will help other people.
[9:39] My pcbs are being manufactured by PCBWay - we’ve used them for a lot

[9:42] of projects and they keep on delivering the goods.
[9:44] There’s now a really nice plug in for KiCad that will submit your order directly to them - so

[9:49] there’s no more messing around trying to work out how to export the Gerber files properly.
[9:53] If you haven’t tried them out then give them a go.
[9:55] I should get the pcbs pretty soon, so we’ll soon see if what I’ve done has any chance of working.
[10:00] I’ll see you in the next video!

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Chris Greening


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A collection of slightly mad projects, instructive/educational videos, and generally interesting stuff. Building projects around the Arduino and ESP32 platforms - we'll be exploring AI, Computer Vision, Audio, 3D Printing - it may get a bit eclectic...

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