# Facet detection using facet times

A project log for open hardware fast high resolution LASER

bringing additive manufacturing to the next level

Hexastorm 09/03/2019 at 10:410 Comments

In my previous logs, I outlined that the cross scan error is well understood and can be kept less than 40 micrometers. The solution is to keep the spot very close to the facet, so the deviation due to the prism wedge is minimal. In future versions, five times more accurate prisms are used so the deviation should be much less than 40 microns.
In the log titled jitter, I outlined that the facet overlay in time can be a challenge. Although this was not the case for the proof of concept version. Let's present a solution;
For each facet the microprocessor calculates using the photo-diode signal how many ticks it takes per facet. There are about 12500 ticks per facet, so ideally you would always get 12500 ticks.
If i look at the a sequence of the first 16 facets I see;

12468, 12590, 12579, 12347
12473, 12591, 12581, 12349
12472, 12591, 12581, 12348
12472, 12591, 12580, 12348

Here, I ordered the times already in groups of four. It is immediately clear that you can bin the facet by times. Especially, the fourth facet in each row seems suited for a bin.
You can also calculate that the sum of the first group is is 49984, roughly 4X12500 = 5000.
If I take the bin with lower limit 12340 and upper limit 1360, I get 200 items over 800 facets.
The binning seems perfect; there are no outliers.

One could think that setting the counter to zero and illuminating after a certain time should give zero jitter. In practice, there are limitations as the spot speed is not constant over a line and there might be other aberrations.
You could also correct for the cross scan error once you have detected the facet but you might end up with a non-uniform dosage. This is quite critical if you are using multi-patterning techniques.

*** experimental remark ***
Too complicate stuff, a few days later I found out that I could get all the facet times equal. If make the jitter error very low, e.g. 1/3000,  I am not able to detect facets; their times are too close.
I had to modify the code a bit for this to pass the hsync threshold. I didn't do this earlier so therefore didn't see this. This was needed when I blew up the polygon motor and replaced it with the panasonic one. I thought I could see jitter but will ignore this for now and use like 1/200 of period and continue with one facet
****

Although, I could add an interpolation table to the algorithm. I will simply start with one facet and try to expose PCBs.

What further might be of interest to the reader is that I have also created some prior art in the field of door detection systems. A good example of a scanning mirror patent is EP1470314B1.

Claim one reads; " ... which operates according to the reflection principle and has the purpose of monitoring a region assigned to one of the approach areas, by means of a detector beam for obstructing objects ... ".
Prior art is constructed by claiming the same claim but than on basis of the refraction principle. I also claim all the areas where the Belgian company Bea Sensors uses reflective polygon scanners, e.g. the LRZ-H100 and opening & safety sensor for barriers.