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@@ -2,13 +2,29 @@
## Motivation
-w/ AS5304 / AS5306
+I want an encoder! Encoders<sup>1</sup> read position, generally by counting 'ticks' on some reference. For example, an Optical Encoder uses a reference with openings that allow light to pass, and count whenever a photodiode goes high or low to indicate that some motion has occurred.
-Pins for Quadrature Input on ATSAMS70?
+
-A key limitation in my previous work here was encoder positioning. I wanted to use a ring magnet and off-axis linear sensor to decode rotary position - the trouble was I could only find one supplier of ring magnets with the right pole spacing, and it was ~38mm ID. The whole gist of the mechanical design here is to bring the diameter way up, so this was not satisfactory, or, at least, it was a constraint.
+Then, you add the ticks together to get your position. In order to detect direction, a technique called 'quadrature' is used:
-Here I am going to try to use discrete magnets in an assembly of my own... I am basically just building a big ring magnet, and trying to read that.
+
+
+Super simple, very neat. Here's picture of a *fancy* encoder:
+
+
+
+You'll notice this encoder has two other reference markings - those are super interesting as well, and encode *absolute* position into the ring - there is only one uniqe combination of holes-or-not-holes on those rings for every single indicated position. This is good news because then your joint / axis does not have to touchoff on a known reference point - aka a 'home' or 'limit' switch - before it starts counting. If this looks expensive, it's because it is. An encoder like this might run ~ $300, and they're super sensitive to dust, misalignment etc. That's no bueno.
+
+**So** my goal here is develop an encoder that I can put on whatever-I-want without spending too much money, and without fussing too much about setup. Critically, I also want to un-constrain myself from particular sizes - whenever I have to source a particular size ring or strip (for reference) I end up boxed-in in terms of the *~ design spaces ~* I can get into. I don't like that.
+
+## Solution?
+
+So I'm going to try rolling<sup>2</sup> a magnetic encoder<sup>3</sup> with a custom array of magnets. This way I can roll a ring or line of magnets into whatever structure I want, and hover one of these encoders above it to read position.
+
+The chip I'm going to use is an AS5304 - a product offered by AMS that does all of the hard work on the hall-array end for me. I can read quadrature output from this ('ABI' interface - 'I' being an Index Pulse)
+
+So, in summary, I am basically just building a big ring magnet, and trying to read that with the AS5304.
## Problems
@@ -18,17 +34,17 @@ angular resolution = 360 / (((PI x D)/pole_length) x 160)
Of course, I have to set the circumference equal to an integer value of magnets. I wrote a quick spreadsheet for this, and get that w/ a Diameter ~ 100mm (where the gearbox currently stands) I can get 0.017 deg/step with a 2.4mm pole pair, and ~ 0.03 deg/step with a 4mm pole spacing. Nice.
-
+
-Now, this is not so straightforward. The important thing to remember is that **Resolution != Accuracy** - while I have ~ 0.03 deg / step, those steps are not exactly mapped perfectly in a circle. I will actually be referencing magnets that are glued in place - so my glueing and magnet placement would have to be *perfect* for Resolution to = Accuracy. In addition, not all magnetic fields will be identical, etc. Everything is crooked.
+Now, this is not so straightforward. The important thing to remember is that **Resolution != Accuracy** - while I have ~ 0.03 deg / step, those steps are not exactly mapped perfectly in a circle. I will actually be referencing magnets that are glued in place - so my glueing and magnet placement would have to be *perfect<sup>4</sup>* for Resolution to = Accuracy. In addition, not all magnetic fields will be identical, etc.
-In addition to these mappings between poles, I will have some deviation, I'm sure, within the poles. As in, the magnets will likely have small gaps (finding a magnet which is perfectly 4mm across... not likely) and so I expect readings to kind of sine-wave around linear.
+In addition to these mappings between poles, I will have some deviation, I'm sure, within the poles. As in, the magnets will likely have small gaps (finding a magnet which is perfectly 2mm across... not likely) and so I expect readings to deviate in a kind of sine-wave around *actual* position... one period of the wave corresponding to one period of the poles.
This can be overcome in implementation by mapping in memory a lookup table, with respect to some 'home' position, of how measured ticks of the encoder match up with reference positions. For example, I would rotate the encoder to known positions (with some other, higher precision system) and map readings -> known values.
This is not ideal! And it requires a deal of labor, some good routines and memory-storage games, and a reference point.
-In this exercise (which I am trying to keep bounded)<sup>1</sup> I will try only to measure this deviation-from-perfection. Or, at least, deviation from some other, better positioning system.
+In this exercise (which I am trying to keep bounded)<sup>5</sup> I will try only to measure this deviation-from-perfection. Or, at least, deviation from some other, better positioning system.
At best, I hope to cancel periodic deviation (i.e. map the sine-wave between individual poles into a more linear interpolation. my encoders have an Index pulse (on every top-of-pole) that I can use as a reference point for this.
@@ -54,15 +70,15 @@ And started laying out a schematic. Had to build the footprint in Eagle.
-If you're wondering why I have SPI pins labelled, it's because I'm actually connecting this to a plug on my Networking Switch - these are the pinouts. Instead of SPI I will just configure those pins to do GPIO.
+If you're wondering why I have SPI pins labelled, it's because I'm actually connecting this to a plug on my [Networking Switch](https://github.com/jakeread/tinynets/tree/master/circuit) - these are the pinouts. Instead of SPI I will just configure those pins to do GPIO.
THIS is a big what-if for me. As in, what if I get my switch boards back from the fab and it turns out I can't get GPIO to work on these particular pins? I have trapped myself. !
I also have voltage dividers set up here - the AS5304 drives 5v, and I don't want to pump that into my puny 3v3 logic lines on the ATSAMS70.
-I also have a jumper / pin header footprint setup on the Analog Out pin - this will let me read the strength of the magnetic fields, making sure I am aligned.
+I also have a jumper / pin header footprint setup on the Analog Out pin - this will let me read the strength of the magnetic fields, making sure I am aligned, and that the field strength does not deviate too wildly between poles.
-Ideally, I would tie these to an ADC on the ATSAM, but I haven't broken any of those out on the BLDC encoder board... this is a long chain of interconnects. I am rethinking my overall switch -> peripheral strategies now. A good learning / systems design moment. OR I could tie it to some kind of comparator, and have an LED turn on only when that voltage was in a particular range. *shrugguy*
+Ideally, I would tie this voltage out to an ADC on the ATSAM, but I haven't broken any of those out on the BLDC encoder board... this is a long chain of interconnects. I am rethinking my overall switch -> peripheral strategies now. A good learning / systems design moment. OR I could tie it to some kind of comparator, and have an LED turn on only when that voltage was in a particular range. *shrugguy*. This is helping me design the modular switch, so that's good news.
OK, Routed
@@ -70,6 +86,10 @@ OK, Routed
Exporting to mods from Eagle is a bit of a pain. In the fullness of time I would write a MOD to do GERBERS -> Milling, however.
+In Eagle, I do this
+
+Options -> Settings -> Misc -> don't display pad names or signals on traces
+
Layers -> none
Layers -> Top, Pads
export image monochrome
@@ -86,7 +106,7 @@ Back to my layout, and working back into my sheet, I'm going to add magnets here
And then to test this against some reference, I'm going to use a stepper motor. With the current diameter I'll have 0.05 deg resolution (or, I would ideally) - this means I would need 1/32 microstepping to match up on a stepper motor - I want my reference to have a bit more resolution than what I'm measuring, so I'll try to find a stepper motor with 400 steps / rev, and do 1/32 microstepping on that. Then I have 0.028 deg / step. I'll also bring the diameter down on my encoder to 58 poles, so I'll have 0.077 deg / step there. This means I have about 3x the resolution on my reference than I do on my measured-thing, and intuitively that feels like a good spec. Thumbs up.
-ALSO - I knew I should check this, and I'm glad I did - here's a #2 and a #0 screw - the #2 contacts the encoder ring. Noice.<sup>2</sup>
+ALSO - I knew I should check this, and I'm glad I did - here's a #2 and a #0 screw - the #2 contacts the encoder ring. Noice.<sup>6</sup>
@@ -118,24 +138,43 @@ Also, a moment of appreciation for the new *Leica* microscope that showed up at
I printed the hardware on the Eden. The Eden's washing-up station is currently broken, and I spent the better part of an afternoon trying to cobble together a fittings-and-adapters solution to unbreak it, to no avail. I await McMaster parts.
-## Programming
+Update: I borrow Biomechatronics' eden waterjet blaster. I have parts now.
+
+First order is getting these magnets in. They're small!
+
+I accidentally put a few poles side-by-side (rather than alternating) - a bit of a bummer. But this is good enough for my test... Here's the ring
+
+
+
+Ok I got this mostly together - made one mistake, where the motor floats a bit behind the mount. I kind of knew this might be an issue - also, generally, I am overconstrained w/ the motor bearings fighting my ring bearing. In hindsight I didn't even need this ring bearing, but it's something I also wanted to prototype for later development, so here we are. Belt and Suspenders.
+
+
+
+## Testing
-I'm using my ATSAMS70 Switch (link?) - I actually used this project as an excuse to bring the GPIO (in SAM-speak, this is just 'PIO' and stands for Parallel Input and Output). Looks like it's datasheet-o-clock again...
+Before I get into the meat of programming interrupts etc for this, I'm going to first get some voltage on this board alone and scope the lines to see that I am getting (1) the voltage I want out of the magnetic-field sensing pin, and (2) the ticks I want on the ABI lines.
-## Measurements
+## Programming, Measurements & Corrections
I want to be like Sam. Sam measures *everything* and he does it really beautifully. This is some of the functionality I really want to be able to bring into mods - the ability to quickly pull test data out of systems, do computation with it, and learn.
With that motivation, I want to take this chance to run a test cast of that. The Goal here:
- - Use one of the 'network switches' (or are they routers? vagueness rules in this project at the moment)<sup>3</sup>
- - Gateway it to MODS
- - Build a basic interface to do this test, where I spin the stepper around a few times and measure devation from 'linear' rotation between the stepper and the encoder.
- - Pull this data out and chart it in mods, using javascript to do the maths.
- - Have some conclusions, or solutions, to whatever I see in this data.
+- First, do ATSAM Router -> MODS pipeline
+- Read Encoder with GPIO Interrupts or QED<sup>7</sup> and report periodically to MODS
+- Bones of step & dir control on the ATSAMS70
+- Stateless Stepper Motor Drive on MODS -> ATSAMS70
+- Graphs & Charts & Realtime Data Capture in MODS
+- Push a line to the stepper, read encoder, watch linear != linear
+- Think long &