With two way digital communication what are your modding plans?

After the latest update (51), it got me thinking of new mods and limitations we can have fun overcoming that can now be done.

For example, the self emptying build reservoir after print completion is now an easier mod(I'll link once I find the thread/post)

I'm sure there's going to more than few limitations that can be fixed with some home DIYing.

An example is that the Peachy's closed looped, something I'm sure we could fix (I'm not convinced using variable capacity to correct laser position is the best way), or the self emptying build reservoir after print completion is now an easier mod(I'll link once I find the thread/post)

So. What mod ideas do you have in mind?

How would you you do a real time reading of the laser position or dealing with a build reservoir that doesn't have a constant cross sectional area (like most container I know and it seems the capacity/frequency mapping is not in this version, from what I could see in the video).

So, to get the ball rolling. Some thoughts I had while washing the pots (living the dream).
There's a few ways we can get feedback from the mirror position, but they have to be passive and not interferer with the mirror in little or no way.
There are (the ones I could think of):

• Capacitance

given the current mirror and it's small size, would not hold well to added to that much added weight, also I'm sure normal air movement will add to much noise to the sensing.

• Magnetic field polarity strength

Not even sure how this one would work, at all....

• Light, IR/visible

I'm thinking either another laser (CD drive?) either bouncing off the back of the mirror (Will require the mirror to be aluminised both side or something to the same effect) or have two polarizing sheets along the axis of rotation, one being static on the support arm and the other being on the back of the mirror (this would be small).
At a guess the distance the peach mirror is from the printing layer is around 40cm(?) and with (what looks like) only a projection area of 5cm that's only going to move the mirror by 7deg... That's only a 12% sample range to fit 262,143 points in(18bit), which is would mean a stupidly sensitive sensor (ignoring noise), which brings me to my next point.

From my understanding to get a good reading to feedback you have to have twice the resolution, I'm not sure on the specs of the micro-controller being used but that's a big hurdle to tackle no matter that sensing approach. Maybe an additional micro-controller would be needed which serially feeds back.... Food for thought.

Quote:

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Originally Posted by jontelling

• Light, IR/visible

I'm thinking either another laser (CD drive?) either bouncing off the back of the mirror (Will require the mirror to be aluminised both side or something to the same effect)

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Ive been working on this ... jezz jontelling your ideas are going to take the element of supprize out of some future updates lol
well what can i say, This is one of my favorite ideas because it adds no mass to the mirror assembly.

and actualy as much as I liked how far we got with capacitive, i didn't like how hard that delicate bit of aluminum foil was to manufacture, assemble and Im woried about how it would do in shipping.

I had assumed you'd be using something similar to this with a photointerrupter to determine absolute position. it'd add a bit of mass and a bit of cost, but might be worth it for the accuracy... a 5000 cpr 1" wheel might be a bit overkill though... (that's one line every .072°)

Quote:

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Originally Posted by 3dspider

I had assumed you'd be using something similar to this with a photointerrupter to determine absolute position. it'd add a bit of mass and a bit of cost, but might be worth it for the accuracy... a 5000 cpr 1" wheel might be a bit overkill though... (that's one line every .072°)

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I’d say under kill, by a lot..

Current number of points is: 262143 (18bit), also using an interrupter would require the blockage of the laser more times than this if you’re going for a binary encoder. As you can guess it becomes a case of more points bigger the wheel as you have to block the whole laser, very tight focus required.


On a side note I have found 24bit ADC, :D
EDIT: Something to note, i think the peachy uses it's whole 18bit over a range of only 10deg, those interrupters are at 5000 for 360deg.. But I do like those.

Hey guys,

a little bit of googling...
At this point, you might want to use a Differential Plane Mirror Interferometer
this would give you a resolution around of 2*10^-5 ° (0.00002°) when using high quality products
;)

EDIT:
I just realized that dividing 10° by (2¹⁸) gives you about 3.8*10^-5 ° (0.000038°) degrees so this might actually be the way you have to go if you want the full overkilled resolution

quertz

Hmmm....

I think I found an interesting alternative that also might be easier to be DIY:

The general idea is to 'fold' a long way for the light to travel so that at the end you get a large deflection for small angles that can be picked up by a sensor stripe.

In this case you could just use two parallel mirrors.
http://jsbin.com/revepicigu/31/edit?output
I think you get the idea of having a sensor stripe on the right end of the tunnel.

This would of course have some problems like:
You cannot measure absolute values (you have to keep track and count)
You cannot measure the direction (in this configuration, but you could also replace the last part of the upper mirror with a sensor stripe to allow this)
The accuracy of the measurement depends a lot on the angle you want to measure

Many of these (+ skewed mirrors) can be solved by extensive calibration.

This is just a quick idea.
Please discuss, slate and find other ways ;)

quertz

Quote:

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Originally Posted by quertz

Hmmm....

In this case you could just use two parallel mirrors.
http://jsbin.com/revepicigu/31/edit?output
I think you get the idea of having a sensor stripe on the right end of the tunnel.


quertz

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I could be wrong but this doesn't amplify the deflection, just measures it from 90deg angle. The best to to amplify in this manner would to simply place the sensor at a greater distance from the laser diode. Buttt.... then we're needing a sensor with many points.

I'm going to have a look later at the range on MRD510 or something like that. :)

Quote:

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Originally Posted by jontelling

The best to to amplify in this manner would to simply place the sensor at a greater distance from the laser diode.

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That's exactly what I'm doing.
The only difference is that I'm using two mirrors to 'fold' the long distance into a short package.
In the end, this would also allow us to use a single, smaller sensor with less points that gets used over and over again.

Maybe I'll find some time and write a better simulation that also contains a sensor

Greetings,
quertz

How about just doing direct feedback on the laser position with a camera?

The Peachy team has shown videos of the Peachy illuminating the resin as it prints. Point two webcams into the container and (once calibrated) you can easily pull out both the absolute height of the resin in the container and the position that the laser's hitting it. Worst-case is that this has to go through the PC, so there could be a bit of lag (although that can be handled). Best-case is that they actually used one of the STM32 microcontrollers with a camera port on it or USB host, but that's probably pushing our luck.

Hey Slatye,

I think a camera would have just a way too small resolution.
If the image would cover the build area and nothing else, you would theoretically get HD resolution.
This won't be the case as you also have to analyse the image (the laser point will look a lot larger that it actually is) plus you will have calibration errors (skew camera, ...).

My current problem is that everything I can think of requires precise optics...

Quote:

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Originally Posted by quertz

Hey Slatye,

I think a camera would have just a way too small resolution.
If the image would cover the build area and nothing else, you would theoretically get HD resolution.
This won't be the case as you also have to analyse the image (the laser point will look a lot larger that it actually is) plus you will have calibration errors (skew camera, ...).

My current problem is that everything I can think of requires precise optics...

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I originally thought this but I’ve done some numbers (some made up).

If you had a CCD (or CMOS) of 15M and every pixel is sensitive to 405nm (they're not as they have filters for each colour) that would give you a square of 3872pixels.

The peachy at 18bit per channel is 262143 bit per channel. So, the dynamic range would need to be 67points per CCD pixel.

From what I could find based on my old Canon 40D the CMOS is 16bit.
So even if we only listen to the blue pixels the location of the laser could be worked out as we know there’s only one laser per image, that’s ignoring focusing of the camera..

food for thought.

The nice thing about using cameras is that there's a bunch of research already into making cheap-and-nasty cameras (with horrible optics that cause odd distortions) behave like "ideal cameras" (zero distortion).

I was thinking about using them along the lines of how the compass is used in UAVs - it's not updated very frequently (10 - 80Hz or so, compared to 400Hz+ for the main control loops and inertial measurements) and when it is updated it's not very accurate (to within a couple of degrees if there's not too much magnetic interference), but you can be fairly confident that it doesn't drift over time (unlike inertial and barometric measurements). There are a couple of filter designs that very neatly integrate this information with that from gyros and accelerometers (both very fast and very accurate in the short-term, but prone to drift in the long-term) to produce fast and accurate and drift-free outputs. In the case of the Peachy, the high-speed control is done with the drip sensor (which gives you immediate information about incremental water level changes, but may well drift over time if the drops are not quite the expected size relative to the container) and straight feed-forward on the mirrors (also prone to drift, from things like the DACs and lubricants warming up). The cameras won't immediately fix drift there, but over the course of a print they'll prevent anything drifting too far.

With regards to camera resolution: take a standard (cheap) 640x480 webcam with a 60-degree field of view horizontally. That should allow you to measure angles down to 0.1 degrees, give or take a bit. If the cameras are mounted 30cm above the printing surface then they'll be able to measure the laser dot position to an accuracy of roughly half a millimetre (if I've done the maths right). As Quertz said, the laser dot won't just be one pixel, but it wouldn't surprise me if you could consistently lock down the laser position to within a few square millimetres. No good for actually positioning the laser in realtime (especially because cheap cameras can't go anywhere near fast enough) but fine for slower control (eg. sensing that the design appears to be a little bit bigger than it should be at this stage, and therefore the output amplitude should be slightly reduced).

Using twin cameras for height estimation appears to (again, if my maths is right) give you 0.2mm height resolution, assuming cameras 30cm above the print surface, separated by 20cm. Height resolution will change over the build height, but getting it accurate to within 1mm should be straightforward. Again, it's not a perfect high-speed feedback system, but after the resin has risen a couple of millimetres you should be able to make a decent estimate of whether or not the "vertical rise per drop of water" is being calculated correctly.

I think the best option for mirror position feedback would be to use a patterned disk attached to the thread and bounce a laser off it to focus on an optical mouse chip. And there you have it 2000 dpi resolution for about £1 sterling (just cannibalise a mouse)

Quote:

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Originally Posted by mike_biddell

I think the best option for mirror position feedback would be to use a patterned disk attached to the thread and bounce a laser off it to focus on an optical mouse chip. And there you have it 2000 dpi resolution for about £1 sterling (just cannibalise a mouse)

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I think you're on to something, but not how I read your post. Rather than use the sensor directly from an optical mouse maybe we could use the whole unit, after all the mouse just sends movement information back down the USB not the RAW data it collects.

I'll put my thinking hat on as I've never worked with data from a mouse.

EDIT: Just the thing!
http://www.instructables.com/id/Opti...obot/?ALLSTEPS

Interesting article.... 1,900 counts per rev... pretty good resolution

http://www.sciencedirect.com/science...24424709003483

Quote:

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Originally Posted by mike_biddell

Interesting article.... 1,900 counts per rev... pretty good resolution

http://www.sciencedirect.com/science...24424709003483

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I think I've mentioned this, even using 5000points around 360deg it isn't enough, the peachy is 18bits, 262K points which is uses over 40deg. That's 2,358K per 360. I'll find my post... :)

errr not sure the number of A to D bits is relevant for this approach. We are reading digital values (x and y) serially. There is no analogue to digital conversion involved as the signal from a mouse is already digital. We can store values in long words if necessary. I think there are some very high resolution mice, so I think if I was monitoring mirror position, I would try this optical approach.

Mike - the problem is that you can't measure the position of the mirrors as accurately as the Peachy can drive them. As jontelling has said, the Peachy can apparently do 262144 counts over a range of about 40 degrees, giving it an angular resolution of 0.00015 degrees. The encoder you linked manages just under 0.2 degrees angular resolution. If you're trying to use all of the Peachy's precision, you might drive the mirror to 8.20305 degrees - but the encoder will only be able to tell you that it's somewhere between 8.2 and 8.4 degrees. You can't verify that it's at exactly the right position.

With that said, the encoder could be used as a relatively coarse check that the mirrors are in the right place. With 0.2° angular resolution, you can guarantee that (with the distances I assumed for the cameras) the laser is within about 1mm of the target.

Yes.... understand..... 5000 dpi is about the best resolution from the current breed. But optical is the way to go..... I think it would require a custom chip..... expensive !!!!!

Thinking this through, with the mirror mounted at right angles to, but rigidly fixed to a 2 inch diameter disk. Monitoring the disk at its periphery with an optical mouse type setup, we could get 30,000 steps. Pi*D = roughly 6 inches and at 5000 dots per inch, that is 30,000 dots. Therefore steps per degree = 30000/360 = 83 steps per degree. An angular resolution of .012.

Yes, you can definitely improve resolution with a bigger disc. The limiting factors will be how fast the mouse sensor can read it (although modern ones are very quick) and how badly the extra inertia of the disc affects the Peachy (those mirrors have to accelerate and decelerate pretty fast). Sounds like something to test when the first Peachys turn up.

the disk would be extremely thin plastic , and therefore very little inertia. In fact , you would only have three spokes and the main areas would be cut out (leaving only the rim).

I suspect that it'll still be an issue. The inertia of the mirrors must be very close to zero (because they're very light and also very small). I wouldn't be surprised if even the smallest practical plastic disc doubled or tripled the mirror inertia.